New eBook "An Introduction to plastic injection molding"
An Introduction to
A resource to help designers, engineers and purchasing professionals
navigate the world of plastic injection molding
A publication of
Table of Contents
Chapter 1 – Plastics and society
Chapter 2 – Types of plastic molding
Chapter 3 – Key ingredients to achieving perfect parts
Chapter 4 – The basics of an injection molding machine
Chapter 5 – Cold runners versus hot runner systems
Chapter 6 – Determining the cost of an injection mold
Chapter 7 – Common part defects
Glossary of terms
Learn more about us
We developed this eBook with designers,
engineers and purchasing specialists in
mind. It is written to provide a basic
understanding of plastic injection molding
presses, processes and costs. Our goal is
to make our customers more
knowledgeable about what goes into
making a plastic part. We hope you find
this eBook informative and useful. Please
feel free to share it with your colleagues.
PLASTICS AND SOCIETY
Plastics in modern society
Many products that are a part of everyday life go unnoticed, either
because they are components of larger items or they are so commonly
used that little thought is given to their existence. Items manufactured
by plastic injection molding often fall into this category. Looking around
a home or business, you will find many products that exist because of
the injection molding process. From toys to cars, plastics play an
Plastic injection molding is a process of forming this durable, resinous
material into just about any form of fashion imaginable. The first
injection molding machine was invented and patented by brothers John
and Isaiah Hyatt in 1872. It resembled a large hypodermic needle, with
a heated cylinder through which a large plunger forced the gooey mass
into a mold. Today, the process is more complicated although, the basic
principle of plastic being injected into a waiting mold is still the same.
One of the biggest advancements has come by way of the materials
used, and there are now thousands of different formulations available
for making ‘plastic.’ Raw materials used in the plastic injection molding
process include thermoplastics, thermosets, and elastomers. Also
called polymers or resins, there are more than 20,000 unique
formulations that can be injected into molds to produce parts with
specific properties to be utilized for specific purposes. Examples of
common thermosetting plastics include polymers such as epoxy and
phenolic. Common thermoplastics are nylon, polyethylene and
Injection molding machines are fairly simple and straightforward,
consisting of a hopper where raw material is placed, a heating cylinder
and an injection plunger. Molds are typically made from steel or
aluminum. Major advantages to using plastic injection molding for the
manufacture of parts include:
• Ability to complete high-production rates
• Repeatability of high tolerances
• Labor costs
• Minimal material loss
• Minimal finishing
• Wide range of materials available for specific applications
Injection molding is the most common plastic molding process and is
used to create a huge variety of complex parts of different size and
shape. Whether it’s a snowboard or a vinyl window part being produced,
injection molding is efficient and economical, especially if large numbers
of items are being made. Highly complex parts can be produced at a low
cost. The only real disadvantage is the initial start-up costs.
“When you think
about it, plastics
play a very
important role in our
lives and the
products we use
TYPES OF PLASTIC MOLDING
Types of plastic
In today’s manufacturing environment, plastics are being used to make
everything from automotive body parts to human body parts. Each application
requires a special manufacturing process that can mold the part based on
specifications. This article provides a brief overview of the different types of
molding and their advantages and applications.
Blow Molding – Well suited for hollow objects, like bottles
The process follows the basic steps found in glass blowing. A parison (heated
plastic mass, generally a tube) is inflated by air. The air pushes the plastic against
the mold to form the desired shape. Once cooled, the plastic is ejected.
The blow molding process is designed to manufacture high volume, one-piece
hollow objects. If you need to make lots of bottles, this is the process for you.
Blow molding creates very uniform, thin walled containers. And, it can do so very
Compression Molding – Well suited for larger objects like auto parts
The name of this molding method says everything. A heated plastic material is
placed in a heated mold and is then compressed into shape. The plastic can be in
bulk but often comes in sheets. The heating process, called curing, insures the
final part will maintain its integrity. As with other molding methods, once the part
has been shaped, it is then removed from the mold. If sheeting plastic material is
used, the material is first trimmed in the mold before the part is removed.
This method of molding is very suitable to high-strength compounds like
thermosetting resins as well as fiberglass and reinforced plastics. The superior
strength properties of the materials used in compression molding make it an
invaluable process for the automotive industry.
Extrusion Molding – Well suited for long hollow formed applications like
tubing, pipes and straws
While other forms of molding uses extrusion to get the plastic resins into a
mold, this process extrudes the melted plastic directly into a die. The die
shape, not a mold, determines the shape of the final product. The extruded
“tubing” is cooled and can be cut or rolled for shipment.
Injection molding - Well suited for high-quality, high-volume part
Injection molding is by far the most versatile of all injection molding
techniques. The presses used in this process vary in size and are rated based
on pressure or tonnage. Larger machines can injection mold car parts.
Smaller machines can produce very precise plastic parts for surgical
applications. In addition, there are many types of plastic resins and additives
that can be used in the injection molding process, increasing its flexibility for
designers and engineers.
The process itself is fairly straightforward; however, there are many
enhancements and customization techniques that can be used to produce
the desired finish and structure. Injection molds, which are usually made
from steel, contain cavities that will form the parts. Melted plastic is injected
into the mold, filling the cavities. The mold is cooled, and the parts are
ejected by pins. This process is similar to a jello mold which is filled then
cooled to create the final product.
The mold making costs in this method are relatively high; however, the cost
per part is very economical. Low part cost along with resin and finish options
have all contributed to injection molding’s popularity in today’s
Rotational Molding (Rotomolding) – Well suited for large, hollow,
This process uses high temperatures and rotational movement to coat the
inside of the mold and form the part. The constant rotation of the mold creates
centrifugal force forming even-walled products. Because it is ideally suited to
large hollow containers, such as tanks, it is not a fast moving process. However,
it is a very economical process for particular applications and can be cheaper
than other types of molding. Very little material is wasted using this process,
and excess material can often be re-used, making it an environmentally viable
Each type of molding has its strengths and weaknesses. Designers and
engineers need to understand these differences and the production options
available. There are always several approaches to a final manufacturing
solution. The molding company who consults on a specific project should be
able to provide additional insights into the applications and materials that are
best suited to an individual project.
“There are always several
approaches to a final
FIND OUT HOW THE RODON
GROUP CAN HELP YOU WITH
YOUR NEXT PROJECT
Answer some simple questions about your project and we will call you to
schedule a consultation.
Key ingredients to achieving
perfect plastic parts
The adage “If it can go wrong, it will go wrong” should never be true in the
world of injection molding. In fact, problems can be easily avoided from the
very beginning as long you are working with a turnkey precision molding
manufacturer. Some companies opt to use a separate firm to design the
mold, then contract another vendor to build the mold (often these are
offshore mold builders) and another to run the parts. By separating these
responsibilities, they often sacrifice control, accountability and quality.
Often problems arise at the very beginning of a project. You may have
drawings and even a prototype, but without the expert advice of design
engineers who understand how to optimize an injection mold, you may
experience costly defects and delays.
Many designers begin their careers as tool makers before gaining the
knowledge and experience needed in CAD/CAM systems to become
engineers. This expertise helps to develop molds that will perform at the
highest production levels. The design team you choose to work with should
conduct a Design for Manufacturability (DFM) analysis to ensure your parts
meet the highest quality standards.
Choosing the right material for a project is one of the most important factors
in creating perfect parts. The advances in polymer science have helped create
a wide variety of resins to choose from based on the final application of the
part. It is important to work with an injection molder that has experience with
a wide range of resins and applications including resins that are compliant with
FDA, RoHS, REACH and NSF. Reputable companies will have established
strategic relationships with the best resin suppliers in the country. They
should have a great deal of experience using certified commodity and
engineering resins that adhere to stringent manufacturing standards.
Mold Building and Testing
Without careful attention to mold design, the end product may be non-
conforming. It is important to create molds that accommodate enough draft
for the selected resin and finish, for example. Plastic injection molders should
create pre-production molds. These molds offer several benefits to the design
and manufacture process. They are single cavity molds that are created using
the same 3D software and tools as production molds; however they are made
with less durable metal and steel. Pre-production molds can be modified to
help determine the best production solution for the project including finishes
and coloration. Various resins can also be “tested” in this environment.
Alternatively, SLA models can be created using 3-D printers, though these
parts cannot be used as pre-production samples.
Once the pre-production sample parts have passed rigorous quality
inspections and have satisfied the client’s expectations, the manufacturer
can move onto the final stage of tooling, creating the production mold.
Production and Quality
Once the multi-cavity production molds are completed, a
full cycle of samples are produced and checked for the
quality standards outlined by the client. Adjustments are
made as required before full production begins. Quality
checks continue throughout the part production
process. Working with a “Just in Time” manufacturer,
they can monitor and adjust quantities.
At The Rodon Group, we maintain our client’s inventory in-
house until they require the parts. Our production molds
are built to last, and we guarantee them for as long as we
manufacture the parts. Our clients return year after year
because our injection molds maintain their integrity. We
use 420 stainless steel on all of our molds, and our highly-
trained operators insure each tool is properly maintained
to maximize quality and output.
The Rodon Group is ISO 9001:2008 certified and we are
very proud of our 99.8% part acceptance rate.
“At Rodon, our production molds
are built to last and we
guarantee them for as long as
We manufacture the parts.”
BASICS OF A PLASTIC
Basics of an injection
While plastic injection molders will help you determine the size of the
machine needed to get the best results, a project designer or engineer can get
a good estimate based on some basic information. By knowing approximately
what size machine will be required, you can better source a plastic injection
molder that will meet your needs.
First, let’s take a quick look at how plastic injection molding presses are
rated or classified.
Often plastic injection companies will provide a molding equipment list on
their website. It may look something like this:
3- 68 Ton Injection Molding Presses
5- 123 Ton Injection Molding Presses
5- 154 Ton Injection Molding Presses
5- 202 Ton Injection Molding Presses
5- 233 Ton Injection Molding Presses
4- 400 Ton Injection Molding Presses
So, what does this mean?
Plastic injection molding presses are classified or rated based on tonnage, or
more specifically, the clamping pressure or force. Presses can run in size from
less than 5 tons of clamping pressure to over 4000. The higher the press ton
rating, the larger the machine.
A machine rated for 68 tons can deliver 68 tons of clamping pressure. This
pressure keeps the mold closed during the injection process. Too much or too
little pressure can cause quality issues. Too much or too little pressure can
also cause flashing, where excess material appears on the part edge. Pressure
also impacts the viscosity of the plastic being used in the project. Melt Flow
Index or MFI is a measure of the ease of flow of the melt of a thermoplastic
polymer. Plastic compounds react differently to pressure based on their
MFI. The higher the MFI, the higher the pressure needed.
Second, let’s figure out how much clamping force or pressure is required.
There are many factors that are taken into consideration when determining
the size of the press. The size of the part, the polymer being used and
something called the safety factor. The safety factor is an additional
numerical percentage buffer that is added to the calculation to help avoid
defects in the final part. Some recommend adding 10% to allow for the safety
factor. As mentioned earlier, the MFI (Melt Flow Index) of the plastic
compound will also impact the pressure needed to produce the part. Many
calculations include the platen size as well as the mold and part size, however,
to get an estimate of the press size your project will need, we have simplified
it even further.
Many plastic injection professionals use a general rule of thumb of 2.5 times
the surface square inches of the part to be produced. So, if you have a part
with 42 square inches than you would need a press size with 105 tons of
pressure. If you add 10% for a safety factor, you will need to use a press with
a minimum of 115 tons of clamping force. A press size of 120 tons would be
able to accommodate your plastic injection molded product.
Lastly, let’s look at how you can identify a plastic injection molder that is right
for your project.
Once you have an estimate of the press size you will need, you can identify
plastic injection molding companies that will meet your requirements. In
general, molders with a greater number and wider selection of press sizes will be
able to accommodate the needs of your project. If you are not working with a
completed mold, look for a plastic injection company who can design and build
the mold. They will have a better understanding of how to maximize the
manufacturing process and will often offer tooling allowances. This, in turn, will
minimize the overall cost of your project. In the end, your plastic injection
molder will determine which machine would be best suited for your project.
Larger presses can accommodate larger molds and multi-cavity molds often
reducing the cost per part. However, larger molds are more expensive. Choosing
the right press size can balance the upfront tooling expenditures with long-term
COLD RUNNER VS.
HOT RUNNER SYSTEMS
Cold runner vs. hot runner
Every plastic part starts in a mold. Molds are classified into two main types,
cold runner and hot runner. Each has its advantages and
disadvantages. Your plastic injection molder will be able to give you the
costs and benefits of using these different systems. However, by
understanding the key differences of these technologies, you can have a
more educated discussion on the type of mold that would best fit your
First, let’s discuss cold runner molding systems
Cold runner molds usually consist of 2 or 3 plates that are held within the
mold base. The plastic is injected into the mold via the sprue and fills the
runners which lead to the parts in the cavity. In 2 plate molds, the runner
system and parts are attached, and an ejection system is used to separate
the pair from the mold.
For those of you who assembled a model car at some point in your youth,
the runners and the parts were not separated. The child assembling the
model was responsible for that final part of the process. In 3 plate molds, the
runner is contained on a separate plate, leaving the parts to be ejected
alone. In both systems, the runner is recycled and reground, reducing plastic
waste. However, these processes can increase cycle time.
Hot runner molding systems
Hot runner molds consist of 2 plates that are heated with a manifold
system. The manifold sends the melted plastic to nozzles which fill the part
cavities. There are several types of hot runner systems, however, in general,
they fall into two main categories; externally heated and internally
heated. The externally heated systems are well suited to polymers that are
sensitive to thermal variations. Internally heated systems offer better flow
The hot runner process eliminates runners entirely, so recycling and regrind
(which can only be done with virgin plastics) do not impact cycle times. A
variation of this system is called an insulated runner. The insulation, rather
than heat, keeps the plastic in a molten state. This system can only
accommodate a few types of plastics, specifically semi-crystalline polymers
which have a low thermal conductivity.
Let’s look at a list of advantages and disadvantages of each type of
injection molding system.
Cold runner systems
-Comparatively cheaper to produce and maintain
-Accommodate a wide variety of polymers, both commodity and engineered
-Color changes can be made quickly
-Fast cycle times if the systems include robotic assist in removing runners
-Cycle times are slower than hot runner systems
-Plastic waste from runners (if they cannot be reground and recycled)
Advantages and disadvantages continued
Hot runner systems
-Potential faster cycle times
-Eliminates runners and potential waste
-No need for robotics to remove runners
-Can accommodate larger parts
-More expensive molds to produce
-Color cannot be easily changed
-Higher maintenance costs and potential downtime
-May not be suited to certain thermally sensitive materials
Professionals in the field of plastic injection molding should be your primary
resource for determining the best injection molding system for your
project. Look for injection molders who are familiar with all types of plastics
processing. They will be able to provide you with a cost/benefit analysis of
the various systems available based on the part and the material used.
machine at work at
The Rodon Group
“It pays to work with injection molders
who are familiar with all types of
FIND OUT HOW THE RODON
GROUP CAN HELP YOU WITH
YOUR NEXT PROJECT
Answer some simple questions about your project and we will call you to
schedule a consultation.
DETERMINING THE COST OF AN
Cost of an injection mold
Key determining factors
A common question for designers and engineers is “How much will a plastic
injection mold cost?” It makes sense. Injection molds represent the
greatest expense in upfront production costs. And, there are many factors
that go into determining the cost. With any custom injection molding
project, your injection molder will be able to give you the final price tag. In
this article, we will review the variables that can impact the cost so that
you can be better informed in making a mold purchasing decision.
Not all quotes are created equal
Procurement and purchasing managers have the unenviable task of
obtaining quotes from a few mold makers for each project. Depending on
the input (in terms of drawings, prototypes or sample parts), the cost
quotes can vary greatly. Designers should look at all of these inputs and
determine the best molding solution. They may re-design the part to
maximize manufacturing efficiency and increase the number of parts that
can be made with each molding cycle. Generally, molds made with tighter
tolerances, more cavities and a longer production life will take longer to
build and will cost more upfront. The savings with a high-quality mold are
long-term. These molds require less maintenance and last longer than
lower quality molds.
Here are some variables that impact the cost of a plastic injection mold.
The core metal. For shorter production runs, some mold makers will use
molds made from aluminum. This is a perfectly reasonable choice if you will
not need the mold to perform long-term. However, if a project requires that
a mold lasts for several years, an aluminum mold may cost more in the long-
The number of cavities. It is pretty intuitive when you think about it. Fewer
cavities in a mold require less tooling work and time and ultimately less
cost. A reputable experienced molder will be able to maximize cavitations in
the mold to maintain the highest level of productivity. In general, most
molders recommend creating one mold per part versus creating a family
mold. Family molds are created with various cavities for assorted
parts. They tend to produce inferior products and have more downtime due
to maintenance issues.
Mold base. Think of the mold base as a case that holds all of the mold
cavities, inserts and components together. The cost of the base is estimated
based on the size of the mold and the type of steel used to make the base as
well as the customization required. Most mold bases come in standard sizes
and are further machined to meet the requirements of a specific project.
Core/Cavity machining. All molds must also be customized. Customization
includes the placement of cores, cavities, ejectors, cooling lines, etc. The
steel used in the tool also impacts cost. Hardened steel molds lasts the
longest and are more expensive to machine. Once done, however, they have
a long production life.
Part complexity. Just as the number of cavities plays a role in determining
the cost of the mold, so does part complexity. This includes the surface
finish of the final part as well as the number of undercuts required. Parts,
which demand tight tolerances, also contribute to the mold complexity.
Turnkey or vertically integrated injection molders.
Some mold builders also manufacture the parts. This type of integration can
help defray the mold building cost. Often full service molding manufacturers will
subsidize a portion or all of the cost of the mold based on the full term and
value of the manufacturing contract. They will amortize the cost of the mold so
they can maintain profit margins while providing the lowest possible per piece
cost to their clients.
The cost of a quality injection mold is certainly a major expense. However, tight-
tolerance, precision molds that are made from the best steel available should
last for years to come. The upfront cost must be calculated or amortized into
the lifetime value of the project. Will these parts be in production for several
years or several months? Does the project require a high-volume of parts? Are
faster cycle times required? If you answered yes to these questions, then the
initial investment in a quality mold will lower the per part cost and will end up
saving money in the long-run.
“Injection molds represent the
greatest expense in upfront
COMMON PART DEFECTS
Common parts defects
When purchasing injection molded parts, it is important to understand some of
the common problems and defects that impact product quality. Being familiar
with these imperfections and their causes can help you work with injection
molders to insure the highest quality part production is achieved.
Most defects in plastic parts can be traced back to three sources:
1. The material being used to make the part
2. The processing of the material in the mold
3. The mold itself
We have grouped the defects together by their most common cause; however
one or more factors may contribute to a defect.
Common defects linked to the plastic resins or additives being used to
manufacture a part include:
• Color streaks – Just like the name implies, color streaks are random areas of color change
that are often attributed to the non-uniform mixing of resins and colorants.
• Delamination – This defect appears as a flaky surface layer on the part and is often
caused by contamination or moisture in the resin pellets.
• Discoloration - This can occur when the hopper and feed zone have not been flushed
properly to remove any residual color.
• Embedded contaminates – Particles or flecks of residual foreign material that can
originate in the barrel of the press.
• Splay marks or silver streaks – Circular marks appearing where the molten plastic enters
the mold cavity. This is often caused by excessive moisture in the resin.
Common defects linked to the processing of the plastic resin in the mold
• Blistering – Raised imperfections that are generally caused improper heating and/or
cooling or by gas/air bubbles.
• Burn marks – Black or brown blemishes (which are carbon deposits) that are caused by
improper ventilation or prolonged heating in the mold.
• Cold slugs – A small non-uniform area on the part caused by an improperly heated piece
of plastic becoming attached to the part.
• Flow marks – A wavy pattern or discoloration caused by a slow injection speed which
allows the material to cool too quickly.
• Sink marks or shrinkage voids – Depressions or hollows in a part that can be attributed to
excessive press pressure, non-uniform heating, inadequate cooling time or part design.
• Stress cracking or stress crazing – This defect usually occurs as a result of over exposure
to a high temperature.
• Stringing – A thin strand of material attached to a part generally caused by a nozzle that is
Common defects linked to improper mold design and/or maintenance
• Drag marks – Scratches that occur when the part is ejected from the mold. This is usually
due to an improperly designed ejector system or one that is out of alignment.
• Flash or burrs – A thin lip or protrusion beyond the body of the part that is generally
caused by poor clamping force, improper mold design and/or mold damage.
• Jetting –A snakelike line of material that cools independently of the material around it.
This defect is generally due to poor tool design often relating to incorrect gate size and
length or placement.
• Short shot – An incomplete part due to lack of a filled mold. This problem is often
attributed to a blockage or improper injection pressure.
• Warping – A part with a distorted shape can be due to a poor cooling system in the mold.
When the plastic material is cooled unevenly, the result is a bowing effect.
Most of the defects listed here can be addressed by making changes to the
processing, the material or the mold itself. The best way to avoid these part
defects is to work with a plastic injection molder that has a great deal of
experience with various resins and their applications. Using a turn-key
manufacturer, who also builds and maintains the molds can help avoid any
costly machining charges or mold replacements.
“When purchasing injection molded parts, it is
important to understand the common problems
and defects that impact product quality.”
Plastic injection molding may not be rocket science, but it comes pretty
close. There are hundreds of terms used in the industry. We have chosen to
highlight the most common nomenclature used when discussing mold parts,
materials and problems.
Additives – These compounds are added to resins to improve the overall performance and
appearance of finished products.
Alloy – A plastic alloy is a physical modification of an existing plastic to achieve higher
performance and or functionality. These alloys are often used in the automobile industry and to
replace metal parts.
Annealing - Annealing is the heating and slow cooling of a plastic part which allows the polymer
chains to recoil and relieve internal stresses.
Assembly – A secondary manufacturing function of joining finished parts together.
Backing plate – A plate, which supports the mold, pins and bushings in the injection machine.
Back Pressure – The resistance of a melted plastic to move forward. This impacts not only the
temperature but the cycle time as well.
Blister – As the name says, this is a part defect which appears as a small bubble or blister on the
surface of a part and it generally created by improper heating or cooling of by gas or air bubbles.
Blow molding – The process follows the basic steps found in glass blowing. A parison (heated
plastic mass, generally a tube) is inflated by air. The air pushes the plastic against the mold cavity
to form the desired shape. Once cooled, the plastic is ejected. This method is used to make
Bridge tool – An injection mold that makes parts until the final tool is completed. These molds or
tools are not meant to be production tools.
Bubbles – Similar to blisters, gas pockets, or voids that have formed inside the plastic.
Cavity - The machined shape within a mold which creates the form of the plastic part.
Check valve – A device located at the end of the injection screw. The check valve makes sure that resin
does not flow back into the machine after it is pumped into the mold.
Clamp – The mechanism that holds the mold in location during the molding process.
Cold slug – A defect characterized by a small non-uniform area on the part caused by an improperly
heated piece of plastic becoming attached to the part.
Colorant – A pigment system, usually in pelletized form, powder or liquid, which is mixed with resin to
produce the desired color.
Compression molding - The name of this molding method says everything. A heated plastic material is
placed in a heated mold and is then compressed into shape. The plastic can be in bulk but often comes
in sheets. The heating process, called curing, insures the final part will maintain its integrity. This
molding method is often used to make large objects such as automobile components.
Copolymer - A polymer derived from more than one type of monomer.
Core - A protrusion or set of matching protrusions, which form the
inner surface of a plastic part.
Core pull – A device that retracts a core in a direction that is not
parallel to the opening direction of the mold. Often referred to
as a side action, this functionality assists in the manufacture of
more complex parts.
Crazing – A defect that causes small cracks often caused by
over-stressing the plastic material.
Creep – The “set” that a molded part takes under stress,
and does not return to its original shape. Also known as “memory”.
Crush Ring – A contact ring on the inner surface of the sprue bushing used to eliminate nozzle leakage.
Cure – The process of allowing a plastic to harden or stabilize.
Cushion – A space between the screw and the nozzle that provides a pressure pathway to enabling
packing out the part.
Cycle – The time it takes for the plastic injection process to complete a finished part.
Degassing – Opening and closing of a mold to allow gas to escape. Trapped gas and/or air can cause
parts defects such as blistering.
Delamination - This defect appears as a flaky surface layer on the part and is often caused by
contamination or moisture in the resin pellets.
Density – Mass per unit volume of a substance.
Dimensional stability - Ability of a plastic part to retain the precise shape in which it was molded.
Draft – The angle or degree of taper in a side wall to help facilitate removal of the parts from the mold.
Ejection pin – Metal rods in the mold which push the parts from the mold.
Ejector return pins – Pins that push the ejectors back into position once the parts have been released.
Ejector rod – A bar that engages the ejector assembly and pins when the mold opens.
Elasticity – The ability of a material to return to its original state when stretched.
Elastomer – A rubber-like material, which is highly elastic.
Extrusion – The process of forming tubes or continuous shapes by pushing melted material through a
Fabricating – The process of manufacturing plastic products through various molding and forming
Family mold – A mold which contains cavities for various parts.
Fan gate – A gate with a wider width that helps reduce warping through stress.
Fill – The packing of material into the mold.
Fill Imbalance – Generally occurs in multi-cavity molds, when the plastic material fails to fill all of the
Filler – An inert additive that adds strength or hardness to a part.
Finish – The surface texture to a part.
Flash gate – An alternative to a fan gate, which conveys the melted resins into a thinner gate
section creating a linear melt flow into the cavity.
Flash or burrs – A thin lip or protrusion beyond the body of the part that is generally caused by
poor clamping force, improper mold design and/or mold damage.
Flow marks - A wavy pattern or discoloration caused by a slow injection speed which allows
the material to cool too quickly.
Flow rate – The volume of material passing a fixed point per unit time.
Gate – The channel into which melted plastic flows into a mold.
Gate seal or freeze – The holding time and pressure needed to insure the plastic material in
the mold is set.
Hardness – The resistance of a material to compression, indentation and scratching.
Hot-runner mold – Hot runner molds consist of 2 plates that are heated with a manifold
system. The manifold sends the melted plastic to nozzles which fill the part cavities.
Hot to cold mold - Using a hot runner system to gate into a cold runner which in turn feeds
each cavity. This configuration can be used when full cold runner molds aren't viable and full
hot runner molds are cost prohibitive.
Injection blow molding - A blow molding process in which the parison to be blown is formed
by injection molding.
Injection molding – A manufacturing process in which melted plastic is injected into a mold to
form a part.
Insert – An object, such as a magnet or screw, which is inserted into the molded part.
Intensification ratio – A calculation used to convert hydraulic pressure to plastics pressure
Line of draw - The parallel direction of the moving platen.
Machine shot capacity – The maximum volume of resin which can be injected in a single stroke.
Masterbatch – A solid or liquid additive for plastic used for coloring plastics or imparting other properties
Melt temperature – Melt temperature is the actual temperature of the polymer as it exits the nozzle and
enters the mold
Memory – The action of plastic returning to its previous size and form.
Mold – A hollow form that plastic is injected or inserted into to
manufacture a plastic part.
Mold release – A surface preparation used to aid in the ejection of the
part from the mold.
Molecular orientation - The manner in which polymer chains position themselves in the mold cavity.
Polymers near the wall of the mold orient themselves by straightening out, while polymers near the
center tend to stay coiled.
Multi-shot molding – A process where two or more plastic substances are injected into the mold to form
a part. Toothbrushes are often manufactured using this technique.
Non-return valve – Also called a check ring, a valve that allows rapid material shut off for part weight
consistency; and a smooth, high-flow profile to prevent material degradation.
Nozzle - The hollow-cored metal nose screwed into the injection end of the barrel which forms a seal
Orange peel – A patchy rough surface defect caused by moisture in the mold cavity, or by incomplete
Over molding – A two-shot process, in which two plastic substances, are injected into a mold sequentially,
usually a harder base material with a coating of softer material.
Parting line – A line on a part formed when the two sides of the mold come together.
Pinpoint gate – A very small gate, used in hot runner molds, to control the flow of material.
Plastic – A polymeric substance of large molecular weight.
Plasticity - The quality of being easily shaped or molded.
Platens – Steel plates in the molding machine onto which the mold is fastened.
Polymer - A substance that has a molecular structure consisting chiefly or entirely of a large number of
similar units bonded together, e.g., many synthetic organic materials used as plastics and resins.
Projected area – The area of the mold that will be filled with plastic at the mold’s parting line.
Prototype tool – Also called a soft tool, a preliminary mold built to produce prototype parts and used
to make adjustments to the final production tool.
Purging – The process of cleaning the injection machine of remnant color or materials prior to running
a new part.
Ram – A plunger-like part which pushes the melted material into the mold.
Reciprocating screw – An injection molding machine mechanism which compresses melts, and
conveys the material to the sprue and mold.
Regrind – In thermoplastic resins, scrap material that is ground and recycled back into finished parts.
Relative viscosity - Peak hydraulic pressure fill time
Release agent – A compound, which is sprayed on the mold, or as an additive, molded into the part to
help facilitate the release of the part.
Retainer plate – A plate onto which the removable parts of the mold are mounted.
Runner system – The channel system that allows the flow of the melted material to fill the part
Screw decompression – Also called “Suck Back” the action of the screw return toward the hopper to
eliminate drooling of the melted plastic from the nozzle.
Short shot – A defect where the material does not fully fill the part cavity.
Shot – A complete cycle of the injection machine.
Shrinkage – The amount of volume reduction that takes place when a plastic material cools.
Side-action, slide or cam - a mechanical, pneumatic or hydraulic action within the mold that forms a
plastic part detail that is not in the line of draw of the mold.
Sprue – The opening feed that conveys material from the nozzle to runner system in the mold.
Thermoplastic - A material that can be heated and cooled repeatedly without changing the material
structure. Highly recyclable.
Sprue bushing – This part seals tightly against the nozzle of the injection barrel of the molding machine
to allow molten plastic to flow from the barrel into the mold.
Thermoset – A material, which when heated, is pressed or molded into a shape. The heating process
changes the structure of these materials, and for this reason they cannot by re-heated.
Tie bars - Bars which provide structural support to the mold in the press. The spacing between the tie
bars dictates the size of the mold that can be placed into the injection machine. The mold opens and
closes riding on the tie bars.
Toggle – A mechanism that is used to mechanically close the mold, as opposed to hydraulic clamping.
Tool – The mold used to form plastic parts in an injection machine.
Undercut – Can be a design flaw that results in an indentation or protrusion that inhibits the ejection of
the part from the mold. Other times undercuts are designed into a mold to ensure a part holds onto
the correct side of the mold.
Velocity – The speed at which the plastic enters the cavity of the mold.
Vent – A channel from the mold cavity that allows gas and air to escape as resin is being injected into
the cavity to prevent many types of defects from occurring.
Viscosity – The resistance to liquid flow.
Weld line - Also called a knit line, the juncture where two flow fronts meet and are unable to join
together during the molding process. These lines usually occur around holes or obstructions and cause
localized weak areas in the molded part.
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A resource guide to plastic manufacturing for designers, engineers and purchasing agents.