Injection-mold-design

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BDD 40903
INJECTION MOLD 
DESIGN
BDD 40903 Prepared by : Mohd Hilmi Othman 1
CHAPTER 1
• Introduction(2 hours)
• Injection molding concept
• Mold structure and components
• Mold type
• Mold material
• Surface treatment
• Mold development process
• Type 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)
Injection molding concept (3)
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Injection Mold Design
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Injection molding concept (4)
Mold structure and components
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Mold structure and components (2)
Mold type
Standard Molds
The standard mold is the most simple design, basically
divided in two side :
a) Cavity side
Cavity side is the side that construct to flowing plastic
material from nozzle to cavity parts, basically they consist
of sprue, runner.
b) Core side
Core side construct to make shape for core, de-molding
system and ejection system, at this side it the ejection
system will be designed.
Standard mold have one parting line, and have one
opening direction. This type of mold use in all kinds of
plastic parts that doesn't have undercut, inner and outer
screw.
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http://bp1.blogger.com/_oF_1Qcfku_w/R2xigNC7BEI/AAAAAAAAAV8/7AApHY-BIbg/s1600-h/standard-2-plate-mold.jpg
http://bp1.blogger.com/_oF_1Qcfku_w/R2xigNC7BEI/AAAAAAAAAV8/7AApHY-BIbg/s1600-h/standard-2-plate-mold.jpg
Slide mold
Development from this mold type is the used slider parts in various molds types; basic slide mold is transfer
horizontal movement of mold to vertical movement, this types of molds is used to make parts with
undercut.
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Mold type (2)
http://bp2.blogger.com/_oF_1Qcfku_w/R2xs7dC7BFI/AAAAAAAAAWE/8O_7UtSrmZc/s1600-h/slider-mold.jpg
http://bp2.blogger.com/_oF_1Qcfku_w/R2xs7dC7BFI/AAAAAAAAAWE/8O_7UtSrmZc/s1600-h/slider-mold.jpg
Three plate mold
Basically three plate molds has two parting line, and
floating plate, whereby the floating plate was supported
by support pin. Since the mold has two parting planes,
the runner system can be located on one side of floating
plate or by making a special plate that attach in floating
plate, known as runner plate. Three plate molds are
used because of their flexibility in gating location. This
types of molds is flexible even use in multiple cavity.
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Mold type (3)
http://bp2.blogger.com/_oF_1Qcfku_w/R2xxRdC7BGI/AAAAAAAAAWM/rDA-ApWqEp4/s1600-h/3plate.jpg
http://bp2.blogger.com/_oF_1Qcfku_w/R2xxRdC7BGI/AAAAAAAAAWM/rDA-ApWqEp4/s1600-h/3plate.jpg
Split Cavity Mold
Basically the split cavity is same as 2 plate standard
mold, but the cavity have split block to make undercut
product or external threads. This type of mold is used
when the slider is not enough to make the undercut or
the threads. the disadvantages of this mold is when use
for little parts, the construction will difficult
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Mold type (4)
http://bp3.blogger.com/_oF_1Qcfku_w/R2zVhtC7BHI/AAAAAAAAAWU/EhBk6H2Fe9w/s1600-h/split-cavity.jpg
http://bp3.blogger.com/_oF_1Qcfku_w/R2zVhtC7BHI/AAAAAAAAAWU/EhBk6H2Fe9w/s1600-h/split-cavity.jpg
Mold Material
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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.
Mold Material (2)
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How Material Selection Effects Mold Design
One aspect to consider is that each material has its own specific shrinkage factors. Every material with the
exception of water expands when it is heated and contracts when it is cooled. In injection molding the
contraction was labeled as the materials shrinkage. The shrinkage factor of the material is taken into
account when designing the cavity of the mold so that when the part contracts it will meet the expected
dimensional requirements of the original design.
Some materials dissipate heat differently than others, which can result in variation in material cooling
during the molding process. This has a big impact on how the water cooling lines are placed in the mold to
ensure even cooling.
Another related factor is the viscosity of the material. Viscosity is the measurement of the thickness of the
material in its liquid form. Higher viscosity means thicker material, and greater injection pressure to push it
through the mold. Also these materials require larger diameter in the runners and gates to allow the material
to flow easily into the cavity. The melt index is a good tool in determining the flowability of a particular
plastic.
Mold Material (3)
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http://injectionmold.cc/mold-design-and-manufacturing-process
Mold design and manufacturing process
Studies have shown that life of the mold depends on heat
treatment, material selection, construction, machining and
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 treatment 
Selection and mold structure 
Process problems
Lubricants and equipment 
Failure of mold design 
Mold Material (4)
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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.
zinc alloy
bismuth-tin alloy
Mold Material (5)
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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.
Mold Material (6)
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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.
Electrical Discharge 
Machining (EDM) 
Machine
Surface Treatment
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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
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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 correct
shape 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
“flat” surface can look perfect despite the fact that it is not
geometrically 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 Interference Techniques.
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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 mirror polishability
Mold Development Process
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Initial 
Design
Review part design 
and specifications
Develop preliminary 
mold design and 
quote
Layout Design
Project OK?
Feed System Design
Structural System 
Design
Ejector System 
Design
Cooling System 
Design
Machining, Polishing, 
Assembly and Trials
Moldings 
OK?
Close 
Project
Source: David O. Kazmer, “Injection Mold Design Engineering”, Carl Hanser Verlag, 2007. (Page 14)
Type and classification of plastic materials
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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 can be straight or
branched chains.
• Different monomers may combine together to form copolymers, which also may be branched or straight.
Type and classification of plastic materials (2)
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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.
Type and classification of plastic materials (3)
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Two types of plastic are thermoplastic and
thermosetting.
For thermoplastics the chemical structure remains
unchanged during heating and shaping. It can be
reused after re-melting.
Examples of Thermoplastics
Acrylonitrile butadiene styrene – ABS
Polycarbonate - PC
Polyethylene - PE
Polyethylene terephthalate - PET
Poly(vinyl chloride) - PVC
Poly(methyl methacrylate) - PMMA
Polypropylene - PP
Polystyrene - PS
Expanded Polystyrene – EPS
Type and classification of plastic materials (4)
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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)
Melting and solidification
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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 melting
or 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
Short question
• What is the difference between liquid and fluid?
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Melting and solidification (2)
Polymer Melts
• To shape a thermoplastic polymer it must be
heated so that it softens to the consistency of
a liquid
• In this form, it is called a polymer melt
• Important properties of polymer melts:
• Viscosity
• Viscoelasticity
Melting and solidification (3)
Polymer melt is forced to flow through a die 
Polymer melt spinning 
Viscosity of Polymer Melts
• Fluid property that relates shear stress to shear rate
during flow
• Due to its high molecular weight, a polymer melt is a
thick fluidwith high viscosity
• Most polymer shaping processes involve flow through
small channels or die openings
• Flow rates are often large, leading to high shear rates
and 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
Melting and solidification (4)
Viscosity relationships for Newtonian fluid and 
typical polymer melt.
Viscosity and Temperature
• Viscosity decreases with temperature, thus the
thinner at higher temperatures fluid becomes
Viscoelasticity
• It is the property of a material that determines the
strain it experiences when subjected to combination
of stress and strain
• Possessed by both polymer solids and polymer melts
• Example: die swell in extrusion, in which the hot
plastic expands when exiting the die opening
Melting and solidification (5)
Viscosity as a function of temperature for 
selected polymers at a shear rate of 103 s-1.
Shrinkage
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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 the
cavity 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 percentage for different 
type of material
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Shrinkage to 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.
Shrinkage (2)
3 Dimension pVT-diagrams
Shrinkage Factors
Shrinkage (3)
Fillers in the plastic tend to reduce shrinkage
Injection pressure – higher pressures force more material
into mold cavity to reduce shrinkage
Compaction time - similar effect – longer time forces more
material into cavity to reduce shrinkage
Molding temperature - higher temperatures lower polymer
melt viscosity, allowing more material to be packed into
mold to reduce shrinkage.