Injection molding is the most commonly used
manufacturing process for the fabrication of plastic parts. A
wide variety of products are manufactured using injection
molding, which vary greatly in their size, complexity, and
application. The injection molding process requires the use of
an injection molding machine, raw plastic material, and a
mold. The plastic is melted in the injection molding machine and
then injected into the mold, where it cools and solidifies into
the final part. The steps in this process are described in
greater detail in the next section.
Injection molding is used to produce thin-walled plastic parts
for a wide variety of applications, one of the most
common being plastic housings. Plastic housing is a thin-walled
enclosure, often requiring many ribs
on the interior. These housings are used in a variety of products including household
appliances, consumer electronics, power tools, and
as automotive dashboards. Other common thin-walled products include different types
of open containers, such as buckets.
Injection molding is also used to produce several everyday items
such as toothbrushes or small plastic toys. Many medical
devices, including valves and syringes, are manufactured using
injection molding as well.
Injection molding overview
The process cycle
for injection molding is very short, typically
between 2 seconds and 2 minutes, and consists of the
following four stages:
- Clamping - The two mold halves are securely closed
- Injection - The raw plastic is melted and injected into the mold
- Cooling - The molten plastic solidifies into the shape of the part
- Ejection - The mold halves separate and the part is ejected
Injection molding overview
Stage 1: Clamping
Prior to the injection of the material into the mold,
the two halves of the mold must first be securely closed
by the clamping unit. Each half of the mold is attached
to the injection molding machine and one half is
allowed to slide. The hydraulically powered
clamping unit pushes the mold halves together and
exerts sufficient force to
keep the mold securely closed while the material is
injected. The time required
to close and clamp the mold is dependent upon
the machine - larger machines (those with
greater clamping forces) will require more time. This
time can be estimated from the dry cycle time
of the machine.
Injection molding clamping
Stage 2: Injection
The raw plastic material,
usually in the form of pellets, is fed into the
injection molding machine, and advanced
towards the mold by the injection unit. During
this process, the material is melted by heat and
The molten plastic is then injected into the mold
very quickly and the buildup of pressure packs and holds
the material. The amount of material that is injected is
referred to as the shot. The injection time is difficult to calculate
accurately due to the complex and changing flow of the molten
plastic into the mold. However, the injection time can be estimated
by the shot volume, injection
pressure, and injection power.
Injection molding injection
Stage 3: Cooling
The molten plastic that is inside the mold begins to
cool as soon as it makes contact with the interior mold
surfaces. As the plastic cools, it will solidify into
the shape of the desired part. However, during cooling
of the part may occur. The packing of
material in the injection stage allows additional material to flow
into the mold and reduce the amount of visible shrinkage. The mold
can not be opened until the required cooling time has elapsed. The cooling
time can be estimated from several thermodynamic properties of the
plastic and the
maximum wall thickness
of the part.
Stage 4: Ejection
After sufficient time has passed, the cooled part may be
ejected from the mold by the ejection system, which is
attached to the rear half of the mold. When the mold is
opened, a mechanism is used to push the part out of the
mold. Force must be applied to eject the part because
during cooling the part shrinks and adheres to the mold.
In order to facilitate the ejection of the part, a mold
release agent can be sprayed onto the surfaces of the
mold cavity prior to injection of the material. The time
that is required to open the mold and eject the part can
be estimated from the dry cycle time of the
machine and should include time for the
part to fall free of the mold. Once the part is ejected, the mold
can be clamped shut for the next shot to be injected.
Injection molding ejection
After the injection molding cycle, some
post processing is typically required. During cooling, the material
in the channels of the mold will solidify attached to the part. This excess material,
along with any flash that has
occurred, must be trimmed from the part, typically by using cutters.
For some types of material, such as thermoplastics, the scrap material
that results from this trimming can be recycled
by being placed into a plastic grinder, also
called regrind machines or granulators, which regrinds
the scrap material into pellets. Due to some
degradation of the material properties, the
regrind must be mixed with raw material in the
proper regrind ratio to
be reused in the injection molding process.
Injection molding cycle
1. Which part is most likely to be injection molded?
- Soda bottle
- Bicycle handle grip
- Remote control cover
- Solid toy ball
Answer: c - Injection molding is not well suited for parts with
enclosed spaces (soda bottle), rubber parts (bicycle handle grip),
or thick parts (solid toy ball). Thin plastic covers and housings
are typically injection molded.
2. How long does it take to injection mold a typical part?
- 1 second
- 30 seconds
- 3 minutes
- 6 minutes
Answer: b - Typical injection molding cycle times range from 2 seconds
to 2 minutes, with most parts requiring around 30 seconds.
3. What is the correct process cycle?
- Injection, Cooling, Clamping, Ejection
- Injection, Clamping, Cooling, Ejection
- Clamping, Cooling, Injection, Ejection
- Clamping, Injection, Cooling, Ejection
Answer: d - The correct process cycle is clamping, injection, cooling, ejection
4. What happens in the clamping stage?
- The front mold half is pushed towards the rear mold half
- The rear mold half is pushed towards the front mold half
- Both mold halves are pushed towards each other
- The mold is one piece and is clamped in place
Answer: b - The rear mold half is pushed by the clamping unit towards the
front mold half, which remains stationary.
5. In what form is the raw plastic fed into the injection molding machine?
- Molten plastic is poured into the machine
- Solid plastic pellets are poured into the machine
- Solid plastic sheets are loaded into the machine
- One solid block is loaded into the machine
Answer: b - The raw plastic material for injection molding is typically small
solid pellets that are poured into the machine. In some cases, granulated plastic powder can be used.
6. What primarily determines the cooling time of a part?
- The thickness of the part
- The volume of the part
- The complexity of the part
- The number of parts in the mold
Answer: a - The cooling time depends primarily upon the maximum thickness of the part and the
material being used. The volume, complexity, and quantity of parts do not greatly effect the cooling time.
7. How is the part removed from the mold?
- The part falls free from the mold on its own
- The machine operator pulls the part out manually
- An ejection machine reaches into the mold and pulls out the part
- A mechanism attached to the mold pushes out the part when it opens
Answer: d - The ejection system is attached to the rear half of the mold and is
pushed forward into the part as the mold opens.
8. What post-processing is typically required for injection molded parts?
- Milling to improve tolerances
- Grinding to improve surface finish
- Trimming to remove excess material
- Painting to add color
Answer: c - Injection molded parts have material from the mold
channels and flash attached, which must be trimmed. Molded parts
offer high tolerances and surface finish and rarely require milling
or grinding. Colorant can be added to the molten plastic to add color,
so painting isn't required either.
Injection Molding Machines
Injection molding machines have many
components and are available in different
configurations, including a horizontal configuration and
a vertical configuration. However, regardless of their
design, all injection molding machines utilize a power
source, injection unit, mold assembly, and clamping unit
to perform the four stages of the process cycle.
Injection molding machine - Overview
The injection unit is responsible for both
heating and injecting the material into the mold. The first part of
this unit is the hopper, a large container into which the raw
plastic is poured. The hopper has an open bottom, which allows the
material to feed into the barrel. The barrel contains the mechanism
for heating and injecting the material into the mold. This mechanism
is usually a ram injector or a reciprocating screw. A ram injector
forces the material forward through a heated section with a ram or
plunger that is usually hydraulically powered. Today, the more
common technique is the use of a reciprocating screw. A
reciprocating screw moves the material forward by both rotating and
sliding axially, being powered by either a hydraulic or electric
motor. The material enters the grooves of the screw from the hopper
and is advanced towards the mold as the screw rotates. While it is
advanced, the material is melted by pressure, friction, and
additional heaters that surround the reciprocating screw. The molten
plastic is then injected very quickly into the mold through the nozzle at the end of the barrel
by the buildup of pressure and the forward action of
the screw. This increasing pressure allows the material to be packed
and forcibly held in the mold. Once the material has solidified
inside the mold, the screw can retract and fill with more material
for the next shot.
Injection molding machine - Injection unit
Prior to the injection of the molten plastic
into the mold, the two halves of the mold must first be securely closed by the
clamping unit. When the mold is attached to the injection molding
machine, each half is fixed to a large plate, called a
platen. The front half of the
mold, called the mold cavity, is mounted to a stationary platen and
aligns with the nozzle of the injection unit. The rear half of the mold, called the mold
core, is mounted to a movable platen, which slides along the tie
bars. The hydraulically powered clamping motor
actuates clamping bars that push the moveable platen
towards the stationary platen and exert sufficient force to
keep the mold securely closed while the material is
injected and subsequently cools. After the required
cooling time, the mold is then opened by the clamping
motor. An ejection system, which is attached to the
rear half of the mold, is actuated by the ejector bar
and pushes the solidified part out of
the open cavity.
Injection molding machine - Clamping unit
Injection molding machines are typically
characterized by the tonnage of the
clamp force they provide. The required
clamp force is determined by the
of the parts in the mold and the pressure with which the material is
injected. Therefore, a larger part will require a larger
clamping force. Also, certain materials that require
high injection pressures may require higher tonnage
machines. The size of the part must also comply with
other machine specifications, such as
minimum mold thickness, and platen
Injection molded parts can vary greatly in size
and therefore require these measures to cover a very large range. As
a result, injection molding machines are designed to each
accommodate a small range of this larger spectrum of values. Sample
specifications are shown below for three different models
(Babyplast, Powerline, and Maxima) of injection molding machine
that are manufactured by Cincinnati Milacron.
|Clamp force (ton)
|Shot capacity (oz.)
||0.13 - 0.50
||8 - 34
||413 - 1054
|Clamp stroke (in.)
|Min. mold thickness (in.)
|Platen size (in.)
||2.95 x 2.95
||40.55 x 40.55
||122.0 x 106.3
Injection molding machine
1. The injection unit includes which of the following components?
- Stationary platen, Mold cavity, Mold core
- Sprue, Runner, Gate
- Hopper, Barrel, Nozzle
- Clamping bar, Ejector box, Ejector pins
Answer: c - Components of answer A are part of the mold assembly, answer B
is part of the mold channels, and answer D is part of the clamping unit. The
injection unit includes the hopper, screw motor drive, reciprocating screw,
barrel, and nozzle.
2. In which component, is the raw plastic material melted?
Answer: b - The raw material is poured into the hopper, which feeds into the barrel, where it
is then melted by heat and pressure. The material is already melted when it passes through
the nozzle into the mold.
3. What is the primary function of the reciprocating screw?
- Storing the raw plastic to be melted
- Melting the raw plastic
- Pushing the molten plastic into the mold
- Opening and closing the mold
Answer: c - The hopper stores the raw material, which is then melted by the heated barrel. The
reciprocating screw pushes the molten plastic into the mold, which is opened and closed by the
4. What is the function of the clamping unit
- Opening the mold
- Closing the mold
- Applying force to keep the mold closed
- All of the above
Answer: d - The clamping unit is responsible for both opening and closing the
mold and maintains the application of force during injection to keep the mold closed.
5. On which component is the mold core mounted?
- Moveable platen
- Stationary platen
- Mold cavity
- Clamping bar
Answer: a - The mold core, which is part of the rear half of the mold, is mounted on the moveable platen,
which is pushed by the clamping bar. The mold cavity is mounted on the stationary platen.
6. What is the primary function of the tie bars?
- Providing a surface for the mold halves to be mounted in the machine
- Pushing the mold core towards the mold cavity
- Keeping the mold halves clamped together
- Guiding the mold core towards the mold cavity
Answer: d - The platens provide a surface for the mold halves to be mounted and the
clamping bars push the mold halves together and keep them closed. The tie bars run
through both mold halves and platens and guide the mold core in a straight path towards the mold cavity.
7. Which factor does NOT affect the amount of required clamping force?
- Part size
- Production quantity
- Number of parts per mold
Answer: b - The clamping force is determined by the projected area
of the parts in the mold (affected by the part size and number of parts
in the mold) and the injection pressure (affected by the material being injected).
The total number of parts being molded does not affect the amount of clamping force required.
8. If you were to mold a very large, very thin disc, why would most machines be incompatible?
- The shot capacity must be very high
- The clamp stroke must be very long
- The minimum mold thickness must be very thick
- The platen size must be very large
Answer: d - A very thin disc will have a small volume and therefore
require a small shot capacity. Also, a flat part will not require a large
clamp stroke or mold thickness, which are both affected by the part depth.
A large part, in terms of length and width, will require a large platen size to
accommodate the mold. Also, for such a part, the clamping force is a concern
because of the large projected area.