Injection Molding Outline Facts and figures Why injection molding? Goals Advantages Disadvantages Overview of the injection molding process Equipment Materials consideration Critical operation parameters Mathematical modeling Mold flow analysis Commercial software In essence, injection molding is a process whereby a solid thermoplastic material is heateduntil it reaches a state of fluidity, is then transferred under pressure (injected) into a closed hollow space (mold cavity), and then cooled in the mold till it again reaches a solid state, conforming in shape to the mold cavity. Introduction to Injection Molding by Clifford I. Weir Facts and Figures Thermoplastics: Injection molding-32% by weight Extrusion-36% All others-32% History John Hyatt patented the first injection molder in 1872. Further advances were made in Europe through the 1920s. In 1951, William H. Willert invented the first molder machine to use a screw to provide continuous feed of liquid material. From the 1980s on, the most significant developments in injection molding have been in the area of computerization of the process. (Rosato & Rosato) I also invented the first synthetic material, Celluloid, which lead to many important advances in the plastics industry, including the injection molding industry. John Hyatt William Willert My invention, which allowedcontinuous injection, made injection+blowmolding feasible and fast. This is the method used to create bottles, jars and many other containers. Goals Produce high quality parts in terms of aesthetic, functional, and material properties Maximize profits by reducing cycle time Achieve the proper balance between quality and cost Advantages Tight tolerance parts Complex geometry Various surface textures Highly repeatable process Low cost in high volume production Automated process, low labor cost Net shape Parts consolidation Disadvantages High initial cost-Mold, Injection machine, Auxiliary equipment Economical for high volume production High amount of scrap High level of competition Overview PART DESIGN MOLD EQUIPMENTMATERIAL Injection Molding Machine Equipment Injection molding machine Auxiliary Equipment: Drier Chiller Heater Granulator Mixer/Blender Robots Mold change system Injection Molding Machine Main Components: Injection system Mold system Clamping system Controls Process & machine schematics * * Source: http://www.idsa-mp.org/proc/plastic/injection/injection_process.htm * Schematic of thermoplastic Injection molding machine Injection System Function Plastification Injection (pump) Types Conventional Piston type Screw type Reciprocating Screw Different Type of IM Machines Two Types of Injection Systems 1. Reciprocating Screw System 2. Ram Injection Molding Plastic is pre-melted in the hopper, fed into a chamber, and forced into the mold with a piston. Plastic is melted through shearing and external heat, forced into the mold with a the screw. CYLINDER AND PLUNGER Most Machines are Reciprocating Screw Injection Molding machines because: -More uniform melting -Better mixing of resin with additives -Lower injections pressures -Faster cycle times -Fewer stresses in parts -Larger permissible part area Clamping System Important in maintaining the right pressure when the the polymer is injected Hydraulic Clamp-Varying amounts of pressure can be applied Mechanical Clamp-Less expensive and faster clamping/unclamping action Clamping System Function Sizing the clamping unit: F = P x A p injection pressure, psi A cavity projected area, in2 F clamping force, pounds 1 ton = 2000 pounds Clamping system Typical machines: Small size machines 10-100 tons Medium size machines100-500 tons Large size machines 500-10,000 tons Types Hydraulic Toggle Calculate clamp force, & shot size F=P X A = 420 tons 3.8 lbs = 2245 cm3 =75 oz What does this mean? Clamp force and machine cost Reciprocating Screw Injection System Function Injection cycle Shot size Screw design Processing parameters Screw Design L/D Ratio,General purpose 20:1 Helix angle 17.8o Compression ratio, range 1.5:1 to 4.5:1 Screw profile, e.g 10-5-5 Channel depth in metering section Screw tip pitch Processing Parameters Injection Molding Process Simple Definition:Plastic is melted and then forced into a closed cavity mold which gives the cooled plastic shape. 1- Melting 2- Injecting Resin 3- Part Cooling 4- Part Ejection Processing Parameters Screw speed Injection speed Injection pressure Barrel temperature Mold temperature Back pressure Process Operation Temperature: barrel zones, tool, die zone Pressures: injection max, hold Times: injection, hold, tool opening Shot size: screw travel Flash Melt Thermal degradation Short-shot Temp. Pressure Processing window Typical pressure/temperature cycle ( )23 3half thickness10 sec for polymerscooltcmoo==Time(sec) Cooling time generally dominates cycle time Time(sec) * Source: http://islnotes.cps.msu.edu/trp/inj/inj_time.html* * Effects of mold temperature and pressure on shrinkage 0.030 0.000 0.010 0.005 0.015 0.020 0.025 100120140160180200220240 Mold Temperature (F) LDPEPP Nylon 6/6 PMMA Acetal Shrinkage 0.030 0.000 0.010 0.005 0.015 0.020 0.025 Shrinkage 6000 8000 10000 12000 14000 16000 Pressure on injection plunger (psi) Acetal LDPE Nylon6/6 PP with flow 18000 PP across flow PMMA Injection Mold Overview PART DESIGN MOLD EQUIPMENTMATERIAL Injection Molds Tooling Basics Cavity Plate Cavity Moulding Core Core Plate Basic mould consisting of cavity and core plate Runner Cavity Gate Nozzle Sprue Melt Delivery Part Cavity Core Stripper plate Tooling for a plastic cup Runner Knob Nozzle Tooling for a plastic cup Runner Part Cavity Nozzle Part Cavity Knob Stripper plate Runner Part Cavity Nozzle Tooling * Source: http://www.idsa-mp.org/proc/plastic/injection/; ** http://www.hzs.co.jp/english/products/e_trainer/mold/basic/basic.htm (E-trainer by HZS Co.,Ltd.) * * * * * *** Mold Design Considerations Cavity:-Single or Multi The cavity design is dependent upon the design specifications of your part, the equipment and your budget higher viscosity polymer = larger runnersRunners: -Must be the right size and shape to achieve proper flow characteristics of the polymer used Mold Design Considerations Gates: Entry path between runner and part cavity Edge Gatelow cost Submarine Gate part separation from runners Tab Gatelarge parts Fan Gateintermediate size part Ring Gatehollow cylinder parts Most common Tool Steel, grade P20 and P21High TemperaturesH13 steelMirror FinishA2 and A6 steelsLow cost and low Production AluminumMold Material: Ideal properties vs. design specs Gate Location Center gate: radial flow severe distortion Diagonal gate: radial flow twistingEnd gates: linear flow minimum warping Gate Air entrapment Edge gate: warp free, air entrapment Sprue 2.0 2.0 60 Before shrinkage 60.32 1.96 1.976 After shrinkage Shrinkage Direction of flow 0.020 in/in Perpendicular to flow 0.012 Where would you gate this part? Weld line, Sink mark * Source: http://www.idsa-mp.org/proc/plastic/injection/injection_design_7.htmWeld line Mold Filling Gate Solidified part Sink mark Basic rules in designing ribs to minimize sink marks MOLD COST ESTIMATION Mold Base Costs Cb = 1000 + 0.45 Ac*hp^(0.4) where Cb = cost of mold base, $ Ac = area of mold base cavity plate, cm^2 hp = combined thickness of cavity and core plates in mold base, cm MOLD COST ESTIMATION Cavity and core manufacturing costs Initial cost estimates are based on the use of a standard two-plate mold.Decisions regarding more complex molds can only be made by comparing the increased cost of the mold system with the reduced machine supervision associated with semiautomatic or fully automatic operation MOLD COST ESTIMATION Cavity and core manufacturing costs Transform a pre-assembled mold into a working mold deep hole drilling of the cooling channels milling of pockets in the plates to receive the cavity and core inserts work on ejector plate and housing to receive the ejection system insertion of extra support pillars where necessary and the fitting of electrical and coolant systems Formulas to estimate cost MOLD COST POINT SYSTEM A point system for mold cavity and core cost estimating Establishes a point scale for various attributes of the mold These points are added to yield the Total Points Score Total Point Score is then multiplied by average $/hr for mold manufacturing Material Material and Design Considerations Factors in Material Choice -Shrinkage -Thermal Properties -Viscosity -Viscoelasticty When choosing a material it is important to consider: Design specs of the part Mold design Material properties Special issues about injection molding of reinforced materials Microstructure Fiber orientation Description Fiber orientation prediction Fiber orientation measurementFiber orientation control Issues with long fibers Part Design Injection Molding * * * Source: http://www.idsa-mp.org/proc/plastic/injection/injection_design_2.htmPart design rules Simple shapes to reduce tooling cost No undercuts, etc. Draft angle to remove part In some cases, small angles (1/4) will do Problem for gears Even wall thickness Minimum wall thickness ~ 0.025 in Avoid sharp corners Hide weld lines Holes may be molded 2/3 of the way through the wall only, with final drilling to eliminate weld lines Part Design Hand out from Steinwall, Inc.Cost Cost Analysis Cost Considerations: Material and additives Tooling Cost Machine Cost Labor cost Finishing cost Injection Molded Parts Process Modeling Modeling Dissection of the process Fundamentals of fluid flow and heat transfer Analytical flow modeling in simple molds Use of Moldflow simulation products Fiber orientation modelling Reynolds Number * Source: http://www.idsa-mp.org/proc/plastic/injection/injection_process.htm 22inertiaReviscousVVLLVL= =Reynolds Number: For typical injection molding 3 3 4 2 313 21 10 ;10 thicknessPart length 10; 10Fill time 1Zgcm Nm s L mV Nsms= = =~ = = 4Re 10=For Die casting 3 1 333 10 10 10Re 30010 ~ =Viscous Shearing of Fluids v F h F vA hF/A v/h 1 Newtonian Viscosity vht =Generalization:t =( )tq =Injection molding :shear rate Typical shear rate forPolymer processes (sec)-1 Extrusion102~103 Calendering10~102 Injection molding 103~104 Comp. Molding1~10 Shear Thinning ~ 1 sec-1 for PE Viscous Heating Rate of Heating= Rate of Viscous Work 2P Fv F v vVol Vol A h h| |= = = |\ .Rate of Temperature rise 2 2 or ppdT v dT vcdt h dt c h | | | | = = ||\ . \ .Rate of Conduction out 22 2~p pdT k d T k Tdt c dx c h A= 2Viscous heatingConductionvk T=ABrinkman number For injection molding, order of magnitude ~ 0.1 to 10 Non-Isothermal Flow v Flow rate: 1/t ~V/Lx Heat transfer rate: 1/t ~a/(Lz/2)2 2Flow rate 1~Heat xfer rate 4 4z z zx xV L VL LL L o o= For injection molding 3 2Flow rate 1 10 / 0.1 0.1~ 2.5Heat xfer rate 4 10 / 10cms cm cmcm s cm =For Die casting of aluminum 22Flow rate 1 10 / 0.1 0.1~ 10Heat xfer rate 4 0.3 / 10cms cm cmcm s cm ~* Very small, therefore it requires thick runners Small value => Short shot Fountain Flow * Source: http://islnotes.cps.msu.edu/trp/inj/flw_froz.html ; ** Z. Tadmore and C. Gogos, Principles of Polymer Processing* ** Heat transfer Note; oTool > opolymer

( )xpqc T x y x yt xc c A A = A Ac c1st kind ( ') constant2nd kind ( ') constant3rd kind ( ') ( )Tx xTk x xxTk x x hT Tx= =c = =cc = = cBoundary Conditions: 1-dimensional heat conduction equation : The boundary condition of 1st kind applies to injection molding since the tool is often maintained at a constant temperature xTq kxc= cqxqx + Aqx

2 22 2or pT T T Tc kt x t x oc c c c = =c c c cFouriers law Heat transfer TW Tii t x +L-L Let Lch = H/2 (half thickness) = L ; tch = L2/o ;ATch = Ti TW (initial temp. wall temp.) Non-dimensionalize: 2;1; WOi WT T x tFT T L Lou = = + =22OFu uc c=c cDimensionless equation: Initial condition01OF u = =Boundary condition 0020 u u= == =Separation of variables ;matching B.C.; matching I.C. ( , ) ( ) ( )O OF f F g u =Centerline, u = 0.1,Fo = ot/L2 = 1 Temperature in a slab Bi-1 =k/hL Analytical Modeling Flow simulation in a center gated round mold Computer Simulation Flow Simulation Project Use MPI software Design project for the course New Developments New developments- Gas assisted injection molding New developments ; injection molding with cores Cores and Part Molded in Clear PlasticCores used in Injection MoldingInjection Molded Housing shown in class