BDD 40903 Injection Mold Design Chapter 1.pdf

7/21/2019 BDD 40903 Injection Mold Design Chapter 1.pdf

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BDD 4 9 3

INJECTION MOLDDESIGN

BDD 40903 Prepared by : Mohd Hilmi Othman


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CHAPTER 1

Introduction(2 hours)

Injection molding concept

Mold structure and components

Mold type

Mold material

Surface treatment

Mold development processType and classification of plastic materials

Melting and solidification

Shrinkage



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Advantages:

Short production cycle Achievable accuracy

Highly efficient production

Producing large variety

Extreme tolerances

Intricate shapes.

Good strength-to-weight ratio,

Good appearance and surface

definition

Good in quality control.

1. Groover, M.P.Fundamentals of Modern Manufacturing: Materials, Process, and System.3rd Ed. Asia: John & Wiley Sons, Pte Ltd, 2007.

2. Osswald, T.A. Turng, L.S. Gramann, P.Injection Molding Handbook. 2nd Ed. Hanser Verlag Publications. 2007.

Injection molding concept


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Injection molding concept (2)


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Injection


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Mold structure and components



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Mold structure and components (


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Mold type

Standard Molds

The standard mold is the most simple d

divided in two side :

a) Cavity side

Cavity side is the side that construct to

material from nozzle to cavity parts, basic

of sprue, runner.

b) Core side

Core side construct to make shape for c

system and ejection system, at this sid

system will be designed.

Standard mold have one parting line,

opening direction. This type of mold us

plastic parts that doesn't have undercut,

screw.

http://bp1.blogger.com/_oF_1Qcfku_w/R2xigNC7BEI/AAAAAAAAAV8/7AApHY-BIbg/s1600-h/standard-2-plate-mold.jpghttp://bp1.blogger.com/_oF_1Qcfku_w/R2xigNC7BEI/AAAAAAAAAV8/7AApHY-BIbg/s1600-h/standard-2-plate-mold.jpg


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Slide mold

Development from this mold type is the used slider parts in various molds types; basic sli

horizontal movement of mold to vertical movement, this types of molds is used to

undercut.


Mold type (2)
http://bp2.blogger.com/_oF_1Qcfku_w/R2xs7dC7BFI/AAAAAAAAAWE/8O_7UtSrmZc/s1600-h/slider-mold.jpghttp://bp2.blogger.com/_oF_1Qcfku_w/R2xs7dC7BFI/AAAAAAAAAWE/8O_7UtSrmZc/s1600-h/slider-mold.jpg


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Three plate mold

Basically three plate molds has two p

floating plate, whereby the floating pla

by support pin. Since the mold has tw

the runner system can be located on on

plate or by making a special plate that

plate, known as runner plate. Three

used because of their flexibility in gati

types of molds is flexible even use in m


Mold type (3)
http://bp2.blogger.com/_oF_1Qcfku_w/R2xxRdC7BGI/AAAAAAAAAWM/rDA-ApWqEp4/s1600-h/3plate.jpghttp://bp2.blogger.com/_oF_1Qcfku_w/R2xxRdC7BGI/AAAAAAAAAWM/rDA-ApWqEp4/s1600-h/3plate.jpg


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Split Cavity Mold

Basically the split cavity is same as

mold, but the cavity have split block tproduct or external threads. This type

when the slider is not enough to make

the threads. the disadvantages of this m

for little parts, the construction will diff


Mold type (4)
http://bp3.blogger.com/_oF_1Qcfku_w/R2zVhtC7BHI/AAAAAAAAAWU/EhBk6H2Fe9w/s1600-h/split-cavity.jpghttp://bp3.blogger.com/_oF_1Qcfku_w/R2zVhtC7BHI/AAAAAAAAAWU/EhBk6H2Fe9w/s1600-h/split-cavity.jpg


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Mold Material


The plastic part is when a plastic material is

heated until it becomes soft enough to be injected

into a closed mold.

The finished part is ejected out once it has cooled

enough to solidify and is completely formed.

It is possible to create a part with intricate

features, although the more complex the part the

more complex the mold, and ultimately more

expensive it is.

There are six materials that are commonly used

for creating mold, which are Aluminum, Copper

Alloy, Plaster, Polymers, Steel and Wood.

i ( )


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Mold Material (2)


How Material Selection Effects Mold DesignOne aspect to consider is that each material has its own specific shrinkage factors. Eve

exception of water expands when it is heated and contracts when it is cooled. In inj

contraction was labeled as the materials shrinkage. The shrinkage factor of the ma

account when designing the cavity of the mold so that when the part contracts it will

dimensional requirements of the original design.

Some materials dissipate heat differently than others, which can result in variation i

during the molding process. This has a big impact on how the water cooling lines are p

ensure even cooling.

Another related factor is the viscosity of the material. Viscosity is the measurement of

material in its liquid form. Higher viscosity means thicker material, and greater injection

through the mold. Also these materials require larger diameter in the runners and gates t

to flow easily into the cavity. The melt index is a good tool in determining the flowab

plastic.

M ld M i l (3)


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Mold Material (3)


http://injectionmold.cc/mold-design-and-manufacturing-process

Mold design and manufacturing processStudies have shown that life of the mold depends on heat

treatment, material selection, construction, machiningand

many other factors.

Based on the statistical analysis of a large number of failures

mold, the failure of mold design due to improper heat

treatment accounts is about 45%, selection and mold structure

is irrational about 25%, about 10% of process problems, and

lubricants and equipment questions about 20%. Therefore, in

the mold design and manufacturing process, material

selection, mold structure, heat treatment, machining processes

and improve working conditions for mold can improve the

quality and service life.

Improper heat

Selectio

Lu

Failure of m

M ld M t i l (4)


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Mold Material (4)


The rational use of mold material Selection of mold material, should be selected according to different

production batches and process methods.

In mass production, the mold should be used durable materials, such as

cemented carbide, high toughness & high wear-resistant tool steel.

For small quantities or new product trial can be zinc alloy, bismuth-tin

alloy.

As mold material it should be easily deformed, easy to fracture failure

common mold, the need to use high-strength, high toughness material;

good toughness, and strength.

Die-casting mold was good to adopt high thermal fatigue resistance, high

temperature strength alloy steel.

Plastic mold should be chosen for easy cutting, dense, and good polished

performance material.

In addition, by using different hardness or different materials for matched

mold, a mold life can be increased by 5 to 6 times. b

M ld M t i l (5)


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Mold Material (5)


The mold heat treatment process From the die failure analysis, it shows that 45% of die failure is due to

improper heat treatment.

Abrasion usually occurred at the surface. Fatigue and fracture often startfrom the surface, so the surface processing quality requirements are very

high.

But in fact due to the presence of traces of machining, surface oxidation and

decarbonization of inevitable heat treatment, the surface properties of the

mold was worse than the substrate.

Adoption of new technology heat treatment is an effective measure to

improve mold performance. Mold heat treatment process includes a substrate and a surface hardening

treatment toughening.

The main purpose is to strengthen the surface of the mold surface, by

improving the wear resistance, corrosion resistance, and lubricating

properties.

M ld M t i l (6)


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Mold Material (6)


Mold machining process Mold machining process directly affects the quality of life of the

mold and products.

Complex structure and the process requires special tooling, new

processing methods. Due to the variety of shapes and molds, with

high precision dimension, therefore the needs of additional process

such as EDM, WEDM and precision grinders was required.

Therefore, a special die EDM has been developing rapidly. This

method does not require the tool material and application of

mechanical forces in the process, but the direct use of electrical,

chemical, light and sound on the work piece can be processed to

achieve the shape and size and the surface roughness of the request.

Practice has proved that with the right process, the surface

roughness improvement had doubled the die life span, increased by

50%, which is particularly important for the plastic mold.

Electr

Mac

S f T t t


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Surface Treatment


Why strive for a high surface finish?

The increased use of plastic products has created a

higher demand for mirror finish of molding tools. The

highest demands for surface finish are in the optical

lens mold whereby an extreme requirement on

polishability is desired.

However, in general there are other advantages for

high surface finish, including:

a) Easier ejection of the plastic parts from the

molding tool

b) Reduced risk of local corrosion

c) Reduced risk of fracture or cracking due to

temporary overloading or pure fatigue.

www.acerosuddeholm.com/spanish/files/polishing-english.pdf

S f T t t (2)


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Judging surface finish Two things are important when judging the surface of the

mold:

a) The surface must first have a geometrically correctshape without any long macro waves.

b) Secondly, the mirror finish of the mold surface must be

free from scratches, pores, orange peel, pitting (pin-

holes) etc.

The surface finish is normally judged by the naked eye. There

are certain difficulties involved in such a visual evaluation. A

flatsurface can look perfect despite the fact that it is notgeometrically completely flat. Thus, the eye can be fooled.

In more sophisticated cases, the finish can be judged by

instrumental methods, such as optical interference techniques.

Surface Treatment (2)

Optical I nterf e

S f T t t (3)


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Factors which affect polishability

The surface smoothness which can be achieved by

polishing steel depends on factors such as:

a) Tool steel quality

b) Heat treatment

c) Polishing technique.

In general, it can be stated that polishing technique is the

most important factor.

If a suitable polishing technique is used, it is almost

possible to achieve acceptable results, providing the

correct heat treatment and the good quality tool steel is

used.

However, if an unsuitable technique is used, even the best

steels can be ruined.

Surface Treatment (3)

Rating of hardness and

Mold Development Process


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Mold Development Process


Initial

Design

Review part design

and specifications

Develop preliminary

mold design and

quote

Feed System Design

Structural System

Design

Ejector System

Design

Cooling System

Design

Machining, Polishing,

Assembly and Trials

Moldings

OK?

Source: David O. Kazmer,Injection Mold DesignEngineering, Carl Hanser Verlag, 2007.

Type and classification of plastic mate


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Type and classification of plastic mate


Polymers

All plastics are polymers, but not all polymers are plastics.

The simplified diagram below shows the relationship between monomers and polymers.

Identical monomers can combine with each other to form homopolymer, which cbranched chains.

Different monomers may combine together to form copolymers, which also may be bran

Type and classification of plastic materi


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The chemical properties of a polymer depends on:

The type of monomer or monomers that make up the polymer. The

chemical properties of homopolymer 1 are different from those of

homopolymer 2 or the copolymers. The arrangement of monomers within the polymer. The chemical

properties of the straight polymers are different from those of the

branched polymers

The monomers that are found in many plastics include organic compounds

like ethylene, propylene, styrene, phenol, formaldehyde, ethylene glycol,

vinyl chloride and acetonitrile.

Because there are so many different monomers that can combine in many

different ways, it can make many kinds of plastics.



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Two types of plastic are thermoplastic and

thermosetting.

For thermoplastics the chemical structure remainsunchanged during heating and shaping. It can be

reused after re-melting.

Examples of Thermoplastics

Acrylonitrile butadiene styreneABS

Polycarbonate - PC

Polyethylene - PEPolyethylene terephthalate - PET

Poly(vinyl chloride) - PVC

Poly(methyl methacrylate) - PMMA

Polypropylene - PP

Polystyrene - PS

Expanded PolystyreneEPS



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For thermosets, it will undergo a curing process

during heating and shaping, causing a permanent

change (cross-linking) in molecular structure. Once

cured, they cannot be re-melted.

Examples of Thermosets

Epoxide (EP)

Phenol-formaldehyde (PF)

Polyurethane (PUR)

Polytetrafluoroethylene - PTFE

Unsaturated polyester resins (UP)



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Melting and solidification are the two phenomena that

frequently occur in nature and in many technological

processes. Good examples of the phenomena that occur in

nature are ice melting and water freezing, ground freezing(the uppermost surface layer), solidification of vulcanic

lava and the melting processes that evolve deep under the

earth surface. Examples of the phenomena that occur in

many technological processes are the freezing and thawing

of food products, casting, production of plastic products,

welding, electrolytic machining and thermal energy

accumulators, which make use of the metal or wax meltingor freezing phenomena. In the cases when pure metals, ice

or eutectic alloys undergo a phase change, one can observe

a clear cut line between a liquid and a solid and a definite

melting point tm

Melting and solidification (2)


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Short question

What is the difference between liquid and fluid?





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Polymer Melts

To shape a thermoplastic polymer it must beheated so that it softens to the consistency of

a liquid

In this form, it is called apolymer melt

Important properties of polymer melts:

Viscosity

Viscoelasticity


Polymer meltis forced to flow

Polymer meltspinning



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Viscosity of Polymer Melts

Fluid property that relatesshear stress to shear rateduring flow

Due to its high molecular weight, apolymer melt is athick fluid with high viscosity

Most polymer shaping processes involve flow throughsmall channels or die openings

Flow rates are often large, leading to high shear ratesand shear stresses, so significant pressures are

required to accomplish the processes. Viscosity of a polymer melt decreases with shear rate,

thus the fluid becomes thinner at higher shear rates


Viscosity relationships fotypical polym



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Viscosity and Temperature

Viscosity decreases with temperature, thus thethinner at highertemperatures fluid becomes

Viscoelasticity

It is the property of a material that determines thestrain it experiences when subjected to combinationof stress and strain

Possessed by both polymer solids and polymer melts

Example: die swell in extrusion, in which the hotplastic expands when exiting the die opening


Viscosity as a function of temselected polymers at a shear r

Shrinkage


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Shrinkage


When plastic material is injected into a cavity, it starts to

cool down, whereby its volume decreases. A measure for

this volume decrease is given by the difference between

melt density and solid density. As cooling rates in thecavity are very high and non-uniform, the frozen material

will also incorporate internal stresses. These stresses may

relieve after ejection from the cavity, which process can be

accelerated by keeping the part at elevated temperatures.

Shrinkage can be defined by the following formula:

S = (D - d) / D ( 100%).

D = dimension of mould cavity.

d = dimension of moulded part.Mold shrinkage percen

type of ma

Shrinkage (2)


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Shrinkageto be distinguished are:

shrinkage in flow direction ;

shrinkage normal to flow;

shrinkage in thickness direction.

The sum of these three shrinkages must be equal to the volumetric

shrinkage of a material, which can be obtained from the difference

between melt density and solid density, or from pVT-diagrams.

Besides on material, shrinkage is further dependent on processing

conditions, (such as injection-speed, hold-pressure, hold-pressure time,

runner/gate-dimensions and mold temperature), on part shape, (during

injection the flow-direction may change) and part thickness, (thicker

parts have usually a thicker central layer with less orientation). The

shrinkage contribution caused by stress relieve after ejection from the

mould is called Post Mold Shrinkage.

g ( )

3 Dimens

Shrinkage (3)


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Shrinkage Factors

g ( )

Fillers in the plastictend to reduce shrinkage

Injection pressure higher pressures force more material

into mold cavity to reduce shrinkage

Compaction time- similar effect

longer time forces morematerial into cavity to reduce shrinkage

Molding temperature- higher temperatures lower polymer

melt viscosity, allowing more material to be packed into

mold to reduce shrinkage.

Documents

BDD 40903 Injection Mold Design Chapter 1.pdf