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Moldflow DesignGuide
Jay Shoemaker (Ed)
A Resource for Plastics Engineers
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Jay Shoemaker (Editor)
Mold1047298ow Design Guide
8102019 L-G-0004410537-0002634439 Moldflow
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Mold1047298ow Design
Guide
A Resource for Plastics Engineers
Jay Shoemaker (Editor)
Hanser Publishers Munich bull Hanser Gardner Publications Cincinnati
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The Editor
Jay Shoemaker Mold1047298ow Corporation Framingham Massachusetts USA
Distributed in the USA and in Canada by Hanser Gardner Publications Inc6915 Valley Avenue Cincinnati Ohio 45244-3029 USAFax (513) 527-8801Phone (513) 527-8977 or 1-800-950-8977 wwwhansergardnercom
Distributed in all other countries by Carl Hanser Verlag Postfach 86 04 20 81631 Muumlnchen Germany Fax +49 (89) 98 48 09 wwwhanserde
The use of general descriptive names trademarks etc in this publication even if the former are not especiallyidenti1047297ed is not to be taken as a sign that such names as understood by the Trade Marks and Merchandise Marks Actmay accordingly be used freely by anyone While the advice and information in this book are believed to be true and accurate at the date of going to press neitherthe authors nor the editors nor the publisher can accept any legal responsibility for any errors or omissions that may bemade The publisher makes no warranty express or implied with respect to the material contained herein
Library of Congress Cataloging-in-Publication Data
Shoemaker Jay
Mold1047298ow design guide a resource for plastic engineers Jay Shoemaker p cm ISBN-13 978-1-56990-403-9 (hardcover) ISBN-10 1-56990-403-0 (hardcover)1 Injection molding of plastics I Title TP1120S486 2006 6684lsquo12--dc22 2006012438
Bibliogra1047297sche Information Der Deutschen Bibliothek Die Deutsche Bibliothek verzeichnet diese Publikation in der Deutschen Nationalbibliogra1047297e
detaillierte bibliogra1047297sche Daten sind im Internet uumlber lthttpdnbddbdegt abrufbar
ISBN-10 3-446-40640-9ISBN-13 978-3-446-40640-7
All rights reserved No part of this book may be reproduced or transmitted in any form or by any means electronic ormechanical including photocopying or by any information storage and retrieval system without permission in writingfrom the publisher
copy Mold1047298ow Corporation USA 2006Production Management Oswald Immel Typeset by Marcia Swan USACoverconcept Marc Muumlller-Bremer Rebranding Muumlnchen Germany Coverdesign MCP bull Susanne Kraus GbR Holzkirchen Germany Printed and bound by Koumlsel Krugzell Germany
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Foreword
The drive toward fast cost-effective and reliable plastics manufacturing has been Moldflowssole guiding goal since the company was founded over 25 years ago
This focused determination led us to introduce many new and exciting tools into the marketeach contributing to achieving our goal in some way whether by driving cost out of production with reduced material usage or shortened cycle times reducing mold delivery timeby minimizing re-work or increasing the reliability of supply by enabling higher quality products to be manufactured with greater surety in scheduling
The artificially balanced multi-cavity and family molds that are now commonplace were madepractical through the advent of our early simulation and runner balancing capabilities which
were introduced in the late 1970s and early 1980s As these tools evolved we were able to visualize and therefore control flow patterns and weld lines This evolution continued until we arrived in the 2000s with an array of sophisticated technology to control warpage accountfor heat transfer predict core shift adapt to new molding processes and much more Fromtraditional midplane technology to fully three-dimensional simulations all our solutions are well integrated into a solid-modeling design environment
As the technology has evolved so has its usage When Moldflow simulation technology wasintroduced its primary purpose was to search for remedies to pre-existing molding problemsIt soon became evident that the insight the software provided to solve molding problems would be better applied ahead of actual molding during the design process This
methodology which we call problem avoidance was the primary use for Moldflow technology for the first 20 years of its existence
For Moldflow this created a unique challenge to open the world of manufacturing to thedesigners of parts and molds What constitutes an ineffective design for molding may beapparent to a seasoned processing engineer looking retrospectively at a poorly performing tool but how can design engineers use the CAE tools to visualize diagnose and solve thesesame issues ahead of timewithout 20 years of molding experience How can manufacturersgo further and use information that cannot be seen in the real molding process but is revealed via simulation
The key that unlocked this puzzle began its life as the Moldflow Design Philosophy This is widely
viewed as the most important publication Moldflow has ever produced and has spawnedfollow-on works on related subjects Rather than provide insight into the operation of thesimulation tools Moldflow Design Philosophy set forth simple principles that transcend any specific software application and as a result are as valid with todays advanced simulationproducts as they were over two decades ago
In more recent years another transition has occurred The global imperative to drive down thecost of manufacturing has led to the use of molding simulation as a cost optimization toolrather than for problem avoidance This change has increased the number of Moldflow usersby an order of magnitude across a far broader cross-section of the plastics industry Greaterdesign-centricity leads to even more dependence on the plastics design principles which can
be used to drive optimization
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VI Foreword
Despite a quarter of a century of technological advances the golden years of CAE are aheadof us as our industry takes a broader and more integrated view of what it takes to manage a
products life cycle Moldflow is proud of its contributions to date and will continue to focuson developing innovative technology coupled with practical design principles to deliver moreprofitable manufacturing
Roland Thomas
President amp CEO Moldflow Corporation
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Preface VII
Preface
About this Book
The origins of this book include not only Moldflow Design Principles but also Warpage Design
Principles published by Moldflow and the C-MOLD Design Guide Collectively thesedocuments are based on years of experience in the research theory and practice of injectionmolding These documents are now combined into this book the Moldflow Design Guide The Moldflow Design Guide is intended to help practicing engineers solve problems they frequently encounter in the design of parts and molds as well as during production This book can alsobe used as a reference for training purposes at industrial and educational institutions
How to Use this Book
This book has several chapters and appendices that deal with different stages of the designprocess and provides background on the injection-molding process and plastic materials
bull The first three chapters introduce injection molding how polymers flow inside injectionmolds and how molding conditions and injection pressure influence the process
bull Chapter 4 discusses Moldflow design principles and how they relate to making quality parts
bull Chapter 5 introduces the finite element mesh technology used by Moldflow and how thesemeshes influence the quality of the analysis
bull Chapters 6 to 9 introduce design concepts for the product gates runners and coolingsystems
bull Chapter 10 introduces concepts relating to shrinkage and warpage and how Moldflow isused to determine the amount of shrinkage and warpage a molded part will have and whatcauses the warpage
bull
Chapter 11 discusses the design procedure for analyzing injection-molded partsbull Chapter 12 discusses major part defects found on injection-molded parts
bull Finally the four appendices discuss basic injection-molding machine operation processcontrol variants of the standard injection-molding process and plastic materials
Benefits of Using CAE
The injection-molding industry has recognized that computer-aided engineering (CAE)
enhances an engineers ability to handle all aspects of the polymer injection-molding processbenefiting productivity product quality timeliness and cost This is illustrated by a wealth of
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VIII Preface
literature and the ever-growing number of CAE software users in the injection-molding industry
CAE Predicts Process Behavior
Ideally CAE analysis provides insight that is useful in designing parts molds and molding processes Without it we rely on previous experience intuition prototyping or molding trialsto obtain information such as polymer melt filling patterns weld-line and air-trap locationsrequired injection pressure and clamp tonnage fiber orientation cycle time final part shapeand deformation and mechanical properties of molded parts just to name a few WithoutCAE analysis other equally important design data such as spatial distributions of pressuretemperature shear rate shear stress and velocity are more difficult to obtain even with a well-instrumented mold The process behavior predicted by CAE can help novice engineers
overcome the lack of previous experience and assist experienced engineers in pinpointing factors that may otherwise be overlooked By using CAE analysis to iterate and evaluatealternative designs and competing materials engineering know-how in the form of designguidelines can be established relatively faster and more cost-effectively
User Proficiency Determines the Benefits of CAE
While CAE technology helps save time money and raw material as well as cuts scrap reducesthe rejection rate improves product quality and gets new products to market faster it is by nomeans a panacea for solving all molding problems Rather it should be recognized that CAEanalysis is essentially a tool designed to assist engineers instead of taking over theirresponsibilities or replacing them Like many other tools the usefulness of CAE technology depends on the proficiency of the user The benefits mentioned above will not be realizedunless the CAE tool is used properly To be more specific the accuracy of CAE analysisdepends greatly on the input data provided by the user In addition the results generated by CAE analysis need to be correctly and intelligently interpreted by the user before soundjudgments and rational decisions are made Otherwise users will simply be swamped by the vast amount of data without getting any useful information
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Acknowledgements IX
Acknowledgements
The Moldflow Design Guide would not have been accomplished were it not for the vision of Ken Welch Ken and I have discussed the value of assembling the best of the Moldflow Design
Principles Warpage Design Principles and the C-MOLD Design Guide into a single book for severalyears With Kens leadership he gave the project to Steve Thompsons training group of which I am a part Steve helped me coordinate the resources necessary to get this projectdone I could not have done this project without Steves help and guidance
A review of the content was part of the development of the Moldflow Design Guide Moldflow developers including Peter Kennedy Rong Zheng Zhongshuang Yuan and Xiaoshi Jin havereviewed sections of the book Moldflows application engineers and other technical staff withMoldflow have also reviewed sections These reviewers include Chad Fuhrman Matt JaworskiChristine Roedlich Eric Henry Olivier Anninos Paul Larter and Ana Maria Marin A specialthanks goes to Mike Rogers who reviewed the entire book for me and provided criticalfeedback on the content and organization of the book I would also like to thank Kurt Haydenof Western Michigan University for reviewing the appendix on process control His many years of experience of process setup and optimization was invaluable
Finally I would like to thank members of Moldflows Technology Transfer group forsupporting me in the writing of this book These members include Marcia Swan CarolineDorin Robert Ashley Melissa Haslam Darren Seamons and Gregory Brown
On a personal note I would like to acknowledge and thank Paul Engelmann Professor and
Department Chair Western Michigan University Department of Industrial and Manufacturing Engineering for being my friend and mentor during my career With Paul I have been able toteach and participate in research he has done on injection molding tooling and processing at Western Michigan University I have found working with Paul has made me a better Moldflow user and engineer by providing another perspective on how Moldflow can be used to solveinjection molding problemsJay Shoemaker Editor
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Contents
1 Polymer Flow Behavior in Injection Molds 1
11 Phases of Injection Molding 1111 How Plastic Fills a Mold 1112 The Filling Phase 3113 The Pressurization Phase 6114 The Compensation Phase 612 How Do Plastics Flow 7121 Material Behavior 7122 Deformation 8
123 Viscoelastic Behavior 8124 Melt Shear Viscosity 9125 Newtonian Fluid vs Non-Newtonian Fluid 10126 Shear-thinning Behavior 10127 Shear Rate Distribution 11128 Pressure-driven Flow 11129 Pressure Gradient and Injection Times 131210 Melt Flow Length 131211 Injection Pressure vs Fill Time 131212 Flow Instability 15
2 Molding Conditions and Injection Pressure 17
21 Injection-pressure Overview 17211 Pressure Drives the Flow Front 1822 Factors Influencing Injection-pressure Requirements 1823 Equations 20231 Circular Channel Flow 20232 Strip Channel Flow 2024 Effect of Molding Conditions 21
241 Part Quality 21242 Melt Temperature 22243 Mold Temperature 22244 Fill Time 23245 Shear Stress Variation 23246 Packing Pressure and Time 24247 Summary 25248 Back Flow 2625 Using Moldflow to Determine Optimum Molding Conditions 26251 Part 27
252 Molding Window Size 27253 Injection Pressure 28254 Flow Front Temperature 29
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XII Contents
255 Cooling Time 30256 Summary 31
3 Filling Pattern 33
31 Filling Pattern Overview 33311 What Is the Filling Pattern 3332 Flow in Complex Molds 34321 Overpack 34322 Racetrack Effect 35323 Varying Injection Rate 35324 Underflow Effect 36325 Hesitation Effect 37326 Weld Lines 38327 Meld Lines 38328 Sink Marks 38329 Multidirectional Flow 393210 Unstable Flow 403211 Simple Flow Pattern 4133 Flow-front Velocity and Flow-front Area 41331 What are FFV and FFA 41332 Flow-front Velocity Influences Filling Pattern 41
333 Equation 4234 Using Moldflow to Determine the Filling Pattern 43341 Computer Simulation Can Eliminate Molding Trials 43342 Using a Flow Analysis 4335 Using Moldflow to Achieve Constant FFV 44351 Controlling the FFV Through Ram Speed 44
4 Moldflow Design Principles 47
41 Product Design and Moldflow 4742 Sequence of Analysis 47421 Part Filling Optimization 48422 Molding Conditions 48423 Runner Design 48424 Cooling Optimization 49425 Packing Optimization 49426 Warpage Optimization 4943 Moldflow Flow Concepts 49431 Unidirectional and Controlled Flow Pattern 50432 Flow Balancing 51
433 Constant Pressure Gradient 52434 Maximum Shear Stress 53435 Uniform Cooling 54
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Contents XIII
436 Positioning Weld and Meld Lines 55437 Avoid Hesitation Effects 55
438 Avoid Underflow 55439 Balancing with Flow Leaders and Flow Deflectors 574310 Controlled Frictional Heat 584311 Thermal Shutoff of Runners 584312 Acceptable RunnerCavity Ratio 59
5 Meshes Used In Moldf low Analyses 61
51 Mesh Types Used by Moldflow 61511 Finite Elements Used in Moldflow 61512 Mesh Types 62513 Solver Assumptions 6352 Mesh Requirements 64521 Mesh Density Considerations 64522 Part Details 6753 Geometry Creation 6854 Importing Geometry 6955 Using Different Mesh Types 69551 Door Panel 69552 Manifold 70
6 Product Design 71
61 Material Properties for Product Design 71611 Plastics Are Sensitive to Operating Conditions 71612 Stress-Strain Behavior 72613 Creep and Stress Relaxation 77614 Fatigue 79615 Impact strength 80616 Thermal Mechanical Behavior 8062 Design for Strength 82621 Predicting Part Strength 82622 LoadingOperating Conditions 8263 Part Thickness 86631 Part Thickness Drives Quality and Cost 86632 Cycle Time Increases with Thickness 86633 Thick Parts Tend to Warp 86634 Thin Uniform Parts Improve Surface Quality 87635 Reducing Part Thickness 8764 Boosting Structural Integrity with Ribs 88
641 Structural Integrity the Goal of Every Design 88642 Designing Ribs 88
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XIV Contents
65 Design for Assembly 90651 Molding One Part vs Separate Components 90
652 Tolerances Fit between Parts 90653 Press-fit Joints 91654 Snap-fit Joints 93655 Cantilever Snap Joints 95656 Torsion Snap-fit Joints 96657 Fasteners 98658 Inserts 101659 Welding Processes 101
7 Gate Design 103
71 Gate Design Overview 103711 What Is a Gate 103712 Single vs Multiple Gates 103713 Gate Dimensions 103714 Gate Location 10472 Gate Types 104721 Manually Trimmed Gates 104722 Automatically Trimmed Gates 11073 Design Rules 113
731 Determining the Number of Gates 113732 Flow Patterns 115733 Gate Position 116734 Avoiding Common Problems 122735 Gate Length 122736 Gate Thickness 122737 Freeze-off Time 12374 Using Moldflow for Gate Design 123741 Gate Location 123742 Molding Window Size for the Three Gate Locations 124
743 Filling Pattern 125744 Gate Size Based on Shear Rate 125
8 Runner System Design 127
81 Definitions 127811 Feed System 127812 Runner System 127813 Cold Runner 127814 Hot Runner 128
815 Hot Manifold 128816 Hot Drop 128817 Sprue 128
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Contents XV
82 Runner System Design Principles 129821 Benefits of Good Runner Design 129
822 Runner Design Philosophy 129823 Flow Balancing 130824 Flow Control 130825 Frictional Heating in Runners 132826 Thermal Shutoff 132827 System and Runner Pressures 132828 Constant Pressure Gradient 133829 Cold Slug Wells 1338210 Easy Ejection 13483 Runner Types 134
831 Cold Runners 134832 Hot Runner Systems 13584 Runner Layout 136841 Determining the Number of Cavities 136842 Planning the Runner System Layout 138843 Partially Balanced Runners 139844 Geometrically Balanced Runners 14185 Initial Runner Sizing 141851 Determining Sprue Dimensions 141852 Designing Runner Cross Sections 142
853 Determining Runner Diameters 14586 Runner Balancing 147861 How Runner Balancing Works 147862 When Are the Runner Sizes Optimized 147863 Validating the Balance 147864 Processing Window 14887 Using Moldflow for Runner Balancing 149871 Runner Balancing a 48-cavity Tool 149872 Runner Balancing for a Family Mold 151873 Runner Balancing for a Multigated Part 152
9 Cooling System Design 153
91 Mold Cooling System Overview 153911 Importance of Cooling System Design 153912 Mold Cooling System Components 15492 Cooling-channel Configuration 156921 Types of Cooling Channels 15693 Alternative Cooling Devices 157931 What Do They Do 157
932 Baffles 158933 Bubblers 159934 Thermal Pins 159
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XVI Contents
935 Cooling Slender Cores 160936 Cooling Large Cores 161
937 Cooling Cylinder Cores 16294 Cooling System Equations 162941 Cooling Time 162942 Reynolds Number and Coolant Flow 16395 Design Rules 163951 Mold Cooling Design Considerations 163952 Location and Size of Channels 164953 Flow Rate and Heat Transfer 16696 Using Moldflow for Cooling System Design 167961 Example Setup 168
962 Cycle Time Determined by Design and Processing Parameters 169
10 Shrinkage and Warpage 173
101 Injection Molding and Shrinkage 1731011 What Are Shrinkage and Warpage 1731012 Shrinkage and Machine Settings 1731013 Mold Filling and Packing 1741014 How Pressure and Time Affect Shrinkage 1751015 Thermally Unstable Flow 176
102 Basic Causes of Shrinkage and Warpage 1771021 Causes of Shrinkage 1771022 Causes of Warpage 1811023 Relating Orientation and Area Shrinkage to Warpage 182103 Designing Accurate Parts Considering Warpage 1831031 Material Selection 1841032 Wall Thickness Variation 1841033 Gate Position and Runner Dimensions 1851034 Molding Conditions 1871035 Cooling Line Layout 190
11 Moldflow Design Procedure 191
111 Determine Analysis Objectives 191112 Moldflow Analysis Steps Framework 1921121 The Whole Process 1921122 Optimize Fill 1921123 Balance and Size the Runners 1971124 Optimize Cooling 1991125 Optimize the Packing Profile 200
1126 Optimize Warpage 201113 Using Moldflow to Evaluate an Initial Design 2051131 Description of this Example 205
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Contents XVII
1132 Molding Window 2051133 Filling Analysis 206
1134 Gate and Runner Design 2061135 Cooling System Design 2071136 Packing Analysis 2091137 Warpage Analysis 209114 Using Moldflow to Optimize the Design 2101141 Determine the Cause of Warpage 2101142 Investigating Different Gate Locations 2101143 Validating the Best Gate Location 211
12 Part Defects 215
121 Air Traps 2151211 What Is an Air Trap 2151212 Problems Caused by Air Traps 2161213 Remedies 216122 Black Specks and Black Streaks 2171221 What Are Black Specks and Black Streaks 2171222 Causes of Black Specks and Black Streaks 2171223 Remedies 218123 Brittleness 219
1231 What Is Brittleness 2191232 Causes of Brittleness 2191233 Remedies 219124 Burn Marks 2201241 What Is a Burn Mark 2201242 Causes of Burn Marks 2211243 Remedies 222125 Delamination 2221251 What Is Delamination 2221252 Causes of Delamination 223
1253 Remedies 223126 Dimensional Variation 2241261 What Is Dimensional Variation 2241262 Causes of Dimensional Variation 2241263 Remedies 224127 Discoloration 2251271 What Is Discoloration 2251272 Causes of Discoloration 2251273 Remedies 226128 Fish Eyes 227
1281 What Are Fish Eyes 2271282 Causes of Fish Eyes 2271283 Remedies 227
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XVIII Contents
129 Flash 2281291 What Is Flash 228
1292 Causes of Flash 2281293 Remedies 2291210 Flow Marks 23012101 What Is A Flow Mark 23012102 Causes of Flow Marks 23012103 Remedies 2301211 Hesitation 23112111 What Is Hesitation 23112112 Problems Caused by Hesitation 23112113 Remedies 232
1212 Jetting 23212121 What Is Jetting 23212122 Problems Caused by Jetting 23312123 Remedies 2331213 Ripples 23512131 What Are Ripples 23512132 Cause of Ripples 23512133 Remedies 2361214 Short Shots 23612141 What Is a Short Shot 236
12142 Causes of Short Shots 23712143 Remedies 2371215 Silver Streaks 23812151 What Are Silver Streaks 23812152 Causes of Silver Streaks 23912153 Remedies 2391216 Sink Marks and Voids 24012161 What Are Sink Marks and Voids 24012162 Causes of Sink Marks and Voids 24012163 Remedies 2411217 Weld Lines and Meld Lines 24312171 What Are Weld Lines and Meld Lines 24312172 Problems Caused by Weld Lines 24312173 Strength of Weld Lines 24412174 Remedies 245
Appendix A Injection Molding 247
A1 Injection-molding Overview 247 A11 Process 247
A12 Applications 248 A2 Development of the Injection-molding Machine 248 A21 Benefits of the Reciprocating Screw 248
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Contents XIX
A3 Development of the Injection-molding Process 249 A4 Alternative Injection-molding Processes 249
A41 Co-injection (Sandwich) Molding 249 A42 Fusible Core Injection Molding 250 A43 Gas-assisted Injection Molding 251 A44 Injection-compression Molding 252 A45 Lamellar (Microlayer) Injection Molding 253 A46 Live-feed Injection Molding 253 A47 Low-pressure Injection Molding 254 A48 Push-pull Injection Molding 254 A49 Reactive Molding 255 A410 Structural Foam Injection Molding 256
A411 Thin-wall Molding 256
Appendix B Injection-molding Machine System and Operations 259
B1 Injection-molding Machine 259B11 Components 259B12 Machine Specification 260B13 Machine Function 260B14 Auxiliary Equipment 260B2 Machine Components 261
B21 Injection System 261B22 Mold System 263B23 Hydraulic System 264B24 Control System 264B25 Clamping System 264B3 Molded System 265B31 The Delivery System 265B4 Machine Operating Sequence 266B41 Process Cycle 266B5 Screw Operation 268
B51 Back Pressure 268B52 Injection Speed 268B53 Screw Rotation Speed 268B54 Cushion 269B6 Secondary Operations 269B61 Assembly 269B62 Decoration 269B63 Other Secondary Operations 270
Appendix C Injection-molding Process Control 271
C1 Importance of Process Conditions 271C11 Setting Machine Process Conditions 271
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XX Contents
Appendix D Plastic Materials 281
D1 What Are Plastics 281
D11 Polymerization Process 281D12 Structure of Polymers 281D13 Polymer Alloys and Blends 283D14 Polymer Composites 283D2 Classification of Plastics 283D21 Classes of Plastics 284D22 Structures and Properties of Plastics 284D3 Thermoplastics 285D31 Market Share Distribution of Thermoplastics 285D32 Structures and Properties of Thermoplastics 286D33 Amorphous Polymers 286D34 Semicrystalline Polymers 287D4 Thermosets 288D5 Properties and Applications of Thermoplastics 289D51 ABS 290D52 PA 12 291D53 PA 6 293D54 PA 66 294D55 PBT 296
D56 PC 297D57 PCABS 299D58 PCPBT 300D59 HDPE 301D510 LDPE 303D511 PEI 304D512 PET 305D513 PETG 306D514 PMMA 307D515 POM 309
D516 PP 310D517 PPEPPO 312D518 PS 313D519 PVC 314D520 SAN 316D521 Additives Fillers and Reinforcements 317D522 Modifying Polymer Properties 318D523 Low-aspect Fillers 319D524 High-aspect Fillers Fibers 319
Index 321
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1 Polymer Flow Behavior in Injection Molds
bull Phases of injection molding
bull How do plastics flow
11 Phases of Injection Molding
Any molder can prove that all the conditions and effects discussed in this chapter do indeedoccur during the injection molding process While this knowledge alone can somewhatimprove quality it is only with the use of Moldflow analysis during the initial design stage with the mold designed for the optimum filling pattern that these effects can be controlledand the full benefits obtained
Flow technology is concerned with the behavior of plastics during the mold filling process Aplastic parts properties depend on how the part is molded Two parts having identicaldimensions and made from the same material but molded under different conditions will havedifferent stress and shrinkage levels and will behave differently in the field meaning that they
are in practice two different parts The way the plastic flows into the mold is of paramount importance in determining the quality of the part The process of filling the mold can be distinctly analyzed with the ability to predictpressure temperature and stress
111 How Plastic Fills a Mold
This was investigated using a centrally gated mold shaped like a dinner plate with a thick rim
around the outside as shown in Figure 11 It was found that the injection molding processalthough complex could be divided into three phases (we use the word phase to avoidconfusion with injection stage as used with programmed injection)
Figure 1ure 1 1 Cross-section of disk mold used to investigate flow
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2 Polymer Flow Behavior in Injection Molds
1111 Filling Phase
As the ram moves forward it first moves at a steady speed as the plastic flows into the cavity
This is the filling phase This phase lasts until the mold is just filled See Figure 12 andFigure 13
1112 Pressurization Phase
The pressurization phase begins when the ram moves forward after the filling phase to bring the mold up to pressure When the mold is filled the ram will slow down but it still movesquite some distance because plastics are very compressible materials At injection molding pressure an extra 15 volume of material can be forced into the cavity See Figure 12 andFigure 13
Although fluids are usually assumed to be incompressible molten plastics have to beconsidered to be more like a gas The compressibility of plastics can be observed by blocking off the nozzle and attempting to purge the barrel The ram will jump forward when thepressure is applied but will spring back when the pressure is released
1113 Compensation Phase
After the pressurization phase the ram still does not stop completely continuing to creepforward for some time Plastics have a very large volumetric change of about 25 from themelt to the solid This can be seen in a short shot the difference in volume between themolding and the cavity is due to this volumetric change See Figure 12 and Figure 13
The ram moving forward to compensate for the volumetric change in the part is called thecompensation phase As the volumetric change is 25 and at the most only an extra 15 canbe injected in the pressurization phase there must always be some compensation phase
Figure 1igure 1 2 Phases of injection molding
Filling phase
Pressurization phase
Compensation phase
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Phases of Injection Molding 3
Figure 13ure 1 3 Phases of injection molding detail
112 The Filling Phase
A two-color technique best demonstrates this phase After emptying the barrel of an injection-molding machine a small amount of red plastic was charged followed by green plastic
Consider the closed mold with the plastic front just starting to flow from the nozzle Theplastic first fills the sprue and runner system then enters the mold cavity itself forming a smallbubble of molten plastic
The skin of the plastic in contact with the cool mold freezes rapidly while the central coreremains molten When additional material is injected it flows into this central core displacing
the material already there which then forms a new flow front The flow of this displacedmaterial is a combination of forward flow and outward flow The outward flow contacts the wall freezes and forms the next section of skin while the forward flow forms the new moltencore When more material enters the mold it flows along a channel lined with these frozen walls of plastic illustrated in Figure 14
This flow pattern is often called fountain flow or bubble flow because the flow front is like abubble being inflated with hot plastic from the center The frozen layer is formed by the flow front inflating and so is subject to only a low shear stress and therefore has a very low levelof molecular orientation Once it is frozen it cannot be orientated any further so the frozen
layer in the finished part has a low level of orientation
Figure 14ure 1 4 Fountain flow and heat transfer
Filling Pressurization Compensation
Hot plastic
Frozen layer
Heat loss to mold
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4 Polymer Flow Behavior in Injection Molds
Now consider what happens upstream Hot plastic is continuously flowing bringing new hotmaterial along and generating significant frictional heat At the same time heat is being lost
through the frozen layer to the cold mold surfaceInitially the frozen layer is very thin so heat is lost very rapidly This results in more plasticfreezing and the frozen layer getting thicker cutting down the heat flow After a time thefrozen layer will reach a thickness such that the heat lost by conduction is equal to the heatinput from plastic flow and frictional heating ie an equilibrium condition is reached(Figure 14)
It is interesting to do some calculations on the time taken to reach this state of equilibrium The actual rate of heat flow is very large in comparison with the small heat content of theplastic in the frozen layer The result is that equilibrium is reached very quickly often in a timemeasured in a few tenths of a second As the total filling time is measured in seconds thefrozen layer reaches an equilibrium state early in the filling cycle
It is useful to think about how the thickness of this frozen layer will vary If the injection rate were slowed less heat would be generated by friction along the flow path with less heat inputfrom the flow The heat loss would be at the same rate and with less heat input the frozenlayer would grow in thickness If the injection rate were raised the frozen layer would bethinner (Figure 15) Similarly higher melt and mold temperatures would reduce the thicknessof the frozen layer This can be seen experimentally using the two-color technique
Figure 1igure 1 5 Influence of injection rate on frozen layer thickness
1121 Flow Shear Stress
It is easy to get confused between the various stress levels and orientation of the polymer Asthe plastic flows it is subject to shear stress also called flow shear stress This flow shear stress willorient the material ie cause the molecules to align themselves in the general direction of flow
The shear stress varies from a maximum at the outside dropping off to zero at the center
3Shear stress is purely a function of force and area This must not be confused withshear rate which is the rate of plastic sliding over the next layer Shear rate is zero
= Frozen
= Molten cooler
= Molten hotter
Slowerinjection
rate
Faster
injection
rate
vs
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Phases of Injection Molding 5
at the outer edge where the plastic is frozen rises to a maximum just inwards of thefrozen layer then drops toward the center as shown in Figure 16
Figure 16ure 1 6 Shear rate distribution
If the flow were stopped and the plastic allowed to cool down very slowly this orientation would have time to relax giving a very low level of residual orientation On the other hand if the material were kept under stress and the plastic snap frozen most of the orientation would
be trapped in the frozen plastic (Figure 17)
Figure 17ure 1 7 Molecular orientation through the thickness of the part
Min MaxShear rate
= Tension
= Compression
Molecular orientation trapped
in pressurization phase
Gradual reduction of orientation
toward the center of the
mold as stress levels are lower
and the cooling rate is slower
which allows more time for
orientation to relax
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6 Polymer Flow Behavior in Injection Molds
Now consider the orientation from the mold surface toward the center
The frozen layer itself formed with very little shear and therefore low orientation immediately
freezes setting the low level of orientation
The layer of plastic just on the inside of the frozen layer is subject to maximum shear stressand freezes the instant flow stops trapping almost all the orientation
This is the orientation pattern the further toward the center the more the shear stress dropsand the slower the rate of cooling This allows more time for the level of orientation to relaxso the residual orientation drops rapidly toward the center Consider how this pattern willaffect the residual stress level Oriented material (normally) will shrink more than nonorientedmaterial On the inner surface of the original frozen layer highly oriented material wants toshrink a great deal but it is prevented from doing so by the less-oriented material The highly
oriented layer ends up being in tension while the less-oriented material is in compression This residual stress pattern is a common cause of part warpage
3 There is a connectionthrough orientationbetween the shear stress during
filling (flow stress) and the residual stress in the final molded part This meansshear stress during filling shown on Moldflow plots can be used as a designparameter
113 The Pressurization Phase
The pressurization phasefrom the point of view of flow behavioris very similar to thefilling phase The flow rate may drop somewhat as the mold builds up to pressure resulting inan increase in the thickness of the frozen layer
The main difference of course is the increase in hydrostatic (isotropic) pressure We shall seein chapter 2 section 24 Effect of Molding Conditions that hydrostatic pressure in itself doesnot cause any residual stress
114 The Compensation Phase
Compensating flow is unstable Consider the plate molding again (see Figure 11) You wouldthink that plastic flowing uniformly through the thin diaphragm would top up the thick rim Inpractice the plastic during the compensation phase flows in rivers that spread out like a deltaas illustrated in Figure 18 This may seem surprising at first but it can be explained by temperature instability
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How Do Plastics Flow 7
Figure 18ure 1 8 River flow
1141 Temperature Variation
There is always some variation in melt temperature coming from the barrel of the injectionmachine In exceptional cases up to 40 ordmC variation has been measured using a high-speedthermocouple
1142 Natural Instability
However slight the temperature variation natural instability will amplify it If for exampleone part of the melt is slightly hotter than the rest then the plastic flow in that area will beslightly greater bringing hotter material into the area and maintaining the temperature If onthe other hand there is another area that is cooler the flow will be less so there will be lessheat input and the plastic will get colder until it eventually freezes off
However balanced the initial conditions this natural instability will result in a river-type flow
This is a very important consideration The first material to freeze off will shrink early in thecycle By the time the material in the river flows freezes the bulk of the material will havealready frozen off and shrinkage will have occurred The rivers will shrink relative to the bulk of the molding and because they are highly orientated shrinkage will be very high The resultis high-stress tensile members throughout the molding a common cause of warpage
1143 Optimum Part Quality
Most of the stress in plastic parts occurs during the compensation phase By controlling flow and minimizing stress it is possible to design for optimum part quality This important point is
at the heart of the Moldflow philosophy
12 How Do Plastics Flow
121 Material Behavior
Molten thermoplastics exhibit viscoelastic behavior which combines flow characteristics of both viscous liquids and elastic solids When a viscous liquid flows the energy that causes thedeformation is dissipated and becomes viscous heat On the other hand when an elastic solid
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8 Polymer Flow Behavior in Injection Molds
is deformed the driving energy is stored For example the flow of water is a typical viscousflow whereas the deformation of a rubber cube falls into the elastic category
122 Deformation
In addition to the two types of material flow behavior there are two types of deformationsimple shear and simple extension (elongation) as shown in Figure 19 (a) and (b) below Theflow of molten thermoplastics during injection-molding filling is predominantly shear flow asshown in Figure 19 (c) in which layers of material elements slide over each other Theextensional flow however becomes significant as the material elements undergo elongation when the melt passes areas of abrupt dimensional change (eg a gate region) as shown inFigure 19 (d)
Figure 1igure 1 9 (a) Simple shear flow (b) Simple extensional flow (c) Shear flow in cavity filling (d) Extensional flow in cavity filling
123 Viscoelastic Behavior
In response to an applied stress (force per unit area) molten thermoplastics exhibit viscoelastic behavior which combines characteristics of an ideal viscous liquid with those of an ideal elastic solid In other words under certain conditions molten thermoplastics behavelike a liquid and will continuously deform while shear stress is applied as shown in
Figure 110 Upon the removal of the stress however the materials behave somewhat like anelastic solid with partial recovery of the deformation as shown in Figure 110 (b) and (c) This viscoelastic behavior stems from the random-coil configuration of polymer molecules in the
Stress
Stress Stress
Mold wall
Material element before
deformation begins
Material element during
deformation
Mold wall
Material elements at rest
Moving
materialelements
Velocity
profile
(a) (b)
(c) (d)
Velocity
profile
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How Do Plastics Flow 9
molten state which allows the movement and slippage of molecular chains under theinfluence of an applied load However the entanglement of the polymer molecular chains also
makes the system behave like an elastic solid upon the application and removal of the externalload Namely on removal of the stress chains will tend to return to the equilibrium random-coil state and thus will be a component of stress recovery The recovery is not instantaneousbecause of the entanglements still present in the system
Figure 1ure 1 10 (a) Ideal viscous liquid deforms continuously under applied stress (b) Ideal elasticsolid deforms immediately upon the application of stress but fully recovers when the
stress is removed (c) Molten thermoplastic deforms continuously under the applied
stress (like a viscous liquid) but also recovers partially from the deformation upon
removal of the applied stress (like an elastic solid)
124 Melt Shear Viscosity
1241 What Is Shear Viscosity
Melt shear viscosity is a materials resistance to shear flow In general polymer melts are highly viscous because of their long molecular chain structure The viscosity of a polymer melt rangesfrom 2 to 3000 Pas (water 10^-1 Pas glass 10^20 Pas) Viscosity can be thought of as thethickness of a fluid or how much it resists flow Viscosity is expressed as the ratio of shearstress (force per unit area) to the shear rate (rate change of shear strain) as shown inEquation 11 and Figure 111
(a) Viscous
liquid
(b) Elastic
solid
(c) Viscoelastic
polymer
melt
Initial shape
(shear stress
to be applied)
Time = Dt Time = 2 x Dt
(shear stress to
be removed)
Final deformed
shape
d
d
d
d
2d
2d
2d
2d
total
recovery
partial
recovery
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Jay Shoemaker (Editor)
Mold1047298ow Design Guide
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Mold1047298ow Design
Guide
A Resource for Plastics Engineers
Jay Shoemaker (Editor)
Hanser Publishers Munich bull Hanser Gardner Publications Cincinnati
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The Editor
Jay Shoemaker Mold1047298ow Corporation Framingham Massachusetts USA
Distributed in the USA and in Canada by Hanser Gardner Publications Inc6915 Valley Avenue Cincinnati Ohio 45244-3029 USAFax (513) 527-8801Phone (513) 527-8977 or 1-800-950-8977 wwwhansergardnercom
Distributed in all other countries by Carl Hanser Verlag Postfach 86 04 20 81631 Muumlnchen Germany Fax +49 (89) 98 48 09 wwwhanserde
The use of general descriptive names trademarks etc in this publication even if the former are not especiallyidenti1047297ed is not to be taken as a sign that such names as understood by the Trade Marks and Merchandise Marks Actmay accordingly be used freely by anyone While the advice and information in this book are believed to be true and accurate at the date of going to press neitherthe authors nor the editors nor the publisher can accept any legal responsibility for any errors or omissions that may bemade The publisher makes no warranty express or implied with respect to the material contained herein
Library of Congress Cataloging-in-Publication Data
Shoemaker Jay
Mold1047298ow design guide a resource for plastic engineers Jay Shoemaker p cm ISBN-13 978-1-56990-403-9 (hardcover) ISBN-10 1-56990-403-0 (hardcover)1 Injection molding of plastics I Title TP1120S486 2006 6684lsquo12--dc22 2006012438
Bibliogra1047297sche Information Der Deutschen Bibliothek Die Deutsche Bibliothek verzeichnet diese Publikation in der Deutschen Nationalbibliogra1047297e
detaillierte bibliogra1047297sche Daten sind im Internet uumlber lthttpdnbddbdegt abrufbar
ISBN-10 3-446-40640-9ISBN-13 978-3-446-40640-7
All rights reserved No part of this book may be reproduced or transmitted in any form or by any means electronic ormechanical including photocopying or by any information storage and retrieval system without permission in writingfrom the publisher
copy Mold1047298ow Corporation USA 2006Production Management Oswald Immel Typeset by Marcia Swan USACoverconcept Marc Muumlller-Bremer Rebranding Muumlnchen Germany Coverdesign MCP bull Susanne Kraus GbR Holzkirchen Germany Printed and bound by Koumlsel Krugzell Germany
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Foreword
The drive toward fast cost-effective and reliable plastics manufacturing has been Moldflowssole guiding goal since the company was founded over 25 years ago
This focused determination led us to introduce many new and exciting tools into the marketeach contributing to achieving our goal in some way whether by driving cost out of production with reduced material usage or shortened cycle times reducing mold delivery timeby minimizing re-work or increasing the reliability of supply by enabling higher quality products to be manufactured with greater surety in scheduling
The artificially balanced multi-cavity and family molds that are now commonplace were madepractical through the advent of our early simulation and runner balancing capabilities which
were introduced in the late 1970s and early 1980s As these tools evolved we were able to visualize and therefore control flow patterns and weld lines This evolution continued until we arrived in the 2000s with an array of sophisticated technology to control warpage accountfor heat transfer predict core shift adapt to new molding processes and much more Fromtraditional midplane technology to fully three-dimensional simulations all our solutions are well integrated into a solid-modeling design environment
As the technology has evolved so has its usage When Moldflow simulation technology wasintroduced its primary purpose was to search for remedies to pre-existing molding problemsIt soon became evident that the insight the software provided to solve molding problems would be better applied ahead of actual molding during the design process This
methodology which we call problem avoidance was the primary use for Moldflow technology for the first 20 years of its existence
For Moldflow this created a unique challenge to open the world of manufacturing to thedesigners of parts and molds What constitutes an ineffective design for molding may beapparent to a seasoned processing engineer looking retrospectively at a poorly performing tool but how can design engineers use the CAE tools to visualize diagnose and solve thesesame issues ahead of timewithout 20 years of molding experience How can manufacturersgo further and use information that cannot be seen in the real molding process but is revealed via simulation
The key that unlocked this puzzle began its life as the Moldflow Design Philosophy This is widely
viewed as the most important publication Moldflow has ever produced and has spawnedfollow-on works on related subjects Rather than provide insight into the operation of thesimulation tools Moldflow Design Philosophy set forth simple principles that transcend any specific software application and as a result are as valid with todays advanced simulationproducts as they were over two decades ago
In more recent years another transition has occurred The global imperative to drive down thecost of manufacturing has led to the use of molding simulation as a cost optimization toolrather than for problem avoidance This change has increased the number of Moldflow usersby an order of magnitude across a far broader cross-section of the plastics industry Greaterdesign-centricity leads to even more dependence on the plastics design principles which can
be used to drive optimization
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VI Foreword
Despite a quarter of a century of technological advances the golden years of CAE are aheadof us as our industry takes a broader and more integrated view of what it takes to manage a
products life cycle Moldflow is proud of its contributions to date and will continue to focuson developing innovative technology coupled with practical design principles to deliver moreprofitable manufacturing
Roland Thomas
President amp CEO Moldflow Corporation
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Preface VII
Preface
About this Book
The origins of this book include not only Moldflow Design Principles but also Warpage Design
Principles published by Moldflow and the C-MOLD Design Guide Collectively thesedocuments are based on years of experience in the research theory and practice of injectionmolding These documents are now combined into this book the Moldflow Design Guide The Moldflow Design Guide is intended to help practicing engineers solve problems they frequently encounter in the design of parts and molds as well as during production This book can alsobe used as a reference for training purposes at industrial and educational institutions
How to Use this Book
This book has several chapters and appendices that deal with different stages of the designprocess and provides background on the injection-molding process and plastic materials
bull The first three chapters introduce injection molding how polymers flow inside injectionmolds and how molding conditions and injection pressure influence the process
bull Chapter 4 discusses Moldflow design principles and how they relate to making quality parts
bull Chapter 5 introduces the finite element mesh technology used by Moldflow and how thesemeshes influence the quality of the analysis
bull Chapters 6 to 9 introduce design concepts for the product gates runners and coolingsystems
bull Chapter 10 introduces concepts relating to shrinkage and warpage and how Moldflow isused to determine the amount of shrinkage and warpage a molded part will have and whatcauses the warpage
bull
Chapter 11 discusses the design procedure for analyzing injection-molded partsbull Chapter 12 discusses major part defects found on injection-molded parts
bull Finally the four appendices discuss basic injection-molding machine operation processcontrol variants of the standard injection-molding process and plastic materials
Benefits of Using CAE
The injection-molding industry has recognized that computer-aided engineering (CAE)
enhances an engineers ability to handle all aspects of the polymer injection-molding processbenefiting productivity product quality timeliness and cost This is illustrated by a wealth of
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VIII Preface
literature and the ever-growing number of CAE software users in the injection-molding industry
CAE Predicts Process Behavior
Ideally CAE analysis provides insight that is useful in designing parts molds and molding processes Without it we rely on previous experience intuition prototyping or molding trialsto obtain information such as polymer melt filling patterns weld-line and air-trap locationsrequired injection pressure and clamp tonnage fiber orientation cycle time final part shapeand deformation and mechanical properties of molded parts just to name a few WithoutCAE analysis other equally important design data such as spatial distributions of pressuretemperature shear rate shear stress and velocity are more difficult to obtain even with a well-instrumented mold The process behavior predicted by CAE can help novice engineers
overcome the lack of previous experience and assist experienced engineers in pinpointing factors that may otherwise be overlooked By using CAE analysis to iterate and evaluatealternative designs and competing materials engineering know-how in the form of designguidelines can be established relatively faster and more cost-effectively
User Proficiency Determines the Benefits of CAE
While CAE technology helps save time money and raw material as well as cuts scrap reducesthe rejection rate improves product quality and gets new products to market faster it is by nomeans a panacea for solving all molding problems Rather it should be recognized that CAEanalysis is essentially a tool designed to assist engineers instead of taking over theirresponsibilities or replacing them Like many other tools the usefulness of CAE technology depends on the proficiency of the user The benefits mentioned above will not be realizedunless the CAE tool is used properly To be more specific the accuracy of CAE analysisdepends greatly on the input data provided by the user In addition the results generated by CAE analysis need to be correctly and intelligently interpreted by the user before soundjudgments and rational decisions are made Otherwise users will simply be swamped by the vast amount of data without getting any useful information
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Acknowledgements IX
Acknowledgements
The Moldflow Design Guide would not have been accomplished were it not for the vision of Ken Welch Ken and I have discussed the value of assembling the best of the Moldflow Design
Principles Warpage Design Principles and the C-MOLD Design Guide into a single book for severalyears With Kens leadership he gave the project to Steve Thompsons training group of which I am a part Steve helped me coordinate the resources necessary to get this projectdone I could not have done this project without Steves help and guidance
A review of the content was part of the development of the Moldflow Design Guide Moldflow developers including Peter Kennedy Rong Zheng Zhongshuang Yuan and Xiaoshi Jin havereviewed sections of the book Moldflows application engineers and other technical staff withMoldflow have also reviewed sections These reviewers include Chad Fuhrman Matt JaworskiChristine Roedlich Eric Henry Olivier Anninos Paul Larter and Ana Maria Marin A specialthanks goes to Mike Rogers who reviewed the entire book for me and provided criticalfeedback on the content and organization of the book I would also like to thank Kurt Haydenof Western Michigan University for reviewing the appendix on process control His many years of experience of process setup and optimization was invaluable
Finally I would like to thank members of Moldflows Technology Transfer group forsupporting me in the writing of this book These members include Marcia Swan CarolineDorin Robert Ashley Melissa Haslam Darren Seamons and Gregory Brown
On a personal note I would like to acknowledge and thank Paul Engelmann Professor and
Department Chair Western Michigan University Department of Industrial and Manufacturing Engineering for being my friend and mentor during my career With Paul I have been able toteach and participate in research he has done on injection molding tooling and processing at Western Michigan University I have found working with Paul has made me a better Moldflow user and engineer by providing another perspective on how Moldflow can be used to solveinjection molding problemsJay Shoemaker Editor
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Contents
1 Polymer Flow Behavior in Injection Molds 1
11 Phases of Injection Molding 1111 How Plastic Fills a Mold 1112 The Filling Phase 3113 The Pressurization Phase 6114 The Compensation Phase 612 How Do Plastics Flow 7121 Material Behavior 7122 Deformation 8
123 Viscoelastic Behavior 8124 Melt Shear Viscosity 9125 Newtonian Fluid vs Non-Newtonian Fluid 10126 Shear-thinning Behavior 10127 Shear Rate Distribution 11128 Pressure-driven Flow 11129 Pressure Gradient and Injection Times 131210 Melt Flow Length 131211 Injection Pressure vs Fill Time 131212 Flow Instability 15
2 Molding Conditions and Injection Pressure 17
21 Injection-pressure Overview 17211 Pressure Drives the Flow Front 1822 Factors Influencing Injection-pressure Requirements 1823 Equations 20231 Circular Channel Flow 20232 Strip Channel Flow 2024 Effect of Molding Conditions 21
241 Part Quality 21242 Melt Temperature 22243 Mold Temperature 22244 Fill Time 23245 Shear Stress Variation 23246 Packing Pressure and Time 24247 Summary 25248 Back Flow 2625 Using Moldflow to Determine Optimum Molding Conditions 26251 Part 27
252 Molding Window Size 27253 Injection Pressure 28254 Flow Front Temperature 29
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XII Contents
255 Cooling Time 30256 Summary 31
3 Filling Pattern 33
31 Filling Pattern Overview 33311 What Is the Filling Pattern 3332 Flow in Complex Molds 34321 Overpack 34322 Racetrack Effect 35323 Varying Injection Rate 35324 Underflow Effect 36325 Hesitation Effect 37326 Weld Lines 38327 Meld Lines 38328 Sink Marks 38329 Multidirectional Flow 393210 Unstable Flow 403211 Simple Flow Pattern 4133 Flow-front Velocity and Flow-front Area 41331 What are FFV and FFA 41332 Flow-front Velocity Influences Filling Pattern 41
333 Equation 4234 Using Moldflow to Determine the Filling Pattern 43341 Computer Simulation Can Eliminate Molding Trials 43342 Using a Flow Analysis 4335 Using Moldflow to Achieve Constant FFV 44351 Controlling the FFV Through Ram Speed 44
4 Moldflow Design Principles 47
41 Product Design and Moldflow 4742 Sequence of Analysis 47421 Part Filling Optimization 48422 Molding Conditions 48423 Runner Design 48424 Cooling Optimization 49425 Packing Optimization 49426 Warpage Optimization 4943 Moldflow Flow Concepts 49431 Unidirectional and Controlled Flow Pattern 50432 Flow Balancing 51
433 Constant Pressure Gradient 52434 Maximum Shear Stress 53435 Uniform Cooling 54
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Contents XIII
436 Positioning Weld and Meld Lines 55437 Avoid Hesitation Effects 55
438 Avoid Underflow 55439 Balancing with Flow Leaders and Flow Deflectors 574310 Controlled Frictional Heat 584311 Thermal Shutoff of Runners 584312 Acceptable RunnerCavity Ratio 59
5 Meshes Used In Moldf low Analyses 61
51 Mesh Types Used by Moldflow 61511 Finite Elements Used in Moldflow 61512 Mesh Types 62513 Solver Assumptions 6352 Mesh Requirements 64521 Mesh Density Considerations 64522 Part Details 6753 Geometry Creation 6854 Importing Geometry 6955 Using Different Mesh Types 69551 Door Panel 69552 Manifold 70
6 Product Design 71
61 Material Properties for Product Design 71611 Plastics Are Sensitive to Operating Conditions 71612 Stress-Strain Behavior 72613 Creep and Stress Relaxation 77614 Fatigue 79615 Impact strength 80616 Thermal Mechanical Behavior 8062 Design for Strength 82621 Predicting Part Strength 82622 LoadingOperating Conditions 8263 Part Thickness 86631 Part Thickness Drives Quality and Cost 86632 Cycle Time Increases with Thickness 86633 Thick Parts Tend to Warp 86634 Thin Uniform Parts Improve Surface Quality 87635 Reducing Part Thickness 8764 Boosting Structural Integrity with Ribs 88
641 Structural Integrity the Goal of Every Design 88642 Designing Ribs 88
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XIV Contents
65 Design for Assembly 90651 Molding One Part vs Separate Components 90
652 Tolerances Fit between Parts 90653 Press-fit Joints 91654 Snap-fit Joints 93655 Cantilever Snap Joints 95656 Torsion Snap-fit Joints 96657 Fasteners 98658 Inserts 101659 Welding Processes 101
7 Gate Design 103
71 Gate Design Overview 103711 What Is a Gate 103712 Single vs Multiple Gates 103713 Gate Dimensions 103714 Gate Location 10472 Gate Types 104721 Manually Trimmed Gates 104722 Automatically Trimmed Gates 11073 Design Rules 113
731 Determining the Number of Gates 113732 Flow Patterns 115733 Gate Position 116734 Avoiding Common Problems 122735 Gate Length 122736 Gate Thickness 122737 Freeze-off Time 12374 Using Moldflow for Gate Design 123741 Gate Location 123742 Molding Window Size for the Three Gate Locations 124
743 Filling Pattern 125744 Gate Size Based on Shear Rate 125
8 Runner System Design 127
81 Definitions 127811 Feed System 127812 Runner System 127813 Cold Runner 127814 Hot Runner 128
815 Hot Manifold 128816 Hot Drop 128817 Sprue 128
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Contents XV
82 Runner System Design Principles 129821 Benefits of Good Runner Design 129
822 Runner Design Philosophy 129823 Flow Balancing 130824 Flow Control 130825 Frictional Heating in Runners 132826 Thermal Shutoff 132827 System and Runner Pressures 132828 Constant Pressure Gradient 133829 Cold Slug Wells 1338210 Easy Ejection 13483 Runner Types 134
831 Cold Runners 134832 Hot Runner Systems 13584 Runner Layout 136841 Determining the Number of Cavities 136842 Planning the Runner System Layout 138843 Partially Balanced Runners 139844 Geometrically Balanced Runners 14185 Initial Runner Sizing 141851 Determining Sprue Dimensions 141852 Designing Runner Cross Sections 142
853 Determining Runner Diameters 14586 Runner Balancing 147861 How Runner Balancing Works 147862 When Are the Runner Sizes Optimized 147863 Validating the Balance 147864 Processing Window 14887 Using Moldflow for Runner Balancing 149871 Runner Balancing a 48-cavity Tool 149872 Runner Balancing for a Family Mold 151873 Runner Balancing for a Multigated Part 152
9 Cooling System Design 153
91 Mold Cooling System Overview 153911 Importance of Cooling System Design 153912 Mold Cooling System Components 15492 Cooling-channel Configuration 156921 Types of Cooling Channels 15693 Alternative Cooling Devices 157931 What Do They Do 157
932 Baffles 158933 Bubblers 159934 Thermal Pins 159
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XVI Contents
935 Cooling Slender Cores 160936 Cooling Large Cores 161
937 Cooling Cylinder Cores 16294 Cooling System Equations 162941 Cooling Time 162942 Reynolds Number and Coolant Flow 16395 Design Rules 163951 Mold Cooling Design Considerations 163952 Location and Size of Channels 164953 Flow Rate and Heat Transfer 16696 Using Moldflow for Cooling System Design 167961 Example Setup 168
962 Cycle Time Determined by Design and Processing Parameters 169
10 Shrinkage and Warpage 173
101 Injection Molding and Shrinkage 1731011 What Are Shrinkage and Warpage 1731012 Shrinkage and Machine Settings 1731013 Mold Filling and Packing 1741014 How Pressure and Time Affect Shrinkage 1751015 Thermally Unstable Flow 176
102 Basic Causes of Shrinkage and Warpage 1771021 Causes of Shrinkage 1771022 Causes of Warpage 1811023 Relating Orientation and Area Shrinkage to Warpage 182103 Designing Accurate Parts Considering Warpage 1831031 Material Selection 1841032 Wall Thickness Variation 1841033 Gate Position and Runner Dimensions 1851034 Molding Conditions 1871035 Cooling Line Layout 190
11 Moldflow Design Procedure 191
111 Determine Analysis Objectives 191112 Moldflow Analysis Steps Framework 1921121 The Whole Process 1921122 Optimize Fill 1921123 Balance and Size the Runners 1971124 Optimize Cooling 1991125 Optimize the Packing Profile 200
1126 Optimize Warpage 201113 Using Moldflow to Evaluate an Initial Design 2051131 Description of this Example 205
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Contents XVII
1132 Molding Window 2051133 Filling Analysis 206
1134 Gate and Runner Design 2061135 Cooling System Design 2071136 Packing Analysis 2091137 Warpage Analysis 209114 Using Moldflow to Optimize the Design 2101141 Determine the Cause of Warpage 2101142 Investigating Different Gate Locations 2101143 Validating the Best Gate Location 211
12 Part Defects 215
121 Air Traps 2151211 What Is an Air Trap 2151212 Problems Caused by Air Traps 2161213 Remedies 216122 Black Specks and Black Streaks 2171221 What Are Black Specks and Black Streaks 2171222 Causes of Black Specks and Black Streaks 2171223 Remedies 218123 Brittleness 219
1231 What Is Brittleness 2191232 Causes of Brittleness 2191233 Remedies 219124 Burn Marks 2201241 What Is a Burn Mark 2201242 Causes of Burn Marks 2211243 Remedies 222125 Delamination 2221251 What Is Delamination 2221252 Causes of Delamination 223
1253 Remedies 223126 Dimensional Variation 2241261 What Is Dimensional Variation 2241262 Causes of Dimensional Variation 2241263 Remedies 224127 Discoloration 2251271 What Is Discoloration 2251272 Causes of Discoloration 2251273 Remedies 226128 Fish Eyes 227
1281 What Are Fish Eyes 2271282 Causes of Fish Eyes 2271283 Remedies 227
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XVIII Contents
129 Flash 2281291 What Is Flash 228
1292 Causes of Flash 2281293 Remedies 2291210 Flow Marks 23012101 What Is A Flow Mark 23012102 Causes of Flow Marks 23012103 Remedies 2301211 Hesitation 23112111 What Is Hesitation 23112112 Problems Caused by Hesitation 23112113 Remedies 232
1212 Jetting 23212121 What Is Jetting 23212122 Problems Caused by Jetting 23312123 Remedies 2331213 Ripples 23512131 What Are Ripples 23512132 Cause of Ripples 23512133 Remedies 2361214 Short Shots 23612141 What Is a Short Shot 236
12142 Causes of Short Shots 23712143 Remedies 2371215 Silver Streaks 23812151 What Are Silver Streaks 23812152 Causes of Silver Streaks 23912153 Remedies 2391216 Sink Marks and Voids 24012161 What Are Sink Marks and Voids 24012162 Causes of Sink Marks and Voids 24012163 Remedies 2411217 Weld Lines and Meld Lines 24312171 What Are Weld Lines and Meld Lines 24312172 Problems Caused by Weld Lines 24312173 Strength of Weld Lines 24412174 Remedies 245
Appendix A Injection Molding 247
A1 Injection-molding Overview 247 A11 Process 247
A12 Applications 248 A2 Development of the Injection-molding Machine 248 A21 Benefits of the Reciprocating Screw 248
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Contents XIX
A3 Development of the Injection-molding Process 249 A4 Alternative Injection-molding Processes 249
A41 Co-injection (Sandwich) Molding 249 A42 Fusible Core Injection Molding 250 A43 Gas-assisted Injection Molding 251 A44 Injection-compression Molding 252 A45 Lamellar (Microlayer) Injection Molding 253 A46 Live-feed Injection Molding 253 A47 Low-pressure Injection Molding 254 A48 Push-pull Injection Molding 254 A49 Reactive Molding 255 A410 Structural Foam Injection Molding 256
A411 Thin-wall Molding 256
Appendix B Injection-molding Machine System and Operations 259
B1 Injection-molding Machine 259B11 Components 259B12 Machine Specification 260B13 Machine Function 260B14 Auxiliary Equipment 260B2 Machine Components 261
B21 Injection System 261B22 Mold System 263B23 Hydraulic System 264B24 Control System 264B25 Clamping System 264B3 Molded System 265B31 The Delivery System 265B4 Machine Operating Sequence 266B41 Process Cycle 266B5 Screw Operation 268
B51 Back Pressure 268B52 Injection Speed 268B53 Screw Rotation Speed 268B54 Cushion 269B6 Secondary Operations 269B61 Assembly 269B62 Decoration 269B63 Other Secondary Operations 270
Appendix C Injection-molding Process Control 271
C1 Importance of Process Conditions 271C11 Setting Machine Process Conditions 271
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XX Contents
Appendix D Plastic Materials 281
D1 What Are Plastics 281
D11 Polymerization Process 281D12 Structure of Polymers 281D13 Polymer Alloys and Blends 283D14 Polymer Composites 283D2 Classification of Plastics 283D21 Classes of Plastics 284D22 Structures and Properties of Plastics 284D3 Thermoplastics 285D31 Market Share Distribution of Thermoplastics 285D32 Structures and Properties of Thermoplastics 286D33 Amorphous Polymers 286D34 Semicrystalline Polymers 287D4 Thermosets 288D5 Properties and Applications of Thermoplastics 289D51 ABS 290D52 PA 12 291D53 PA 6 293D54 PA 66 294D55 PBT 296
D56 PC 297D57 PCABS 299D58 PCPBT 300D59 HDPE 301D510 LDPE 303D511 PEI 304D512 PET 305D513 PETG 306D514 PMMA 307D515 POM 309
D516 PP 310D517 PPEPPO 312D518 PS 313D519 PVC 314D520 SAN 316D521 Additives Fillers and Reinforcements 317D522 Modifying Polymer Properties 318D523 Low-aspect Fillers 319D524 High-aspect Fillers Fibers 319
Index 321
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1 Polymer Flow Behavior in Injection Molds
bull Phases of injection molding
bull How do plastics flow
11 Phases of Injection Molding
Any molder can prove that all the conditions and effects discussed in this chapter do indeedoccur during the injection molding process While this knowledge alone can somewhatimprove quality it is only with the use of Moldflow analysis during the initial design stage with the mold designed for the optimum filling pattern that these effects can be controlledand the full benefits obtained
Flow technology is concerned with the behavior of plastics during the mold filling process Aplastic parts properties depend on how the part is molded Two parts having identicaldimensions and made from the same material but molded under different conditions will havedifferent stress and shrinkage levels and will behave differently in the field meaning that they
are in practice two different parts The way the plastic flows into the mold is of paramount importance in determining the quality of the part The process of filling the mold can be distinctly analyzed with the ability to predictpressure temperature and stress
111 How Plastic Fills a Mold
This was investigated using a centrally gated mold shaped like a dinner plate with a thick rim
around the outside as shown in Figure 11 It was found that the injection molding processalthough complex could be divided into three phases (we use the word phase to avoidconfusion with injection stage as used with programmed injection)
Figure 1ure 1 1 Cross-section of disk mold used to investigate flow
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2 Polymer Flow Behavior in Injection Molds
1111 Filling Phase
As the ram moves forward it first moves at a steady speed as the plastic flows into the cavity
This is the filling phase This phase lasts until the mold is just filled See Figure 12 andFigure 13
1112 Pressurization Phase
The pressurization phase begins when the ram moves forward after the filling phase to bring the mold up to pressure When the mold is filled the ram will slow down but it still movesquite some distance because plastics are very compressible materials At injection molding pressure an extra 15 volume of material can be forced into the cavity See Figure 12 andFigure 13
Although fluids are usually assumed to be incompressible molten plastics have to beconsidered to be more like a gas The compressibility of plastics can be observed by blocking off the nozzle and attempting to purge the barrel The ram will jump forward when thepressure is applied but will spring back when the pressure is released
1113 Compensation Phase
After the pressurization phase the ram still does not stop completely continuing to creepforward for some time Plastics have a very large volumetric change of about 25 from themelt to the solid This can be seen in a short shot the difference in volume between themolding and the cavity is due to this volumetric change See Figure 12 and Figure 13
The ram moving forward to compensate for the volumetric change in the part is called thecompensation phase As the volumetric change is 25 and at the most only an extra 15 canbe injected in the pressurization phase there must always be some compensation phase
Figure 1igure 1 2 Phases of injection molding
Filling phase
Pressurization phase
Compensation phase
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Phases of Injection Molding 3
Figure 13ure 1 3 Phases of injection molding detail
112 The Filling Phase
A two-color technique best demonstrates this phase After emptying the barrel of an injection-molding machine a small amount of red plastic was charged followed by green plastic
Consider the closed mold with the plastic front just starting to flow from the nozzle Theplastic first fills the sprue and runner system then enters the mold cavity itself forming a smallbubble of molten plastic
The skin of the plastic in contact with the cool mold freezes rapidly while the central coreremains molten When additional material is injected it flows into this central core displacing
the material already there which then forms a new flow front The flow of this displacedmaterial is a combination of forward flow and outward flow The outward flow contacts the wall freezes and forms the next section of skin while the forward flow forms the new moltencore When more material enters the mold it flows along a channel lined with these frozen walls of plastic illustrated in Figure 14
This flow pattern is often called fountain flow or bubble flow because the flow front is like abubble being inflated with hot plastic from the center The frozen layer is formed by the flow front inflating and so is subject to only a low shear stress and therefore has a very low levelof molecular orientation Once it is frozen it cannot be orientated any further so the frozen
layer in the finished part has a low level of orientation
Figure 14ure 1 4 Fountain flow and heat transfer
Filling Pressurization Compensation
Hot plastic
Frozen layer
Heat loss to mold
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4 Polymer Flow Behavior in Injection Molds
Now consider what happens upstream Hot plastic is continuously flowing bringing new hotmaterial along and generating significant frictional heat At the same time heat is being lost
through the frozen layer to the cold mold surfaceInitially the frozen layer is very thin so heat is lost very rapidly This results in more plasticfreezing and the frozen layer getting thicker cutting down the heat flow After a time thefrozen layer will reach a thickness such that the heat lost by conduction is equal to the heatinput from plastic flow and frictional heating ie an equilibrium condition is reached(Figure 14)
It is interesting to do some calculations on the time taken to reach this state of equilibrium The actual rate of heat flow is very large in comparison with the small heat content of theplastic in the frozen layer The result is that equilibrium is reached very quickly often in a timemeasured in a few tenths of a second As the total filling time is measured in seconds thefrozen layer reaches an equilibrium state early in the filling cycle
It is useful to think about how the thickness of this frozen layer will vary If the injection rate were slowed less heat would be generated by friction along the flow path with less heat inputfrom the flow The heat loss would be at the same rate and with less heat input the frozenlayer would grow in thickness If the injection rate were raised the frozen layer would bethinner (Figure 15) Similarly higher melt and mold temperatures would reduce the thicknessof the frozen layer This can be seen experimentally using the two-color technique
Figure 1igure 1 5 Influence of injection rate on frozen layer thickness
1121 Flow Shear Stress
It is easy to get confused between the various stress levels and orientation of the polymer Asthe plastic flows it is subject to shear stress also called flow shear stress This flow shear stress willorient the material ie cause the molecules to align themselves in the general direction of flow
The shear stress varies from a maximum at the outside dropping off to zero at the center
3Shear stress is purely a function of force and area This must not be confused withshear rate which is the rate of plastic sliding over the next layer Shear rate is zero
= Frozen
= Molten cooler
= Molten hotter
Slowerinjection
rate
Faster
injection
rate
vs
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Phases of Injection Molding 5
at the outer edge where the plastic is frozen rises to a maximum just inwards of thefrozen layer then drops toward the center as shown in Figure 16
Figure 16ure 1 6 Shear rate distribution
If the flow were stopped and the plastic allowed to cool down very slowly this orientation would have time to relax giving a very low level of residual orientation On the other hand if the material were kept under stress and the plastic snap frozen most of the orientation would
be trapped in the frozen plastic (Figure 17)
Figure 17ure 1 7 Molecular orientation through the thickness of the part
Min MaxShear rate
= Tension
= Compression
Molecular orientation trapped
in pressurization phase
Gradual reduction of orientation
toward the center of the
mold as stress levels are lower
and the cooling rate is slower
which allows more time for
orientation to relax
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6 Polymer Flow Behavior in Injection Molds
Now consider the orientation from the mold surface toward the center
The frozen layer itself formed with very little shear and therefore low orientation immediately
freezes setting the low level of orientation
The layer of plastic just on the inside of the frozen layer is subject to maximum shear stressand freezes the instant flow stops trapping almost all the orientation
This is the orientation pattern the further toward the center the more the shear stress dropsand the slower the rate of cooling This allows more time for the level of orientation to relaxso the residual orientation drops rapidly toward the center Consider how this pattern willaffect the residual stress level Oriented material (normally) will shrink more than nonorientedmaterial On the inner surface of the original frozen layer highly oriented material wants toshrink a great deal but it is prevented from doing so by the less-oriented material The highly
oriented layer ends up being in tension while the less-oriented material is in compression This residual stress pattern is a common cause of part warpage
3 There is a connectionthrough orientationbetween the shear stress during
filling (flow stress) and the residual stress in the final molded part This meansshear stress during filling shown on Moldflow plots can be used as a designparameter
113 The Pressurization Phase
The pressurization phasefrom the point of view of flow behavioris very similar to thefilling phase The flow rate may drop somewhat as the mold builds up to pressure resulting inan increase in the thickness of the frozen layer
The main difference of course is the increase in hydrostatic (isotropic) pressure We shall seein chapter 2 section 24 Effect of Molding Conditions that hydrostatic pressure in itself doesnot cause any residual stress
114 The Compensation Phase
Compensating flow is unstable Consider the plate molding again (see Figure 11) You wouldthink that plastic flowing uniformly through the thin diaphragm would top up the thick rim Inpractice the plastic during the compensation phase flows in rivers that spread out like a deltaas illustrated in Figure 18 This may seem surprising at first but it can be explained by temperature instability
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How Do Plastics Flow 7
Figure 18ure 1 8 River flow
1141 Temperature Variation
There is always some variation in melt temperature coming from the barrel of the injectionmachine In exceptional cases up to 40 ordmC variation has been measured using a high-speedthermocouple
1142 Natural Instability
However slight the temperature variation natural instability will amplify it If for exampleone part of the melt is slightly hotter than the rest then the plastic flow in that area will beslightly greater bringing hotter material into the area and maintaining the temperature If onthe other hand there is another area that is cooler the flow will be less so there will be lessheat input and the plastic will get colder until it eventually freezes off
However balanced the initial conditions this natural instability will result in a river-type flow
This is a very important consideration The first material to freeze off will shrink early in thecycle By the time the material in the river flows freezes the bulk of the material will havealready frozen off and shrinkage will have occurred The rivers will shrink relative to the bulk of the molding and because they are highly orientated shrinkage will be very high The resultis high-stress tensile members throughout the molding a common cause of warpage
1143 Optimum Part Quality
Most of the stress in plastic parts occurs during the compensation phase By controlling flow and minimizing stress it is possible to design for optimum part quality This important point is
at the heart of the Moldflow philosophy
12 How Do Plastics Flow
121 Material Behavior
Molten thermoplastics exhibit viscoelastic behavior which combines flow characteristics of both viscous liquids and elastic solids When a viscous liquid flows the energy that causes thedeformation is dissipated and becomes viscous heat On the other hand when an elastic solid
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8 Polymer Flow Behavior in Injection Molds
is deformed the driving energy is stored For example the flow of water is a typical viscousflow whereas the deformation of a rubber cube falls into the elastic category
122 Deformation
In addition to the two types of material flow behavior there are two types of deformationsimple shear and simple extension (elongation) as shown in Figure 19 (a) and (b) below Theflow of molten thermoplastics during injection-molding filling is predominantly shear flow asshown in Figure 19 (c) in which layers of material elements slide over each other Theextensional flow however becomes significant as the material elements undergo elongation when the melt passes areas of abrupt dimensional change (eg a gate region) as shown inFigure 19 (d)
Figure 1igure 1 9 (a) Simple shear flow (b) Simple extensional flow (c) Shear flow in cavity filling (d) Extensional flow in cavity filling
123 Viscoelastic Behavior
In response to an applied stress (force per unit area) molten thermoplastics exhibit viscoelastic behavior which combines characteristics of an ideal viscous liquid with those of an ideal elastic solid In other words under certain conditions molten thermoplastics behavelike a liquid and will continuously deform while shear stress is applied as shown in
Figure 110 Upon the removal of the stress however the materials behave somewhat like anelastic solid with partial recovery of the deformation as shown in Figure 110 (b) and (c) This viscoelastic behavior stems from the random-coil configuration of polymer molecules in the
Stress
Stress Stress
Mold wall
Material element before
deformation begins
Material element during
deformation
Mold wall
Material elements at rest
Moving
materialelements
Velocity
profile
(a) (b)
(c) (d)
Velocity
profile
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How Do Plastics Flow 9
molten state which allows the movement and slippage of molecular chains under theinfluence of an applied load However the entanglement of the polymer molecular chains also
makes the system behave like an elastic solid upon the application and removal of the externalload Namely on removal of the stress chains will tend to return to the equilibrium random-coil state and thus will be a component of stress recovery The recovery is not instantaneousbecause of the entanglements still present in the system
Figure 1ure 1 10 (a) Ideal viscous liquid deforms continuously under applied stress (b) Ideal elasticsolid deforms immediately upon the application of stress but fully recovers when the
stress is removed (c) Molten thermoplastic deforms continuously under the applied
stress (like a viscous liquid) but also recovers partially from the deformation upon
removal of the applied stress (like an elastic solid)
124 Melt Shear Viscosity
1241 What Is Shear Viscosity
Melt shear viscosity is a materials resistance to shear flow In general polymer melts are highly viscous because of their long molecular chain structure The viscosity of a polymer melt rangesfrom 2 to 3000 Pas (water 10^-1 Pas glass 10^20 Pas) Viscosity can be thought of as thethickness of a fluid or how much it resists flow Viscosity is expressed as the ratio of shearstress (force per unit area) to the shear rate (rate change of shear strain) as shown inEquation 11 and Figure 111
(a) Viscous
liquid
(b) Elastic
solid
(c) Viscoelastic
polymer
melt
Initial shape
(shear stress
to be applied)
Time = Dt Time = 2 x Dt
(shear stress to
be removed)
Final deformed
shape
d
d
d
d
2d
2d
2d
2d
total
recovery
partial
recovery
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Mold1047298ow Design
Guide
A Resource for Plastics Engineers
Jay Shoemaker (Editor)
Hanser Publishers Munich bull Hanser Gardner Publications Cincinnati
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The Editor
Jay Shoemaker Mold1047298ow Corporation Framingham Massachusetts USA
Distributed in the USA and in Canada by Hanser Gardner Publications Inc6915 Valley Avenue Cincinnati Ohio 45244-3029 USAFax (513) 527-8801Phone (513) 527-8977 or 1-800-950-8977 wwwhansergardnercom
Distributed in all other countries by Carl Hanser Verlag Postfach 86 04 20 81631 Muumlnchen Germany Fax +49 (89) 98 48 09 wwwhanserde
The use of general descriptive names trademarks etc in this publication even if the former are not especiallyidenti1047297ed is not to be taken as a sign that such names as understood by the Trade Marks and Merchandise Marks Actmay accordingly be used freely by anyone While the advice and information in this book are believed to be true and accurate at the date of going to press neitherthe authors nor the editors nor the publisher can accept any legal responsibility for any errors or omissions that may bemade The publisher makes no warranty express or implied with respect to the material contained herein
Library of Congress Cataloging-in-Publication Data
Shoemaker Jay
Mold1047298ow design guide a resource for plastic engineers Jay Shoemaker p cm ISBN-13 978-1-56990-403-9 (hardcover) ISBN-10 1-56990-403-0 (hardcover)1 Injection molding of plastics I Title TP1120S486 2006 6684lsquo12--dc22 2006012438
Bibliogra1047297sche Information Der Deutschen Bibliothek Die Deutsche Bibliothek verzeichnet diese Publikation in der Deutschen Nationalbibliogra1047297e
detaillierte bibliogra1047297sche Daten sind im Internet uumlber lthttpdnbddbdegt abrufbar
ISBN-10 3-446-40640-9ISBN-13 978-3-446-40640-7
All rights reserved No part of this book may be reproduced or transmitted in any form or by any means electronic ormechanical including photocopying or by any information storage and retrieval system without permission in writingfrom the publisher
copy Mold1047298ow Corporation USA 2006Production Management Oswald Immel Typeset by Marcia Swan USACoverconcept Marc Muumlller-Bremer Rebranding Muumlnchen Germany Coverdesign MCP bull Susanne Kraus GbR Holzkirchen Germany Printed and bound by Koumlsel Krugzell Germany
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Foreword
The drive toward fast cost-effective and reliable plastics manufacturing has been Moldflowssole guiding goal since the company was founded over 25 years ago
This focused determination led us to introduce many new and exciting tools into the marketeach contributing to achieving our goal in some way whether by driving cost out of production with reduced material usage or shortened cycle times reducing mold delivery timeby minimizing re-work or increasing the reliability of supply by enabling higher quality products to be manufactured with greater surety in scheduling
The artificially balanced multi-cavity and family molds that are now commonplace were madepractical through the advent of our early simulation and runner balancing capabilities which
were introduced in the late 1970s and early 1980s As these tools evolved we were able to visualize and therefore control flow patterns and weld lines This evolution continued until we arrived in the 2000s with an array of sophisticated technology to control warpage accountfor heat transfer predict core shift adapt to new molding processes and much more Fromtraditional midplane technology to fully three-dimensional simulations all our solutions are well integrated into a solid-modeling design environment
As the technology has evolved so has its usage When Moldflow simulation technology wasintroduced its primary purpose was to search for remedies to pre-existing molding problemsIt soon became evident that the insight the software provided to solve molding problems would be better applied ahead of actual molding during the design process This
methodology which we call problem avoidance was the primary use for Moldflow technology for the first 20 years of its existence
For Moldflow this created a unique challenge to open the world of manufacturing to thedesigners of parts and molds What constitutes an ineffective design for molding may beapparent to a seasoned processing engineer looking retrospectively at a poorly performing tool but how can design engineers use the CAE tools to visualize diagnose and solve thesesame issues ahead of timewithout 20 years of molding experience How can manufacturersgo further and use information that cannot be seen in the real molding process but is revealed via simulation
The key that unlocked this puzzle began its life as the Moldflow Design Philosophy This is widely
viewed as the most important publication Moldflow has ever produced and has spawnedfollow-on works on related subjects Rather than provide insight into the operation of thesimulation tools Moldflow Design Philosophy set forth simple principles that transcend any specific software application and as a result are as valid with todays advanced simulationproducts as they were over two decades ago
In more recent years another transition has occurred The global imperative to drive down thecost of manufacturing has led to the use of molding simulation as a cost optimization toolrather than for problem avoidance This change has increased the number of Moldflow usersby an order of magnitude across a far broader cross-section of the plastics industry Greaterdesign-centricity leads to even more dependence on the plastics design principles which can
be used to drive optimization
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VI Foreword
Despite a quarter of a century of technological advances the golden years of CAE are aheadof us as our industry takes a broader and more integrated view of what it takes to manage a
products life cycle Moldflow is proud of its contributions to date and will continue to focuson developing innovative technology coupled with practical design principles to deliver moreprofitable manufacturing
Roland Thomas
President amp CEO Moldflow Corporation
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Preface VII
Preface
About this Book
The origins of this book include not only Moldflow Design Principles but also Warpage Design
Principles published by Moldflow and the C-MOLD Design Guide Collectively thesedocuments are based on years of experience in the research theory and practice of injectionmolding These documents are now combined into this book the Moldflow Design Guide The Moldflow Design Guide is intended to help practicing engineers solve problems they frequently encounter in the design of parts and molds as well as during production This book can alsobe used as a reference for training purposes at industrial and educational institutions
How to Use this Book
This book has several chapters and appendices that deal with different stages of the designprocess and provides background on the injection-molding process and plastic materials
bull The first three chapters introduce injection molding how polymers flow inside injectionmolds and how molding conditions and injection pressure influence the process
bull Chapter 4 discusses Moldflow design principles and how they relate to making quality parts
bull Chapter 5 introduces the finite element mesh technology used by Moldflow and how thesemeshes influence the quality of the analysis
bull Chapters 6 to 9 introduce design concepts for the product gates runners and coolingsystems
bull Chapter 10 introduces concepts relating to shrinkage and warpage and how Moldflow isused to determine the amount of shrinkage and warpage a molded part will have and whatcauses the warpage
bull
Chapter 11 discusses the design procedure for analyzing injection-molded partsbull Chapter 12 discusses major part defects found on injection-molded parts
bull Finally the four appendices discuss basic injection-molding machine operation processcontrol variants of the standard injection-molding process and plastic materials
Benefits of Using CAE
The injection-molding industry has recognized that computer-aided engineering (CAE)
enhances an engineers ability to handle all aspects of the polymer injection-molding processbenefiting productivity product quality timeliness and cost This is illustrated by a wealth of
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VIII Preface
literature and the ever-growing number of CAE software users in the injection-molding industry
CAE Predicts Process Behavior
Ideally CAE analysis provides insight that is useful in designing parts molds and molding processes Without it we rely on previous experience intuition prototyping or molding trialsto obtain information such as polymer melt filling patterns weld-line and air-trap locationsrequired injection pressure and clamp tonnage fiber orientation cycle time final part shapeand deformation and mechanical properties of molded parts just to name a few WithoutCAE analysis other equally important design data such as spatial distributions of pressuretemperature shear rate shear stress and velocity are more difficult to obtain even with a well-instrumented mold The process behavior predicted by CAE can help novice engineers
overcome the lack of previous experience and assist experienced engineers in pinpointing factors that may otherwise be overlooked By using CAE analysis to iterate and evaluatealternative designs and competing materials engineering know-how in the form of designguidelines can be established relatively faster and more cost-effectively
User Proficiency Determines the Benefits of CAE
While CAE technology helps save time money and raw material as well as cuts scrap reducesthe rejection rate improves product quality and gets new products to market faster it is by nomeans a panacea for solving all molding problems Rather it should be recognized that CAEanalysis is essentially a tool designed to assist engineers instead of taking over theirresponsibilities or replacing them Like many other tools the usefulness of CAE technology depends on the proficiency of the user The benefits mentioned above will not be realizedunless the CAE tool is used properly To be more specific the accuracy of CAE analysisdepends greatly on the input data provided by the user In addition the results generated by CAE analysis need to be correctly and intelligently interpreted by the user before soundjudgments and rational decisions are made Otherwise users will simply be swamped by the vast amount of data without getting any useful information
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Acknowledgements IX
Acknowledgements
The Moldflow Design Guide would not have been accomplished were it not for the vision of Ken Welch Ken and I have discussed the value of assembling the best of the Moldflow Design
Principles Warpage Design Principles and the C-MOLD Design Guide into a single book for severalyears With Kens leadership he gave the project to Steve Thompsons training group of which I am a part Steve helped me coordinate the resources necessary to get this projectdone I could not have done this project without Steves help and guidance
A review of the content was part of the development of the Moldflow Design Guide Moldflow developers including Peter Kennedy Rong Zheng Zhongshuang Yuan and Xiaoshi Jin havereviewed sections of the book Moldflows application engineers and other technical staff withMoldflow have also reviewed sections These reviewers include Chad Fuhrman Matt JaworskiChristine Roedlich Eric Henry Olivier Anninos Paul Larter and Ana Maria Marin A specialthanks goes to Mike Rogers who reviewed the entire book for me and provided criticalfeedback on the content and organization of the book I would also like to thank Kurt Haydenof Western Michigan University for reviewing the appendix on process control His many years of experience of process setup and optimization was invaluable
Finally I would like to thank members of Moldflows Technology Transfer group forsupporting me in the writing of this book These members include Marcia Swan CarolineDorin Robert Ashley Melissa Haslam Darren Seamons and Gregory Brown
On a personal note I would like to acknowledge and thank Paul Engelmann Professor and
Department Chair Western Michigan University Department of Industrial and Manufacturing Engineering for being my friend and mentor during my career With Paul I have been able toteach and participate in research he has done on injection molding tooling and processing at Western Michigan University I have found working with Paul has made me a better Moldflow user and engineer by providing another perspective on how Moldflow can be used to solveinjection molding problemsJay Shoemaker Editor
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Contents
1 Polymer Flow Behavior in Injection Molds 1
11 Phases of Injection Molding 1111 How Plastic Fills a Mold 1112 The Filling Phase 3113 The Pressurization Phase 6114 The Compensation Phase 612 How Do Plastics Flow 7121 Material Behavior 7122 Deformation 8
123 Viscoelastic Behavior 8124 Melt Shear Viscosity 9125 Newtonian Fluid vs Non-Newtonian Fluid 10126 Shear-thinning Behavior 10127 Shear Rate Distribution 11128 Pressure-driven Flow 11129 Pressure Gradient and Injection Times 131210 Melt Flow Length 131211 Injection Pressure vs Fill Time 131212 Flow Instability 15
2 Molding Conditions and Injection Pressure 17
21 Injection-pressure Overview 17211 Pressure Drives the Flow Front 1822 Factors Influencing Injection-pressure Requirements 1823 Equations 20231 Circular Channel Flow 20232 Strip Channel Flow 2024 Effect of Molding Conditions 21
241 Part Quality 21242 Melt Temperature 22243 Mold Temperature 22244 Fill Time 23245 Shear Stress Variation 23246 Packing Pressure and Time 24247 Summary 25248 Back Flow 2625 Using Moldflow to Determine Optimum Molding Conditions 26251 Part 27
252 Molding Window Size 27253 Injection Pressure 28254 Flow Front Temperature 29
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XII Contents
255 Cooling Time 30256 Summary 31
3 Filling Pattern 33
31 Filling Pattern Overview 33311 What Is the Filling Pattern 3332 Flow in Complex Molds 34321 Overpack 34322 Racetrack Effect 35323 Varying Injection Rate 35324 Underflow Effect 36325 Hesitation Effect 37326 Weld Lines 38327 Meld Lines 38328 Sink Marks 38329 Multidirectional Flow 393210 Unstable Flow 403211 Simple Flow Pattern 4133 Flow-front Velocity and Flow-front Area 41331 What are FFV and FFA 41332 Flow-front Velocity Influences Filling Pattern 41
333 Equation 4234 Using Moldflow to Determine the Filling Pattern 43341 Computer Simulation Can Eliminate Molding Trials 43342 Using a Flow Analysis 4335 Using Moldflow to Achieve Constant FFV 44351 Controlling the FFV Through Ram Speed 44
4 Moldflow Design Principles 47
41 Product Design and Moldflow 4742 Sequence of Analysis 47421 Part Filling Optimization 48422 Molding Conditions 48423 Runner Design 48424 Cooling Optimization 49425 Packing Optimization 49426 Warpage Optimization 4943 Moldflow Flow Concepts 49431 Unidirectional and Controlled Flow Pattern 50432 Flow Balancing 51
433 Constant Pressure Gradient 52434 Maximum Shear Stress 53435 Uniform Cooling 54
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Contents XIII
436 Positioning Weld and Meld Lines 55437 Avoid Hesitation Effects 55
438 Avoid Underflow 55439 Balancing with Flow Leaders and Flow Deflectors 574310 Controlled Frictional Heat 584311 Thermal Shutoff of Runners 584312 Acceptable RunnerCavity Ratio 59
5 Meshes Used In Moldf low Analyses 61
51 Mesh Types Used by Moldflow 61511 Finite Elements Used in Moldflow 61512 Mesh Types 62513 Solver Assumptions 6352 Mesh Requirements 64521 Mesh Density Considerations 64522 Part Details 6753 Geometry Creation 6854 Importing Geometry 6955 Using Different Mesh Types 69551 Door Panel 69552 Manifold 70
6 Product Design 71
61 Material Properties for Product Design 71611 Plastics Are Sensitive to Operating Conditions 71612 Stress-Strain Behavior 72613 Creep and Stress Relaxation 77614 Fatigue 79615 Impact strength 80616 Thermal Mechanical Behavior 8062 Design for Strength 82621 Predicting Part Strength 82622 LoadingOperating Conditions 8263 Part Thickness 86631 Part Thickness Drives Quality and Cost 86632 Cycle Time Increases with Thickness 86633 Thick Parts Tend to Warp 86634 Thin Uniform Parts Improve Surface Quality 87635 Reducing Part Thickness 8764 Boosting Structural Integrity with Ribs 88
641 Structural Integrity the Goal of Every Design 88642 Designing Ribs 88
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XIV Contents
65 Design for Assembly 90651 Molding One Part vs Separate Components 90
652 Tolerances Fit between Parts 90653 Press-fit Joints 91654 Snap-fit Joints 93655 Cantilever Snap Joints 95656 Torsion Snap-fit Joints 96657 Fasteners 98658 Inserts 101659 Welding Processes 101
7 Gate Design 103
71 Gate Design Overview 103711 What Is a Gate 103712 Single vs Multiple Gates 103713 Gate Dimensions 103714 Gate Location 10472 Gate Types 104721 Manually Trimmed Gates 104722 Automatically Trimmed Gates 11073 Design Rules 113
731 Determining the Number of Gates 113732 Flow Patterns 115733 Gate Position 116734 Avoiding Common Problems 122735 Gate Length 122736 Gate Thickness 122737 Freeze-off Time 12374 Using Moldflow for Gate Design 123741 Gate Location 123742 Molding Window Size for the Three Gate Locations 124
743 Filling Pattern 125744 Gate Size Based on Shear Rate 125
8 Runner System Design 127
81 Definitions 127811 Feed System 127812 Runner System 127813 Cold Runner 127814 Hot Runner 128
815 Hot Manifold 128816 Hot Drop 128817 Sprue 128
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Contents XV
82 Runner System Design Principles 129821 Benefits of Good Runner Design 129
822 Runner Design Philosophy 129823 Flow Balancing 130824 Flow Control 130825 Frictional Heating in Runners 132826 Thermal Shutoff 132827 System and Runner Pressures 132828 Constant Pressure Gradient 133829 Cold Slug Wells 1338210 Easy Ejection 13483 Runner Types 134
831 Cold Runners 134832 Hot Runner Systems 13584 Runner Layout 136841 Determining the Number of Cavities 136842 Planning the Runner System Layout 138843 Partially Balanced Runners 139844 Geometrically Balanced Runners 14185 Initial Runner Sizing 141851 Determining Sprue Dimensions 141852 Designing Runner Cross Sections 142
853 Determining Runner Diameters 14586 Runner Balancing 147861 How Runner Balancing Works 147862 When Are the Runner Sizes Optimized 147863 Validating the Balance 147864 Processing Window 14887 Using Moldflow for Runner Balancing 149871 Runner Balancing a 48-cavity Tool 149872 Runner Balancing for a Family Mold 151873 Runner Balancing for a Multigated Part 152
9 Cooling System Design 153
91 Mold Cooling System Overview 153911 Importance of Cooling System Design 153912 Mold Cooling System Components 15492 Cooling-channel Configuration 156921 Types of Cooling Channels 15693 Alternative Cooling Devices 157931 What Do They Do 157
932 Baffles 158933 Bubblers 159934 Thermal Pins 159
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XVI Contents
935 Cooling Slender Cores 160936 Cooling Large Cores 161
937 Cooling Cylinder Cores 16294 Cooling System Equations 162941 Cooling Time 162942 Reynolds Number and Coolant Flow 16395 Design Rules 163951 Mold Cooling Design Considerations 163952 Location and Size of Channels 164953 Flow Rate and Heat Transfer 16696 Using Moldflow for Cooling System Design 167961 Example Setup 168
962 Cycle Time Determined by Design and Processing Parameters 169
10 Shrinkage and Warpage 173
101 Injection Molding and Shrinkage 1731011 What Are Shrinkage and Warpage 1731012 Shrinkage and Machine Settings 1731013 Mold Filling and Packing 1741014 How Pressure and Time Affect Shrinkage 1751015 Thermally Unstable Flow 176
102 Basic Causes of Shrinkage and Warpage 1771021 Causes of Shrinkage 1771022 Causes of Warpage 1811023 Relating Orientation and Area Shrinkage to Warpage 182103 Designing Accurate Parts Considering Warpage 1831031 Material Selection 1841032 Wall Thickness Variation 1841033 Gate Position and Runner Dimensions 1851034 Molding Conditions 1871035 Cooling Line Layout 190
11 Moldflow Design Procedure 191
111 Determine Analysis Objectives 191112 Moldflow Analysis Steps Framework 1921121 The Whole Process 1921122 Optimize Fill 1921123 Balance and Size the Runners 1971124 Optimize Cooling 1991125 Optimize the Packing Profile 200
1126 Optimize Warpage 201113 Using Moldflow to Evaluate an Initial Design 2051131 Description of this Example 205
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Contents XVII
1132 Molding Window 2051133 Filling Analysis 206
1134 Gate and Runner Design 2061135 Cooling System Design 2071136 Packing Analysis 2091137 Warpage Analysis 209114 Using Moldflow to Optimize the Design 2101141 Determine the Cause of Warpage 2101142 Investigating Different Gate Locations 2101143 Validating the Best Gate Location 211
12 Part Defects 215
121 Air Traps 2151211 What Is an Air Trap 2151212 Problems Caused by Air Traps 2161213 Remedies 216122 Black Specks and Black Streaks 2171221 What Are Black Specks and Black Streaks 2171222 Causes of Black Specks and Black Streaks 2171223 Remedies 218123 Brittleness 219
1231 What Is Brittleness 2191232 Causes of Brittleness 2191233 Remedies 219124 Burn Marks 2201241 What Is a Burn Mark 2201242 Causes of Burn Marks 2211243 Remedies 222125 Delamination 2221251 What Is Delamination 2221252 Causes of Delamination 223
1253 Remedies 223126 Dimensional Variation 2241261 What Is Dimensional Variation 2241262 Causes of Dimensional Variation 2241263 Remedies 224127 Discoloration 2251271 What Is Discoloration 2251272 Causes of Discoloration 2251273 Remedies 226128 Fish Eyes 227
1281 What Are Fish Eyes 2271282 Causes of Fish Eyes 2271283 Remedies 227
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XVIII Contents
129 Flash 2281291 What Is Flash 228
1292 Causes of Flash 2281293 Remedies 2291210 Flow Marks 23012101 What Is A Flow Mark 23012102 Causes of Flow Marks 23012103 Remedies 2301211 Hesitation 23112111 What Is Hesitation 23112112 Problems Caused by Hesitation 23112113 Remedies 232
1212 Jetting 23212121 What Is Jetting 23212122 Problems Caused by Jetting 23312123 Remedies 2331213 Ripples 23512131 What Are Ripples 23512132 Cause of Ripples 23512133 Remedies 2361214 Short Shots 23612141 What Is a Short Shot 236
12142 Causes of Short Shots 23712143 Remedies 2371215 Silver Streaks 23812151 What Are Silver Streaks 23812152 Causes of Silver Streaks 23912153 Remedies 2391216 Sink Marks and Voids 24012161 What Are Sink Marks and Voids 24012162 Causes of Sink Marks and Voids 24012163 Remedies 2411217 Weld Lines and Meld Lines 24312171 What Are Weld Lines and Meld Lines 24312172 Problems Caused by Weld Lines 24312173 Strength of Weld Lines 24412174 Remedies 245
Appendix A Injection Molding 247
A1 Injection-molding Overview 247 A11 Process 247
A12 Applications 248 A2 Development of the Injection-molding Machine 248 A21 Benefits of the Reciprocating Screw 248
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Contents XIX
A3 Development of the Injection-molding Process 249 A4 Alternative Injection-molding Processes 249
A41 Co-injection (Sandwich) Molding 249 A42 Fusible Core Injection Molding 250 A43 Gas-assisted Injection Molding 251 A44 Injection-compression Molding 252 A45 Lamellar (Microlayer) Injection Molding 253 A46 Live-feed Injection Molding 253 A47 Low-pressure Injection Molding 254 A48 Push-pull Injection Molding 254 A49 Reactive Molding 255 A410 Structural Foam Injection Molding 256
A411 Thin-wall Molding 256
Appendix B Injection-molding Machine System and Operations 259
B1 Injection-molding Machine 259B11 Components 259B12 Machine Specification 260B13 Machine Function 260B14 Auxiliary Equipment 260B2 Machine Components 261
B21 Injection System 261B22 Mold System 263B23 Hydraulic System 264B24 Control System 264B25 Clamping System 264B3 Molded System 265B31 The Delivery System 265B4 Machine Operating Sequence 266B41 Process Cycle 266B5 Screw Operation 268
B51 Back Pressure 268B52 Injection Speed 268B53 Screw Rotation Speed 268B54 Cushion 269B6 Secondary Operations 269B61 Assembly 269B62 Decoration 269B63 Other Secondary Operations 270
Appendix C Injection-molding Process Control 271
C1 Importance of Process Conditions 271C11 Setting Machine Process Conditions 271
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XX Contents
Appendix D Plastic Materials 281
D1 What Are Plastics 281
D11 Polymerization Process 281D12 Structure of Polymers 281D13 Polymer Alloys and Blends 283D14 Polymer Composites 283D2 Classification of Plastics 283D21 Classes of Plastics 284D22 Structures and Properties of Plastics 284D3 Thermoplastics 285D31 Market Share Distribution of Thermoplastics 285D32 Structures and Properties of Thermoplastics 286D33 Amorphous Polymers 286D34 Semicrystalline Polymers 287D4 Thermosets 288D5 Properties and Applications of Thermoplastics 289D51 ABS 290D52 PA 12 291D53 PA 6 293D54 PA 66 294D55 PBT 296
D56 PC 297D57 PCABS 299D58 PCPBT 300D59 HDPE 301D510 LDPE 303D511 PEI 304D512 PET 305D513 PETG 306D514 PMMA 307D515 POM 309
D516 PP 310D517 PPEPPO 312D518 PS 313D519 PVC 314D520 SAN 316D521 Additives Fillers and Reinforcements 317D522 Modifying Polymer Properties 318D523 Low-aspect Fillers 319D524 High-aspect Fillers Fibers 319
Index 321
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1 Polymer Flow Behavior in Injection Molds
bull Phases of injection molding
bull How do plastics flow
11 Phases of Injection Molding
Any molder can prove that all the conditions and effects discussed in this chapter do indeedoccur during the injection molding process While this knowledge alone can somewhatimprove quality it is only with the use of Moldflow analysis during the initial design stage with the mold designed for the optimum filling pattern that these effects can be controlledand the full benefits obtained
Flow technology is concerned with the behavior of plastics during the mold filling process Aplastic parts properties depend on how the part is molded Two parts having identicaldimensions and made from the same material but molded under different conditions will havedifferent stress and shrinkage levels and will behave differently in the field meaning that they
are in practice two different parts The way the plastic flows into the mold is of paramount importance in determining the quality of the part The process of filling the mold can be distinctly analyzed with the ability to predictpressure temperature and stress
111 How Plastic Fills a Mold
This was investigated using a centrally gated mold shaped like a dinner plate with a thick rim
around the outside as shown in Figure 11 It was found that the injection molding processalthough complex could be divided into three phases (we use the word phase to avoidconfusion with injection stage as used with programmed injection)
Figure 1ure 1 1 Cross-section of disk mold used to investigate flow
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2 Polymer Flow Behavior in Injection Molds
1111 Filling Phase
As the ram moves forward it first moves at a steady speed as the plastic flows into the cavity
This is the filling phase This phase lasts until the mold is just filled See Figure 12 andFigure 13
1112 Pressurization Phase
The pressurization phase begins when the ram moves forward after the filling phase to bring the mold up to pressure When the mold is filled the ram will slow down but it still movesquite some distance because plastics are very compressible materials At injection molding pressure an extra 15 volume of material can be forced into the cavity See Figure 12 andFigure 13
Although fluids are usually assumed to be incompressible molten plastics have to beconsidered to be more like a gas The compressibility of plastics can be observed by blocking off the nozzle and attempting to purge the barrel The ram will jump forward when thepressure is applied but will spring back when the pressure is released
1113 Compensation Phase
After the pressurization phase the ram still does not stop completely continuing to creepforward for some time Plastics have a very large volumetric change of about 25 from themelt to the solid This can be seen in a short shot the difference in volume between themolding and the cavity is due to this volumetric change See Figure 12 and Figure 13
The ram moving forward to compensate for the volumetric change in the part is called thecompensation phase As the volumetric change is 25 and at the most only an extra 15 canbe injected in the pressurization phase there must always be some compensation phase
Figure 1igure 1 2 Phases of injection molding
Filling phase
Pressurization phase
Compensation phase
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Phases of Injection Molding 3
Figure 13ure 1 3 Phases of injection molding detail
112 The Filling Phase
A two-color technique best demonstrates this phase After emptying the barrel of an injection-molding machine a small amount of red plastic was charged followed by green plastic
Consider the closed mold with the plastic front just starting to flow from the nozzle Theplastic first fills the sprue and runner system then enters the mold cavity itself forming a smallbubble of molten plastic
The skin of the plastic in contact with the cool mold freezes rapidly while the central coreremains molten When additional material is injected it flows into this central core displacing
the material already there which then forms a new flow front The flow of this displacedmaterial is a combination of forward flow and outward flow The outward flow contacts the wall freezes and forms the next section of skin while the forward flow forms the new moltencore When more material enters the mold it flows along a channel lined with these frozen walls of plastic illustrated in Figure 14
This flow pattern is often called fountain flow or bubble flow because the flow front is like abubble being inflated with hot plastic from the center The frozen layer is formed by the flow front inflating and so is subject to only a low shear stress and therefore has a very low levelof molecular orientation Once it is frozen it cannot be orientated any further so the frozen
layer in the finished part has a low level of orientation
Figure 14ure 1 4 Fountain flow and heat transfer
Filling Pressurization Compensation
Hot plastic
Frozen layer
Heat loss to mold
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4 Polymer Flow Behavior in Injection Molds
Now consider what happens upstream Hot plastic is continuously flowing bringing new hotmaterial along and generating significant frictional heat At the same time heat is being lost
through the frozen layer to the cold mold surfaceInitially the frozen layer is very thin so heat is lost very rapidly This results in more plasticfreezing and the frozen layer getting thicker cutting down the heat flow After a time thefrozen layer will reach a thickness such that the heat lost by conduction is equal to the heatinput from plastic flow and frictional heating ie an equilibrium condition is reached(Figure 14)
It is interesting to do some calculations on the time taken to reach this state of equilibrium The actual rate of heat flow is very large in comparison with the small heat content of theplastic in the frozen layer The result is that equilibrium is reached very quickly often in a timemeasured in a few tenths of a second As the total filling time is measured in seconds thefrozen layer reaches an equilibrium state early in the filling cycle
It is useful to think about how the thickness of this frozen layer will vary If the injection rate were slowed less heat would be generated by friction along the flow path with less heat inputfrom the flow The heat loss would be at the same rate and with less heat input the frozenlayer would grow in thickness If the injection rate were raised the frozen layer would bethinner (Figure 15) Similarly higher melt and mold temperatures would reduce the thicknessof the frozen layer This can be seen experimentally using the two-color technique
Figure 1igure 1 5 Influence of injection rate on frozen layer thickness
1121 Flow Shear Stress
It is easy to get confused between the various stress levels and orientation of the polymer Asthe plastic flows it is subject to shear stress also called flow shear stress This flow shear stress willorient the material ie cause the molecules to align themselves in the general direction of flow
The shear stress varies from a maximum at the outside dropping off to zero at the center
3Shear stress is purely a function of force and area This must not be confused withshear rate which is the rate of plastic sliding over the next layer Shear rate is zero
= Frozen
= Molten cooler
= Molten hotter
Slowerinjection
rate
Faster
injection
rate
vs
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Phases of Injection Molding 5
at the outer edge where the plastic is frozen rises to a maximum just inwards of thefrozen layer then drops toward the center as shown in Figure 16
Figure 16ure 1 6 Shear rate distribution
If the flow were stopped and the plastic allowed to cool down very slowly this orientation would have time to relax giving a very low level of residual orientation On the other hand if the material were kept under stress and the plastic snap frozen most of the orientation would
be trapped in the frozen plastic (Figure 17)
Figure 17ure 1 7 Molecular orientation through the thickness of the part
Min MaxShear rate
= Tension
= Compression
Molecular orientation trapped
in pressurization phase
Gradual reduction of orientation
toward the center of the
mold as stress levels are lower
and the cooling rate is slower
which allows more time for
orientation to relax
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6 Polymer Flow Behavior in Injection Molds
Now consider the orientation from the mold surface toward the center
The frozen layer itself formed with very little shear and therefore low orientation immediately
freezes setting the low level of orientation
The layer of plastic just on the inside of the frozen layer is subject to maximum shear stressand freezes the instant flow stops trapping almost all the orientation
This is the orientation pattern the further toward the center the more the shear stress dropsand the slower the rate of cooling This allows more time for the level of orientation to relaxso the residual orientation drops rapidly toward the center Consider how this pattern willaffect the residual stress level Oriented material (normally) will shrink more than nonorientedmaterial On the inner surface of the original frozen layer highly oriented material wants toshrink a great deal but it is prevented from doing so by the less-oriented material The highly
oriented layer ends up being in tension while the less-oriented material is in compression This residual stress pattern is a common cause of part warpage
3 There is a connectionthrough orientationbetween the shear stress during
filling (flow stress) and the residual stress in the final molded part This meansshear stress during filling shown on Moldflow plots can be used as a designparameter
113 The Pressurization Phase
The pressurization phasefrom the point of view of flow behavioris very similar to thefilling phase The flow rate may drop somewhat as the mold builds up to pressure resulting inan increase in the thickness of the frozen layer
The main difference of course is the increase in hydrostatic (isotropic) pressure We shall seein chapter 2 section 24 Effect of Molding Conditions that hydrostatic pressure in itself doesnot cause any residual stress
114 The Compensation Phase
Compensating flow is unstable Consider the plate molding again (see Figure 11) You wouldthink that plastic flowing uniformly through the thin diaphragm would top up the thick rim Inpractice the plastic during the compensation phase flows in rivers that spread out like a deltaas illustrated in Figure 18 This may seem surprising at first but it can be explained by temperature instability
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How Do Plastics Flow 7
Figure 18ure 1 8 River flow
1141 Temperature Variation
There is always some variation in melt temperature coming from the barrel of the injectionmachine In exceptional cases up to 40 ordmC variation has been measured using a high-speedthermocouple
1142 Natural Instability
However slight the temperature variation natural instability will amplify it If for exampleone part of the melt is slightly hotter than the rest then the plastic flow in that area will beslightly greater bringing hotter material into the area and maintaining the temperature If onthe other hand there is another area that is cooler the flow will be less so there will be lessheat input and the plastic will get colder until it eventually freezes off
However balanced the initial conditions this natural instability will result in a river-type flow
This is a very important consideration The first material to freeze off will shrink early in thecycle By the time the material in the river flows freezes the bulk of the material will havealready frozen off and shrinkage will have occurred The rivers will shrink relative to the bulk of the molding and because they are highly orientated shrinkage will be very high The resultis high-stress tensile members throughout the molding a common cause of warpage
1143 Optimum Part Quality
Most of the stress in plastic parts occurs during the compensation phase By controlling flow and minimizing stress it is possible to design for optimum part quality This important point is
at the heart of the Moldflow philosophy
12 How Do Plastics Flow
121 Material Behavior
Molten thermoplastics exhibit viscoelastic behavior which combines flow characteristics of both viscous liquids and elastic solids When a viscous liquid flows the energy that causes thedeformation is dissipated and becomes viscous heat On the other hand when an elastic solid
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8 Polymer Flow Behavior in Injection Molds
is deformed the driving energy is stored For example the flow of water is a typical viscousflow whereas the deformation of a rubber cube falls into the elastic category
122 Deformation
In addition to the two types of material flow behavior there are two types of deformationsimple shear and simple extension (elongation) as shown in Figure 19 (a) and (b) below Theflow of molten thermoplastics during injection-molding filling is predominantly shear flow asshown in Figure 19 (c) in which layers of material elements slide over each other Theextensional flow however becomes significant as the material elements undergo elongation when the melt passes areas of abrupt dimensional change (eg a gate region) as shown inFigure 19 (d)
Figure 1igure 1 9 (a) Simple shear flow (b) Simple extensional flow (c) Shear flow in cavity filling (d) Extensional flow in cavity filling
123 Viscoelastic Behavior
In response to an applied stress (force per unit area) molten thermoplastics exhibit viscoelastic behavior which combines characteristics of an ideal viscous liquid with those of an ideal elastic solid In other words under certain conditions molten thermoplastics behavelike a liquid and will continuously deform while shear stress is applied as shown in
Figure 110 Upon the removal of the stress however the materials behave somewhat like anelastic solid with partial recovery of the deformation as shown in Figure 110 (b) and (c) This viscoelastic behavior stems from the random-coil configuration of polymer molecules in the
Stress
Stress Stress
Mold wall
Material element before
deformation begins
Material element during
deformation
Mold wall
Material elements at rest
Moving
materialelements
Velocity
profile
(a) (b)
(c) (d)
Velocity
profile
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How Do Plastics Flow 9
molten state which allows the movement and slippage of molecular chains under theinfluence of an applied load However the entanglement of the polymer molecular chains also
makes the system behave like an elastic solid upon the application and removal of the externalload Namely on removal of the stress chains will tend to return to the equilibrium random-coil state and thus will be a component of stress recovery The recovery is not instantaneousbecause of the entanglements still present in the system
Figure 1ure 1 10 (a) Ideal viscous liquid deforms continuously under applied stress (b) Ideal elasticsolid deforms immediately upon the application of stress but fully recovers when the
stress is removed (c) Molten thermoplastic deforms continuously under the applied
stress (like a viscous liquid) but also recovers partially from the deformation upon
removal of the applied stress (like an elastic solid)
124 Melt Shear Viscosity
1241 What Is Shear Viscosity
Melt shear viscosity is a materials resistance to shear flow In general polymer melts are highly viscous because of their long molecular chain structure The viscosity of a polymer melt rangesfrom 2 to 3000 Pas (water 10^-1 Pas glass 10^20 Pas) Viscosity can be thought of as thethickness of a fluid or how much it resists flow Viscosity is expressed as the ratio of shearstress (force per unit area) to the shear rate (rate change of shear strain) as shown inEquation 11 and Figure 111
(a) Viscous
liquid
(b) Elastic
solid
(c) Viscoelastic
polymer
melt
Initial shape
(shear stress
to be applied)
Time = Dt Time = 2 x Dt
(shear stress to
be removed)
Final deformed
shape
d
d
d
d
2d
2d
2d
2d
total
recovery
partial
recovery
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Mold1047298ow Design
Guide
A Resource for Plastics Engineers
Jay Shoemaker (Editor)
Hanser Publishers Munich bull Hanser Gardner Publications Cincinnati
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The Editor
Jay Shoemaker Mold1047298ow Corporation Framingham Massachusetts USA
Distributed in the USA and in Canada by Hanser Gardner Publications Inc6915 Valley Avenue Cincinnati Ohio 45244-3029 USAFax (513) 527-8801Phone (513) 527-8977 or 1-800-950-8977 wwwhansergardnercom
Distributed in all other countries by Carl Hanser Verlag Postfach 86 04 20 81631 Muumlnchen Germany Fax +49 (89) 98 48 09 wwwhanserde
The use of general descriptive names trademarks etc in this publication even if the former are not especiallyidenti1047297ed is not to be taken as a sign that such names as understood by the Trade Marks and Merchandise Marks Actmay accordingly be used freely by anyone While the advice and information in this book are believed to be true and accurate at the date of going to press neitherthe authors nor the editors nor the publisher can accept any legal responsibility for any errors or omissions that may bemade The publisher makes no warranty express or implied with respect to the material contained herein
Library of Congress Cataloging-in-Publication Data
Shoemaker Jay
Mold1047298ow design guide a resource for plastic engineers Jay Shoemaker p cm ISBN-13 978-1-56990-403-9 (hardcover) ISBN-10 1-56990-403-0 (hardcover)1 Injection molding of plastics I Title TP1120S486 2006 6684lsquo12--dc22 2006012438
Bibliogra1047297sche Information Der Deutschen Bibliothek Die Deutsche Bibliothek verzeichnet diese Publikation in der Deutschen Nationalbibliogra1047297e
detaillierte bibliogra1047297sche Daten sind im Internet uumlber lthttpdnbddbdegt abrufbar
ISBN-10 3-446-40640-9ISBN-13 978-3-446-40640-7
All rights reserved No part of this book may be reproduced or transmitted in any form or by any means electronic ormechanical including photocopying or by any information storage and retrieval system without permission in writingfrom the publisher
copy Mold1047298ow Corporation USA 2006Production Management Oswald Immel Typeset by Marcia Swan USACoverconcept Marc Muumlller-Bremer Rebranding Muumlnchen Germany Coverdesign MCP bull Susanne Kraus GbR Holzkirchen Germany Printed and bound by Koumlsel Krugzell Germany
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Foreword
The drive toward fast cost-effective and reliable plastics manufacturing has been Moldflowssole guiding goal since the company was founded over 25 years ago
This focused determination led us to introduce many new and exciting tools into the marketeach contributing to achieving our goal in some way whether by driving cost out of production with reduced material usage or shortened cycle times reducing mold delivery timeby minimizing re-work or increasing the reliability of supply by enabling higher quality products to be manufactured with greater surety in scheduling
The artificially balanced multi-cavity and family molds that are now commonplace were madepractical through the advent of our early simulation and runner balancing capabilities which
were introduced in the late 1970s and early 1980s As these tools evolved we were able to visualize and therefore control flow patterns and weld lines This evolution continued until we arrived in the 2000s with an array of sophisticated technology to control warpage accountfor heat transfer predict core shift adapt to new molding processes and much more Fromtraditional midplane technology to fully three-dimensional simulations all our solutions are well integrated into a solid-modeling design environment
As the technology has evolved so has its usage When Moldflow simulation technology wasintroduced its primary purpose was to search for remedies to pre-existing molding problemsIt soon became evident that the insight the software provided to solve molding problems would be better applied ahead of actual molding during the design process This
methodology which we call problem avoidance was the primary use for Moldflow technology for the first 20 years of its existence
For Moldflow this created a unique challenge to open the world of manufacturing to thedesigners of parts and molds What constitutes an ineffective design for molding may beapparent to a seasoned processing engineer looking retrospectively at a poorly performing tool but how can design engineers use the CAE tools to visualize diagnose and solve thesesame issues ahead of timewithout 20 years of molding experience How can manufacturersgo further and use information that cannot be seen in the real molding process but is revealed via simulation
The key that unlocked this puzzle began its life as the Moldflow Design Philosophy This is widely
viewed as the most important publication Moldflow has ever produced and has spawnedfollow-on works on related subjects Rather than provide insight into the operation of thesimulation tools Moldflow Design Philosophy set forth simple principles that transcend any specific software application and as a result are as valid with todays advanced simulationproducts as they were over two decades ago
In more recent years another transition has occurred The global imperative to drive down thecost of manufacturing has led to the use of molding simulation as a cost optimization toolrather than for problem avoidance This change has increased the number of Moldflow usersby an order of magnitude across a far broader cross-section of the plastics industry Greaterdesign-centricity leads to even more dependence on the plastics design principles which can
be used to drive optimization
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VI Foreword
Despite a quarter of a century of technological advances the golden years of CAE are aheadof us as our industry takes a broader and more integrated view of what it takes to manage a
products life cycle Moldflow is proud of its contributions to date and will continue to focuson developing innovative technology coupled with practical design principles to deliver moreprofitable manufacturing
Roland Thomas
President amp CEO Moldflow Corporation
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Preface VII
Preface
About this Book
The origins of this book include not only Moldflow Design Principles but also Warpage Design
Principles published by Moldflow and the C-MOLD Design Guide Collectively thesedocuments are based on years of experience in the research theory and practice of injectionmolding These documents are now combined into this book the Moldflow Design Guide The Moldflow Design Guide is intended to help practicing engineers solve problems they frequently encounter in the design of parts and molds as well as during production This book can alsobe used as a reference for training purposes at industrial and educational institutions
How to Use this Book
This book has several chapters and appendices that deal with different stages of the designprocess and provides background on the injection-molding process and plastic materials
bull The first three chapters introduce injection molding how polymers flow inside injectionmolds and how molding conditions and injection pressure influence the process
bull Chapter 4 discusses Moldflow design principles and how they relate to making quality parts
bull Chapter 5 introduces the finite element mesh technology used by Moldflow and how thesemeshes influence the quality of the analysis
bull Chapters 6 to 9 introduce design concepts for the product gates runners and coolingsystems
bull Chapter 10 introduces concepts relating to shrinkage and warpage and how Moldflow isused to determine the amount of shrinkage and warpage a molded part will have and whatcauses the warpage
bull
Chapter 11 discusses the design procedure for analyzing injection-molded partsbull Chapter 12 discusses major part defects found on injection-molded parts
bull Finally the four appendices discuss basic injection-molding machine operation processcontrol variants of the standard injection-molding process and plastic materials
Benefits of Using CAE
The injection-molding industry has recognized that computer-aided engineering (CAE)
enhances an engineers ability to handle all aspects of the polymer injection-molding processbenefiting productivity product quality timeliness and cost This is illustrated by a wealth of
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VIII Preface
literature and the ever-growing number of CAE software users in the injection-molding industry
CAE Predicts Process Behavior
Ideally CAE analysis provides insight that is useful in designing parts molds and molding processes Without it we rely on previous experience intuition prototyping or molding trialsto obtain information such as polymer melt filling patterns weld-line and air-trap locationsrequired injection pressure and clamp tonnage fiber orientation cycle time final part shapeand deformation and mechanical properties of molded parts just to name a few WithoutCAE analysis other equally important design data such as spatial distributions of pressuretemperature shear rate shear stress and velocity are more difficult to obtain even with a well-instrumented mold The process behavior predicted by CAE can help novice engineers
overcome the lack of previous experience and assist experienced engineers in pinpointing factors that may otherwise be overlooked By using CAE analysis to iterate and evaluatealternative designs and competing materials engineering know-how in the form of designguidelines can be established relatively faster and more cost-effectively
User Proficiency Determines the Benefits of CAE
While CAE technology helps save time money and raw material as well as cuts scrap reducesthe rejection rate improves product quality and gets new products to market faster it is by nomeans a panacea for solving all molding problems Rather it should be recognized that CAEanalysis is essentially a tool designed to assist engineers instead of taking over theirresponsibilities or replacing them Like many other tools the usefulness of CAE technology depends on the proficiency of the user The benefits mentioned above will not be realizedunless the CAE tool is used properly To be more specific the accuracy of CAE analysisdepends greatly on the input data provided by the user In addition the results generated by CAE analysis need to be correctly and intelligently interpreted by the user before soundjudgments and rational decisions are made Otherwise users will simply be swamped by the vast amount of data without getting any useful information
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Acknowledgements IX
Acknowledgements
The Moldflow Design Guide would not have been accomplished were it not for the vision of Ken Welch Ken and I have discussed the value of assembling the best of the Moldflow Design
Principles Warpage Design Principles and the C-MOLD Design Guide into a single book for severalyears With Kens leadership he gave the project to Steve Thompsons training group of which I am a part Steve helped me coordinate the resources necessary to get this projectdone I could not have done this project without Steves help and guidance
A review of the content was part of the development of the Moldflow Design Guide Moldflow developers including Peter Kennedy Rong Zheng Zhongshuang Yuan and Xiaoshi Jin havereviewed sections of the book Moldflows application engineers and other technical staff withMoldflow have also reviewed sections These reviewers include Chad Fuhrman Matt JaworskiChristine Roedlich Eric Henry Olivier Anninos Paul Larter and Ana Maria Marin A specialthanks goes to Mike Rogers who reviewed the entire book for me and provided criticalfeedback on the content and organization of the book I would also like to thank Kurt Haydenof Western Michigan University for reviewing the appendix on process control His many years of experience of process setup and optimization was invaluable
Finally I would like to thank members of Moldflows Technology Transfer group forsupporting me in the writing of this book These members include Marcia Swan CarolineDorin Robert Ashley Melissa Haslam Darren Seamons and Gregory Brown
On a personal note I would like to acknowledge and thank Paul Engelmann Professor and
Department Chair Western Michigan University Department of Industrial and Manufacturing Engineering for being my friend and mentor during my career With Paul I have been able toteach and participate in research he has done on injection molding tooling and processing at Western Michigan University I have found working with Paul has made me a better Moldflow user and engineer by providing another perspective on how Moldflow can be used to solveinjection molding problemsJay Shoemaker Editor
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Contents
1 Polymer Flow Behavior in Injection Molds 1
11 Phases of Injection Molding 1111 How Plastic Fills a Mold 1112 The Filling Phase 3113 The Pressurization Phase 6114 The Compensation Phase 612 How Do Plastics Flow 7121 Material Behavior 7122 Deformation 8
123 Viscoelastic Behavior 8124 Melt Shear Viscosity 9125 Newtonian Fluid vs Non-Newtonian Fluid 10126 Shear-thinning Behavior 10127 Shear Rate Distribution 11128 Pressure-driven Flow 11129 Pressure Gradient and Injection Times 131210 Melt Flow Length 131211 Injection Pressure vs Fill Time 131212 Flow Instability 15
2 Molding Conditions and Injection Pressure 17
21 Injection-pressure Overview 17211 Pressure Drives the Flow Front 1822 Factors Influencing Injection-pressure Requirements 1823 Equations 20231 Circular Channel Flow 20232 Strip Channel Flow 2024 Effect of Molding Conditions 21
241 Part Quality 21242 Melt Temperature 22243 Mold Temperature 22244 Fill Time 23245 Shear Stress Variation 23246 Packing Pressure and Time 24247 Summary 25248 Back Flow 2625 Using Moldflow to Determine Optimum Molding Conditions 26251 Part 27
252 Molding Window Size 27253 Injection Pressure 28254 Flow Front Temperature 29
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XII Contents
255 Cooling Time 30256 Summary 31
3 Filling Pattern 33
31 Filling Pattern Overview 33311 What Is the Filling Pattern 3332 Flow in Complex Molds 34321 Overpack 34322 Racetrack Effect 35323 Varying Injection Rate 35324 Underflow Effect 36325 Hesitation Effect 37326 Weld Lines 38327 Meld Lines 38328 Sink Marks 38329 Multidirectional Flow 393210 Unstable Flow 403211 Simple Flow Pattern 4133 Flow-front Velocity and Flow-front Area 41331 What are FFV and FFA 41332 Flow-front Velocity Influences Filling Pattern 41
333 Equation 4234 Using Moldflow to Determine the Filling Pattern 43341 Computer Simulation Can Eliminate Molding Trials 43342 Using a Flow Analysis 4335 Using Moldflow to Achieve Constant FFV 44351 Controlling the FFV Through Ram Speed 44
4 Moldflow Design Principles 47
41 Product Design and Moldflow 4742 Sequence of Analysis 47421 Part Filling Optimization 48422 Molding Conditions 48423 Runner Design 48424 Cooling Optimization 49425 Packing Optimization 49426 Warpage Optimization 4943 Moldflow Flow Concepts 49431 Unidirectional and Controlled Flow Pattern 50432 Flow Balancing 51
433 Constant Pressure Gradient 52434 Maximum Shear Stress 53435 Uniform Cooling 54
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Contents XIII
436 Positioning Weld and Meld Lines 55437 Avoid Hesitation Effects 55
438 Avoid Underflow 55439 Balancing with Flow Leaders and Flow Deflectors 574310 Controlled Frictional Heat 584311 Thermal Shutoff of Runners 584312 Acceptable RunnerCavity Ratio 59
5 Meshes Used In Moldf low Analyses 61
51 Mesh Types Used by Moldflow 61511 Finite Elements Used in Moldflow 61512 Mesh Types 62513 Solver Assumptions 6352 Mesh Requirements 64521 Mesh Density Considerations 64522 Part Details 6753 Geometry Creation 6854 Importing Geometry 6955 Using Different Mesh Types 69551 Door Panel 69552 Manifold 70
6 Product Design 71
61 Material Properties for Product Design 71611 Plastics Are Sensitive to Operating Conditions 71612 Stress-Strain Behavior 72613 Creep and Stress Relaxation 77614 Fatigue 79615 Impact strength 80616 Thermal Mechanical Behavior 8062 Design for Strength 82621 Predicting Part Strength 82622 LoadingOperating Conditions 8263 Part Thickness 86631 Part Thickness Drives Quality and Cost 86632 Cycle Time Increases with Thickness 86633 Thick Parts Tend to Warp 86634 Thin Uniform Parts Improve Surface Quality 87635 Reducing Part Thickness 8764 Boosting Structural Integrity with Ribs 88
641 Structural Integrity the Goal of Every Design 88642 Designing Ribs 88
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XIV Contents
65 Design for Assembly 90651 Molding One Part vs Separate Components 90
652 Tolerances Fit between Parts 90653 Press-fit Joints 91654 Snap-fit Joints 93655 Cantilever Snap Joints 95656 Torsion Snap-fit Joints 96657 Fasteners 98658 Inserts 101659 Welding Processes 101
7 Gate Design 103
71 Gate Design Overview 103711 What Is a Gate 103712 Single vs Multiple Gates 103713 Gate Dimensions 103714 Gate Location 10472 Gate Types 104721 Manually Trimmed Gates 104722 Automatically Trimmed Gates 11073 Design Rules 113
731 Determining the Number of Gates 113732 Flow Patterns 115733 Gate Position 116734 Avoiding Common Problems 122735 Gate Length 122736 Gate Thickness 122737 Freeze-off Time 12374 Using Moldflow for Gate Design 123741 Gate Location 123742 Molding Window Size for the Three Gate Locations 124
743 Filling Pattern 125744 Gate Size Based on Shear Rate 125
8 Runner System Design 127
81 Definitions 127811 Feed System 127812 Runner System 127813 Cold Runner 127814 Hot Runner 128
815 Hot Manifold 128816 Hot Drop 128817 Sprue 128
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Contents XV
82 Runner System Design Principles 129821 Benefits of Good Runner Design 129
822 Runner Design Philosophy 129823 Flow Balancing 130824 Flow Control 130825 Frictional Heating in Runners 132826 Thermal Shutoff 132827 System and Runner Pressures 132828 Constant Pressure Gradient 133829 Cold Slug Wells 1338210 Easy Ejection 13483 Runner Types 134
831 Cold Runners 134832 Hot Runner Systems 13584 Runner Layout 136841 Determining the Number of Cavities 136842 Planning the Runner System Layout 138843 Partially Balanced Runners 139844 Geometrically Balanced Runners 14185 Initial Runner Sizing 141851 Determining Sprue Dimensions 141852 Designing Runner Cross Sections 142
853 Determining Runner Diameters 14586 Runner Balancing 147861 How Runner Balancing Works 147862 When Are the Runner Sizes Optimized 147863 Validating the Balance 147864 Processing Window 14887 Using Moldflow for Runner Balancing 149871 Runner Balancing a 48-cavity Tool 149872 Runner Balancing for a Family Mold 151873 Runner Balancing for a Multigated Part 152
9 Cooling System Design 153
91 Mold Cooling System Overview 153911 Importance of Cooling System Design 153912 Mold Cooling System Components 15492 Cooling-channel Configuration 156921 Types of Cooling Channels 15693 Alternative Cooling Devices 157931 What Do They Do 157
932 Baffles 158933 Bubblers 159934 Thermal Pins 159
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XVI Contents
935 Cooling Slender Cores 160936 Cooling Large Cores 161
937 Cooling Cylinder Cores 16294 Cooling System Equations 162941 Cooling Time 162942 Reynolds Number and Coolant Flow 16395 Design Rules 163951 Mold Cooling Design Considerations 163952 Location and Size of Channels 164953 Flow Rate and Heat Transfer 16696 Using Moldflow for Cooling System Design 167961 Example Setup 168
962 Cycle Time Determined by Design and Processing Parameters 169
10 Shrinkage and Warpage 173
101 Injection Molding and Shrinkage 1731011 What Are Shrinkage and Warpage 1731012 Shrinkage and Machine Settings 1731013 Mold Filling and Packing 1741014 How Pressure and Time Affect Shrinkage 1751015 Thermally Unstable Flow 176
102 Basic Causes of Shrinkage and Warpage 1771021 Causes of Shrinkage 1771022 Causes of Warpage 1811023 Relating Orientation and Area Shrinkage to Warpage 182103 Designing Accurate Parts Considering Warpage 1831031 Material Selection 1841032 Wall Thickness Variation 1841033 Gate Position and Runner Dimensions 1851034 Molding Conditions 1871035 Cooling Line Layout 190
11 Moldflow Design Procedure 191
111 Determine Analysis Objectives 191112 Moldflow Analysis Steps Framework 1921121 The Whole Process 1921122 Optimize Fill 1921123 Balance and Size the Runners 1971124 Optimize Cooling 1991125 Optimize the Packing Profile 200
1126 Optimize Warpage 201113 Using Moldflow to Evaluate an Initial Design 2051131 Description of this Example 205
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Contents XVII
1132 Molding Window 2051133 Filling Analysis 206
1134 Gate and Runner Design 2061135 Cooling System Design 2071136 Packing Analysis 2091137 Warpage Analysis 209114 Using Moldflow to Optimize the Design 2101141 Determine the Cause of Warpage 2101142 Investigating Different Gate Locations 2101143 Validating the Best Gate Location 211
12 Part Defects 215
121 Air Traps 2151211 What Is an Air Trap 2151212 Problems Caused by Air Traps 2161213 Remedies 216122 Black Specks and Black Streaks 2171221 What Are Black Specks and Black Streaks 2171222 Causes of Black Specks and Black Streaks 2171223 Remedies 218123 Brittleness 219
1231 What Is Brittleness 2191232 Causes of Brittleness 2191233 Remedies 219124 Burn Marks 2201241 What Is a Burn Mark 2201242 Causes of Burn Marks 2211243 Remedies 222125 Delamination 2221251 What Is Delamination 2221252 Causes of Delamination 223
1253 Remedies 223126 Dimensional Variation 2241261 What Is Dimensional Variation 2241262 Causes of Dimensional Variation 2241263 Remedies 224127 Discoloration 2251271 What Is Discoloration 2251272 Causes of Discoloration 2251273 Remedies 226128 Fish Eyes 227
1281 What Are Fish Eyes 2271282 Causes of Fish Eyes 2271283 Remedies 227
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XVIII Contents
129 Flash 2281291 What Is Flash 228
1292 Causes of Flash 2281293 Remedies 2291210 Flow Marks 23012101 What Is A Flow Mark 23012102 Causes of Flow Marks 23012103 Remedies 2301211 Hesitation 23112111 What Is Hesitation 23112112 Problems Caused by Hesitation 23112113 Remedies 232
1212 Jetting 23212121 What Is Jetting 23212122 Problems Caused by Jetting 23312123 Remedies 2331213 Ripples 23512131 What Are Ripples 23512132 Cause of Ripples 23512133 Remedies 2361214 Short Shots 23612141 What Is a Short Shot 236
12142 Causes of Short Shots 23712143 Remedies 2371215 Silver Streaks 23812151 What Are Silver Streaks 23812152 Causes of Silver Streaks 23912153 Remedies 2391216 Sink Marks and Voids 24012161 What Are Sink Marks and Voids 24012162 Causes of Sink Marks and Voids 24012163 Remedies 2411217 Weld Lines and Meld Lines 24312171 What Are Weld Lines and Meld Lines 24312172 Problems Caused by Weld Lines 24312173 Strength of Weld Lines 24412174 Remedies 245
Appendix A Injection Molding 247
A1 Injection-molding Overview 247 A11 Process 247
A12 Applications 248 A2 Development of the Injection-molding Machine 248 A21 Benefits of the Reciprocating Screw 248
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Contents XIX
A3 Development of the Injection-molding Process 249 A4 Alternative Injection-molding Processes 249
A41 Co-injection (Sandwich) Molding 249 A42 Fusible Core Injection Molding 250 A43 Gas-assisted Injection Molding 251 A44 Injection-compression Molding 252 A45 Lamellar (Microlayer) Injection Molding 253 A46 Live-feed Injection Molding 253 A47 Low-pressure Injection Molding 254 A48 Push-pull Injection Molding 254 A49 Reactive Molding 255 A410 Structural Foam Injection Molding 256
A411 Thin-wall Molding 256
Appendix B Injection-molding Machine System and Operations 259
B1 Injection-molding Machine 259B11 Components 259B12 Machine Specification 260B13 Machine Function 260B14 Auxiliary Equipment 260B2 Machine Components 261
B21 Injection System 261B22 Mold System 263B23 Hydraulic System 264B24 Control System 264B25 Clamping System 264B3 Molded System 265B31 The Delivery System 265B4 Machine Operating Sequence 266B41 Process Cycle 266B5 Screw Operation 268
B51 Back Pressure 268B52 Injection Speed 268B53 Screw Rotation Speed 268B54 Cushion 269B6 Secondary Operations 269B61 Assembly 269B62 Decoration 269B63 Other Secondary Operations 270
Appendix C Injection-molding Process Control 271
C1 Importance of Process Conditions 271C11 Setting Machine Process Conditions 271
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XX Contents
Appendix D Plastic Materials 281
D1 What Are Plastics 281
D11 Polymerization Process 281D12 Structure of Polymers 281D13 Polymer Alloys and Blends 283D14 Polymer Composites 283D2 Classification of Plastics 283D21 Classes of Plastics 284D22 Structures and Properties of Plastics 284D3 Thermoplastics 285D31 Market Share Distribution of Thermoplastics 285D32 Structures and Properties of Thermoplastics 286D33 Amorphous Polymers 286D34 Semicrystalline Polymers 287D4 Thermosets 288D5 Properties and Applications of Thermoplastics 289D51 ABS 290D52 PA 12 291D53 PA 6 293D54 PA 66 294D55 PBT 296
D56 PC 297D57 PCABS 299D58 PCPBT 300D59 HDPE 301D510 LDPE 303D511 PEI 304D512 PET 305D513 PETG 306D514 PMMA 307D515 POM 309
D516 PP 310D517 PPEPPO 312D518 PS 313D519 PVC 314D520 SAN 316D521 Additives Fillers and Reinforcements 317D522 Modifying Polymer Properties 318D523 Low-aspect Fillers 319D524 High-aspect Fillers Fibers 319
Index 321
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1 Polymer Flow Behavior in Injection Molds
bull Phases of injection molding
bull How do plastics flow
11 Phases of Injection Molding
Any molder can prove that all the conditions and effects discussed in this chapter do indeedoccur during the injection molding process While this knowledge alone can somewhatimprove quality it is only with the use of Moldflow analysis during the initial design stage with the mold designed for the optimum filling pattern that these effects can be controlledand the full benefits obtained
Flow technology is concerned with the behavior of plastics during the mold filling process Aplastic parts properties depend on how the part is molded Two parts having identicaldimensions and made from the same material but molded under different conditions will havedifferent stress and shrinkage levels and will behave differently in the field meaning that they
are in practice two different parts The way the plastic flows into the mold is of paramount importance in determining the quality of the part The process of filling the mold can be distinctly analyzed with the ability to predictpressure temperature and stress
111 How Plastic Fills a Mold
This was investigated using a centrally gated mold shaped like a dinner plate with a thick rim
around the outside as shown in Figure 11 It was found that the injection molding processalthough complex could be divided into three phases (we use the word phase to avoidconfusion with injection stage as used with programmed injection)
Figure 1ure 1 1 Cross-section of disk mold used to investigate flow
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2 Polymer Flow Behavior in Injection Molds
1111 Filling Phase
As the ram moves forward it first moves at a steady speed as the plastic flows into the cavity
This is the filling phase This phase lasts until the mold is just filled See Figure 12 andFigure 13
1112 Pressurization Phase
The pressurization phase begins when the ram moves forward after the filling phase to bring the mold up to pressure When the mold is filled the ram will slow down but it still movesquite some distance because plastics are very compressible materials At injection molding pressure an extra 15 volume of material can be forced into the cavity See Figure 12 andFigure 13
Although fluids are usually assumed to be incompressible molten plastics have to beconsidered to be more like a gas The compressibility of plastics can be observed by blocking off the nozzle and attempting to purge the barrel The ram will jump forward when thepressure is applied but will spring back when the pressure is released
1113 Compensation Phase
After the pressurization phase the ram still does not stop completely continuing to creepforward for some time Plastics have a very large volumetric change of about 25 from themelt to the solid This can be seen in a short shot the difference in volume between themolding and the cavity is due to this volumetric change See Figure 12 and Figure 13
The ram moving forward to compensate for the volumetric change in the part is called thecompensation phase As the volumetric change is 25 and at the most only an extra 15 canbe injected in the pressurization phase there must always be some compensation phase
Figure 1igure 1 2 Phases of injection molding
Filling phase
Pressurization phase
Compensation phase
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Phases of Injection Molding 3
Figure 13ure 1 3 Phases of injection molding detail
112 The Filling Phase
A two-color technique best demonstrates this phase After emptying the barrel of an injection-molding machine a small amount of red plastic was charged followed by green plastic
Consider the closed mold with the plastic front just starting to flow from the nozzle Theplastic first fills the sprue and runner system then enters the mold cavity itself forming a smallbubble of molten plastic
The skin of the plastic in contact with the cool mold freezes rapidly while the central coreremains molten When additional material is injected it flows into this central core displacing
the material already there which then forms a new flow front The flow of this displacedmaterial is a combination of forward flow and outward flow The outward flow contacts the wall freezes and forms the next section of skin while the forward flow forms the new moltencore When more material enters the mold it flows along a channel lined with these frozen walls of plastic illustrated in Figure 14
This flow pattern is often called fountain flow or bubble flow because the flow front is like abubble being inflated with hot plastic from the center The frozen layer is formed by the flow front inflating and so is subject to only a low shear stress and therefore has a very low levelof molecular orientation Once it is frozen it cannot be orientated any further so the frozen
layer in the finished part has a low level of orientation
Figure 14ure 1 4 Fountain flow and heat transfer
Filling Pressurization Compensation
Hot plastic
Frozen layer
Heat loss to mold
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4 Polymer Flow Behavior in Injection Molds
Now consider what happens upstream Hot plastic is continuously flowing bringing new hotmaterial along and generating significant frictional heat At the same time heat is being lost
through the frozen layer to the cold mold surfaceInitially the frozen layer is very thin so heat is lost very rapidly This results in more plasticfreezing and the frozen layer getting thicker cutting down the heat flow After a time thefrozen layer will reach a thickness such that the heat lost by conduction is equal to the heatinput from plastic flow and frictional heating ie an equilibrium condition is reached(Figure 14)
It is interesting to do some calculations on the time taken to reach this state of equilibrium The actual rate of heat flow is very large in comparison with the small heat content of theplastic in the frozen layer The result is that equilibrium is reached very quickly often in a timemeasured in a few tenths of a second As the total filling time is measured in seconds thefrozen layer reaches an equilibrium state early in the filling cycle
It is useful to think about how the thickness of this frozen layer will vary If the injection rate were slowed less heat would be generated by friction along the flow path with less heat inputfrom the flow The heat loss would be at the same rate and with less heat input the frozenlayer would grow in thickness If the injection rate were raised the frozen layer would bethinner (Figure 15) Similarly higher melt and mold temperatures would reduce the thicknessof the frozen layer This can be seen experimentally using the two-color technique
Figure 1igure 1 5 Influence of injection rate on frozen layer thickness
1121 Flow Shear Stress
It is easy to get confused between the various stress levels and orientation of the polymer Asthe plastic flows it is subject to shear stress also called flow shear stress This flow shear stress willorient the material ie cause the molecules to align themselves in the general direction of flow
The shear stress varies from a maximum at the outside dropping off to zero at the center
3Shear stress is purely a function of force and area This must not be confused withshear rate which is the rate of plastic sliding over the next layer Shear rate is zero
= Frozen
= Molten cooler
= Molten hotter
Slowerinjection
rate
Faster
injection
rate
vs
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Phases of Injection Molding 5
at the outer edge where the plastic is frozen rises to a maximum just inwards of thefrozen layer then drops toward the center as shown in Figure 16
Figure 16ure 1 6 Shear rate distribution
If the flow were stopped and the plastic allowed to cool down very slowly this orientation would have time to relax giving a very low level of residual orientation On the other hand if the material were kept under stress and the plastic snap frozen most of the orientation would
be trapped in the frozen plastic (Figure 17)
Figure 17ure 1 7 Molecular orientation through the thickness of the part
Min MaxShear rate
= Tension
= Compression
Molecular orientation trapped
in pressurization phase
Gradual reduction of orientation
toward the center of the
mold as stress levels are lower
and the cooling rate is slower
which allows more time for
orientation to relax
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6 Polymer Flow Behavior in Injection Molds
Now consider the orientation from the mold surface toward the center
The frozen layer itself formed with very little shear and therefore low orientation immediately
freezes setting the low level of orientation
The layer of plastic just on the inside of the frozen layer is subject to maximum shear stressand freezes the instant flow stops trapping almost all the orientation
This is the orientation pattern the further toward the center the more the shear stress dropsand the slower the rate of cooling This allows more time for the level of orientation to relaxso the residual orientation drops rapidly toward the center Consider how this pattern willaffect the residual stress level Oriented material (normally) will shrink more than nonorientedmaterial On the inner surface of the original frozen layer highly oriented material wants toshrink a great deal but it is prevented from doing so by the less-oriented material The highly
oriented layer ends up being in tension while the less-oriented material is in compression This residual stress pattern is a common cause of part warpage
3 There is a connectionthrough orientationbetween the shear stress during
filling (flow stress) and the residual stress in the final molded part This meansshear stress during filling shown on Moldflow plots can be used as a designparameter
113 The Pressurization Phase
The pressurization phasefrom the point of view of flow behavioris very similar to thefilling phase The flow rate may drop somewhat as the mold builds up to pressure resulting inan increase in the thickness of the frozen layer
The main difference of course is the increase in hydrostatic (isotropic) pressure We shall seein chapter 2 section 24 Effect of Molding Conditions that hydrostatic pressure in itself doesnot cause any residual stress
114 The Compensation Phase
Compensating flow is unstable Consider the plate molding again (see Figure 11) You wouldthink that plastic flowing uniformly through the thin diaphragm would top up the thick rim Inpractice the plastic during the compensation phase flows in rivers that spread out like a deltaas illustrated in Figure 18 This may seem surprising at first but it can be explained by temperature instability
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How Do Plastics Flow 7
Figure 18ure 1 8 River flow
1141 Temperature Variation
There is always some variation in melt temperature coming from the barrel of the injectionmachine In exceptional cases up to 40 ordmC variation has been measured using a high-speedthermocouple
1142 Natural Instability
However slight the temperature variation natural instability will amplify it If for exampleone part of the melt is slightly hotter than the rest then the plastic flow in that area will beslightly greater bringing hotter material into the area and maintaining the temperature If onthe other hand there is another area that is cooler the flow will be less so there will be lessheat input and the plastic will get colder until it eventually freezes off
However balanced the initial conditions this natural instability will result in a river-type flow
This is a very important consideration The first material to freeze off will shrink early in thecycle By the time the material in the river flows freezes the bulk of the material will havealready frozen off and shrinkage will have occurred The rivers will shrink relative to the bulk of the molding and because they are highly orientated shrinkage will be very high The resultis high-stress tensile members throughout the molding a common cause of warpage
1143 Optimum Part Quality
Most of the stress in plastic parts occurs during the compensation phase By controlling flow and minimizing stress it is possible to design for optimum part quality This important point is
at the heart of the Moldflow philosophy
12 How Do Plastics Flow
121 Material Behavior
Molten thermoplastics exhibit viscoelastic behavior which combines flow characteristics of both viscous liquids and elastic solids When a viscous liquid flows the energy that causes thedeformation is dissipated and becomes viscous heat On the other hand when an elastic solid
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8 Polymer Flow Behavior in Injection Molds
is deformed the driving energy is stored For example the flow of water is a typical viscousflow whereas the deformation of a rubber cube falls into the elastic category
122 Deformation
In addition to the two types of material flow behavior there are two types of deformationsimple shear and simple extension (elongation) as shown in Figure 19 (a) and (b) below Theflow of molten thermoplastics during injection-molding filling is predominantly shear flow asshown in Figure 19 (c) in which layers of material elements slide over each other Theextensional flow however becomes significant as the material elements undergo elongation when the melt passes areas of abrupt dimensional change (eg a gate region) as shown inFigure 19 (d)
Figure 1igure 1 9 (a) Simple shear flow (b) Simple extensional flow (c) Shear flow in cavity filling (d) Extensional flow in cavity filling
123 Viscoelastic Behavior
In response to an applied stress (force per unit area) molten thermoplastics exhibit viscoelastic behavior which combines characteristics of an ideal viscous liquid with those of an ideal elastic solid In other words under certain conditions molten thermoplastics behavelike a liquid and will continuously deform while shear stress is applied as shown in
Figure 110 Upon the removal of the stress however the materials behave somewhat like anelastic solid with partial recovery of the deformation as shown in Figure 110 (b) and (c) This viscoelastic behavior stems from the random-coil configuration of polymer molecules in the
Stress
Stress Stress
Mold wall
Material element before
deformation begins
Material element during
deformation
Mold wall
Material elements at rest
Moving
materialelements
Velocity
profile
(a) (b)
(c) (d)
Velocity
profile
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How Do Plastics Flow 9
molten state which allows the movement and slippage of molecular chains under theinfluence of an applied load However the entanglement of the polymer molecular chains also
makes the system behave like an elastic solid upon the application and removal of the externalload Namely on removal of the stress chains will tend to return to the equilibrium random-coil state and thus will be a component of stress recovery The recovery is not instantaneousbecause of the entanglements still present in the system
Figure 1ure 1 10 (a) Ideal viscous liquid deforms continuously under applied stress (b) Ideal elasticsolid deforms immediately upon the application of stress but fully recovers when the
stress is removed (c) Molten thermoplastic deforms continuously under the applied
stress (like a viscous liquid) but also recovers partially from the deformation upon
removal of the applied stress (like an elastic solid)
124 Melt Shear Viscosity
1241 What Is Shear Viscosity
Melt shear viscosity is a materials resistance to shear flow In general polymer melts are highly viscous because of their long molecular chain structure The viscosity of a polymer melt rangesfrom 2 to 3000 Pas (water 10^-1 Pas glass 10^20 Pas) Viscosity can be thought of as thethickness of a fluid or how much it resists flow Viscosity is expressed as the ratio of shearstress (force per unit area) to the shear rate (rate change of shear strain) as shown inEquation 11 and Figure 111
(a) Viscous
liquid
(b) Elastic
solid
(c) Viscoelastic
polymer
melt
Initial shape
(shear stress
to be applied)
Time = Dt Time = 2 x Dt
(shear stress to
be removed)
Final deformed
shape
d
d
d
d
2d
2d
2d
2d
total
recovery
partial
recovery
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The Editor
Jay Shoemaker Mold1047298ow Corporation Framingham Massachusetts USA
Distributed in the USA and in Canada by Hanser Gardner Publications Inc6915 Valley Avenue Cincinnati Ohio 45244-3029 USAFax (513) 527-8801Phone (513) 527-8977 or 1-800-950-8977 wwwhansergardnercom
Distributed in all other countries by Carl Hanser Verlag Postfach 86 04 20 81631 Muumlnchen Germany Fax +49 (89) 98 48 09 wwwhanserde
The use of general descriptive names trademarks etc in this publication even if the former are not especiallyidenti1047297ed is not to be taken as a sign that such names as understood by the Trade Marks and Merchandise Marks Actmay accordingly be used freely by anyone While the advice and information in this book are believed to be true and accurate at the date of going to press neitherthe authors nor the editors nor the publisher can accept any legal responsibility for any errors or omissions that may bemade The publisher makes no warranty express or implied with respect to the material contained herein
Library of Congress Cataloging-in-Publication Data
Shoemaker Jay
Mold1047298ow design guide a resource for plastic engineers Jay Shoemaker p cm ISBN-13 978-1-56990-403-9 (hardcover) ISBN-10 1-56990-403-0 (hardcover)1 Injection molding of plastics I Title TP1120S486 2006 6684lsquo12--dc22 2006012438
Bibliogra1047297sche Information Der Deutschen Bibliothek Die Deutsche Bibliothek verzeichnet diese Publikation in der Deutschen Nationalbibliogra1047297e
detaillierte bibliogra1047297sche Daten sind im Internet uumlber lthttpdnbddbdegt abrufbar
ISBN-10 3-446-40640-9ISBN-13 978-3-446-40640-7
All rights reserved No part of this book may be reproduced or transmitted in any form or by any means electronic ormechanical including photocopying or by any information storage and retrieval system without permission in writingfrom the publisher
copy Mold1047298ow Corporation USA 2006Production Management Oswald Immel Typeset by Marcia Swan USACoverconcept Marc Muumlller-Bremer Rebranding Muumlnchen Germany Coverdesign MCP bull Susanne Kraus GbR Holzkirchen Germany Printed and bound by Koumlsel Krugzell Germany
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Foreword
The drive toward fast cost-effective and reliable plastics manufacturing has been Moldflowssole guiding goal since the company was founded over 25 years ago
This focused determination led us to introduce many new and exciting tools into the marketeach contributing to achieving our goal in some way whether by driving cost out of production with reduced material usage or shortened cycle times reducing mold delivery timeby minimizing re-work or increasing the reliability of supply by enabling higher quality products to be manufactured with greater surety in scheduling
The artificially balanced multi-cavity and family molds that are now commonplace were madepractical through the advent of our early simulation and runner balancing capabilities which
were introduced in the late 1970s and early 1980s As these tools evolved we were able to visualize and therefore control flow patterns and weld lines This evolution continued until we arrived in the 2000s with an array of sophisticated technology to control warpage accountfor heat transfer predict core shift adapt to new molding processes and much more Fromtraditional midplane technology to fully three-dimensional simulations all our solutions are well integrated into a solid-modeling design environment
As the technology has evolved so has its usage When Moldflow simulation technology wasintroduced its primary purpose was to search for remedies to pre-existing molding problemsIt soon became evident that the insight the software provided to solve molding problems would be better applied ahead of actual molding during the design process This
methodology which we call problem avoidance was the primary use for Moldflow technology for the first 20 years of its existence
For Moldflow this created a unique challenge to open the world of manufacturing to thedesigners of parts and molds What constitutes an ineffective design for molding may beapparent to a seasoned processing engineer looking retrospectively at a poorly performing tool but how can design engineers use the CAE tools to visualize diagnose and solve thesesame issues ahead of timewithout 20 years of molding experience How can manufacturersgo further and use information that cannot be seen in the real molding process but is revealed via simulation
The key that unlocked this puzzle began its life as the Moldflow Design Philosophy This is widely
viewed as the most important publication Moldflow has ever produced and has spawnedfollow-on works on related subjects Rather than provide insight into the operation of thesimulation tools Moldflow Design Philosophy set forth simple principles that transcend any specific software application and as a result are as valid with todays advanced simulationproducts as they were over two decades ago
In more recent years another transition has occurred The global imperative to drive down thecost of manufacturing has led to the use of molding simulation as a cost optimization toolrather than for problem avoidance This change has increased the number of Moldflow usersby an order of magnitude across a far broader cross-section of the plastics industry Greaterdesign-centricity leads to even more dependence on the plastics design principles which can
be used to drive optimization
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VI Foreword
Despite a quarter of a century of technological advances the golden years of CAE are aheadof us as our industry takes a broader and more integrated view of what it takes to manage a
products life cycle Moldflow is proud of its contributions to date and will continue to focuson developing innovative technology coupled with practical design principles to deliver moreprofitable manufacturing
Roland Thomas
President amp CEO Moldflow Corporation
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Preface VII
Preface
About this Book
The origins of this book include not only Moldflow Design Principles but also Warpage Design
Principles published by Moldflow and the C-MOLD Design Guide Collectively thesedocuments are based on years of experience in the research theory and practice of injectionmolding These documents are now combined into this book the Moldflow Design Guide The Moldflow Design Guide is intended to help practicing engineers solve problems they frequently encounter in the design of parts and molds as well as during production This book can alsobe used as a reference for training purposes at industrial and educational institutions
How to Use this Book
This book has several chapters and appendices that deal with different stages of the designprocess and provides background on the injection-molding process and plastic materials
bull The first three chapters introduce injection molding how polymers flow inside injectionmolds and how molding conditions and injection pressure influence the process
bull Chapter 4 discusses Moldflow design principles and how they relate to making quality parts
bull Chapter 5 introduces the finite element mesh technology used by Moldflow and how thesemeshes influence the quality of the analysis
bull Chapters 6 to 9 introduce design concepts for the product gates runners and coolingsystems
bull Chapter 10 introduces concepts relating to shrinkage and warpage and how Moldflow isused to determine the amount of shrinkage and warpage a molded part will have and whatcauses the warpage
bull
Chapter 11 discusses the design procedure for analyzing injection-molded partsbull Chapter 12 discusses major part defects found on injection-molded parts
bull Finally the four appendices discuss basic injection-molding machine operation processcontrol variants of the standard injection-molding process and plastic materials
Benefits of Using CAE
The injection-molding industry has recognized that computer-aided engineering (CAE)
enhances an engineers ability to handle all aspects of the polymer injection-molding processbenefiting productivity product quality timeliness and cost This is illustrated by a wealth of
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VIII Preface
literature and the ever-growing number of CAE software users in the injection-molding industry
CAE Predicts Process Behavior
Ideally CAE analysis provides insight that is useful in designing parts molds and molding processes Without it we rely on previous experience intuition prototyping or molding trialsto obtain information such as polymer melt filling patterns weld-line and air-trap locationsrequired injection pressure and clamp tonnage fiber orientation cycle time final part shapeand deformation and mechanical properties of molded parts just to name a few WithoutCAE analysis other equally important design data such as spatial distributions of pressuretemperature shear rate shear stress and velocity are more difficult to obtain even with a well-instrumented mold The process behavior predicted by CAE can help novice engineers
overcome the lack of previous experience and assist experienced engineers in pinpointing factors that may otherwise be overlooked By using CAE analysis to iterate and evaluatealternative designs and competing materials engineering know-how in the form of designguidelines can be established relatively faster and more cost-effectively
User Proficiency Determines the Benefits of CAE
While CAE technology helps save time money and raw material as well as cuts scrap reducesthe rejection rate improves product quality and gets new products to market faster it is by nomeans a panacea for solving all molding problems Rather it should be recognized that CAEanalysis is essentially a tool designed to assist engineers instead of taking over theirresponsibilities or replacing them Like many other tools the usefulness of CAE technology depends on the proficiency of the user The benefits mentioned above will not be realizedunless the CAE tool is used properly To be more specific the accuracy of CAE analysisdepends greatly on the input data provided by the user In addition the results generated by CAE analysis need to be correctly and intelligently interpreted by the user before soundjudgments and rational decisions are made Otherwise users will simply be swamped by the vast amount of data without getting any useful information
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Acknowledgements IX
Acknowledgements
The Moldflow Design Guide would not have been accomplished were it not for the vision of Ken Welch Ken and I have discussed the value of assembling the best of the Moldflow Design
Principles Warpage Design Principles and the C-MOLD Design Guide into a single book for severalyears With Kens leadership he gave the project to Steve Thompsons training group of which I am a part Steve helped me coordinate the resources necessary to get this projectdone I could not have done this project without Steves help and guidance
A review of the content was part of the development of the Moldflow Design Guide Moldflow developers including Peter Kennedy Rong Zheng Zhongshuang Yuan and Xiaoshi Jin havereviewed sections of the book Moldflows application engineers and other technical staff withMoldflow have also reviewed sections These reviewers include Chad Fuhrman Matt JaworskiChristine Roedlich Eric Henry Olivier Anninos Paul Larter and Ana Maria Marin A specialthanks goes to Mike Rogers who reviewed the entire book for me and provided criticalfeedback on the content and organization of the book I would also like to thank Kurt Haydenof Western Michigan University for reviewing the appendix on process control His many years of experience of process setup and optimization was invaluable
Finally I would like to thank members of Moldflows Technology Transfer group forsupporting me in the writing of this book These members include Marcia Swan CarolineDorin Robert Ashley Melissa Haslam Darren Seamons and Gregory Brown
On a personal note I would like to acknowledge and thank Paul Engelmann Professor and
Department Chair Western Michigan University Department of Industrial and Manufacturing Engineering for being my friend and mentor during my career With Paul I have been able toteach and participate in research he has done on injection molding tooling and processing at Western Michigan University I have found working with Paul has made me a better Moldflow user and engineer by providing another perspective on how Moldflow can be used to solveinjection molding problemsJay Shoemaker Editor
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Contents
1 Polymer Flow Behavior in Injection Molds 1
11 Phases of Injection Molding 1111 How Plastic Fills a Mold 1112 The Filling Phase 3113 The Pressurization Phase 6114 The Compensation Phase 612 How Do Plastics Flow 7121 Material Behavior 7122 Deformation 8
123 Viscoelastic Behavior 8124 Melt Shear Viscosity 9125 Newtonian Fluid vs Non-Newtonian Fluid 10126 Shear-thinning Behavior 10127 Shear Rate Distribution 11128 Pressure-driven Flow 11129 Pressure Gradient and Injection Times 131210 Melt Flow Length 131211 Injection Pressure vs Fill Time 131212 Flow Instability 15
2 Molding Conditions and Injection Pressure 17
21 Injection-pressure Overview 17211 Pressure Drives the Flow Front 1822 Factors Influencing Injection-pressure Requirements 1823 Equations 20231 Circular Channel Flow 20232 Strip Channel Flow 2024 Effect of Molding Conditions 21
241 Part Quality 21242 Melt Temperature 22243 Mold Temperature 22244 Fill Time 23245 Shear Stress Variation 23246 Packing Pressure and Time 24247 Summary 25248 Back Flow 2625 Using Moldflow to Determine Optimum Molding Conditions 26251 Part 27
252 Molding Window Size 27253 Injection Pressure 28254 Flow Front Temperature 29
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XII Contents
255 Cooling Time 30256 Summary 31
3 Filling Pattern 33
31 Filling Pattern Overview 33311 What Is the Filling Pattern 3332 Flow in Complex Molds 34321 Overpack 34322 Racetrack Effect 35323 Varying Injection Rate 35324 Underflow Effect 36325 Hesitation Effect 37326 Weld Lines 38327 Meld Lines 38328 Sink Marks 38329 Multidirectional Flow 393210 Unstable Flow 403211 Simple Flow Pattern 4133 Flow-front Velocity and Flow-front Area 41331 What are FFV and FFA 41332 Flow-front Velocity Influences Filling Pattern 41
333 Equation 4234 Using Moldflow to Determine the Filling Pattern 43341 Computer Simulation Can Eliminate Molding Trials 43342 Using a Flow Analysis 4335 Using Moldflow to Achieve Constant FFV 44351 Controlling the FFV Through Ram Speed 44
4 Moldflow Design Principles 47
41 Product Design and Moldflow 4742 Sequence of Analysis 47421 Part Filling Optimization 48422 Molding Conditions 48423 Runner Design 48424 Cooling Optimization 49425 Packing Optimization 49426 Warpage Optimization 4943 Moldflow Flow Concepts 49431 Unidirectional and Controlled Flow Pattern 50432 Flow Balancing 51
433 Constant Pressure Gradient 52434 Maximum Shear Stress 53435 Uniform Cooling 54
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Contents XIII
436 Positioning Weld and Meld Lines 55437 Avoid Hesitation Effects 55
438 Avoid Underflow 55439 Balancing with Flow Leaders and Flow Deflectors 574310 Controlled Frictional Heat 584311 Thermal Shutoff of Runners 584312 Acceptable RunnerCavity Ratio 59
5 Meshes Used In Moldf low Analyses 61
51 Mesh Types Used by Moldflow 61511 Finite Elements Used in Moldflow 61512 Mesh Types 62513 Solver Assumptions 6352 Mesh Requirements 64521 Mesh Density Considerations 64522 Part Details 6753 Geometry Creation 6854 Importing Geometry 6955 Using Different Mesh Types 69551 Door Panel 69552 Manifold 70
6 Product Design 71
61 Material Properties for Product Design 71611 Plastics Are Sensitive to Operating Conditions 71612 Stress-Strain Behavior 72613 Creep and Stress Relaxation 77614 Fatigue 79615 Impact strength 80616 Thermal Mechanical Behavior 8062 Design for Strength 82621 Predicting Part Strength 82622 LoadingOperating Conditions 8263 Part Thickness 86631 Part Thickness Drives Quality and Cost 86632 Cycle Time Increases with Thickness 86633 Thick Parts Tend to Warp 86634 Thin Uniform Parts Improve Surface Quality 87635 Reducing Part Thickness 8764 Boosting Structural Integrity with Ribs 88
641 Structural Integrity the Goal of Every Design 88642 Designing Ribs 88
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XIV Contents
65 Design for Assembly 90651 Molding One Part vs Separate Components 90
652 Tolerances Fit between Parts 90653 Press-fit Joints 91654 Snap-fit Joints 93655 Cantilever Snap Joints 95656 Torsion Snap-fit Joints 96657 Fasteners 98658 Inserts 101659 Welding Processes 101
7 Gate Design 103
71 Gate Design Overview 103711 What Is a Gate 103712 Single vs Multiple Gates 103713 Gate Dimensions 103714 Gate Location 10472 Gate Types 104721 Manually Trimmed Gates 104722 Automatically Trimmed Gates 11073 Design Rules 113
731 Determining the Number of Gates 113732 Flow Patterns 115733 Gate Position 116734 Avoiding Common Problems 122735 Gate Length 122736 Gate Thickness 122737 Freeze-off Time 12374 Using Moldflow for Gate Design 123741 Gate Location 123742 Molding Window Size for the Three Gate Locations 124
743 Filling Pattern 125744 Gate Size Based on Shear Rate 125
8 Runner System Design 127
81 Definitions 127811 Feed System 127812 Runner System 127813 Cold Runner 127814 Hot Runner 128
815 Hot Manifold 128816 Hot Drop 128817 Sprue 128
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Contents XV
82 Runner System Design Principles 129821 Benefits of Good Runner Design 129
822 Runner Design Philosophy 129823 Flow Balancing 130824 Flow Control 130825 Frictional Heating in Runners 132826 Thermal Shutoff 132827 System and Runner Pressures 132828 Constant Pressure Gradient 133829 Cold Slug Wells 1338210 Easy Ejection 13483 Runner Types 134
831 Cold Runners 134832 Hot Runner Systems 13584 Runner Layout 136841 Determining the Number of Cavities 136842 Planning the Runner System Layout 138843 Partially Balanced Runners 139844 Geometrically Balanced Runners 14185 Initial Runner Sizing 141851 Determining Sprue Dimensions 141852 Designing Runner Cross Sections 142
853 Determining Runner Diameters 14586 Runner Balancing 147861 How Runner Balancing Works 147862 When Are the Runner Sizes Optimized 147863 Validating the Balance 147864 Processing Window 14887 Using Moldflow for Runner Balancing 149871 Runner Balancing a 48-cavity Tool 149872 Runner Balancing for a Family Mold 151873 Runner Balancing for a Multigated Part 152
9 Cooling System Design 153
91 Mold Cooling System Overview 153911 Importance of Cooling System Design 153912 Mold Cooling System Components 15492 Cooling-channel Configuration 156921 Types of Cooling Channels 15693 Alternative Cooling Devices 157931 What Do They Do 157
932 Baffles 158933 Bubblers 159934 Thermal Pins 159
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XVI Contents
935 Cooling Slender Cores 160936 Cooling Large Cores 161
937 Cooling Cylinder Cores 16294 Cooling System Equations 162941 Cooling Time 162942 Reynolds Number and Coolant Flow 16395 Design Rules 163951 Mold Cooling Design Considerations 163952 Location and Size of Channels 164953 Flow Rate and Heat Transfer 16696 Using Moldflow for Cooling System Design 167961 Example Setup 168
962 Cycle Time Determined by Design and Processing Parameters 169
10 Shrinkage and Warpage 173
101 Injection Molding and Shrinkage 1731011 What Are Shrinkage and Warpage 1731012 Shrinkage and Machine Settings 1731013 Mold Filling and Packing 1741014 How Pressure and Time Affect Shrinkage 1751015 Thermally Unstable Flow 176
102 Basic Causes of Shrinkage and Warpage 1771021 Causes of Shrinkage 1771022 Causes of Warpage 1811023 Relating Orientation and Area Shrinkage to Warpage 182103 Designing Accurate Parts Considering Warpage 1831031 Material Selection 1841032 Wall Thickness Variation 1841033 Gate Position and Runner Dimensions 1851034 Molding Conditions 1871035 Cooling Line Layout 190
11 Moldflow Design Procedure 191
111 Determine Analysis Objectives 191112 Moldflow Analysis Steps Framework 1921121 The Whole Process 1921122 Optimize Fill 1921123 Balance and Size the Runners 1971124 Optimize Cooling 1991125 Optimize the Packing Profile 200
1126 Optimize Warpage 201113 Using Moldflow to Evaluate an Initial Design 2051131 Description of this Example 205
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Contents XVII
1132 Molding Window 2051133 Filling Analysis 206
1134 Gate and Runner Design 2061135 Cooling System Design 2071136 Packing Analysis 2091137 Warpage Analysis 209114 Using Moldflow to Optimize the Design 2101141 Determine the Cause of Warpage 2101142 Investigating Different Gate Locations 2101143 Validating the Best Gate Location 211
12 Part Defects 215
121 Air Traps 2151211 What Is an Air Trap 2151212 Problems Caused by Air Traps 2161213 Remedies 216122 Black Specks and Black Streaks 2171221 What Are Black Specks and Black Streaks 2171222 Causes of Black Specks and Black Streaks 2171223 Remedies 218123 Brittleness 219
1231 What Is Brittleness 2191232 Causes of Brittleness 2191233 Remedies 219124 Burn Marks 2201241 What Is a Burn Mark 2201242 Causes of Burn Marks 2211243 Remedies 222125 Delamination 2221251 What Is Delamination 2221252 Causes of Delamination 223
1253 Remedies 223126 Dimensional Variation 2241261 What Is Dimensional Variation 2241262 Causes of Dimensional Variation 2241263 Remedies 224127 Discoloration 2251271 What Is Discoloration 2251272 Causes of Discoloration 2251273 Remedies 226128 Fish Eyes 227
1281 What Are Fish Eyes 2271282 Causes of Fish Eyes 2271283 Remedies 227
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XVIII Contents
129 Flash 2281291 What Is Flash 228
1292 Causes of Flash 2281293 Remedies 2291210 Flow Marks 23012101 What Is A Flow Mark 23012102 Causes of Flow Marks 23012103 Remedies 2301211 Hesitation 23112111 What Is Hesitation 23112112 Problems Caused by Hesitation 23112113 Remedies 232
1212 Jetting 23212121 What Is Jetting 23212122 Problems Caused by Jetting 23312123 Remedies 2331213 Ripples 23512131 What Are Ripples 23512132 Cause of Ripples 23512133 Remedies 2361214 Short Shots 23612141 What Is a Short Shot 236
12142 Causes of Short Shots 23712143 Remedies 2371215 Silver Streaks 23812151 What Are Silver Streaks 23812152 Causes of Silver Streaks 23912153 Remedies 2391216 Sink Marks and Voids 24012161 What Are Sink Marks and Voids 24012162 Causes of Sink Marks and Voids 24012163 Remedies 2411217 Weld Lines and Meld Lines 24312171 What Are Weld Lines and Meld Lines 24312172 Problems Caused by Weld Lines 24312173 Strength of Weld Lines 24412174 Remedies 245
Appendix A Injection Molding 247
A1 Injection-molding Overview 247 A11 Process 247
A12 Applications 248 A2 Development of the Injection-molding Machine 248 A21 Benefits of the Reciprocating Screw 248
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Contents XIX
A3 Development of the Injection-molding Process 249 A4 Alternative Injection-molding Processes 249
A41 Co-injection (Sandwich) Molding 249 A42 Fusible Core Injection Molding 250 A43 Gas-assisted Injection Molding 251 A44 Injection-compression Molding 252 A45 Lamellar (Microlayer) Injection Molding 253 A46 Live-feed Injection Molding 253 A47 Low-pressure Injection Molding 254 A48 Push-pull Injection Molding 254 A49 Reactive Molding 255 A410 Structural Foam Injection Molding 256
A411 Thin-wall Molding 256
Appendix B Injection-molding Machine System and Operations 259
B1 Injection-molding Machine 259B11 Components 259B12 Machine Specification 260B13 Machine Function 260B14 Auxiliary Equipment 260B2 Machine Components 261
B21 Injection System 261B22 Mold System 263B23 Hydraulic System 264B24 Control System 264B25 Clamping System 264B3 Molded System 265B31 The Delivery System 265B4 Machine Operating Sequence 266B41 Process Cycle 266B5 Screw Operation 268
B51 Back Pressure 268B52 Injection Speed 268B53 Screw Rotation Speed 268B54 Cushion 269B6 Secondary Operations 269B61 Assembly 269B62 Decoration 269B63 Other Secondary Operations 270
Appendix C Injection-molding Process Control 271
C1 Importance of Process Conditions 271C11 Setting Machine Process Conditions 271
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XX Contents
Appendix D Plastic Materials 281
D1 What Are Plastics 281
D11 Polymerization Process 281D12 Structure of Polymers 281D13 Polymer Alloys and Blends 283D14 Polymer Composites 283D2 Classification of Plastics 283D21 Classes of Plastics 284D22 Structures and Properties of Plastics 284D3 Thermoplastics 285D31 Market Share Distribution of Thermoplastics 285D32 Structures and Properties of Thermoplastics 286D33 Amorphous Polymers 286D34 Semicrystalline Polymers 287D4 Thermosets 288D5 Properties and Applications of Thermoplastics 289D51 ABS 290D52 PA 12 291D53 PA 6 293D54 PA 66 294D55 PBT 296
D56 PC 297D57 PCABS 299D58 PCPBT 300D59 HDPE 301D510 LDPE 303D511 PEI 304D512 PET 305D513 PETG 306D514 PMMA 307D515 POM 309
D516 PP 310D517 PPEPPO 312D518 PS 313D519 PVC 314D520 SAN 316D521 Additives Fillers and Reinforcements 317D522 Modifying Polymer Properties 318D523 Low-aspect Fillers 319D524 High-aspect Fillers Fibers 319
Index 321
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1 Polymer Flow Behavior in Injection Molds
bull Phases of injection molding
bull How do plastics flow
11 Phases of Injection Molding
Any molder can prove that all the conditions and effects discussed in this chapter do indeedoccur during the injection molding process While this knowledge alone can somewhatimprove quality it is only with the use of Moldflow analysis during the initial design stage with the mold designed for the optimum filling pattern that these effects can be controlledand the full benefits obtained
Flow technology is concerned with the behavior of plastics during the mold filling process Aplastic parts properties depend on how the part is molded Two parts having identicaldimensions and made from the same material but molded under different conditions will havedifferent stress and shrinkage levels and will behave differently in the field meaning that they
are in practice two different parts The way the plastic flows into the mold is of paramount importance in determining the quality of the part The process of filling the mold can be distinctly analyzed with the ability to predictpressure temperature and stress
111 How Plastic Fills a Mold
This was investigated using a centrally gated mold shaped like a dinner plate with a thick rim
around the outside as shown in Figure 11 It was found that the injection molding processalthough complex could be divided into three phases (we use the word phase to avoidconfusion with injection stage as used with programmed injection)
Figure 1ure 1 1 Cross-section of disk mold used to investigate flow
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2 Polymer Flow Behavior in Injection Molds
1111 Filling Phase
As the ram moves forward it first moves at a steady speed as the plastic flows into the cavity
This is the filling phase This phase lasts until the mold is just filled See Figure 12 andFigure 13
1112 Pressurization Phase
The pressurization phase begins when the ram moves forward after the filling phase to bring the mold up to pressure When the mold is filled the ram will slow down but it still movesquite some distance because plastics are very compressible materials At injection molding pressure an extra 15 volume of material can be forced into the cavity See Figure 12 andFigure 13
Although fluids are usually assumed to be incompressible molten plastics have to beconsidered to be more like a gas The compressibility of plastics can be observed by blocking off the nozzle and attempting to purge the barrel The ram will jump forward when thepressure is applied but will spring back when the pressure is released
1113 Compensation Phase
After the pressurization phase the ram still does not stop completely continuing to creepforward for some time Plastics have a very large volumetric change of about 25 from themelt to the solid This can be seen in a short shot the difference in volume between themolding and the cavity is due to this volumetric change See Figure 12 and Figure 13
The ram moving forward to compensate for the volumetric change in the part is called thecompensation phase As the volumetric change is 25 and at the most only an extra 15 canbe injected in the pressurization phase there must always be some compensation phase
Figure 1igure 1 2 Phases of injection molding
Filling phase
Pressurization phase
Compensation phase
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Phases of Injection Molding 3
Figure 13ure 1 3 Phases of injection molding detail
112 The Filling Phase
A two-color technique best demonstrates this phase After emptying the barrel of an injection-molding machine a small amount of red plastic was charged followed by green plastic
Consider the closed mold with the plastic front just starting to flow from the nozzle Theplastic first fills the sprue and runner system then enters the mold cavity itself forming a smallbubble of molten plastic
The skin of the plastic in contact with the cool mold freezes rapidly while the central coreremains molten When additional material is injected it flows into this central core displacing
the material already there which then forms a new flow front The flow of this displacedmaterial is a combination of forward flow and outward flow The outward flow contacts the wall freezes and forms the next section of skin while the forward flow forms the new moltencore When more material enters the mold it flows along a channel lined with these frozen walls of plastic illustrated in Figure 14
This flow pattern is often called fountain flow or bubble flow because the flow front is like abubble being inflated with hot plastic from the center The frozen layer is formed by the flow front inflating and so is subject to only a low shear stress and therefore has a very low levelof molecular orientation Once it is frozen it cannot be orientated any further so the frozen
layer in the finished part has a low level of orientation
Figure 14ure 1 4 Fountain flow and heat transfer
Filling Pressurization Compensation
Hot plastic
Frozen layer
Heat loss to mold
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4 Polymer Flow Behavior in Injection Molds
Now consider what happens upstream Hot plastic is continuously flowing bringing new hotmaterial along and generating significant frictional heat At the same time heat is being lost
through the frozen layer to the cold mold surfaceInitially the frozen layer is very thin so heat is lost very rapidly This results in more plasticfreezing and the frozen layer getting thicker cutting down the heat flow After a time thefrozen layer will reach a thickness such that the heat lost by conduction is equal to the heatinput from plastic flow and frictional heating ie an equilibrium condition is reached(Figure 14)
It is interesting to do some calculations on the time taken to reach this state of equilibrium The actual rate of heat flow is very large in comparison with the small heat content of theplastic in the frozen layer The result is that equilibrium is reached very quickly often in a timemeasured in a few tenths of a second As the total filling time is measured in seconds thefrozen layer reaches an equilibrium state early in the filling cycle
It is useful to think about how the thickness of this frozen layer will vary If the injection rate were slowed less heat would be generated by friction along the flow path with less heat inputfrom the flow The heat loss would be at the same rate and with less heat input the frozenlayer would grow in thickness If the injection rate were raised the frozen layer would bethinner (Figure 15) Similarly higher melt and mold temperatures would reduce the thicknessof the frozen layer This can be seen experimentally using the two-color technique
Figure 1igure 1 5 Influence of injection rate on frozen layer thickness
1121 Flow Shear Stress
It is easy to get confused between the various stress levels and orientation of the polymer Asthe plastic flows it is subject to shear stress also called flow shear stress This flow shear stress willorient the material ie cause the molecules to align themselves in the general direction of flow
The shear stress varies from a maximum at the outside dropping off to zero at the center
3Shear stress is purely a function of force and area This must not be confused withshear rate which is the rate of plastic sliding over the next layer Shear rate is zero
= Frozen
= Molten cooler
= Molten hotter
Slowerinjection
rate
Faster
injection
rate
vs
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Phases of Injection Molding 5
at the outer edge where the plastic is frozen rises to a maximum just inwards of thefrozen layer then drops toward the center as shown in Figure 16
Figure 16ure 1 6 Shear rate distribution
If the flow were stopped and the plastic allowed to cool down very slowly this orientation would have time to relax giving a very low level of residual orientation On the other hand if the material were kept under stress and the plastic snap frozen most of the orientation would
be trapped in the frozen plastic (Figure 17)
Figure 17ure 1 7 Molecular orientation through the thickness of the part
Min MaxShear rate
= Tension
= Compression
Molecular orientation trapped
in pressurization phase
Gradual reduction of orientation
toward the center of the
mold as stress levels are lower
and the cooling rate is slower
which allows more time for
orientation to relax
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6 Polymer Flow Behavior in Injection Molds
Now consider the orientation from the mold surface toward the center
The frozen layer itself formed with very little shear and therefore low orientation immediately
freezes setting the low level of orientation
The layer of plastic just on the inside of the frozen layer is subject to maximum shear stressand freezes the instant flow stops trapping almost all the orientation
This is the orientation pattern the further toward the center the more the shear stress dropsand the slower the rate of cooling This allows more time for the level of orientation to relaxso the residual orientation drops rapidly toward the center Consider how this pattern willaffect the residual stress level Oriented material (normally) will shrink more than nonorientedmaterial On the inner surface of the original frozen layer highly oriented material wants toshrink a great deal but it is prevented from doing so by the less-oriented material The highly
oriented layer ends up being in tension while the less-oriented material is in compression This residual stress pattern is a common cause of part warpage
3 There is a connectionthrough orientationbetween the shear stress during
filling (flow stress) and the residual stress in the final molded part This meansshear stress during filling shown on Moldflow plots can be used as a designparameter
113 The Pressurization Phase
The pressurization phasefrom the point of view of flow behavioris very similar to thefilling phase The flow rate may drop somewhat as the mold builds up to pressure resulting inan increase in the thickness of the frozen layer
The main difference of course is the increase in hydrostatic (isotropic) pressure We shall seein chapter 2 section 24 Effect of Molding Conditions that hydrostatic pressure in itself doesnot cause any residual stress
114 The Compensation Phase
Compensating flow is unstable Consider the plate molding again (see Figure 11) You wouldthink that plastic flowing uniformly through the thin diaphragm would top up the thick rim Inpractice the plastic during the compensation phase flows in rivers that spread out like a deltaas illustrated in Figure 18 This may seem surprising at first but it can be explained by temperature instability
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How Do Plastics Flow 7
Figure 18ure 1 8 River flow
1141 Temperature Variation
There is always some variation in melt temperature coming from the barrel of the injectionmachine In exceptional cases up to 40 ordmC variation has been measured using a high-speedthermocouple
1142 Natural Instability
However slight the temperature variation natural instability will amplify it If for exampleone part of the melt is slightly hotter than the rest then the plastic flow in that area will beslightly greater bringing hotter material into the area and maintaining the temperature If onthe other hand there is another area that is cooler the flow will be less so there will be lessheat input and the plastic will get colder until it eventually freezes off
However balanced the initial conditions this natural instability will result in a river-type flow
This is a very important consideration The first material to freeze off will shrink early in thecycle By the time the material in the river flows freezes the bulk of the material will havealready frozen off and shrinkage will have occurred The rivers will shrink relative to the bulk of the molding and because they are highly orientated shrinkage will be very high The resultis high-stress tensile members throughout the molding a common cause of warpage
1143 Optimum Part Quality
Most of the stress in plastic parts occurs during the compensation phase By controlling flow and minimizing stress it is possible to design for optimum part quality This important point is
at the heart of the Moldflow philosophy
12 How Do Plastics Flow
121 Material Behavior
Molten thermoplastics exhibit viscoelastic behavior which combines flow characteristics of both viscous liquids and elastic solids When a viscous liquid flows the energy that causes thedeformation is dissipated and becomes viscous heat On the other hand when an elastic solid
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8 Polymer Flow Behavior in Injection Molds
is deformed the driving energy is stored For example the flow of water is a typical viscousflow whereas the deformation of a rubber cube falls into the elastic category
122 Deformation
In addition to the two types of material flow behavior there are two types of deformationsimple shear and simple extension (elongation) as shown in Figure 19 (a) and (b) below Theflow of molten thermoplastics during injection-molding filling is predominantly shear flow asshown in Figure 19 (c) in which layers of material elements slide over each other Theextensional flow however becomes significant as the material elements undergo elongation when the melt passes areas of abrupt dimensional change (eg a gate region) as shown inFigure 19 (d)
Figure 1igure 1 9 (a) Simple shear flow (b) Simple extensional flow (c) Shear flow in cavity filling (d) Extensional flow in cavity filling
123 Viscoelastic Behavior
In response to an applied stress (force per unit area) molten thermoplastics exhibit viscoelastic behavior which combines characteristics of an ideal viscous liquid with those of an ideal elastic solid In other words under certain conditions molten thermoplastics behavelike a liquid and will continuously deform while shear stress is applied as shown in
Figure 110 Upon the removal of the stress however the materials behave somewhat like anelastic solid with partial recovery of the deformation as shown in Figure 110 (b) and (c) This viscoelastic behavior stems from the random-coil configuration of polymer molecules in the
Stress
Stress Stress
Mold wall
Material element before
deformation begins
Material element during
deformation
Mold wall
Material elements at rest
Moving
materialelements
Velocity
profile
(a) (b)
(c) (d)
Velocity
profile
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How Do Plastics Flow 9
molten state which allows the movement and slippage of molecular chains under theinfluence of an applied load However the entanglement of the polymer molecular chains also
makes the system behave like an elastic solid upon the application and removal of the externalload Namely on removal of the stress chains will tend to return to the equilibrium random-coil state and thus will be a component of stress recovery The recovery is not instantaneousbecause of the entanglements still present in the system
Figure 1ure 1 10 (a) Ideal viscous liquid deforms continuously under applied stress (b) Ideal elasticsolid deforms immediately upon the application of stress but fully recovers when the
stress is removed (c) Molten thermoplastic deforms continuously under the applied
stress (like a viscous liquid) but also recovers partially from the deformation upon
removal of the applied stress (like an elastic solid)
124 Melt Shear Viscosity
1241 What Is Shear Viscosity
Melt shear viscosity is a materials resistance to shear flow In general polymer melts are highly viscous because of their long molecular chain structure The viscosity of a polymer melt rangesfrom 2 to 3000 Pas (water 10^-1 Pas glass 10^20 Pas) Viscosity can be thought of as thethickness of a fluid or how much it resists flow Viscosity is expressed as the ratio of shearstress (force per unit area) to the shear rate (rate change of shear strain) as shown inEquation 11 and Figure 111
(a) Viscous
liquid
(b) Elastic
solid
(c) Viscoelastic
polymer
melt
Initial shape
(shear stress
to be applied)
Time = Dt Time = 2 x Dt
(shear stress to
be removed)
Final deformed
shape
d
d
d
d
2d
2d
2d
2d
total
recovery
partial
recovery
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Foreword
The drive toward fast cost-effective and reliable plastics manufacturing has been Moldflowssole guiding goal since the company was founded over 25 years ago
This focused determination led us to introduce many new and exciting tools into the marketeach contributing to achieving our goal in some way whether by driving cost out of production with reduced material usage or shortened cycle times reducing mold delivery timeby minimizing re-work or increasing the reliability of supply by enabling higher quality products to be manufactured with greater surety in scheduling
The artificially balanced multi-cavity and family molds that are now commonplace were madepractical through the advent of our early simulation and runner balancing capabilities which
were introduced in the late 1970s and early 1980s As these tools evolved we were able to visualize and therefore control flow patterns and weld lines This evolution continued until we arrived in the 2000s with an array of sophisticated technology to control warpage accountfor heat transfer predict core shift adapt to new molding processes and much more Fromtraditional midplane technology to fully three-dimensional simulations all our solutions are well integrated into a solid-modeling design environment
As the technology has evolved so has its usage When Moldflow simulation technology wasintroduced its primary purpose was to search for remedies to pre-existing molding problemsIt soon became evident that the insight the software provided to solve molding problems would be better applied ahead of actual molding during the design process This
methodology which we call problem avoidance was the primary use for Moldflow technology for the first 20 years of its existence
For Moldflow this created a unique challenge to open the world of manufacturing to thedesigners of parts and molds What constitutes an ineffective design for molding may beapparent to a seasoned processing engineer looking retrospectively at a poorly performing tool but how can design engineers use the CAE tools to visualize diagnose and solve thesesame issues ahead of timewithout 20 years of molding experience How can manufacturersgo further and use information that cannot be seen in the real molding process but is revealed via simulation
The key that unlocked this puzzle began its life as the Moldflow Design Philosophy This is widely
viewed as the most important publication Moldflow has ever produced and has spawnedfollow-on works on related subjects Rather than provide insight into the operation of thesimulation tools Moldflow Design Philosophy set forth simple principles that transcend any specific software application and as a result are as valid with todays advanced simulationproducts as they were over two decades ago
In more recent years another transition has occurred The global imperative to drive down thecost of manufacturing has led to the use of molding simulation as a cost optimization toolrather than for problem avoidance This change has increased the number of Moldflow usersby an order of magnitude across a far broader cross-section of the plastics industry Greaterdesign-centricity leads to even more dependence on the plastics design principles which can
be used to drive optimization
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VI Foreword
Despite a quarter of a century of technological advances the golden years of CAE are aheadof us as our industry takes a broader and more integrated view of what it takes to manage a
products life cycle Moldflow is proud of its contributions to date and will continue to focuson developing innovative technology coupled with practical design principles to deliver moreprofitable manufacturing
Roland Thomas
President amp CEO Moldflow Corporation
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Preface VII
Preface
About this Book
The origins of this book include not only Moldflow Design Principles but also Warpage Design
Principles published by Moldflow and the C-MOLD Design Guide Collectively thesedocuments are based on years of experience in the research theory and practice of injectionmolding These documents are now combined into this book the Moldflow Design Guide The Moldflow Design Guide is intended to help practicing engineers solve problems they frequently encounter in the design of parts and molds as well as during production This book can alsobe used as a reference for training purposes at industrial and educational institutions
How to Use this Book
This book has several chapters and appendices that deal with different stages of the designprocess and provides background on the injection-molding process and plastic materials
bull The first three chapters introduce injection molding how polymers flow inside injectionmolds and how molding conditions and injection pressure influence the process
bull Chapter 4 discusses Moldflow design principles and how they relate to making quality parts
bull Chapter 5 introduces the finite element mesh technology used by Moldflow and how thesemeshes influence the quality of the analysis
bull Chapters 6 to 9 introduce design concepts for the product gates runners and coolingsystems
bull Chapter 10 introduces concepts relating to shrinkage and warpage and how Moldflow isused to determine the amount of shrinkage and warpage a molded part will have and whatcauses the warpage
bull
Chapter 11 discusses the design procedure for analyzing injection-molded partsbull Chapter 12 discusses major part defects found on injection-molded parts
bull Finally the four appendices discuss basic injection-molding machine operation processcontrol variants of the standard injection-molding process and plastic materials
Benefits of Using CAE
The injection-molding industry has recognized that computer-aided engineering (CAE)
enhances an engineers ability to handle all aspects of the polymer injection-molding processbenefiting productivity product quality timeliness and cost This is illustrated by a wealth of
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VIII Preface
literature and the ever-growing number of CAE software users in the injection-molding industry
CAE Predicts Process Behavior
Ideally CAE analysis provides insight that is useful in designing parts molds and molding processes Without it we rely on previous experience intuition prototyping or molding trialsto obtain information such as polymer melt filling patterns weld-line and air-trap locationsrequired injection pressure and clamp tonnage fiber orientation cycle time final part shapeand deformation and mechanical properties of molded parts just to name a few WithoutCAE analysis other equally important design data such as spatial distributions of pressuretemperature shear rate shear stress and velocity are more difficult to obtain even with a well-instrumented mold The process behavior predicted by CAE can help novice engineers
overcome the lack of previous experience and assist experienced engineers in pinpointing factors that may otherwise be overlooked By using CAE analysis to iterate and evaluatealternative designs and competing materials engineering know-how in the form of designguidelines can be established relatively faster and more cost-effectively
User Proficiency Determines the Benefits of CAE
While CAE technology helps save time money and raw material as well as cuts scrap reducesthe rejection rate improves product quality and gets new products to market faster it is by nomeans a panacea for solving all molding problems Rather it should be recognized that CAEanalysis is essentially a tool designed to assist engineers instead of taking over theirresponsibilities or replacing them Like many other tools the usefulness of CAE technology depends on the proficiency of the user The benefits mentioned above will not be realizedunless the CAE tool is used properly To be more specific the accuracy of CAE analysisdepends greatly on the input data provided by the user In addition the results generated by CAE analysis need to be correctly and intelligently interpreted by the user before soundjudgments and rational decisions are made Otherwise users will simply be swamped by the vast amount of data without getting any useful information
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Acknowledgements IX
Acknowledgements
The Moldflow Design Guide would not have been accomplished were it not for the vision of Ken Welch Ken and I have discussed the value of assembling the best of the Moldflow Design
Principles Warpage Design Principles and the C-MOLD Design Guide into a single book for severalyears With Kens leadership he gave the project to Steve Thompsons training group of which I am a part Steve helped me coordinate the resources necessary to get this projectdone I could not have done this project without Steves help and guidance
A review of the content was part of the development of the Moldflow Design Guide Moldflow developers including Peter Kennedy Rong Zheng Zhongshuang Yuan and Xiaoshi Jin havereviewed sections of the book Moldflows application engineers and other technical staff withMoldflow have also reviewed sections These reviewers include Chad Fuhrman Matt JaworskiChristine Roedlich Eric Henry Olivier Anninos Paul Larter and Ana Maria Marin A specialthanks goes to Mike Rogers who reviewed the entire book for me and provided criticalfeedback on the content and organization of the book I would also like to thank Kurt Haydenof Western Michigan University for reviewing the appendix on process control His many years of experience of process setup and optimization was invaluable
Finally I would like to thank members of Moldflows Technology Transfer group forsupporting me in the writing of this book These members include Marcia Swan CarolineDorin Robert Ashley Melissa Haslam Darren Seamons and Gregory Brown
On a personal note I would like to acknowledge and thank Paul Engelmann Professor and
Department Chair Western Michigan University Department of Industrial and Manufacturing Engineering for being my friend and mentor during my career With Paul I have been able toteach and participate in research he has done on injection molding tooling and processing at Western Michigan University I have found working with Paul has made me a better Moldflow user and engineer by providing another perspective on how Moldflow can be used to solveinjection molding problemsJay Shoemaker Editor
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Contents
1 Polymer Flow Behavior in Injection Molds 1
11 Phases of Injection Molding 1111 How Plastic Fills a Mold 1112 The Filling Phase 3113 The Pressurization Phase 6114 The Compensation Phase 612 How Do Plastics Flow 7121 Material Behavior 7122 Deformation 8
123 Viscoelastic Behavior 8124 Melt Shear Viscosity 9125 Newtonian Fluid vs Non-Newtonian Fluid 10126 Shear-thinning Behavior 10127 Shear Rate Distribution 11128 Pressure-driven Flow 11129 Pressure Gradient and Injection Times 131210 Melt Flow Length 131211 Injection Pressure vs Fill Time 131212 Flow Instability 15
2 Molding Conditions and Injection Pressure 17
21 Injection-pressure Overview 17211 Pressure Drives the Flow Front 1822 Factors Influencing Injection-pressure Requirements 1823 Equations 20231 Circular Channel Flow 20232 Strip Channel Flow 2024 Effect of Molding Conditions 21
241 Part Quality 21242 Melt Temperature 22243 Mold Temperature 22244 Fill Time 23245 Shear Stress Variation 23246 Packing Pressure and Time 24247 Summary 25248 Back Flow 2625 Using Moldflow to Determine Optimum Molding Conditions 26251 Part 27
252 Molding Window Size 27253 Injection Pressure 28254 Flow Front Temperature 29
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XII Contents
255 Cooling Time 30256 Summary 31
3 Filling Pattern 33
31 Filling Pattern Overview 33311 What Is the Filling Pattern 3332 Flow in Complex Molds 34321 Overpack 34322 Racetrack Effect 35323 Varying Injection Rate 35324 Underflow Effect 36325 Hesitation Effect 37326 Weld Lines 38327 Meld Lines 38328 Sink Marks 38329 Multidirectional Flow 393210 Unstable Flow 403211 Simple Flow Pattern 4133 Flow-front Velocity and Flow-front Area 41331 What are FFV and FFA 41332 Flow-front Velocity Influences Filling Pattern 41
333 Equation 4234 Using Moldflow to Determine the Filling Pattern 43341 Computer Simulation Can Eliminate Molding Trials 43342 Using a Flow Analysis 4335 Using Moldflow to Achieve Constant FFV 44351 Controlling the FFV Through Ram Speed 44
4 Moldflow Design Principles 47
41 Product Design and Moldflow 4742 Sequence of Analysis 47421 Part Filling Optimization 48422 Molding Conditions 48423 Runner Design 48424 Cooling Optimization 49425 Packing Optimization 49426 Warpage Optimization 4943 Moldflow Flow Concepts 49431 Unidirectional and Controlled Flow Pattern 50432 Flow Balancing 51
433 Constant Pressure Gradient 52434 Maximum Shear Stress 53435 Uniform Cooling 54
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Contents XIII
436 Positioning Weld and Meld Lines 55437 Avoid Hesitation Effects 55
438 Avoid Underflow 55439 Balancing with Flow Leaders and Flow Deflectors 574310 Controlled Frictional Heat 584311 Thermal Shutoff of Runners 584312 Acceptable RunnerCavity Ratio 59
5 Meshes Used In Moldf low Analyses 61
51 Mesh Types Used by Moldflow 61511 Finite Elements Used in Moldflow 61512 Mesh Types 62513 Solver Assumptions 6352 Mesh Requirements 64521 Mesh Density Considerations 64522 Part Details 6753 Geometry Creation 6854 Importing Geometry 6955 Using Different Mesh Types 69551 Door Panel 69552 Manifold 70
6 Product Design 71
61 Material Properties for Product Design 71611 Plastics Are Sensitive to Operating Conditions 71612 Stress-Strain Behavior 72613 Creep and Stress Relaxation 77614 Fatigue 79615 Impact strength 80616 Thermal Mechanical Behavior 8062 Design for Strength 82621 Predicting Part Strength 82622 LoadingOperating Conditions 8263 Part Thickness 86631 Part Thickness Drives Quality and Cost 86632 Cycle Time Increases with Thickness 86633 Thick Parts Tend to Warp 86634 Thin Uniform Parts Improve Surface Quality 87635 Reducing Part Thickness 8764 Boosting Structural Integrity with Ribs 88
641 Structural Integrity the Goal of Every Design 88642 Designing Ribs 88
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XIV Contents
65 Design for Assembly 90651 Molding One Part vs Separate Components 90
652 Tolerances Fit between Parts 90653 Press-fit Joints 91654 Snap-fit Joints 93655 Cantilever Snap Joints 95656 Torsion Snap-fit Joints 96657 Fasteners 98658 Inserts 101659 Welding Processes 101
7 Gate Design 103
71 Gate Design Overview 103711 What Is a Gate 103712 Single vs Multiple Gates 103713 Gate Dimensions 103714 Gate Location 10472 Gate Types 104721 Manually Trimmed Gates 104722 Automatically Trimmed Gates 11073 Design Rules 113
731 Determining the Number of Gates 113732 Flow Patterns 115733 Gate Position 116734 Avoiding Common Problems 122735 Gate Length 122736 Gate Thickness 122737 Freeze-off Time 12374 Using Moldflow for Gate Design 123741 Gate Location 123742 Molding Window Size for the Three Gate Locations 124
743 Filling Pattern 125744 Gate Size Based on Shear Rate 125
8 Runner System Design 127
81 Definitions 127811 Feed System 127812 Runner System 127813 Cold Runner 127814 Hot Runner 128
815 Hot Manifold 128816 Hot Drop 128817 Sprue 128
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Contents XV
82 Runner System Design Principles 129821 Benefits of Good Runner Design 129
822 Runner Design Philosophy 129823 Flow Balancing 130824 Flow Control 130825 Frictional Heating in Runners 132826 Thermal Shutoff 132827 System and Runner Pressures 132828 Constant Pressure Gradient 133829 Cold Slug Wells 1338210 Easy Ejection 13483 Runner Types 134
831 Cold Runners 134832 Hot Runner Systems 13584 Runner Layout 136841 Determining the Number of Cavities 136842 Planning the Runner System Layout 138843 Partially Balanced Runners 139844 Geometrically Balanced Runners 14185 Initial Runner Sizing 141851 Determining Sprue Dimensions 141852 Designing Runner Cross Sections 142
853 Determining Runner Diameters 14586 Runner Balancing 147861 How Runner Balancing Works 147862 When Are the Runner Sizes Optimized 147863 Validating the Balance 147864 Processing Window 14887 Using Moldflow for Runner Balancing 149871 Runner Balancing a 48-cavity Tool 149872 Runner Balancing for a Family Mold 151873 Runner Balancing for a Multigated Part 152
9 Cooling System Design 153
91 Mold Cooling System Overview 153911 Importance of Cooling System Design 153912 Mold Cooling System Components 15492 Cooling-channel Configuration 156921 Types of Cooling Channels 15693 Alternative Cooling Devices 157931 What Do They Do 157
932 Baffles 158933 Bubblers 159934 Thermal Pins 159
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XVI Contents
935 Cooling Slender Cores 160936 Cooling Large Cores 161
937 Cooling Cylinder Cores 16294 Cooling System Equations 162941 Cooling Time 162942 Reynolds Number and Coolant Flow 16395 Design Rules 163951 Mold Cooling Design Considerations 163952 Location and Size of Channels 164953 Flow Rate and Heat Transfer 16696 Using Moldflow for Cooling System Design 167961 Example Setup 168
962 Cycle Time Determined by Design and Processing Parameters 169
10 Shrinkage and Warpage 173
101 Injection Molding and Shrinkage 1731011 What Are Shrinkage and Warpage 1731012 Shrinkage and Machine Settings 1731013 Mold Filling and Packing 1741014 How Pressure and Time Affect Shrinkage 1751015 Thermally Unstable Flow 176
102 Basic Causes of Shrinkage and Warpage 1771021 Causes of Shrinkage 1771022 Causes of Warpage 1811023 Relating Orientation and Area Shrinkage to Warpage 182103 Designing Accurate Parts Considering Warpage 1831031 Material Selection 1841032 Wall Thickness Variation 1841033 Gate Position and Runner Dimensions 1851034 Molding Conditions 1871035 Cooling Line Layout 190
11 Moldflow Design Procedure 191
111 Determine Analysis Objectives 191112 Moldflow Analysis Steps Framework 1921121 The Whole Process 1921122 Optimize Fill 1921123 Balance and Size the Runners 1971124 Optimize Cooling 1991125 Optimize the Packing Profile 200
1126 Optimize Warpage 201113 Using Moldflow to Evaluate an Initial Design 2051131 Description of this Example 205
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Contents XVII
1132 Molding Window 2051133 Filling Analysis 206
1134 Gate and Runner Design 2061135 Cooling System Design 2071136 Packing Analysis 2091137 Warpage Analysis 209114 Using Moldflow to Optimize the Design 2101141 Determine the Cause of Warpage 2101142 Investigating Different Gate Locations 2101143 Validating the Best Gate Location 211
12 Part Defects 215
121 Air Traps 2151211 What Is an Air Trap 2151212 Problems Caused by Air Traps 2161213 Remedies 216122 Black Specks and Black Streaks 2171221 What Are Black Specks and Black Streaks 2171222 Causes of Black Specks and Black Streaks 2171223 Remedies 218123 Brittleness 219
1231 What Is Brittleness 2191232 Causes of Brittleness 2191233 Remedies 219124 Burn Marks 2201241 What Is a Burn Mark 2201242 Causes of Burn Marks 2211243 Remedies 222125 Delamination 2221251 What Is Delamination 2221252 Causes of Delamination 223
1253 Remedies 223126 Dimensional Variation 2241261 What Is Dimensional Variation 2241262 Causes of Dimensional Variation 2241263 Remedies 224127 Discoloration 2251271 What Is Discoloration 2251272 Causes of Discoloration 2251273 Remedies 226128 Fish Eyes 227
1281 What Are Fish Eyes 2271282 Causes of Fish Eyes 2271283 Remedies 227
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XVIII Contents
129 Flash 2281291 What Is Flash 228
1292 Causes of Flash 2281293 Remedies 2291210 Flow Marks 23012101 What Is A Flow Mark 23012102 Causes of Flow Marks 23012103 Remedies 2301211 Hesitation 23112111 What Is Hesitation 23112112 Problems Caused by Hesitation 23112113 Remedies 232
1212 Jetting 23212121 What Is Jetting 23212122 Problems Caused by Jetting 23312123 Remedies 2331213 Ripples 23512131 What Are Ripples 23512132 Cause of Ripples 23512133 Remedies 2361214 Short Shots 23612141 What Is a Short Shot 236
12142 Causes of Short Shots 23712143 Remedies 2371215 Silver Streaks 23812151 What Are Silver Streaks 23812152 Causes of Silver Streaks 23912153 Remedies 2391216 Sink Marks and Voids 24012161 What Are Sink Marks and Voids 24012162 Causes of Sink Marks and Voids 24012163 Remedies 2411217 Weld Lines and Meld Lines 24312171 What Are Weld Lines and Meld Lines 24312172 Problems Caused by Weld Lines 24312173 Strength of Weld Lines 24412174 Remedies 245
Appendix A Injection Molding 247
A1 Injection-molding Overview 247 A11 Process 247
A12 Applications 248 A2 Development of the Injection-molding Machine 248 A21 Benefits of the Reciprocating Screw 248
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Contents XIX
A3 Development of the Injection-molding Process 249 A4 Alternative Injection-molding Processes 249
A41 Co-injection (Sandwich) Molding 249 A42 Fusible Core Injection Molding 250 A43 Gas-assisted Injection Molding 251 A44 Injection-compression Molding 252 A45 Lamellar (Microlayer) Injection Molding 253 A46 Live-feed Injection Molding 253 A47 Low-pressure Injection Molding 254 A48 Push-pull Injection Molding 254 A49 Reactive Molding 255 A410 Structural Foam Injection Molding 256
A411 Thin-wall Molding 256
Appendix B Injection-molding Machine System and Operations 259
B1 Injection-molding Machine 259B11 Components 259B12 Machine Specification 260B13 Machine Function 260B14 Auxiliary Equipment 260B2 Machine Components 261
B21 Injection System 261B22 Mold System 263B23 Hydraulic System 264B24 Control System 264B25 Clamping System 264B3 Molded System 265B31 The Delivery System 265B4 Machine Operating Sequence 266B41 Process Cycle 266B5 Screw Operation 268
B51 Back Pressure 268B52 Injection Speed 268B53 Screw Rotation Speed 268B54 Cushion 269B6 Secondary Operations 269B61 Assembly 269B62 Decoration 269B63 Other Secondary Operations 270
Appendix C Injection-molding Process Control 271
C1 Importance of Process Conditions 271C11 Setting Machine Process Conditions 271
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XX Contents
Appendix D Plastic Materials 281
D1 What Are Plastics 281
D11 Polymerization Process 281D12 Structure of Polymers 281D13 Polymer Alloys and Blends 283D14 Polymer Composites 283D2 Classification of Plastics 283D21 Classes of Plastics 284D22 Structures and Properties of Plastics 284D3 Thermoplastics 285D31 Market Share Distribution of Thermoplastics 285D32 Structures and Properties of Thermoplastics 286D33 Amorphous Polymers 286D34 Semicrystalline Polymers 287D4 Thermosets 288D5 Properties and Applications of Thermoplastics 289D51 ABS 290D52 PA 12 291D53 PA 6 293D54 PA 66 294D55 PBT 296
D56 PC 297D57 PCABS 299D58 PCPBT 300D59 HDPE 301D510 LDPE 303D511 PEI 304D512 PET 305D513 PETG 306D514 PMMA 307D515 POM 309
D516 PP 310D517 PPEPPO 312D518 PS 313D519 PVC 314D520 SAN 316D521 Additives Fillers and Reinforcements 317D522 Modifying Polymer Properties 318D523 Low-aspect Fillers 319D524 High-aspect Fillers Fibers 319
Index 321
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1 Polymer Flow Behavior in Injection Molds
bull Phases of injection molding
bull How do plastics flow
11 Phases of Injection Molding
Any molder can prove that all the conditions and effects discussed in this chapter do indeedoccur during the injection molding process While this knowledge alone can somewhatimprove quality it is only with the use of Moldflow analysis during the initial design stage with the mold designed for the optimum filling pattern that these effects can be controlledand the full benefits obtained
Flow technology is concerned with the behavior of plastics during the mold filling process Aplastic parts properties depend on how the part is molded Two parts having identicaldimensions and made from the same material but molded under different conditions will havedifferent stress and shrinkage levels and will behave differently in the field meaning that they
are in practice two different parts The way the plastic flows into the mold is of paramount importance in determining the quality of the part The process of filling the mold can be distinctly analyzed with the ability to predictpressure temperature and stress
111 How Plastic Fills a Mold
This was investigated using a centrally gated mold shaped like a dinner plate with a thick rim
around the outside as shown in Figure 11 It was found that the injection molding processalthough complex could be divided into three phases (we use the word phase to avoidconfusion with injection stage as used with programmed injection)
Figure 1ure 1 1 Cross-section of disk mold used to investigate flow
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2 Polymer Flow Behavior in Injection Molds
1111 Filling Phase
As the ram moves forward it first moves at a steady speed as the plastic flows into the cavity
This is the filling phase This phase lasts until the mold is just filled See Figure 12 andFigure 13
1112 Pressurization Phase
The pressurization phase begins when the ram moves forward after the filling phase to bring the mold up to pressure When the mold is filled the ram will slow down but it still movesquite some distance because plastics are very compressible materials At injection molding pressure an extra 15 volume of material can be forced into the cavity See Figure 12 andFigure 13
Although fluids are usually assumed to be incompressible molten plastics have to beconsidered to be more like a gas The compressibility of plastics can be observed by blocking off the nozzle and attempting to purge the barrel The ram will jump forward when thepressure is applied but will spring back when the pressure is released
1113 Compensation Phase
After the pressurization phase the ram still does not stop completely continuing to creepforward for some time Plastics have a very large volumetric change of about 25 from themelt to the solid This can be seen in a short shot the difference in volume between themolding and the cavity is due to this volumetric change See Figure 12 and Figure 13
The ram moving forward to compensate for the volumetric change in the part is called thecompensation phase As the volumetric change is 25 and at the most only an extra 15 canbe injected in the pressurization phase there must always be some compensation phase
Figure 1igure 1 2 Phases of injection molding
Filling phase
Pressurization phase
Compensation phase
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Phases of Injection Molding 3
Figure 13ure 1 3 Phases of injection molding detail
112 The Filling Phase
A two-color technique best demonstrates this phase After emptying the barrel of an injection-molding machine a small amount of red plastic was charged followed by green plastic
Consider the closed mold with the plastic front just starting to flow from the nozzle Theplastic first fills the sprue and runner system then enters the mold cavity itself forming a smallbubble of molten plastic
The skin of the plastic in contact with the cool mold freezes rapidly while the central coreremains molten When additional material is injected it flows into this central core displacing
the material already there which then forms a new flow front The flow of this displacedmaterial is a combination of forward flow and outward flow The outward flow contacts the wall freezes and forms the next section of skin while the forward flow forms the new moltencore When more material enters the mold it flows along a channel lined with these frozen walls of plastic illustrated in Figure 14
This flow pattern is often called fountain flow or bubble flow because the flow front is like abubble being inflated with hot plastic from the center The frozen layer is formed by the flow front inflating and so is subject to only a low shear stress and therefore has a very low levelof molecular orientation Once it is frozen it cannot be orientated any further so the frozen
layer in the finished part has a low level of orientation
Figure 14ure 1 4 Fountain flow and heat transfer
Filling Pressurization Compensation
Hot plastic
Frozen layer
Heat loss to mold
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4 Polymer Flow Behavior in Injection Molds
Now consider what happens upstream Hot plastic is continuously flowing bringing new hotmaterial along and generating significant frictional heat At the same time heat is being lost
through the frozen layer to the cold mold surfaceInitially the frozen layer is very thin so heat is lost very rapidly This results in more plasticfreezing and the frozen layer getting thicker cutting down the heat flow After a time thefrozen layer will reach a thickness such that the heat lost by conduction is equal to the heatinput from plastic flow and frictional heating ie an equilibrium condition is reached(Figure 14)
It is interesting to do some calculations on the time taken to reach this state of equilibrium The actual rate of heat flow is very large in comparison with the small heat content of theplastic in the frozen layer The result is that equilibrium is reached very quickly often in a timemeasured in a few tenths of a second As the total filling time is measured in seconds thefrozen layer reaches an equilibrium state early in the filling cycle
It is useful to think about how the thickness of this frozen layer will vary If the injection rate were slowed less heat would be generated by friction along the flow path with less heat inputfrom the flow The heat loss would be at the same rate and with less heat input the frozenlayer would grow in thickness If the injection rate were raised the frozen layer would bethinner (Figure 15) Similarly higher melt and mold temperatures would reduce the thicknessof the frozen layer This can be seen experimentally using the two-color technique
Figure 1igure 1 5 Influence of injection rate on frozen layer thickness
1121 Flow Shear Stress
It is easy to get confused between the various stress levels and orientation of the polymer Asthe plastic flows it is subject to shear stress also called flow shear stress This flow shear stress willorient the material ie cause the molecules to align themselves in the general direction of flow
The shear stress varies from a maximum at the outside dropping off to zero at the center
3Shear stress is purely a function of force and area This must not be confused withshear rate which is the rate of plastic sliding over the next layer Shear rate is zero
= Frozen
= Molten cooler
= Molten hotter
Slowerinjection
rate
Faster
injection
rate
vs
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Phases of Injection Molding 5
at the outer edge where the plastic is frozen rises to a maximum just inwards of thefrozen layer then drops toward the center as shown in Figure 16
Figure 16ure 1 6 Shear rate distribution
If the flow were stopped and the plastic allowed to cool down very slowly this orientation would have time to relax giving a very low level of residual orientation On the other hand if the material were kept under stress and the plastic snap frozen most of the orientation would
be trapped in the frozen plastic (Figure 17)
Figure 17ure 1 7 Molecular orientation through the thickness of the part
Min MaxShear rate
= Tension
= Compression
Molecular orientation trapped
in pressurization phase
Gradual reduction of orientation
toward the center of the
mold as stress levels are lower
and the cooling rate is slower
which allows more time for
orientation to relax
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6 Polymer Flow Behavior in Injection Molds
Now consider the orientation from the mold surface toward the center
The frozen layer itself formed with very little shear and therefore low orientation immediately
freezes setting the low level of orientation
The layer of plastic just on the inside of the frozen layer is subject to maximum shear stressand freezes the instant flow stops trapping almost all the orientation
This is the orientation pattern the further toward the center the more the shear stress dropsand the slower the rate of cooling This allows more time for the level of orientation to relaxso the residual orientation drops rapidly toward the center Consider how this pattern willaffect the residual stress level Oriented material (normally) will shrink more than nonorientedmaterial On the inner surface of the original frozen layer highly oriented material wants toshrink a great deal but it is prevented from doing so by the less-oriented material The highly
oriented layer ends up being in tension while the less-oriented material is in compression This residual stress pattern is a common cause of part warpage
3 There is a connectionthrough orientationbetween the shear stress during
filling (flow stress) and the residual stress in the final molded part This meansshear stress during filling shown on Moldflow plots can be used as a designparameter
113 The Pressurization Phase
The pressurization phasefrom the point of view of flow behavioris very similar to thefilling phase The flow rate may drop somewhat as the mold builds up to pressure resulting inan increase in the thickness of the frozen layer
The main difference of course is the increase in hydrostatic (isotropic) pressure We shall seein chapter 2 section 24 Effect of Molding Conditions that hydrostatic pressure in itself doesnot cause any residual stress
114 The Compensation Phase
Compensating flow is unstable Consider the plate molding again (see Figure 11) You wouldthink that plastic flowing uniformly through the thin diaphragm would top up the thick rim Inpractice the plastic during the compensation phase flows in rivers that spread out like a deltaas illustrated in Figure 18 This may seem surprising at first but it can be explained by temperature instability
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How Do Plastics Flow 7
Figure 18ure 1 8 River flow
1141 Temperature Variation
There is always some variation in melt temperature coming from the barrel of the injectionmachine In exceptional cases up to 40 ordmC variation has been measured using a high-speedthermocouple
1142 Natural Instability
However slight the temperature variation natural instability will amplify it If for exampleone part of the melt is slightly hotter than the rest then the plastic flow in that area will beslightly greater bringing hotter material into the area and maintaining the temperature If onthe other hand there is another area that is cooler the flow will be less so there will be lessheat input and the plastic will get colder until it eventually freezes off
However balanced the initial conditions this natural instability will result in a river-type flow
This is a very important consideration The first material to freeze off will shrink early in thecycle By the time the material in the river flows freezes the bulk of the material will havealready frozen off and shrinkage will have occurred The rivers will shrink relative to the bulk of the molding and because they are highly orientated shrinkage will be very high The resultis high-stress tensile members throughout the molding a common cause of warpage
1143 Optimum Part Quality
Most of the stress in plastic parts occurs during the compensation phase By controlling flow and minimizing stress it is possible to design for optimum part quality This important point is
at the heart of the Moldflow philosophy
12 How Do Plastics Flow
121 Material Behavior
Molten thermoplastics exhibit viscoelastic behavior which combines flow characteristics of both viscous liquids and elastic solids When a viscous liquid flows the energy that causes thedeformation is dissipated and becomes viscous heat On the other hand when an elastic solid
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8 Polymer Flow Behavior in Injection Molds
is deformed the driving energy is stored For example the flow of water is a typical viscousflow whereas the deformation of a rubber cube falls into the elastic category
122 Deformation
In addition to the two types of material flow behavior there are two types of deformationsimple shear and simple extension (elongation) as shown in Figure 19 (a) and (b) below Theflow of molten thermoplastics during injection-molding filling is predominantly shear flow asshown in Figure 19 (c) in which layers of material elements slide over each other Theextensional flow however becomes significant as the material elements undergo elongation when the melt passes areas of abrupt dimensional change (eg a gate region) as shown inFigure 19 (d)
Figure 1igure 1 9 (a) Simple shear flow (b) Simple extensional flow (c) Shear flow in cavity filling (d) Extensional flow in cavity filling
123 Viscoelastic Behavior
In response to an applied stress (force per unit area) molten thermoplastics exhibit viscoelastic behavior which combines characteristics of an ideal viscous liquid with those of an ideal elastic solid In other words under certain conditions molten thermoplastics behavelike a liquid and will continuously deform while shear stress is applied as shown in
Figure 110 Upon the removal of the stress however the materials behave somewhat like anelastic solid with partial recovery of the deformation as shown in Figure 110 (b) and (c) This viscoelastic behavior stems from the random-coil configuration of polymer molecules in the
Stress
Stress Stress
Mold wall
Material element before
deformation begins
Material element during
deformation
Mold wall
Material elements at rest
Moving
materialelements
Velocity
profile
(a) (b)
(c) (d)
Velocity
profile
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How Do Plastics Flow 9
molten state which allows the movement and slippage of molecular chains under theinfluence of an applied load However the entanglement of the polymer molecular chains also
makes the system behave like an elastic solid upon the application and removal of the externalload Namely on removal of the stress chains will tend to return to the equilibrium random-coil state and thus will be a component of stress recovery The recovery is not instantaneousbecause of the entanglements still present in the system
Figure 1ure 1 10 (a) Ideal viscous liquid deforms continuously under applied stress (b) Ideal elasticsolid deforms immediately upon the application of stress but fully recovers when the
stress is removed (c) Molten thermoplastic deforms continuously under the applied
stress (like a viscous liquid) but also recovers partially from the deformation upon
removal of the applied stress (like an elastic solid)
124 Melt Shear Viscosity
1241 What Is Shear Viscosity
Melt shear viscosity is a materials resistance to shear flow In general polymer melts are highly viscous because of their long molecular chain structure The viscosity of a polymer melt rangesfrom 2 to 3000 Pas (water 10^-1 Pas glass 10^20 Pas) Viscosity can be thought of as thethickness of a fluid or how much it resists flow Viscosity is expressed as the ratio of shearstress (force per unit area) to the shear rate (rate change of shear strain) as shown inEquation 11 and Figure 111
(a) Viscous
liquid
(b) Elastic
solid
(c) Viscoelastic
polymer
melt
Initial shape
(shear stress
to be applied)
Time = Dt Time = 2 x Dt
(shear stress to
be removed)
Final deformed
shape
d
d
d
d
2d
2d
2d
2d
total
recovery
partial
recovery
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VI Foreword
Despite a quarter of a century of technological advances the golden years of CAE are aheadof us as our industry takes a broader and more integrated view of what it takes to manage a
products life cycle Moldflow is proud of its contributions to date and will continue to focuson developing innovative technology coupled with practical design principles to deliver moreprofitable manufacturing
Roland Thomas
President amp CEO Moldflow Corporation
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Preface VII
Preface
About this Book
The origins of this book include not only Moldflow Design Principles but also Warpage Design
Principles published by Moldflow and the C-MOLD Design Guide Collectively thesedocuments are based on years of experience in the research theory and practice of injectionmolding These documents are now combined into this book the Moldflow Design Guide The Moldflow Design Guide is intended to help practicing engineers solve problems they frequently encounter in the design of parts and molds as well as during production This book can alsobe used as a reference for training purposes at industrial and educational institutions
How to Use this Book
This book has several chapters and appendices that deal with different stages of the designprocess and provides background on the injection-molding process and plastic materials
bull The first three chapters introduce injection molding how polymers flow inside injectionmolds and how molding conditions and injection pressure influence the process
bull Chapter 4 discusses Moldflow design principles and how they relate to making quality parts
bull Chapter 5 introduces the finite element mesh technology used by Moldflow and how thesemeshes influence the quality of the analysis
bull Chapters 6 to 9 introduce design concepts for the product gates runners and coolingsystems
bull Chapter 10 introduces concepts relating to shrinkage and warpage and how Moldflow isused to determine the amount of shrinkage and warpage a molded part will have and whatcauses the warpage
bull
Chapter 11 discusses the design procedure for analyzing injection-molded partsbull Chapter 12 discusses major part defects found on injection-molded parts
bull Finally the four appendices discuss basic injection-molding machine operation processcontrol variants of the standard injection-molding process and plastic materials
Benefits of Using CAE
The injection-molding industry has recognized that computer-aided engineering (CAE)
enhances an engineers ability to handle all aspects of the polymer injection-molding processbenefiting productivity product quality timeliness and cost This is illustrated by a wealth of
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VIII Preface
literature and the ever-growing number of CAE software users in the injection-molding industry
CAE Predicts Process Behavior
Ideally CAE analysis provides insight that is useful in designing parts molds and molding processes Without it we rely on previous experience intuition prototyping or molding trialsto obtain information such as polymer melt filling patterns weld-line and air-trap locationsrequired injection pressure and clamp tonnage fiber orientation cycle time final part shapeand deformation and mechanical properties of molded parts just to name a few WithoutCAE analysis other equally important design data such as spatial distributions of pressuretemperature shear rate shear stress and velocity are more difficult to obtain even with a well-instrumented mold The process behavior predicted by CAE can help novice engineers
overcome the lack of previous experience and assist experienced engineers in pinpointing factors that may otherwise be overlooked By using CAE analysis to iterate and evaluatealternative designs and competing materials engineering know-how in the form of designguidelines can be established relatively faster and more cost-effectively
User Proficiency Determines the Benefits of CAE
While CAE technology helps save time money and raw material as well as cuts scrap reducesthe rejection rate improves product quality and gets new products to market faster it is by nomeans a panacea for solving all molding problems Rather it should be recognized that CAEanalysis is essentially a tool designed to assist engineers instead of taking over theirresponsibilities or replacing them Like many other tools the usefulness of CAE technology depends on the proficiency of the user The benefits mentioned above will not be realizedunless the CAE tool is used properly To be more specific the accuracy of CAE analysisdepends greatly on the input data provided by the user In addition the results generated by CAE analysis need to be correctly and intelligently interpreted by the user before soundjudgments and rational decisions are made Otherwise users will simply be swamped by the vast amount of data without getting any useful information
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Acknowledgements IX
Acknowledgements
The Moldflow Design Guide would not have been accomplished were it not for the vision of Ken Welch Ken and I have discussed the value of assembling the best of the Moldflow Design
Principles Warpage Design Principles and the C-MOLD Design Guide into a single book for severalyears With Kens leadership he gave the project to Steve Thompsons training group of which I am a part Steve helped me coordinate the resources necessary to get this projectdone I could not have done this project without Steves help and guidance
A review of the content was part of the development of the Moldflow Design Guide Moldflow developers including Peter Kennedy Rong Zheng Zhongshuang Yuan and Xiaoshi Jin havereviewed sections of the book Moldflows application engineers and other technical staff withMoldflow have also reviewed sections These reviewers include Chad Fuhrman Matt JaworskiChristine Roedlich Eric Henry Olivier Anninos Paul Larter and Ana Maria Marin A specialthanks goes to Mike Rogers who reviewed the entire book for me and provided criticalfeedback on the content and organization of the book I would also like to thank Kurt Haydenof Western Michigan University for reviewing the appendix on process control His many years of experience of process setup and optimization was invaluable
Finally I would like to thank members of Moldflows Technology Transfer group forsupporting me in the writing of this book These members include Marcia Swan CarolineDorin Robert Ashley Melissa Haslam Darren Seamons and Gregory Brown
On a personal note I would like to acknowledge and thank Paul Engelmann Professor and
Department Chair Western Michigan University Department of Industrial and Manufacturing Engineering for being my friend and mentor during my career With Paul I have been able toteach and participate in research he has done on injection molding tooling and processing at Western Michigan University I have found working with Paul has made me a better Moldflow user and engineer by providing another perspective on how Moldflow can be used to solveinjection molding problemsJay Shoemaker Editor
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Contents
1 Polymer Flow Behavior in Injection Molds 1
11 Phases of Injection Molding 1111 How Plastic Fills a Mold 1112 The Filling Phase 3113 The Pressurization Phase 6114 The Compensation Phase 612 How Do Plastics Flow 7121 Material Behavior 7122 Deformation 8
123 Viscoelastic Behavior 8124 Melt Shear Viscosity 9125 Newtonian Fluid vs Non-Newtonian Fluid 10126 Shear-thinning Behavior 10127 Shear Rate Distribution 11128 Pressure-driven Flow 11129 Pressure Gradient and Injection Times 131210 Melt Flow Length 131211 Injection Pressure vs Fill Time 131212 Flow Instability 15
2 Molding Conditions and Injection Pressure 17
21 Injection-pressure Overview 17211 Pressure Drives the Flow Front 1822 Factors Influencing Injection-pressure Requirements 1823 Equations 20231 Circular Channel Flow 20232 Strip Channel Flow 2024 Effect of Molding Conditions 21
241 Part Quality 21242 Melt Temperature 22243 Mold Temperature 22244 Fill Time 23245 Shear Stress Variation 23246 Packing Pressure and Time 24247 Summary 25248 Back Flow 2625 Using Moldflow to Determine Optimum Molding Conditions 26251 Part 27
252 Molding Window Size 27253 Injection Pressure 28254 Flow Front Temperature 29
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XII Contents
255 Cooling Time 30256 Summary 31
3 Filling Pattern 33
31 Filling Pattern Overview 33311 What Is the Filling Pattern 3332 Flow in Complex Molds 34321 Overpack 34322 Racetrack Effect 35323 Varying Injection Rate 35324 Underflow Effect 36325 Hesitation Effect 37326 Weld Lines 38327 Meld Lines 38328 Sink Marks 38329 Multidirectional Flow 393210 Unstable Flow 403211 Simple Flow Pattern 4133 Flow-front Velocity and Flow-front Area 41331 What are FFV and FFA 41332 Flow-front Velocity Influences Filling Pattern 41
333 Equation 4234 Using Moldflow to Determine the Filling Pattern 43341 Computer Simulation Can Eliminate Molding Trials 43342 Using a Flow Analysis 4335 Using Moldflow to Achieve Constant FFV 44351 Controlling the FFV Through Ram Speed 44
4 Moldflow Design Principles 47
41 Product Design and Moldflow 4742 Sequence of Analysis 47421 Part Filling Optimization 48422 Molding Conditions 48423 Runner Design 48424 Cooling Optimization 49425 Packing Optimization 49426 Warpage Optimization 4943 Moldflow Flow Concepts 49431 Unidirectional and Controlled Flow Pattern 50432 Flow Balancing 51
433 Constant Pressure Gradient 52434 Maximum Shear Stress 53435 Uniform Cooling 54
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Contents XIII
436 Positioning Weld and Meld Lines 55437 Avoid Hesitation Effects 55
438 Avoid Underflow 55439 Balancing with Flow Leaders and Flow Deflectors 574310 Controlled Frictional Heat 584311 Thermal Shutoff of Runners 584312 Acceptable RunnerCavity Ratio 59
5 Meshes Used In Moldf low Analyses 61
51 Mesh Types Used by Moldflow 61511 Finite Elements Used in Moldflow 61512 Mesh Types 62513 Solver Assumptions 6352 Mesh Requirements 64521 Mesh Density Considerations 64522 Part Details 6753 Geometry Creation 6854 Importing Geometry 6955 Using Different Mesh Types 69551 Door Panel 69552 Manifold 70
6 Product Design 71
61 Material Properties for Product Design 71611 Plastics Are Sensitive to Operating Conditions 71612 Stress-Strain Behavior 72613 Creep and Stress Relaxation 77614 Fatigue 79615 Impact strength 80616 Thermal Mechanical Behavior 8062 Design for Strength 82621 Predicting Part Strength 82622 LoadingOperating Conditions 8263 Part Thickness 86631 Part Thickness Drives Quality and Cost 86632 Cycle Time Increases with Thickness 86633 Thick Parts Tend to Warp 86634 Thin Uniform Parts Improve Surface Quality 87635 Reducing Part Thickness 8764 Boosting Structural Integrity with Ribs 88
641 Structural Integrity the Goal of Every Design 88642 Designing Ribs 88
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XIV Contents
65 Design for Assembly 90651 Molding One Part vs Separate Components 90
652 Tolerances Fit between Parts 90653 Press-fit Joints 91654 Snap-fit Joints 93655 Cantilever Snap Joints 95656 Torsion Snap-fit Joints 96657 Fasteners 98658 Inserts 101659 Welding Processes 101
7 Gate Design 103
71 Gate Design Overview 103711 What Is a Gate 103712 Single vs Multiple Gates 103713 Gate Dimensions 103714 Gate Location 10472 Gate Types 104721 Manually Trimmed Gates 104722 Automatically Trimmed Gates 11073 Design Rules 113
731 Determining the Number of Gates 113732 Flow Patterns 115733 Gate Position 116734 Avoiding Common Problems 122735 Gate Length 122736 Gate Thickness 122737 Freeze-off Time 12374 Using Moldflow for Gate Design 123741 Gate Location 123742 Molding Window Size for the Three Gate Locations 124
743 Filling Pattern 125744 Gate Size Based on Shear Rate 125
8 Runner System Design 127
81 Definitions 127811 Feed System 127812 Runner System 127813 Cold Runner 127814 Hot Runner 128
815 Hot Manifold 128816 Hot Drop 128817 Sprue 128
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Contents XV
82 Runner System Design Principles 129821 Benefits of Good Runner Design 129
822 Runner Design Philosophy 129823 Flow Balancing 130824 Flow Control 130825 Frictional Heating in Runners 132826 Thermal Shutoff 132827 System and Runner Pressures 132828 Constant Pressure Gradient 133829 Cold Slug Wells 1338210 Easy Ejection 13483 Runner Types 134
831 Cold Runners 134832 Hot Runner Systems 13584 Runner Layout 136841 Determining the Number of Cavities 136842 Planning the Runner System Layout 138843 Partially Balanced Runners 139844 Geometrically Balanced Runners 14185 Initial Runner Sizing 141851 Determining Sprue Dimensions 141852 Designing Runner Cross Sections 142
853 Determining Runner Diameters 14586 Runner Balancing 147861 How Runner Balancing Works 147862 When Are the Runner Sizes Optimized 147863 Validating the Balance 147864 Processing Window 14887 Using Moldflow for Runner Balancing 149871 Runner Balancing a 48-cavity Tool 149872 Runner Balancing for a Family Mold 151873 Runner Balancing for a Multigated Part 152
9 Cooling System Design 153
91 Mold Cooling System Overview 153911 Importance of Cooling System Design 153912 Mold Cooling System Components 15492 Cooling-channel Configuration 156921 Types of Cooling Channels 15693 Alternative Cooling Devices 157931 What Do They Do 157
932 Baffles 158933 Bubblers 159934 Thermal Pins 159
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XVI Contents
935 Cooling Slender Cores 160936 Cooling Large Cores 161
937 Cooling Cylinder Cores 16294 Cooling System Equations 162941 Cooling Time 162942 Reynolds Number and Coolant Flow 16395 Design Rules 163951 Mold Cooling Design Considerations 163952 Location and Size of Channels 164953 Flow Rate and Heat Transfer 16696 Using Moldflow for Cooling System Design 167961 Example Setup 168
962 Cycle Time Determined by Design and Processing Parameters 169
10 Shrinkage and Warpage 173
101 Injection Molding and Shrinkage 1731011 What Are Shrinkage and Warpage 1731012 Shrinkage and Machine Settings 1731013 Mold Filling and Packing 1741014 How Pressure and Time Affect Shrinkage 1751015 Thermally Unstable Flow 176
102 Basic Causes of Shrinkage and Warpage 1771021 Causes of Shrinkage 1771022 Causes of Warpage 1811023 Relating Orientation and Area Shrinkage to Warpage 182103 Designing Accurate Parts Considering Warpage 1831031 Material Selection 1841032 Wall Thickness Variation 1841033 Gate Position and Runner Dimensions 1851034 Molding Conditions 1871035 Cooling Line Layout 190
11 Moldflow Design Procedure 191
111 Determine Analysis Objectives 191112 Moldflow Analysis Steps Framework 1921121 The Whole Process 1921122 Optimize Fill 1921123 Balance and Size the Runners 1971124 Optimize Cooling 1991125 Optimize the Packing Profile 200
1126 Optimize Warpage 201113 Using Moldflow to Evaluate an Initial Design 2051131 Description of this Example 205
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Contents XVII
1132 Molding Window 2051133 Filling Analysis 206
1134 Gate and Runner Design 2061135 Cooling System Design 2071136 Packing Analysis 2091137 Warpage Analysis 209114 Using Moldflow to Optimize the Design 2101141 Determine the Cause of Warpage 2101142 Investigating Different Gate Locations 2101143 Validating the Best Gate Location 211
12 Part Defects 215
121 Air Traps 2151211 What Is an Air Trap 2151212 Problems Caused by Air Traps 2161213 Remedies 216122 Black Specks and Black Streaks 2171221 What Are Black Specks and Black Streaks 2171222 Causes of Black Specks and Black Streaks 2171223 Remedies 218123 Brittleness 219
1231 What Is Brittleness 2191232 Causes of Brittleness 2191233 Remedies 219124 Burn Marks 2201241 What Is a Burn Mark 2201242 Causes of Burn Marks 2211243 Remedies 222125 Delamination 2221251 What Is Delamination 2221252 Causes of Delamination 223
1253 Remedies 223126 Dimensional Variation 2241261 What Is Dimensional Variation 2241262 Causes of Dimensional Variation 2241263 Remedies 224127 Discoloration 2251271 What Is Discoloration 2251272 Causes of Discoloration 2251273 Remedies 226128 Fish Eyes 227
1281 What Are Fish Eyes 2271282 Causes of Fish Eyes 2271283 Remedies 227
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XVIII Contents
129 Flash 2281291 What Is Flash 228
1292 Causes of Flash 2281293 Remedies 2291210 Flow Marks 23012101 What Is A Flow Mark 23012102 Causes of Flow Marks 23012103 Remedies 2301211 Hesitation 23112111 What Is Hesitation 23112112 Problems Caused by Hesitation 23112113 Remedies 232
1212 Jetting 23212121 What Is Jetting 23212122 Problems Caused by Jetting 23312123 Remedies 2331213 Ripples 23512131 What Are Ripples 23512132 Cause of Ripples 23512133 Remedies 2361214 Short Shots 23612141 What Is a Short Shot 236
12142 Causes of Short Shots 23712143 Remedies 2371215 Silver Streaks 23812151 What Are Silver Streaks 23812152 Causes of Silver Streaks 23912153 Remedies 2391216 Sink Marks and Voids 24012161 What Are Sink Marks and Voids 24012162 Causes of Sink Marks and Voids 24012163 Remedies 2411217 Weld Lines and Meld Lines 24312171 What Are Weld Lines and Meld Lines 24312172 Problems Caused by Weld Lines 24312173 Strength of Weld Lines 24412174 Remedies 245
Appendix A Injection Molding 247
A1 Injection-molding Overview 247 A11 Process 247
A12 Applications 248 A2 Development of the Injection-molding Machine 248 A21 Benefits of the Reciprocating Screw 248
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Contents XIX
A3 Development of the Injection-molding Process 249 A4 Alternative Injection-molding Processes 249
A41 Co-injection (Sandwich) Molding 249 A42 Fusible Core Injection Molding 250 A43 Gas-assisted Injection Molding 251 A44 Injection-compression Molding 252 A45 Lamellar (Microlayer) Injection Molding 253 A46 Live-feed Injection Molding 253 A47 Low-pressure Injection Molding 254 A48 Push-pull Injection Molding 254 A49 Reactive Molding 255 A410 Structural Foam Injection Molding 256
A411 Thin-wall Molding 256
Appendix B Injection-molding Machine System and Operations 259
B1 Injection-molding Machine 259B11 Components 259B12 Machine Specification 260B13 Machine Function 260B14 Auxiliary Equipment 260B2 Machine Components 261
B21 Injection System 261B22 Mold System 263B23 Hydraulic System 264B24 Control System 264B25 Clamping System 264B3 Molded System 265B31 The Delivery System 265B4 Machine Operating Sequence 266B41 Process Cycle 266B5 Screw Operation 268
B51 Back Pressure 268B52 Injection Speed 268B53 Screw Rotation Speed 268B54 Cushion 269B6 Secondary Operations 269B61 Assembly 269B62 Decoration 269B63 Other Secondary Operations 270
Appendix C Injection-molding Process Control 271
C1 Importance of Process Conditions 271C11 Setting Machine Process Conditions 271
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XX Contents
Appendix D Plastic Materials 281
D1 What Are Plastics 281
D11 Polymerization Process 281D12 Structure of Polymers 281D13 Polymer Alloys and Blends 283D14 Polymer Composites 283D2 Classification of Plastics 283D21 Classes of Plastics 284D22 Structures and Properties of Plastics 284D3 Thermoplastics 285D31 Market Share Distribution of Thermoplastics 285D32 Structures and Properties of Thermoplastics 286D33 Amorphous Polymers 286D34 Semicrystalline Polymers 287D4 Thermosets 288D5 Properties and Applications of Thermoplastics 289D51 ABS 290D52 PA 12 291D53 PA 6 293D54 PA 66 294D55 PBT 296
D56 PC 297D57 PCABS 299D58 PCPBT 300D59 HDPE 301D510 LDPE 303D511 PEI 304D512 PET 305D513 PETG 306D514 PMMA 307D515 POM 309
D516 PP 310D517 PPEPPO 312D518 PS 313D519 PVC 314D520 SAN 316D521 Additives Fillers and Reinforcements 317D522 Modifying Polymer Properties 318D523 Low-aspect Fillers 319D524 High-aspect Fillers Fibers 319
Index 321
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1 Polymer Flow Behavior in Injection Molds
bull Phases of injection molding
bull How do plastics flow
11 Phases of Injection Molding
Any molder can prove that all the conditions and effects discussed in this chapter do indeedoccur during the injection molding process While this knowledge alone can somewhatimprove quality it is only with the use of Moldflow analysis during the initial design stage with the mold designed for the optimum filling pattern that these effects can be controlledand the full benefits obtained
Flow technology is concerned with the behavior of plastics during the mold filling process Aplastic parts properties depend on how the part is molded Two parts having identicaldimensions and made from the same material but molded under different conditions will havedifferent stress and shrinkage levels and will behave differently in the field meaning that they
are in practice two different parts The way the plastic flows into the mold is of paramount importance in determining the quality of the part The process of filling the mold can be distinctly analyzed with the ability to predictpressure temperature and stress
111 How Plastic Fills a Mold
This was investigated using a centrally gated mold shaped like a dinner plate with a thick rim
around the outside as shown in Figure 11 It was found that the injection molding processalthough complex could be divided into three phases (we use the word phase to avoidconfusion with injection stage as used with programmed injection)
Figure 1ure 1 1 Cross-section of disk mold used to investigate flow
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2 Polymer Flow Behavior in Injection Molds
1111 Filling Phase
As the ram moves forward it first moves at a steady speed as the plastic flows into the cavity
This is the filling phase This phase lasts until the mold is just filled See Figure 12 andFigure 13
1112 Pressurization Phase
The pressurization phase begins when the ram moves forward after the filling phase to bring the mold up to pressure When the mold is filled the ram will slow down but it still movesquite some distance because plastics are very compressible materials At injection molding pressure an extra 15 volume of material can be forced into the cavity See Figure 12 andFigure 13
Although fluids are usually assumed to be incompressible molten plastics have to beconsidered to be more like a gas The compressibility of plastics can be observed by blocking off the nozzle and attempting to purge the barrel The ram will jump forward when thepressure is applied but will spring back when the pressure is released
1113 Compensation Phase
After the pressurization phase the ram still does not stop completely continuing to creepforward for some time Plastics have a very large volumetric change of about 25 from themelt to the solid This can be seen in a short shot the difference in volume between themolding and the cavity is due to this volumetric change See Figure 12 and Figure 13
The ram moving forward to compensate for the volumetric change in the part is called thecompensation phase As the volumetric change is 25 and at the most only an extra 15 canbe injected in the pressurization phase there must always be some compensation phase
Figure 1igure 1 2 Phases of injection molding
Filling phase
Pressurization phase
Compensation phase
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Phases of Injection Molding 3
Figure 13ure 1 3 Phases of injection molding detail
112 The Filling Phase
A two-color technique best demonstrates this phase After emptying the barrel of an injection-molding machine a small amount of red plastic was charged followed by green plastic
Consider the closed mold with the plastic front just starting to flow from the nozzle Theplastic first fills the sprue and runner system then enters the mold cavity itself forming a smallbubble of molten plastic
The skin of the plastic in contact with the cool mold freezes rapidly while the central coreremains molten When additional material is injected it flows into this central core displacing
the material already there which then forms a new flow front The flow of this displacedmaterial is a combination of forward flow and outward flow The outward flow contacts the wall freezes and forms the next section of skin while the forward flow forms the new moltencore When more material enters the mold it flows along a channel lined with these frozen walls of plastic illustrated in Figure 14
This flow pattern is often called fountain flow or bubble flow because the flow front is like abubble being inflated with hot plastic from the center The frozen layer is formed by the flow front inflating and so is subject to only a low shear stress and therefore has a very low levelof molecular orientation Once it is frozen it cannot be orientated any further so the frozen
layer in the finished part has a low level of orientation
Figure 14ure 1 4 Fountain flow and heat transfer
Filling Pressurization Compensation
Hot plastic
Frozen layer
Heat loss to mold
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4 Polymer Flow Behavior in Injection Molds
Now consider what happens upstream Hot plastic is continuously flowing bringing new hotmaterial along and generating significant frictional heat At the same time heat is being lost
through the frozen layer to the cold mold surfaceInitially the frozen layer is very thin so heat is lost very rapidly This results in more plasticfreezing and the frozen layer getting thicker cutting down the heat flow After a time thefrozen layer will reach a thickness such that the heat lost by conduction is equal to the heatinput from plastic flow and frictional heating ie an equilibrium condition is reached(Figure 14)
It is interesting to do some calculations on the time taken to reach this state of equilibrium The actual rate of heat flow is very large in comparison with the small heat content of theplastic in the frozen layer The result is that equilibrium is reached very quickly often in a timemeasured in a few tenths of a second As the total filling time is measured in seconds thefrozen layer reaches an equilibrium state early in the filling cycle
It is useful to think about how the thickness of this frozen layer will vary If the injection rate were slowed less heat would be generated by friction along the flow path with less heat inputfrom the flow The heat loss would be at the same rate and with less heat input the frozenlayer would grow in thickness If the injection rate were raised the frozen layer would bethinner (Figure 15) Similarly higher melt and mold temperatures would reduce the thicknessof the frozen layer This can be seen experimentally using the two-color technique
Figure 1igure 1 5 Influence of injection rate on frozen layer thickness
1121 Flow Shear Stress
It is easy to get confused between the various stress levels and orientation of the polymer Asthe plastic flows it is subject to shear stress also called flow shear stress This flow shear stress willorient the material ie cause the molecules to align themselves in the general direction of flow
The shear stress varies from a maximum at the outside dropping off to zero at the center
3Shear stress is purely a function of force and area This must not be confused withshear rate which is the rate of plastic sliding over the next layer Shear rate is zero
= Frozen
= Molten cooler
= Molten hotter
Slowerinjection
rate
Faster
injection
rate
vs
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Phases of Injection Molding 5
at the outer edge where the plastic is frozen rises to a maximum just inwards of thefrozen layer then drops toward the center as shown in Figure 16
Figure 16ure 1 6 Shear rate distribution
If the flow were stopped and the plastic allowed to cool down very slowly this orientation would have time to relax giving a very low level of residual orientation On the other hand if the material were kept under stress and the plastic snap frozen most of the orientation would
be trapped in the frozen plastic (Figure 17)
Figure 17ure 1 7 Molecular orientation through the thickness of the part
Min MaxShear rate
= Tension
= Compression
Molecular orientation trapped
in pressurization phase
Gradual reduction of orientation
toward the center of the
mold as stress levels are lower
and the cooling rate is slower
which allows more time for
orientation to relax
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6 Polymer Flow Behavior in Injection Molds
Now consider the orientation from the mold surface toward the center
The frozen layer itself formed with very little shear and therefore low orientation immediately
freezes setting the low level of orientation
The layer of plastic just on the inside of the frozen layer is subject to maximum shear stressand freezes the instant flow stops trapping almost all the orientation
This is the orientation pattern the further toward the center the more the shear stress dropsand the slower the rate of cooling This allows more time for the level of orientation to relaxso the residual orientation drops rapidly toward the center Consider how this pattern willaffect the residual stress level Oriented material (normally) will shrink more than nonorientedmaterial On the inner surface of the original frozen layer highly oriented material wants toshrink a great deal but it is prevented from doing so by the less-oriented material The highly
oriented layer ends up being in tension while the less-oriented material is in compression This residual stress pattern is a common cause of part warpage
3 There is a connectionthrough orientationbetween the shear stress during
filling (flow stress) and the residual stress in the final molded part This meansshear stress during filling shown on Moldflow plots can be used as a designparameter
113 The Pressurization Phase
The pressurization phasefrom the point of view of flow behavioris very similar to thefilling phase The flow rate may drop somewhat as the mold builds up to pressure resulting inan increase in the thickness of the frozen layer
The main difference of course is the increase in hydrostatic (isotropic) pressure We shall seein chapter 2 section 24 Effect of Molding Conditions that hydrostatic pressure in itself doesnot cause any residual stress
114 The Compensation Phase
Compensating flow is unstable Consider the plate molding again (see Figure 11) You wouldthink that plastic flowing uniformly through the thin diaphragm would top up the thick rim Inpractice the plastic during the compensation phase flows in rivers that spread out like a deltaas illustrated in Figure 18 This may seem surprising at first but it can be explained by temperature instability
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How Do Plastics Flow 7
Figure 18ure 1 8 River flow
1141 Temperature Variation
There is always some variation in melt temperature coming from the barrel of the injectionmachine In exceptional cases up to 40 ordmC variation has been measured using a high-speedthermocouple
1142 Natural Instability
However slight the temperature variation natural instability will amplify it If for exampleone part of the melt is slightly hotter than the rest then the plastic flow in that area will beslightly greater bringing hotter material into the area and maintaining the temperature If onthe other hand there is another area that is cooler the flow will be less so there will be lessheat input and the plastic will get colder until it eventually freezes off
However balanced the initial conditions this natural instability will result in a river-type flow
This is a very important consideration The first material to freeze off will shrink early in thecycle By the time the material in the river flows freezes the bulk of the material will havealready frozen off and shrinkage will have occurred The rivers will shrink relative to the bulk of the molding and because they are highly orientated shrinkage will be very high The resultis high-stress tensile members throughout the molding a common cause of warpage
1143 Optimum Part Quality
Most of the stress in plastic parts occurs during the compensation phase By controlling flow and minimizing stress it is possible to design for optimum part quality This important point is
at the heart of the Moldflow philosophy
12 How Do Plastics Flow
121 Material Behavior
Molten thermoplastics exhibit viscoelastic behavior which combines flow characteristics of both viscous liquids and elastic solids When a viscous liquid flows the energy that causes thedeformation is dissipated and becomes viscous heat On the other hand when an elastic solid
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8 Polymer Flow Behavior in Injection Molds
is deformed the driving energy is stored For example the flow of water is a typical viscousflow whereas the deformation of a rubber cube falls into the elastic category
122 Deformation
In addition to the two types of material flow behavior there are two types of deformationsimple shear and simple extension (elongation) as shown in Figure 19 (a) and (b) below Theflow of molten thermoplastics during injection-molding filling is predominantly shear flow asshown in Figure 19 (c) in which layers of material elements slide over each other Theextensional flow however becomes significant as the material elements undergo elongation when the melt passes areas of abrupt dimensional change (eg a gate region) as shown inFigure 19 (d)
Figure 1igure 1 9 (a) Simple shear flow (b) Simple extensional flow (c) Shear flow in cavity filling (d) Extensional flow in cavity filling
123 Viscoelastic Behavior
In response to an applied stress (force per unit area) molten thermoplastics exhibit viscoelastic behavior which combines characteristics of an ideal viscous liquid with those of an ideal elastic solid In other words under certain conditions molten thermoplastics behavelike a liquid and will continuously deform while shear stress is applied as shown in
Figure 110 Upon the removal of the stress however the materials behave somewhat like anelastic solid with partial recovery of the deformation as shown in Figure 110 (b) and (c) This viscoelastic behavior stems from the random-coil configuration of polymer molecules in the
Stress
Stress Stress
Mold wall
Material element before
deformation begins
Material element during
deformation
Mold wall
Material elements at rest
Moving
materialelements
Velocity
profile
(a) (b)
(c) (d)
Velocity
profile
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How Do Plastics Flow 9
molten state which allows the movement and slippage of molecular chains under theinfluence of an applied load However the entanglement of the polymer molecular chains also
makes the system behave like an elastic solid upon the application and removal of the externalload Namely on removal of the stress chains will tend to return to the equilibrium random-coil state and thus will be a component of stress recovery The recovery is not instantaneousbecause of the entanglements still present in the system
Figure 1ure 1 10 (a) Ideal viscous liquid deforms continuously under applied stress (b) Ideal elasticsolid deforms immediately upon the application of stress but fully recovers when the
stress is removed (c) Molten thermoplastic deforms continuously under the applied
stress (like a viscous liquid) but also recovers partially from the deformation upon
removal of the applied stress (like an elastic solid)
124 Melt Shear Viscosity
1241 What Is Shear Viscosity
Melt shear viscosity is a materials resistance to shear flow In general polymer melts are highly viscous because of their long molecular chain structure The viscosity of a polymer melt rangesfrom 2 to 3000 Pas (water 10^-1 Pas glass 10^20 Pas) Viscosity can be thought of as thethickness of a fluid or how much it resists flow Viscosity is expressed as the ratio of shearstress (force per unit area) to the shear rate (rate change of shear strain) as shown inEquation 11 and Figure 111
(a) Viscous
liquid
(b) Elastic
solid
(c) Viscoelastic
polymer
melt
Initial shape
(shear stress
to be applied)
Time = Dt Time = 2 x Dt
(shear stress to
be removed)
Final deformed
shape
d
d
d
d
2d
2d
2d
2d
total
recovery
partial
recovery
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Preface VII
Preface
About this Book
The origins of this book include not only Moldflow Design Principles but also Warpage Design
Principles published by Moldflow and the C-MOLD Design Guide Collectively thesedocuments are based on years of experience in the research theory and practice of injectionmolding These documents are now combined into this book the Moldflow Design Guide The Moldflow Design Guide is intended to help practicing engineers solve problems they frequently encounter in the design of parts and molds as well as during production This book can alsobe used as a reference for training purposes at industrial and educational institutions
How to Use this Book
This book has several chapters and appendices that deal with different stages of the designprocess and provides background on the injection-molding process and plastic materials
bull The first three chapters introduce injection molding how polymers flow inside injectionmolds and how molding conditions and injection pressure influence the process
bull Chapter 4 discusses Moldflow design principles and how they relate to making quality parts
bull Chapter 5 introduces the finite element mesh technology used by Moldflow and how thesemeshes influence the quality of the analysis
bull Chapters 6 to 9 introduce design concepts for the product gates runners and coolingsystems
bull Chapter 10 introduces concepts relating to shrinkage and warpage and how Moldflow isused to determine the amount of shrinkage and warpage a molded part will have and whatcauses the warpage
bull
Chapter 11 discusses the design procedure for analyzing injection-molded partsbull Chapter 12 discusses major part defects found on injection-molded parts
bull Finally the four appendices discuss basic injection-molding machine operation processcontrol variants of the standard injection-molding process and plastic materials
Benefits of Using CAE
The injection-molding industry has recognized that computer-aided engineering (CAE)
enhances an engineers ability to handle all aspects of the polymer injection-molding processbenefiting productivity product quality timeliness and cost This is illustrated by a wealth of
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VIII Preface
literature and the ever-growing number of CAE software users in the injection-molding industry
CAE Predicts Process Behavior
Ideally CAE analysis provides insight that is useful in designing parts molds and molding processes Without it we rely on previous experience intuition prototyping or molding trialsto obtain information such as polymer melt filling patterns weld-line and air-trap locationsrequired injection pressure and clamp tonnage fiber orientation cycle time final part shapeand deformation and mechanical properties of molded parts just to name a few WithoutCAE analysis other equally important design data such as spatial distributions of pressuretemperature shear rate shear stress and velocity are more difficult to obtain even with a well-instrumented mold The process behavior predicted by CAE can help novice engineers
overcome the lack of previous experience and assist experienced engineers in pinpointing factors that may otherwise be overlooked By using CAE analysis to iterate and evaluatealternative designs and competing materials engineering know-how in the form of designguidelines can be established relatively faster and more cost-effectively
User Proficiency Determines the Benefits of CAE
While CAE technology helps save time money and raw material as well as cuts scrap reducesthe rejection rate improves product quality and gets new products to market faster it is by nomeans a panacea for solving all molding problems Rather it should be recognized that CAEanalysis is essentially a tool designed to assist engineers instead of taking over theirresponsibilities or replacing them Like many other tools the usefulness of CAE technology depends on the proficiency of the user The benefits mentioned above will not be realizedunless the CAE tool is used properly To be more specific the accuracy of CAE analysisdepends greatly on the input data provided by the user In addition the results generated by CAE analysis need to be correctly and intelligently interpreted by the user before soundjudgments and rational decisions are made Otherwise users will simply be swamped by the vast amount of data without getting any useful information
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Acknowledgements IX
Acknowledgements
The Moldflow Design Guide would not have been accomplished were it not for the vision of Ken Welch Ken and I have discussed the value of assembling the best of the Moldflow Design
Principles Warpage Design Principles and the C-MOLD Design Guide into a single book for severalyears With Kens leadership he gave the project to Steve Thompsons training group of which I am a part Steve helped me coordinate the resources necessary to get this projectdone I could not have done this project without Steves help and guidance
A review of the content was part of the development of the Moldflow Design Guide Moldflow developers including Peter Kennedy Rong Zheng Zhongshuang Yuan and Xiaoshi Jin havereviewed sections of the book Moldflows application engineers and other technical staff withMoldflow have also reviewed sections These reviewers include Chad Fuhrman Matt JaworskiChristine Roedlich Eric Henry Olivier Anninos Paul Larter and Ana Maria Marin A specialthanks goes to Mike Rogers who reviewed the entire book for me and provided criticalfeedback on the content and organization of the book I would also like to thank Kurt Haydenof Western Michigan University for reviewing the appendix on process control His many years of experience of process setup and optimization was invaluable
Finally I would like to thank members of Moldflows Technology Transfer group forsupporting me in the writing of this book These members include Marcia Swan CarolineDorin Robert Ashley Melissa Haslam Darren Seamons and Gregory Brown
On a personal note I would like to acknowledge and thank Paul Engelmann Professor and
Department Chair Western Michigan University Department of Industrial and Manufacturing Engineering for being my friend and mentor during my career With Paul I have been able toteach and participate in research he has done on injection molding tooling and processing at Western Michigan University I have found working with Paul has made me a better Moldflow user and engineer by providing another perspective on how Moldflow can be used to solveinjection molding problemsJay Shoemaker Editor
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Contents
1 Polymer Flow Behavior in Injection Molds 1
11 Phases of Injection Molding 1111 How Plastic Fills a Mold 1112 The Filling Phase 3113 The Pressurization Phase 6114 The Compensation Phase 612 How Do Plastics Flow 7121 Material Behavior 7122 Deformation 8
123 Viscoelastic Behavior 8124 Melt Shear Viscosity 9125 Newtonian Fluid vs Non-Newtonian Fluid 10126 Shear-thinning Behavior 10127 Shear Rate Distribution 11128 Pressure-driven Flow 11129 Pressure Gradient and Injection Times 131210 Melt Flow Length 131211 Injection Pressure vs Fill Time 131212 Flow Instability 15
2 Molding Conditions and Injection Pressure 17
21 Injection-pressure Overview 17211 Pressure Drives the Flow Front 1822 Factors Influencing Injection-pressure Requirements 1823 Equations 20231 Circular Channel Flow 20232 Strip Channel Flow 2024 Effect of Molding Conditions 21
241 Part Quality 21242 Melt Temperature 22243 Mold Temperature 22244 Fill Time 23245 Shear Stress Variation 23246 Packing Pressure and Time 24247 Summary 25248 Back Flow 2625 Using Moldflow to Determine Optimum Molding Conditions 26251 Part 27
252 Molding Window Size 27253 Injection Pressure 28254 Flow Front Temperature 29
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XII Contents
255 Cooling Time 30256 Summary 31
3 Filling Pattern 33
31 Filling Pattern Overview 33311 What Is the Filling Pattern 3332 Flow in Complex Molds 34321 Overpack 34322 Racetrack Effect 35323 Varying Injection Rate 35324 Underflow Effect 36325 Hesitation Effect 37326 Weld Lines 38327 Meld Lines 38328 Sink Marks 38329 Multidirectional Flow 393210 Unstable Flow 403211 Simple Flow Pattern 4133 Flow-front Velocity and Flow-front Area 41331 What are FFV and FFA 41332 Flow-front Velocity Influences Filling Pattern 41
333 Equation 4234 Using Moldflow to Determine the Filling Pattern 43341 Computer Simulation Can Eliminate Molding Trials 43342 Using a Flow Analysis 4335 Using Moldflow to Achieve Constant FFV 44351 Controlling the FFV Through Ram Speed 44
4 Moldflow Design Principles 47
41 Product Design and Moldflow 4742 Sequence of Analysis 47421 Part Filling Optimization 48422 Molding Conditions 48423 Runner Design 48424 Cooling Optimization 49425 Packing Optimization 49426 Warpage Optimization 4943 Moldflow Flow Concepts 49431 Unidirectional and Controlled Flow Pattern 50432 Flow Balancing 51
433 Constant Pressure Gradient 52434 Maximum Shear Stress 53435 Uniform Cooling 54
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Contents XIII
436 Positioning Weld and Meld Lines 55437 Avoid Hesitation Effects 55
438 Avoid Underflow 55439 Balancing with Flow Leaders and Flow Deflectors 574310 Controlled Frictional Heat 584311 Thermal Shutoff of Runners 584312 Acceptable RunnerCavity Ratio 59
5 Meshes Used In Moldf low Analyses 61
51 Mesh Types Used by Moldflow 61511 Finite Elements Used in Moldflow 61512 Mesh Types 62513 Solver Assumptions 6352 Mesh Requirements 64521 Mesh Density Considerations 64522 Part Details 6753 Geometry Creation 6854 Importing Geometry 6955 Using Different Mesh Types 69551 Door Panel 69552 Manifold 70
6 Product Design 71
61 Material Properties for Product Design 71611 Plastics Are Sensitive to Operating Conditions 71612 Stress-Strain Behavior 72613 Creep and Stress Relaxation 77614 Fatigue 79615 Impact strength 80616 Thermal Mechanical Behavior 8062 Design for Strength 82621 Predicting Part Strength 82622 LoadingOperating Conditions 8263 Part Thickness 86631 Part Thickness Drives Quality and Cost 86632 Cycle Time Increases with Thickness 86633 Thick Parts Tend to Warp 86634 Thin Uniform Parts Improve Surface Quality 87635 Reducing Part Thickness 8764 Boosting Structural Integrity with Ribs 88
641 Structural Integrity the Goal of Every Design 88642 Designing Ribs 88
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XIV Contents
65 Design for Assembly 90651 Molding One Part vs Separate Components 90
652 Tolerances Fit between Parts 90653 Press-fit Joints 91654 Snap-fit Joints 93655 Cantilever Snap Joints 95656 Torsion Snap-fit Joints 96657 Fasteners 98658 Inserts 101659 Welding Processes 101
7 Gate Design 103
71 Gate Design Overview 103711 What Is a Gate 103712 Single vs Multiple Gates 103713 Gate Dimensions 103714 Gate Location 10472 Gate Types 104721 Manually Trimmed Gates 104722 Automatically Trimmed Gates 11073 Design Rules 113
731 Determining the Number of Gates 113732 Flow Patterns 115733 Gate Position 116734 Avoiding Common Problems 122735 Gate Length 122736 Gate Thickness 122737 Freeze-off Time 12374 Using Moldflow for Gate Design 123741 Gate Location 123742 Molding Window Size for the Three Gate Locations 124
743 Filling Pattern 125744 Gate Size Based on Shear Rate 125
8 Runner System Design 127
81 Definitions 127811 Feed System 127812 Runner System 127813 Cold Runner 127814 Hot Runner 128
815 Hot Manifold 128816 Hot Drop 128817 Sprue 128
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Contents XV
82 Runner System Design Principles 129821 Benefits of Good Runner Design 129
822 Runner Design Philosophy 129823 Flow Balancing 130824 Flow Control 130825 Frictional Heating in Runners 132826 Thermal Shutoff 132827 System and Runner Pressures 132828 Constant Pressure Gradient 133829 Cold Slug Wells 1338210 Easy Ejection 13483 Runner Types 134
831 Cold Runners 134832 Hot Runner Systems 13584 Runner Layout 136841 Determining the Number of Cavities 136842 Planning the Runner System Layout 138843 Partially Balanced Runners 139844 Geometrically Balanced Runners 14185 Initial Runner Sizing 141851 Determining Sprue Dimensions 141852 Designing Runner Cross Sections 142
853 Determining Runner Diameters 14586 Runner Balancing 147861 How Runner Balancing Works 147862 When Are the Runner Sizes Optimized 147863 Validating the Balance 147864 Processing Window 14887 Using Moldflow for Runner Balancing 149871 Runner Balancing a 48-cavity Tool 149872 Runner Balancing for a Family Mold 151873 Runner Balancing for a Multigated Part 152
9 Cooling System Design 153
91 Mold Cooling System Overview 153911 Importance of Cooling System Design 153912 Mold Cooling System Components 15492 Cooling-channel Configuration 156921 Types of Cooling Channels 15693 Alternative Cooling Devices 157931 What Do They Do 157
932 Baffles 158933 Bubblers 159934 Thermal Pins 159
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XVI Contents
935 Cooling Slender Cores 160936 Cooling Large Cores 161
937 Cooling Cylinder Cores 16294 Cooling System Equations 162941 Cooling Time 162942 Reynolds Number and Coolant Flow 16395 Design Rules 163951 Mold Cooling Design Considerations 163952 Location and Size of Channels 164953 Flow Rate and Heat Transfer 16696 Using Moldflow for Cooling System Design 167961 Example Setup 168
962 Cycle Time Determined by Design and Processing Parameters 169
10 Shrinkage and Warpage 173
101 Injection Molding and Shrinkage 1731011 What Are Shrinkage and Warpage 1731012 Shrinkage and Machine Settings 1731013 Mold Filling and Packing 1741014 How Pressure and Time Affect Shrinkage 1751015 Thermally Unstable Flow 176
102 Basic Causes of Shrinkage and Warpage 1771021 Causes of Shrinkage 1771022 Causes of Warpage 1811023 Relating Orientation and Area Shrinkage to Warpage 182103 Designing Accurate Parts Considering Warpage 1831031 Material Selection 1841032 Wall Thickness Variation 1841033 Gate Position and Runner Dimensions 1851034 Molding Conditions 1871035 Cooling Line Layout 190
11 Moldflow Design Procedure 191
111 Determine Analysis Objectives 191112 Moldflow Analysis Steps Framework 1921121 The Whole Process 1921122 Optimize Fill 1921123 Balance and Size the Runners 1971124 Optimize Cooling 1991125 Optimize the Packing Profile 200
1126 Optimize Warpage 201113 Using Moldflow to Evaluate an Initial Design 2051131 Description of this Example 205
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Contents XVII
1132 Molding Window 2051133 Filling Analysis 206
1134 Gate and Runner Design 2061135 Cooling System Design 2071136 Packing Analysis 2091137 Warpage Analysis 209114 Using Moldflow to Optimize the Design 2101141 Determine the Cause of Warpage 2101142 Investigating Different Gate Locations 2101143 Validating the Best Gate Location 211
12 Part Defects 215
121 Air Traps 2151211 What Is an Air Trap 2151212 Problems Caused by Air Traps 2161213 Remedies 216122 Black Specks and Black Streaks 2171221 What Are Black Specks and Black Streaks 2171222 Causes of Black Specks and Black Streaks 2171223 Remedies 218123 Brittleness 219
1231 What Is Brittleness 2191232 Causes of Brittleness 2191233 Remedies 219124 Burn Marks 2201241 What Is a Burn Mark 2201242 Causes of Burn Marks 2211243 Remedies 222125 Delamination 2221251 What Is Delamination 2221252 Causes of Delamination 223
1253 Remedies 223126 Dimensional Variation 2241261 What Is Dimensional Variation 2241262 Causes of Dimensional Variation 2241263 Remedies 224127 Discoloration 2251271 What Is Discoloration 2251272 Causes of Discoloration 2251273 Remedies 226128 Fish Eyes 227
1281 What Are Fish Eyes 2271282 Causes of Fish Eyes 2271283 Remedies 227
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XVIII Contents
129 Flash 2281291 What Is Flash 228
1292 Causes of Flash 2281293 Remedies 2291210 Flow Marks 23012101 What Is A Flow Mark 23012102 Causes of Flow Marks 23012103 Remedies 2301211 Hesitation 23112111 What Is Hesitation 23112112 Problems Caused by Hesitation 23112113 Remedies 232
1212 Jetting 23212121 What Is Jetting 23212122 Problems Caused by Jetting 23312123 Remedies 2331213 Ripples 23512131 What Are Ripples 23512132 Cause of Ripples 23512133 Remedies 2361214 Short Shots 23612141 What Is a Short Shot 236
12142 Causes of Short Shots 23712143 Remedies 2371215 Silver Streaks 23812151 What Are Silver Streaks 23812152 Causes of Silver Streaks 23912153 Remedies 2391216 Sink Marks and Voids 24012161 What Are Sink Marks and Voids 24012162 Causes of Sink Marks and Voids 24012163 Remedies 2411217 Weld Lines and Meld Lines 24312171 What Are Weld Lines and Meld Lines 24312172 Problems Caused by Weld Lines 24312173 Strength of Weld Lines 24412174 Remedies 245
Appendix A Injection Molding 247
A1 Injection-molding Overview 247 A11 Process 247
A12 Applications 248 A2 Development of the Injection-molding Machine 248 A21 Benefits of the Reciprocating Screw 248
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Contents XIX
A3 Development of the Injection-molding Process 249 A4 Alternative Injection-molding Processes 249
A41 Co-injection (Sandwich) Molding 249 A42 Fusible Core Injection Molding 250 A43 Gas-assisted Injection Molding 251 A44 Injection-compression Molding 252 A45 Lamellar (Microlayer) Injection Molding 253 A46 Live-feed Injection Molding 253 A47 Low-pressure Injection Molding 254 A48 Push-pull Injection Molding 254 A49 Reactive Molding 255 A410 Structural Foam Injection Molding 256
A411 Thin-wall Molding 256
Appendix B Injection-molding Machine System and Operations 259
B1 Injection-molding Machine 259B11 Components 259B12 Machine Specification 260B13 Machine Function 260B14 Auxiliary Equipment 260B2 Machine Components 261
B21 Injection System 261B22 Mold System 263B23 Hydraulic System 264B24 Control System 264B25 Clamping System 264B3 Molded System 265B31 The Delivery System 265B4 Machine Operating Sequence 266B41 Process Cycle 266B5 Screw Operation 268
B51 Back Pressure 268B52 Injection Speed 268B53 Screw Rotation Speed 268B54 Cushion 269B6 Secondary Operations 269B61 Assembly 269B62 Decoration 269B63 Other Secondary Operations 270
Appendix C Injection-molding Process Control 271
C1 Importance of Process Conditions 271C11 Setting Machine Process Conditions 271
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XX Contents
Appendix D Plastic Materials 281
D1 What Are Plastics 281
D11 Polymerization Process 281D12 Structure of Polymers 281D13 Polymer Alloys and Blends 283D14 Polymer Composites 283D2 Classification of Plastics 283D21 Classes of Plastics 284D22 Structures and Properties of Plastics 284D3 Thermoplastics 285D31 Market Share Distribution of Thermoplastics 285D32 Structures and Properties of Thermoplastics 286D33 Amorphous Polymers 286D34 Semicrystalline Polymers 287D4 Thermosets 288D5 Properties and Applications of Thermoplastics 289D51 ABS 290D52 PA 12 291D53 PA 6 293D54 PA 66 294D55 PBT 296
D56 PC 297D57 PCABS 299D58 PCPBT 300D59 HDPE 301D510 LDPE 303D511 PEI 304D512 PET 305D513 PETG 306D514 PMMA 307D515 POM 309
D516 PP 310D517 PPEPPO 312D518 PS 313D519 PVC 314D520 SAN 316D521 Additives Fillers and Reinforcements 317D522 Modifying Polymer Properties 318D523 Low-aspect Fillers 319D524 High-aspect Fillers Fibers 319
Index 321
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1 Polymer Flow Behavior in Injection Molds
bull Phases of injection molding
bull How do plastics flow
11 Phases of Injection Molding
Any molder can prove that all the conditions and effects discussed in this chapter do indeedoccur during the injection molding process While this knowledge alone can somewhatimprove quality it is only with the use of Moldflow analysis during the initial design stage with the mold designed for the optimum filling pattern that these effects can be controlledand the full benefits obtained
Flow technology is concerned with the behavior of plastics during the mold filling process Aplastic parts properties depend on how the part is molded Two parts having identicaldimensions and made from the same material but molded under different conditions will havedifferent stress and shrinkage levels and will behave differently in the field meaning that they
are in practice two different parts The way the plastic flows into the mold is of paramount importance in determining the quality of the part The process of filling the mold can be distinctly analyzed with the ability to predictpressure temperature and stress
111 How Plastic Fills a Mold
This was investigated using a centrally gated mold shaped like a dinner plate with a thick rim
around the outside as shown in Figure 11 It was found that the injection molding processalthough complex could be divided into three phases (we use the word phase to avoidconfusion with injection stage as used with programmed injection)
Figure 1ure 1 1 Cross-section of disk mold used to investigate flow
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2 Polymer Flow Behavior in Injection Molds
1111 Filling Phase
As the ram moves forward it first moves at a steady speed as the plastic flows into the cavity
This is the filling phase This phase lasts until the mold is just filled See Figure 12 andFigure 13
1112 Pressurization Phase
The pressurization phase begins when the ram moves forward after the filling phase to bring the mold up to pressure When the mold is filled the ram will slow down but it still movesquite some distance because plastics are very compressible materials At injection molding pressure an extra 15 volume of material can be forced into the cavity See Figure 12 andFigure 13
Although fluids are usually assumed to be incompressible molten plastics have to beconsidered to be more like a gas The compressibility of plastics can be observed by blocking off the nozzle and attempting to purge the barrel The ram will jump forward when thepressure is applied but will spring back when the pressure is released
1113 Compensation Phase
After the pressurization phase the ram still does not stop completely continuing to creepforward for some time Plastics have a very large volumetric change of about 25 from themelt to the solid This can be seen in a short shot the difference in volume between themolding and the cavity is due to this volumetric change See Figure 12 and Figure 13
The ram moving forward to compensate for the volumetric change in the part is called thecompensation phase As the volumetric change is 25 and at the most only an extra 15 canbe injected in the pressurization phase there must always be some compensation phase
Figure 1igure 1 2 Phases of injection molding
Filling phase
Pressurization phase
Compensation phase
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Phases of Injection Molding 3
Figure 13ure 1 3 Phases of injection molding detail
112 The Filling Phase
A two-color technique best demonstrates this phase After emptying the barrel of an injection-molding machine a small amount of red plastic was charged followed by green plastic
Consider the closed mold with the plastic front just starting to flow from the nozzle Theplastic first fills the sprue and runner system then enters the mold cavity itself forming a smallbubble of molten plastic
The skin of the plastic in contact with the cool mold freezes rapidly while the central coreremains molten When additional material is injected it flows into this central core displacing
the material already there which then forms a new flow front The flow of this displacedmaterial is a combination of forward flow and outward flow The outward flow contacts the wall freezes and forms the next section of skin while the forward flow forms the new moltencore When more material enters the mold it flows along a channel lined with these frozen walls of plastic illustrated in Figure 14
This flow pattern is often called fountain flow or bubble flow because the flow front is like abubble being inflated with hot plastic from the center The frozen layer is formed by the flow front inflating and so is subject to only a low shear stress and therefore has a very low levelof molecular orientation Once it is frozen it cannot be orientated any further so the frozen
layer in the finished part has a low level of orientation
Figure 14ure 1 4 Fountain flow and heat transfer
Filling Pressurization Compensation
Hot plastic
Frozen layer
Heat loss to mold
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4 Polymer Flow Behavior in Injection Molds
Now consider what happens upstream Hot plastic is continuously flowing bringing new hotmaterial along and generating significant frictional heat At the same time heat is being lost
through the frozen layer to the cold mold surfaceInitially the frozen layer is very thin so heat is lost very rapidly This results in more plasticfreezing and the frozen layer getting thicker cutting down the heat flow After a time thefrozen layer will reach a thickness such that the heat lost by conduction is equal to the heatinput from plastic flow and frictional heating ie an equilibrium condition is reached(Figure 14)
It is interesting to do some calculations on the time taken to reach this state of equilibrium The actual rate of heat flow is very large in comparison with the small heat content of theplastic in the frozen layer The result is that equilibrium is reached very quickly often in a timemeasured in a few tenths of a second As the total filling time is measured in seconds thefrozen layer reaches an equilibrium state early in the filling cycle
It is useful to think about how the thickness of this frozen layer will vary If the injection rate were slowed less heat would be generated by friction along the flow path with less heat inputfrom the flow The heat loss would be at the same rate and with less heat input the frozenlayer would grow in thickness If the injection rate were raised the frozen layer would bethinner (Figure 15) Similarly higher melt and mold temperatures would reduce the thicknessof the frozen layer This can be seen experimentally using the two-color technique
Figure 1igure 1 5 Influence of injection rate on frozen layer thickness
1121 Flow Shear Stress
It is easy to get confused between the various stress levels and orientation of the polymer Asthe plastic flows it is subject to shear stress also called flow shear stress This flow shear stress willorient the material ie cause the molecules to align themselves in the general direction of flow
The shear stress varies from a maximum at the outside dropping off to zero at the center
3Shear stress is purely a function of force and area This must not be confused withshear rate which is the rate of plastic sliding over the next layer Shear rate is zero
= Frozen
= Molten cooler
= Molten hotter
Slowerinjection
rate
Faster
injection
rate
vs
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Phases of Injection Molding 5
at the outer edge where the plastic is frozen rises to a maximum just inwards of thefrozen layer then drops toward the center as shown in Figure 16
Figure 16ure 1 6 Shear rate distribution
If the flow were stopped and the plastic allowed to cool down very slowly this orientation would have time to relax giving a very low level of residual orientation On the other hand if the material were kept under stress and the plastic snap frozen most of the orientation would
be trapped in the frozen plastic (Figure 17)
Figure 17ure 1 7 Molecular orientation through the thickness of the part
Min MaxShear rate
= Tension
= Compression
Molecular orientation trapped
in pressurization phase
Gradual reduction of orientation
toward the center of the
mold as stress levels are lower
and the cooling rate is slower
which allows more time for
orientation to relax
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6 Polymer Flow Behavior in Injection Molds
Now consider the orientation from the mold surface toward the center
The frozen layer itself formed with very little shear and therefore low orientation immediately
freezes setting the low level of orientation
The layer of plastic just on the inside of the frozen layer is subject to maximum shear stressand freezes the instant flow stops trapping almost all the orientation
This is the orientation pattern the further toward the center the more the shear stress dropsand the slower the rate of cooling This allows more time for the level of orientation to relaxso the residual orientation drops rapidly toward the center Consider how this pattern willaffect the residual stress level Oriented material (normally) will shrink more than nonorientedmaterial On the inner surface of the original frozen layer highly oriented material wants toshrink a great deal but it is prevented from doing so by the less-oriented material The highly
oriented layer ends up being in tension while the less-oriented material is in compression This residual stress pattern is a common cause of part warpage
3 There is a connectionthrough orientationbetween the shear stress during
filling (flow stress) and the residual stress in the final molded part This meansshear stress during filling shown on Moldflow plots can be used as a designparameter
113 The Pressurization Phase
The pressurization phasefrom the point of view of flow behavioris very similar to thefilling phase The flow rate may drop somewhat as the mold builds up to pressure resulting inan increase in the thickness of the frozen layer
The main difference of course is the increase in hydrostatic (isotropic) pressure We shall seein chapter 2 section 24 Effect of Molding Conditions that hydrostatic pressure in itself doesnot cause any residual stress
114 The Compensation Phase
Compensating flow is unstable Consider the plate molding again (see Figure 11) You wouldthink that plastic flowing uniformly through the thin diaphragm would top up the thick rim Inpractice the plastic during the compensation phase flows in rivers that spread out like a deltaas illustrated in Figure 18 This may seem surprising at first but it can be explained by temperature instability
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How Do Plastics Flow 7
Figure 18ure 1 8 River flow
1141 Temperature Variation
There is always some variation in melt temperature coming from the barrel of the injectionmachine In exceptional cases up to 40 ordmC variation has been measured using a high-speedthermocouple
1142 Natural Instability
However slight the temperature variation natural instability will amplify it If for exampleone part of the melt is slightly hotter than the rest then the plastic flow in that area will beslightly greater bringing hotter material into the area and maintaining the temperature If onthe other hand there is another area that is cooler the flow will be less so there will be lessheat input and the plastic will get colder until it eventually freezes off
However balanced the initial conditions this natural instability will result in a river-type flow
This is a very important consideration The first material to freeze off will shrink early in thecycle By the time the material in the river flows freezes the bulk of the material will havealready frozen off and shrinkage will have occurred The rivers will shrink relative to the bulk of the molding and because they are highly orientated shrinkage will be very high The resultis high-stress tensile members throughout the molding a common cause of warpage
1143 Optimum Part Quality
Most of the stress in plastic parts occurs during the compensation phase By controlling flow and minimizing stress it is possible to design for optimum part quality This important point is
at the heart of the Moldflow philosophy
12 How Do Plastics Flow
121 Material Behavior
Molten thermoplastics exhibit viscoelastic behavior which combines flow characteristics of both viscous liquids and elastic solids When a viscous liquid flows the energy that causes thedeformation is dissipated and becomes viscous heat On the other hand when an elastic solid
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8 Polymer Flow Behavior in Injection Molds
is deformed the driving energy is stored For example the flow of water is a typical viscousflow whereas the deformation of a rubber cube falls into the elastic category
122 Deformation
In addition to the two types of material flow behavior there are two types of deformationsimple shear and simple extension (elongation) as shown in Figure 19 (a) and (b) below Theflow of molten thermoplastics during injection-molding filling is predominantly shear flow asshown in Figure 19 (c) in which layers of material elements slide over each other Theextensional flow however becomes significant as the material elements undergo elongation when the melt passes areas of abrupt dimensional change (eg a gate region) as shown inFigure 19 (d)
Figure 1igure 1 9 (a) Simple shear flow (b) Simple extensional flow (c) Shear flow in cavity filling (d) Extensional flow in cavity filling
123 Viscoelastic Behavior
In response to an applied stress (force per unit area) molten thermoplastics exhibit viscoelastic behavior which combines characteristics of an ideal viscous liquid with those of an ideal elastic solid In other words under certain conditions molten thermoplastics behavelike a liquid and will continuously deform while shear stress is applied as shown in
Figure 110 Upon the removal of the stress however the materials behave somewhat like anelastic solid with partial recovery of the deformation as shown in Figure 110 (b) and (c) This viscoelastic behavior stems from the random-coil configuration of polymer molecules in the
Stress
Stress Stress
Mold wall
Material element before
deformation begins
Material element during
deformation
Mold wall
Material elements at rest
Moving
materialelements
Velocity
profile
(a) (b)
(c) (d)
Velocity
profile
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How Do Plastics Flow 9
molten state which allows the movement and slippage of molecular chains under theinfluence of an applied load However the entanglement of the polymer molecular chains also
makes the system behave like an elastic solid upon the application and removal of the externalload Namely on removal of the stress chains will tend to return to the equilibrium random-coil state and thus will be a component of stress recovery The recovery is not instantaneousbecause of the entanglements still present in the system
Figure 1ure 1 10 (a) Ideal viscous liquid deforms continuously under applied stress (b) Ideal elasticsolid deforms immediately upon the application of stress but fully recovers when the
stress is removed (c) Molten thermoplastic deforms continuously under the applied
stress (like a viscous liquid) but also recovers partially from the deformation upon
removal of the applied stress (like an elastic solid)
124 Melt Shear Viscosity
1241 What Is Shear Viscosity
Melt shear viscosity is a materials resistance to shear flow In general polymer melts are highly viscous because of their long molecular chain structure The viscosity of a polymer melt rangesfrom 2 to 3000 Pas (water 10^-1 Pas glass 10^20 Pas) Viscosity can be thought of as thethickness of a fluid or how much it resists flow Viscosity is expressed as the ratio of shearstress (force per unit area) to the shear rate (rate change of shear strain) as shown inEquation 11 and Figure 111
(a) Viscous
liquid
(b) Elastic
solid
(c) Viscoelastic
polymer
melt
Initial shape
(shear stress
to be applied)
Time = Dt Time = 2 x Dt
(shear stress to
be removed)
Final deformed
shape
d
d
d
d
2d
2d
2d
2d
total
recovery
partial
recovery
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VIII Preface
literature and the ever-growing number of CAE software users in the injection-molding industry
CAE Predicts Process Behavior
Ideally CAE analysis provides insight that is useful in designing parts molds and molding processes Without it we rely on previous experience intuition prototyping or molding trialsto obtain information such as polymer melt filling patterns weld-line and air-trap locationsrequired injection pressure and clamp tonnage fiber orientation cycle time final part shapeand deformation and mechanical properties of molded parts just to name a few WithoutCAE analysis other equally important design data such as spatial distributions of pressuretemperature shear rate shear stress and velocity are more difficult to obtain even with a well-instrumented mold The process behavior predicted by CAE can help novice engineers
overcome the lack of previous experience and assist experienced engineers in pinpointing factors that may otherwise be overlooked By using CAE analysis to iterate and evaluatealternative designs and competing materials engineering know-how in the form of designguidelines can be established relatively faster and more cost-effectively
User Proficiency Determines the Benefits of CAE
While CAE technology helps save time money and raw material as well as cuts scrap reducesthe rejection rate improves product quality and gets new products to market faster it is by nomeans a panacea for solving all molding problems Rather it should be recognized that CAEanalysis is essentially a tool designed to assist engineers instead of taking over theirresponsibilities or replacing them Like many other tools the usefulness of CAE technology depends on the proficiency of the user The benefits mentioned above will not be realizedunless the CAE tool is used properly To be more specific the accuracy of CAE analysisdepends greatly on the input data provided by the user In addition the results generated by CAE analysis need to be correctly and intelligently interpreted by the user before soundjudgments and rational decisions are made Otherwise users will simply be swamped by the vast amount of data without getting any useful information
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Acknowledgements IX
Acknowledgements
The Moldflow Design Guide would not have been accomplished were it not for the vision of Ken Welch Ken and I have discussed the value of assembling the best of the Moldflow Design
Principles Warpage Design Principles and the C-MOLD Design Guide into a single book for severalyears With Kens leadership he gave the project to Steve Thompsons training group of which I am a part Steve helped me coordinate the resources necessary to get this projectdone I could not have done this project without Steves help and guidance
A review of the content was part of the development of the Moldflow Design Guide Moldflow developers including Peter Kennedy Rong Zheng Zhongshuang Yuan and Xiaoshi Jin havereviewed sections of the book Moldflows application engineers and other technical staff withMoldflow have also reviewed sections These reviewers include Chad Fuhrman Matt JaworskiChristine Roedlich Eric Henry Olivier Anninos Paul Larter and Ana Maria Marin A specialthanks goes to Mike Rogers who reviewed the entire book for me and provided criticalfeedback on the content and organization of the book I would also like to thank Kurt Haydenof Western Michigan University for reviewing the appendix on process control His many years of experience of process setup and optimization was invaluable
Finally I would like to thank members of Moldflows Technology Transfer group forsupporting me in the writing of this book These members include Marcia Swan CarolineDorin Robert Ashley Melissa Haslam Darren Seamons and Gregory Brown
On a personal note I would like to acknowledge and thank Paul Engelmann Professor and
Department Chair Western Michigan University Department of Industrial and Manufacturing Engineering for being my friend and mentor during my career With Paul I have been able toteach and participate in research he has done on injection molding tooling and processing at Western Michigan University I have found working with Paul has made me a better Moldflow user and engineer by providing another perspective on how Moldflow can be used to solveinjection molding problemsJay Shoemaker Editor
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Contents
1 Polymer Flow Behavior in Injection Molds 1
11 Phases of Injection Molding 1111 How Plastic Fills a Mold 1112 The Filling Phase 3113 The Pressurization Phase 6114 The Compensation Phase 612 How Do Plastics Flow 7121 Material Behavior 7122 Deformation 8
123 Viscoelastic Behavior 8124 Melt Shear Viscosity 9125 Newtonian Fluid vs Non-Newtonian Fluid 10126 Shear-thinning Behavior 10127 Shear Rate Distribution 11128 Pressure-driven Flow 11129 Pressure Gradient and Injection Times 131210 Melt Flow Length 131211 Injection Pressure vs Fill Time 131212 Flow Instability 15
2 Molding Conditions and Injection Pressure 17
21 Injection-pressure Overview 17211 Pressure Drives the Flow Front 1822 Factors Influencing Injection-pressure Requirements 1823 Equations 20231 Circular Channel Flow 20232 Strip Channel Flow 2024 Effect of Molding Conditions 21
241 Part Quality 21242 Melt Temperature 22243 Mold Temperature 22244 Fill Time 23245 Shear Stress Variation 23246 Packing Pressure and Time 24247 Summary 25248 Back Flow 2625 Using Moldflow to Determine Optimum Molding Conditions 26251 Part 27
252 Molding Window Size 27253 Injection Pressure 28254 Flow Front Temperature 29
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XII Contents
255 Cooling Time 30256 Summary 31
3 Filling Pattern 33
31 Filling Pattern Overview 33311 What Is the Filling Pattern 3332 Flow in Complex Molds 34321 Overpack 34322 Racetrack Effect 35323 Varying Injection Rate 35324 Underflow Effect 36325 Hesitation Effect 37326 Weld Lines 38327 Meld Lines 38328 Sink Marks 38329 Multidirectional Flow 393210 Unstable Flow 403211 Simple Flow Pattern 4133 Flow-front Velocity and Flow-front Area 41331 What are FFV and FFA 41332 Flow-front Velocity Influences Filling Pattern 41
333 Equation 4234 Using Moldflow to Determine the Filling Pattern 43341 Computer Simulation Can Eliminate Molding Trials 43342 Using a Flow Analysis 4335 Using Moldflow to Achieve Constant FFV 44351 Controlling the FFV Through Ram Speed 44
4 Moldflow Design Principles 47
41 Product Design and Moldflow 4742 Sequence of Analysis 47421 Part Filling Optimization 48422 Molding Conditions 48423 Runner Design 48424 Cooling Optimization 49425 Packing Optimization 49426 Warpage Optimization 4943 Moldflow Flow Concepts 49431 Unidirectional and Controlled Flow Pattern 50432 Flow Balancing 51
433 Constant Pressure Gradient 52434 Maximum Shear Stress 53435 Uniform Cooling 54
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Contents XIII
436 Positioning Weld and Meld Lines 55437 Avoid Hesitation Effects 55
438 Avoid Underflow 55439 Balancing with Flow Leaders and Flow Deflectors 574310 Controlled Frictional Heat 584311 Thermal Shutoff of Runners 584312 Acceptable RunnerCavity Ratio 59
5 Meshes Used In Moldf low Analyses 61
51 Mesh Types Used by Moldflow 61511 Finite Elements Used in Moldflow 61512 Mesh Types 62513 Solver Assumptions 6352 Mesh Requirements 64521 Mesh Density Considerations 64522 Part Details 6753 Geometry Creation 6854 Importing Geometry 6955 Using Different Mesh Types 69551 Door Panel 69552 Manifold 70
6 Product Design 71
61 Material Properties for Product Design 71611 Plastics Are Sensitive to Operating Conditions 71612 Stress-Strain Behavior 72613 Creep and Stress Relaxation 77614 Fatigue 79615 Impact strength 80616 Thermal Mechanical Behavior 8062 Design for Strength 82621 Predicting Part Strength 82622 LoadingOperating Conditions 8263 Part Thickness 86631 Part Thickness Drives Quality and Cost 86632 Cycle Time Increases with Thickness 86633 Thick Parts Tend to Warp 86634 Thin Uniform Parts Improve Surface Quality 87635 Reducing Part Thickness 8764 Boosting Structural Integrity with Ribs 88
641 Structural Integrity the Goal of Every Design 88642 Designing Ribs 88
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XIV Contents
65 Design for Assembly 90651 Molding One Part vs Separate Components 90
652 Tolerances Fit between Parts 90653 Press-fit Joints 91654 Snap-fit Joints 93655 Cantilever Snap Joints 95656 Torsion Snap-fit Joints 96657 Fasteners 98658 Inserts 101659 Welding Processes 101
7 Gate Design 103
71 Gate Design Overview 103711 What Is a Gate 103712 Single vs Multiple Gates 103713 Gate Dimensions 103714 Gate Location 10472 Gate Types 104721 Manually Trimmed Gates 104722 Automatically Trimmed Gates 11073 Design Rules 113
731 Determining the Number of Gates 113732 Flow Patterns 115733 Gate Position 116734 Avoiding Common Problems 122735 Gate Length 122736 Gate Thickness 122737 Freeze-off Time 12374 Using Moldflow for Gate Design 123741 Gate Location 123742 Molding Window Size for the Three Gate Locations 124
743 Filling Pattern 125744 Gate Size Based on Shear Rate 125
8 Runner System Design 127
81 Definitions 127811 Feed System 127812 Runner System 127813 Cold Runner 127814 Hot Runner 128
815 Hot Manifold 128816 Hot Drop 128817 Sprue 128
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Contents XV
82 Runner System Design Principles 129821 Benefits of Good Runner Design 129
822 Runner Design Philosophy 129823 Flow Balancing 130824 Flow Control 130825 Frictional Heating in Runners 132826 Thermal Shutoff 132827 System and Runner Pressures 132828 Constant Pressure Gradient 133829 Cold Slug Wells 1338210 Easy Ejection 13483 Runner Types 134
831 Cold Runners 134832 Hot Runner Systems 13584 Runner Layout 136841 Determining the Number of Cavities 136842 Planning the Runner System Layout 138843 Partially Balanced Runners 139844 Geometrically Balanced Runners 14185 Initial Runner Sizing 141851 Determining Sprue Dimensions 141852 Designing Runner Cross Sections 142
853 Determining Runner Diameters 14586 Runner Balancing 147861 How Runner Balancing Works 147862 When Are the Runner Sizes Optimized 147863 Validating the Balance 147864 Processing Window 14887 Using Moldflow for Runner Balancing 149871 Runner Balancing a 48-cavity Tool 149872 Runner Balancing for a Family Mold 151873 Runner Balancing for a Multigated Part 152
9 Cooling System Design 153
91 Mold Cooling System Overview 153911 Importance of Cooling System Design 153912 Mold Cooling System Components 15492 Cooling-channel Configuration 156921 Types of Cooling Channels 15693 Alternative Cooling Devices 157931 What Do They Do 157
932 Baffles 158933 Bubblers 159934 Thermal Pins 159
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XVI Contents
935 Cooling Slender Cores 160936 Cooling Large Cores 161
937 Cooling Cylinder Cores 16294 Cooling System Equations 162941 Cooling Time 162942 Reynolds Number and Coolant Flow 16395 Design Rules 163951 Mold Cooling Design Considerations 163952 Location and Size of Channels 164953 Flow Rate and Heat Transfer 16696 Using Moldflow for Cooling System Design 167961 Example Setup 168
962 Cycle Time Determined by Design and Processing Parameters 169
10 Shrinkage and Warpage 173
101 Injection Molding and Shrinkage 1731011 What Are Shrinkage and Warpage 1731012 Shrinkage and Machine Settings 1731013 Mold Filling and Packing 1741014 How Pressure and Time Affect Shrinkage 1751015 Thermally Unstable Flow 176
102 Basic Causes of Shrinkage and Warpage 1771021 Causes of Shrinkage 1771022 Causes of Warpage 1811023 Relating Orientation and Area Shrinkage to Warpage 182103 Designing Accurate Parts Considering Warpage 1831031 Material Selection 1841032 Wall Thickness Variation 1841033 Gate Position and Runner Dimensions 1851034 Molding Conditions 1871035 Cooling Line Layout 190
11 Moldflow Design Procedure 191
111 Determine Analysis Objectives 191112 Moldflow Analysis Steps Framework 1921121 The Whole Process 1921122 Optimize Fill 1921123 Balance and Size the Runners 1971124 Optimize Cooling 1991125 Optimize the Packing Profile 200
1126 Optimize Warpage 201113 Using Moldflow to Evaluate an Initial Design 2051131 Description of this Example 205
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Contents XVII
1132 Molding Window 2051133 Filling Analysis 206
1134 Gate and Runner Design 2061135 Cooling System Design 2071136 Packing Analysis 2091137 Warpage Analysis 209114 Using Moldflow to Optimize the Design 2101141 Determine the Cause of Warpage 2101142 Investigating Different Gate Locations 2101143 Validating the Best Gate Location 211
12 Part Defects 215
121 Air Traps 2151211 What Is an Air Trap 2151212 Problems Caused by Air Traps 2161213 Remedies 216122 Black Specks and Black Streaks 2171221 What Are Black Specks and Black Streaks 2171222 Causes of Black Specks and Black Streaks 2171223 Remedies 218123 Brittleness 219
1231 What Is Brittleness 2191232 Causes of Brittleness 2191233 Remedies 219124 Burn Marks 2201241 What Is a Burn Mark 2201242 Causes of Burn Marks 2211243 Remedies 222125 Delamination 2221251 What Is Delamination 2221252 Causes of Delamination 223
1253 Remedies 223126 Dimensional Variation 2241261 What Is Dimensional Variation 2241262 Causes of Dimensional Variation 2241263 Remedies 224127 Discoloration 2251271 What Is Discoloration 2251272 Causes of Discoloration 2251273 Remedies 226128 Fish Eyes 227
1281 What Are Fish Eyes 2271282 Causes of Fish Eyes 2271283 Remedies 227
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XVIII Contents
129 Flash 2281291 What Is Flash 228
1292 Causes of Flash 2281293 Remedies 2291210 Flow Marks 23012101 What Is A Flow Mark 23012102 Causes of Flow Marks 23012103 Remedies 2301211 Hesitation 23112111 What Is Hesitation 23112112 Problems Caused by Hesitation 23112113 Remedies 232
1212 Jetting 23212121 What Is Jetting 23212122 Problems Caused by Jetting 23312123 Remedies 2331213 Ripples 23512131 What Are Ripples 23512132 Cause of Ripples 23512133 Remedies 2361214 Short Shots 23612141 What Is a Short Shot 236
12142 Causes of Short Shots 23712143 Remedies 2371215 Silver Streaks 23812151 What Are Silver Streaks 23812152 Causes of Silver Streaks 23912153 Remedies 2391216 Sink Marks and Voids 24012161 What Are Sink Marks and Voids 24012162 Causes of Sink Marks and Voids 24012163 Remedies 2411217 Weld Lines and Meld Lines 24312171 What Are Weld Lines and Meld Lines 24312172 Problems Caused by Weld Lines 24312173 Strength of Weld Lines 24412174 Remedies 245
Appendix A Injection Molding 247
A1 Injection-molding Overview 247 A11 Process 247
A12 Applications 248 A2 Development of the Injection-molding Machine 248 A21 Benefits of the Reciprocating Screw 248
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Contents XIX
A3 Development of the Injection-molding Process 249 A4 Alternative Injection-molding Processes 249
A41 Co-injection (Sandwich) Molding 249 A42 Fusible Core Injection Molding 250 A43 Gas-assisted Injection Molding 251 A44 Injection-compression Molding 252 A45 Lamellar (Microlayer) Injection Molding 253 A46 Live-feed Injection Molding 253 A47 Low-pressure Injection Molding 254 A48 Push-pull Injection Molding 254 A49 Reactive Molding 255 A410 Structural Foam Injection Molding 256
A411 Thin-wall Molding 256
Appendix B Injection-molding Machine System and Operations 259
B1 Injection-molding Machine 259B11 Components 259B12 Machine Specification 260B13 Machine Function 260B14 Auxiliary Equipment 260B2 Machine Components 261
B21 Injection System 261B22 Mold System 263B23 Hydraulic System 264B24 Control System 264B25 Clamping System 264B3 Molded System 265B31 The Delivery System 265B4 Machine Operating Sequence 266B41 Process Cycle 266B5 Screw Operation 268
B51 Back Pressure 268B52 Injection Speed 268B53 Screw Rotation Speed 268B54 Cushion 269B6 Secondary Operations 269B61 Assembly 269B62 Decoration 269B63 Other Secondary Operations 270
Appendix C Injection-molding Process Control 271
C1 Importance of Process Conditions 271C11 Setting Machine Process Conditions 271
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XX Contents
Appendix D Plastic Materials 281
D1 What Are Plastics 281
D11 Polymerization Process 281D12 Structure of Polymers 281D13 Polymer Alloys and Blends 283D14 Polymer Composites 283D2 Classification of Plastics 283D21 Classes of Plastics 284D22 Structures and Properties of Plastics 284D3 Thermoplastics 285D31 Market Share Distribution of Thermoplastics 285D32 Structures and Properties of Thermoplastics 286D33 Amorphous Polymers 286D34 Semicrystalline Polymers 287D4 Thermosets 288D5 Properties and Applications of Thermoplastics 289D51 ABS 290D52 PA 12 291D53 PA 6 293D54 PA 66 294D55 PBT 296
D56 PC 297D57 PCABS 299D58 PCPBT 300D59 HDPE 301D510 LDPE 303D511 PEI 304D512 PET 305D513 PETG 306D514 PMMA 307D515 POM 309
D516 PP 310D517 PPEPPO 312D518 PS 313D519 PVC 314D520 SAN 316D521 Additives Fillers and Reinforcements 317D522 Modifying Polymer Properties 318D523 Low-aspect Fillers 319D524 High-aspect Fillers Fibers 319
Index 321
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1 Polymer Flow Behavior in Injection Molds
bull Phases of injection molding
bull How do plastics flow
11 Phases of Injection Molding
Any molder can prove that all the conditions and effects discussed in this chapter do indeedoccur during the injection molding process While this knowledge alone can somewhatimprove quality it is only with the use of Moldflow analysis during the initial design stage with the mold designed for the optimum filling pattern that these effects can be controlledand the full benefits obtained
Flow technology is concerned with the behavior of plastics during the mold filling process Aplastic parts properties depend on how the part is molded Two parts having identicaldimensions and made from the same material but molded under different conditions will havedifferent stress and shrinkage levels and will behave differently in the field meaning that they
are in practice two different parts The way the plastic flows into the mold is of paramount importance in determining the quality of the part The process of filling the mold can be distinctly analyzed with the ability to predictpressure temperature and stress
111 How Plastic Fills a Mold
This was investigated using a centrally gated mold shaped like a dinner plate with a thick rim
around the outside as shown in Figure 11 It was found that the injection molding processalthough complex could be divided into three phases (we use the word phase to avoidconfusion with injection stage as used with programmed injection)
Figure 1ure 1 1 Cross-section of disk mold used to investigate flow
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2 Polymer Flow Behavior in Injection Molds
1111 Filling Phase
As the ram moves forward it first moves at a steady speed as the plastic flows into the cavity
This is the filling phase This phase lasts until the mold is just filled See Figure 12 andFigure 13
1112 Pressurization Phase
The pressurization phase begins when the ram moves forward after the filling phase to bring the mold up to pressure When the mold is filled the ram will slow down but it still movesquite some distance because plastics are very compressible materials At injection molding pressure an extra 15 volume of material can be forced into the cavity See Figure 12 andFigure 13
Although fluids are usually assumed to be incompressible molten plastics have to beconsidered to be more like a gas The compressibility of plastics can be observed by blocking off the nozzle and attempting to purge the barrel The ram will jump forward when thepressure is applied but will spring back when the pressure is released
1113 Compensation Phase
After the pressurization phase the ram still does not stop completely continuing to creepforward for some time Plastics have a very large volumetric change of about 25 from themelt to the solid This can be seen in a short shot the difference in volume between themolding and the cavity is due to this volumetric change See Figure 12 and Figure 13
The ram moving forward to compensate for the volumetric change in the part is called thecompensation phase As the volumetric change is 25 and at the most only an extra 15 canbe injected in the pressurization phase there must always be some compensation phase
Figure 1igure 1 2 Phases of injection molding
Filling phase
Pressurization phase
Compensation phase
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Phases of Injection Molding 3
Figure 13ure 1 3 Phases of injection molding detail
112 The Filling Phase
A two-color technique best demonstrates this phase After emptying the barrel of an injection-molding machine a small amount of red plastic was charged followed by green plastic
Consider the closed mold with the plastic front just starting to flow from the nozzle Theplastic first fills the sprue and runner system then enters the mold cavity itself forming a smallbubble of molten plastic
The skin of the plastic in contact with the cool mold freezes rapidly while the central coreremains molten When additional material is injected it flows into this central core displacing
the material already there which then forms a new flow front The flow of this displacedmaterial is a combination of forward flow and outward flow The outward flow contacts the wall freezes and forms the next section of skin while the forward flow forms the new moltencore When more material enters the mold it flows along a channel lined with these frozen walls of plastic illustrated in Figure 14
This flow pattern is often called fountain flow or bubble flow because the flow front is like abubble being inflated with hot plastic from the center The frozen layer is formed by the flow front inflating and so is subject to only a low shear stress and therefore has a very low levelof molecular orientation Once it is frozen it cannot be orientated any further so the frozen
layer in the finished part has a low level of orientation
Figure 14ure 1 4 Fountain flow and heat transfer
Filling Pressurization Compensation
Hot plastic
Frozen layer
Heat loss to mold
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4 Polymer Flow Behavior in Injection Molds
Now consider what happens upstream Hot plastic is continuously flowing bringing new hotmaterial along and generating significant frictional heat At the same time heat is being lost
through the frozen layer to the cold mold surfaceInitially the frozen layer is very thin so heat is lost very rapidly This results in more plasticfreezing and the frozen layer getting thicker cutting down the heat flow After a time thefrozen layer will reach a thickness such that the heat lost by conduction is equal to the heatinput from plastic flow and frictional heating ie an equilibrium condition is reached(Figure 14)
It is interesting to do some calculations on the time taken to reach this state of equilibrium The actual rate of heat flow is very large in comparison with the small heat content of theplastic in the frozen layer The result is that equilibrium is reached very quickly often in a timemeasured in a few tenths of a second As the total filling time is measured in seconds thefrozen layer reaches an equilibrium state early in the filling cycle
It is useful to think about how the thickness of this frozen layer will vary If the injection rate were slowed less heat would be generated by friction along the flow path with less heat inputfrom the flow The heat loss would be at the same rate and with less heat input the frozenlayer would grow in thickness If the injection rate were raised the frozen layer would bethinner (Figure 15) Similarly higher melt and mold temperatures would reduce the thicknessof the frozen layer This can be seen experimentally using the two-color technique
Figure 1igure 1 5 Influence of injection rate on frozen layer thickness
1121 Flow Shear Stress
It is easy to get confused between the various stress levels and orientation of the polymer Asthe plastic flows it is subject to shear stress also called flow shear stress This flow shear stress willorient the material ie cause the molecules to align themselves in the general direction of flow
The shear stress varies from a maximum at the outside dropping off to zero at the center
3Shear stress is purely a function of force and area This must not be confused withshear rate which is the rate of plastic sliding over the next layer Shear rate is zero
= Frozen
= Molten cooler
= Molten hotter
Slowerinjection
rate
Faster
injection
rate
vs
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Phases of Injection Molding 5
at the outer edge where the plastic is frozen rises to a maximum just inwards of thefrozen layer then drops toward the center as shown in Figure 16
Figure 16ure 1 6 Shear rate distribution
If the flow were stopped and the plastic allowed to cool down very slowly this orientation would have time to relax giving a very low level of residual orientation On the other hand if the material were kept under stress and the plastic snap frozen most of the orientation would
be trapped in the frozen plastic (Figure 17)
Figure 17ure 1 7 Molecular orientation through the thickness of the part
Min MaxShear rate
= Tension
= Compression
Molecular orientation trapped
in pressurization phase
Gradual reduction of orientation
toward the center of the
mold as stress levels are lower
and the cooling rate is slower
which allows more time for
orientation to relax
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6 Polymer Flow Behavior in Injection Molds
Now consider the orientation from the mold surface toward the center
The frozen layer itself formed with very little shear and therefore low orientation immediately
freezes setting the low level of orientation
The layer of plastic just on the inside of the frozen layer is subject to maximum shear stressand freezes the instant flow stops trapping almost all the orientation
This is the orientation pattern the further toward the center the more the shear stress dropsand the slower the rate of cooling This allows more time for the level of orientation to relaxso the residual orientation drops rapidly toward the center Consider how this pattern willaffect the residual stress level Oriented material (normally) will shrink more than nonorientedmaterial On the inner surface of the original frozen layer highly oriented material wants toshrink a great deal but it is prevented from doing so by the less-oriented material The highly
oriented layer ends up being in tension while the less-oriented material is in compression This residual stress pattern is a common cause of part warpage
3 There is a connectionthrough orientationbetween the shear stress during
filling (flow stress) and the residual stress in the final molded part This meansshear stress during filling shown on Moldflow plots can be used as a designparameter
113 The Pressurization Phase
The pressurization phasefrom the point of view of flow behavioris very similar to thefilling phase The flow rate may drop somewhat as the mold builds up to pressure resulting inan increase in the thickness of the frozen layer
The main difference of course is the increase in hydrostatic (isotropic) pressure We shall seein chapter 2 section 24 Effect of Molding Conditions that hydrostatic pressure in itself doesnot cause any residual stress
114 The Compensation Phase
Compensating flow is unstable Consider the plate molding again (see Figure 11) You wouldthink that plastic flowing uniformly through the thin diaphragm would top up the thick rim Inpractice the plastic during the compensation phase flows in rivers that spread out like a deltaas illustrated in Figure 18 This may seem surprising at first but it can be explained by temperature instability
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How Do Plastics Flow 7
Figure 18ure 1 8 River flow
1141 Temperature Variation
There is always some variation in melt temperature coming from the barrel of the injectionmachine In exceptional cases up to 40 ordmC variation has been measured using a high-speedthermocouple
1142 Natural Instability
However slight the temperature variation natural instability will amplify it If for exampleone part of the melt is slightly hotter than the rest then the plastic flow in that area will beslightly greater bringing hotter material into the area and maintaining the temperature If onthe other hand there is another area that is cooler the flow will be less so there will be lessheat input and the plastic will get colder until it eventually freezes off
However balanced the initial conditions this natural instability will result in a river-type flow
This is a very important consideration The first material to freeze off will shrink early in thecycle By the time the material in the river flows freezes the bulk of the material will havealready frozen off and shrinkage will have occurred The rivers will shrink relative to the bulk of the molding and because they are highly orientated shrinkage will be very high The resultis high-stress tensile members throughout the molding a common cause of warpage
1143 Optimum Part Quality
Most of the stress in plastic parts occurs during the compensation phase By controlling flow and minimizing stress it is possible to design for optimum part quality This important point is
at the heart of the Moldflow philosophy
12 How Do Plastics Flow
121 Material Behavior
Molten thermoplastics exhibit viscoelastic behavior which combines flow characteristics of both viscous liquids and elastic solids When a viscous liquid flows the energy that causes thedeformation is dissipated and becomes viscous heat On the other hand when an elastic solid
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8 Polymer Flow Behavior in Injection Molds
is deformed the driving energy is stored For example the flow of water is a typical viscousflow whereas the deformation of a rubber cube falls into the elastic category
122 Deformation
In addition to the two types of material flow behavior there are two types of deformationsimple shear and simple extension (elongation) as shown in Figure 19 (a) and (b) below Theflow of molten thermoplastics during injection-molding filling is predominantly shear flow asshown in Figure 19 (c) in which layers of material elements slide over each other Theextensional flow however becomes significant as the material elements undergo elongation when the melt passes areas of abrupt dimensional change (eg a gate region) as shown inFigure 19 (d)
Figure 1igure 1 9 (a) Simple shear flow (b) Simple extensional flow (c) Shear flow in cavity filling (d) Extensional flow in cavity filling
123 Viscoelastic Behavior
In response to an applied stress (force per unit area) molten thermoplastics exhibit viscoelastic behavior which combines characteristics of an ideal viscous liquid with those of an ideal elastic solid In other words under certain conditions molten thermoplastics behavelike a liquid and will continuously deform while shear stress is applied as shown in
Figure 110 Upon the removal of the stress however the materials behave somewhat like anelastic solid with partial recovery of the deformation as shown in Figure 110 (b) and (c) This viscoelastic behavior stems from the random-coil configuration of polymer molecules in the
Stress
Stress Stress
Mold wall
Material element before
deformation begins
Material element during
deformation
Mold wall
Material elements at rest
Moving
materialelements
Velocity
profile
(a) (b)
(c) (d)
Velocity
profile
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How Do Plastics Flow 9
molten state which allows the movement and slippage of molecular chains under theinfluence of an applied load However the entanglement of the polymer molecular chains also
makes the system behave like an elastic solid upon the application and removal of the externalload Namely on removal of the stress chains will tend to return to the equilibrium random-coil state and thus will be a component of stress recovery The recovery is not instantaneousbecause of the entanglements still present in the system
Figure 1ure 1 10 (a) Ideal viscous liquid deforms continuously under applied stress (b) Ideal elasticsolid deforms immediately upon the application of stress but fully recovers when the
stress is removed (c) Molten thermoplastic deforms continuously under the applied
stress (like a viscous liquid) but also recovers partially from the deformation upon
removal of the applied stress (like an elastic solid)
124 Melt Shear Viscosity
1241 What Is Shear Viscosity
Melt shear viscosity is a materials resistance to shear flow In general polymer melts are highly viscous because of their long molecular chain structure The viscosity of a polymer melt rangesfrom 2 to 3000 Pas (water 10^-1 Pas glass 10^20 Pas) Viscosity can be thought of as thethickness of a fluid or how much it resists flow Viscosity is expressed as the ratio of shearstress (force per unit area) to the shear rate (rate change of shear strain) as shown inEquation 11 and Figure 111
(a) Viscous
liquid
(b) Elastic
solid
(c) Viscoelastic
polymer
melt
Initial shape
(shear stress
to be applied)
Time = Dt Time = 2 x Dt
(shear stress to
be removed)
Final deformed
shape
d
d
d
d
2d
2d
2d
2d
total
recovery
partial
recovery
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Acknowledgements IX
Acknowledgements
The Moldflow Design Guide would not have been accomplished were it not for the vision of Ken Welch Ken and I have discussed the value of assembling the best of the Moldflow Design
Principles Warpage Design Principles and the C-MOLD Design Guide into a single book for severalyears With Kens leadership he gave the project to Steve Thompsons training group of which I am a part Steve helped me coordinate the resources necessary to get this projectdone I could not have done this project without Steves help and guidance
A review of the content was part of the development of the Moldflow Design Guide Moldflow developers including Peter Kennedy Rong Zheng Zhongshuang Yuan and Xiaoshi Jin havereviewed sections of the book Moldflows application engineers and other technical staff withMoldflow have also reviewed sections These reviewers include Chad Fuhrman Matt JaworskiChristine Roedlich Eric Henry Olivier Anninos Paul Larter and Ana Maria Marin A specialthanks goes to Mike Rogers who reviewed the entire book for me and provided criticalfeedback on the content and organization of the book I would also like to thank Kurt Haydenof Western Michigan University for reviewing the appendix on process control His many years of experience of process setup and optimization was invaluable
Finally I would like to thank members of Moldflows Technology Transfer group forsupporting me in the writing of this book These members include Marcia Swan CarolineDorin Robert Ashley Melissa Haslam Darren Seamons and Gregory Brown
On a personal note I would like to acknowledge and thank Paul Engelmann Professor and
Department Chair Western Michigan University Department of Industrial and Manufacturing Engineering for being my friend and mentor during my career With Paul I have been able toteach and participate in research he has done on injection molding tooling and processing at Western Michigan University I have found working with Paul has made me a better Moldflow user and engineer by providing another perspective on how Moldflow can be used to solveinjection molding problemsJay Shoemaker Editor
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Contents
1 Polymer Flow Behavior in Injection Molds 1
11 Phases of Injection Molding 1111 How Plastic Fills a Mold 1112 The Filling Phase 3113 The Pressurization Phase 6114 The Compensation Phase 612 How Do Plastics Flow 7121 Material Behavior 7122 Deformation 8
123 Viscoelastic Behavior 8124 Melt Shear Viscosity 9125 Newtonian Fluid vs Non-Newtonian Fluid 10126 Shear-thinning Behavior 10127 Shear Rate Distribution 11128 Pressure-driven Flow 11129 Pressure Gradient and Injection Times 131210 Melt Flow Length 131211 Injection Pressure vs Fill Time 131212 Flow Instability 15
2 Molding Conditions and Injection Pressure 17
21 Injection-pressure Overview 17211 Pressure Drives the Flow Front 1822 Factors Influencing Injection-pressure Requirements 1823 Equations 20231 Circular Channel Flow 20232 Strip Channel Flow 2024 Effect of Molding Conditions 21
241 Part Quality 21242 Melt Temperature 22243 Mold Temperature 22244 Fill Time 23245 Shear Stress Variation 23246 Packing Pressure and Time 24247 Summary 25248 Back Flow 2625 Using Moldflow to Determine Optimum Molding Conditions 26251 Part 27
252 Molding Window Size 27253 Injection Pressure 28254 Flow Front Temperature 29
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XII Contents
255 Cooling Time 30256 Summary 31
3 Filling Pattern 33
31 Filling Pattern Overview 33311 What Is the Filling Pattern 3332 Flow in Complex Molds 34321 Overpack 34322 Racetrack Effect 35323 Varying Injection Rate 35324 Underflow Effect 36325 Hesitation Effect 37326 Weld Lines 38327 Meld Lines 38328 Sink Marks 38329 Multidirectional Flow 393210 Unstable Flow 403211 Simple Flow Pattern 4133 Flow-front Velocity and Flow-front Area 41331 What are FFV and FFA 41332 Flow-front Velocity Influences Filling Pattern 41
333 Equation 4234 Using Moldflow to Determine the Filling Pattern 43341 Computer Simulation Can Eliminate Molding Trials 43342 Using a Flow Analysis 4335 Using Moldflow to Achieve Constant FFV 44351 Controlling the FFV Through Ram Speed 44
4 Moldflow Design Principles 47
41 Product Design and Moldflow 4742 Sequence of Analysis 47421 Part Filling Optimization 48422 Molding Conditions 48423 Runner Design 48424 Cooling Optimization 49425 Packing Optimization 49426 Warpage Optimization 4943 Moldflow Flow Concepts 49431 Unidirectional and Controlled Flow Pattern 50432 Flow Balancing 51
433 Constant Pressure Gradient 52434 Maximum Shear Stress 53435 Uniform Cooling 54
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Contents XIII
436 Positioning Weld and Meld Lines 55437 Avoid Hesitation Effects 55
438 Avoid Underflow 55439 Balancing with Flow Leaders and Flow Deflectors 574310 Controlled Frictional Heat 584311 Thermal Shutoff of Runners 584312 Acceptable RunnerCavity Ratio 59
5 Meshes Used In Moldf low Analyses 61
51 Mesh Types Used by Moldflow 61511 Finite Elements Used in Moldflow 61512 Mesh Types 62513 Solver Assumptions 6352 Mesh Requirements 64521 Mesh Density Considerations 64522 Part Details 6753 Geometry Creation 6854 Importing Geometry 6955 Using Different Mesh Types 69551 Door Panel 69552 Manifold 70
6 Product Design 71
61 Material Properties for Product Design 71611 Plastics Are Sensitive to Operating Conditions 71612 Stress-Strain Behavior 72613 Creep and Stress Relaxation 77614 Fatigue 79615 Impact strength 80616 Thermal Mechanical Behavior 8062 Design for Strength 82621 Predicting Part Strength 82622 LoadingOperating Conditions 8263 Part Thickness 86631 Part Thickness Drives Quality and Cost 86632 Cycle Time Increases with Thickness 86633 Thick Parts Tend to Warp 86634 Thin Uniform Parts Improve Surface Quality 87635 Reducing Part Thickness 8764 Boosting Structural Integrity with Ribs 88
641 Structural Integrity the Goal of Every Design 88642 Designing Ribs 88
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XIV Contents
65 Design for Assembly 90651 Molding One Part vs Separate Components 90
652 Tolerances Fit between Parts 90653 Press-fit Joints 91654 Snap-fit Joints 93655 Cantilever Snap Joints 95656 Torsion Snap-fit Joints 96657 Fasteners 98658 Inserts 101659 Welding Processes 101
7 Gate Design 103
71 Gate Design Overview 103711 What Is a Gate 103712 Single vs Multiple Gates 103713 Gate Dimensions 103714 Gate Location 10472 Gate Types 104721 Manually Trimmed Gates 104722 Automatically Trimmed Gates 11073 Design Rules 113
731 Determining the Number of Gates 113732 Flow Patterns 115733 Gate Position 116734 Avoiding Common Problems 122735 Gate Length 122736 Gate Thickness 122737 Freeze-off Time 12374 Using Moldflow for Gate Design 123741 Gate Location 123742 Molding Window Size for the Three Gate Locations 124
743 Filling Pattern 125744 Gate Size Based on Shear Rate 125
8 Runner System Design 127
81 Definitions 127811 Feed System 127812 Runner System 127813 Cold Runner 127814 Hot Runner 128
815 Hot Manifold 128816 Hot Drop 128817 Sprue 128
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Contents XV
82 Runner System Design Principles 129821 Benefits of Good Runner Design 129
822 Runner Design Philosophy 129823 Flow Balancing 130824 Flow Control 130825 Frictional Heating in Runners 132826 Thermal Shutoff 132827 System and Runner Pressures 132828 Constant Pressure Gradient 133829 Cold Slug Wells 1338210 Easy Ejection 13483 Runner Types 134
831 Cold Runners 134832 Hot Runner Systems 13584 Runner Layout 136841 Determining the Number of Cavities 136842 Planning the Runner System Layout 138843 Partially Balanced Runners 139844 Geometrically Balanced Runners 14185 Initial Runner Sizing 141851 Determining Sprue Dimensions 141852 Designing Runner Cross Sections 142
853 Determining Runner Diameters 14586 Runner Balancing 147861 How Runner Balancing Works 147862 When Are the Runner Sizes Optimized 147863 Validating the Balance 147864 Processing Window 14887 Using Moldflow for Runner Balancing 149871 Runner Balancing a 48-cavity Tool 149872 Runner Balancing for a Family Mold 151873 Runner Balancing for a Multigated Part 152
9 Cooling System Design 153
91 Mold Cooling System Overview 153911 Importance of Cooling System Design 153912 Mold Cooling System Components 15492 Cooling-channel Configuration 156921 Types of Cooling Channels 15693 Alternative Cooling Devices 157931 What Do They Do 157
932 Baffles 158933 Bubblers 159934 Thermal Pins 159
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XVI Contents
935 Cooling Slender Cores 160936 Cooling Large Cores 161
937 Cooling Cylinder Cores 16294 Cooling System Equations 162941 Cooling Time 162942 Reynolds Number and Coolant Flow 16395 Design Rules 163951 Mold Cooling Design Considerations 163952 Location and Size of Channels 164953 Flow Rate and Heat Transfer 16696 Using Moldflow for Cooling System Design 167961 Example Setup 168
962 Cycle Time Determined by Design and Processing Parameters 169
10 Shrinkage and Warpage 173
101 Injection Molding and Shrinkage 1731011 What Are Shrinkage and Warpage 1731012 Shrinkage and Machine Settings 1731013 Mold Filling and Packing 1741014 How Pressure and Time Affect Shrinkage 1751015 Thermally Unstable Flow 176
102 Basic Causes of Shrinkage and Warpage 1771021 Causes of Shrinkage 1771022 Causes of Warpage 1811023 Relating Orientation and Area Shrinkage to Warpage 182103 Designing Accurate Parts Considering Warpage 1831031 Material Selection 1841032 Wall Thickness Variation 1841033 Gate Position and Runner Dimensions 1851034 Molding Conditions 1871035 Cooling Line Layout 190
11 Moldflow Design Procedure 191
111 Determine Analysis Objectives 191112 Moldflow Analysis Steps Framework 1921121 The Whole Process 1921122 Optimize Fill 1921123 Balance and Size the Runners 1971124 Optimize Cooling 1991125 Optimize the Packing Profile 200
1126 Optimize Warpage 201113 Using Moldflow to Evaluate an Initial Design 2051131 Description of this Example 205
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Contents XVII
1132 Molding Window 2051133 Filling Analysis 206
1134 Gate and Runner Design 2061135 Cooling System Design 2071136 Packing Analysis 2091137 Warpage Analysis 209114 Using Moldflow to Optimize the Design 2101141 Determine the Cause of Warpage 2101142 Investigating Different Gate Locations 2101143 Validating the Best Gate Location 211
12 Part Defects 215
121 Air Traps 2151211 What Is an Air Trap 2151212 Problems Caused by Air Traps 2161213 Remedies 216122 Black Specks and Black Streaks 2171221 What Are Black Specks and Black Streaks 2171222 Causes of Black Specks and Black Streaks 2171223 Remedies 218123 Brittleness 219
1231 What Is Brittleness 2191232 Causes of Brittleness 2191233 Remedies 219124 Burn Marks 2201241 What Is a Burn Mark 2201242 Causes of Burn Marks 2211243 Remedies 222125 Delamination 2221251 What Is Delamination 2221252 Causes of Delamination 223
1253 Remedies 223126 Dimensional Variation 2241261 What Is Dimensional Variation 2241262 Causes of Dimensional Variation 2241263 Remedies 224127 Discoloration 2251271 What Is Discoloration 2251272 Causes of Discoloration 2251273 Remedies 226128 Fish Eyes 227
1281 What Are Fish Eyes 2271282 Causes of Fish Eyes 2271283 Remedies 227
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XVIII Contents
129 Flash 2281291 What Is Flash 228
1292 Causes of Flash 2281293 Remedies 2291210 Flow Marks 23012101 What Is A Flow Mark 23012102 Causes of Flow Marks 23012103 Remedies 2301211 Hesitation 23112111 What Is Hesitation 23112112 Problems Caused by Hesitation 23112113 Remedies 232
1212 Jetting 23212121 What Is Jetting 23212122 Problems Caused by Jetting 23312123 Remedies 2331213 Ripples 23512131 What Are Ripples 23512132 Cause of Ripples 23512133 Remedies 2361214 Short Shots 23612141 What Is a Short Shot 236
12142 Causes of Short Shots 23712143 Remedies 2371215 Silver Streaks 23812151 What Are Silver Streaks 23812152 Causes of Silver Streaks 23912153 Remedies 2391216 Sink Marks and Voids 24012161 What Are Sink Marks and Voids 24012162 Causes of Sink Marks and Voids 24012163 Remedies 2411217 Weld Lines and Meld Lines 24312171 What Are Weld Lines and Meld Lines 24312172 Problems Caused by Weld Lines 24312173 Strength of Weld Lines 24412174 Remedies 245
Appendix A Injection Molding 247
A1 Injection-molding Overview 247 A11 Process 247
A12 Applications 248 A2 Development of the Injection-molding Machine 248 A21 Benefits of the Reciprocating Screw 248
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Contents XIX
A3 Development of the Injection-molding Process 249 A4 Alternative Injection-molding Processes 249
A41 Co-injection (Sandwich) Molding 249 A42 Fusible Core Injection Molding 250 A43 Gas-assisted Injection Molding 251 A44 Injection-compression Molding 252 A45 Lamellar (Microlayer) Injection Molding 253 A46 Live-feed Injection Molding 253 A47 Low-pressure Injection Molding 254 A48 Push-pull Injection Molding 254 A49 Reactive Molding 255 A410 Structural Foam Injection Molding 256
A411 Thin-wall Molding 256
Appendix B Injection-molding Machine System and Operations 259
B1 Injection-molding Machine 259B11 Components 259B12 Machine Specification 260B13 Machine Function 260B14 Auxiliary Equipment 260B2 Machine Components 261
B21 Injection System 261B22 Mold System 263B23 Hydraulic System 264B24 Control System 264B25 Clamping System 264B3 Molded System 265B31 The Delivery System 265B4 Machine Operating Sequence 266B41 Process Cycle 266B5 Screw Operation 268
B51 Back Pressure 268B52 Injection Speed 268B53 Screw Rotation Speed 268B54 Cushion 269B6 Secondary Operations 269B61 Assembly 269B62 Decoration 269B63 Other Secondary Operations 270
Appendix C Injection-molding Process Control 271
C1 Importance of Process Conditions 271C11 Setting Machine Process Conditions 271
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XX Contents
Appendix D Plastic Materials 281
D1 What Are Plastics 281
D11 Polymerization Process 281D12 Structure of Polymers 281D13 Polymer Alloys and Blends 283D14 Polymer Composites 283D2 Classification of Plastics 283D21 Classes of Plastics 284D22 Structures and Properties of Plastics 284D3 Thermoplastics 285D31 Market Share Distribution of Thermoplastics 285D32 Structures and Properties of Thermoplastics 286D33 Amorphous Polymers 286D34 Semicrystalline Polymers 287D4 Thermosets 288D5 Properties and Applications of Thermoplastics 289D51 ABS 290D52 PA 12 291D53 PA 6 293D54 PA 66 294D55 PBT 296
D56 PC 297D57 PCABS 299D58 PCPBT 300D59 HDPE 301D510 LDPE 303D511 PEI 304D512 PET 305D513 PETG 306D514 PMMA 307D515 POM 309
D516 PP 310D517 PPEPPO 312D518 PS 313D519 PVC 314D520 SAN 316D521 Additives Fillers and Reinforcements 317D522 Modifying Polymer Properties 318D523 Low-aspect Fillers 319D524 High-aspect Fillers Fibers 319
Index 321
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1 Polymer Flow Behavior in Injection Molds
bull Phases of injection molding
bull How do plastics flow
11 Phases of Injection Molding
Any molder can prove that all the conditions and effects discussed in this chapter do indeedoccur during the injection molding process While this knowledge alone can somewhatimprove quality it is only with the use of Moldflow analysis during the initial design stage with the mold designed for the optimum filling pattern that these effects can be controlledand the full benefits obtained
Flow technology is concerned with the behavior of plastics during the mold filling process Aplastic parts properties depend on how the part is molded Two parts having identicaldimensions and made from the same material but molded under different conditions will havedifferent stress and shrinkage levels and will behave differently in the field meaning that they
are in practice two different parts The way the plastic flows into the mold is of paramount importance in determining the quality of the part The process of filling the mold can be distinctly analyzed with the ability to predictpressure temperature and stress
111 How Plastic Fills a Mold
This was investigated using a centrally gated mold shaped like a dinner plate with a thick rim
around the outside as shown in Figure 11 It was found that the injection molding processalthough complex could be divided into three phases (we use the word phase to avoidconfusion with injection stage as used with programmed injection)
Figure 1ure 1 1 Cross-section of disk mold used to investigate flow
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2 Polymer Flow Behavior in Injection Molds
1111 Filling Phase
As the ram moves forward it first moves at a steady speed as the plastic flows into the cavity
This is the filling phase This phase lasts until the mold is just filled See Figure 12 andFigure 13
1112 Pressurization Phase
The pressurization phase begins when the ram moves forward after the filling phase to bring the mold up to pressure When the mold is filled the ram will slow down but it still movesquite some distance because plastics are very compressible materials At injection molding pressure an extra 15 volume of material can be forced into the cavity See Figure 12 andFigure 13
Although fluids are usually assumed to be incompressible molten plastics have to beconsidered to be more like a gas The compressibility of plastics can be observed by blocking off the nozzle and attempting to purge the barrel The ram will jump forward when thepressure is applied but will spring back when the pressure is released
1113 Compensation Phase
After the pressurization phase the ram still does not stop completely continuing to creepforward for some time Plastics have a very large volumetric change of about 25 from themelt to the solid This can be seen in a short shot the difference in volume between themolding and the cavity is due to this volumetric change See Figure 12 and Figure 13
The ram moving forward to compensate for the volumetric change in the part is called thecompensation phase As the volumetric change is 25 and at the most only an extra 15 canbe injected in the pressurization phase there must always be some compensation phase
Figure 1igure 1 2 Phases of injection molding
Filling phase
Pressurization phase
Compensation phase
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Phases of Injection Molding 3
Figure 13ure 1 3 Phases of injection molding detail
112 The Filling Phase
A two-color technique best demonstrates this phase After emptying the barrel of an injection-molding machine a small amount of red plastic was charged followed by green plastic
Consider the closed mold with the plastic front just starting to flow from the nozzle Theplastic first fills the sprue and runner system then enters the mold cavity itself forming a smallbubble of molten plastic
The skin of the plastic in contact with the cool mold freezes rapidly while the central coreremains molten When additional material is injected it flows into this central core displacing
the material already there which then forms a new flow front The flow of this displacedmaterial is a combination of forward flow and outward flow The outward flow contacts the wall freezes and forms the next section of skin while the forward flow forms the new moltencore When more material enters the mold it flows along a channel lined with these frozen walls of plastic illustrated in Figure 14
This flow pattern is often called fountain flow or bubble flow because the flow front is like abubble being inflated with hot plastic from the center The frozen layer is formed by the flow front inflating and so is subject to only a low shear stress and therefore has a very low levelof molecular orientation Once it is frozen it cannot be orientated any further so the frozen
layer in the finished part has a low level of orientation
Figure 14ure 1 4 Fountain flow and heat transfer
Filling Pressurization Compensation
Hot plastic
Frozen layer
Heat loss to mold
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4 Polymer Flow Behavior in Injection Molds
Now consider what happens upstream Hot plastic is continuously flowing bringing new hotmaterial along and generating significant frictional heat At the same time heat is being lost
through the frozen layer to the cold mold surfaceInitially the frozen layer is very thin so heat is lost very rapidly This results in more plasticfreezing and the frozen layer getting thicker cutting down the heat flow After a time thefrozen layer will reach a thickness such that the heat lost by conduction is equal to the heatinput from plastic flow and frictional heating ie an equilibrium condition is reached(Figure 14)
It is interesting to do some calculations on the time taken to reach this state of equilibrium The actual rate of heat flow is very large in comparison with the small heat content of theplastic in the frozen layer The result is that equilibrium is reached very quickly often in a timemeasured in a few tenths of a second As the total filling time is measured in seconds thefrozen layer reaches an equilibrium state early in the filling cycle
It is useful to think about how the thickness of this frozen layer will vary If the injection rate were slowed less heat would be generated by friction along the flow path with less heat inputfrom the flow The heat loss would be at the same rate and with less heat input the frozenlayer would grow in thickness If the injection rate were raised the frozen layer would bethinner (Figure 15) Similarly higher melt and mold temperatures would reduce the thicknessof the frozen layer This can be seen experimentally using the two-color technique
Figure 1igure 1 5 Influence of injection rate on frozen layer thickness
1121 Flow Shear Stress
It is easy to get confused between the various stress levels and orientation of the polymer Asthe plastic flows it is subject to shear stress also called flow shear stress This flow shear stress willorient the material ie cause the molecules to align themselves in the general direction of flow
The shear stress varies from a maximum at the outside dropping off to zero at the center
3Shear stress is purely a function of force and area This must not be confused withshear rate which is the rate of plastic sliding over the next layer Shear rate is zero
= Frozen
= Molten cooler
= Molten hotter
Slowerinjection
rate
Faster
injection
rate
vs
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Phases of Injection Molding 5
at the outer edge where the plastic is frozen rises to a maximum just inwards of thefrozen layer then drops toward the center as shown in Figure 16
Figure 16ure 1 6 Shear rate distribution
If the flow were stopped and the plastic allowed to cool down very slowly this orientation would have time to relax giving a very low level of residual orientation On the other hand if the material were kept under stress and the plastic snap frozen most of the orientation would
be trapped in the frozen plastic (Figure 17)
Figure 17ure 1 7 Molecular orientation through the thickness of the part
Min MaxShear rate
= Tension
= Compression
Molecular orientation trapped
in pressurization phase
Gradual reduction of orientation
toward the center of the
mold as stress levels are lower
and the cooling rate is slower
which allows more time for
orientation to relax
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6 Polymer Flow Behavior in Injection Molds
Now consider the orientation from the mold surface toward the center
The frozen layer itself formed with very little shear and therefore low orientation immediately
freezes setting the low level of orientation
The layer of plastic just on the inside of the frozen layer is subject to maximum shear stressand freezes the instant flow stops trapping almost all the orientation
This is the orientation pattern the further toward the center the more the shear stress dropsand the slower the rate of cooling This allows more time for the level of orientation to relaxso the residual orientation drops rapidly toward the center Consider how this pattern willaffect the residual stress level Oriented material (normally) will shrink more than nonorientedmaterial On the inner surface of the original frozen layer highly oriented material wants toshrink a great deal but it is prevented from doing so by the less-oriented material The highly
oriented layer ends up being in tension while the less-oriented material is in compression This residual stress pattern is a common cause of part warpage
3 There is a connectionthrough orientationbetween the shear stress during
filling (flow stress) and the residual stress in the final molded part This meansshear stress during filling shown on Moldflow plots can be used as a designparameter
113 The Pressurization Phase
The pressurization phasefrom the point of view of flow behavioris very similar to thefilling phase The flow rate may drop somewhat as the mold builds up to pressure resulting inan increase in the thickness of the frozen layer
The main difference of course is the increase in hydrostatic (isotropic) pressure We shall seein chapter 2 section 24 Effect of Molding Conditions that hydrostatic pressure in itself doesnot cause any residual stress
114 The Compensation Phase
Compensating flow is unstable Consider the plate molding again (see Figure 11) You wouldthink that plastic flowing uniformly through the thin diaphragm would top up the thick rim Inpractice the plastic during the compensation phase flows in rivers that spread out like a deltaas illustrated in Figure 18 This may seem surprising at first but it can be explained by temperature instability
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How Do Plastics Flow 7
Figure 18ure 1 8 River flow
1141 Temperature Variation
There is always some variation in melt temperature coming from the barrel of the injectionmachine In exceptional cases up to 40 ordmC variation has been measured using a high-speedthermocouple
1142 Natural Instability
However slight the temperature variation natural instability will amplify it If for exampleone part of the melt is slightly hotter than the rest then the plastic flow in that area will beslightly greater bringing hotter material into the area and maintaining the temperature If onthe other hand there is another area that is cooler the flow will be less so there will be lessheat input and the plastic will get colder until it eventually freezes off
However balanced the initial conditions this natural instability will result in a river-type flow
This is a very important consideration The first material to freeze off will shrink early in thecycle By the time the material in the river flows freezes the bulk of the material will havealready frozen off and shrinkage will have occurred The rivers will shrink relative to the bulk of the molding and because they are highly orientated shrinkage will be very high The resultis high-stress tensile members throughout the molding a common cause of warpage
1143 Optimum Part Quality
Most of the stress in plastic parts occurs during the compensation phase By controlling flow and minimizing stress it is possible to design for optimum part quality This important point is
at the heart of the Moldflow philosophy
12 How Do Plastics Flow
121 Material Behavior
Molten thermoplastics exhibit viscoelastic behavior which combines flow characteristics of both viscous liquids and elastic solids When a viscous liquid flows the energy that causes thedeformation is dissipated and becomes viscous heat On the other hand when an elastic solid
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8 Polymer Flow Behavior in Injection Molds
is deformed the driving energy is stored For example the flow of water is a typical viscousflow whereas the deformation of a rubber cube falls into the elastic category
122 Deformation
In addition to the two types of material flow behavior there are two types of deformationsimple shear and simple extension (elongation) as shown in Figure 19 (a) and (b) below Theflow of molten thermoplastics during injection-molding filling is predominantly shear flow asshown in Figure 19 (c) in which layers of material elements slide over each other Theextensional flow however becomes significant as the material elements undergo elongation when the melt passes areas of abrupt dimensional change (eg a gate region) as shown inFigure 19 (d)
Figure 1igure 1 9 (a) Simple shear flow (b) Simple extensional flow (c) Shear flow in cavity filling (d) Extensional flow in cavity filling
123 Viscoelastic Behavior
In response to an applied stress (force per unit area) molten thermoplastics exhibit viscoelastic behavior which combines characteristics of an ideal viscous liquid with those of an ideal elastic solid In other words under certain conditions molten thermoplastics behavelike a liquid and will continuously deform while shear stress is applied as shown in
Figure 110 Upon the removal of the stress however the materials behave somewhat like anelastic solid with partial recovery of the deformation as shown in Figure 110 (b) and (c) This viscoelastic behavior stems from the random-coil configuration of polymer molecules in the
Stress
Stress Stress
Mold wall
Material element before
deformation begins
Material element during
deformation
Mold wall
Material elements at rest
Moving
materialelements
Velocity
profile
(a) (b)
(c) (d)
Velocity
profile
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How Do Plastics Flow 9
molten state which allows the movement and slippage of molecular chains under theinfluence of an applied load However the entanglement of the polymer molecular chains also
makes the system behave like an elastic solid upon the application and removal of the externalload Namely on removal of the stress chains will tend to return to the equilibrium random-coil state and thus will be a component of stress recovery The recovery is not instantaneousbecause of the entanglements still present in the system
Figure 1ure 1 10 (a) Ideal viscous liquid deforms continuously under applied stress (b) Ideal elasticsolid deforms immediately upon the application of stress but fully recovers when the
stress is removed (c) Molten thermoplastic deforms continuously under the applied
stress (like a viscous liquid) but also recovers partially from the deformation upon
removal of the applied stress (like an elastic solid)
124 Melt Shear Viscosity
1241 What Is Shear Viscosity
Melt shear viscosity is a materials resistance to shear flow In general polymer melts are highly viscous because of their long molecular chain structure The viscosity of a polymer melt rangesfrom 2 to 3000 Pas (water 10^-1 Pas glass 10^20 Pas) Viscosity can be thought of as thethickness of a fluid or how much it resists flow Viscosity is expressed as the ratio of shearstress (force per unit area) to the shear rate (rate change of shear strain) as shown inEquation 11 and Figure 111
(a) Viscous
liquid
(b) Elastic
solid
(c) Viscoelastic
polymer
melt
Initial shape
(shear stress
to be applied)
Time = Dt Time = 2 x Dt
(shear stress to
be removed)
Final deformed
shape
d
d
d
d
2d
2d
2d
2d
total
recovery
partial
recovery
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Contents
1 Polymer Flow Behavior in Injection Molds 1
11 Phases of Injection Molding 1111 How Plastic Fills a Mold 1112 The Filling Phase 3113 The Pressurization Phase 6114 The Compensation Phase 612 How Do Plastics Flow 7121 Material Behavior 7122 Deformation 8
123 Viscoelastic Behavior 8124 Melt Shear Viscosity 9125 Newtonian Fluid vs Non-Newtonian Fluid 10126 Shear-thinning Behavior 10127 Shear Rate Distribution 11128 Pressure-driven Flow 11129 Pressure Gradient and Injection Times 131210 Melt Flow Length 131211 Injection Pressure vs Fill Time 131212 Flow Instability 15
2 Molding Conditions and Injection Pressure 17
21 Injection-pressure Overview 17211 Pressure Drives the Flow Front 1822 Factors Influencing Injection-pressure Requirements 1823 Equations 20231 Circular Channel Flow 20232 Strip Channel Flow 2024 Effect of Molding Conditions 21
241 Part Quality 21242 Melt Temperature 22243 Mold Temperature 22244 Fill Time 23245 Shear Stress Variation 23246 Packing Pressure and Time 24247 Summary 25248 Back Flow 2625 Using Moldflow to Determine Optimum Molding Conditions 26251 Part 27
252 Molding Window Size 27253 Injection Pressure 28254 Flow Front Temperature 29
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XII Contents
255 Cooling Time 30256 Summary 31
3 Filling Pattern 33
31 Filling Pattern Overview 33311 What Is the Filling Pattern 3332 Flow in Complex Molds 34321 Overpack 34322 Racetrack Effect 35323 Varying Injection Rate 35324 Underflow Effect 36325 Hesitation Effect 37326 Weld Lines 38327 Meld Lines 38328 Sink Marks 38329 Multidirectional Flow 393210 Unstable Flow 403211 Simple Flow Pattern 4133 Flow-front Velocity and Flow-front Area 41331 What are FFV and FFA 41332 Flow-front Velocity Influences Filling Pattern 41
333 Equation 4234 Using Moldflow to Determine the Filling Pattern 43341 Computer Simulation Can Eliminate Molding Trials 43342 Using a Flow Analysis 4335 Using Moldflow to Achieve Constant FFV 44351 Controlling the FFV Through Ram Speed 44
4 Moldflow Design Principles 47
41 Product Design and Moldflow 4742 Sequence of Analysis 47421 Part Filling Optimization 48422 Molding Conditions 48423 Runner Design 48424 Cooling Optimization 49425 Packing Optimization 49426 Warpage Optimization 4943 Moldflow Flow Concepts 49431 Unidirectional and Controlled Flow Pattern 50432 Flow Balancing 51
433 Constant Pressure Gradient 52434 Maximum Shear Stress 53435 Uniform Cooling 54
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Contents XIII
436 Positioning Weld and Meld Lines 55437 Avoid Hesitation Effects 55
438 Avoid Underflow 55439 Balancing with Flow Leaders and Flow Deflectors 574310 Controlled Frictional Heat 584311 Thermal Shutoff of Runners 584312 Acceptable RunnerCavity Ratio 59
5 Meshes Used In Moldf low Analyses 61
51 Mesh Types Used by Moldflow 61511 Finite Elements Used in Moldflow 61512 Mesh Types 62513 Solver Assumptions 6352 Mesh Requirements 64521 Mesh Density Considerations 64522 Part Details 6753 Geometry Creation 6854 Importing Geometry 6955 Using Different Mesh Types 69551 Door Panel 69552 Manifold 70
6 Product Design 71
61 Material Properties for Product Design 71611 Plastics Are Sensitive to Operating Conditions 71612 Stress-Strain Behavior 72613 Creep and Stress Relaxation 77614 Fatigue 79615 Impact strength 80616 Thermal Mechanical Behavior 8062 Design for Strength 82621 Predicting Part Strength 82622 LoadingOperating Conditions 8263 Part Thickness 86631 Part Thickness Drives Quality and Cost 86632 Cycle Time Increases with Thickness 86633 Thick Parts Tend to Warp 86634 Thin Uniform Parts Improve Surface Quality 87635 Reducing Part Thickness 8764 Boosting Structural Integrity with Ribs 88
641 Structural Integrity the Goal of Every Design 88642 Designing Ribs 88
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XIV Contents
65 Design for Assembly 90651 Molding One Part vs Separate Components 90
652 Tolerances Fit between Parts 90653 Press-fit Joints 91654 Snap-fit Joints 93655 Cantilever Snap Joints 95656 Torsion Snap-fit Joints 96657 Fasteners 98658 Inserts 101659 Welding Processes 101
7 Gate Design 103
71 Gate Design Overview 103711 What Is a Gate 103712 Single vs Multiple Gates 103713 Gate Dimensions 103714 Gate Location 10472 Gate Types 104721 Manually Trimmed Gates 104722 Automatically Trimmed Gates 11073 Design Rules 113
731 Determining the Number of Gates 113732 Flow Patterns 115733 Gate Position 116734 Avoiding Common Problems 122735 Gate Length 122736 Gate Thickness 122737 Freeze-off Time 12374 Using Moldflow for Gate Design 123741 Gate Location 123742 Molding Window Size for the Three Gate Locations 124
743 Filling Pattern 125744 Gate Size Based on Shear Rate 125
8 Runner System Design 127
81 Definitions 127811 Feed System 127812 Runner System 127813 Cold Runner 127814 Hot Runner 128
815 Hot Manifold 128816 Hot Drop 128817 Sprue 128
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Contents XV
82 Runner System Design Principles 129821 Benefits of Good Runner Design 129
822 Runner Design Philosophy 129823 Flow Balancing 130824 Flow Control 130825 Frictional Heating in Runners 132826 Thermal Shutoff 132827 System and Runner Pressures 132828 Constant Pressure Gradient 133829 Cold Slug Wells 1338210 Easy Ejection 13483 Runner Types 134
831 Cold Runners 134832 Hot Runner Systems 13584 Runner Layout 136841 Determining the Number of Cavities 136842 Planning the Runner System Layout 138843 Partially Balanced Runners 139844 Geometrically Balanced Runners 14185 Initial Runner Sizing 141851 Determining Sprue Dimensions 141852 Designing Runner Cross Sections 142
853 Determining Runner Diameters 14586 Runner Balancing 147861 How Runner Balancing Works 147862 When Are the Runner Sizes Optimized 147863 Validating the Balance 147864 Processing Window 14887 Using Moldflow for Runner Balancing 149871 Runner Balancing a 48-cavity Tool 149872 Runner Balancing for a Family Mold 151873 Runner Balancing for a Multigated Part 152
9 Cooling System Design 153
91 Mold Cooling System Overview 153911 Importance of Cooling System Design 153912 Mold Cooling System Components 15492 Cooling-channel Configuration 156921 Types of Cooling Channels 15693 Alternative Cooling Devices 157931 What Do They Do 157
932 Baffles 158933 Bubblers 159934 Thermal Pins 159
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XVI Contents
935 Cooling Slender Cores 160936 Cooling Large Cores 161
937 Cooling Cylinder Cores 16294 Cooling System Equations 162941 Cooling Time 162942 Reynolds Number and Coolant Flow 16395 Design Rules 163951 Mold Cooling Design Considerations 163952 Location and Size of Channels 164953 Flow Rate and Heat Transfer 16696 Using Moldflow for Cooling System Design 167961 Example Setup 168
962 Cycle Time Determined by Design and Processing Parameters 169
10 Shrinkage and Warpage 173
101 Injection Molding and Shrinkage 1731011 What Are Shrinkage and Warpage 1731012 Shrinkage and Machine Settings 1731013 Mold Filling and Packing 1741014 How Pressure and Time Affect Shrinkage 1751015 Thermally Unstable Flow 176
102 Basic Causes of Shrinkage and Warpage 1771021 Causes of Shrinkage 1771022 Causes of Warpage 1811023 Relating Orientation and Area Shrinkage to Warpage 182103 Designing Accurate Parts Considering Warpage 1831031 Material Selection 1841032 Wall Thickness Variation 1841033 Gate Position and Runner Dimensions 1851034 Molding Conditions 1871035 Cooling Line Layout 190
11 Moldflow Design Procedure 191
111 Determine Analysis Objectives 191112 Moldflow Analysis Steps Framework 1921121 The Whole Process 1921122 Optimize Fill 1921123 Balance and Size the Runners 1971124 Optimize Cooling 1991125 Optimize the Packing Profile 200
1126 Optimize Warpage 201113 Using Moldflow to Evaluate an Initial Design 2051131 Description of this Example 205
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Contents XVII
1132 Molding Window 2051133 Filling Analysis 206
1134 Gate and Runner Design 2061135 Cooling System Design 2071136 Packing Analysis 2091137 Warpage Analysis 209114 Using Moldflow to Optimize the Design 2101141 Determine the Cause of Warpage 2101142 Investigating Different Gate Locations 2101143 Validating the Best Gate Location 211
12 Part Defects 215
121 Air Traps 2151211 What Is an Air Trap 2151212 Problems Caused by Air Traps 2161213 Remedies 216122 Black Specks and Black Streaks 2171221 What Are Black Specks and Black Streaks 2171222 Causes of Black Specks and Black Streaks 2171223 Remedies 218123 Brittleness 219
1231 What Is Brittleness 2191232 Causes of Brittleness 2191233 Remedies 219124 Burn Marks 2201241 What Is a Burn Mark 2201242 Causes of Burn Marks 2211243 Remedies 222125 Delamination 2221251 What Is Delamination 2221252 Causes of Delamination 223
1253 Remedies 223126 Dimensional Variation 2241261 What Is Dimensional Variation 2241262 Causes of Dimensional Variation 2241263 Remedies 224127 Discoloration 2251271 What Is Discoloration 2251272 Causes of Discoloration 2251273 Remedies 226128 Fish Eyes 227
1281 What Are Fish Eyes 2271282 Causes of Fish Eyes 2271283 Remedies 227
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XVIII Contents
129 Flash 2281291 What Is Flash 228
1292 Causes of Flash 2281293 Remedies 2291210 Flow Marks 23012101 What Is A Flow Mark 23012102 Causes of Flow Marks 23012103 Remedies 2301211 Hesitation 23112111 What Is Hesitation 23112112 Problems Caused by Hesitation 23112113 Remedies 232
1212 Jetting 23212121 What Is Jetting 23212122 Problems Caused by Jetting 23312123 Remedies 2331213 Ripples 23512131 What Are Ripples 23512132 Cause of Ripples 23512133 Remedies 2361214 Short Shots 23612141 What Is a Short Shot 236
12142 Causes of Short Shots 23712143 Remedies 2371215 Silver Streaks 23812151 What Are Silver Streaks 23812152 Causes of Silver Streaks 23912153 Remedies 2391216 Sink Marks and Voids 24012161 What Are Sink Marks and Voids 24012162 Causes of Sink Marks and Voids 24012163 Remedies 2411217 Weld Lines and Meld Lines 24312171 What Are Weld Lines and Meld Lines 24312172 Problems Caused by Weld Lines 24312173 Strength of Weld Lines 24412174 Remedies 245
Appendix A Injection Molding 247
A1 Injection-molding Overview 247 A11 Process 247
A12 Applications 248 A2 Development of the Injection-molding Machine 248 A21 Benefits of the Reciprocating Screw 248
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Contents XIX
A3 Development of the Injection-molding Process 249 A4 Alternative Injection-molding Processes 249
A41 Co-injection (Sandwich) Molding 249 A42 Fusible Core Injection Molding 250 A43 Gas-assisted Injection Molding 251 A44 Injection-compression Molding 252 A45 Lamellar (Microlayer) Injection Molding 253 A46 Live-feed Injection Molding 253 A47 Low-pressure Injection Molding 254 A48 Push-pull Injection Molding 254 A49 Reactive Molding 255 A410 Structural Foam Injection Molding 256
A411 Thin-wall Molding 256
Appendix B Injection-molding Machine System and Operations 259
B1 Injection-molding Machine 259B11 Components 259B12 Machine Specification 260B13 Machine Function 260B14 Auxiliary Equipment 260B2 Machine Components 261
B21 Injection System 261B22 Mold System 263B23 Hydraulic System 264B24 Control System 264B25 Clamping System 264B3 Molded System 265B31 The Delivery System 265B4 Machine Operating Sequence 266B41 Process Cycle 266B5 Screw Operation 268
B51 Back Pressure 268B52 Injection Speed 268B53 Screw Rotation Speed 268B54 Cushion 269B6 Secondary Operations 269B61 Assembly 269B62 Decoration 269B63 Other Secondary Operations 270
Appendix C Injection-molding Process Control 271
C1 Importance of Process Conditions 271C11 Setting Machine Process Conditions 271
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XX Contents
Appendix D Plastic Materials 281
D1 What Are Plastics 281
D11 Polymerization Process 281D12 Structure of Polymers 281D13 Polymer Alloys and Blends 283D14 Polymer Composites 283D2 Classification of Plastics 283D21 Classes of Plastics 284D22 Structures and Properties of Plastics 284D3 Thermoplastics 285D31 Market Share Distribution of Thermoplastics 285D32 Structures and Properties of Thermoplastics 286D33 Amorphous Polymers 286D34 Semicrystalline Polymers 287D4 Thermosets 288D5 Properties and Applications of Thermoplastics 289D51 ABS 290D52 PA 12 291D53 PA 6 293D54 PA 66 294D55 PBT 296
D56 PC 297D57 PCABS 299D58 PCPBT 300D59 HDPE 301D510 LDPE 303D511 PEI 304D512 PET 305D513 PETG 306D514 PMMA 307D515 POM 309
D516 PP 310D517 PPEPPO 312D518 PS 313D519 PVC 314D520 SAN 316D521 Additives Fillers and Reinforcements 317D522 Modifying Polymer Properties 318D523 Low-aspect Fillers 319D524 High-aspect Fillers Fibers 319
Index 321
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1 Polymer Flow Behavior in Injection Molds
bull Phases of injection molding
bull How do plastics flow
11 Phases of Injection Molding
Any molder can prove that all the conditions and effects discussed in this chapter do indeedoccur during the injection molding process While this knowledge alone can somewhatimprove quality it is only with the use of Moldflow analysis during the initial design stage with the mold designed for the optimum filling pattern that these effects can be controlledand the full benefits obtained
Flow technology is concerned with the behavior of plastics during the mold filling process Aplastic parts properties depend on how the part is molded Two parts having identicaldimensions and made from the same material but molded under different conditions will havedifferent stress and shrinkage levels and will behave differently in the field meaning that they
are in practice two different parts The way the plastic flows into the mold is of paramount importance in determining the quality of the part The process of filling the mold can be distinctly analyzed with the ability to predictpressure temperature and stress
111 How Plastic Fills a Mold
This was investigated using a centrally gated mold shaped like a dinner plate with a thick rim
around the outside as shown in Figure 11 It was found that the injection molding processalthough complex could be divided into three phases (we use the word phase to avoidconfusion with injection stage as used with programmed injection)
Figure 1ure 1 1 Cross-section of disk mold used to investigate flow
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2 Polymer Flow Behavior in Injection Molds
1111 Filling Phase
As the ram moves forward it first moves at a steady speed as the plastic flows into the cavity
This is the filling phase This phase lasts until the mold is just filled See Figure 12 andFigure 13
1112 Pressurization Phase
The pressurization phase begins when the ram moves forward after the filling phase to bring the mold up to pressure When the mold is filled the ram will slow down but it still movesquite some distance because plastics are very compressible materials At injection molding pressure an extra 15 volume of material can be forced into the cavity See Figure 12 andFigure 13
Although fluids are usually assumed to be incompressible molten plastics have to beconsidered to be more like a gas The compressibility of plastics can be observed by blocking off the nozzle and attempting to purge the barrel The ram will jump forward when thepressure is applied but will spring back when the pressure is released
1113 Compensation Phase
After the pressurization phase the ram still does not stop completely continuing to creepforward for some time Plastics have a very large volumetric change of about 25 from themelt to the solid This can be seen in a short shot the difference in volume between themolding and the cavity is due to this volumetric change See Figure 12 and Figure 13
The ram moving forward to compensate for the volumetric change in the part is called thecompensation phase As the volumetric change is 25 and at the most only an extra 15 canbe injected in the pressurization phase there must always be some compensation phase
Figure 1igure 1 2 Phases of injection molding
Filling phase
Pressurization phase
Compensation phase
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Phases of Injection Molding 3
Figure 13ure 1 3 Phases of injection molding detail
112 The Filling Phase
A two-color technique best demonstrates this phase After emptying the barrel of an injection-molding machine a small amount of red plastic was charged followed by green plastic
Consider the closed mold with the plastic front just starting to flow from the nozzle Theplastic first fills the sprue and runner system then enters the mold cavity itself forming a smallbubble of molten plastic
The skin of the plastic in contact with the cool mold freezes rapidly while the central coreremains molten When additional material is injected it flows into this central core displacing
the material already there which then forms a new flow front The flow of this displacedmaterial is a combination of forward flow and outward flow The outward flow contacts the wall freezes and forms the next section of skin while the forward flow forms the new moltencore When more material enters the mold it flows along a channel lined with these frozen walls of plastic illustrated in Figure 14
This flow pattern is often called fountain flow or bubble flow because the flow front is like abubble being inflated with hot plastic from the center The frozen layer is formed by the flow front inflating and so is subject to only a low shear stress and therefore has a very low levelof molecular orientation Once it is frozen it cannot be orientated any further so the frozen
layer in the finished part has a low level of orientation
Figure 14ure 1 4 Fountain flow and heat transfer
Filling Pressurization Compensation
Hot plastic
Frozen layer
Heat loss to mold
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4 Polymer Flow Behavior in Injection Molds
Now consider what happens upstream Hot plastic is continuously flowing bringing new hotmaterial along and generating significant frictional heat At the same time heat is being lost
through the frozen layer to the cold mold surfaceInitially the frozen layer is very thin so heat is lost very rapidly This results in more plasticfreezing and the frozen layer getting thicker cutting down the heat flow After a time thefrozen layer will reach a thickness such that the heat lost by conduction is equal to the heatinput from plastic flow and frictional heating ie an equilibrium condition is reached(Figure 14)
It is interesting to do some calculations on the time taken to reach this state of equilibrium The actual rate of heat flow is very large in comparison with the small heat content of theplastic in the frozen layer The result is that equilibrium is reached very quickly often in a timemeasured in a few tenths of a second As the total filling time is measured in seconds thefrozen layer reaches an equilibrium state early in the filling cycle
It is useful to think about how the thickness of this frozen layer will vary If the injection rate were slowed less heat would be generated by friction along the flow path with less heat inputfrom the flow The heat loss would be at the same rate and with less heat input the frozenlayer would grow in thickness If the injection rate were raised the frozen layer would bethinner (Figure 15) Similarly higher melt and mold temperatures would reduce the thicknessof the frozen layer This can be seen experimentally using the two-color technique
Figure 1igure 1 5 Influence of injection rate on frozen layer thickness
1121 Flow Shear Stress
It is easy to get confused between the various stress levels and orientation of the polymer Asthe plastic flows it is subject to shear stress also called flow shear stress This flow shear stress willorient the material ie cause the molecules to align themselves in the general direction of flow
The shear stress varies from a maximum at the outside dropping off to zero at the center
3Shear stress is purely a function of force and area This must not be confused withshear rate which is the rate of plastic sliding over the next layer Shear rate is zero
= Frozen
= Molten cooler
= Molten hotter
Slowerinjection
rate
Faster
injection
rate
vs
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Phases of Injection Molding 5
at the outer edge where the plastic is frozen rises to a maximum just inwards of thefrozen layer then drops toward the center as shown in Figure 16
Figure 16ure 1 6 Shear rate distribution
If the flow were stopped and the plastic allowed to cool down very slowly this orientation would have time to relax giving a very low level of residual orientation On the other hand if the material were kept under stress and the plastic snap frozen most of the orientation would
be trapped in the frozen plastic (Figure 17)
Figure 17ure 1 7 Molecular orientation through the thickness of the part
Min MaxShear rate
= Tension
= Compression
Molecular orientation trapped
in pressurization phase
Gradual reduction of orientation
toward the center of the
mold as stress levels are lower
and the cooling rate is slower
which allows more time for
orientation to relax
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6 Polymer Flow Behavior in Injection Molds
Now consider the orientation from the mold surface toward the center
The frozen layer itself formed with very little shear and therefore low orientation immediately
freezes setting the low level of orientation
The layer of plastic just on the inside of the frozen layer is subject to maximum shear stressand freezes the instant flow stops trapping almost all the orientation
This is the orientation pattern the further toward the center the more the shear stress dropsand the slower the rate of cooling This allows more time for the level of orientation to relaxso the residual orientation drops rapidly toward the center Consider how this pattern willaffect the residual stress level Oriented material (normally) will shrink more than nonorientedmaterial On the inner surface of the original frozen layer highly oriented material wants toshrink a great deal but it is prevented from doing so by the less-oriented material The highly
oriented layer ends up being in tension while the less-oriented material is in compression This residual stress pattern is a common cause of part warpage
3 There is a connectionthrough orientationbetween the shear stress during
filling (flow stress) and the residual stress in the final molded part This meansshear stress during filling shown on Moldflow plots can be used as a designparameter
113 The Pressurization Phase
The pressurization phasefrom the point of view of flow behavioris very similar to thefilling phase The flow rate may drop somewhat as the mold builds up to pressure resulting inan increase in the thickness of the frozen layer
The main difference of course is the increase in hydrostatic (isotropic) pressure We shall seein chapter 2 section 24 Effect of Molding Conditions that hydrostatic pressure in itself doesnot cause any residual stress
114 The Compensation Phase
Compensating flow is unstable Consider the plate molding again (see Figure 11) You wouldthink that plastic flowing uniformly through the thin diaphragm would top up the thick rim Inpractice the plastic during the compensation phase flows in rivers that spread out like a deltaas illustrated in Figure 18 This may seem surprising at first but it can be explained by temperature instability
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How Do Plastics Flow 7
Figure 18ure 1 8 River flow
1141 Temperature Variation
There is always some variation in melt temperature coming from the barrel of the injectionmachine In exceptional cases up to 40 ordmC variation has been measured using a high-speedthermocouple
1142 Natural Instability
However slight the temperature variation natural instability will amplify it If for exampleone part of the melt is slightly hotter than the rest then the plastic flow in that area will beslightly greater bringing hotter material into the area and maintaining the temperature If onthe other hand there is another area that is cooler the flow will be less so there will be lessheat input and the plastic will get colder until it eventually freezes off
However balanced the initial conditions this natural instability will result in a river-type flow
This is a very important consideration The first material to freeze off will shrink early in thecycle By the time the material in the river flows freezes the bulk of the material will havealready frozen off and shrinkage will have occurred The rivers will shrink relative to the bulk of the molding and because they are highly orientated shrinkage will be very high The resultis high-stress tensile members throughout the molding a common cause of warpage
1143 Optimum Part Quality
Most of the stress in plastic parts occurs during the compensation phase By controlling flow and minimizing stress it is possible to design for optimum part quality This important point is
at the heart of the Moldflow philosophy
12 How Do Plastics Flow
121 Material Behavior
Molten thermoplastics exhibit viscoelastic behavior which combines flow characteristics of both viscous liquids and elastic solids When a viscous liquid flows the energy that causes thedeformation is dissipated and becomes viscous heat On the other hand when an elastic solid
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8 Polymer Flow Behavior in Injection Molds
is deformed the driving energy is stored For example the flow of water is a typical viscousflow whereas the deformation of a rubber cube falls into the elastic category
122 Deformation
In addition to the two types of material flow behavior there are two types of deformationsimple shear and simple extension (elongation) as shown in Figure 19 (a) and (b) below Theflow of molten thermoplastics during injection-molding filling is predominantly shear flow asshown in Figure 19 (c) in which layers of material elements slide over each other Theextensional flow however becomes significant as the material elements undergo elongation when the melt passes areas of abrupt dimensional change (eg a gate region) as shown inFigure 19 (d)
Figure 1igure 1 9 (a) Simple shear flow (b) Simple extensional flow (c) Shear flow in cavity filling (d) Extensional flow in cavity filling
123 Viscoelastic Behavior
In response to an applied stress (force per unit area) molten thermoplastics exhibit viscoelastic behavior which combines characteristics of an ideal viscous liquid with those of an ideal elastic solid In other words under certain conditions molten thermoplastics behavelike a liquid and will continuously deform while shear stress is applied as shown in
Figure 110 Upon the removal of the stress however the materials behave somewhat like anelastic solid with partial recovery of the deformation as shown in Figure 110 (b) and (c) This viscoelastic behavior stems from the random-coil configuration of polymer molecules in the
Stress
Stress Stress
Mold wall
Material element before
deformation begins
Material element during
deformation
Mold wall
Material elements at rest
Moving
materialelements
Velocity
profile
(a) (b)
(c) (d)
Velocity
profile
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How Do Plastics Flow 9
molten state which allows the movement and slippage of molecular chains under theinfluence of an applied load However the entanglement of the polymer molecular chains also
makes the system behave like an elastic solid upon the application and removal of the externalload Namely on removal of the stress chains will tend to return to the equilibrium random-coil state and thus will be a component of stress recovery The recovery is not instantaneousbecause of the entanglements still present in the system
Figure 1ure 1 10 (a) Ideal viscous liquid deforms continuously under applied stress (b) Ideal elasticsolid deforms immediately upon the application of stress but fully recovers when the
stress is removed (c) Molten thermoplastic deforms continuously under the applied
stress (like a viscous liquid) but also recovers partially from the deformation upon
removal of the applied stress (like an elastic solid)
124 Melt Shear Viscosity
1241 What Is Shear Viscosity
Melt shear viscosity is a materials resistance to shear flow In general polymer melts are highly viscous because of their long molecular chain structure The viscosity of a polymer melt rangesfrom 2 to 3000 Pas (water 10^-1 Pas glass 10^20 Pas) Viscosity can be thought of as thethickness of a fluid or how much it resists flow Viscosity is expressed as the ratio of shearstress (force per unit area) to the shear rate (rate change of shear strain) as shown inEquation 11 and Figure 111
(a) Viscous
liquid
(b) Elastic
solid
(c) Viscoelastic
polymer
melt
Initial shape
(shear stress
to be applied)
Time = Dt Time = 2 x Dt
(shear stress to
be removed)
Final deformed
shape
d
d
d
d
2d
2d
2d
2d
total
recovery
partial
recovery
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Contents
1 Polymer Flow Behavior in Injection Molds 1
11 Phases of Injection Molding 1111 How Plastic Fills a Mold 1112 The Filling Phase 3113 The Pressurization Phase 6114 The Compensation Phase 612 How Do Plastics Flow 7121 Material Behavior 7122 Deformation 8
123 Viscoelastic Behavior 8124 Melt Shear Viscosity 9125 Newtonian Fluid vs Non-Newtonian Fluid 10126 Shear-thinning Behavior 10127 Shear Rate Distribution 11128 Pressure-driven Flow 11129 Pressure Gradient and Injection Times 131210 Melt Flow Length 131211 Injection Pressure vs Fill Time 131212 Flow Instability 15
2 Molding Conditions and Injection Pressure 17
21 Injection-pressure Overview 17211 Pressure Drives the Flow Front 1822 Factors Influencing Injection-pressure Requirements 1823 Equations 20231 Circular Channel Flow 20232 Strip Channel Flow 2024 Effect of Molding Conditions 21
241 Part Quality 21242 Melt Temperature 22243 Mold Temperature 22244 Fill Time 23245 Shear Stress Variation 23246 Packing Pressure and Time 24247 Summary 25248 Back Flow 2625 Using Moldflow to Determine Optimum Molding Conditions 26251 Part 27
252 Molding Window Size 27253 Injection Pressure 28254 Flow Front Temperature 29
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XII Contents
255 Cooling Time 30256 Summary 31
3 Filling Pattern 33
31 Filling Pattern Overview 33311 What Is the Filling Pattern 3332 Flow in Complex Molds 34321 Overpack 34322 Racetrack Effect 35323 Varying Injection Rate 35324 Underflow Effect 36325 Hesitation Effect 37326 Weld Lines 38327 Meld Lines 38328 Sink Marks 38329 Multidirectional Flow 393210 Unstable Flow 403211 Simple Flow Pattern 4133 Flow-front Velocity and Flow-front Area 41331 What are FFV and FFA 41332 Flow-front Velocity Influences Filling Pattern 41
333 Equation 4234 Using Moldflow to Determine the Filling Pattern 43341 Computer Simulation Can Eliminate Molding Trials 43342 Using a Flow Analysis 4335 Using Moldflow to Achieve Constant FFV 44351 Controlling the FFV Through Ram Speed 44
4 Moldflow Design Principles 47
41 Product Design and Moldflow 4742 Sequence of Analysis 47421 Part Filling Optimization 48422 Molding Conditions 48423 Runner Design 48424 Cooling Optimization 49425 Packing Optimization 49426 Warpage Optimization 4943 Moldflow Flow Concepts 49431 Unidirectional and Controlled Flow Pattern 50432 Flow Balancing 51
433 Constant Pressure Gradient 52434 Maximum Shear Stress 53435 Uniform Cooling 54
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Contents XIII
436 Positioning Weld and Meld Lines 55437 Avoid Hesitation Effects 55
438 Avoid Underflow 55439 Balancing with Flow Leaders and Flow Deflectors 574310 Controlled Frictional Heat 584311 Thermal Shutoff of Runners 584312 Acceptable RunnerCavity Ratio 59
5 Meshes Used In Moldf low Analyses 61
51 Mesh Types Used by Moldflow 61511 Finite Elements Used in Moldflow 61512 Mesh Types 62513 Solver Assumptions 6352 Mesh Requirements 64521 Mesh Density Considerations 64522 Part Details 6753 Geometry Creation 6854 Importing Geometry 6955 Using Different Mesh Types 69551 Door Panel 69552 Manifold 70
6 Product Design 71
61 Material Properties for Product Design 71611 Plastics Are Sensitive to Operating Conditions 71612 Stress-Strain Behavior 72613 Creep and Stress Relaxation 77614 Fatigue 79615 Impact strength 80616 Thermal Mechanical Behavior 8062 Design for Strength 82621 Predicting Part Strength 82622 LoadingOperating Conditions 8263 Part Thickness 86631 Part Thickness Drives Quality and Cost 86632 Cycle Time Increases with Thickness 86633 Thick Parts Tend to Warp 86634 Thin Uniform Parts Improve Surface Quality 87635 Reducing Part Thickness 8764 Boosting Structural Integrity with Ribs 88
641 Structural Integrity the Goal of Every Design 88642 Designing Ribs 88
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XIV Contents
65 Design for Assembly 90651 Molding One Part vs Separate Components 90
652 Tolerances Fit between Parts 90653 Press-fit Joints 91654 Snap-fit Joints 93655 Cantilever Snap Joints 95656 Torsion Snap-fit Joints 96657 Fasteners 98658 Inserts 101659 Welding Processes 101
7 Gate Design 103
71 Gate Design Overview 103711 What Is a Gate 103712 Single vs Multiple Gates 103713 Gate Dimensions 103714 Gate Location 10472 Gate Types 104721 Manually Trimmed Gates 104722 Automatically Trimmed Gates 11073 Design Rules 113
731 Determining the Number of Gates 113732 Flow Patterns 115733 Gate Position 116734 Avoiding Common Problems 122735 Gate Length 122736 Gate Thickness 122737 Freeze-off Time 12374 Using Moldflow for Gate Design 123741 Gate Location 123742 Molding Window Size for the Three Gate Locations 124
743 Filling Pattern 125744 Gate Size Based on Shear Rate 125
8 Runner System Design 127
81 Definitions 127811 Feed System 127812 Runner System 127813 Cold Runner 127814 Hot Runner 128
815 Hot Manifold 128816 Hot Drop 128817 Sprue 128
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Contents XV
82 Runner System Design Principles 129821 Benefits of Good Runner Design 129
822 Runner Design Philosophy 129823 Flow Balancing 130824 Flow Control 130825 Frictional Heating in Runners 132826 Thermal Shutoff 132827 System and Runner Pressures 132828 Constant Pressure Gradient 133829 Cold Slug Wells 1338210 Easy Ejection 13483 Runner Types 134
831 Cold Runners 134832 Hot Runner Systems 13584 Runner Layout 136841 Determining the Number of Cavities 136842 Planning the Runner System Layout 138843 Partially Balanced Runners 139844 Geometrically Balanced Runners 14185 Initial Runner Sizing 141851 Determining Sprue Dimensions 141852 Designing Runner Cross Sections 142
853 Determining Runner Diameters 14586 Runner Balancing 147861 How Runner Balancing Works 147862 When Are the Runner Sizes Optimized 147863 Validating the Balance 147864 Processing Window 14887 Using Moldflow for Runner Balancing 149871 Runner Balancing a 48-cavity Tool 149872 Runner Balancing for a Family Mold 151873 Runner Balancing for a Multigated Part 152
9 Cooling System Design 153
91 Mold Cooling System Overview 153911 Importance of Cooling System Design 153912 Mold Cooling System Components 15492 Cooling-channel Configuration 156921 Types of Cooling Channels 15693 Alternative Cooling Devices 157931 What Do They Do 157
932 Baffles 158933 Bubblers 159934 Thermal Pins 159
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XVI Contents
935 Cooling Slender Cores 160936 Cooling Large Cores 161
937 Cooling Cylinder Cores 16294 Cooling System Equations 162941 Cooling Time 162942 Reynolds Number and Coolant Flow 16395 Design Rules 163951 Mold Cooling Design Considerations 163952 Location and Size of Channels 164953 Flow Rate and Heat Transfer 16696 Using Moldflow for Cooling System Design 167961 Example Setup 168
962 Cycle Time Determined by Design and Processing Parameters 169
10 Shrinkage and Warpage 173
101 Injection Molding and Shrinkage 1731011 What Are Shrinkage and Warpage 1731012 Shrinkage and Machine Settings 1731013 Mold Filling and Packing 1741014 How Pressure and Time Affect Shrinkage 1751015 Thermally Unstable Flow 176
102 Basic Causes of Shrinkage and Warpage 1771021 Causes of Shrinkage 1771022 Causes of Warpage 1811023 Relating Orientation and Area Shrinkage to Warpage 182103 Designing Accurate Parts Considering Warpage 1831031 Material Selection 1841032 Wall Thickness Variation 1841033 Gate Position and Runner Dimensions 1851034 Molding Conditions 1871035 Cooling Line Layout 190
11 Moldflow Design Procedure 191
111 Determine Analysis Objectives 191112 Moldflow Analysis Steps Framework 1921121 The Whole Process 1921122 Optimize Fill 1921123 Balance and Size the Runners 1971124 Optimize Cooling 1991125 Optimize the Packing Profile 200
1126 Optimize Warpage 201113 Using Moldflow to Evaluate an Initial Design 2051131 Description of this Example 205
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Contents XVII
1132 Molding Window 2051133 Filling Analysis 206
1134 Gate and Runner Design 2061135 Cooling System Design 2071136 Packing Analysis 2091137 Warpage Analysis 209114 Using Moldflow to Optimize the Design 2101141 Determine the Cause of Warpage 2101142 Investigating Different Gate Locations 2101143 Validating the Best Gate Location 211
12 Part Defects 215
121 Air Traps 2151211 What Is an Air Trap 2151212 Problems Caused by Air Traps 2161213 Remedies 216122 Black Specks and Black Streaks 2171221 What Are Black Specks and Black Streaks 2171222 Causes of Black Specks and Black Streaks 2171223 Remedies 218123 Brittleness 219
1231 What Is Brittleness 2191232 Causes of Brittleness 2191233 Remedies 219124 Burn Marks 2201241 What Is a Burn Mark 2201242 Causes of Burn Marks 2211243 Remedies 222125 Delamination 2221251 What Is Delamination 2221252 Causes of Delamination 223
1253 Remedies 223126 Dimensional Variation 2241261 What Is Dimensional Variation 2241262 Causes of Dimensional Variation 2241263 Remedies 224127 Discoloration 2251271 What Is Discoloration 2251272 Causes of Discoloration 2251273 Remedies 226128 Fish Eyes 227
1281 What Are Fish Eyes 2271282 Causes of Fish Eyes 2271283 Remedies 227
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XVIII Contents
129 Flash 2281291 What Is Flash 228
1292 Causes of Flash 2281293 Remedies 2291210 Flow Marks 23012101 What Is A Flow Mark 23012102 Causes of Flow Marks 23012103 Remedies 2301211 Hesitation 23112111 What Is Hesitation 23112112 Problems Caused by Hesitation 23112113 Remedies 232
1212 Jetting 23212121 What Is Jetting 23212122 Problems Caused by Jetting 23312123 Remedies 2331213 Ripples 23512131 What Are Ripples 23512132 Cause of Ripples 23512133 Remedies 2361214 Short Shots 23612141 What Is a Short Shot 236
12142 Causes of Short Shots 23712143 Remedies 2371215 Silver Streaks 23812151 What Are Silver Streaks 23812152 Causes of Silver Streaks 23912153 Remedies 2391216 Sink Marks and Voids 24012161 What Are Sink Marks and Voids 24012162 Causes of Sink Marks and Voids 24012163 Remedies 2411217 Weld Lines and Meld Lines 24312171 What Are Weld Lines and Meld Lines 24312172 Problems Caused by Weld Lines 24312173 Strength of Weld Lines 24412174 Remedies 245
Appendix A Injection Molding 247
A1 Injection-molding Overview 247 A11 Process 247
A12 Applications 248 A2 Development of the Injection-molding Machine 248 A21 Benefits of the Reciprocating Screw 248
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Contents XIX
A3 Development of the Injection-molding Process 249 A4 Alternative Injection-molding Processes 249
A41 Co-injection (Sandwich) Molding 249 A42 Fusible Core Injection Molding 250 A43 Gas-assisted Injection Molding 251 A44 Injection-compression Molding 252 A45 Lamellar (Microlayer) Injection Molding 253 A46 Live-feed Injection Molding 253 A47 Low-pressure Injection Molding 254 A48 Push-pull Injection Molding 254 A49 Reactive Molding 255 A410 Structural Foam Injection Molding 256
A411 Thin-wall Molding 256
Appendix B Injection-molding Machine System and Operations 259
B1 Injection-molding Machine 259B11 Components 259B12 Machine Specification 260B13 Machine Function 260B14 Auxiliary Equipment 260B2 Machine Components 261
B21 Injection System 261B22 Mold System 263B23 Hydraulic System 264B24 Control System 264B25 Clamping System 264B3 Molded System 265B31 The Delivery System 265B4 Machine Operating Sequence 266B41 Process Cycle 266B5 Screw Operation 268
B51 Back Pressure 268B52 Injection Speed 268B53 Screw Rotation Speed 268B54 Cushion 269B6 Secondary Operations 269B61 Assembly 269B62 Decoration 269B63 Other Secondary Operations 270
Appendix C Injection-molding Process Control 271
C1 Importance of Process Conditions 271C11 Setting Machine Process Conditions 271
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XX Contents
Appendix D Plastic Materials 281
D1 What Are Plastics 281
D11 Polymerization Process 281D12 Structure of Polymers 281D13 Polymer Alloys and Blends 283D14 Polymer Composites 283D2 Classification of Plastics 283D21 Classes of Plastics 284D22 Structures and Properties of Plastics 284D3 Thermoplastics 285D31 Market Share Distribution of Thermoplastics 285D32 Structures and Properties of Thermoplastics 286D33 Amorphous Polymers 286D34 Semicrystalline Polymers 287D4 Thermosets 288D5 Properties and Applications of Thermoplastics 289D51 ABS 290D52 PA 12 291D53 PA 6 293D54 PA 66 294D55 PBT 296
D56 PC 297D57 PCABS 299D58 PCPBT 300D59 HDPE 301D510 LDPE 303D511 PEI 304D512 PET 305D513 PETG 306D514 PMMA 307D515 POM 309
D516 PP 310D517 PPEPPO 312D518 PS 313D519 PVC 314D520 SAN 316D521 Additives Fillers and Reinforcements 317D522 Modifying Polymer Properties 318D523 Low-aspect Fillers 319D524 High-aspect Fillers Fibers 319
Index 321
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1 Polymer Flow Behavior in Injection Molds
bull Phases of injection molding
bull How do plastics flow
11 Phases of Injection Molding
Any molder can prove that all the conditions and effects discussed in this chapter do indeedoccur during the injection molding process While this knowledge alone can somewhatimprove quality it is only with the use of Moldflow analysis during the initial design stage with the mold designed for the optimum filling pattern that these effects can be controlledand the full benefits obtained
Flow technology is concerned with the behavior of plastics during the mold filling process Aplastic parts properties depend on how the part is molded Two parts having identicaldimensions and made from the same material but molded under different conditions will havedifferent stress and shrinkage levels and will behave differently in the field meaning that they
are in practice two different parts The way the plastic flows into the mold is of paramount importance in determining the quality of the part The process of filling the mold can be distinctly analyzed with the ability to predictpressure temperature and stress
111 How Plastic Fills a Mold
This was investigated using a centrally gated mold shaped like a dinner plate with a thick rim
around the outside as shown in Figure 11 It was found that the injection molding processalthough complex could be divided into three phases (we use the word phase to avoidconfusion with injection stage as used with programmed injection)
Figure 1ure 1 1 Cross-section of disk mold used to investigate flow
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2 Polymer Flow Behavior in Injection Molds
1111 Filling Phase
As the ram moves forward it first moves at a steady speed as the plastic flows into the cavity
This is the filling phase This phase lasts until the mold is just filled See Figure 12 andFigure 13
1112 Pressurization Phase
The pressurization phase begins when the ram moves forward after the filling phase to bring the mold up to pressure When the mold is filled the ram will slow down but it still movesquite some distance because plastics are very compressible materials At injection molding pressure an extra 15 volume of material can be forced into the cavity See Figure 12 andFigure 13
Although fluids are usually assumed to be incompressible molten plastics have to beconsidered to be more like a gas The compressibility of plastics can be observed by blocking off the nozzle and attempting to purge the barrel The ram will jump forward when thepressure is applied but will spring back when the pressure is released
1113 Compensation Phase
After the pressurization phase the ram still does not stop completely continuing to creepforward for some time Plastics have a very large volumetric change of about 25 from themelt to the solid This can be seen in a short shot the difference in volume between themolding and the cavity is due to this volumetric change See Figure 12 and Figure 13
The ram moving forward to compensate for the volumetric change in the part is called thecompensation phase As the volumetric change is 25 and at the most only an extra 15 canbe injected in the pressurization phase there must always be some compensation phase
Figure 1igure 1 2 Phases of injection molding
Filling phase
Pressurization phase
Compensation phase
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Phases of Injection Molding 3
Figure 13ure 1 3 Phases of injection molding detail
112 The Filling Phase
A two-color technique best demonstrates this phase After emptying the barrel of an injection-molding machine a small amount of red plastic was charged followed by green plastic
Consider the closed mold with the plastic front just starting to flow from the nozzle Theplastic first fills the sprue and runner system then enters the mold cavity itself forming a smallbubble of molten plastic
The skin of the plastic in contact with the cool mold freezes rapidly while the central coreremains molten When additional material is injected it flows into this central core displacing
the material already there which then forms a new flow front The flow of this displacedmaterial is a combination of forward flow and outward flow The outward flow contacts the wall freezes and forms the next section of skin while the forward flow forms the new moltencore When more material enters the mold it flows along a channel lined with these frozen walls of plastic illustrated in Figure 14
This flow pattern is often called fountain flow or bubble flow because the flow front is like abubble being inflated with hot plastic from the center The frozen layer is formed by the flow front inflating and so is subject to only a low shear stress and therefore has a very low levelof molecular orientation Once it is frozen it cannot be orientated any further so the frozen
layer in the finished part has a low level of orientation
Figure 14ure 1 4 Fountain flow and heat transfer
Filling Pressurization Compensation
Hot plastic
Frozen layer
Heat loss to mold
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4 Polymer Flow Behavior in Injection Molds
Now consider what happens upstream Hot plastic is continuously flowing bringing new hotmaterial along and generating significant frictional heat At the same time heat is being lost
through the frozen layer to the cold mold surfaceInitially the frozen layer is very thin so heat is lost very rapidly This results in more plasticfreezing and the frozen layer getting thicker cutting down the heat flow After a time thefrozen layer will reach a thickness such that the heat lost by conduction is equal to the heatinput from plastic flow and frictional heating ie an equilibrium condition is reached(Figure 14)
It is interesting to do some calculations on the time taken to reach this state of equilibrium The actual rate of heat flow is very large in comparison with the small heat content of theplastic in the frozen layer The result is that equilibrium is reached very quickly often in a timemeasured in a few tenths of a second As the total filling time is measured in seconds thefrozen layer reaches an equilibrium state early in the filling cycle
It is useful to think about how the thickness of this frozen layer will vary If the injection rate were slowed less heat would be generated by friction along the flow path with less heat inputfrom the flow The heat loss would be at the same rate and with less heat input the frozenlayer would grow in thickness If the injection rate were raised the frozen layer would bethinner (Figure 15) Similarly higher melt and mold temperatures would reduce the thicknessof the frozen layer This can be seen experimentally using the two-color technique
Figure 1igure 1 5 Influence of injection rate on frozen layer thickness
1121 Flow Shear Stress
It is easy to get confused between the various stress levels and orientation of the polymer Asthe plastic flows it is subject to shear stress also called flow shear stress This flow shear stress willorient the material ie cause the molecules to align themselves in the general direction of flow
The shear stress varies from a maximum at the outside dropping off to zero at the center
3Shear stress is purely a function of force and area This must not be confused withshear rate which is the rate of plastic sliding over the next layer Shear rate is zero
= Frozen
= Molten cooler
= Molten hotter
Slowerinjection
rate
Faster
injection
rate
vs
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Phases of Injection Molding 5
at the outer edge where the plastic is frozen rises to a maximum just inwards of thefrozen layer then drops toward the center as shown in Figure 16
Figure 16ure 1 6 Shear rate distribution
If the flow were stopped and the plastic allowed to cool down very slowly this orientation would have time to relax giving a very low level of residual orientation On the other hand if the material were kept under stress and the plastic snap frozen most of the orientation would
be trapped in the frozen plastic (Figure 17)
Figure 17ure 1 7 Molecular orientation through the thickness of the part
Min MaxShear rate
= Tension
= Compression
Molecular orientation trapped
in pressurization phase
Gradual reduction of orientation
toward the center of the
mold as stress levels are lower
and the cooling rate is slower
which allows more time for
orientation to relax
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6 Polymer Flow Behavior in Injection Molds
Now consider the orientation from the mold surface toward the center
The frozen layer itself formed with very little shear and therefore low orientation immediately
freezes setting the low level of orientation
The layer of plastic just on the inside of the frozen layer is subject to maximum shear stressand freezes the instant flow stops trapping almost all the orientation
This is the orientation pattern the further toward the center the more the shear stress dropsand the slower the rate of cooling This allows more time for the level of orientation to relaxso the residual orientation drops rapidly toward the center Consider how this pattern willaffect the residual stress level Oriented material (normally) will shrink more than nonorientedmaterial On the inner surface of the original frozen layer highly oriented material wants toshrink a great deal but it is prevented from doing so by the less-oriented material The highly
oriented layer ends up being in tension while the less-oriented material is in compression This residual stress pattern is a common cause of part warpage
3 There is a connectionthrough orientationbetween the shear stress during
filling (flow stress) and the residual stress in the final molded part This meansshear stress during filling shown on Moldflow plots can be used as a designparameter
113 The Pressurization Phase
The pressurization phasefrom the point of view of flow behavioris very similar to thefilling phase The flow rate may drop somewhat as the mold builds up to pressure resulting inan increase in the thickness of the frozen layer
The main difference of course is the increase in hydrostatic (isotropic) pressure We shall seein chapter 2 section 24 Effect of Molding Conditions that hydrostatic pressure in itself doesnot cause any residual stress
114 The Compensation Phase
Compensating flow is unstable Consider the plate molding again (see Figure 11) You wouldthink that plastic flowing uniformly through the thin diaphragm would top up the thick rim Inpractice the plastic during the compensation phase flows in rivers that spread out like a deltaas illustrated in Figure 18 This may seem surprising at first but it can be explained by temperature instability
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How Do Plastics Flow 7
Figure 18ure 1 8 River flow
1141 Temperature Variation
There is always some variation in melt temperature coming from the barrel of the injectionmachine In exceptional cases up to 40 ordmC variation has been measured using a high-speedthermocouple
1142 Natural Instability
However slight the temperature variation natural instability will amplify it If for exampleone part of the melt is slightly hotter than the rest then the plastic flow in that area will beslightly greater bringing hotter material into the area and maintaining the temperature If onthe other hand there is another area that is cooler the flow will be less so there will be lessheat input and the plastic will get colder until it eventually freezes off
However balanced the initial conditions this natural instability will result in a river-type flow
This is a very important consideration The first material to freeze off will shrink early in thecycle By the time the material in the river flows freezes the bulk of the material will havealready frozen off and shrinkage will have occurred The rivers will shrink relative to the bulk of the molding and because they are highly orientated shrinkage will be very high The resultis high-stress tensile members throughout the molding a common cause of warpage
1143 Optimum Part Quality
Most of the stress in plastic parts occurs during the compensation phase By controlling flow and minimizing stress it is possible to design for optimum part quality This important point is
at the heart of the Moldflow philosophy
12 How Do Plastics Flow
121 Material Behavior
Molten thermoplastics exhibit viscoelastic behavior which combines flow characteristics of both viscous liquids and elastic solids When a viscous liquid flows the energy that causes thedeformation is dissipated and becomes viscous heat On the other hand when an elastic solid
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8 Polymer Flow Behavior in Injection Molds
is deformed the driving energy is stored For example the flow of water is a typical viscousflow whereas the deformation of a rubber cube falls into the elastic category
122 Deformation
In addition to the two types of material flow behavior there are two types of deformationsimple shear and simple extension (elongation) as shown in Figure 19 (a) and (b) below Theflow of molten thermoplastics during injection-molding filling is predominantly shear flow asshown in Figure 19 (c) in which layers of material elements slide over each other Theextensional flow however becomes significant as the material elements undergo elongation when the melt passes areas of abrupt dimensional change (eg a gate region) as shown inFigure 19 (d)
Figure 1igure 1 9 (a) Simple shear flow (b) Simple extensional flow (c) Shear flow in cavity filling (d) Extensional flow in cavity filling
123 Viscoelastic Behavior
In response to an applied stress (force per unit area) molten thermoplastics exhibit viscoelastic behavior which combines characteristics of an ideal viscous liquid with those of an ideal elastic solid In other words under certain conditions molten thermoplastics behavelike a liquid and will continuously deform while shear stress is applied as shown in
Figure 110 Upon the removal of the stress however the materials behave somewhat like anelastic solid with partial recovery of the deformation as shown in Figure 110 (b) and (c) This viscoelastic behavior stems from the random-coil configuration of polymer molecules in the
Stress
Stress Stress
Mold wall
Material element before
deformation begins
Material element during
deformation
Mold wall
Material elements at rest
Moving
materialelements
Velocity
profile
(a) (b)
(c) (d)
Velocity
profile
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How Do Plastics Flow 9
molten state which allows the movement and slippage of molecular chains under theinfluence of an applied load However the entanglement of the polymer molecular chains also
makes the system behave like an elastic solid upon the application and removal of the externalload Namely on removal of the stress chains will tend to return to the equilibrium random-coil state and thus will be a component of stress recovery The recovery is not instantaneousbecause of the entanglements still present in the system
Figure 1ure 1 10 (a) Ideal viscous liquid deforms continuously under applied stress (b) Ideal elasticsolid deforms immediately upon the application of stress but fully recovers when the
stress is removed (c) Molten thermoplastic deforms continuously under the applied
stress (like a viscous liquid) but also recovers partially from the deformation upon
removal of the applied stress (like an elastic solid)
124 Melt Shear Viscosity
1241 What Is Shear Viscosity
Melt shear viscosity is a materials resistance to shear flow In general polymer melts are highly viscous because of their long molecular chain structure The viscosity of a polymer melt rangesfrom 2 to 3000 Pas (water 10^-1 Pas glass 10^20 Pas) Viscosity can be thought of as thethickness of a fluid or how much it resists flow Viscosity is expressed as the ratio of shearstress (force per unit area) to the shear rate (rate change of shear strain) as shown inEquation 11 and Figure 111
(a) Viscous
liquid
(b) Elastic
solid
(c) Viscoelastic
polymer
melt
Initial shape
(shear stress
to be applied)
Time = Dt Time = 2 x Dt
(shear stress to
be removed)
Final deformed
shape
d
d
d
d
2d
2d
2d
2d
total
recovery
partial
recovery
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XII Contents
255 Cooling Time 30256 Summary 31
3 Filling Pattern 33
31 Filling Pattern Overview 33311 What Is the Filling Pattern 3332 Flow in Complex Molds 34321 Overpack 34322 Racetrack Effect 35323 Varying Injection Rate 35324 Underflow Effect 36325 Hesitation Effect 37326 Weld Lines 38327 Meld Lines 38328 Sink Marks 38329 Multidirectional Flow 393210 Unstable Flow 403211 Simple Flow Pattern 4133 Flow-front Velocity and Flow-front Area 41331 What are FFV and FFA 41332 Flow-front Velocity Influences Filling Pattern 41
333 Equation 4234 Using Moldflow to Determine the Filling Pattern 43341 Computer Simulation Can Eliminate Molding Trials 43342 Using a Flow Analysis 4335 Using Moldflow to Achieve Constant FFV 44351 Controlling the FFV Through Ram Speed 44
4 Moldflow Design Principles 47
41 Product Design and Moldflow 4742 Sequence of Analysis 47421 Part Filling Optimization 48422 Molding Conditions 48423 Runner Design 48424 Cooling Optimization 49425 Packing Optimization 49426 Warpage Optimization 4943 Moldflow Flow Concepts 49431 Unidirectional and Controlled Flow Pattern 50432 Flow Balancing 51
433 Constant Pressure Gradient 52434 Maximum Shear Stress 53435 Uniform Cooling 54
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Contents XIII
436 Positioning Weld and Meld Lines 55437 Avoid Hesitation Effects 55
438 Avoid Underflow 55439 Balancing with Flow Leaders and Flow Deflectors 574310 Controlled Frictional Heat 584311 Thermal Shutoff of Runners 584312 Acceptable RunnerCavity Ratio 59
5 Meshes Used In Moldf low Analyses 61
51 Mesh Types Used by Moldflow 61511 Finite Elements Used in Moldflow 61512 Mesh Types 62513 Solver Assumptions 6352 Mesh Requirements 64521 Mesh Density Considerations 64522 Part Details 6753 Geometry Creation 6854 Importing Geometry 6955 Using Different Mesh Types 69551 Door Panel 69552 Manifold 70
6 Product Design 71
61 Material Properties for Product Design 71611 Plastics Are Sensitive to Operating Conditions 71612 Stress-Strain Behavior 72613 Creep and Stress Relaxation 77614 Fatigue 79615 Impact strength 80616 Thermal Mechanical Behavior 8062 Design for Strength 82621 Predicting Part Strength 82622 LoadingOperating Conditions 8263 Part Thickness 86631 Part Thickness Drives Quality and Cost 86632 Cycle Time Increases with Thickness 86633 Thick Parts Tend to Warp 86634 Thin Uniform Parts Improve Surface Quality 87635 Reducing Part Thickness 8764 Boosting Structural Integrity with Ribs 88
641 Structural Integrity the Goal of Every Design 88642 Designing Ribs 88
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XIV Contents
65 Design for Assembly 90651 Molding One Part vs Separate Components 90
652 Tolerances Fit between Parts 90653 Press-fit Joints 91654 Snap-fit Joints 93655 Cantilever Snap Joints 95656 Torsion Snap-fit Joints 96657 Fasteners 98658 Inserts 101659 Welding Processes 101
7 Gate Design 103
71 Gate Design Overview 103711 What Is a Gate 103712 Single vs Multiple Gates 103713 Gate Dimensions 103714 Gate Location 10472 Gate Types 104721 Manually Trimmed Gates 104722 Automatically Trimmed Gates 11073 Design Rules 113
731 Determining the Number of Gates 113732 Flow Patterns 115733 Gate Position 116734 Avoiding Common Problems 122735 Gate Length 122736 Gate Thickness 122737 Freeze-off Time 12374 Using Moldflow for Gate Design 123741 Gate Location 123742 Molding Window Size for the Three Gate Locations 124
743 Filling Pattern 125744 Gate Size Based on Shear Rate 125
8 Runner System Design 127
81 Definitions 127811 Feed System 127812 Runner System 127813 Cold Runner 127814 Hot Runner 128
815 Hot Manifold 128816 Hot Drop 128817 Sprue 128
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Contents XV
82 Runner System Design Principles 129821 Benefits of Good Runner Design 129
822 Runner Design Philosophy 129823 Flow Balancing 130824 Flow Control 130825 Frictional Heating in Runners 132826 Thermal Shutoff 132827 System and Runner Pressures 132828 Constant Pressure Gradient 133829 Cold Slug Wells 1338210 Easy Ejection 13483 Runner Types 134
831 Cold Runners 134832 Hot Runner Systems 13584 Runner Layout 136841 Determining the Number of Cavities 136842 Planning the Runner System Layout 138843 Partially Balanced Runners 139844 Geometrically Balanced Runners 14185 Initial Runner Sizing 141851 Determining Sprue Dimensions 141852 Designing Runner Cross Sections 142
853 Determining Runner Diameters 14586 Runner Balancing 147861 How Runner Balancing Works 147862 When Are the Runner Sizes Optimized 147863 Validating the Balance 147864 Processing Window 14887 Using Moldflow for Runner Balancing 149871 Runner Balancing a 48-cavity Tool 149872 Runner Balancing for a Family Mold 151873 Runner Balancing for a Multigated Part 152
9 Cooling System Design 153
91 Mold Cooling System Overview 153911 Importance of Cooling System Design 153912 Mold Cooling System Components 15492 Cooling-channel Configuration 156921 Types of Cooling Channels 15693 Alternative Cooling Devices 157931 What Do They Do 157
932 Baffles 158933 Bubblers 159934 Thermal Pins 159
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XVI Contents
935 Cooling Slender Cores 160936 Cooling Large Cores 161
937 Cooling Cylinder Cores 16294 Cooling System Equations 162941 Cooling Time 162942 Reynolds Number and Coolant Flow 16395 Design Rules 163951 Mold Cooling Design Considerations 163952 Location and Size of Channels 164953 Flow Rate and Heat Transfer 16696 Using Moldflow for Cooling System Design 167961 Example Setup 168
962 Cycle Time Determined by Design and Processing Parameters 169
10 Shrinkage and Warpage 173
101 Injection Molding and Shrinkage 1731011 What Are Shrinkage and Warpage 1731012 Shrinkage and Machine Settings 1731013 Mold Filling and Packing 1741014 How Pressure and Time Affect Shrinkage 1751015 Thermally Unstable Flow 176
102 Basic Causes of Shrinkage and Warpage 1771021 Causes of Shrinkage 1771022 Causes of Warpage 1811023 Relating Orientation and Area Shrinkage to Warpage 182103 Designing Accurate Parts Considering Warpage 1831031 Material Selection 1841032 Wall Thickness Variation 1841033 Gate Position and Runner Dimensions 1851034 Molding Conditions 1871035 Cooling Line Layout 190
11 Moldflow Design Procedure 191
111 Determine Analysis Objectives 191112 Moldflow Analysis Steps Framework 1921121 The Whole Process 1921122 Optimize Fill 1921123 Balance and Size the Runners 1971124 Optimize Cooling 1991125 Optimize the Packing Profile 200
1126 Optimize Warpage 201113 Using Moldflow to Evaluate an Initial Design 2051131 Description of this Example 205
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Contents XVII
1132 Molding Window 2051133 Filling Analysis 206
1134 Gate and Runner Design 2061135 Cooling System Design 2071136 Packing Analysis 2091137 Warpage Analysis 209114 Using Moldflow to Optimize the Design 2101141 Determine the Cause of Warpage 2101142 Investigating Different Gate Locations 2101143 Validating the Best Gate Location 211
12 Part Defects 215
121 Air Traps 2151211 What Is an Air Trap 2151212 Problems Caused by Air Traps 2161213 Remedies 216122 Black Specks and Black Streaks 2171221 What Are Black Specks and Black Streaks 2171222 Causes of Black Specks and Black Streaks 2171223 Remedies 218123 Brittleness 219
1231 What Is Brittleness 2191232 Causes of Brittleness 2191233 Remedies 219124 Burn Marks 2201241 What Is a Burn Mark 2201242 Causes of Burn Marks 2211243 Remedies 222125 Delamination 2221251 What Is Delamination 2221252 Causes of Delamination 223
1253 Remedies 223126 Dimensional Variation 2241261 What Is Dimensional Variation 2241262 Causes of Dimensional Variation 2241263 Remedies 224127 Discoloration 2251271 What Is Discoloration 2251272 Causes of Discoloration 2251273 Remedies 226128 Fish Eyes 227
1281 What Are Fish Eyes 2271282 Causes of Fish Eyes 2271283 Remedies 227
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XVIII Contents
129 Flash 2281291 What Is Flash 228
1292 Causes of Flash 2281293 Remedies 2291210 Flow Marks 23012101 What Is A Flow Mark 23012102 Causes of Flow Marks 23012103 Remedies 2301211 Hesitation 23112111 What Is Hesitation 23112112 Problems Caused by Hesitation 23112113 Remedies 232
1212 Jetting 23212121 What Is Jetting 23212122 Problems Caused by Jetting 23312123 Remedies 2331213 Ripples 23512131 What Are Ripples 23512132 Cause of Ripples 23512133 Remedies 2361214 Short Shots 23612141 What Is a Short Shot 236
12142 Causes of Short Shots 23712143 Remedies 2371215 Silver Streaks 23812151 What Are Silver Streaks 23812152 Causes of Silver Streaks 23912153 Remedies 2391216 Sink Marks and Voids 24012161 What Are Sink Marks and Voids 24012162 Causes of Sink Marks and Voids 24012163 Remedies 2411217 Weld Lines and Meld Lines 24312171 What Are Weld Lines and Meld Lines 24312172 Problems Caused by Weld Lines 24312173 Strength of Weld Lines 24412174 Remedies 245
Appendix A Injection Molding 247
A1 Injection-molding Overview 247 A11 Process 247
A12 Applications 248 A2 Development of the Injection-molding Machine 248 A21 Benefits of the Reciprocating Screw 248
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Contents XIX
A3 Development of the Injection-molding Process 249 A4 Alternative Injection-molding Processes 249
A41 Co-injection (Sandwich) Molding 249 A42 Fusible Core Injection Molding 250 A43 Gas-assisted Injection Molding 251 A44 Injection-compression Molding 252 A45 Lamellar (Microlayer) Injection Molding 253 A46 Live-feed Injection Molding 253 A47 Low-pressure Injection Molding 254 A48 Push-pull Injection Molding 254 A49 Reactive Molding 255 A410 Structural Foam Injection Molding 256
A411 Thin-wall Molding 256
Appendix B Injection-molding Machine System and Operations 259
B1 Injection-molding Machine 259B11 Components 259B12 Machine Specification 260B13 Machine Function 260B14 Auxiliary Equipment 260B2 Machine Components 261
B21 Injection System 261B22 Mold System 263B23 Hydraulic System 264B24 Control System 264B25 Clamping System 264B3 Molded System 265B31 The Delivery System 265B4 Machine Operating Sequence 266B41 Process Cycle 266B5 Screw Operation 268
B51 Back Pressure 268B52 Injection Speed 268B53 Screw Rotation Speed 268B54 Cushion 269B6 Secondary Operations 269B61 Assembly 269B62 Decoration 269B63 Other Secondary Operations 270
Appendix C Injection-molding Process Control 271
C1 Importance of Process Conditions 271C11 Setting Machine Process Conditions 271
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XX Contents
Appendix D Plastic Materials 281
D1 What Are Plastics 281
D11 Polymerization Process 281D12 Structure of Polymers 281D13 Polymer Alloys and Blends 283D14 Polymer Composites 283D2 Classification of Plastics 283D21 Classes of Plastics 284D22 Structures and Properties of Plastics 284D3 Thermoplastics 285D31 Market Share Distribution of Thermoplastics 285D32 Structures and Properties of Thermoplastics 286D33 Amorphous Polymers 286D34 Semicrystalline Polymers 287D4 Thermosets 288D5 Properties and Applications of Thermoplastics 289D51 ABS 290D52 PA 12 291D53 PA 6 293D54 PA 66 294D55 PBT 296
D56 PC 297D57 PCABS 299D58 PCPBT 300D59 HDPE 301D510 LDPE 303D511 PEI 304D512 PET 305D513 PETG 306D514 PMMA 307D515 POM 309
D516 PP 310D517 PPEPPO 312D518 PS 313D519 PVC 314D520 SAN 316D521 Additives Fillers and Reinforcements 317D522 Modifying Polymer Properties 318D523 Low-aspect Fillers 319D524 High-aspect Fillers Fibers 319
Index 321
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1 Polymer Flow Behavior in Injection Molds
bull Phases of injection molding
bull How do plastics flow
11 Phases of Injection Molding
Any molder can prove that all the conditions and effects discussed in this chapter do indeedoccur during the injection molding process While this knowledge alone can somewhatimprove quality it is only with the use of Moldflow analysis during the initial design stage with the mold designed for the optimum filling pattern that these effects can be controlledand the full benefits obtained
Flow technology is concerned with the behavior of plastics during the mold filling process Aplastic parts properties depend on how the part is molded Two parts having identicaldimensions and made from the same material but molded under different conditions will havedifferent stress and shrinkage levels and will behave differently in the field meaning that they
are in practice two different parts The way the plastic flows into the mold is of paramount importance in determining the quality of the part The process of filling the mold can be distinctly analyzed with the ability to predictpressure temperature and stress
111 How Plastic Fills a Mold
This was investigated using a centrally gated mold shaped like a dinner plate with a thick rim
around the outside as shown in Figure 11 It was found that the injection molding processalthough complex could be divided into three phases (we use the word phase to avoidconfusion with injection stage as used with programmed injection)
Figure 1ure 1 1 Cross-section of disk mold used to investigate flow
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2 Polymer Flow Behavior in Injection Molds
1111 Filling Phase
As the ram moves forward it first moves at a steady speed as the plastic flows into the cavity
This is the filling phase This phase lasts until the mold is just filled See Figure 12 andFigure 13
1112 Pressurization Phase
The pressurization phase begins when the ram moves forward after the filling phase to bring the mold up to pressure When the mold is filled the ram will slow down but it still movesquite some distance because plastics are very compressible materials At injection molding pressure an extra 15 volume of material can be forced into the cavity See Figure 12 andFigure 13
Although fluids are usually assumed to be incompressible molten plastics have to beconsidered to be more like a gas The compressibility of plastics can be observed by blocking off the nozzle and attempting to purge the barrel The ram will jump forward when thepressure is applied but will spring back when the pressure is released
1113 Compensation Phase
After the pressurization phase the ram still does not stop completely continuing to creepforward for some time Plastics have a very large volumetric change of about 25 from themelt to the solid This can be seen in a short shot the difference in volume between themolding and the cavity is due to this volumetric change See Figure 12 and Figure 13
The ram moving forward to compensate for the volumetric change in the part is called thecompensation phase As the volumetric change is 25 and at the most only an extra 15 canbe injected in the pressurization phase there must always be some compensation phase
Figure 1igure 1 2 Phases of injection molding
Filling phase
Pressurization phase
Compensation phase
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Phases of Injection Molding 3
Figure 13ure 1 3 Phases of injection molding detail
112 The Filling Phase
A two-color technique best demonstrates this phase After emptying the barrel of an injection-molding machine a small amount of red plastic was charged followed by green plastic
Consider the closed mold with the plastic front just starting to flow from the nozzle Theplastic first fills the sprue and runner system then enters the mold cavity itself forming a smallbubble of molten plastic
The skin of the plastic in contact with the cool mold freezes rapidly while the central coreremains molten When additional material is injected it flows into this central core displacing
the material already there which then forms a new flow front The flow of this displacedmaterial is a combination of forward flow and outward flow The outward flow contacts the wall freezes and forms the next section of skin while the forward flow forms the new moltencore When more material enters the mold it flows along a channel lined with these frozen walls of plastic illustrated in Figure 14
This flow pattern is often called fountain flow or bubble flow because the flow front is like abubble being inflated with hot plastic from the center The frozen layer is formed by the flow front inflating and so is subject to only a low shear stress and therefore has a very low levelof molecular orientation Once it is frozen it cannot be orientated any further so the frozen
layer in the finished part has a low level of orientation
Figure 14ure 1 4 Fountain flow and heat transfer
Filling Pressurization Compensation
Hot plastic
Frozen layer
Heat loss to mold
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4 Polymer Flow Behavior in Injection Molds
Now consider what happens upstream Hot plastic is continuously flowing bringing new hotmaterial along and generating significant frictional heat At the same time heat is being lost
through the frozen layer to the cold mold surfaceInitially the frozen layer is very thin so heat is lost very rapidly This results in more plasticfreezing and the frozen layer getting thicker cutting down the heat flow After a time thefrozen layer will reach a thickness such that the heat lost by conduction is equal to the heatinput from plastic flow and frictional heating ie an equilibrium condition is reached(Figure 14)
It is interesting to do some calculations on the time taken to reach this state of equilibrium The actual rate of heat flow is very large in comparison with the small heat content of theplastic in the frozen layer The result is that equilibrium is reached very quickly often in a timemeasured in a few tenths of a second As the total filling time is measured in seconds thefrozen layer reaches an equilibrium state early in the filling cycle
It is useful to think about how the thickness of this frozen layer will vary If the injection rate were slowed less heat would be generated by friction along the flow path with less heat inputfrom the flow The heat loss would be at the same rate and with less heat input the frozenlayer would grow in thickness If the injection rate were raised the frozen layer would bethinner (Figure 15) Similarly higher melt and mold temperatures would reduce the thicknessof the frozen layer This can be seen experimentally using the two-color technique
Figure 1igure 1 5 Influence of injection rate on frozen layer thickness
1121 Flow Shear Stress
It is easy to get confused between the various stress levels and orientation of the polymer Asthe plastic flows it is subject to shear stress also called flow shear stress This flow shear stress willorient the material ie cause the molecules to align themselves in the general direction of flow
The shear stress varies from a maximum at the outside dropping off to zero at the center
3Shear stress is purely a function of force and area This must not be confused withshear rate which is the rate of plastic sliding over the next layer Shear rate is zero
= Frozen
= Molten cooler
= Molten hotter
Slowerinjection
rate
Faster
injection
rate
vs
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Phases of Injection Molding 5
at the outer edge where the plastic is frozen rises to a maximum just inwards of thefrozen layer then drops toward the center as shown in Figure 16
Figure 16ure 1 6 Shear rate distribution
If the flow were stopped and the plastic allowed to cool down very slowly this orientation would have time to relax giving a very low level of residual orientation On the other hand if the material were kept under stress and the plastic snap frozen most of the orientation would
be trapped in the frozen plastic (Figure 17)
Figure 17ure 1 7 Molecular orientation through the thickness of the part
Min MaxShear rate
= Tension
= Compression
Molecular orientation trapped
in pressurization phase
Gradual reduction of orientation
toward the center of the
mold as stress levels are lower
and the cooling rate is slower
which allows more time for
orientation to relax
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6 Polymer Flow Behavior in Injection Molds
Now consider the orientation from the mold surface toward the center
The frozen layer itself formed with very little shear and therefore low orientation immediately
freezes setting the low level of orientation
The layer of plastic just on the inside of the frozen layer is subject to maximum shear stressand freezes the instant flow stops trapping almost all the orientation
This is the orientation pattern the further toward the center the more the shear stress dropsand the slower the rate of cooling This allows more time for the level of orientation to relaxso the residual orientation drops rapidly toward the center Consider how this pattern willaffect the residual stress level Oriented material (normally) will shrink more than nonorientedmaterial On the inner surface of the original frozen layer highly oriented material wants toshrink a great deal but it is prevented from doing so by the less-oriented material The highly
oriented layer ends up being in tension while the less-oriented material is in compression This residual stress pattern is a common cause of part warpage
3 There is a connectionthrough orientationbetween the shear stress during
filling (flow stress) and the residual stress in the final molded part This meansshear stress during filling shown on Moldflow plots can be used as a designparameter
113 The Pressurization Phase
The pressurization phasefrom the point of view of flow behavioris very similar to thefilling phase The flow rate may drop somewhat as the mold builds up to pressure resulting inan increase in the thickness of the frozen layer
The main difference of course is the increase in hydrostatic (isotropic) pressure We shall seein chapter 2 section 24 Effect of Molding Conditions that hydrostatic pressure in itself doesnot cause any residual stress
114 The Compensation Phase
Compensating flow is unstable Consider the plate molding again (see Figure 11) You wouldthink that plastic flowing uniformly through the thin diaphragm would top up the thick rim Inpractice the plastic during the compensation phase flows in rivers that spread out like a deltaas illustrated in Figure 18 This may seem surprising at first but it can be explained by temperature instability
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How Do Plastics Flow 7
Figure 18ure 1 8 River flow
1141 Temperature Variation
There is always some variation in melt temperature coming from the barrel of the injectionmachine In exceptional cases up to 40 ordmC variation has been measured using a high-speedthermocouple
1142 Natural Instability
However slight the temperature variation natural instability will amplify it If for exampleone part of the melt is slightly hotter than the rest then the plastic flow in that area will beslightly greater bringing hotter material into the area and maintaining the temperature If onthe other hand there is another area that is cooler the flow will be less so there will be lessheat input and the plastic will get colder until it eventually freezes off
However balanced the initial conditions this natural instability will result in a river-type flow
This is a very important consideration The first material to freeze off will shrink early in thecycle By the time the material in the river flows freezes the bulk of the material will havealready frozen off and shrinkage will have occurred The rivers will shrink relative to the bulk of the molding and because they are highly orientated shrinkage will be very high The resultis high-stress tensile members throughout the molding a common cause of warpage
1143 Optimum Part Quality
Most of the stress in plastic parts occurs during the compensation phase By controlling flow and minimizing stress it is possible to design for optimum part quality This important point is
at the heart of the Moldflow philosophy
12 How Do Plastics Flow
121 Material Behavior
Molten thermoplastics exhibit viscoelastic behavior which combines flow characteristics of both viscous liquids and elastic solids When a viscous liquid flows the energy that causes thedeformation is dissipated and becomes viscous heat On the other hand when an elastic solid
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8 Polymer Flow Behavior in Injection Molds
is deformed the driving energy is stored For example the flow of water is a typical viscousflow whereas the deformation of a rubber cube falls into the elastic category
122 Deformation
In addition to the two types of material flow behavior there are two types of deformationsimple shear and simple extension (elongation) as shown in Figure 19 (a) and (b) below Theflow of molten thermoplastics during injection-molding filling is predominantly shear flow asshown in Figure 19 (c) in which layers of material elements slide over each other Theextensional flow however becomes significant as the material elements undergo elongation when the melt passes areas of abrupt dimensional change (eg a gate region) as shown inFigure 19 (d)
Figure 1igure 1 9 (a) Simple shear flow (b) Simple extensional flow (c) Shear flow in cavity filling (d) Extensional flow in cavity filling
123 Viscoelastic Behavior
In response to an applied stress (force per unit area) molten thermoplastics exhibit viscoelastic behavior which combines characteristics of an ideal viscous liquid with those of an ideal elastic solid In other words under certain conditions molten thermoplastics behavelike a liquid and will continuously deform while shear stress is applied as shown in
Figure 110 Upon the removal of the stress however the materials behave somewhat like anelastic solid with partial recovery of the deformation as shown in Figure 110 (b) and (c) This viscoelastic behavior stems from the random-coil configuration of polymer molecules in the
Stress
Stress Stress
Mold wall
Material element before
deformation begins
Material element during
deformation
Mold wall
Material elements at rest
Moving
materialelements
Velocity
profile
(a) (b)
(c) (d)
Velocity
profile
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How Do Plastics Flow 9
molten state which allows the movement and slippage of molecular chains under theinfluence of an applied load However the entanglement of the polymer molecular chains also
makes the system behave like an elastic solid upon the application and removal of the externalload Namely on removal of the stress chains will tend to return to the equilibrium random-coil state and thus will be a component of stress recovery The recovery is not instantaneousbecause of the entanglements still present in the system
Figure 1ure 1 10 (a) Ideal viscous liquid deforms continuously under applied stress (b) Ideal elasticsolid deforms immediately upon the application of stress but fully recovers when the
stress is removed (c) Molten thermoplastic deforms continuously under the applied
stress (like a viscous liquid) but also recovers partially from the deformation upon
removal of the applied stress (like an elastic solid)
124 Melt Shear Viscosity
1241 What Is Shear Viscosity
Melt shear viscosity is a materials resistance to shear flow In general polymer melts are highly viscous because of their long molecular chain structure The viscosity of a polymer melt rangesfrom 2 to 3000 Pas (water 10^-1 Pas glass 10^20 Pas) Viscosity can be thought of as thethickness of a fluid or how much it resists flow Viscosity is expressed as the ratio of shearstress (force per unit area) to the shear rate (rate change of shear strain) as shown inEquation 11 and Figure 111
(a) Viscous
liquid
(b) Elastic
solid
(c) Viscoelastic
polymer
melt
Initial shape
(shear stress
to be applied)
Time = Dt Time = 2 x Dt
(shear stress to
be removed)
Final deformed
shape
d
d
d
d
2d
2d
2d
2d
total
recovery
partial
recovery
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Contents XIII
436 Positioning Weld and Meld Lines 55437 Avoid Hesitation Effects 55
438 Avoid Underflow 55439 Balancing with Flow Leaders and Flow Deflectors 574310 Controlled Frictional Heat 584311 Thermal Shutoff of Runners 584312 Acceptable RunnerCavity Ratio 59
5 Meshes Used In Moldf low Analyses 61
51 Mesh Types Used by Moldflow 61511 Finite Elements Used in Moldflow 61512 Mesh Types 62513 Solver Assumptions 6352 Mesh Requirements 64521 Mesh Density Considerations 64522 Part Details 6753 Geometry Creation 6854 Importing Geometry 6955 Using Different Mesh Types 69551 Door Panel 69552 Manifold 70
6 Product Design 71
61 Material Properties for Product Design 71611 Plastics Are Sensitive to Operating Conditions 71612 Stress-Strain Behavior 72613 Creep and Stress Relaxation 77614 Fatigue 79615 Impact strength 80616 Thermal Mechanical Behavior 8062 Design for Strength 82621 Predicting Part Strength 82622 LoadingOperating Conditions 8263 Part Thickness 86631 Part Thickness Drives Quality and Cost 86632 Cycle Time Increases with Thickness 86633 Thick Parts Tend to Warp 86634 Thin Uniform Parts Improve Surface Quality 87635 Reducing Part Thickness 8764 Boosting Structural Integrity with Ribs 88
641 Structural Integrity the Goal of Every Design 88642 Designing Ribs 88
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XIV Contents
65 Design for Assembly 90651 Molding One Part vs Separate Components 90
652 Tolerances Fit between Parts 90653 Press-fit Joints 91654 Snap-fit Joints 93655 Cantilever Snap Joints 95656 Torsion Snap-fit Joints 96657 Fasteners 98658 Inserts 101659 Welding Processes 101
7 Gate Design 103
71 Gate Design Overview 103711 What Is a Gate 103712 Single vs Multiple Gates 103713 Gate Dimensions 103714 Gate Location 10472 Gate Types 104721 Manually Trimmed Gates 104722 Automatically Trimmed Gates 11073 Design Rules 113
731 Determining the Number of Gates 113732 Flow Patterns 115733 Gate Position 116734 Avoiding Common Problems 122735 Gate Length 122736 Gate Thickness 122737 Freeze-off Time 12374 Using Moldflow for Gate Design 123741 Gate Location 123742 Molding Window Size for the Three Gate Locations 124
743 Filling Pattern 125744 Gate Size Based on Shear Rate 125
8 Runner System Design 127
81 Definitions 127811 Feed System 127812 Runner System 127813 Cold Runner 127814 Hot Runner 128
815 Hot Manifold 128816 Hot Drop 128817 Sprue 128
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Contents XV
82 Runner System Design Principles 129821 Benefits of Good Runner Design 129
822 Runner Design Philosophy 129823 Flow Balancing 130824 Flow Control 130825 Frictional Heating in Runners 132826 Thermal Shutoff 132827 System and Runner Pressures 132828 Constant Pressure Gradient 133829 Cold Slug Wells 1338210 Easy Ejection 13483 Runner Types 134
831 Cold Runners 134832 Hot Runner Systems 13584 Runner Layout 136841 Determining the Number of Cavities 136842 Planning the Runner System Layout 138843 Partially Balanced Runners 139844 Geometrically Balanced Runners 14185 Initial Runner Sizing 141851 Determining Sprue Dimensions 141852 Designing Runner Cross Sections 142
853 Determining Runner Diameters 14586 Runner Balancing 147861 How Runner Balancing Works 147862 When Are the Runner Sizes Optimized 147863 Validating the Balance 147864 Processing Window 14887 Using Moldflow for Runner Balancing 149871 Runner Balancing a 48-cavity Tool 149872 Runner Balancing for a Family Mold 151873 Runner Balancing for a Multigated Part 152
9 Cooling System Design 153
91 Mold Cooling System Overview 153911 Importance of Cooling System Design 153912 Mold Cooling System Components 15492 Cooling-channel Configuration 156921 Types of Cooling Channels 15693 Alternative Cooling Devices 157931 What Do They Do 157
932 Baffles 158933 Bubblers 159934 Thermal Pins 159
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XVI Contents
935 Cooling Slender Cores 160936 Cooling Large Cores 161
937 Cooling Cylinder Cores 16294 Cooling System Equations 162941 Cooling Time 162942 Reynolds Number and Coolant Flow 16395 Design Rules 163951 Mold Cooling Design Considerations 163952 Location and Size of Channels 164953 Flow Rate and Heat Transfer 16696 Using Moldflow for Cooling System Design 167961 Example Setup 168
962 Cycle Time Determined by Design and Processing Parameters 169
10 Shrinkage and Warpage 173
101 Injection Molding and Shrinkage 1731011 What Are Shrinkage and Warpage 1731012 Shrinkage and Machine Settings 1731013 Mold Filling and Packing 1741014 How Pressure and Time Affect Shrinkage 1751015 Thermally Unstable Flow 176
102 Basic Causes of Shrinkage and Warpage 1771021 Causes of Shrinkage 1771022 Causes of Warpage 1811023 Relating Orientation and Area Shrinkage to Warpage 182103 Designing Accurate Parts Considering Warpage 1831031 Material Selection 1841032 Wall Thickness Variation 1841033 Gate Position and Runner Dimensions 1851034 Molding Conditions 1871035 Cooling Line Layout 190
11 Moldflow Design Procedure 191
111 Determine Analysis Objectives 191112 Moldflow Analysis Steps Framework 1921121 The Whole Process 1921122 Optimize Fill 1921123 Balance and Size the Runners 1971124 Optimize Cooling 1991125 Optimize the Packing Profile 200
1126 Optimize Warpage 201113 Using Moldflow to Evaluate an Initial Design 2051131 Description of this Example 205
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Contents XVII
1132 Molding Window 2051133 Filling Analysis 206
1134 Gate and Runner Design 2061135 Cooling System Design 2071136 Packing Analysis 2091137 Warpage Analysis 209114 Using Moldflow to Optimize the Design 2101141 Determine the Cause of Warpage 2101142 Investigating Different Gate Locations 2101143 Validating the Best Gate Location 211
12 Part Defects 215
121 Air Traps 2151211 What Is an Air Trap 2151212 Problems Caused by Air Traps 2161213 Remedies 216122 Black Specks and Black Streaks 2171221 What Are Black Specks and Black Streaks 2171222 Causes of Black Specks and Black Streaks 2171223 Remedies 218123 Brittleness 219
1231 What Is Brittleness 2191232 Causes of Brittleness 2191233 Remedies 219124 Burn Marks 2201241 What Is a Burn Mark 2201242 Causes of Burn Marks 2211243 Remedies 222125 Delamination 2221251 What Is Delamination 2221252 Causes of Delamination 223
1253 Remedies 223126 Dimensional Variation 2241261 What Is Dimensional Variation 2241262 Causes of Dimensional Variation 2241263 Remedies 224127 Discoloration 2251271 What Is Discoloration 2251272 Causes of Discoloration 2251273 Remedies 226128 Fish Eyes 227
1281 What Are Fish Eyes 2271282 Causes of Fish Eyes 2271283 Remedies 227
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XVIII Contents
129 Flash 2281291 What Is Flash 228
1292 Causes of Flash 2281293 Remedies 2291210 Flow Marks 23012101 What Is A Flow Mark 23012102 Causes of Flow Marks 23012103 Remedies 2301211 Hesitation 23112111 What Is Hesitation 23112112 Problems Caused by Hesitation 23112113 Remedies 232
1212 Jetting 23212121 What Is Jetting 23212122 Problems Caused by Jetting 23312123 Remedies 2331213 Ripples 23512131 What Are Ripples 23512132 Cause of Ripples 23512133 Remedies 2361214 Short Shots 23612141 What Is a Short Shot 236
12142 Causes of Short Shots 23712143 Remedies 2371215 Silver Streaks 23812151 What Are Silver Streaks 23812152 Causes of Silver Streaks 23912153 Remedies 2391216 Sink Marks and Voids 24012161 What Are Sink Marks and Voids 24012162 Causes of Sink Marks and Voids 24012163 Remedies 2411217 Weld Lines and Meld Lines 24312171 What Are Weld Lines and Meld Lines 24312172 Problems Caused by Weld Lines 24312173 Strength of Weld Lines 24412174 Remedies 245
Appendix A Injection Molding 247
A1 Injection-molding Overview 247 A11 Process 247
A12 Applications 248 A2 Development of the Injection-molding Machine 248 A21 Benefits of the Reciprocating Screw 248
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Contents XIX
A3 Development of the Injection-molding Process 249 A4 Alternative Injection-molding Processes 249
A41 Co-injection (Sandwich) Molding 249 A42 Fusible Core Injection Molding 250 A43 Gas-assisted Injection Molding 251 A44 Injection-compression Molding 252 A45 Lamellar (Microlayer) Injection Molding 253 A46 Live-feed Injection Molding 253 A47 Low-pressure Injection Molding 254 A48 Push-pull Injection Molding 254 A49 Reactive Molding 255 A410 Structural Foam Injection Molding 256
A411 Thin-wall Molding 256
Appendix B Injection-molding Machine System and Operations 259
B1 Injection-molding Machine 259B11 Components 259B12 Machine Specification 260B13 Machine Function 260B14 Auxiliary Equipment 260B2 Machine Components 261
B21 Injection System 261B22 Mold System 263B23 Hydraulic System 264B24 Control System 264B25 Clamping System 264B3 Molded System 265B31 The Delivery System 265B4 Machine Operating Sequence 266B41 Process Cycle 266B5 Screw Operation 268
B51 Back Pressure 268B52 Injection Speed 268B53 Screw Rotation Speed 268B54 Cushion 269B6 Secondary Operations 269B61 Assembly 269B62 Decoration 269B63 Other Secondary Operations 270
Appendix C Injection-molding Process Control 271
C1 Importance of Process Conditions 271C11 Setting Machine Process Conditions 271
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XX Contents
Appendix D Plastic Materials 281
D1 What Are Plastics 281
D11 Polymerization Process 281D12 Structure of Polymers 281D13 Polymer Alloys and Blends 283D14 Polymer Composites 283D2 Classification of Plastics 283D21 Classes of Plastics 284D22 Structures and Properties of Plastics 284D3 Thermoplastics 285D31 Market Share Distribution of Thermoplastics 285D32 Structures and Properties of Thermoplastics 286D33 Amorphous Polymers 286D34 Semicrystalline Polymers 287D4 Thermosets 288D5 Properties and Applications of Thermoplastics 289D51 ABS 290D52 PA 12 291D53 PA 6 293D54 PA 66 294D55 PBT 296
D56 PC 297D57 PCABS 299D58 PCPBT 300D59 HDPE 301D510 LDPE 303D511 PEI 304D512 PET 305D513 PETG 306D514 PMMA 307D515 POM 309
D516 PP 310D517 PPEPPO 312D518 PS 313D519 PVC 314D520 SAN 316D521 Additives Fillers and Reinforcements 317D522 Modifying Polymer Properties 318D523 Low-aspect Fillers 319D524 High-aspect Fillers Fibers 319
Index 321
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1 Polymer Flow Behavior in Injection Molds
bull Phases of injection molding
bull How do plastics flow
11 Phases of Injection Molding
Any molder can prove that all the conditions and effects discussed in this chapter do indeedoccur during the injection molding process While this knowledge alone can somewhatimprove quality it is only with the use of Moldflow analysis during the initial design stage with the mold designed for the optimum filling pattern that these effects can be controlledand the full benefits obtained
Flow technology is concerned with the behavior of plastics during the mold filling process Aplastic parts properties depend on how the part is molded Two parts having identicaldimensions and made from the same material but molded under different conditions will havedifferent stress and shrinkage levels and will behave differently in the field meaning that they
are in practice two different parts The way the plastic flows into the mold is of paramount importance in determining the quality of the part The process of filling the mold can be distinctly analyzed with the ability to predictpressure temperature and stress
111 How Plastic Fills a Mold
This was investigated using a centrally gated mold shaped like a dinner plate with a thick rim
around the outside as shown in Figure 11 It was found that the injection molding processalthough complex could be divided into three phases (we use the word phase to avoidconfusion with injection stage as used with programmed injection)
Figure 1ure 1 1 Cross-section of disk mold used to investigate flow
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2 Polymer Flow Behavior in Injection Molds
1111 Filling Phase
As the ram moves forward it first moves at a steady speed as the plastic flows into the cavity
This is the filling phase This phase lasts until the mold is just filled See Figure 12 andFigure 13
1112 Pressurization Phase
The pressurization phase begins when the ram moves forward after the filling phase to bring the mold up to pressure When the mold is filled the ram will slow down but it still movesquite some distance because plastics are very compressible materials At injection molding pressure an extra 15 volume of material can be forced into the cavity See Figure 12 andFigure 13
Although fluids are usually assumed to be incompressible molten plastics have to beconsidered to be more like a gas The compressibility of plastics can be observed by blocking off the nozzle and attempting to purge the barrel The ram will jump forward when thepressure is applied but will spring back when the pressure is released
1113 Compensation Phase
After the pressurization phase the ram still does not stop completely continuing to creepforward for some time Plastics have a very large volumetric change of about 25 from themelt to the solid This can be seen in a short shot the difference in volume between themolding and the cavity is due to this volumetric change See Figure 12 and Figure 13
The ram moving forward to compensate for the volumetric change in the part is called thecompensation phase As the volumetric change is 25 and at the most only an extra 15 canbe injected in the pressurization phase there must always be some compensation phase
Figure 1igure 1 2 Phases of injection molding
Filling phase
Pressurization phase
Compensation phase
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Phases of Injection Molding 3
Figure 13ure 1 3 Phases of injection molding detail
112 The Filling Phase
A two-color technique best demonstrates this phase After emptying the barrel of an injection-molding machine a small amount of red plastic was charged followed by green plastic
Consider the closed mold with the plastic front just starting to flow from the nozzle Theplastic first fills the sprue and runner system then enters the mold cavity itself forming a smallbubble of molten plastic
The skin of the plastic in contact with the cool mold freezes rapidly while the central coreremains molten When additional material is injected it flows into this central core displacing
the material already there which then forms a new flow front The flow of this displacedmaterial is a combination of forward flow and outward flow The outward flow contacts the wall freezes and forms the next section of skin while the forward flow forms the new moltencore When more material enters the mold it flows along a channel lined with these frozen walls of plastic illustrated in Figure 14
This flow pattern is often called fountain flow or bubble flow because the flow front is like abubble being inflated with hot plastic from the center The frozen layer is formed by the flow front inflating and so is subject to only a low shear stress and therefore has a very low levelof molecular orientation Once it is frozen it cannot be orientated any further so the frozen
layer in the finished part has a low level of orientation
Figure 14ure 1 4 Fountain flow and heat transfer
Filling Pressurization Compensation
Hot plastic
Frozen layer
Heat loss to mold
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4 Polymer Flow Behavior in Injection Molds
Now consider what happens upstream Hot plastic is continuously flowing bringing new hotmaterial along and generating significant frictional heat At the same time heat is being lost
through the frozen layer to the cold mold surfaceInitially the frozen layer is very thin so heat is lost very rapidly This results in more plasticfreezing and the frozen layer getting thicker cutting down the heat flow After a time thefrozen layer will reach a thickness such that the heat lost by conduction is equal to the heatinput from plastic flow and frictional heating ie an equilibrium condition is reached(Figure 14)
It is interesting to do some calculations on the time taken to reach this state of equilibrium The actual rate of heat flow is very large in comparison with the small heat content of theplastic in the frozen layer The result is that equilibrium is reached very quickly often in a timemeasured in a few tenths of a second As the total filling time is measured in seconds thefrozen layer reaches an equilibrium state early in the filling cycle
It is useful to think about how the thickness of this frozen layer will vary If the injection rate were slowed less heat would be generated by friction along the flow path with less heat inputfrom the flow The heat loss would be at the same rate and with less heat input the frozenlayer would grow in thickness If the injection rate were raised the frozen layer would bethinner (Figure 15) Similarly higher melt and mold temperatures would reduce the thicknessof the frozen layer This can be seen experimentally using the two-color technique
Figure 1igure 1 5 Influence of injection rate on frozen layer thickness
1121 Flow Shear Stress
It is easy to get confused between the various stress levels and orientation of the polymer Asthe plastic flows it is subject to shear stress also called flow shear stress This flow shear stress willorient the material ie cause the molecules to align themselves in the general direction of flow
The shear stress varies from a maximum at the outside dropping off to zero at the center
3Shear stress is purely a function of force and area This must not be confused withshear rate which is the rate of plastic sliding over the next layer Shear rate is zero
= Frozen
= Molten cooler
= Molten hotter
Slowerinjection
rate
Faster
injection
rate
vs
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Phases of Injection Molding 5
at the outer edge where the plastic is frozen rises to a maximum just inwards of thefrozen layer then drops toward the center as shown in Figure 16
Figure 16ure 1 6 Shear rate distribution
If the flow were stopped and the plastic allowed to cool down very slowly this orientation would have time to relax giving a very low level of residual orientation On the other hand if the material were kept under stress and the plastic snap frozen most of the orientation would
be trapped in the frozen plastic (Figure 17)
Figure 17ure 1 7 Molecular orientation through the thickness of the part
Min MaxShear rate
= Tension
= Compression
Molecular orientation trapped
in pressurization phase
Gradual reduction of orientation
toward the center of the
mold as stress levels are lower
and the cooling rate is slower
which allows more time for
orientation to relax
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6 Polymer Flow Behavior in Injection Molds
Now consider the orientation from the mold surface toward the center
The frozen layer itself formed with very little shear and therefore low orientation immediately
freezes setting the low level of orientation
The layer of plastic just on the inside of the frozen layer is subject to maximum shear stressand freezes the instant flow stops trapping almost all the orientation
This is the orientation pattern the further toward the center the more the shear stress dropsand the slower the rate of cooling This allows more time for the level of orientation to relaxso the residual orientation drops rapidly toward the center Consider how this pattern willaffect the residual stress level Oriented material (normally) will shrink more than nonorientedmaterial On the inner surface of the original frozen layer highly oriented material wants toshrink a great deal but it is prevented from doing so by the less-oriented material The highly
oriented layer ends up being in tension while the less-oriented material is in compression This residual stress pattern is a common cause of part warpage
3 There is a connectionthrough orientationbetween the shear stress during
filling (flow stress) and the residual stress in the final molded part This meansshear stress during filling shown on Moldflow plots can be used as a designparameter
113 The Pressurization Phase
The pressurization phasefrom the point of view of flow behavioris very similar to thefilling phase The flow rate may drop somewhat as the mold builds up to pressure resulting inan increase in the thickness of the frozen layer
The main difference of course is the increase in hydrostatic (isotropic) pressure We shall seein chapter 2 section 24 Effect of Molding Conditions that hydrostatic pressure in itself doesnot cause any residual stress
114 The Compensation Phase
Compensating flow is unstable Consider the plate molding again (see Figure 11) You wouldthink that plastic flowing uniformly through the thin diaphragm would top up the thick rim Inpractice the plastic during the compensation phase flows in rivers that spread out like a deltaas illustrated in Figure 18 This may seem surprising at first but it can be explained by temperature instability
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How Do Plastics Flow 7
Figure 18ure 1 8 River flow
1141 Temperature Variation
There is always some variation in melt temperature coming from the barrel of the injectionmachine In exceptional cases up to 40 ordmC variation has been measured using a high-speedthermocouple
1142 Natural Instability
However slight the temperature variation natural instability will amplify it If for exampleone part of the melt is slightly hotter than the rest then the plastic flow in that area will beslightly greater bringing hotter material into the area and maintaining the temperature If onthe other hand there is another area that is cooler the flow will be less so there will be lessheat input and the plastic will get colder until it eventually freezes off
However balanced the initial conditions this natural instability will result in a river-type flow
This is a very important consideration The first material to freeze off will shrink early in thecycle By the time the material in the river flows freezes the bulk of the material will havealready frozen off and shrinkage will have occurred The rivers will shrink relative to the bulk of the molding and because they are highly orientated shrinkage will be very high The resultis high-stress tensile members throughout the molding a common cause of warpage
1143 Optimum Part Quality
Most of the stress in plastic parts occurs during the compensation phase By controlling flow and minimizing stress it is possible to design for optimum part quality This important point is
at the heart of the Moldflow philosophy
12 How Do Plastics Flow
121 Material Behavior
Molten thermoplastics exhibit viscoelastic behavior which combines flow characteristics of both viscous liquids and elastic solids When a viscous liquid flows the energy that causes thedeformation is dissipated and becomes viscous heat On the other hand when an elastic solid
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8 Polymer Flow Behavior in Injection Molds
is deformed the driving energy is stored For example the flow of water is a typical viscousflow whereas the deformation of a rubber cube falls into the elastic category
122 Deformation
In addition to the two types of material flow behavior there are two types of deformationsimple shear and simple extension (elongation) as shown in Figure 19 (a) and (b) below Theflow of molten thermoplastics during injection-molding filling is predominantly shear flow asshown in Figure 19 (c) in which layers of material elements slide over each other Theextensional flow however becomes significant as the material elements undergo elongation when the melt passes areas of abrupt dimensional change (eg a gate region) as shown inFigure 19 (d)
Figure 1igure 1 9 (a) Simple shear flow (b) Simple extensional flow (c) Shear flow in cavity filling (d) Extensional flow in cavity filling
123 Viscoelastic Behavior
In response to an applied stress (force per unit area) molten thermoplastics exhibit viscoelastic behavior which combines characteristics of an ideal viscous liquid with those of an ideal elastic solid In other words under certain conditions molten thermoplastics behavelike a liquid and will continuously deform while shear stress is applied as shown in
Figure 110 Upon the removal of the stress however the materials behave somewhat like anelastic solid with partial recovery of the deformation as shown in Figure 110 (b) and (c) This viscoelastic behavior stems from the random-coil configuration of polymer molecules in the
Stress
Stress Stress
Mold wall
Material element before
deformation begins
Material element during
deformation
Mold wall
Material elements at rest
Moving
materialelements
Velocity
profile
(a) (b)
(c) (d)
Velocity
profile
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How Do Plastics Flow 9
molten state which allows the movement and slippage of molecular chains under theinfluence of an applied load However the entanglement of the polymer molecular chains also
makes the system behave like an elastic solid upon the application and removal of the externalload Namely on removal of the stress chains will tend to return to the equilibrium random-coil state and thus will be a component of stress recovery The recovery is not instantaneousbecause of the entanglements still present in the system
Figure 1ure 1 10 (a) Ideal viscous liquid deforms continuously under applied stress (b) Ideal elasticsolid deforms immediately upon the application of stress but fully recovers when the
stress is removed (c) Molten thermoplastic deforms continuously under the applied
stress (like a viscous liquid) but also recovers partially from the deformation upon
removal of the applied stress (like an elastic solid)
124 Melt Shear Viscosity
1241 What Is Shear Viscosity
Melt shear viscosity is a materials resistance to shear flow In general polymer melts are highly viscous because of their long molecular chain structure The viscosity of a polymer melt rangesfrom 2 to 3000 Pas (water 10^-1 Pas glass 10^20 Pas) Viscosity can be thought of as thethickness of a fluid or how much it resists flow Viscosity is expressed as the ratio of shearstress (force per unit area) to the shear rate (rate change of shear strain) as shown inEquation 11 and Figure 111
(a) Viscous
liquid
(b) Elastic
solid
(c) Viscoelastic
polymer
melt
Initial shape
(shear stress
to be applied)
Time = Dt Time = 2 x Dt
(shear stress to
be removed)
Final deformed
shape
d
d
d
d
2d
2d
2d
2d
total
recovery
partial
recovery
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XIV Contents
65 Design for Assembly 90651 Molding One Part vs Separate Components 90
652 Tolerances Fit between Parts 90653 Press-fit Joints 91654 Snap-fit Joints 93655 Cantilever Snap Joints 95656 Torsion Snap-fit Joints 96657 Fasteners 98658 Inserts 101659 Welding Processes 101
7 Gate Design 103
71 Gate Design Overview 103711 What Is a Gate 103712 Single vs Multiple Gates 103713 Gate Dimensions 103714 Gate Location 10472 Gate Types 104721 Manually Trimmed Gates 104722 Automatically Trimmed Gates 11073 Design Rules 113
731 Determining the Number of Gates 113732 Flow Patterns 115733 Gate Position 116734 Avoiding Common Problems 122735 Gate Length 122736 Gate Thickness 122737 Freeze-off Time 12374 Using Moldflow for Gate Design 123741 Gate Location 123742 Molding Window Size for the Three Gate Locations 124
743 Filling Pattern 125744 Gate Size Based on Shear Rate 125
8 Runner System Design 127
81 Definitions 127811 Feed System 127812 Runner System 127813 Cold Runner 127814 Hot Runner 128
815 Hot Manifold 128816 Hot Drop 128817 Sprue 128
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Contents XV
82 Runner System Design Principles 129821 Benefits of Good Runner Design 129
822 Runner Design Philosophy 129823 Flow Balancing 130824 Flow Control 130825 Frictional Heating in Runners 132826 Thermal Shutoff 132827 System and Runner Pressures 132828 Constant Pressure Gradient 133829 Cold Slug Wells 1338210 Easy Ejection 13483 Runner Types 134
831 Cold Runners 134832 Hot Runner Systems 13584 Runner Layout 136841 Determining the Number of Cavities 136842 Planning the Runner System Layout 138843 Partially Balanced Runners 139844 Geometrically Balanced Runners 14185 Initial Runner Sizing 141851 Determining Sprue Dimensions 141852 Designing Runner Cross Sections 142
853 Determining Runner Diameters 14586 Runner Balancing 147861 How Runner Balancing Works 147862 When Are the Runner Sizes Optimized 147863 Validating the Balance 147864 Processing Window 14887 Using Moldflow for Runner Balancing 149871 Runner Balancing a 48-cavity Tool 149872 Runner Balancing for a Family Mold 151873 Runner Balancing for a Multigated Part 152
9 Cooling System Design 153
91 Mold Cooling System Overview 153911 Importance of Cooling System Design 153912 Mold Cooling System Components 15492 Cooling-channel Configuration 156921 Types of Cooling Channels 15693 Alternative Cooling Devices 157931 What Do They Do 157
932 Baffles 158933 Bubblers 159934 Thermal Pins 159
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XVI Contents
935 Cooling Slender Cores 160936 Cooling Large Cores 161
937 Cooling Cylinder Cores 16294 Cooling System Equations 162941 Cooling Time 162942 Reynolds Number and Coolant Flow 16395 Design Rules 163951 Mold Cooling Design Considerations 163952 Location and Size of Channels 164953 Flow Rate and Heat Transfer 16696 Using Moldflow for Cooling System Design 167961 Example Setup 168
962 Cycle Time Determined by Design and Processing Parameters 169
10 Shrinkage and Warpage 173
101 Injection Molding and Shrinkage 1731011 What Are Shrinkage and Warpage 1731012 Shrinkage and Machine Settings 1731013 Mold Filling and Packing 1741014 How Pressure and Time Affect Shrinkage 1751015 Thermally Unstable Flow 176
102 Basic Causes of Shrinkage and Warpage 1771021 Causes of Shrinkage 1771022 Causes of Warpage 1811023 Relating Orientation and Area Shrinkage to Warpage 182103 Designing Accurate Parts Considering Warpage 1831031 Material Selection 1841032 Wall Thickness Variation 1841033 Gate Position and Runner Dimensions 1851034 Molding Conditions 1871035 Cooling Line Layout 190
11 Moldflow Design Procedure 191
111 Determine Analysis Objectives 191112 Moldflow Analysis Steps Framework 1921121 The Whole Process 1921122 Optimize Fill 1921123 Balance and Size the Runners 1971124 Optimize Cooling 1991125 Optimize the Packing Profile 200
1126 Optimize Warpage 201113 Using Moldflow to Evaluate an Initial Design 2051131 Description of this Example 205
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Contents XVII
1132 Molding Window 2051133 Filling Analysis 206
1134 Gate and Runner Design 2061135 Cooling System Design 2071136 Packing Analysis 2091137 Warpage Analysis 209114 Using Moldflow to Optimize the Design 2101141 Determine the Cause of Warpage 2101142 Investigating Different Gate Locations 2101143 Validating the Best Gate Location 211
12 Part Defects 215
121 Air Traps 2151211 What Is an Air Trap 2151212 Problems Caused by Air Traps 2161213 Remedies 216122 Black Specks and Black Streaks 2171221 What Are Black Specks and Black Streaks 2171222 Causes of Black Specks and Black Streaks 2171223 Remedies 218123 Brittleness 219
1231 What Is Brittleness 2191232 Causes of Brittleness 2191233 Remedies 219124 Burn Marks 2201241 What Is a Burn Mark 2201242 Causes of Burn Marks 2211243 Remedies 222125 Delamination 2221251 What Is Delamination 2221252 Causes of Delamination 223
1253 Remedies 223126 Dimensional Variation 2241261 What Is Dimensional Variation 2241262 Causes of Dimensional Variation 2241263 Remedies 224127 Discoloration 2251271 What Is Discoloration 2251272 Causes of Discoloration 2251273 Remedies 226128 Fish Eyes 227
1281 What Are Fish Eyes 2271282 Causes of Fish Eyes 2271283 Remedies 227
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XVIII Contents
129 Flash 2281291 What Is Flash 228
1292 Causes of Flash 2281293 Remedies 2291210 Flow Marks 23012101 What Is A Flow Mark 23012102 Causes of Flow Marks 23012103 Remedies 2301211 Hesitation 23112111 What Is Hesitation 23112112 Problems Caused by Hesitation 23112113 Remedies 232
1212 Jetting 23212121 What Is Jetting 23212122 Problems Caused by Jetting 23312123 Remedies 2331213 Ripples 23512131 What Are Ripples 23512132 Cause of Ripples 23512133 Remedies 2361214 Short Shots 23612141 What Is a Short Shot 236
12142 Causes of Short Shots 23712143 Remedies 2371215 Silver Streaks 23812151 What Are Silver Streaks 23812152 Causes of Silver Streaks 23912153 Remedies 2391216 Sink Marks and Voids 24012161 What Are Sink Marks and Voids 24012162 Causes of Sink Marks and Voids 24012163 Remedies 2411217 Weld Lines and Meld Lines 24312171 What Are Weld Lines and Meld Lines 24312172 Problems Caused by Weld Lines 24312173 Strength of Weld Lines 24412174 Remedies 245
Appendix A Injection Molding 247
A1 Injection-molding Overview 247 A11 Process 247
A12 Applications 248 A2 Development of the Injection-molding Machine 248 A21 Benefits of the Reciprocating Screw 248
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Contents XIX
A3 Development of the Injection-molding Process 249 A4 Alternative Injection-molding Processes 249
A41 Co-injection (Sandwich) Molding 249 A42 Fusible Core Injection Molding 250 A43 Gas-assisted Injection Molding 251 A44 Injection-compression Molding 252 A45 Lamellar (Microlayer) Injection Molding 253 A46 Live-feed Injection Molding 253 A47 Low-pressure Injection Molding 254 A48 Push-pull Injection Molding 254 A49 Reactive Molding 255 A410 Structural Foam Injection Molding 256
A411 Thin-wall Molding 256
Appendix B Injection-molding Machine System and Operations 259
B1 Injection-molding Machine 259B11 Components 259B12 Machine Specification 260B13 Machine Function 260B14 Auxiliary Equipment 260B2 Machine Components 261
B21 Injection System 261B22 Mold System 263B23 Hydraulic System 264B24 Control System 264B25 Clamping System 264B3 Molded System 265B31 The Delivery System 265B4 Machine Operating Sequence 266B41 Process Cycle 266B5 Screw Operation 268
B51 Back Pressure 268B52 Injection Speed 268B53 Screw Rotation Speed 268B54 Cushion 269B6 Secondary Operations 269B61 Assembly 269B62 Decoration 269B63 Other Secondary Operations 270
Appendix C Injection-molding Process Control 271
C1 Importance of Process Conditions 271C11 Setting Machine Process Conditions 271
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XX Contents
Appendix D Plastic Materials 281
D1 What Are Plastics 281
D11 Polymerization Process 281D12 Structure of Polymers 281D13 Polymer Alloys and Blends 283D14 Polymer Composites 283D2 Classification of Plastics 283D21 Classes of Plastics 284D22 Structures and Properties of Plastics 284D3 Thermoplastics 285D31 Market Share Distribution of Thermoplastics 285D32 Structures and Properties of Thermoplastics 286D33 Amorphous Polymers 286D34 Semicrystalline Polymers 287D4 Thermosets 288D5 Properties and Applications of Thermoplastics 289D51 ABS 290D52 PA 12 291D53 PA 6 293D54 PA 66 294D55 PBT 296
D56 PC 297D57 PCABS 299D58 PCPBT 300D59 HDPE 301D510 LDPE 303D511 PEI 304D512 PET 305D513 PETG 306D514 PMMA 307D515 POM 309
D516 PP 310D517 PPEPPO 312D518 PS 313D519 PVC 314D520 SAN 316D521 Additives Fillers and Reinforcements 317D522 Modifying Polymer Properties 318D523 Low-aspect Fillers 319D524 High-aspect Fillers Fibers 319
Index 321
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1 Polymer Flow Behavior in Injection Molds
bull Phases of injection molding
bull How do plastics flow
11 Phases of Injection Molding
Any molder can prove that all the conditions and effects discussed in this chapter do indeedoccur during the injection molding process While this knowledge alone can somewhatimprove quality it is only with the use of Moldflow analysis during the initial design stage with the mold designed for the optimum filling pattern that these effects can be controlledand the full benefits obtained
Flow technology is concerned with the behavior of plastics during the mold filling process Aplastic parts properties depend on how the part is molded Two parts having identicaldimensions and made from the same material but molded under different conditions will havedifferent stress and shrinkage levels and will behave differently in the field meaning that they
are in practice two different parts The way the plastic flows into the mold is of paramount importance in determining the quality of the part The process of filling the mold can be distinctly analyzed with the ability to predictpressure temperature and stress
111 How Plastic Fills a Mold
This was investigated using a centrally gated mold shaped like a dinner plate with a thick rim
around the outside as shown in Figure 11 It was found that the injection molding processalthough complex could be divided into three phases (we use the word phase to avoidconfusion with injection stage as used with programmed injection)
Figure 1ure 1 1 Cross-section of disk mold used to investigate flow
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2 Polymer Flow Behavior in Injection Molds
1111 Filling Phase
As the ram moves forward it first moves at a steady speed as the plastic flows into the cavity
This is the filling phase This phase lasts until the mold is just filled See Figure 12 andFigure 13
1112 Pressurization Phase
The pressurization phase begins when the ram moves forward after the filling phase to bring the mold up to pressure When the mold is filled the ram will slow down but it still movesquite some distance because plastics are very compressible materials At injection molding pressure an extra 15 volume of material can be forced into the cavity See Figure 12 andFigure 13
Although fluids are usually assumed to be incompressible molten plastics have to beconsidered to be more like a gas The compressibility of plastics can be observed by blocking off the nozzle and attempting to purge the barrel The ram will jump forward when thepressure is applied but will spring back when the pressure is released
1113 Compensation Phase
After the pressurization phase the ram still does not stop completely continuing to creepforward for some time Plastics have a very large volumetric change of about 25 from themelt to the solid This can be seen in a short shot the difference in volume between themolding and the cavity is due to this volumetric change See Figure 12 and Figure 13
The ram moving forward to compensate for the volumetric change in the part is called thecompensation phase As the volumetric change is 25 and at the most only an extra 15 canbe injected in the pressurization phase there must always be some compensation phase
Figure 1igure 1 2 Phases of injection molding
Filling phase
Pressurization phase
Compensation phase
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Phases of Injection Molding 3
Figure 13ure 1 3 Phases of injection molding detail
112 The Filling Phase
A two-color technique best demonstrates this phase After emptying the barrel of an injection-molding machine a small amount of red plastic was charged followed by green plastic
Consider the closed mold with the plastic front just starting to flow from the nozzle Theplastic first fills the sprue and runner system then enters the mold cavity itself forming a smallbubble of molten plastic
The skin of the plastic in contact with the cool mold freezes rapidly while the central coreremains molten When additional material is injected it flows into this central core displacing
the material already there which then forms a new flow front The flow of this displacedmaterial is a combination of forward flow and outward flow The outward flow contacts the wall freezes and forms the next section of skin while the forward flow forms the new moltencore When more material enters the mold it flows along a channel lined with these frozen walls of plastic illustrated in Figure 14
This flow pattern is often called fountain flow or bubble flow because the flow front is like abubble being inflated with hot plastic from the center The frozen layer is formed by the flow front inflating and so is subject to only a low shear stress and therefore has a very low levelof molecular orientation Once it is frozen it cannot be orientated any further so the frozen
layer in the finished part has a low level of orientation
Figure 14ure 1 4 Fountain flow and heat transfer
Filling Pressurization Compensation
Hot plastic
Frozen layer
Heat loss to mold
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4 Polymer Flow Behavior in Injection Molds
Now consider what happens upstream Hot plastic is continuously flowing bringing new hotmaterial along and generating significant frictional heat At the same time heat is being lost
through the frozen layer to the cold mold surfaceInitially the frozen layer is very thin so heat is lost very rapidly This results in more plasticfreezing and the frozen layer getting thicker cutting down the heat flow After a time thefrozen layer will reach a thickness such that the heat lost by conduction is equal to the heatinput from plastic flow and frictional heating ie an equilibrium condition is reached(Figure 14)
It is interesting to do some calculations on the time taken to reach this state of equilibrium The actual rate of heat flow is very large in comparison with the small heat content of theplastic in the frozen layer The result is that equilibrium is reached very quickly often in a timemeasured in a few tenths of a second As the total filling time is measured in seconds thefrozen layer reaches an equilibrium state early in the filling cycle
It is useful to think about how the thickness of this frozen layer will vary If the injection rate were slowed less heat would be generated by friction along the flow path with less heat inputfrom the flow The heat loss would be at the same rate and with less heat input the frozenlayer would grow in thickness If the injection rate were raised the frozen layer would bethinner (Figure 15) Similarly higher melt and mold temperatures would reduce the thicknessof the frozen layer This can be seen experimentally using the two-color technique
Figure 1igure 1 5 Influence of injection rate on frozen layer thickness
1121 Flow Shear Stress
It is easy to get confused between the various stress levels and orientation of the polymer Asthe plastic flows it is subject to shear stress also called flow shear stress This flow shear stress willorient the material ie cause the molecules to align themselves in the general direction of flow
The shear stress varies from a maximum at the outside dropping off to zero at the center
3Shear stress is purely a function of force and area This must not be confused withshear rate which is the rate of plastic sliding over the next layer Shear rate is zero
= Frozen
= Molten cooler
= Molten hotter
Slowerinjection
rate
Faster
injection
rate
vs
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Phases of Injection Molding 5
at the outer edge where the plastic is frozen rises to a maximum just inwards of thefrozen layer then drops toward the center as shown in Figure 16
Figure 16ure 1 6 Shear rate distribution
If the flow were stopped and the plastic allowed to cool down very slowly this orientation would have time to relax giving a very low level of residual orientation On the other hand if the material were kept under stress and the plastic snap frozen most of the orientation would
be trapped in the frozen plastic (Figure 17)
Figure 17ure 1 7 Molecular orientation through the thickness of the part
Min MaxShear rate
= Tension
= Compression
Molecular orientation trapped
in pressurization phase
Gradual reduction of orientation
toward the center of the
mold as stress levels are lower
and the cooling rate is slower
which allows more time for
orientation to relax
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6 Polymer Flow Behavior in Injection Molds
Now consider the orientation from the mold surface toward the center
The frozen layer itself formed with very little shear and therefore low orientation immediately
freezes setting the low level of orientation
The layer of plastic just on the inside of the frozen layer is subject to maximum shear stressand freezes the instant flow stops trapping almost all the orientation
This is the orientation pattern the further toward the center the more the shear stress dropsand the slower the rate of cooling This allows more time for the level of orientation to relaxso the residual orientation drops rapidly toward the center Consider how this pattern willaffect the residual stress level Oriented material (normally) will shrink more than nonorientedmaterial On the inner surface of the original frozen layer highly oriented material wants toshrink a great deal but it is prevented from doing so by the less-oriented material The highly
oriented layer ends up being in tension while the less-oriented material is in compression This residual stress pattern is a common cause of part warpage
3 There is a connectionthrough orientationbetween the shear stress during
filling (flow stress) and the residual stress in the final molded part This meansshear stress during filling shown on Moldflow plots can be used as a designparameter
113 The Pressurization Phase
The pressurization phasefrom the point of view of flow behavioris very similar to thefilling phase The flow rate may drop somewhat as the mold builds up to pressure resulting inan increase in the thickness of the frozen layer
The main difference of course is the increase in hydrostatic (isotropic) pressure We shall seein chapter 2 section 24 Effect of Molding Conditions that hydrostatic pressure in itself doesnot cause any residual stress
114 The Compensation Phase
Compensating flow is unstable Consider the plate molding again (see Figure 11) You wouldthink that plastic flowing uniformly through the thin diaphragm would top up the thick rim Inpractice the plastic during the compensation phase flows in rivers that spread out like a deltaas illustrated in Figure 18 This may seem surprising at first but it can be explained by temperature instability
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How Do Plastics Flow 7
Figure 18ure 1 8 River flow
1141 Temperature Variation
There is always some variation in melt temperature coming from the barrel of the injectionmachine In exceptional cases up to 40 ordmC variation has been measured using a high-speedthermocouple
1142 Natural Instability
However slight the temperature variation natural instability will amplify it If for exampleone part of the melt is slightly hotter than the rest then the plastic flow in that area will beslightly greater bringing hotter material into the area and maintaining the temperature If onthe other hand there is another area that is cooler the flow will be less so there will be lessheat input and the plastic will get colder until it eventually freezes off
However balanced the initial conditions this natural instability will result in a river-type flow
This is a very important consideration The first material to freeze off will shrink early in thecycle By the time the material in the river flows freezes the bulk of the material will havealready frozen off and shrinkage will have occurred The rivers will shrink relative to the bulk of the molding and because they are highly orientated shrinkage will be very high The resultis high-stress tensile members throughout the molding a common cause of warpage
1143 Optimum Part Quality
Most of the stress in plastic parts occurs during the compensation phase By controlling flow and minimizing stress it is possible to design for optimum part quality This important point is
at the heart of the Moldflow philosophy
12 How Do Plastics Flow
121 Material Behavior
Molten thermoplastics exhibit viscoelastic behavior which combines flow characteristics of both viscous liquids and elastic solids When a viscous liquid flows the energy that causes thedeformation is dissipated and becomes viscous heat On the other hand when an elastic solid
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8 Polymer Flow Behavior in Injection Molds
is deformed the driving energy is stored For example the flow of water is a typical viscousflow whereas the deformation of a rubber cube falls into the elastic category
122 Deformation
In addition to the two types of material flow behavior there are two types of deformationsimple shear and simple extension (elongation) as shown in Figure 19 (a) and (b) below Theflow of molten thermoplastics during injection-molding filling is predominantly shear flow asshown in Figure 19 (c) in which layers of material elements slide over each other Theextensional flow however becomes significant as the material elements undergo elongation when the melt passes areas of abrupt dimensional change (eg a gate region) as shown inFigure 19 (d)
Figure 1igure 1 9 (a) Simple shear flow (b) Simple extensional flow (c) Shear flow in cavity filling (d) Extensional flow in cavity filling
123 Viscoelastic Behavior
In response to an applied stress (force per unit area) molten thermoplastics exhibit viscoelastic behavior which combines characteristics of an ideal viscous liquid with those of an ideal elastic solid In other words under certain conditions molten thermoplastics behavelike a liquid and will continuously deform while shear stress is applied as shown in
Figure 110 Upon the removal of the stress however the materials behave somewhat like anelastic solid with partial recovery of the deformation as shown in Figure 110 (b) and (c) This viscoelastic behavior stems from the random-coil configuration of polymer molecules in the
Stress
Stress Stress
Mold wall
Material element before
deformation begins
Material element during
deformation
Mold wall
Material elements at rest
Moving
materialelements
Velocity
profile
(a) (b)
(c) (d)
Velocity
profile
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How Do Plastics Flow 9
molten state which allows the movement and slippage of molecular chains under theinfluence of an applied load However the entanglement of the polymer molecular chains also
makes the system behave like an elastic solid upon the application and removal of the externalload Namely on removal of the stress chains will tend to return to the equilibrium random-coil state and thus will be a component of stress recovery The recovery is not instantaneousbecause of the entanglements still present in the system
Figure 1ure 1 10 (a) Ideal viscous liquid deforms continuously under applied stress (b) Ideal elasticsolid deforms immediately upon the application of stress but fully recovers when the
stress is removed (c) Molten thermoplastic deforms continuously under the applied
stress (like a viscous liquid) but also recovers partially from the deformation upon
removal of the applied stress (like an elastic solid)
124 Melt Shear Viscosity
1241 What Is Shear Viscosity
Melt shear viscosity is a materials resistance to shear flow In general polymer melts are highly viscous because of their long molecular chain structure The viscosity of a polymer melt rangesfrom 2 to 3000 Pas (water 10^-1 Pas glass 10^20 Pas) Viscosity can be thought of as thethickness of a fluid or how much it resists flow Viscosity is expressed as the ratio of shearstress (force per unit area) to the shear rate (rate change of shear strain) as shown inEquation 11 and Figure 111
(a) Viscous
liquid
(b) Elastic
solid
(c) Viscoelastic
polymer
melt
Initial shape
(shear stress
to be applied)
Time = Dt Time = 2 x Dt
(shear stress to
be removed)
Final deformed
shape
d
d
d
d
2d
2d
2d
2d
total
recovery
partial
recovery
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Contents XV
82 Runner System Design Principles 129821 Benefits of Good Runner Design 129
822 Runner Design Philosophy 129823 Flow Balancing 130824 Flow Control 130825 Frictional Heating in Runners 132826 Thermal Shutoff 132827 System and Runner Pressures 132828 Constant Pressure Gradient 133829 Cold Slug Wells 1338210 Easy Ejection 13483 Runner Types 134
831 Cold Runners 134832 Hot Runner Systems 13584 Runner Layout 136841 Determining the Number of Cavities 136842 Planning the Runner System Layout 138843 Partially Balanced Runners 139844 Geometrically Balanced Runners 14185 Initial Runner Sizing 141851 Determining Sprue Dimensions 141852 Designing Runner Cross Sections 142
853 Determining Runner Diameters 14586 Runner Balancing 147861 How Runner Balancing Works 147862 When Are the Runner Sizes Optimized 147863 Validating the Balance 147864 Processing Window 14887 Using Moldflow for Runner Balancing 149871 Runner Balancing a 48-cavity Tool 149872 Runner Balancing for a Family Mold 151873 Runner Balancing for a Multigated Part 152
9 Cooling System Design 153
91 Mold Cooling System Overview 153911 Importance of Cooling System Design 153912 Mold Cooling System Components 15492 Cooling-channel Configuration 156921 Types of Cooling Channels 15693 Alternative Cooling Devices 157931 What Do They Do 157
932 Baffles 158933 Bubblers 159934 Thermal Pins 159
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XVI Contents
935 Cooling Slender Cores 160936 Cooling Large Cores 161
937 Cooling Cylinder Cores 16294 Cooling System Equations 162941 Cooling Time 162942 Reynolds Number and Coolant Flow 16395 Design Rules 163951 Mold Cooling Design Considerations 163952 Location and Size of Channels 164953 Flow Rate and Heat Transfer 16696 Using Moldflow for Cooling System Design 167961 Example Setup 168
962 Cycle Time Determined by Design and Processing Parameters 169
10 Shrinkage and Warpage 173
101 Injection Molding and Shrinkage 1731011 What Are Shrinkage and Warpage 1731012 Shrinkage and Machine Settings 1731013 Mold Filling and Packing 1741014 How Pressure and Time Affect Shrinkage 1751015 Thermally Unstable Flow 176
102 Basic Causes of Shrinkage and Warpage 1771021 Causes of Shrinkage 1771022 Causes of Warpage 1811023 Relating Orientation and Area Shrinkage to Warpage 182103 Designing Accurate Parts Considering Warpage 1831031 Material Selection 1841032 Wall Thickness Variation 1841033 Gate Position and Runner Dimensions 1851034 Molding Conditions 1871035 Cooling Line Layout 190
11 Moldflow Design Procedure 191
111 Determine Analysis Objectives 191112 Moldflow Analysis Steps Framework 1921121 The Whole Process 1921122 Optimize Fill 1921123 Balance and Size the Runners 1971124 Optimize Cooling 1991125 Optimize the Packing Profile 200
1126 Optimize Warpage 201113 Using Moldflow to Evaluate an Initial Design 2051131 Description of this Example 205
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Contents XVII
1132 Molding Window 2051133 Filling Analysis 206
1134 Gate and Runner Design 2061135 Cooling System Design 2071136 Packing Analysis 2091137 Warpage Analysis 209114 Using Moldflow to Optimize the Design 2101141 Determine the Cause of Warpage 2101142 Investigating Different Gate Locations 2101143 Validating the Best Gate Location 211
12 Part Defects 215
121 Air Traps 2151211 What Is an Air Trap 2151212 Problems Caused by Air Traps 2161213 Remedies 216122 Black Specks and Black Streaks 2171221 What Are Black Specks and Black Streaks 2171222 Causes of Black Specks and Black Streaks 2171223 Remedies 218123 Brittleness 219
1231 What Is Brittleness 2191232 Causes of Brittleness 2191233 Remedies 219124 Burn Marks 2201241 What Is a Burn Mark 2201242 Causes of Burn Marks 2211243 Remedies 222125 Delamination 2221251 What Is Delamination 2221252 Causes of Delamination 223
1253 Remedies 223126 Dimensional Variation 2241261 What Is Dimensional Variation 2241262 Causes of Dimensional Variation 2241263 Remedies 224127 Discoloration 2251271 What Is Discoloration 2251272 Causes of Discoloration 2251273 Remedies 226128 Fish Eyes 227
1281 What Are Fish Eyes 2271282 Causes of Fish Eyes 2271283 Remedies 227
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XVIII Contents
129 Flash 2281291 What Is Flash 228
1292 Causes of Flash 2281293 Remedies 2291210 Flow Marks 23012101 What Is A Flow Mark 23012102 Causes of Flow Marks 23012103 Remedies 2301211 Hesitation 23112111 What Is Hesitation 23112112 Problems Caused by Hesitation 23112113 Remedies 232
1212 Jetting 23212121 What Is Jetting 23212122 Problems Caused by Jetting 23312123 Remedies 2331213 Ripples 23512131 What Are Ripples 23512132 Cause of Ripples 23512133 Remedies 2361214 Short Shots 23612141 What Is a Short Shot 236
12142 Causes of Short Shots 23712143 Remedies 2371215 Silver Streaks 23812151 What Are Silver Streaks 23812152 Causes of Silver Streaks 23912153 Remedies 2391216 Sink Marks and Voids 24012161 What Are Sink Marks and Voids 24012162 Causes of Sink Marks and Voids 24012163 Remedies 2411217 Weld Lines and Meld Lines 24312171 What Are Weld Lines and Meld Lines 24312172 Problems Caused by Weld Lines 24312173 Strength of Weld Lines 24412174 Remedies 245
Appendix A Injection Molding 247
A1 Injection-molding Overview 247 A11 Process 247
A12 Applications 248 A2 Development of the Injection-molding Machine 248 A21 Benefits of the Reciprocating Screw 248
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Contents XIX
A3 Development of the Injection-molding Process 249 A4 Alternative Injection-molding Processes 249
A41 Co-injection (Sandwich) Molding 249 A42 Fusible Core Injection Molding 250 A43 Gas-assisted Injection Molding 251 A44 Injection-compression Molding 252 A45 Lamellar (Microlayer) Injection Molding 253 A46 Live-feed Injection Molding 253 A47 Low-pressure Injection Molding 254 A48 Push-pull Injection Molding 254 A49 Reactive Molding 255 A410 Structural Foam Injection Molding 256
A411 Thin-wall Molding 256
Appendix B Injection-molding Machine System and Operations 259
B1 Injection-molding Machine 259B11 Components 259B12 Machine Specification 260B13 Machine Function 260B14 Auxiliary Equipment 260B2 Machine Components 261
B21 Injection System 261B22 Mold System 263B23 Hydraulic System 264B24 Control System 264B25 Clamping System 264B3 Molded System 265B31 The Delivery System 265B4 Machine Operating Sequence 266B41 Process Cycle 266B5 Screw Operation 268
B51 Back Pressure 268B52 Injection Speed 268B53 Screw Rotation Speed 268B54 Cushion 269B6 Secondary Operations 269B61 Assembly 269B62 Decoration 269B63 Other Secondary Operations 270
Appendix C Injection-molding Process Control 271
C1 Importance of Process Conditions 271C11 Setting Machine Process Conditions 271
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XX Contents
Appendix D Plastic Materials 281
D1 What Are Plastics 281
D11 Polymerization Process 281D12 Structure of Polymers 281D13 Polymer Alloys and Blends 283D14 Polymer Composites 283D2 Classification of Plastics 283D21 Classes of Plastics 284D22 Structures and Properties of Plastics 284D3 Thermoplastics 285D31 Market Share Distribution of Thermoplastics 285D32 Structures and Properties of Thermoplastics 286D33 Amorphous Polymers 286D34 Semicrystalline Polymers 287D4 Thermosets 288D5 Properties and Applications of Thermoplastics 289D51 ABS 290D52 PA 12 291D53 PA 6 293D54 PA 66 294D55 PBT 296
D56 PC 297D57 PCABS 299D58 PCPBT 300D59 HDPE 301D510 LDPE 303D511 PEI 304D512 PET 305D513 PETG 306D514 PMMA 307D515 POM 309
D516 PP 310D517 PPEPPO 312D518 PS 313D519 PVC 314D520 SAN 316D521 Additives Fillers and Reinforcements 317D522 Modifying Polymer Properties 318D523 Low-aspect Fillers 319D524 High-aspect Fillers Fibers 319
Index 321
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1 Polymer Flow Behavior in Injection Molds
bull Phases of injection molding
bull How do plastics flow
11 Phases of Injection Molding
Any molder can prove that all the conditions and effects discussed in this chapter do indeedoccur during the injection molding process While this knowledge alone can somewhatimprove quality it is only with the use of Moldflow analysis during the initial design stage with the mold designed for the optimum filling pattern that these effects can be controlledand the full benefits obtained
Flow technology is concerned with the behavior of plastics during the mold filling process Aplastic parts properties depend on how the part is molded Two parts having identicaldimensions and made from the same material but molded under different conditions will havedifferent stress and shrinkage levels and will behave differently in the field meaning that they
are in practice two different parts The way the plastic flows into the mold is of paramount importance in determining the quality of the part The process of filling the mold can be distinctly analyzed with the ability to predictpressure temperature and stress
111 How Plastic Fills a Mold
This was investigated using a centrally gated mold shaped like a dinner plate with a thick rim
around the outside as shown in Figure 11 It was found that the injection molding processalthough complex could be divided into three phases (we use the word phase to avoidconfusion with injection stage as used with programmed injection)
Figure 1ure 1 1 Cross-section of disk mold used to investigate flow
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2 Polymer Flow Behavior in Injection Molds
1111 Filling Phase
As the ram moves forward it first moves at a steady speed as the plastic flows into the cavity
This is the filling phase This phase lasts until the mold is just filled See Figure 12 andFigure 13
1112 Pressurization Phase
The pressurization phase begins when the ram moves forward after the filling phase to bring the mold up to pressure When the mold is filled the ram will slow down but it still movesquite some distance because plastics are very compressible materials At injection molding pressure an extra 15 volume of material can be forced into the cavity See Figure 12 andFigure 13
Although fluids are usually assumed to be incompressible molten plastics have to beconsidered to be more like a gas The compressibility of plastics can be observed by blocking off the nozzle and attempting to purge the barrel The ram will jump forward when thepressure is applied but will spring back when the pressure is released
1113 Compensation Phase
After the pressurization phase the ram still does not stop completely continuing to creepforward for some time Plastics have a very large volumetric change of about 25 from themelt to the solid This can be seen in a short shot the difference in volume between themolding and the cavity is due to this volumetric change See Figure 12 and Figure 13
The ram moving forward to compensate for the volumetric change in the part is called thecompensation phase As the volumetric change is 25 and at the most only an extra 15 canbe injected in the pressurization phase there must always be some compensation phase
Figure 1igure 1 2 Phases of injection molding
Filling phase
Pressurization phase
Compensation phase
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Phases of Injection Molding 3
Figure 13ure 1 3 Phases of injection molding detail
112 The Filling Phase
A two-color technique best demonstrates this phase After emptying the barrel of an injection-molding machine a small amount of red plastic was charged followed by green plastic
Consider the closed mold with the plastic front just starting to flow from the nozzle Theplastic first fills the sprue and runner system then enters the mold cavity itself forming a smallbubble of molten plastic
The skin of the plastic in contact with the cool mold freezes rapidly while the central coreremains molten When additional material is injected it flows into this central core displacing
the material already there which then forms a new flow front The flow of this displacedmaterial is a combination of forward flow and outward flow The outward flow contacts the wall freezes and forms the next section of skin while the forward flow forms the new moltencore When more material enters the mold it flows along a channel lined with these frozen walls of plastic illustrated in Figure 14
This flow pattern is often called fountain flow or bubble flow because the flow front is like abubble being inflated with hot plastic from the center The frozen layer is formed by the flow front inflating and so is subject to only a low shear stress and therefore has a very low levelof molecular orientation Once it is frozen it cannot be orientated any further so the frozen
layer in the finished part has a low level of orientation
Figure 14ure 1 4 Fountain flow and heat transfer
Filling Pressurization Compensation
Hot plastic
Frozen layer
Heat loss to mold
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4 Polymer Flow Behavior in Injection Molds
Now consider what happens upstream Hot plastic is continuously flowing bringing new hotmaterial along and generating significant frictional heat At the same time heat is being lost
through the frozen layer to the cold mold surfaceInitially the frozen layer is very thin so heat is lost very rapidly This results in more plasticfreezing and the frozen layer getting thicker cutting down the heat flow After a time thefrozen layer will reach a thickness such that the heat lost by conduction is equal to the heatinput from plastic flow and frictional heating ie an equilibrium condition is reached(Figure 14)
It is interesting to do some calculations on the time taken to reach this state of equilibrium The actual rate of heat flow is very large in comparison with the small heat content of theplastic in the frozen layer The result is that equilibrium is reached very quickly often in a timemeasured in a few tenths of a second As the total filling time is measured in seconds thefrozen layer reaches an equilibrium state early in the filling cycle
It is useful to think about how the thickness of this frozen layer will vary If the injection rate were slowed less heat would be generated by friction along the flow path with less heat inputfrom the flow The heat loss would be at the same rate and with less heat input the frozenlayer would grow in thickness If the injection rate were raised the frozen layer would bethinner (Figure 15) Similarly higher melt and mold temperatures would reduce the thicknessof the frozen layer This can be seen experimentally using the two-color technique
Figure 1igure 1 5 Influence of injection rate on frozen layer thickness
1121 Flow Shear Stress
It is easy to get confused between the various stress levels and orientation of the polymer Asthe plastic flows it is subject to shear stress also called flow shear stress This flow shear stress willorient the material ie cause the molecules to align themselves in the general direction of flow
The shear stress varies from a maximum at the outside dropping off to zero at the center
3Shear stress is purely a function of force and area This must not be confused withshear rate which is the rate of plastic sliding over the next layer Shear rate is zero
= Frozen
= Molten cooler
= Molten hotter
Slowerinjection
rate
Faster
injection
rate
vs
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Phases of Injection Molding 5
at the outer edge where the plastic is frozen rises to a maximum just inwards of thefrozen layer then drops toward the center as shown in Figure 16
Figure 16ure 1 6 Shear rate distribution
If the flow were stopped and the plastic allowed to cool down very slowly this orientation would have time to relax giving a very low level of residual orientation On the other hand if the material were kept under stress and the plastic snap frozen most of the orientation would
be trapped in the frozen plastic (Figure 17)
Figure 17ure 1 7 Molecular orientation through the thickness of the part
Min MaxShear rate
= Tension
= Compression
Molecular orientation trapped
in pressurization phase
Gradual reduction of orientation
toward the center of the
mold as stress levels are lower
and the cooling rate is slower
which allows more time for
orientation to relax
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6 Polymer Flow Behavior in Injection Molds
Now consider the orientation from the mold surface toward the center
The frozen layer itself formed with very little shear and therefore low orientation immediately
freezes setting the low level of orientation
The layer of plastic just on the inside of the frozen layer is subject to maximum shear stressand freezes the instant flow stops trapping almost all the orientation
This is the orientation pattern the further toward the center the more the shear stress dropsand the slower the rate of cooling This allows more time for the level of orientation to relaxso the residual orientation drops rapidly toward the center Consider how this pattern willaffect the residual stress level Oriented material (normally) will shrink more than nonorientedmaterial On the inner surface of the original frozen layer highly oriented material wants toshrink a great deal but it is prevented from doing so by the less-oriented material The highly
oriented layer ends up being in tension while the less-oriented material is in compression This residual stress pattern is a common cause of part warpage
3 There is a connectionthrough orientationbetween the shear stress during
filling (flow stress) and the residual stress in the final molded part This meansshear stress during filling shown on Moldflow plots can be used as a designparameter
113 The Pressurization Phase
The pressurization phasefrom the point of view of flow behavioris very similar to thefilling phase The flow rate may drop somewhat as the mold builds up to pressure resulting inan increase in the thickness of the frozen layer
The main difference of course is the increase in hydrostatic (isotropic) pressure We shall seein chapter 2 section 24 Effect of Molding Conditions that hydrostatic pressure in itself doesnot cause any residual stress
114 The Compensation Phase
Compensating flow is unstable Consider the plate molding again (see Figure 11) You wouldthink that plastic flowing uniformly through the thin diaphragm would top up the thick rim Inpractice the plastic during the compensation phase flows in rivers that spread out like a deltaas illustrated in Figure 18 This may seem surprising at first but it can be explained by temperature instability
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How Do Plastics Flow 7
Figure 18ure 1 8 River flow
1141 Temperature Variation
There is always some variation in melt temperature coming from the barrel of the injectionmachine In exceptional cases up to 40 ordmC variation has been measured using a high-speedthermocouple
1142 Natural Instability
However slight the temperature variation natural instability will amplify it If for exampleone part of the melt is slightly hotter than the rest then the plastic flow in that area will beslightly greater bringing hotter material into the area and maintaining the temperature If onthe other hand there is another area that is cooler the flow will be less so there will be lessheat input and the plastic will get colder until it eventually freezes off
However balanced the initial conditions this natural instability will result in a river-type flow
This is a very important consideration The first material to freeze off will shrink early in thecycle By the time the material in the river flows freezes the bulk of the material will havealready frozen off and shrinkage will have occurred The rivers will shrink relative to the bulk of the molding and because they are highly orientated shrinkage will be very high The resultis high-stress tensile members throughout the molding a common cause of warpage
1143 Optimum Part Quality
Most of the stress in plastic parts occurs during the compensation phase By controlling flow and minimizing stress it is possible to design for optimum part quality This important point is
at the heart of the Moldflow philosophy
12 How Do Plastics Flow
121 Material Behavior
Molten thermoplastics exhibit viscoelastic behavior which combines flow characteristics of both viscous liquids and elastic solids When a viscous liquid flows the energy that causes thedeformation is dissipated and becomes viscous heat On the other hand when an elastic solid
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8 Polymer Flow Behavior in Injection Molds
is deformed the driving energy is stored For example the flow of water is a typical viscousflow whereas the deformation of a rubber cube falls into the elastic category
122 Deformation
In addition to the two types of material flow behavior there are two types of deformationsimple shear and simple extension (elongation) as shown in Figure 19 (a) and (b) below Theflow of molten thermoplastics during injection-molding filling is predominantly shear flow asshown in Figure 19 (c) in which layers of material elements slide over each other Theextensional flow however becomes significant as the material elements undergo elongation when the melt passes areas of abrupt dimensional change (eg a gate region) as shown inFigure 19 (d)
Figure 1igure 1 9 (a) Simple shear flow (b) Simple extensional flow (c) Shear flow in cavity filling (d) Extensional flow in cavity filling
123 Viscoelastic Behavior
In response to an applied stress (force per unit area) molten thermoplastics exhibit viscoelastic behavior which combines characteristics of an ideal viscous liquid with those of an ideal elastic solid In other words under certain conditions molten thermoplastics behavelike a liquid and will continuously deform while shear stress is applied as shown in
Figure 110 Upon the removal of the stress however the materials behave somewhat like anelastic solid with partial recovery of the deformation as shown in Figure 110 (b) and (c) This viscoelastic behavior stems from the random-coil configuration of polymer molecules in the
Stress
Stress Stress
Mold wall
Material element before
deformation begins
Material element during
deformation
Mold wall
Material elements at rest
Moving
materialelements
Velocity
profile
(a) (b)
(c) (d)
Velocity
profile
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How Do Plastics Flow 9
molten state which allows the movement and slippage of molecular chains under theinfluence of an applied load However the entanglement of the polymer molecular chains also
makes the system behave like an elastic solid upon the application and removal of the externalload Namely on removal of the stress chains will tend to return to the equilibrium random-coil state and thus will be a component of stress recovery The recovery is not instantaneousbecause of the entanglements still present in the system
Figure 1ure 1 10 (a) Ideal viscous liquid deforms continuously under applied stress (b) Ideal elasticsolid deforms immediately upon the application of stress but fully recovers when the
stress is removed (c) Molten thermoplastic deforms continuously under the applied
stress (like a viscous liquid) but also recovers partially from the deformation upon
removal of the applied stress (like an elastic solid)
124 Melt Shear Viscosity
1241 What Is Shear Viscosity
Melt shear viscosity is a materials resistance to shear flow In general polymer melts are highly viscous because of their long molecular chain structure The viscosity of a polymer melt rangesfrom 2 to 3000 Pas (water 10^-1 Pas glass 10^20 Pas) Viscosity can be thought of as thethickness of a fluid or how much it resists flow Viscosity is expressed as the ratio of shearstress (force per unit area) to the shear rate (rate change of shear strain) as shown inEquation 11 and Figure 111
(a) Viscous
liquid
(b) Elastic
solid
(c) Viscoelastic
polymer
melt
Initial shape
(shear stress
to be applied)
Time = Dt Time = 2 x Dt
(shear stress to
be removed)
Final deformed
shape
d
d
d
d
2d
2d
2d
2d
total
recovery
partial
recovery
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XVI Contents
935 Cooling Slender Cores 160936 Cooling Large Cores 161
937 Cooling Cylinder Cores 16294 Cooling System Equations 162941 Cooling Time 162942 Reynolds Number and Coolant Flow 16395 Design Rules 163951 Mold Cooling Design Considerations 163952 Location and Size of Channels 164953 Flow Rate and Heat Transfer 16696 Using Moldflow for Cooling System Design 167961 Example Setup 168
962 Cycle Time Determined by Design and Processing Parameters 169
10 Shrinkage and Warpage 173
101 Injection Molding and Shrinkage 1731011 What Are Shrinkage and Warpage 1731012 Shrinkage and Machine Settings 1731013 Mold Filling and Packing 1741014 How Pressure and Time Affect Shrinkage 1751015 Thermally Unstable Flow 176
102 Basic Causes of Shrinkage and Warpage 1771021 Causes of Shrinkage 1771022 Causes of Warpage 1811023 Relating Orientation and Area Shrinkage to Warpage 182103 Designing Accurate Parts Considering Warpage 1831031 Material Selection 1841032 Wall Thickness Variation 1841033 Gate Position and Runner Dimensions 1851034 Molding Conditions 1871035 Cooling Line Layout 190
11 Moldflow Design Procedure 191
111 Determine Analysis Objectives 191112 Moldflow Analysis Steps Framework 1921121 The Whole Process 1921122 Optimize Fill 1921123 Balance and Size the Runners 1971124 Optimize Cooling 1991125 Optimize the Packing Profile 200
1126 Optimize Warpage 201113 Using Moldflow to Evaluate an Initial Design 2051131 Description of this Example 205
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Contents XVII
1132 Molding Window 2051133 Filling Analysis 206
1134 Gate and Runner Design 2061135 Cooling System Design 2071136 Packing Analysis 2091137 Warpage Analysis 209114 Using Moldflow to Optimize the Design 2101141 Determine the Cause of Warpage 2101142 Investigating Different Gate Locations 2101143 Validating the Best Gate Location 211
12 Part Defects 215
121 Air Traps 2151211 What Is an Air Trap 2151212 Problems Caused by Air Traps 2161213 Remedies 216122 Black Specks and Black Streaks 2171221 What Are Black Specks and Black Streaks 2171222 Causes of Black Specks and Black Streaks 2171223 Remedies 218123 Brittleness 219
1231 What Is Brittleness 2191232 Causes of Brittleness 2191233 Remedies 219124 Burn Marks 2201241 What Is a Burn Mark 2201242 Causes of Burn Marks 2211243 Remedies 222125 Delamination 2221251 What Is Delamination 2221252 Causes of Delamination 223
1253 Remedies 223126 Dimensional Variation 2241261 What Is Dimensional Variation 2241262 Causes of Dimensional Variation 2241263 Remedies 224127 Discoloration 2251271 What Is Discoloration 2251272 Causes of Discoloration 2251273 Remedies 226128 Fish Eyes 227
1281 What Are Fish Eyes 2271282 Causes of Fish Eyes 2271283 Remedies 227
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XVIII Contents
129 Flash 2281291 What Is Flash 228
1292 Causes of Flash 2281293 Remedies 2291210 Flow Marks 23012101 What Is A Flow Mark 23012102 Causes of Flow Marks 23012103 Remedies 2301211 Hesitation 23112111 What Is Hesitation 23112112 Problems Caused by Hesitation 23112113 Remedies 232
1212 Jetting 23212121 What Is Jetting 23212122 Problems Caused by Jetting 23312123 Remedies 2331213 Ripples 23512131 What Are Ripples 23512132 Cause of Ripples 23512133 Remedies 2361214 Short Shots 23612141 What Is a Short Shot 236
12142 Causes of Short Shots 23712143 Remedies 2371215 Silver Streaks 23812151 What Are Silver Streaks 23812152 Causes of Silver Streaks 23912153 Remedies 2391216 Sink Marks and Voids 24012161 What Are Sink Marks and Voids 24012162 Causes of Sink Marks and Voids 24012163 Remedies 2411217 Weld Lines and Meld Lines 24312171 What Are Weld Lines and Meld Lines 24312172 Problems Caused by Weld Lines 24312173 Strength of Weld Lines 24412174 Remedies 245
Appendix A Injection Molding 247
A1 Injection-molding Overview 247 A11 Process 247
A12 Applications 248 A2 Development of the Injection-molding Machine 248 A21 Benefits of the Reciprocating Screw 248
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Contents XIX
A3 Development of the Injection-molding Process 249 A4 Alternative Injection-molding Processes 249
A41 Co-injection (Sandwich) Molding 249 A42 Fusible Core Injection Molding 250 A43 Gas-assisted Injection Molding 251 A44 Injection-compression Molding 252 A45 Lamellar (Microlayer) Injection Molding 253 A46 Live-feed Injection Molding 253 A47 Low-pressure Injection Molding 254 A48 Push-pull Injection Molding 254 A49 Reactive Molding 255 A410 Structural Foam Injection Molding 256
A411 Thin-wall Molding 256
Appendix B Injection-molding Machine System and Operations 259
B1 Injection-molding Machine 259B11 Components 259B12 Machine Specification 260B13 Machine Function 260B14 Auxiliary Equipment 260B2 Machine Components 261
B21 Injection System 261B22 Mold System 263B23 Hydraulic System 264B24 Control System 264B25 Clamping System 264B3 Molded System 265B31 The Delivery System 265B4 Machine Operating Sequence 266B41 Process Cycle 266B5 Screw Operation 268
B51 Back Pressure 268B52 Injection Speed 268B53 Screw Rotation Speed 268B54 Cushion 269B6 Secondary Operations 269B61 Assembly 269B62 Decoration 269B63 Other Secondary Operations 270
Appendix C Injection-molding Process Control 271
C1 Importance of Process Conditions 271C11 Setting Machine Process Conditions 271
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XX Contents
Appendix D Plastic Materials 281
D1 What Are Plastics 281
D11 Polymerization Process 281D12 Structure of Polymers 281D13 Polymer Alloys and Blends 283D14 Polymer Composites 283D2 Classification of Plastics 283D21 Classes of Plastics 284D22 Structures and Properties of Plastics 284D3 Thermoplastics 285D31 Market Share Distribution of Thermoplastics 285D32 Structures and Properties of Thermoplastics 286D33 Amorphous Polymers 286D34 Semicrystalline Polymers 287D4 Thermosets 288D5 Properties and Applications of Thermoplastics 289D51 ABS 290D52 PA 12 291D53 PA 6 293D54 PA 66 294D55 PBT 296
D56 PC 297D57 PCABS 299D58 PCPBT 300D59 HDPE 301D510 LDPE 303D511 PEI 304D512 PET 305D513 PETG 306D514 PMMA 307D515 POM 309
D516 PP 310D517 PPEPPO 312D518 PS 313D519 PVC 314D520 SAN 316D521 Additives Fillers and Reinforcements 317D522 Modifying Polymer Properties 318D523 Low-aspect Fillers 319D524 High-aspect Fillers Fibers 319
Index 321
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1 Polymer Flow Behavior in Injection Molds
bull Phases of injection molding
bull How do plastics flow
11 Phases of Injection Molding
Any molder can prove that all the conditions and effects discussed in this chapter do indeedoccur during the injection molding process While this knowledge alone can somewhatimprove quality it is only with the use of Moldflow analysis during the initial design stage with the mold designed for the optimum filling pattern that these effects can be controlledand the full benefits obtained
Flow technology is concerned with the behavior of plastics during the mold filling process Aplastic parts properties depend on how the part is molded Two parts having identicaldimensions and made from the same material but molded under different conditions will havedifferent stress and shrinkage levels and will behave differently in the field meaning that they
are in practice two different parts The way the plastic flows into the mold is of paramount importance in determining the quality of the part The process of filling the mold can be distinctly analyzed with the ability to predictpressure temperature and stress
111 How Plastic Fills a Mold
This was investigated using a centrally gated mold shaped like a dinner plate with a thick rim
around the outside as shown in Figure 11 It was found that the injection molding processalthough complex could be divided into three phases (we use the word phase to avoidconfusion with injection stage as used with programmed injection)
Figure 1ure 1 1 Cross-section of disk mold used to investigate flow
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2 Polymer Flow Behavior in Injection Molds
1111 Filling Phase
As the ram moves forward it first moves at a steady speed as the plastic flows into the cavity
This is the filling phase This phase lasts until the mold is just filled See Figure 12 andFigure 13
1112 Pressurization Phase
The pressurization phase begins when the ram moves forward after the filling phase to bring the mold up to pressure When the mold is filled the ram will slow down but it still movesquite some distance because plastics are very compressible materials At injection molding pressure an extra 15 volume of material can be forced into the cavity See Figure 12 andFigure 13
Although fluids are usually assumed to be incompressible molten plastics have to beconsidered to be more like a gas The compressibility of plastics can be observed by blocking off the nozzle and attempting to purge the barrel The ram will jump forward when thepressure is applied but will spring back when the pressure is released
1113 Compensation Phase
After the pressurization phase the ram still does not stop completely continuing to creepforward for some time Plastics have a very large volumetric change of about 25 from themelt to the solid This can be seen in a short shot the difference in volume between themolding and the cavity is due to this volumetric change See Figure 12 and Figure 13
The ram moving forward to compensate for the volumetric change in the part is called thecompensation phase As the volumetric change is 25 and at the most only an extra 15 canbe injected in the pressurization phase there must always be some compensation phase
Figure 1igure 1 2 Phases of injection molding
Filling phase
Pressurization phase
Compensation phase
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Phases of Injection Molding 3
Figure 13ure 1 3 Phases of injection molding detail
112 The Filling Phase
A two-color technique best demonstrates this phase After emptying the barrel of an injection-molding machine a small amount of red plastic was charged followed by green plastic
Consider the closed mold with the plastic front just starting to flow from the nozzle Theplastic first fills the sprue and runner system then enters the mold cavity itself forming a smallbubble of molten plastic
The skin of the plastic in contact with the cool mold freezes rapidly while the central coreremains molten When additional material is injected it flows into this central core displacing
the material already there which then forms a new flow front The flow of this displacedmaterial is a combination of forward flow and outward flow The outward flow contacts the wall freezes and forms the next section of skin while the forward flow forms the new moltencore When more material enters the mold it flows along a channel lined with these frozen walls of plastic illustrated in Figure 14
This flow pattern is often called fountain flow or bubble flow because the flow front is like abubble being inflated with hot plastic from the center The frozen layer is formed by the flow front inflating and so is subject to only a low shear stress and therefore has a very low levelof molecular orientation Once it is frozen it cannot be orientated any further so the frozen
layer in the finished part has a low level of orientation
Figure 14ure 1 4 Fountain flow and heat transfer
Filling Pressurization Compensation
Hot plastic
Frozen layer
Heat loss to mold
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4 Polymer Flow Behavior in Injection Molds
Now consider what happens upstream Hot plastic is continuously flowing bringing new hotmaterial along and generating significant frictional heat At the same time heat is being lost
through the frozen layer to the cold mold surfaceInitially the frozen layer is very thin so heat is lost very rapidly This results in more plasticfreezing and the frozen layer getting thicker cutting down the heat flow After a time thefrozen layer will reach a thickness such that the heat lost by conduction is equal to the heatinput from plastic flow and frictional heating ie an equilibrium condition is reached(Figure 14)
It is interesting to do some calculations on the time taken to reach this state of equilibrium The actual rate of heat flow is very large in comparison with the small heat content of theplastic in the frozen layer The result is that equilibrium is reached very quickly often in a timemeasured in a few tenths of a second As the total filling time is measured in seconds thefrozen layer reaches an equilibrium state early in the filling cycle
It is useful to think about how the thickness of this frozen layer will vary If the injection rate were slowed less heat would be generated by friction along the flow path with less heat inputfrom the flow The heat loss would be at the same rate and with less heat input the frozenlayer would grow in thickness If the injection rate were raised the frozen layer would bethinner (Figure 15) Similarly higher melt and mold temperatures would reduce the thicknessof the frozen layer This can be seen experimentally using the two-color technique
Figure 1igure 1 5 Influence of injection rate on frozen layer thickness
1121 Flow Shear Stress
It is easy to get confused between the various stress levels and orientation of the polymer Asthe plastic flows it is subject to shear stress also called flow shear stress This flow shear stress willorient the material ie cause the molecules to align themselves in the general direction of flow
The shear stress varies from a maximum at the outside dropping off to zero at the center
3Shear stress is purely a function of force and area This must not be confused withshear rate which is the rate of plastic sliding over the next layer Shear rate is zero
= Frozen
= Molten cooler
= Molten hotter
Slowerinjection
rate
Faster
injection
rate
vs
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Phases of Injection Molding 5
at the outer edge where the plastic is frozen rises to a maximum just inwards of thefrozen layer then drops toward the center as shown in Figure 16
Figure 16ure 1 6 Shear rate distribution
If the flow were stopped and the plastic allowed to cool down very slowly this orientation would have time to relax giving a very low level of residual orientation On the other hand if the material were kept under stress and the plastic snap frozen most of the orientation would
be trapped in the frozen plastic (Figure 17)
Figure 17ure 1 7 Molecular orientation through the thickness of the part
Min MaxShear rate
= Tension
= Compression
Molecular orientation trapped
in pressurization phase
Gradual reduction of orientation
toward the center of the
mold as stress levels are lower
and the cooling rate is slower
which allows more time for
orientation to relax
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6 Polymer Flow Behavior in Injection Molds
Now consider the orientation from the mold surface toward the center
The frozen layer itself formed with very little shear and therefore low orientation immediately
freezes setting the low level of orientation
The layer of plastic just on the inside of the frozen layer is subject to maximum shear stressand freezes the instant flow stops trapping almost all the orientation
This is the orientation pattern the further toward the center the more the shear stress dropsand the slower the rate of cooling This allows more time for the level of orientation to relaxso the residual orientation drops rapidly toward the center Consider how this pattern willaffect the residual stress level Oriented material (normally) will shrink more than nonorientedmaterial On the inner surface of the original frozen layer highly oriented material wants toshrink a great deal but it is prevented from doing so by the less-oriented material The highly
oriented layer ends up being in tension while the less-oriented material is in compression This residual stress pattern is a common cause of part warpage
3 There is a connectionthrough orientationbetween the shear stress during
filling (flow stress) and the residual stress in the final molded part This meansshear stress during filling shown on Moldflow plots can be used as a designparameter
113 The Pressurization Phase
The pressurization phasefrom the point of view of flow behavioris very similar to thefilling phase The flow rate may drop somewhat as the mold builds up to pressure resulting inan increase in the thickness of the frozen layer
The main difference of course is the increase in hydrostatic (isotropic) pressure We shall seein chapter 2 section 24 Effect of Molding Conditions that hydrostatic pressure in itself doesnot cause any residual stress
114 The Compensation Phase
Compensating flow is unstable Consider the plate molding again (see Figure 11) You wouldthink that plastic flowing uniformly through the thin diaphragm would top up the thick rim Inpractice the plastic during the compensation phase flows in rivers that spread out like a deltaas illustrated in Figure 18 This may seem surprising at first but it can be explained by temperature instability
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How Do Plastics Flow 7
Figure 18ure 1 8 River flow
1141 Temperature Variation
There is always some variation in melt temperature coming from the barrel of the injectionmachine In exceptional cases up to 40 ordmC variation has been measured using a high-speedthermocouple
1142 Natural Instability
However slight the temperature variation natural instability will amplify it If for exampleone part of the melt is slightly hotter than the rest then the plastic flow in that area will beslightly greater bringing hotter material into the area and maintaining the temperature If onthe other hand there is another area that is cooler the flow will be less so there will be lessheat input and the plastic will get colder until it eventually freezes off
However balanced the initial conditions this natural instability will result in a river-type flow
This is a very important consideration The first material to freeze off will shrink early in thecycle By the time the material in the river flows freezes the bulk of the material will havealready frozen off and shrinkage will have occurred The rivers will shrink relative to the bulk of the molding and because they are highly orientated shrinkage will be very high The resultis high-stress tensile members throughout the molding a common cause of warpage
1143 Optimum Part Quality
Most of the stress in plastic parts occurs during the compensation phase By controlling flow and minimizing stress it is possible to design for optimum part quality This important point is
at the heart of the Moldflow philosophy
12 How Do Plastics Flow
121 Material Behavior
Molten thermoplastics exhibit viscoelastic behavior which combines flow characteristics of both viscous liquids and elastic solids When a viscous liquid flows the energy that causes thedeformation is dissipated and becomes viscous heat On the other hand when an elastic solid
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8 Polymer Flow Behavior in Injection Molds
is deformed the driving energy is stored For example the flow of water is a typical viscousflow whereas the deformation of a rubber cube falls into the elastic category
122 Deformation
In addition to the two types of material flow behavior there are two types of deformationsimple shear and simple extension (elongation) as shown in Figure 19 (a) and (b) below Theflow of molten thermoplastics during injection-molding filling is predominantly shear flow asshown in Figure 19 (c) in which layers of material elements slide over each other Theextensional flow however becomes significant as the material elements undergo elongation when the melt passes areas of abrupt dimensional change (eg a gate region) as shown inFigure 19 (d)
Figure 1igure 1 9 (a) Simple shear flow (b) Simple extensional flow (c) Shear flow in cavity filling (d) Extensional flow in cavity filling
123 Viscoelastic Behavior
In response to an applied stress (force per unit area) molten thermoplastics exhibit viscoelastic behavior which combines characteristics of an ideal viscous liquid with those of an ideal elastic solid In other words under certain conditions molten thermoplastics behavelike a liquid and will continuously deform while shear stress is applied as shown in
Figure 110 Upon the removal of the stress however the materials behave somewhat like anelastic solid with partial recovery of the deformation as shown in Figure 110 (b) and (c) This viscoelastic behavior stems from the random-coil configuration of polymer molecules in the
Stress
Stress Stress
Mold wall
Material element before
deformation begins
Material element during
deformation
Mold wall
Material elements at rest
Moving
materialelements
Velocity
profile
(a) (b)
(c) (d)
Velocity
profile
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How Do Plastics Flow 9
molten state which allows the movement and slippage of molecular chains under theinfluence of an applied load However the entanglement of the polymer molecular chains also
makes the system behave like an elastic solid upon the application and removal of the externalload Namely on removal of the stress chains will tend to return to the equilibrium random-coil state and thus will be a component of stress recovery The recovery is not instantaneousbecause of the entanglements still present in the system
Figure 1ure 1 10 (a) Ideal viscous liquid deforms continuously under applied stress (b) Ideal elasticsolid deforms immediately upon the application of stress but fully recovers when the
stress is removed (c) Molten thermoplastic deforms continuously under the applied
stress (like a viscous liquid) but also recovers partially from the deformation upon
removal of the applied stress (like an elastic solid)
124 Melt Shear Viscosity
1241 What Is Shear Viscosity
Melt shear viscosity is a materials resistance to shear flow In general polymer melts are highly viscous because of their long molecular chain structure The viscosity of a polymer melt rangesfrom 2 to 3000 Pas (water 10^-1 Pas glass 10^20 Pas) Viscosity can be thought of as thethickness of a fluid or how much it resists flow Viscosity is expressed as the ratio of shearstress (force per unit area) to the shear rate (rate change of shear strain) as shown inEquation 11 and Figure 111
(a) Viscous
liquid
(b) Elastic
solid
(c) Viscoelastic
polymer
melt
Initial shape
(shear stress
to be applied)
Time = Dt Time = 2 x Dt
(shear stress to
be removed)
Final deformed
shape
d
d
d
d
2d
2d
2d
2d
total
recovery
partial
recovery
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Contents XVII
1132 Molding Window 2051133 Filling Analysis 206
1134 Gate and Runner Design 2061135 Cooling System Design 2071136 Packing Analysis 2091137 Warpage Analysis 209114 Using Moldflow to Optimize the Design 2101141 Determine the Cause of Warpage 2101142 Investigating Different Gate Locations 2101143 Validating the Best Gate Location 211
12 Part Defects 215
121 Air Traps 2151211 What Is an Air Trap 2151212 Problems Caused by Air Traps 2161213 Remedies 216122 Black Specks and Black Streaks 2171221 What Are Black Specks and Black Streaks 2171222 Causes of Black Specks and Black Streaks 2171223 Remedies 218123 Brittleness 219
1231 What Is Brittleness 2191232 Causes of Brittleness 2191233 Remedies 219124 Burn Marks 2201241 What Is a Burn Mark 2201242 Causes of Burn Marks 2211243 Remedies 222125 Delamination 2221251 What Is Delamination 2221252 Causes of Delamination 223
1253 Remedies 223126 Dimensional Variation 2241261 What Is Dimensional Variation 2241262 Causes of Dimensional Variation 2241263 Remedies 224127 Discoloration 2251271 What Is Discoloration 2251272 Causes of Discoloration 2251273 Remedies 226128 Fish Eyes 227
1281 What Are Fish Eyes 2271282 Causes of Fish Eyes 2271283 Remedies 227
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XVIII Contents
129 Flash 2281291 What Is Flash 228
1292 Causes of Flash 2281293 Remedies 2291210 Flow Marks 23012101 What Is A Flow Mark 23012102 Causes of Flow Marks 23012103 Remedies 2301211 Hesitation 23112111 What Is Hesitation 23112112 Problems Caused by Hesitation 23112113 Remedies 232
1212 Jetting 23212121 What Is Jetting 23212122 Problems Caused by Jetting 23312123 Remedies 2331213 Ripples 23512131 What Are Ripples 23512132 Cause of Ripples 23512133 Remedies 2361214 Short Shots 23612141 What Is a Short Shot 236
12142 Causes of Short Shots 23712143 Remedies 2371215 Silver Streaks 23812151 What Are Silver Streaks 23812152 Causes of Silver Streaks 23912153 Remedies 2391216 Sink Marks and Voids 24012161 What Are Sink Marks and Voids 24012162 Causes of Sink Marks and Voids 24012163 Remedies 2411217 Weld Lines and Meld Lines 24312171 What Are Weld Lines and Meld Lines 24312172 Problems Caused by Weld Lines 24312173 Strength of Weld Lines 24412174 Remedies 245
Appendix A Injection Molding 247
A1 Injection-molding Overview 247 A11 Process 247
A12 Applications 248 A2 Development of the Injection-molding Machine 248 A21 Benefits of the Reciprocating Screw 248
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Contents XIX
A3 Development of the Injection-molding Process 249 A4 Alternative Injection-molding Processes 249
A41 Co-injection (Sandwich) Molding 249 A42 Fusible Core Injection Molding 250 A43 Gas-assisted Injection Molding 251 A44 Injection-compression Molding 252 A45 Lamellar (Microlayer) Injection Molding 253 A46 Live-feed Injection Molding 253 A47 Low-pressure Injection Molding 254 A48 Push-pull Injection Molding 254 A49 Reactive Molding 255 A410 Structural Foam Injection Molding 256
A411 Thin-wall Molding 256
Appendix B Injection-molding Machine System and Operations 259
B1 Injection-molding Machine 259B11 Components 259B12 Machine Specification 260B13 Machine Function 260B14 Auxiliary Equipment 260B2 Machine Components 261
B21 Injection System 261B22 Mold System 263B23 Hydraulic System 264B24 Control System 264B25 Clamping System 264B3 Molded System 265B31 The Delivery System 265B4 Machine Operating Sequence 266B41 Process Cycle 266B5 Screw Operation 268
B51 Back Pressure 268B52 Injection Speed 268B53 Screw Rotation Speed 268B54 Cushion 269B6 Secondary Operations 269B61 Assembly 269B62 Decoration 269B63 Other Secondary Operations 270
Appendix C Injection-molding Process Control 271
C1 Importance of Process Conditions 271C11 Setting Machine Process Conditions 271
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XX Contents
Appendix D Plastic Materials 281
D1 What Are Plastics 281
D11 Polymerization Process 281D12 Structure of Polymers 281D13 Polymer Alloys and Blends 283D14 Polymer Composites 283D2 Classification of Plastics 283D21 Classes of Plastics 284D22 Structures and Properties of Plastics 284D3 Thermoplastics 285D31 Market Share Distribution of Thermoplastics 285D32 Structures and Properties of Thermoplastics 286D33 Amorphous Polymers 286D34 Semicrystalline Polymers 287D4 Thermosets 288D5 Properties and Applications of Thermoplastics 289D51 ABS 290D52 PA 12 291D53 PA 6 293D54 PA 66 294D55 PBT 296
D56 PC 297D57 PCABS 299D58 PCPBT 300D59 HDPE 301D510 LDPE 303D511 PEI 304D512 PET 305D513 PETG 306D514 PMMA 307D515 POM 309
D516 PP 310D517 PPEPPO 312D518 PS 313D519 PVC 314D520 SAN 316D521 Additives Fillers and Reinforcements 317D522 Modifying Polymer Properties 318D523 Low-aspect Fillers 319D524 High-aspect Fillers Fibers 319
Index 321
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1 Polymer Flow Behavior in Injection Molds
bull Phases of injection molding
bull How do plastics flow
11 Phases of Injection Molding
Any molder can prove that all the conditions and effects discussed in this chapter do indeedoccur during the injection molding process While this knowledge alone can somewhatimprove quality it is only with the use of Moldflow analysis during the initial design stage with the mold designed for the optimum filling pattern that these effects can be controlledand the full benefits obtained
Flow technology is concerned with the behavior of plastics during the mold filling process Aplastic parts properties depend on how the part is molded Two parts having identicaldimensions and made from the same material but molded under different conditions will havedifferent stress and shrinkage levels and will behave differently in the field meaning that they
are in practice two different parts The way the plastic flows into the mold is of paramount importance in determining the quality of the part The process of filling the mold can be distinctly analyzed with the ability to predictpressure temperature and stress
111 How Plastic Fills a Mold
This was investigated using a centrally gated mold shaped like a dinner plate with a thick rim
around the outside as shown in Figure 11 It was found that the injection molding processalthough complex could be divided into three phases (we use the word phase to avoidconfusion with injection stage as used with programmed injection)
Figure 1ure 1 1 Cross-section of disk mold used to investigate flow
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2 Polymer Flow Behavior in Injection Molds
1111 Filling Phase
As the ram moves forward it first moves at a steady speed as the plastic flows into the cavity
This is the filling phase This phase lasts until the mold is just filled See Figure 12 andFigure 13
1112 Pressurization Phase
The pressurization phase begins when the ram moves forward after the filling phase to bring the mold up to pressure When the mold is filled the ram will slow down but it still movesquite some distance because plastics are very compressible materials At injection molding pressure an extra 15 volume of material can be forced into the cavity See Figure 12 andFigure 13
Although fluids are usually assumed to be incompressible molten plastics have to beconsidered to be more like a gas The compressibility of plastics can be observed by blocking off the nozzle and attempting to purge the barrel The ram will jump forward when thepressure is applied but will spring back when the pressure is released
1113 Compensation Phase
After the pressurization phase the ram still does not stop completely continuing to creepforward for some time Plastics have a very large volumetric change of about 25 from themelt to the solid This can be seen in a short shot the difference in volume between themolding and the cavity is due to this volumetric change See Figure 12 and Figure 13
The ram moving forward to compensate for the volumetric change in the part is called thecompensation phase As the volumetric change is 25 and at the most only an extra 15 canbe injected in the pressurization phase there must always be some compensation phase
Figure 1igure 1 2 Phases of injection molding
Filling phase
Pressurization phase
Compensation phase
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Phases of Injection Molding 3
Figure 13ure 1 3 Phases of injection molding detail
112 The Filling Phase
A two-color technique best demonstrates this phase After emptying the barrel of an injection-molding machine a small amount of red plastic was charged followed by green plastic
Consider the closed mold with the plastic front just starting to flow from the nozzle Theplastic first fills the sprue and runner system then enters the mold cavity itself forming a smallbubble of molten plastic
The skin of the plastic in contact with the cool mold freezes rapidly while the central coreremains molten When additional material is injected it flows into this central core displacing
the material already there which then forms a new flow front The flow of this displacedmaterial is a combination of forward flow and outward flow The outward flow contacts the wall freezes and forms the next section of skin while the forward flow forms the new moltencore When more material enters the mold it flows along a channel lined with these frozen walls of plastic illustrated in Figure 14
This flow pattern is often called fountain flow or bubble flow because the flow front is like abubble being inflated with hot plastic from the center The frozen layer is formed by the flow front inflating and so is subject to only a low shear stress and therefore has a very low levelof molecular orientation Once it is frozen it cannot be orientated any further so the frozen
layer in the finished part has a low level of orientation
Figure 14ure 1 4 Fountain flow and heat transfer
Filling Pressurization Compensation
Hot plastic
Frozen layer
Heat loss to mold
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4 Polymer Flow Behavior in Injection Molds
Now consider what happens upstream Hot plastic is continuously flowing bringing new hotmaterial along and generating significant frictional heat At the same time heat is being lost
through the frozen layer to the cold mold surfaceInitially the frozen layer is very thin so heat is lost very rapidly This results in more plasticfreezing and the frozen layer getting thicker cutting down the heat flow After a time thefrozen layer will reach a thickness such that the heat lost by conduction is equal to the heatinput from plastic flow and frictional heating ie an equilibrium condition is reached(Figure 14)
It is interesting to do some calculations on the time taken to reach this state of equilibrium The actual rate of heat flow is very large in comparison with the small heat content of theplastic in the frozen layer The result is that equilibrium is reached very quickly often in a timemeasured in a few tenths of a second As the total filling time is measured in seconds thefrozen layer reaches an equilibrium state early in the filling cycle
It is useful to think about how the thickness of this frozen layer will vary If the injection rate were slowed less heat would be generated by friction along the flow path with less heat inputfrom the flow The heat loss would be at the same rate and with less heat input the frozenlayer would grow in thickness If the injection rate were raised the frozen layer would bethinner (Figure 15) Similarly higher melt and mold temperatures would reduce the thicknessof the frozen layer This can be seen experimentally using the two-color technique
Figure 1igure 1 5 Influence of injection rate on frozen layer thickness
1121 Flow Shear Stress
It is easy to get confused between the various stress levels and orientation of the polymer Asthe plastic flows it is subject to shear stress also called flow shear stress This flow shear stress willorient the material ie cause the molecules to align themselves in the general direction of flow
The shear stress varies from a maximum at the outside dropping off to zero at the center
3Shear stress is purely a function of force and area This must not be confused withshear rate which is the rate of plastic sliding over the next layer Shear rate is zero
= Frozen
= Molten cooler
= Molten hotter
Slowerinjection
rate
Faster
injection
rate
vs
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Phases of Injection Molding 5
at the outer edge where the plastic is frozen rises to a maximum just inwards of thefrozen layer then drops toward the center as shown in Figure 16
Figure 16ure 1 6 Shear rate distribution
If the flow were stopped and the plastic allowed to cool down very slowly this orientation would have time to relax giving a very low level of residual orientation On the other hand if the material were kept under stress and the plastic snap frozen most of the orientation would
be trapped in the frozen plastic (Figure 17)
Figure 17ure 1 7 Molecular orientation through the thickness of the part
Min MaxShear rate
= Tension
= Compression
Molecular orientation trapped
in pressurization phase
Gradual reduction of orientation
toward the center of the
mold as stress levels are lower
and the cooling rate is slower
which allows more time for
orientation to relax
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6 Polymer Flow Behavior in Injection Molds
Now consider the orientation from the mold surface toward the center
The frozen layer itself formed with very little shear and therefore low orientation immediately
freezes setting the low level of orientation
The layer of plastic just on the inside of the frozen layer is subject to maximum shear stressand freezes the instant flow stops trapping almost all the orientation
This is the orientation pattern the further toward the center the more the shear stress dropsand the slower the rate of cooling This allows more time for the level of orientation to relaxso the residual orientation drops rapidly toward the center Consider how this pattern willaffect the residual stress level Oriented material (normally) will shrink more than nonorientedmaterial On the inner surface of the original frozen layer highly oriented material wants toshrink a great deal but it is prevented from doing so by the less-oriented material The highly
oriented layer ends up being in tension while the less-oriented material is in compression This residual stress pattern is a common cause of part warpage
3 There is a connectionthrough orientationbetween the shear stress during
filling (flow stress) and the residual stress in the final molded part This meansshear stress during filling shown on Moldflow plots can be used as a designparameter
113 The Pressurization Phase
The pressurization phasefrom the point of view of flow behavioris very similar to thefilling phase The flow rate may drop somewhat as the mold builds up to pressure resulting inan increase in the thickness of the frozen layer
The main difference of course is the increase in hydrostatic (isotropic) pressure We shall seein chapter 2 section 24 Effect of Molding Conditions that hydrostatic pressure in itself doesnot cause any residual stress
114 The Compensation Phase
Compensating flow is unstable Consider the plate molding again (see Figure 11) You wouldthink that plastic flowing uniformly through the thin diaphragm would top up the thick rim Inpractice the plastic during the compensation phase flows in rivers that spread out like a deltaas illustrated in Figure 18 This may seem surprising at first but it can be explained by temperature instability
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How Do Plastics Flow 7
Figure 18ure 1 8 River flow
1141 Temperature Variation
There is always some variation in melt temperature coming from the barrel of the injectionmachine In exceptional cases up to 40 ordmC variation has been measured using a high-speedthermocouple
1142 Natural Instability
However slight the temperature variation natural instability will amplify it If for exampleone part of the melt is slightly hotter than the rest then the plastic flow in that area will beslightly greater bringing hotter material into the area and maintaining the temperature If onthe other hand there is another area that is cooler the flow will be less so there will be lessheat input and the plastic will get colder until it eventually freezes off
However balanced the initial conditions this natural instability will result in a river-type flow
This is a very important consideration The first material to freeze off will shrink early in thecycle By the time the material in the river flows freezes the bulk of the material will havealready frozen off and shrinkage will have occurred The rivers will shrink relative to the bulk of the molding and because they are highly orientated shrinkage will be very high The resultis high-stress tensile members throughout the molding a common cause of warpage
1143 Optimum Part Quality
Most of the stress in plastic parts occurs during the compensation phase By controlling flow and minimizing stress it is possible to design for optimum part quality This important point is
at the heart of the Moldflow philosophy
12 How Do Plastics Flow
121 Material Behavior
Molten thermoplastics exhibit viscoelastic behavior which combines flow characteristics of both viscous liquids and elastic solids When a viscous liquid flows the energy that causes thedeformation is dissipated and becomes viscous heat On the other hand when an elastic solid
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8 Polymer Flow Behavior in Injection Molds
is deformed the driving energy is stored For example the flow of water is a typical viscousflow whereas the deformation of a rubber cube falls into the elastic category
122 Deformation
In addition to the two types of material flow behavior there are two types of deformationsimple shear and simple extension (elongation) as shown in Figure 19 (a) and (b) below Theflow of molten thermoplastics during injection-molding filling is predominantly shear flow asshown in Figure 19 (c) in which layers of material elements slide over each other Theextensional flow however becomes significant as the material elements undergo elongation when the melt passes areas of abrupt dimensional change (eg a gate region) as shown inFigure 19 (d)
Figure 1igure 1 9 (a) Simple shear flow (b) Simple extensional flow (c) Shear flow in cavity filling (d) Extensional flow in cavity filling
123 Viscoelastic Behavior
In response to an applied stress (force per unit area) molten thermoplastics exhibit viscoelastic behavior which combines characteristics of an ideal viscous liquid with those of an ideal elastic solid In other words under certain conditions molten thermoplastics behavelike a liquid and will continuously deform while shear stress is applied as shown in
Figure 110 Upon the removal of the stress however the materials behave somewhat like anelastic solid with partial recovery of the deformation as shown in Figure 110 (b) and (c) This viscoelastic behavior stems from the random-coil configuration of polymer molecules in the
Stress
Stress Stress
Mold wall
Material element before
deformation begins
Material element during
deformation
Mold wall
Material elements at rest
Moving
materialelements
Velocity
profile
(a) (b)
(c) (d)
Velocity
profile
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How Do Plastics Flow 9
molten state which allows the movement and slippage of molecular chains under theinfluence of an applied load However the entanglement of the polymer molecular chains also
makes the system behave like an elastic solid upon the application and removal of the externalload Namely on removal of the stress chains will tend to return to the equilibrium random-coil state and thus will be a component of stress recovery The recovery is not instantaneousbecause of the entanglements still present in the system
Figure 1ure 1 10 (a) Ideal viscous liquid deforms continuously under applied stress (b) Ideal elasticsolid deforms immediately upon the application of stress but fully recovers when the
stress is removed (c) Molten thermoplastic deforms continuously under the applied
stress (like a viscous liquid) but also recovers partially from the deformation upon
removal of the applied stress (like an elastic solid)
124 Melt Shear Viscosity
1241 What Is Shear Viscosity
Melt shear viscosity is a materials resistance to shear flow In general polymer melts are highly viscous because of their long molecular chain structure The viscosity of a polymer melt rangesfrom 2 to 3000 Pas (water 10^-1 Pas glass 10^20 Pas) Viscosity can be thought of as thethickness of a fluid or how much it resists flow Viscosity is expressed as the ratio of shearstress (force per unit area) to the shear rate (rate change of shear strain) as shown inEquation 11 and Figure 111
(a) Viscous
liquid
(b) Elastic
solid
(c) Viscoelastic
polymer
melt
Initial shape
(shear stress
to be applied)
Time = Dt Time = 2 x Dt
(shear stress to
be removed)
Final deformed
shape
d
d
d
d
2d
2d
2d
2d
total
recovery
partial
recovery
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XVIII Contents
129 Flash 2281291 What Is Flash 228
1292 Causes of Flash 2281293 Remedies 2291210 Flow Marks 23012101 What Is A Flow Mark 23012102 Causes of Flow Marks 23012103 Remedies 2301211 Hesitation 23112111 What Is Hesitation 23112112 Problems Caused by Hesitation 23112113 Remedies 232
1212 Jetting 23212121 What Is Jetting 23212122 Problems Caused by Jetting 23312123 Remedies 2331213 Ripples 23512131 What Are Ripples 23512132 Cause of Ripples 23512133 Remedies 2361214 Short Shots 23612141 What Is a Short Shot 236
12142 Causes of Short Shots 23712143 Remedies 2371215 Silver Streaks 23812151 What Are Silver Streaks 23812152 Causes of Silver Streaks 23912153 Remedies 2391216 Sink Marks and Voids 24012161 What Are Sink Marks and Voids 24012162 Causes of Sink Marks and Voids 24012163 Remedies 2411217 Weld Lines and Meld Lines 24312171 What Are Weld Lines and Meld Lines 24312172 Problems Caused by Weld Lines 24312173 Strength of Weld Lines 24412174 Remedies 245
Appendix A Injection Molding 247
A1 Injection-molding Overview 247 A11 Process 247
A12 Applications 248 A2 Development of the Injection-molding Machine 248 A21 Benefits of the Reciprocating Screw 248
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Contents XIX
A3 Development of the Injection-molding Process 249 A4 Alternative Injection-molding Processes 249
A41 Co-injection (Sandwich) Molding 249 A42 Fusible Core Injection Molding 250 A43 Gas-assisted Injection Molding 251 A44 Injection-compression Molding 252 A45 Lamellar (Microlayer) Injection Molding 253 A46 Live-feed Injection Molding 253 A47 Low-pressure Injection Molding 254 A48 Push-pull Injection Molding 254 A49 Reactive Molding 255 A410 Structural Foam Injection Molding 256
A411 Thin-wall Molding 256
Appendix B Injection-molding Machine System and Operations 259
B1 Injection-molding Machine 259B11 Components 259B12 Machine Specification 260B13 Machine Function 260B14 Auxiliary Equipment 260B2 Machine Components 261
B21 Injection System 261B22 Mold System 263B23 Hydraulic System 264B24 Control System 264B25 Clamping System 264B3 Molded System 265B31 The Delivery System 265B4 Machine Operating Sequence 266B41 Process Cycle 266B5 Screw Operation 268
B51 Back Pressure 268B52 Injection Speed 268B53 Screw Rotation Speed 268B54 Cushion 269B6 Secondary Operations 269B61 Assembly 269B62 Decoration 269B63 Other Secondary Operations 270
Appendix C Injection-molding Process Control 271
C1 Importance of Process Conditions 271C11 Setting Machine Process Conditions 271
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XX Contents
Appendix D Plastic Materials 281
D1 What Are Plastics 281
D11 Polymerization Process 281D12 Structure of Polymers 281D13 Polymer Alloys and Blends 283D14 Polymer Composites 283D2 Classification of Plastics 283D21 Classes of Plastics 284D22 Structures and Properties of Plastics 284D3 Thermoplastics 285D31 Market Share Distribution of Thermoplastics 285D32 Structures and Properties of Thermoplastics 286D33 Amorphous Polymers 286D34 Semicrystalline Polymers 287D4 Thermosets 288D5 Properties and Applications of Thermoplastics 289D51 ABS 290D52 PA 12 291D53 PA 6 293D54 PA 66 294D55 PBT 296
D56 PC 297D57 PCABS 299D58 PCPBT 300D59 HDPE 301D510 LDPE 303D511 PEI 304D512 PET 305D513 PETG 306D514 PMMA 307D515 POM 309
D516 PP 310D517 PPEPPO 312D518 PS 313D519 PVC 314D520 SAN 316D521 Additives Fillers and Reinforcements 317D522 Modifying Polymer Properties 318D523 Low-aspect Fillers 319D524 High-aspect Fillers Fibers 319
Index 321
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1 Polymer Flow Behavior in Injection Molds
bull Phases of injection molding
bull How do plastics flow
11 Phases of Injection Molding
Any molder can prove that all the conditions and effects discussed in this chapter do indeedoccur during the injection molding process While this knowledge alone can somewhatimprove quality it is only with the use of Moldflow analysis during the initial design stage with the mold designed for the optimum filling pattern that these effects can be controlledand the full benefits obtained
Flow technology is concerned with the behavior of plastics during the mold filling process Aplastic parts properties depend on how the part is molded Two parts having identicaldimensions and made from the same material but molded under different conditions will havedifferent stress and shrinkage levels and will behave differently in the field meaning that they
are in practice two different parts The way the plastic flows into the mold is of paramount importance in determining the quality of the part The process of filling the mold can be distinctly analyzed with the ability to predictpressure temperature and stress
111 How Plastic Fills a Mold
This was investigated using a centrally gated mold shaped like a dinner plate with a thick rim
around the outside as shown in Figure 11 It was found that the injection molding processalthough complex could be divided into three phases (we use the word phase to avoidconfusion with injection stage as used with programmed injection)
Figure 1ure 1 1 Cross-section of disk mold used to investigate flow
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2 Polymer Flow Behavior in Injection Molds
1111 Filling Phase
As the ram moves forward it first moves at a steady speed as the plastic flows into the cavity
This is the filling phase This phase lasts until the mold is just filled See Figure 12 andFigure 13
1112 Pressurization Phase
The pressurization phase begins when the ram moves forward after the filling phase to bring the mold up to pressure When the mold is filled the ram will slow down but it still movesquite some distance because plastics are very compressible materials At injection molding pressure an extra 15 volume of material can be forced into the cavity See Figure 12 andFigure 13
Although fluids are usually assumed to be incompressible molten plastics have to beconsidered to be more like a gas The compressibility of plastics can be observed by blocking off the nozzle and attempting to purge the barrel The ram will jump forward when thepressure is applied but will spring back when the pressure is released
1113 Compensation Phase
After the pressurization phase the ram still does not stop completely continuing to creepforward for some time Plastics have a very large volumetric change of about 25 from themelt to the solid This can be seen in a short shot the difference in volume between themolding and the cavity is due to this volumetric change See Figure 12 and Figure 13
The ram moving forward to compensate for the volumetric change in the part is called thecompensation phase As the volumetric change is 25 and at the most only an extra 15 canbe injected in the pressurization phase there must always be some compensation phase
Figure 1igure 1 2 Phases of injection molding
Filling phase
Pressurization phase
Compensation phase
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Phases of Injection Molding 3
Figure 13ure 1 3 Phases of injection molding detail
112 The Filling Phase
A two-color technique best demonstrates this phase After emptying the barrel of an injection-molding machine a small amount of red plastic was charged followed by green plastic
Consider the closed mold with the plastic front just starting to flow from the nozzle Theplastic first fills the sprue and runner system then enters the mold cavity itself forming a smallbubble of molten plastic
The skin of the plastic in contact with the cool mold freezes rapidly while the central coreremains molten When additional material is injected it flows into this central core displacing
the material already there which then forms a new flow front The flow of this displacedmaterial is a combination of forward flow and outward flow The outward flow contacts the wall freezes and forms the next section of skin while the forward flow forms the new moltencore When more material enters the mold it flows along a channel lined with these frozen walls of plastic illustrated in Figure 14
This flow pattern is often called fountain flow or bubble flow because the flow front is like abubble being inflated with hot plastic from the center The frozen layer is formed by the flow front inflating and so is subject to only a low shear stress and therefore has a very low levelof molecular orientation Once it is frozen it cannot be orientated any further so the frozen
layer in the finished part has a low level of orientation
Figure 14ure 1 4 Fountain flow and heat transfer
Filling Pressurization Compensation
Hot plastic
Frozen layer
Heat loss to mold
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4 Polymer Flow Behavior in Injection Molds
Now consider what happens upstream Hot plastic is continuously flowing bringing new hotmaterial along and generating significant frictional heat At the same time heat is being lost
through the frozen layer to the cold mold surfaceInitially the frozen layer is very thin so heat is lost very rapidly This results in more plasticfreezing and the frozen layer getting thicker cutting down the heat flow After a time thefrozen layer will reach a thickness such that the heat lost by conduction is equal to the heatinput from plastic flow and frictional heating ie an equilibrium condition is reached(Figure 14)
It is interesting to do some calculations on the time taken to reach this state of equilibrium The actual rate of heat flow is very large in comparison with the small heat content of theplastic in the frozen layer The result is that equilibrium is reached very quickly often in a timemeasured in a few tenths of a second As the total filling time is measured in seconds thefrozen layer reaches an equilibrium state early in the filling cycle
It is useful to think about how the thickness of this frozen layer will vary If the injection rate were slowed less heat would be generated by friction along the flow path with less heat inputfrom the flow The heat loss would be at the same rate and with less heat input the frozenlayer would grow in thickness If the injection rate were raised the frozen layer would bethinner (Figure 15) Similarly higher melt and mold temperatures would reduce the thicknessof the frozen layer This can be seen experimentally using the two-color technique
Figure 1igure 1 5 Influence of injection rate on frozen layer thickness
1121 Flow Shear Stress
It is easy to get confused between the various stress levels and orientation of the polymer Asthe plastic flows it is subject to shear stress also called flow shear stress This flow shear stress willorient the material ie cause the molecules to align themselves in the general direction of flow
The shear stress varies from a maximum at the outside dropping off to zero at the center
3Shear stress is purely a function of force and area This must not be confused withshear rate which is the rate of plastic sliding over the next layer Shear rate is zero
= Frozen
= Molten cooler
= Molten hotter
Slowerinjection
rate
Faster
injection
rate
vs
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Phases of Injection Molding 5
at the outer edge where the plastic is frozen rises to a maximum just inwards of thefrozen layer then drops toward the center as shown in Figure 16
Figure 16ure 1 6 Shear rate distribution
If the flow were stopped and the plastic allowed to cool down very slowly this orientation would have time to relax giving a very low level of residual orientation On the other hand if the material were kept under stress and the plastic snap frozen most of the orientation would
be trapped in the frozen plastic (Figure 17)
Figure 17ure 1 7 Molecular orientation through the thickness of the part
Min MaxShear rate
= Tension
= Compression
Molecular orientation trapped
in pressurization phase
Gradual reduction of orientation
toward the center of the
mold as stress levels are lower
and the cooling rate is slower
which allows more time for
orientation to relax
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6 Polymer Flow Behavior in Injection Molds
Now consider the orientation from the mold surface toward the center
The frozen layer itself formed with very little shear and therefore low orientation immediately
freezes setting the low level of orientation
The layer of plastic just on the inside of the frozen layer is subject to maximum shear stressand freezes the instant flow stops trapping almost all the orientation
This is the orientation pattern the further toward the center the more the shear stress dropsand the slower the rate of cooling This allows more time for the level of orientation to relaxso the residual orientation drops rapidly toward the center Consider how this pattern willaffect the residual stress level Oriented material (normally) will shrink more than nonorientedmaterial On the inner surface of the original frozen layer highly oriented material wants toshrink a great deal but it is prevented from doing so by the less-oriented material The highly
oriented layer ends up being in tension while the less-oriented material is in compression This residual stress pattern is a common cause of part warpage
3 There is a connectionthrough orientationbetween the shear stress during
filling (flow stress) and the residual stress in the final molded part This meansshear stress during filling shown on Moldflow plots can be used as a designparameter
113 The Pressurization Phase
The pressurization phasefrom the point of view of flow behavioris very similar to thefilling phase The flow rate may drop somewhat as the mold builds up to pressure resulting inan increase in the thickness of the frozen layer
The main difference of course is the increase in hydrostatic (isotropic) pressure We shall seein chapter 2 section 24 Effect of Molding Conditions that hydrostatic pressure in itself doesnot cause any residual stress
114 The Compensation Phase
Compensating flow is unstable Consider the plate molding again (see Figure 11) You wouldthink that plastic flowing uniformly through the thin diaphragm would top up the thick rim Inpractice the plastic during the compensation phase flows in rivers that spread out like a deltaas illustrated in Figure 18 This may seem surprising at first but it can be explained by temperature instability
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How Do Plastics Flow 7
Figure 18ure 1 8 River flow
1141 Temperature Variation
There is always some variation in melt temperature coming from the barrel of the injectionmachine In exceptional cases up to 40 ordmC variation has been measured using a high-speedthermocouple
1142 Natural Instability
However slight the temperature variation natural instability will amplify it If for exampleone part of the melt is slightly hotter than the rest then the plastic flow in that area will beslightly greater bringing hotter material into the area and maintaining the temperature If onthe other hand there is another area that is cooler the flow will be less so there will be lessheat input and the plastic will get colder until it eventually freezes off
However balanced the initial conditions this natural instability will result in a river-type flow
This is a very important consideration The first material to freeze off will shrink early in thecycle By the time the material in the river flows freezes the bulk of the material will havealready frozen off and shrinkage will have occurred The rivers will shrink relative to the bulk of the molding and because they are highly orientated shrinkage will be very high The resultis high-stress tensile members throughout the molding a common cause of warpage
1143 Optimum Part Quality
Most of the stress in plastic parts occurs during the compensation phase By controlling flow and minimizing stress it is possible to design for optimum part quality This important point is
at the heart of the Moldflow philosophy
12 How Do Plastics Flow
121 Material Behavior
Molten thermoplastics exhibit viscoelastic behavior which combines flow characteristics of both viscous liquids and elastic solids When a viscous liquid flows the energy that causes thedeformation is dissipated and becomes viscous heat On the other hand when an elastic solid
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8 Polymer Flow Behavior in Injection Molds
is deformed the driving energy is stored For example the flow of water is a typical viscousflow whereas the deformation of a rubber cube falls into the elastic category
122 Deformation
In addition to the two types of material flow behavior there are two types of deformationsimple shear and simple extension (elongation) as shown in Figure 19 (a) and (b) below Theflow of molten thermoplastics during injection-molding filling is predominantly shear flow asshown in Figure 19 (c) in which layers of material elements slide over each other Theextensional flow however becomes significant as the material elements undergo elongation when the melt passes areas of abrupt dimensional change (eg a gate region) as shown inFigure 19 (d)
Figure 1igure 1 9 (a) Simple shear flow (b) Simple extensional flow (c) Shear flow in cavity filling (d) Extensional flow in cavity filling
123 Viscoelastic Behavior
In response to an applied stress (force per unit area) molten thermoplastics exhibit viscoelastic behavior which combines characteristics of an ideal viscous liquid with those of an ideal elastic solid In other words under certain conditions molten thermoplastics behavelike a liquid and will continuously deform while shear stress is applied as shown in
Figure 110 Upon the removal of the stress however the materials behave somewhat like anelastic solid with partial recovery of the deformation as shown in Figure 110 (b) and (c) This viscoelastic behavior stems from the random-coil configuration of polymer molecules in the
Stress
Stress Stress
Mold wall
Material element before
deformation begins
Material element during
deformation
Mold wall
Material elements at rest
Moving
materialelements
Velocity
profile
(a) (b)
(c) (d)
Velocity
profile
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How Do Plastics Flow 9
molten state which allows the movement and slippage of molecular chains under theinfluence of an applied load However the entanglement of the polymer molecular chains also
makes the system behave like an elastic solid upon the application and removal of the externalload Namely on removal of the stress chains will tend to return to the equilibrium random-coil state and thus will be a component of stress recovery The recovery is not instantaneousbecause of the entanglements still present in the system
Figure 1ure 1 10 (a) Ideal viscous liquid deforms continuously under applied stress (b) Ideal elasticsolid deforms immediately upon the application of stress but fully recovers when the
stress is removed (c) Molten thermoplastic deforms continuously under the applied
stress (like a viscous liquid) but also recovers partially from the deformation upon
removal of the applied stress (like an elastic solid)
124 Melt Shear Viscosity
1241 What Is Shear Viscosity
Melt shear viscosity is a materials resistance to shear flow In general polymer melts are highly viscous because of their long molecular chain structure The viscosity of a polymer melt rangesfrom 2 to 3000 Pas (water 10^-1 Pas glass 10^20 Pas) Viscosity can be thought of as thethickness of a fluid or how much it resists flow Viscosity is expressed as the ratio of shearstress (force per unit area) to the shear rate (rate change of shear strain) as shown inEquation 11 and Figure 111
(a) Viscous
liquid
(b) Elastic
solid
(c) Viscoelastic
polymer
melt
Initial shape
(shear stress
to be applied)
Time = Dt Time = 2 x Dt
(shear stress to
be removed)
Final deformed
shape
d
d
d
d
2d
2d
2d
2d
total
recovery
partial
recovery
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Contents XIX
A3 Development of the Injection-molding Process 249 A4 Alternative Injection-molding Processes 249
A41 Co-injection (Sandwich) Molding 249 A42 Fusible Core Injection Molding 250 A43 Gas-assisted Injection Molding 251 A44 Injection-compression Molding 252 A45 Lamellar (Microlayer) Injection Molding 253 A46 Live-feed Injection Molding 253 A47 Low-pressure Injection Molding 254 A48 Push-pull Injection Molding 254 A49 Reactive Molding 255 A410 Structural Foam Injection Molding 256
A411 Thin-wall Molding 256
Appendix B Injection-molding Machine System and Operations 259
B1 Injection-molding Machine 259B11 Components 259B12 Machine Specification 260B13 Machine Function 260B14 Auxiliary Equipment 260B2 Machine Components 261
B21 Injection System 261B22 Mold System 263B23 Hydraulic System 264B24 Control System 264B25 Clamping System 264B3 Molded System 265B31 The Delivery System 265B4 Machine Operating Sequence 266B41 Process Cycle 266B5 Screw Operation 268
B51 Back Pressure 268B52 Injection Speed 268B53 Screw Rotation Speed 268B54 Cushion 269B6 Secondary Operations 269B61 Assembly 269B62 Decoration 269B63 Other Secondary Operations 270
Appendix C Injection-molding Process Control 271
C1 Importance of Process Conditions 271C11 Setting Machine Process Conditions 271
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XX Contents
Appendix D Plastic Materials 281
D1 What Are Plastics 281
D11 Polymerization Process 281D12 Structure of Polymers 281D13 Polymer Alloys and Blends 283D14 Polymer Composites 283D2 Classification of Plastics 283D21 Classes of Plastics 284D22 Structures and Properties of Plastics 284D3 Thermoplastics 285D31 Market Share Distribution of Thermoplastics 285D32 Structures and Properties of Thermoplastics 286D33 Amorphous Polymers 286D34 Semicrystalline Polymers 287D4 Thermosets 288D5 Properties and Applications of Thermoplastics 289D51 ABS 290D52 PA 12 291D53 PA 6 293D54 PA 66 294D55 PBT 296
D56 PC 297D57 PCABS 299D58 PCPBT 300D59 HDPE 301D510 LDPE 303D511 PEI 304D512 PET 305D513 PETG 306D514 PMMA 307D515 POM 309
D516 PP 310D517 PPEPPO 312D518 PS 313D519 PVC 314D520 SAN 316D521 Additives Fillers and Reinforcements 317D522 Modifying Polymer Properties 318D523 Low-aspect Fillers 319D524 High-aspect Fillers Fibers 319
Index 321
8102019 L-G-0004410537-0002634439 Moldflow
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1 Polymer Flow Behavior in Injection Molds
bull Phases of injection molding
bull How do plastics flow
11 Phases of Injection Molding
Any molder can prove that all the conditions and effects discussed in this chapter do indeedoccur during the injection molding process While this knowledge alone can somewhatimprove quality it is only with the use of Moldflow analysis during the initial design stage with the mold designed for the optimum filling pattern that these effects can be controlledand the full benefits obtained
Flow technology is concerned with the behavior of plastics during the mold filling process Aplastic parts properties depend on how the part is molded Two parts having identicaldimensions and made from the same material but molded under different conditions will havedifferent stress and shrinkage levels and will behave differently in the field meaning that they
are in practice two different parts The way the plastic flows into the mold is of paramount importance in determining the quality of the part The process of filling the mold can be distinctly analyzed with the ability to predictpressure temperature and stress
111 How Plastic Fills a Mold
This was investigated using a centrally gated mold shaped like a dinner plate with a thick rim
around the outside as shown in Figure 11 It was found that the injection molding processalthough complex could be divided into three phases (we use the word phase to avoidconfusion with injection stage as used with programmed injection)
Figure 1ure 1 1 Cross-section of disk mold used to investigate flow
8102019 L-G-0004410537-0002634439 Moldflow
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2 Polymer Flow Behavior in Injection Molds
1111 Filling Phase
As the ram moves forward it first moves at a steady speed as the plastic flows into the cavity
This is the filling phase This phase lasts until the mold is just filled See Figure 12 andFigure 13
1112 Pressurization Phase
The pressurization phase begins when the ram moves forward after the filling phase to bring the mold up to pressure When the mold is filled the ram will slow down but it still movesquite some distance because plastics are very compressible materials At injection molding pressure an extra 15 volume of material can be forced into the cavity See Figure 12 andFigure 13
Although fluids are usually assumed to be incompressible molten plastics have to beconsidered to be more like a gas The compressibility of plastics can be observed by blocking off the nozzle and attempting to purge the barrel The ram will jump forward when thepressure is applied but will spring back when the pressure is released
1113 Compensation Phase
After the pressurization phase the ram still does not stop completely continuing to creepforward for some time Plastics have a very large volumetric change of about 25 from themelt to the solid This can be seen in a short shot the difference in volume between themolding and the cavity is due to this volumetric change See Figure 12 and Figure 13
The ram moving forward to compensate for the volumetric change in the part is called thecompensation phase As the volumetric change is 25 and at the most only an extra 15 canbe injected in the pressurization phase there must always be some compensation phase
Figure 1igure 1 2 Phases of injection molding
Filling phase
Pressurization phase
Compensation phase
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Phases of Injection Molding 3
Figure 13ure 1 3 Phases of injection molding detail
112 The Filling Phase
A two-color technique best demonstrates this phase After emptying the barrel of an injection-molding machine a small amount of red plastic was charged followed by green plastic
Consider the closed mold with the plastic front just starting to flow from the nozzle Theplastic first fills the sprue and runner system then enters the mold cavity itself forming a smallbubble of molten plastic
The skin of the plastic in contact with the cool mold freezes rapidly while the central coreremains molten When additional material is injected it flows into this central core displacing
the material already there which then forms a new flow front The flow of this displacedmaterial is a combination of forward flow and outward flow The outward flow contacts the wall freezes and forms the next section of skin while the forward flow forms the new moltencore When more material enters the mold it flows along a channel lined with these frozen walls of plastic illustrated in Figure 14
This flow pattern is often called fountain flow or bubble flow because the flow front is like abubble being inflated with hot plastic from the center The frozen layer is formed by the flow front inflating and so is subject to only a low shear stress and therefore has a very low levelof molecular orientation Once it is frozen it cannot be orientated any further so the frozen
layer in the finished part has a low level of orientation
Figure 14ure 1 4 Fountain flow and heat transfer
Filling Pressurization Compensation
Hot plastic
Frozen layer
Heat loss to mold
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4 Polymer Flow Behavior in Injection Molds
Now consider what happens upstream Hot plastic is continuously flowing bringing new hotmaterial along and generating significant frictional heat At the same time heat is being lost
through the frozen layer to the cold mold surfaceInitially the frozen layer is very thin so heat is lost very rapidly This results in more plasticfreezing and the frozen layer getting thicker cutting down the heat flow After a time thefrozen layer will reach a thickness such that the heat lost by conduction is equal to the heatinput from plastic flow and frictional heating ie an equilibrium condition is reached(Figure 14)
It is interesting to do some calculations on the time taken to reach this state of equilibrium The actual rate of heat flow is very large in comparison with the small heat content of theplastic in the frozen layer The result is that equilibrium is reached very quickly often in a timemeasured in a few tenths of a second As the total filling time is measured in seconds thefrozen layer reaches an equilibrium state early in the filling cycle
It is useful to think about how the thickness of this frozen layer will vary If the injection rate were slowed less heat would be generated by friction along the flow path with less heat inputfrom the flow The heat loss would be at the same rate and with less heat input the frozenlayer would grow in thickness If the injection rate were raised the frozen layer would bethinner (Figure 15) Similarly higher melt and mold temperatures would reduce the thicknessof the frozen layer This can be seen experimentally using the two-color technique
Figure 1igure 1 5 Influence of injection rate on frozen layer thickness
1121 Flow Shear Stress
It is easy to get confused between the various stress levels and orientation of the polymer Asthe plastic flows it is subject to shear stress also called flow shear stress This flow shear stress willorient the material ie cause the molecules to align themselves in the general direction of flow
The shear stress varies from a maximum at the outside dropping off to zero at the center
3Shear stress is purely a function of force and area This must not be confused withshear rate which is the rate of plastic sliding over the next layer Shear rate is zero
= Frozen
= Molten cooler
= Molten hotter
Slowerinjection
rate
Faster
injection
rate
vs
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Phases of Injection Molding 5
at the outer edge where the plastic is frozen rises to a maximum just inwards of thefrozen layer then drops toward the center as shown in Figure 16
Figure 16ure 1 6 Shear rate distribution
If the flow were stopped and the plastic allowed to cool down very slowly this orientation would have time to relax giving a very low level of residual orientation On the other hand if the material were kept under stress and the plastic snap frozen most of the orientation would
be trapped in the frozen plastic (Figure 17)
Figure 17ure 1 7 Molecular orientation through the thickness of the part
Min MaxShear rate
= Tension
= Compression
Molecular orientation trapped
in pressurization phase
Gradual reduction of orientation
toward the center of the
mold as stress levels are lower
and the cooling rate is slower
which allows more time for
orientation to relax
8102019 L-G-0004410537-0002634439 Moldflow
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6 Polymer Flow Behavior in Injection Molds
Now consider the orientation from the mold surface toward the center
The frozen layer itself formed with very little shear and therefore low orientation immediately
freezes setting the low level of orientation
The layer of plastic just on the inside of the frozen layer is subject to maximum shear stressand freezes the instant flow stops trapping almost all the orientation
This is the orientation pattern the further toward the center the more the shear stress dropsand the slower the rate of cooling This allows more time for the level of orientation to relaxso the residual orientation drops rapidly toward the center Consider how this pattern willaffect the residual stress level Oriented material (normally) will shrink more than nonorientedmaterial On the inner surface of the original frozen layer highly oriented material wants toshrink a great deal but it is prevented from doing so by the less-oriented material The highly
oriented layer ends up being in tension while the less-oriented material is in compression This residual stress pattern is a common cause of part warpage
3 There is a connectionthrough orientationbetween the shear stress during
filling (flow stress) and the residual stress in the final molded part This meansshear stress during filling shown on Moldflow plots can be used as a designparameter
113 The Pressurization Phase
The pressurization phasefrom the point of view of flow behavioris very similar to thefilling phase The flow rate may drop somewhat as the mold builds up to pressure resulting inan increase in the thickness of the frozen layer
The main difference of course is the increase in hydrostatic (isotropic) pressure We shall seein chapter 2 section 24 Effect of Molding Conditions that hydrostatic pressure in itself doesnot cause any residual stress
114 The Compensation Phase
Compensating flow is unstable Consider the plate molding again (see Figure 11) You wouldthink that plastic flowing uniformly through the thin diaphragm would top up the thick rim Inpractice the plastic during the compensation phase flows in rivers that spread out like a deltaas illustrated in Figure 18 This may seem surprising at first but it can be explained by temperature instability
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How Do Plastics Flow 7
Figure 18ure 1 8 River flow
1141 Temperature Variation
There is always some variation in melt temperature coming from the barrel of the injectionmachine In exceptional cases up to 40 ordmC variation has been measured using a high-speedthermocouple
1142 Natural Instability
However slight the temperature variation natural instability will amplify it If for exampleone part of the melt is slightly hotter than the rest then the plastic flow in that area will beslightly greater bringing hotter material into the area and maintaining the temperature If onthe other hand there is another area that is cooler the flow will be less so there will be lessheat input and the plastic will get colder until it eventually freezes off
However balanced the initial conditions this natural instability will result in a river-type flow
This is a very important consideration The first material to freeze off will shrink early in thecycle By the time the material in the river flows freezes the bulk of the material will havealready frozen off and shrinkage will have occurred The rivers will shrink relative to the bulk of the molding and because they are highly orientated shrinkage will be very high The resultis high-stress tensile members throughout the molding a common cause of warpage
1143 Optimum Part Quality
Most of the stress in plastic parts occurs during the compensation phase By controlling flow and minimizing stress it is possible to design for optimum part quality This important point is
at the heart of the Moldflow philosophy
12 How Do Plastics Flow
121 Material Behavior
Molten thermoplastics exhibit viscoelastic behavior which combines flow characteristics of both viscous liquids and elastic solids When a viscous liquid flows the energy that causes thedeformation is dissipated and becomes viscous heat On the other hand when an elastic solid
8102019 L-G-0004410537-0002634439 Moldflow
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8 Polymer Flow Behavior in Injection Molds
is deformed the driving energy is stored For example the flow of water is a typical viscousflow whereas the deformation of a rubber cube falls into the elastic category
122 Deformation
In addition to the two types of material flow behavior there are two types of deformationsimple shear and simple extension (elongation) as shown in Figure 19 (a) and (b) below Theflow of molten thermoplastics during injection-molding filling is predominantly shear flow asshown in Figure 19 (c) in which layers of material elements slide over each other Theextensional flow however becomes significant as the material elements undergo elongation when the melt passes areas of abrupt dimensional change (eg a gate region) as shown inFigure 19 (d)
Figure 1igure 1 9 (a) Simple shear flow (b) Simple extensional flow (c) Shear flow in cavity filling (d) Extensional flow in cavity filling
123 Viscoelastic Behavior
In response to an applied stress (force per unit area) molten thermoplastics exhibit viscoelastic behavior which combines characteristics of an ideal viscous liquid with those of an ideal elastic solid In other words under certain conditions molten thermoplastics behavelike a liquid and will continuously deform while shear stress is applied as shown in
Figure 110 Upon the removal of the stress however the materials behave somewhat like anelastic solid with partial recovery of the deformation as shown in Figure 110 (b) and (c) This viscoelastic behavior stems from the random-coil configuration of polymer molecules in the
Stress
Stress Stress
Mold wall
Material element before
deformation begins
Material element during
deformation
Mold wall
Material elements at rest
Moving
materialelements
Velocity
profile
(a) (b)
(c) (d)
Velocity
profile
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How Do Plastics Flow 9
molten state which allows the movement and slippage of molecular chains under theinfluence of an applied load However the entanglement of the polymer molecular chains also
makes the system behave like an elastic solid upon the application and removal of the externalload Namely on removal of the stress chains will tend to return to the equilibrium random-coil state and thus will be a component of stress recovery The recovery is not instantaneousbecause of the entanglements still present in the system
Figure 1ure 1 10 (a) Ideal viscous liquid deforms continuously under applied stress (b) Ideal elasticsolid deforms immediately upon the application of stress but fully recovers when the
stress is removed (c) Molten thermoplastic deforms continuously under the applied
stress (like a viscous liquid) but also recovers partially from the deformation upon
removal of the applied stress (like an elastic solid)
124 Melt Shear Viscosity
1241 What Is Shear Viscosity
Melt shear viscosity is a materials resistance to shear flow In general polymer melts are highly viscous because of their long molecular chain structure The viscosity of a polymer melt rangesfrom 2 to 3000 Pas (water 10^-1 Pas glass 10^20 Pas) Viscosity can be thought of as thethickness of a fluid or how much it resists flow Viscosity is expressed as the ratio of shearstress (force per unit area) to the shear rate (rate change of shear strain) as shown inEquation 11 and Figure 111
(a) Viscous
liquid
(b) Elastic
solid
(c) Viscoelastic
polymer
melt
Initial shape
(shear stress
to be applied)
Time = Dt Time = 2 x Dt
(shear stress to
be removed)
Final deformed
shape
d
d
d
d
2d
2d
2d
2d
total
recovery
partial
recovery
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XX Contents
Appendix D Plastic Materials 281
D1 What Are Plastics 281
D11 Polymerization Process 281D12 Structure of Polymers 281D13 Polymer Alloys and Blends 283D14 Polymer Composites 283D2 Classification of Plastics 283D21 Classes of Plastics 284D22 Structures and Properties of Plastics 284D3 Thermoplastics 285D31 Market Share Distribution of Thermoplastics 285D32 Structures and Properties of Thermoplastics 286D33 Amorphous Polymers 286D34 Semicrystalline Polymers 287D4 Thermosets 288D5 Properties and Applications of Thermoplastics 289D51 ABS 290D52 PA 12 291D53 PA 6 293D54 PA 66 294D55 PBT 296
D56 PC 297D57 PCABS 299D58 PCPBT 300D59 HDPE 301D510 LDPE 303D511 PEI 304D512 PET 305D513 PETG 306D514 PMMA 307D515 POM 309
D516 PP 310D517 PPEPPO 312D518 PS 313D519 PVC 314D520 SAN 316D521 Additives Fillers and Reinforcements 317D522 Modifying Polymer Properties 318D523 Low-aspect Fillers 319D524 High-aspect Fillers Fibers 319
Index 321
8102019 L-G-0004410537-0002634439 Moldflow
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1 Polymer Flow Behavior in Injection Molds
bull Phases of injection molding
bull How do plastics flow
11 Phases of Injection Molding
Any molder can prove that all the conditions and effects discussed in this chapter do indeedoccur during the injection molding process While this knowledge alone can somewhatimprove quality it is only with the use of Moldflow analysis during the initial design stage with the mold designed for the optimum filling pattern that these effects can be controlledand the full benefits obtained
Flow technology is concerned with the behavior of plastics during the mold filling process Aplastic parts properties depend on how the part is molded Two parts having identicaldimensions and made from the same material but molded under different conditions will havedifferent stress and shrinkage levels and will behave differently in the field meaning that they
are in practice two different parts The way the plastic flows into the mold is of paramount importance in determining the quality of the part The process of filling the mold can be distinctly analyzed with the ability to predictpressure temperature and stress
111 How Plastic Fills a Mold
This was investigated using a centrally gated mold shaped like a dinner plate with a thick rim
around the outside as shown in Figure 11 It was found that the injection molding processalthough complex could be divided into three phases (we use the word phase to avoidconfusion with injection stage as used with programmed injection)
Figure 1ure 1 1 Cross-section of disk mold used to investigate flow
8102019 L-G-0004410537-0002634439 Moldflow
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2 Polymer Flow Behavior in Injection Molds
1111 Filling Phase
As the ram moves forward it first moves at a steady speed as the plastic flows into the cavity
This is the filling phase This phase lasts until the mold is just filled See Figure 12 andFigure 13
1112 Pressurization Phase
The pressurization phase begins when the ram moves forward after the filling phase to bring the mold up to pressure When the mold is filled the ram will slow down but it still movesquite some distance because plastics are very compressible materials At injection molding pressure an extra 15 volume of material can be forced into the cavity See Figure 12 andFigure 13
Although fluids are usually assumed to be incompressible molten plastics have to beconsidered to be more like a gas The compressibility of plastics can be observed by blocking off the nozzle and attempting to purge the barrel The ram will jump forward when thepressure is applied but will spring back when the pressure is released
1113 Compensation Phase
After the pressurization phase the ram still does not stop completely continuing to creepforward for some time Plastics have a very large volumetric change of about 25 from themelt to the solid This can be seen in a short shot the difference in volume between themolding and the cavity is due to this volumetric change See Figure 12 and Figure 13
The ram moving forward to compensate for the volumetric change in the part is called thecompensation phase As the volumetric change is 25 and at the most only an extra 15 canbe injected in the pressurization phase there must always be some compensation phase
Figure 1igure 1 2 Phases of injection molding
Filling phase
Pressurization phase
Compensation phase
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Phases of Injection Molding 3
Figure 13ure 1 3 Phases of injection molding detail
112 The Filling Phase
A two-color technique best demonstrates this phase After emptying the barrel of an injection-molding machine a small amount of red plastic was charged followed by green plastic
Consider the closed mold with the plastic front just starting to flow from the nozzle Theplastic first fills the sprue and runner system then enters the mold cavity itself forming a smallbubble of molten plastic
The skin of the plastic in contact with the cool mold freezes rapidly while the central coreremains molten When additional material is injected it flows into this central core displacing
the material already there which then forms a new flow front The flow of this displacedmaterial is a combination of forward flow and outward flow The outward flow contacts the wall freezes and forms the next section of skin while the forward flow forms the new moltencore When more material enters the mold it flows along a channel lined with these frozen walls of plastic illustrated in Figure 14
This flow pattern is often called fountain flow or bubble flow because the flow front is like abubble being inflated with hot plastic from the center The frozen layer is formed by the flow front inflating and so is subject to only a low shear stress and therefore has a very low levelof molecular orientation Once it is frozen it cannot be orientated any further so the frozen
layer in the finished part has a low level of orientation
Figure 14ure 1 4 Fountain flow and heat transfer
Filling Pressurization Compensation
Hot plastic
Frozen layer
Heat loss to mold
8102019 L-G-0004410537-0002634439 Moldflow
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4 Polymer Flow Behavior in Injection Molds
Now consider what happens upstream Hot plastic is continuously flowing bringing new hotmaterial along and generating significant frictional heat At the same time heat is being lost
through the frozen layer to the cold mold surfaceInitially the frozen layer is very thin so heat is lost very rapidly This results in more plasticfreezing and the frozen layer getting thicker cutting down the heat flow After a time thefrozen layer will reach a thickness such that the heat lost by conduction is equal to the heatinput from plastic flow and frictional heating ie an equilibrium condition is reached(Figure 14)
It is interesting to do some calculations on the time taken to reach this state of equilibrium The actual rate of heat flow is very large in comparison with the small heat content of theplastic in the frozen layer The result is that equilibrium is reached very quickly often in a timemeasured in a few tenths of a second As the total filling time is measured in seconds thefrozen layer reaches an equilibrium state early in the filling cycle
It is useful to think about how the thickness of this frozen layer will vary If the injection rate were slowed less heat would be generated by friction along the flow path with less heat inputfrom the flow The heat loss would be at the same rate and with less heat input the frozenlayer would grow in thickness If the injection rate were raised the frozen layer would bethinner (Figure 15) Similarly higher melt and mold temperatures would reduce the thicknessof the frozen layer This can be seen experimentally using the two-color technique
Figure 1igure 1 5 Influence of injection rate on frozen layer thickness
1121 Flow Shear Stress
It is easy to get confused between the various stress levels and orientation of the polymer Asthe plastic flows it is subject to shear stress also called flow shear stress This flow shear stress willorient the material ie cause the molecules to align themselves in the general direction of flow
The shear stress varies from a maximum at the outside dropping off to zero at the center
3Shear stress is purely a function of force and area This must not be confused withshear rate which is the rate of plastic sliding over the next layer Shear rate is zero
= Frozen
= Molten cooler
= Molten hotter
Slowerinjection
rate
Faster
injection
rate
vs
8102019 L-G-0004410537-0002634439 Moldflow
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Phases of Injection Molding 5
at the outer edge where the plastic is frozen rises to a maximum just inwards of thefrozen layer then drops toward the center as shown in Figure 16
Figure 16ure 1 6 Shear rate distribution
If the flow were stopped and the plastic allowed to cool down very slowly this orientation would have time to relax giving a very low level of residual orientation On the other hand if the material were kept under stress and the plastic snap frozen most of the orientation would
be trapped in the frozen plastic (Figure 17)
Figure 17ure 1 7 Molecular orientation through the thickness of the part
Min MaxShear rate
= Tension
= Compression
Molecular orientation trapped
in pressurization phase
Gradual reduction of orientation
toward the center of the
mold as stress levels are lower
and the cooling rate is slower
which allows more time for
orientation to relax
8102019 L-G-0004410537-0002634439 Moldflow
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6 Polymer Flow Behavior in Injection Molds
Now consider the orientation from the mold surface toward the center
The frozen layer itself formed with very little shear and therefore low orientation immediately
freezes setting the low level of orientation
The layer of plastic just on the inside of the frozen layer is subject to maximum shear stressand freezes the instant flow stops trapping almost all the orientation
This is the orientation pattern the further toward the center the more the shear stress dropsand the slower the rate of cooling This allows more time for the level of orientation to relaxso the residual orientation drops rapidly toward the center Consider how this pattern willaffect the residual stress level Oriented material (normally) will shrink more than nonorientedmaterial On the inner surface of the original frozen layer highly oriented material wants toshrink a great deal but it is prevented from doing so by the less-oriented material The highly
oriented layer ends up being in tension while the less-oriented material is in compression This residual stress pattern is a common cause of part warpage
3 There is a connectionthrough orientationbetween the shear stress during
filling (flow stress) and the residual stress in the final molded part This meansshear stress during filling shown on Moldflow plots can be used as a designparameter
113 The Pressurization Phase
The pressurization phasefrom the point of view of flow behavioris very similar to thefilling phase The flow rate may drop somewhat as the mold builds up to pressure resulting inan increase in the thickness of the frozen layer
The main difference of course is the increase in hydrostatic (isotropic) pressure We shall seein chapter 2 section 24 Effect of Molding Conditions that hydrostatic pressure in itself doesnot cause any residual stress
114 The Compensation Phase
Compensating flow is unstable Consider the plate molding again (see Figure 11) You wouldthink that plastic flowing uniformly through the thin diaphragm would top up the thick rim Inpractice the plastic during the compensation phase flows in rivers that spread out like a deltaas illustrated in Figure 18 This may seem surprising at first but it can be explained by temperature instability
8102019 L-G-0004410537-0002634439 Moldflow
httpslidepdfcomreaderfulll-g-0004410537-0002634439-moldflow 2830
How Do Plastics Flow 7
Figure 18ure 1 8 River flow
1141 Temperature Variation
There is always some variation in melt temperature coming from the barrel of the injectionmachine In exceptional cases up to 40 ordmC variation has been measured using a high-speedthermocouple
1142 Natural Instability
However slight the temperature variation natural instability will amplify it If for exampleone part of the melt is slightly hotter than the rest then the plastic flow in that area will beslightly greater bringing hotter material into the area and maintaining the temperature If onthe other hand there is another area that is cooler the flow will be less so there will be lessheat input and the plastic will get colder until it eventually freezes off
However balanced the initial conditions this natural instability will result in a river-type flow
This is a very important consideration The first material to freeze off will shrink early in thecycle By the time the material in the river flows freezes the bulk of the material will havealready frozen off and shrinkage will have occurred The rivers will shrink relative to the bulk of the molding and because they are highly orientated shrinkage will be very high The resultis high-stress tensile members throughout the molding a common cause of warpage
1143 Optimum Part Quality
Most of the stress in plastic parts occurs during the compensation phase By controlling flow and minimizing stress it is possible to design for optimum part quality This important point is
at the heart of the Moldflow philosophy
12 How Do Plastics Flow
121 Material Behavior
Molten thermoplastics exhibit viscoelastic behavior which combines flow characteristics of both viscous liquids and elastic solids When a viscous liquid flows the energy that causes thedeformation is dissipated and becomes viscous heat On the other hand when an elastic solid
8102019 L-G-0004410537-0002634439 Moldflow
httpslidepdfcomreaderfulll-g-0004410537-0002634439-moldflow 2930
8 Polymer Flow Behavior in Injection Molds
is deformed the driving energy is stored For example the flow of water is a typical viscousflow whereas the deformation of a rubber cube falls into the elastic category
122 Deformation
In addition to the two types of material flow behavior there are two types of deformationsimple shear and simple extension (elongation) as shown in Figure 19 (a) and (b) below Theflow of molten thermoplastics during injection-molding filling is predominantly shear flow asshown in Figure 19 (c) in which layers of material elements slide over each other Theextensional flow however becomes significant as the material elements undergo elongation when the melt passes areas of abrupt dimensional change (eg a gate region) as shown inFigure 19 (d)
Figure 1igure 1 9 (a) Simple shear flow (b) Simple extensional flow (c) Shear flow in cavity filling (d) Extensional flow in cavity filling
123 Viscoelastic Behavior
In response to an applied stress (force per unit area) molten thermoplastics exhibit viscoelastic behavior which combines characteristics of an ideal viscous liquid with those of an ideal elastic solid In other words under certain conditions molten thermoplastics behavelike a liquid and will continuously deform while shear stress is applied as shown in
Figure 110 Upon the removal of the stress however the materials behave somewhat like anelastic solid with partial recovery of the deformation as shown in Figure 110 (b) and (c) This viscoelastic behavior stems from the random-coil configuration of polymer molecules in the
Stress
Stress Stress
Mold wall
Material element before
deformation begins
Material element during
deformation
Mold wall
Material elements at rest
Moving
materialelements
Velocity
profile
(a) (b)
(c) (d)
Velocity
profile
8102019 L-G-0004410537-0002634439 Moldflow
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How Do Plastics Flow 9
molten state which allows the movement and slippage of molecular chains under theinfluence of an applied load However the entanglement of the polymer molecular chains also
makes the system behave like an elastic solid upon the application and removal of the externalload Namely on removal of the stress chains will tend to return to the equilibrium random-coil state and thus will be a component of stress recovery The recovery is not instantaneousbecause of the entanglements still present in the system
Figure 1ure 1 10 (a) Ideal viscous liquid deforms continuously under applied stress (b) Ideal elasticsolid deforms immediately upon the application of stress but fully recovers when the
stress is removed (c) Molten thermoplastic deforms continuously under the applied
stress (like a viscous liquid) but also recovers partially from the deformation upon
removal of the applied stress (like an elastic solid)
124 Melt Shear Viscosity
1241 What Is Shear Viscosity
Melt shear viscosity is a materials resistance to shear flow In general polymer melts are highly viscous because of their long molecular chain structure The viscosity of a polymer melt rangesfrom 2 to 3000 Pas (water 10^-1 Pas glass 10^20 Pas) Viscosity can be thought of as thethickness of a fluid or how much it resists flow Viscosity is expressed as the ratio of shearstress (force per unit area) to the shear rate (rate change of shear strain) as shown inEquation 11 and Figure 111
(a) Viscous
liquid
(b) Elastic
solid
(c) Viscoelastic
polymer
melt
Initial shape
(shear stress
to be applied)
Time = Dt Time = 2 x Dt
(shear stress to
be removed)
Final deformed
shape
d
d
d
d
2d
2d
2d
2d
total
recovery
partial
recovery
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1 Polymer Flow Behavior in Injection Molds
bull Phases of injection molding
bull How do plastics flow
11 Phases of Injection Molding
Any molder can prove that all the conditions and effects discussed in this chapter do indeedoccur during the injection molding process While this knowledge alone can somewhatimprove quality it is only with the use of Moldflow analysis during the initial design stage with the mold designed for the optimum filling pattern that these effects can be controlledand the full benefits obtained
Flow technology is concerned with the behavior of plastics during the mold filling process Aplastic parts properties depend on how the part is molded Two parts having identicaldimensions and made from the same material but molded under different conditions will havedifferent stress and shrinkage levels and will behave differently in the field meaning that they
are in practice two different parts The way the plastic flows into the mold is of paramount importance in determining the quality of the part The process of filling the mold can be distinctly analyzed with the ability to predictpressure temperature and stress
111 How Plastic Fills a Mold
This was investigated using a centrally gated mold shaped like a dinner plate with a thick rim
around the outside as shown in Figure 11 It was found that the injection molding processalthough complex could be divided into three phases (we use the word phase to avoidconfusion with injection stage as used with programmed injection)
Figure 1ure 1 1 Cross-section of disk mold used to investigate flow
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2 Polymer Flow Behavior in Injection Molds
1111 Filling Phase
As the ram moves forward it first moves at a steady speed as the plastic flows into the cavity
This is the filling phase This phase lasts until the mold is just filled See Figure 12 andFigure 13
1112 Pressurization Phase
The pressurization phase begins when the ram moves forward after the filling phase to bring the mold up to pressure When the mold is filled the ram will slow down but it still movesquite some distance because plastics are very compressible materials At injection molding pressure an extra 15 volume of material can be forced into the cavity See Figure 12 andFigure 13
Although fluids are usually assumed to be incompressible molten plastics have to beconsidered to be more like a gas The compressibility of plastics can be observed by blocking off the nozzle and attempting to purge the barrel The ram will jump forward when thepressure is applied but will spring back when the pressure is released
1113 Compensation Phase
After the pressurization phase the ram still does not stop completely continuing to creepforward for some time Plastics have a very large volumetric change of about 25 from themelt to the solid This can be seen in a short shot the difference in volume between themolding and the cavity is due to this volumetric change See Figure 12 and Figure 13
The ram moving forward to compensate for the volumetric change in the part is called thecompensation phase As the volumetric change is 25 and at the most only an extra 15 canbe injected in the pressurization phase there must always be some compensation phase
Figure 1igure 1 2 Phases of injection molding
Filling phase
Pressurization phase
Compensation phase
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Phases of Injection Molding 3
Figure 13ure 1 3 Phases of injection molding detail
112 The Filling Phase
A two-color technique best demonstrates this phase After emptying the barrel of an injection-molding machine a small amount of red plastic was charged followed by green plastic
Consider the closed mold with the plastic front just starting to flow from the nozzle Theplastic first fills the sprue and runner system then enters the mold cavity itself forming a smallbubble of molten plastic
The skin of the plastic in contact with the cool mold freezes rapidly while the central coreremains molten When additional material is injected it flows into this central core displacing
the material already there which then forms a new flow front The flow of this displacedmaterial is a combination of forward flow and outward flow The outward flow contacts the wall freezes and forms the next section of skin while the forward flow forms the new moltencore When more material enters the mold it flows along a channel lined with these frozen walls of plastic illustrated in Figure 14
This flow pattern is often called fountain flow or bubble flow because the flow front is like abubble being inflated with hot plastic from the center The frozen layer is formed by the flow front inflating and so is subject to only a low shear stress and therefore has a very low levelof molecular orientation Once it is frozen it cannot be orientated any further so the frozen
layer in the finished part has a low level of orientation
Figure 14ure 1 4 Fountain flow and heat transfer
Filling Pressurization Compensation
Hot plastic
Frozen layer
Heat loss to mold
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4 Polymer Flow Behavior in Injection Molds
Now consider what happens upstream Hot plastic is continuously flowing bringing new hotmaterial along and generating significant frictional heat At the same time heat is being lost
through the frozen layer to the cold mold surfaceInitially the frozen layer is very thin so heat is lost very rapidly This results in more plasticfreezing and the frozen layer getting thicker cutting down the heat flow After a time thefrozen layer will reach a thickness such that the heat lost by conduction is equal to the heatinput from plastic flow and frictional heating ie an equilibrium condition is reached(Figure 14)
It is interesting to do some calculations on the time taken to reach this state of equilibrium The actual rate of heat flow is very large in comparison with the small heat content of theplastic in the frozen layer The result is that equilibrium is reached very quickly often in a timemeasured in a few tenths of a second As the total filling time is measured in seconds thefrozen layer reaches an equilibrium state early in the filling cycle
It is useful to think about how the thickness of this frozen layer will vary If the injection rate were slowed less heat would be generated by friction along the flow path with less heat inputfrom the flow The heat loss would be at the same rate and with less heat input the frozenlayer would grow in thickness If the injection rate were raised the frozen layer would bethinner (Figure 15) Similarly higher melt and mold temperatures would reduce the thicknessof the frozen layer This can be seen experimentally using the two-color technique
Figure 1igure 1 5 Influence of injection rate on frozen layer thickness
1121 Flow Shear Stress
It is easy to get confused between the various stress levels and orientation of the polymer Asthe plastic flows it is subject to shear stress also called flow shear stress This flow shear stress willorient the material ie cause the molecules to align themselves in the general direction of flow
The shear stress varies from a maximum at the outside dropping off to zero at the center
3Shear stress is purely a function of force and area This must not be confused withshear rate which is the rate of plastic sliding over the next layer Shear rate is zero
= Frozen
= Molten cooler
= Molten hotter
Slowerinjection
rate
Faster
injection
rate
vs
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Phases of Injection Molding 5
at the outer edge where the plastic is frozen rises to a maximum just inwards of thefrozen layer then drops toward the center as shown in Figure 16
Figure 16ure 1 6 Shear rate distribution
If the flow were stopped and the plastic allowed to cool down very slowly this orientation would have time to relax giving a very low level of residual orientation On the other hand if the material were kept under stress and the plastic snap frozen most of the orientation would
be trapped in the frozen plastic (Figure 17)
Figure 17ure 1 7 Molecular orientation through the thickness of the part
Min MaxShear rate
= Tension
= Compression
Molecular orientation trapped
in pressurization phase
Gradual reduction of orientation
toward the center of the
mold as stress levels are lower
and the cooling rate is slower
which allows more time for
orientation to relax
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6 Polymer Flow Behavior in Injection Molds
Now consider the orientation from the mold surface toward the center
The frozen layer itself formed with very little shear and therefore low orientation immediately
freezes setting the low level of orientation
The layer of plastic just on the inside of the frozen layer is subject to maximum shear stressand freezes the instant flow stops trapping almost all the orientation
This is the orientation pattern the further toward the center the more the shear stress dropsand the slower the rate of cooling This allows more time for the level of orientation to relaxso the residual orientation drops rapidly toward the center Consider how this pattern willaffect the residual stress level Oriented material (normally) will shrink more than nonorientedmaterial On the inner surface of the original frozen layer highly oriented material wants toshrink a great deal but it is prevented from doing so by the less-oriented material The highly
oriented layer ends up being in tension while the less-oriented material is in compression This residual stress pattern is a common cause of part warpage
3 There is a connectionthrough orientationbetween the shear stress during
filling (flow stress) and the residual stress in the final molded part This meansshear stress during filling shown on Moldflow plots can be used as a designparameter
113 The Pressurization Phase
The pressurization phasefrom the point of view of flow behavioris very similar to thefilling phase The flow rate may drop somewhat as the mold builds up to pressure resulting inan increase in the thickness of the frozen layer
The main difference of course is the increase in hydrostatic (isotropic) pressure We shall seein chapter 2 section 24 Effect of Molding Conditions that hydrostatic pressure in itself doesnot cause any residual stress
114 The Compensation Phase
Compensating flow is unstable Consider the plate molding again (see Figure 11) You wouldthink that plastic flowing uniformly through the thin diaphragm would top up the thick rim Inpractice the plastic during the compensation phase flows in rivers that spread out like a deltaas illustrated in Figure 18 This may seem surprising at first but it can be explained by temperature instability
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How Do Plastics Flow 7
Figure 18ure 1 8 River flow
1141 Temperature Variation
There is always some variation in melt temperature coming from the barrel of the injectionmachine In exceptional cases up to 40 ordmC variation has been measured using a high-speedthermocouple
1142 Natural Instability
However slight the temperature variation natural instability will amplify it If for exampleone part of the melt is slightly hotter than the rest then the plastic flow in that area will beslightly greater bringing hotter material into the area and maintaining the temperature If onthe other hand there is another area that is cooler the flow will be less so there will be lessheat input and the plastic will get colder until it eventually freezes off
However balanced the initial conditions this natural instability will result in a river-type flow
This is a very important consideration The first material to freeze off will shrink early in thecycle By the time the material in the river flows freezes the bulk of the material will havealready frozen off and shrinkage will have occurred The rivers will shrink relative to the bulk of the molding and because they are highly orientated shrinkage will be very high The resultis high-stress tensile members throughout the molding a common cause of warpage
1143 Optimum Part Quality
Most of the stress in plastic parts occurs during the compensation phase By controlling flow and minimizing stress it is possible to design for optimum part quality This important point is
at the heart of the Moldflow philosophy
12 How Do Plastics Flow
121 Material Behavior
Molten thermoplastics exhibit viscoelastic behavior which combines flow characteristics of both viscous liquids and elastic solids When a viscous liquid flows the energy that causes thedeformation is dissipated and becomes viscous heat On the other hand when an elastic solid
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8 Polymer Flow Behavior in Injection Molds
is deformed the driving energy is stored For example the flow of water is a typical viscousflow whereas the deformation of a rubber cube falls into the elastic category
122 Deformation
In addition to the two types of material flow behavior there are two types of deformationsimple shear and simple extension (elongation) as shown in Figure 19 (a) and (b) below Theflow of molten thermoplastics during injection-molding filling is predominantly shear flow asshown in Figure 19 (c) in which layers of material elements slide over each other Theextensional flow however becomes significant as the material elements undergo elongation when the melt passes areas of abrupt dimensional change (eg a gate region) as shown inFigure 19 (d)
Figure 1igure 1 9 (a) Simple shear flow (b) Simple extensional flow (c) Shear flow in cavity filling (d) Extensional flow in cavity filling
123 Viscoelastic Behavior
In response to an applied stress (force per unit area) molten thermoplastics exhibit viscoelastic behavior which combines characteristics of an ideal viscous liquid with those of an ideal elastic solid In other words under certain conditions molten thermoplastics behavelike a liquid and will continuously deform while shear stress is applied as shown in
Figure 110 Upon the removal of the stress however the materials behave somewhat like anelastic solid with partial recovery of the deformation as shown in Figure 110 (b) and (c) This viscoelastic behavior stems from the random-coil configuration of polymer molecules in the
Stress
Stress Stress
Mold wall
Material element before
deformation begins
Material element during
deformation
Mold wall
Material elements at rest
Moving
materialelements
Velocity
profile
(a) (b)
(c) (d)
Velocity
profile
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How Do Plastics Flow 9
molten state which allows the movement and slippage of molecular chains under theinfluence of an applied load However the entanglement of the polymer molecular chains also
makes the system behave like an elastic solid upon the application and removal of the externalload Namely on removal of the stress chains will tend to return to the equilibrium random-coil state and thus will be a component of stress recovery The recovery is not instantaneousbecause of the entanglements still present in the system
Figure 1ure 1 10 (a) Ideal viscous liquid deforms continuously under applied stress (b) Ideal elasticsolid deforms immediately upon the application of stress but fully recovers when the
stress is removed (c) Molten thermoplastic deforms continuously under the applied
stress (like a viscous liquid) but also recovers partially from the deformation upon
removal of the applied stress (like an elastic solid)
124 Melt Shear Viscosity
1241 What Is Shear Viscosity
Melt shear viscosity is a materials resistance to shear flow In general polymer melts are highly viscous because of their long molecular chain structure The viscosity of a polymer melt rangesfrom 2 to 3000 Pas (water 10^-1 Pas glass 10^20 Pas) Viscosity can be thought of as thethickness of a fluid or how much it resists flow Viscosity is expressed as the ratio of shearstress (force per unit area) to the shear rate (rate change of shear strain) as shown inEquation 11 and Figure 111
(a) Viscous
liquid
(b) Elastic
solid
(c) Viscoelastic
polymer
melt
Initial shape
(shear stress
to be applied)
Time = Dt Time = 2 x Dt
(shear stress to
be removed)
Final deformed
shape
d
d
d
d
2d
2d
2d
2d
total
recovery
partial
recovery
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2 Polymer Flow Behavior in Injection Molds
1111 Filling Phase
As the ram moves forward it first moves at a steady speed as the plastic flows into the cavity
This is the filling phase This phase lasts until the mold is just filled See Figure 12 andFigure 13
1112 Pressurization Phase
The pressurization phase begins when the ram moves forward after the filling phase to bring the mold up to pressure When the mold is filled the ram will slow down but it still movesquite some distance because plastics are very compressible materials At injection molding pressure an extra 15 volume of material can be forced into the cavity See Figure 12 andFigure 13
Although fluids are usually assumed to be incompressible molten plastics have to beconsidered to be more like a gas The compressibility of plastics can be observed by blocking off the nozzle and attempting to purge the barrel The ram will jump forward when thepressure is applied but will spring back when the pressure is released
1113 Compensation Phase
After the pressurization phase the ram still does not stop completely continuing to creepforward for some time Plastics have a very large volumetric change of about 25 from themelt to the solid This can be seen in a short shot the difference in volume between themolding and the cavity is due to this volumetric change See Figure 12 and Figure 13
The ram moving forward to compensate for the volumetric change in the part is called thecompensation phase As the volumetric change is 25 and at the most only an extra 15 canbe injected in the pressurization phase there must always be some compensation phase
Figure 1igure 1 2 Phases of injection molding
Filling phase
Pressurization phase
Compensation phase
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Phases of Injection Molding 3
Figure 13ure 1 3 Phases of injection molding detail
112 The Filling Phase
A two-color technique best demonstrates this phase After emptying the barrel of an injection-molding machine a small amount of red plastic was charged followed by green plastic
Consider the closed mold with the plastic front just starting to flow from the nozzle Theplastic first fills the sprue and runner system then enters the mold cavity itself forming a smallbubble of molten plastic
The skin of the plastic in contact with the cool mold freezes rapidly while the central coreremains molten When additional material is injected it flows into this central core displacing
the material already there which then forms a new flow front The flow of this displacedmaterial is a combination of forward flow and outward flow The outward flow contacts the wall freezes and forms the next section of skin while the forward flow forms the new moltencore When more material enters the mold it flows along a channel lined with these frozen walls of plastic illustrated in Figure 14
This flow pattern is often called fountain flow or bubble flow because the flow front is like abubble being inflated with hot plastic from the center The frozen layer is formed by the flow front inflating and so is subject to only a low shear stress and therefore has a very low levelof molecular orientation Once it is frozen it cannot be orientated any further so the frozen
layer in the finished part has a low level of orientation
Figure 14ure 1 4 Fountain flow and heat transfer
Filling Pressurization Compensation
Hot plastic
Frozen layer
Heat loss to mold
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4 Polymer Flow Behavior in Injection Molds
Now consider what happens upstream Hot plastic is continuously flowing bringing new hotmaterial along and generating significant frictional heat At the same time heat is being lost
through the frozen layer to the cold mold surfaceInitially the frozen layer is very thin so heat is lost very rapidly This results in more plasticfreezing and the frozen layer getting thicker cutting down the heat flow After a time thefrozen layer will reach a thickness such that the heat lost by conduction is equal to the heatinput from plastic flow and frictional heating ie an equilibrium condition is reached(Figure 14)
It is interesting to do some calculations on the time taken to reach this state of equilibrium The actual rate of heat flow is very large in comparison with the small heat content of theplastic in the frozen layer The result is that equilibrium is reached very quickly often in a timemeasured in a few tenths of a second As the total filling time is measured in seconds thefrozen layer reaches an equilibrium state early in the filling cycle
It is useful to think about how the thickness of this frozen layer will vary If the injection rate were slowed less heat would be generated by friction along the flow path with less heat inputfrom the flow The heat loss would be at the same rate and with less heat input the frozenlayer would grow in thickness If the injection rate were raised the frozen layer would bethinner (Figure 15) Similarly higher melt and mold temperatures would reduce the thicknessof the frozen layer This can be seen experimentally using the two-color technique
Figure 1igure 1 5 Influence of injection rate on frozen layer thickness
1121 Flow Shear Stress
It is easy to get confused between the various stress levels and orientation of the polymer Asthe plastic flows it is subject to shear stress also called flow shear stress This flow shear stress willorient the material ie cause the molecules to align themselves in the general direction of flow
The shear stress varies from a maximum at the outside dropping off to zero at the center
3Shear stress is purely a function of force and area This must not be confused withshear rate which is the rate of plastic sliding over the next layer Shear rate is zero
= Frozen
= Molten cooler
= Molten hotter
Slowerinjection
rate
Faster
injection
rate
vs
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Phases of Injection Molding 5
at the outer edge where the plastic is frozen rises to a maximum just inwards of thefrozen layer then drops toward the center as shown in Figure 16
Figure 16ure 1 6 Shear rate distribution
If the flow were stopped and the plastic allowed to cool down very slowly this orientation would have time to relax giving a very low level of residual orientation On the other hand if the material were kept under stress and the plastic snap frozen most of the orientation would
be trapped in the frozen plastic (Figure 17)
Figure 17ure 1 7 Molecular orientation through the thickness of the part
Min MaxShear rate
= Tension
= Compression
Molecular orientation trapped
in pressurization phase
Gradual reduction of orientation
toward the center of the
mold as stress levels are lower
and the cooling rate is slower
which allows more time for
orientation to relax
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6 Polymer Flow Behavior in Injection Molds
Now consider the orientation from the mold surface toward the center
The frozen layer itself formed with very little shear and therefore low orientation immediately
freezes setting the low level of orientation
The layer of plastic just on the inside of the frozen layer is subject to maximum shear stressand freezes the instant flow stops trapping almost all the orientation
This is the orientation pattern the further toward the center the more the shear stress dropsand the slower the rate of cooling This allows more time for the level of orientation to relaxso the residual orientation drops rapidly toward the center Consider how this pattern willaffect the residual stress level Oriented material (normally) will shrink more than nonorientedmaterial On the inner surface of the original frozen layer highly oriented material wants toshrink a great deal but it is prevented from doing so by the less-oriented material The highly
oriented layer ends up being in tension while the less-oriented material is in compression This residual stress pattern is a common cause of part warpage
3 There is a connectionthrough orientationbetween the shear stress during
filling (flow stress) and the residual stress in the final molded part This meansshear stress during filling shown on Moldflow plots can be used as a designparameter
113 The Pressurization Phase
The pressurization phasefrom the point of view of flow behavioris very similar to thefilling phase The flow rate may drop somewhat as the mold builds up to pressure resulting inan increase in the thickness of the frozen layer
The main difference of course is the increase in hydrostatic (isotropic) pressure We shall seein chapter 2 section 24 Effect of Molding Conditions that hydrostatic pressure in itself doesnot cause any residual stress
114 The Compensation Phase
Compensating flow is unstable Consider the plate molding again (see Figure 11) You wouldthink that plastic flowing uniformly through the thin diaphragm would top up the thick rim Inpractice the plastic during the compensation phase flows in rivers that spread out like a deltaas illustrated in Figure 18 This may seem surprising at first but it can be explained by temperature instability
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How Do Plastics Flow 7
Figure 18ure 1 8 River flow
1141 Temperature Variation
There is always some variation in melt temperature coming from the barrel of the injectionmachine In exceptional cases up to 40 ordmC variation has been measured using a high-speedthermocouple
1142 Natural Instability
However slight the temperature variation natural instability will amplify it If for exampleone part of the melt is slightly hotter than the rest then the plastic flow in that area will beslightly greater bringing hotter material into the area and maintaining the temperature If onthe other hand there is another area that is cooler the flow will be less so there will be lessheat input and the plastic will get colder until it eventually freezes off
However balanced the initial conditions this natural instability will result in a river-type flow
This is a very important consideration The first material to freeze off will shrink early in thecycle By the time the material in the river flows freezes the bulk of the material will havealready frozen off and shrinkage will have occurred The rivers will shrink relative to the bulk of the molding and because they are highly orientated shrinkage will be very high The resultis high-stress tensile members throughout the molding a common cause of warpage
1143 Optimum Part Quality
Most of the stress in plastic parts occurs during the compensation phase By controlling flow and minimizing stress it is possible to design for optimum part quality This important point is
at the heart of the Moldflow philosophy
12 How Do Plastics Flow
121 Material Behavior
Molten thermoplastics exhibit viscoelastic behavior which combines flow characteristics of both viscous liquids and elastic solids When a viscous liquid flows the energy that causes thedeformation is dissipated and becomes viscous heat On the other hand when an elastic solid
8102019 L-G-0004410537-0002634439 Moldflow
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8 Polymer Flow Behavior in Injection Molds
is deformed the driving energy is stored For example the flow of water is a typical viscousflow whereas the deformation of a rubber cube falls into the elastic category
122 Deformation
In addition to the two types of material flow behavior there are two types of deformationsimple shear and simple extension (elongation) as shown in Figure 19 (a) and (b) below Theflow of molten thermoplastics during injection-molding filling is predominantly shear flow asshown in Figure 19 (c) in which layers of material elements slide over each other Theextensional flow however becomes significant as the material elements undergo elongation when the melt passes areas of abrupt dimensional change (eg a gate region) as shown inFigure 19 (d)
Figure 1igure 1 9 (a) Simple shear flow (b) Simple extensional flow (c) Shear flow in cavity filling (d) Extensional flow in cavity filling
123 Viscoelastic Behavior
In response to an applied stress (force per unit area) molten thermoplastics exhibit viscoelastic behavior which combines characteristics of an ideal viscous liquid with those of an ideal elastic solid In other words under certain conditions molten thermoplastics behavelike a liquid and will continuously deform while shear stress is applied as shown in
Figure 110 Upon the removal of the stress however the materials behave somewhat like anelastic solid with partial recovery of the deformation as shown in Figure 110 (b) and (c) This viscoelastic behavior stems from the random-coil configuration of polymer molecules in the
Stress
Stress Stress
Mold wall
Material element before
deformation begins
Material element during
deformation
Mold wall
Material elements at rest
Moving
materialelements
Velocity
profile
(a) (b)
(c) (d)
Velocity
profile
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How Do Plastics Flow 9
molten state which allows the movement and slippage of molecular chains under theinfluence of an applied load However the entanglement of the polymer molecular chains also
makes the system behave like an elastic solid upon the application and removal of the externalload Namely on removal of the stress chains will tend to return to the equilibrium random-coil state and thus will be a component of stress recovery The recovery is not instantaneousbecause of the entanglements still present in the system
Figure 1ure 1 10 (a) Ideal viscous liquid deforms continuously under applied stress (b) Ideal elasticsolid deforms immediately upon the application of stress but fully recovers when the
stress is removed (c) Molten thermoplastic deforms continuously under the applied
stress (like a viscous liquid) but also recovers partially from the deformation upon
removal of the applied stress (like an elastic solid)
124 Melt Shear Viscosity
1241 What Is Shear Viscosity
Melt shear viscosity is a materials resistance to shear flow In general polymer melts are highly viscous because of their long molecular chain structure The viscosity of a polymer melt rangesfrom 2 to 3000 Pas (water 10^-1 Pas glass 10^20 Pas) Viscosity can be thought of as thethickness of a fluid or how much it resists flow Viscosity is expressed as the ratio of shearstress (force per unit area) to the shear rate (rate change of shear strain) as shown inEquation 11 and Figure 111
(a) Viscous
liquid
(b) Elastic
solid
(c) Viscoelastic
polymer
melt
Initial shape
(shear stress
to be applied)
Time = Dt Time = 2 x Dt
(shear stress to
be removed)
Final deformed
shape
d
d
d
d
2d
2d
2d
2d
total
recovery
partial
recovery
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Phases of Injection Molding 3
Figure 13ure 1 3 Phases of injection molding detail
112 The Filling Phase
A two-color technique best demonstrates this phase After emptying the barrel of an injection-molding machine a small amount of red plastic was charged followed by green plastic
Consider the closed mold with the plastic front just starting to flow from the nozzle Theplastic first fills the sprue and runner system then enters the mold cavity itself forming a smallbubble of molten plastic
The skin of the plastic in contact with the cool mold freezes rapidly while the central coreremains molten When additional material is injected it flows into this central core displacing
the material already there which then forms a new flow front The flow of this displacedmaterial is a combination of forward flow and outward flow The outward flow contacts the wall freezes and forms the next section of skin while the forward flow forms the new moltencore When more material enters the mold it flows along a channel lined with these frozen walls of plastic illustrated in Figure 14
This flow pattern is often called fountain flow or bubble flow because the flow front is like abubble being inflated with hot plastic from the center The frozen layer is formed by the flow front inflating and so is subject to only a low shear stress and therefore has a very low levelof molecular orientation Once it is frozen it cannot be orientated any further so the frozen
layer in the finished part has a low level of orientation
Figure 14ure 1 4 Fountain flow and heat transfer
Filling Pressurization Compensation
Hot plastic
Frozen layer
Heat loss to mold
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4 Polymer Flow Behavior in Injection Molds
Now consider what happens upstream Hot plastic is continuously flowing bringing new hotmaterial along and generating significant frictional heat At the same time heat is being lost
through the frozen layer to the cold mold surfaceInitially the frozen layer is very thin so heat is lost very rapidly This results in more plasticfreezing and the frozen layer getting thicker cutting down the heat flow After a time thefrozen layer will reach a thickness such that the heat lost by conduction is equal to the heatinput from plastic flow and frictional heating ie an equilibrium condition is reached(Figure 14)
It is interesting to do some calculations on the time taken to reach this state of equilibrium The actual rate of heat flow is very large in comparison with the small heat content of theplastic in the frozen layer The result is that equilibrium is reached very quickly often in a timemeasured in a few tenths of a second As the total filling time is measured in seconds thefrozen layer reaches an equilibrium state early in the filling cycle
It is useful to think about how the thickness of this frozen layer will vary If the injection rate were slowed less heat would be generated by friction along the flow path with less heat inputfrom the flow The heat loss would be at the same rate and with less heat input the frozenlayer would grow in thickness If the injection rate were raised the frozen layer would bethinner (Figure 15) Similarly higher melt and mold temperatures would reduce the thicknessof the frozen layer This can be seen experimentally using the two-color technique
Figure 1igure 1 5 Influence of injection rate on frozen layer thickness
1121 Flow Shear Stress
It is easy to get confused between the various stress levels and orientation of the polymer Asthe plastic flows it is subject to shear stress also called flow shear stress This flow shear stress willorient the material ie cause the molecules to align themselves in the general direction of flow
The shear stress varies from a maximum at the outside dropping off to zero at the center
3Shear stress is purely a function of force and area This must not be confused withshear rate which is the rate of plastic sliding over the next layer Shear rate is zero
= Frozen
= Molten cooler
= Molten hotter
Slowerinjection
rate
Faster
injection
rate
vs
8102019 L-G-0004410537-0002634439 Moldflow
httpslidepdfcomreaderfulll-g-0004410537-0002634439-moldflow 2630
Phases of Injection Molding 5
at the outer edge where the plastic is frozen rises to a maximum just inwards of thefrozen layer then drops toward the center as shown in Figure 16
Figure 16ure 1 6 Shear rate distribution
If the flow were stopped and the plastic allowed to cool down very slowly this orientation would have time to relax giving a very low level of residual orientation On the other hand if the material were kept under stress and the plastic snap frozen most of the orientation would
be trapped in the frozen plastic (Figure 17)
Figure 17ure 1 7 Molecular orientation through the thickness of the part
Min MaxShear rate
= Tension
= Compression
Molecular orientation trapped
in pressurization phase
Gradual reduction of orientation
toward the center of the
mold as stress levels are lower
and the cooling rate is slower
which allows more time for
orientation to relax
8102019 L-G-0004410537-0002634439 Moldflow
httpslidepdfcomreaderfulll-g-0004410537-0002634439-moldflow 2730
6 Polymer Flow Behavior in Injection Molds
Now consider the orientation from the mold surface toward the center
The frozen layer itself formed with very little shear and therefore low orientation immediately
freezes setting the low level of orientation
The layer of plastic just on the inside of the frozen layer is subject to maximum shear stressand freezes the instant flow stops trapping almost all the orientation
This is the orientation pattern the further toward the center the more the shear stress dropsand the slower the rate of cooling This allows more time for the level of orientation to relaxso the residual orientation drops rapidly toward the center Consider how this pattern willaffect the residual stress level Oriented material (normally) will shrink more than nonorientedmaterial On the inner surface of the original frozen layer highly oriented material wants toshrink a great deal but it is prevented from doing so by the less-oriented material The highly
oriented layer ends up being in tension while the less-oriented material is in compression This residual stress pattern is a common cause of part warpage
3 There is a connectionthrough orientationbetween the shear stress during
filling (flow stress) and the residual stress in the final molded part This meansshear stress during filling shown on Moldflow plots can be used as a designparameter
113 The Pressurization Phase
The pressurization phasefrom the point of view of flow behavioris very similar to thefilling phase The flow rate may drop somewhat as the mold builds up to pressure resulting inan increase in the thickness of the frozen layer
The main difference of course is the increase in hydrostatic (isotropic) pressure We shall seein chapter 2 section 24 Effect of Molding Conditions that hydrostatic pressure in itself doesnot cause any residual stress
114 The Compensation Phase
Compensating flow is unstable Consider the plate molding again (see Figure 11) You wouldthink that plastic flowing uniformly through the thin diaphragm would top up the thick rim Inpractice the plastic during the compensation phase flows in rivers that spread out like a deltaas illustrated in Figure 18 This may seem surprising at first but it can be explained by temperature instability
8102019 L-G-0004410537-0002634439 Moldflow
httpslidepdfcomreaderfulll-g-0004410537-0002634439-moldflow 2830
How Do Plastics Flow 7
Figure 18ure 1 8 River flow
1141 Temperature Variation
There is always some variation in melt temperature coming from the barrel of the injectionmachine In exceptional cases up to 40 ordmC variation has been measured using a high-speedthermocouple
1142 Natural Instability
However slight the temperature variation natural instability will amplify it If for exampleone part of the melt is slightly hotter than the rest then the plastic flow in that area will beslightly greater bringing hotter material into the area and maintaining the temperature If onthe other hand there is another area that is cooler the flow will be less so there will be lessheat input and the plastic will get colder until it eventually freezes off
However balanced the initial conditions this natural instability will result in a river-type flow
This is a very important consideration The first material to freeze off will shrink early in thecycle By the time the material in the river flows freezes the bulk of the material will havealready frozen off and shrinkage will have occurred The rivers will shrink relative to the bulk of the molding and because they are highly orientated shrinkage will be very high The resultis high-stress tensile members throughout the molding a common cause of warpage
1143 Optimum Part Quality
Most of the stress in plastic parts occurs during the compensation phase By controlling flow and minimizing stress it is possible to design for optimum part quality This important point is
at the heart of the Moldflow philosophy
12 How Do Plastics Flow
121 Material Behavior
Molten thermoplastics exhibit viscoelastic behavior which combines flow characteristics of both viscous liquids and elastic solids When a viscous liquid flows the energy that causes thedeformation is dissipated and becomes viscous heat On the other hand when an elastic solid
8102019 L-G-0004410537-0002634439 Moldflow
httpslidepdfcomreaderfulll-g-0004410537-0002634439-moldflow 2930
8 Polymer Flow Behavior in Injection Molds
is deformed the driving energy is stored For example the flow of water is a typical viscousflow whereas the deformation of a rubber cube falls into the elastic category
122 Deformation
In addition to the two types of material flow behavior there are two types of deformationsimple shear and simple extension (elongation) as shown in Figure 19 (a) and (b) below Theflow of molten thermoplastics during injection-molding filling is predominantly shear flow asshown in Figure 19 (c) in which layers of material elements slide over each other Theextensional flow however becomes significant as the material elements undergo elongation when the melt passes areas of abrupt dimensional change (eg a gate region) as shown inFigure 19 (d)
Figure 1igure 1 9 (a) Simple shear flow (b) Simple extensional flow (c) Shear flow in cavity filling (d) Extensional flow in cavity filling
123 Viscoelastic Behavior
In response to an applied stress (force per unit area) molten thermoplastics exhibit viscoelastic behavior which combines characteristics of an ideal viscous liquid with those of an ideal elastic solid In other words under certain conditions molten thermoplastics behavelike a liquid and will continuously deform while shear stress is applied as shown in
Figure 110 Upon the removal of the stress however the materials behave somewhat like anelastic solid with partial recovery of the deformation as shown in Figure 110 (b) and (c) This viscoelastic behavior stems from the random-coil configuration of polymer molecules in the
Stress
Stress Stress
Mold wall
Material element before
deformation begins
Material element during
deformation
Mold wall
Material elements at rest
Moving
materialelements
Velocity
profile
(a) (b)
(c) (d)
Velocity
profile
8102019 L-G-0004410537-0002634439 Moldflow
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How Do Plastics Flow 9
molten state which allows the movement and slippage of molecular chains under theinfluence of an applied load However the entanglement of the polymer molecular chains also
makes the system behave like an elastic solid upon the application and removal of the externalload Namely on removal of the stress chains will tend to return to the equilibrium random-coil state and thus will be a component of stress recovery The recovery is not instantaneousbecause of the entanglements still present in the system
Figure 1ure 1 10 (a) Ideal viscous liquid deforms continuously under applied stress (b) Ideal elasticsolid deforms immediately upon the application of stress but fully recovers when the
stress is removed (c) Molten thermoplastic deforms continuously under the applied
stress (like a viscous liquid) but also recovers partially from the deformation upon
removal of the applied stress (like an elastic solid)
124 Melt Shear Viscosity
1241 What Is Shear Viscosity
Melt shear viscosity is a materials resistance to shear flow In general polymer melts are highly viscous because of their long molecular chain structure The viscosity of a polymer melt rangesfrom 2 to 3000 Pas (water 10^-1 Pas glass 10^20 Pas) Viscosity can be thought of as thethickness of a fluid or how much it resists flow Viscosity is expressed as the ratio of shearstress (force per unit area) to the shear rate (rate change of shear strain) as shown inEquation 11 and Figure 111
(a) Viscous
liquid
(b) Elastic
solid
(c) Viscoelastic
polymer
melt
Initial shape
(shear stress
to be applied)
Time = Dt Time = 2 x Dt
(shear stress to
be removed)
Final deformed
shape
d
d
d
d
2d
2d
2d
2d
total
recovery
partial
recovery
8102019 L-G-0004410537-0002634439 Moldflow
httpslidepdfcomreaderfulll-g-0004410537-0002634439-moldflow 2530
4 Polymer Flow Behavior in Injection Molds
Now consider what happens upstream Hot plastic is continuously flowing bringing new hotmaterial along and generating significant frictional heat At the same time heat is being lost
through the frozen layer to the cold mold surfaceInitially the frozen layer is very thin so heat is lost very rapidly This results in more plasticfreezing and the frozen layer getting thicker cutting down the heat flow After a time thefrozen layer will reach a thickness such that the heat lost by conduction is equal to the heatinput from plastic flow and frictional heating ie an equilibrium condition is reached(Figure 14)
It is interesting to do some calculations on the time taken to reach this state of equilibrium The actual rate of heat flow is very large in comparison with the small heat content of theplastic in the frozen layer The result is that equilibrium is reached very quickly often in a timemeasured in a few tenths of a second As the total filling time is measured in seconds thefrozen layer reaches an equilibrium state early in the filling cycle
It is useful to think about how the thickness of this frozen layer will vary If the injection rate were slowed less heat would be generated by friction along the flow path with less heat inputfrom the flow The heat loss would be at the same rate and with less heat input the frozenlayer would grow in thickness If the injection rate were raised the frozen layer would bethinner (Figure 15) Similarly higher melt and mold temperatures would reduce the thicknessof the frozen layer This can be seen experimentally using the two-color technique
Figure 1igure 1 5 Influence of injection rate on frozen layer thickness
1121 Flow Shear Stress
It is easy to get confused between the various stress levels and orientation of the polymer Asthe plastic flows it is subject to shear stress also called flow shear stress This flow shear stress willorient the material ie cause the molecules to align themselves in the general direction of flow
The shear stress varies from a maximum at the outside dropping off to zero at the center
3Shear stress is purely a function of force and area This must not be confused withshear rate which is the rate of plastic sliding over the next layer Shear rate is zero
= Frozen
= Molten cooler
= Molten hotter
Slowerinjection
rate
Faster
injection
rate
vs
8102019 L-G-0004410537-0002634439 Moldflow
httpslidepdfcomreaderfulll-g-0004410537-0002634439-moldflow 2630
Phases of Injection Molding 5
at the outer edge where the plastic is frozen rises to a maximum just inwards of thefrozen layer then drops toward the center as shown in Figure 16
Figure 16ure 1 6 Shear rate distribution
If the flow were stopped and the plastic allowed to cool down very slowly this orientation would have time to relax giving a very low level of residual orientation On the other hand if the material were kept under stress and the plastic snap frozen most of the orientation would
be trapped in the frozen plastic (Figure 17)
Figure 17ure 1 7 Molecular orientation through the thickness of the part
Min MaxShear rate
= Tension
= Compression
Molecular orientation trapped
in pressurization phase
Gradual reduction of orientation
toward the center of the
mold as stress levels are lower
and the cooling rate is slower
which allows more time for
orientation to relax
8102019 L-G-0004410537-0002634439 Moldflow
httpslidepdfcomreaderfulll-g-0004410537-0002634439-moldflow 2730
6 Polymer Flow Behavior in Injection Molds
Now consider the orientation from the mold surface toward the center
The frozen layer itself formed with very little shear and therefore low orientation immediately
freezes setting the low level of orientation
The layer of plastic just on the inside of the frozen layer is subject to maximum shear stressand freezes the instant flow stops trapping almost all the orientation
This is the orientation pattern the further toward the center the more the shear stress dropsand the slower the rate of cooling This allows more time for the level of orientation to relaxso the residual orientation drops rapidly toward the center Consider how this pattern willaffect the residual stress level Oriented material (normally) will shrink more than nonorientedmaterial On the inner surface of the original frozen layer highly oriented material wants toshrink a great deal but it is prevented from doing so by the less-oriented material The highly
oriented layer ends up being in tension while the less-oriented material is in compression This residual stress pattern is a common cause of part warpage
3 There is a connectionthrough orientationbetween the shear stress during
filling (flow stress) and the residual stress in the final molded part This meansshear stress during filling shown on Moldflow plots can be used as a designparameter
113 The Pressurization Phase
The pressurization phasefrom the point of view of flow behavioris very similar to thefilling phase The flow rate may drop somewhat as the mold builds up to pressure resulting inan increase in the thickness of the frozen layer
The main difference of course is the increase in hydrostatic (isotropic) pressure We shall seein chapter 2 section 24 Effect of Molding Conditions that hydrostatic pressure in itself doesnot cause any residual stress
114 The Compensation Phase
Compensating flow is unstable Consider the plate molding again (see Figure 11) You wouldthink that plastic flowing uniformly through the thin diaphragm would top up the thick rim Inpractice the plastic during the compensation phase flows in rivers that spread out like a deltaas illustrated in Figure 18 This may seem surprising at first but it can be explained by temperature instability
8102019 L-G-0004410537-0002634439 Moldflow
httpslidepdfcomreaderfulll-g-0004410537-0002634439-moldflow 2830
How Do Plastics Flow 7
Figure 18ure 1 8 River flow
1141 Temperature Variation
There is always some variation in melt temperature coming from the barrel of the injectionmachine In exceptional cases up to 40 ordmC variation has been measured using a high-speedthermocouple
1142 Natural Instability
However slight the temperature variation natural instability will amplify it If for exampleone part of the melt is slightly hotter than the rest then the plastic flow in that area will beslightly greater bringing hotter material into the area and maintaining the temperature If onthe other hand there is another area that is cooler the flow will be less so there will be lessheat input and the plastic will get colder until it eventually freezes off
However balanced the initial conditions this natural instability will result in a river-type flow
This is a very important consideration The first material to freeze off will shrink early in thecycle By the time the material in the river flows freezes the bulk of the material will havealready frozen off and shrinkage will have occurred The rivers will shrink relative to the bulk of the molding and because they are highly orientated shrinkage will be very high The resultis high-stress tensile members throughout the molding a common cause of warpage
1143 Optimum Part Quality
Most of the stress in plastic parts occurs during the compensation phase By controlling flow and minimizing stress it is possible to design for optimum part quality This important point is
at the heart of the Moldflow philosophy
12 How Do Plastics Flow
121 Material Behavior
Molten thermoplastics exhibit viscoelastic behavior which combines flow characteristics of both viscous liquids and elastic solids When a viscous liquid flows the energy that causes thedeformation is dissipated and becomes viscous heat On the other hand when an elastic solid
8102019 L-G-0004410537-0002634439 Moldflow
httpslidepdfcomreaderfulll-g-0004410537-0002634439-moldflow 2930
8 Polymer Flow Behavior in Injection Molds
is deformed the driving energy is stored For example the flow of water is a typical viscousflow whereas the deformation of a rubber cube falls into the elastic category
122 Deformation
In addition to the two types of material flow behavior there are two types of deformationsimple shear and simple extension (elongation) as shown in Figure 19 (a) and (b) below Theflow of molten thermoplastics during injection-molding filling is predominantly shear flow asshown in Figure 19 (c) in which layers of material elements slide over each other Theextensional flow however becomes significant as the material elements undergo elongation when the melt passes areas of abrupt dimensional change (eg a gate region) as shown inFigure 19 (d)
Figure 1igure 1 9 (a) Simple shear flow (b) Simple extensional flow (c) Shear flow in cavity filling (d) Extensional flow in cavity filling
123 Viscoelastic Behavior
In response to an applied stress (force per unit area) molten thermoplastics exhibit viscoelastic behavior which combines characteristics of an ideal viscous liquid with those of an ideal elastic solid In other words under certain conditions molten thermoplastics behavelike a liquid and will continuously deform while shear stress is applied as shown in
Figure 110 Upon the removal of the stress however the materials behave somewhat like anelastic solid with partial recovery of the deformation as shown in Figure 110 (b) and (c) This viscoelastic behavior stems from the random-coil configuration of polymer molecules in the
Stress
Stress Stress
Mold wall
Material element before
deformation begins
Material element during
deformation
Mold wall
Material elements at rest
Moving
materialelements
Velocity
profile
(a) (b)
(c) (d)
Velocity
profile
8102019 L-G-0004410537-0002634439 Moldflow
httpslidepdfcomreaderfulll-g-0004410537-0002634439-moldflow 3030
How Do Plastics Flow 9
molten state which allows the movement and slippage of molecular chains under theinfluence of an applied load However the entanglement of the polymer molecular chains also
makes the system behave like an elastic solid upon the application and removal of the externalload Namely on removal of the stress chains will tend to return to the equilibrium random-coil state and thus will be a component of stress recovery The recovery is not instantaneousbecause of the entanglements still present in the system
Figure 1ure 1 10 (a) Ideal viscous liquid deforms continuously under applied stress (b) Ideal elasticsolid deforms immediately upon the application of stress but fully recovers when the
stress is removed (c) Molten thermoplastic deforms continuously under the applied
stress (like a viscous liquid) but also recovers partially from the deformation upon
removal of the applied stress (like an elastic solid)
124 Melt Shear Viscosity
1241 What Is Shear Viscosity
Melt shear viscosity is a materials resistance to shear flow In general polymer melts are highly viscous because of their long molecular chain structure The viscosity of a polymer melt rangesfrom 2 to 3000 Pas (water 10^-1 Pas glass 10^20 Pas) Viscosity can be thought of as thethickness of a fluid or how much it resists flow Viscosity is expressed as the ratio of shearstress (force per unit area) to the shear rate (rate change of shear strain) as shown inEquation 11 and Figure 111
(a) Viscous
liquid
(b) Elastic
solid
(c) Viscoelastic
polymer
melt
Initial shape
(shear stress
to be applied)
Time = Dt Time = 2 x Dt
(shear stress to
be removed)
Final deformed
shape
d
d
d
d
2d
2d
2d
2d
total
recovery
partial
recovery
8102019 L-G-0004410537-0002634439 Moldflow
httpslidepdfcomreaderfulll-g-0004410537-0002634439-moldflow 2630
Phases of Injection Molding 5
at the outer edge where the plastic is frozen rises to a maximum just inwards of thefrozen layer then drops toward the center as shown in Figure 16
Figure 16ure 1 6 Shear rate distribution
If the flow were stopped and the plastic allowed to cool down very slowly this orientation would have time to relax giving a very low level of residual orientation On the other hand if the material were kept under stress and the plastic snap frozen most of the orientation would
be trapped in the frozen plastic (Figure 17)
Figure 17ure 1 7 Molecular orientation through the thickness of the part
Min MaxShear rate
= Tension
= Compression
Molecular orientation trapped
in pressurization phase
Gradual reduction of orientation
toward the center of the
mold as stress levels are lower
and the cooling rate is slower
which allows more time for
orientation to relax
8102019 L-G-0004410537-0002634439 Moldflow
httpslidepdfcomreaderfulll-g-0004410537-0002634439-moldflow 2730
6 Polymer Flow Behavior in Injection Molds
Now consider the orientation from the mold surface toward the center
The frozen layer itself formed with very little shear and therefore low orientation immediately
freezes setting the low level of orientation
The layer of plastic just on the inside of the frozen layer is subject to maximum shear stressand freezes the instant flow stops trapping almost all the orientation
This is the orientation pattern the further toward the center the more the shear stress dropsand the slower the rate of cooling This allows more time for the level of orientation to relaxso the residual orientation drops rapidly toward the center Consider how this pattern willaffect the residual stress level Oriented material (normally) will shrink more than nonorientedmaterial On the inner surface of the original frozen layer highly oriented material wants toshrink a great deal but it is prevented from doing so by the less-oriented material The highly
oriented layer ends up being in tension while the less-oriented material is in compression This residual stress pattern is a common cause of part warpage
3 There is a connectionthrough orientationbetween the shear stress during
filling (flow stress) and the residual stress in the final molded part This meansshear stress during filling shown on Moldflow plots can be used as a designparameter
113 The Pressurization Phase
The pressurization phasefrom the point of view of flow behavioris very similar to thefilling phase The flow rate may drop somewhat as the mold builds up to pressure resulting inan increase in the thickness of the frozen layer
The main difference of course is the increase in hydrostatic (isotropic) pressure We shall seein chapter 2 section 24 Effect of Molding Conditions that hydrostatic pressure in itself doesnot cause any residual stress
114 The Compensation Phase
Compensating flow is unstable Consider the plate molding again (see Figure 11) You wouldthink that plastic flowing uniformly through the thin diaphragm would top up the thick rim Inpractice the plastic during the compensation phase flows in rivers that spread out like a deltaas illustrated in Figure 18 This may seem surprising at first but it can be explained by temperature instability
8102019 L-G-0004410537-0002634439 Moldflow
httpslidepdfcomreaderfulll-g-0004410537-0002634439-moldflow 2830
How Do Plastics Flow 7
Figure 18ure 1 8 River flow
1141 Temperature Variation
There is always some variation in melt temperature coming from the barrel of the injectionmachine In exceptional cases up to 40 ordmC variation has been measured using a high-speedthermocouple
1142 Natural Instability
However slight the temperature variation natural instability will amplify it If for exampleone part of the melt is slightly hotter than the rest then the plastic flow in that area will beslightly greater bringing hotter material into the area and maintaining the temperature If onthe other hand there is another area that is cooler the flow will be less so there will be lessheat input and the plastic will get colder until it eventually freezes off
However balanced the initial conditions this natural instability will result in a river-type flow
This is a very important consideration The first material to freeze off will shrink early in thecycle By the time the material in the river flows freezes the bulk of the material will havealready frozen off and shrinkage will have occurred The rivers will shrink relative to the bulk of the molding and because they are highly orientated shrinkage will be very high The resultis high-stress tensile members throughout the molding a common cause of warpage
1143 Optimum Part Quality
Most of the stress in plastic parts occurs during the compensation phase By controlling flow and minimizing stress it is possible to design for optimum part quality This important point is
at the heart of the Moldflow philosophy
12 How Do Plastics Flow
121 Material Behavior
Molten thermoplastics exhibit viscoelastic behavior which combines flow characteristics of both viscous liquids and elastic solids When a viscous liquid flows the energy that causes thedeformation is dissipated and becomes viscous heat On the other hand when an elastic solid
8102019 L-G-0004410537-0002634439 Moldflow
httpslidepdfcomreaderfulll-g-0004410537-0002634439-moldflow 2930
8 Polymer Flow Behavior in Injection Molds
is deformed the driving energy is stored For example the flow of water is a typical viscousflow whereas the deformation of a rubber cube falls into the elastic category
122 Deformation
In addition to the two types of material flow behavior there are two types of deformationsimple shear and simple extension (elongation) as shown in Figure 19 (a) and (b) below Theflow of molten thermoplastics during injection-molding filling is predominantly shear flow asshown in Figure 19 (c) in which layers of material elements slide over each other Theextensional flow however becomes significant as the material elements undergo elongation when the melt passes areas of abrupt dimensional change (eg a gate region) as shown inFigure 19 (d)
Figure 1igure 1 9 (a) Simple shear flow (b) Simple extensional flow (c) Shear flow in cavity filling (d) Extensional flow in cavity filling
123 Viscoelastic Behavior
In response to an applied stress (force per unit area) molten thermoplastics exhibit viscoelastic behavior which combines characteristics of an ideal viscous liquid with those of an ideal elastic solid In other words under certain conditions molten thermoplastics behavelike a liquid and will continuously deform while shear stress is applied as shown in
Figure 110 Upon the removal of the stress however the materials behave somewhat like anelastic solid with partial recovery of the deformation as shown in Figure 110 (b) and (c) This viscoelastic behavior stems from the random-coil configuration of polymer molecules in the
Stress
Stress Stress
Mold wall
Material element before
deformation begins
Material element during
deformation
Mold wall
Material elements at rest
Moving
materialelements
Velocity
profile
(a) (b)
(c) (d)
Velocity
profile
8102019 L-G-0004410537-0002634439 Moldflow
httpslidepdfcomreaderfulll-g-0004410537-0002634439-moldflow 3030
How Do Plastics Flow 9
molten state which allows the movement and slippage of molecular chains under theinfluence of an applied load However the entanglement of the polymer molecular chains also
makes the system behave like an elastic solid upon the application and removal of the externalload Namely on removal of the stress chains will tend to return to the equilibrium random-coil state and thus will be a component of stress recovery The recovery is not instantaneousbecause of the entanglements still present in the system
Figure 1ure 1 10 (a) Ideal viscous liquid deforms continuously under applied stress (b) Ideal elasticsolid deforms immediately upon the application of stress but fully recovers when the
stress is removed (c) Molten thermoplastic deforms continuously under the applied
stress (like a viscous liquid) but also recovers partially from the deformation upon
removal of the applied stress (like an elastic solid)
124 Melt Shear Viscosity
1241 What Is Shear Viscosity
Melt shear viscosity is a materials resistance to shear flow In general polymer melts are highly viscous because of their long molecular chain structure The viscosity of a polymer melt rangesfrom 2 to 3000 Pas (water 10^-1 Pas glass 10^20 Pas) Viscosity can be thought of as thethickness of a fluid or how much it resists flow Viscosity is expressed as the ratio of shearstress (force per unit area) to the shear rate (rate change of shear strain) as shown inEquation 11 and Figure 111
(a) Viscous
liquid
(b) Elastic
solid
(c) Viscoelastic
polymer
melt
Initial shape
(shear stress
to be applied)
Time = Dt Time = 2 x Dt
(shear stress to
be removed)
Final deformed
shape
d
d
d
d
2d
2d
2d
2d
total
recovery
partial
recovery
8102019 L-G-0004410537-0002634439 Moldflow
httpslidepdfcomreaderfulll-g-0004410537-0002634439-moldflow 2730
6 Polymer Flow Behavior in Injection Molds
Now consider the orientation from the mold surface toward the center
The frozen layer itself formed with very little shear and therefore low orientation immediately
freezes setting the low level of orientation
The layer of plastic just on the inside of the frozen layer is subject to maximum shear stressand freezes the instant flow stops trapping almost all the orientation
This is the orientation pattern the further toward the center the more the shear stress dropsand the slower the rate of cooling This allows more time for the level of orientation to relaxso the residual orientation drops rapidly toward the center Consider how this pattern willaffect the residual stress level Oriented material (normally) will shrink more than nonorientedmaterial On the inner surface of the original frozen layer highly oriented material wants toshrink a great deal but it is prevented from doing so by the less-oriented material The highly
oriented layer ends up being in tension while the less-oriented material is in compression This residual stress pattern is a common cause of part warpage
3 There is a connectionthrough orientationbetween the shear stress during
filling (flow stress) and the residual stress in the final molded part This meansshear stress during filling shown on Moldflow plots can be used as a designparameter
113 The Pressurization Phase
The pressurization phasefrom the point of view of flow behavioris very similar to thefilling phase The flow rate may drop somewhat as the mold builds up to pressure resulting inan increase in the thickness of the frozen layer
The main difference of course is the increase in hydrostatic (isotropic) pressure We shall seein chapter 2 section 24 Effect of Molding Conditions that hydrostatic pressure in itself doesnot cause any residual stress
114 The Compensation Phase
Compensating flow is unstable Consider the plate molding again (see Figure 11) You wouldthink that plastic flowing uniformly through the thin diaphragm would top up the thick rim Inpractice the plastic during the compensation phase flows in rivers that spread out like a deltaas illustrated in Figure 18 This may seem surprising at first but it can be explained by temperature instability
8102019 L-G-0004410537-0002634439 Moldflow
httpslidepdfcomreaderfulll-g-0004410537-0002634439-moldflow 2830
How Do Plastics Flow 7
Figure 18ure 1 8 River flow
1141 Temperature Variation
There is always some variation in melt temperature coming from the barrel of the injectionmachine In exceptional cases up to 40 ordmC variation has been measured using a high-speedthermocouple
1142 Natural Instability
However slight the temperature variation natural instability will amplify it If for exampleone part of the melt is slightly hotter than the rest then the plastic flow in that area will beslightly greater bringing hotter material into the area and maintaining the temperature If onthe other hand there is another area that is cooler the flow will be less so there will be lessheat input and the plastic will get colder until it eventually freezes off
However balanced the initial conditions this natural instability will result in a river-type flow
This is a very important consideration The first material to freeze off will shrink early in thecycle By the time the material in the river flows freezes the bulk of the material will havealready frozen off and shrinkage will have occurred The rivers will shrink relative to the bulk of the molding and because they are highly orientated shrinkage will be very high The resultis high-stress tensile members throughout the molding a common cause of warpage
1143 Optimum Part Quality
Most of the stress in plastic parts occurs during the compensation phase By controlling flow and minimizing stress it is possible to design for optimum part quality This important point is
at the heart of the Moldflow philosophy
12 How Do Plastics Flow
121 Material Behavior
Molten thermoplastics exhibit viscoelastic behavior which combines flow characteristics of both viscous liquids and elastic solids When a viscous liquid flows the energy that causes thedeformation is dissipated and becomes viscous heat On the other hand when an elastic solid
8102019 L-G-0004410537-0002634439 Moldflow
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8 Polymer Flow Behavior in Injection Molds
is deformed the driving energy is stored For example the flow of water is a typical viscousflow whereas the deformation of a rubber cube falls into the elastic category
122 Deformation
In addition to the two types of material flow behavior there are two types of deformationsimple shear and simple extension (elongation) as shown in Figure 19 (a) and (b) below Theflow of molten thermoplastics during injection-molding filling is predominantly shear flow asshown in Figure 19 (c) in which layers of material elements slide over each other Theextensional flow however becomes significant as the material elements undergo elongation when the melt passes areas of abrupt dimensional change (eg a gate region) as shown inFigure 19 (d)
Figure 1igure 1 9 (a) Simple shear flow (b) Simple extensional flow (c) Shear flow in cavity filling (d) Extensional flow in cavity filling
123 Viscoelastic Behavior
In response to an applied stress (force per unit area) molten thermoplastics exhibit viscoelastic behavior which combines characteristics of an ideal viscous liquid with those of an ideal elastic solid In other words under certain conditions molten thermoplastics behavelike a liquid and will continuously deform while shear stress is applied as shown in
Figure 110 Upon the removal of the stress however the materials behave somewhat like anelastic solid with partial recovery of the deformation as shown in Figure 110 (b) and (c) This viscoelastic behavior stems from the random-coil configuration of polymer molecules in the
Stress
Stress Stress
Mold wall
Material element before
deformation begins
Material element during
deformation
Mold wall
Material elements at rest
Moving
materialelements
Velocity
profile
(a) (b)
(c) (d)
Velocity
profile
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How Do Plastics Flow 9
molten state which allows the movement and slippage of molecular chains under theinfluence of an applied load However the entanglement of the polymer molecular chains also
makes the system behave like an elastic solid upon the application and removal of the externalload Namely on removal of the stress chains will tend to return to the equilibrium random-coil state and thus will be a component of stress recovery The recovery is not instantaneousbecause of the entanglements still present in the system
Figure 1ure 1 10 (a) Ideal viscous liquid deforms continuously under applied stress (b) Ideal elasticsolid deforms immediately upon the application of stress but fully recovers when the
stress is removed (c) Molten thermoplastic deforms continuously under the applied
stress (like a viscous liquid) but also recovers partially from the deformation upon
removal of the applied stress (like an elastic solid)
124 Melt Shear Viscosity
1241 What Is Shear Viscosity
Melt shear viscosity is a materials resistance to shear flow In general polymer melts are highly viscous because of their long molecular chain structure The viscosity of a polymer melt rangesfrom 2 to 3000 Pas (water 10^-1 Pas glass 10^20 Pas) Viscosity can be thought of as thethickness of a fluid or how much it resists flow Viscosity is expressed as the ratio of shearstress (force per unit area) to the shear rate (rate change of shear strain) as shown inEquation 11 and Figure 111
(a) Viscous
liquid
(b) Elastic
solid
(c) Viscoelastic
polymer
melt
Initial shape
(shear stress
to be applied)
Time = Dt Time = 2 x Dt
(shear stress to
be removed)
Final deformed
shape
d
d
d
d
2d
2d
2d
2d
total
recovery
partial
recovery
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How Do Plastics Flow 7
Figure 18ure 1 8 River flow
1141 Temperature Variation
There is always some variation in melt temperature coming from the barrel of the injectionmachine In exceptional cases up to 40 ordmC variation has been measured using a high-speedthermocouple
1142 Natural Instability
However slight the temperature variation natural instability will amplify it If for exampleone part of the melt is slightly hotter than the rest then the plastic flow in that area will beslightly greater bringing hotter material into the area and maintaining the temperature If onthe other hand there is another area that is cooler the flow will be less so there will be lessheat input and the plastic will get colder until it eventually freezes off
However balanced the initial conditions this natural instability will result in a river-type flow
This is a very important consideration The first material to freeze off will shrink early in thecycle By the time the material in the river flows freezes the bulk of the material will havealready frozen off and shrinkage will have occurred The rivers will shrink relative to the bulk of the molding and because they are highly orientated shrinkage will be very high The resultis high-stress tensile members throughout the molding a common cause of warpage
1143 Optimum Part Quality
Most of the stress in plastic parts occurs during the compensation phase By controlling flow and minimizing stress it is possible to design for optimum part quality This important point is
at the heart of the Moldflow philosophy
12 How Do Plastics Flow
121 Material Behavior
Molten thermoplastics exhibit viscoelastic behavior which combines flow characteristics of both viscous liquids and elastic solids When a viscous liquid flows the energy that causes thedeformation is dissipated and becomes viscous heat On the other hand when an elastic solid
8102019 L-G-0004410537-0002634439 Moldflow
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8 Polymer Flow Behavior in Injection Molds
is deformed the driving energy is stored For example the flow of water is a typical viscousflow whereas the deformation of a rubber cube falls into the elastic category
122 Deformation
In addition to the two types of material flow behavior there are two types of deformationsimple shear and simple extension (elongation) as shown in Figure 19 (a) and (b) below Theflow of molten thermoplastics during injection-molding filling is predominantly shear flow asshown in Figure 19 (c) in which layers of material elements slide over each other Theextensional flow however becomes significant as the material elements undergo elongation when the melt passes areas of abrupt dimensional change (eg a gate region) as shown inFigure 19 (d)
Figure 1igure 1 9 (a) Simple shear flow (b) Simple extensional flow (c) Shear flow in cavity filling (d) Extensional flow in cavity filling
123 Viscoelastic Behavior
In response to an applied stress (force per unit area) molten thermoplastics exhibit viscoelastic behavior which combines characteristics of an ideal viscous liquid with those of an ideal elastic solid In other words under certain conditions molten thermoplastics behavelike a liquid and will continuously deform while shear stress is applied as shown in
Figure 110 Upon the removal of the stress however the materials behave somewhat like anelastic solid with partial recovery of the deformation as shown in Figure 110 (b) and (c) This viscoelastic behavior stems from the random-coil configuration of polymer molecules in the
Stress
Stress Stress
Mold wall
Material element before
deformation begins
Material element during
deformation
Mold wall
Material elements at rest
Moving
materialelements
Velocity
profile
(a) (b)
(c) (d)
Velocity
profile
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How Do Plastics Flow 9
molten state which allows the movement and slippage of molecular chains under theinfluence of an applied load However the entanglement of the polymer molecular chains also
makes the system behave like an elastic solid upon the application and removal of the externalload Namely on removal of the stress chains will tend to return to the equilibrium random-coil state and thus will be a component of stress recovery The recovery is not instantaneousbecause of the entanglements still present in the system
Figure 1ure 1 10 (a) Ideal viscous liquid deforms continuously under applied stress (b) Ideal elasticsolid deforms immediately upon the application of stress but fully recovers when the
stress is removed (c) Molten thermoplastic deforms continuously under the applied
stress (like a viscous liquid) but also recovers partially from the deformation upon
removal of the applied stress (like an elastic solid)
124 Melt Shear Viscosity
1241 What Is Shear Viscosity
Melt shear viscosity is a materials resistance to shear flow In general polymer melts are highly viscous because of their long molecular chain structure The viscosity of a polymer melt rangesfrom 2 to 3000 Pas (water 10^-1 Pas glass 10^20 Pas) Viscosity can be thought of as thethickness of a fluid or how much it resists flow Viscosity is expressed as the ratio of shearstress (force per unit area) to the shear rate (rate change of shear strain) as shown inEquation 11 and Figure 111
(a) Viscous
liquid
(b) Elastic
solid
(c) Viscoelastic
polymer
melt
Initial shape
(shear stress
to be applied)
Time = Dt Time = 2 x Dt
(shear stress to
be removed)
Final deformed
shape
d
d
d
d
2d
2d
2d
2d
total
recovery
partial
recovery
8102019 L-G-0004410537-0002634439 Moldflow
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8 Polymer Flow Behavior in Injection Molds
is deformed the driving energy is stored For example the flow of water is a typical viscousflow whereas the deformation of a rubber cube falls into the elastic category
122 Deformation
In addition to the two types of material flow behavior there are two types of deformationsimple shear and simple extension (elongation) as shown in Figure 19 (a) and (b) below Theflow of molten thermoplastics during injection-molding filling is predominantly shear flow asshown in Figure 19 (c) in which layers of material elements slide over each other Theextensional flow however becomes significant as the material elements undergo elongation when the melt passes areas of abrupt dimensional change (eg a gate region) as shown inFigure 19 (d)
Figure 1igure 1 9 (a) Simple shear flow (b) Simple extensional flow (c) Shear flow in cavity filling (d) Extensional flow in cavity filling
123 Viscoelastic Behavior
In response to an applied stress (force per unit area) molten thermoplastics exhibit viscoelastic behavior which combines characteristics of an ideal viscous liquid with those of an ideal elastic solid In other words under certain conditions molten thermoplastics behavelike a liquid and will continuously deform while shear stress is applied as shown in
Figure 110 Upon the removal of the stress however the materials behave somewhat like anelastic solid with partial recovery of the deformation as shown in Figure 110 (b) and (c) This viscoelastic behavior stems from the random-coil configuration of polymer molecules in the
Stress
Stress Stress
Mold wall
Material element before
deformation begins
Material element during
deformation
Mold wall
Material elements at rest
Moving
materialelements
Velocity
profile
(a) (b)
(c) (d)
Velocity
profile
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How Do Plastics Flow 9
molten state which allows the movement and slippage of molecular chains under theinfluence of an applied load However the entanglement of the polymer molecular chains also
makes the system behave like an elastic solid upon the application and removal of the externalload Namely on removal of the stress chains will tend to return to the equilibrium random-coil state and thus will be a component of stress recovery The recovery is not instantaneousbecause of the entanglements still present in the system
Figure 1ure 1 10 (a) Ideal viscous liquid deforms continuously under applied stress (b) Ideal elasticsolid deforms immediately upon the application of stress but fully recovers when the
stress is removed (c) Molten thermoplastic deforms continuously under the applied
stress (like a viscous liquid) but also recovers partially from the deformation upon
removal of the applied stress (like an elastic solid)
124 Melt Shear Viscosity
1241 What Is Shear Viscosity
Melt shear viscosity is a materials resistance to shear flow In general polymer melts are highly viscous because of their long molecular chain structure The viscosity of a polymer melt rangesfrom 2 to 3000 Pas (water 10^-1 Pas glass 10^20 Pas) Viscosity can be thought of as thethickness of a fluid or how much it resists flow Viscosity is expressed as the ratio of shearstress (force per unit area) to the shear rate (rate change of shear strain) as shown inEquation 11 and Figure 111
(a) Viscous
liquid
(b) Elastic
solid
(c) Viscoelastic
polymer
melt
Initial shape
(shear stress
to be applied)
Time = Dt Time = 2 x Dt
(shear stress to
be removed)
Final deformed
shape
d
d
d
d
2d
2d
2d
2d
total
recovery
partial
recovery
8102019 L-G-0004410537-0002634439 Moldflow
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How Do Plastics Flow 9
molten state which allows the movement and slippage of molecular chains under theinfluence of an applied load However the entanglement of the polymer molecular chains also
makes the system behave like an elastic solid upon the application and removal of the externalload Namely on removal of the stress chains will tend to return to the equilibrium random-coil state and thus will be a component of stress recovery The recovery is not instantaneousbecause of the entanglements still present in the system
Figure 1ure 1 10 (a) Ideal viscous liquid deforms continuously under applied stress (b) Ideal elasticsolid deforms immediately upon the application of stress but fully recovers when the
stress is removed (c) Molten thermoplastic deforms continuously under the applied
stress (like a viscous liquid) but also recovers partially from the deformation upon
removal of the applied stress (like an elastic solid)
124 Melt Shear Viscosity
1241 What Is Shear Viscosity
Melt shear viscosity is a materials resistance to shear flow In general polymer melts are highly viscous because of their long molecular chain structure The viscosity of a polymer melt rangesfrom 2 to 3000 Pas (water 10^-1 Pas glass 10^20 Pas) Viscosity can be thought of as thethickness of a fluid or how much it resists flow Viscosity is expressed as the ratio of shearstress (force per unit area) to the shear rate (rate change of shear strain) as shown inEquation 11 and Figure 111
(a) Viscous
liquid
(b) Elastic
solid
(c) Viscoelastic
polymer
melt
Initial shape
(shear stress
to be applied)
Time = Dt Time = 2 x Dt
(shear stress to
be removed)
Final deformed
shape
d
d
d
d
2d
2d
2d
2d
total
recovery
partial
recovery
