Plastic Injection Molding

aboundingdriprockUrban and Civil

Nov 29, 2013 (6 years and 7 months ago)


Plastic Injection Molding

Injection Molding

3 major functional units; injection, mold, clamping

Plastic Injection Molding

is a

technique for making parts
thermoplastic and thermoset material

In contrast to the extrusion (which makes
continuous parts of constant cross section),
injection molding make discrete parts (with
complex and variable cross section)

Molten plastic is injected at high

into a
, which is the inverse of the desired shape.

The mold is made from metal, usually either


widely used for manufacturing a variety of parts,
from the smallest component to entire body
panels of

The key to success in injection molding
are to have;

Proper machine for good melting and injecting
of the resin

The proper resin to appropriate part

A good mold for part definition and removal

Proper operation for efficient molding cycle
(mold cycle depends on the design of the
mold and manufacturing parameters)

Plastic Injection Molding

The most commonly used thermoplastic
materials are;


cost, lacking the strength and
longevity of other materials)

ABS or
acrylonitrile butadiene styrene

(a co
or mixture of compounds used for everything from
Lego parts to electronics housings)


(chemically resistant, heat
resistant, tough and

used for combs)


(tough and flexible

used for

PVC (more common in extrusions as used for pipes,
window frames, or as the insulation on wiring where it
is rendered flexible by the inclusion of a high
proportion of

Injection Unit

Purpose: to liquify the
plastic materials and
then inject the liquid
into mold

Resin is introduced
through hopper

Some machines can
have several hoppers (to
fed filler, colorants, other
molding act as mixer

However, due to limited
size of barrel, mixing
capability is poor

From hopper

hole (feed throat)

Barrel made of heavy steel cylinder to
withstand the pressure and temperature
involved in melting the resin

2 types of system used in injection

Reciprocating screw

similar to extruder screw
but with unique reciprocating action

Ram injector

Injection Unit

Design of screw

similar to an extrusion screw

3 sections;

Feed section

to advance the resin

Compression section

to melt the resin

Metering section

to homogenize the resin and pump it forward

The screw of injection molding machine is shorter than
extruder, L/D ratios are 12:1 and 20:1

Low L/D ratios suggest the mixing is less efficient in the
injection molding machine

The compression ratio (diameter of root at feed zone to
the diameter of root at metering zone) often in the range
of 2:1 and 5:1

Low compression ratio means less mechanical action is
added during melting process

Injection Unit

Important measure of the size of an
injection molding is weight of resin that
can be injected, called shot size

Typical shot size range from 20g to 20 kg

Since shot size depends on the density of
the plastic, PS has been chosed as the
standard for rating the machine

Injection Unit

Reciprocating Screw Injection
Molding Machine

Resin is melt by mechanical shear and thermal
energy from heaters

The molten resin is conveyed to a space at the
end of the screw

collects in a pool

Here, the mold is closed

The entire screw move forward and pushes the
molten resin out through the end of barrel

To ensure the resin does not flow backward, a
check valve

nonreturn valve

is attached to the
end of screw

Normally the screw will stay in the forward
position, until resin began to harden in the mold

Reciprocating Screw Injection
Molding Machine

Retraction of the screw, create space at the end of the

Cooling of the part in the mold, until it can be removed

While the part is cooling, the screw turns and melts
additional resin

Reciprocating Screw Injection
Molding Machine


More uniform melting

Improved mixing or additives and dispersion
throughout the resin

Lower injection pressure

Fewer stresses in the part

Faster total cycle

Reciprocating Screw Injection
Molding Machine

In this type of injection molding, the resin is fed from a
hopper into the barrel, and heated through thermal
energy from the heaters

The molten resin is collect in a pool in a barrel celled
injection chamber

The molten resin is then push forward by the action of
plunger (ram or piston)

To five better mixing, the molten resin is pushed past a
torpedo/spreader, impart shear to the melt

Ram Injection

Injection Molding Machine


Designing and making mold for injection
molding is more complicated than making
extrusion die

Mold Parts

mold is placed in between
stationary plate and the moveable plate

The connection from the injection unit to the
mold is through the nozzle

The channel that run through the stationary plate
of the mold is called the sprue channel (material
that is in the channel is called the sprue)

The solid sprue is removed from the finished
part assembly after the part is ejected from the

Resin flow from the sprue through the runner
(connecting channel) to the mold cavities


Mold Bases

Assembly of various
mold parts

Mold bases can be
purchased as entire
units, then the
cavities are cut from
A & B plates


Distribution system for the resin from the sprue
to the cavities

Flow characteristics (viscosity), temperature and
other factors are important in determining the
runner diameter and length

If the diameter of the runner is too small or the
length is too long,the resin can freeze in the
runner before the mold is completely full

If the runner system is too large, excess material
would be ejected and too much regrind created

If the resins have a high viscosity, larger runners
are needed compared to low viscosity resin

The optimum flow of the resin through the
runner system depends on the shape and
diameter of the channel

Round channel give the best flow characteristics
but difficult to machine

Machining cost can be reduce by machining one
side of the mold plates

Better shape where the depth of the channel is
at least two
thirds the size of the width and the
sides are tapered between 2 to 5

Runner Channel Shapes

Secondary Runners

Secondary runner
channel are used for
multicavity molds

The flow into the
secondary channel
should be
streamlined (angle in
flow direction)

The streamlined
minimizes shear on
the resin


The end of runner and the entry path into
the cavity

The gate shape can also affect the filling
of the cavity, dimension and properties of
the parts

Gate is the most restricted point in
injection molding system, i.e. for
reinforcement and filler + polymer systems

Gate Design

Small rectangular opening at
the end of the runner channel,
connect to the edge of cavity

Edge gate can be below the
parting line if the channel and
part are also below the parting

Or it can be symetricaly about
the parting line, if the runner
channel and part are at both
side of parting line

Edge gate

Submarine Gate

Starts from the edge of the
runner, and goes into the cavity
edge at an angle

It narrows to a point as it moves
from the runner to the cavity

The advantage; separation of
the parts and the runner is

Disadvantage; gate cannot be
used for some resins because
of high shear

Submarine Gate

Tab Gate

By connecting the runner directly
into the cavity with no reduction
in runner cross

Used for very large parts where a
reduction in flow would disturb
the resin’s flow pattern and might
result inadequate flow into the

Fan Gate

Made by reducing the
thickness and not the
diameter of the runner
channel as it goes into the

Used for intermediate size,
and when reinforcement in
the resin cannot flow through
the edge gate

Ring Gate

Used to make hollow
cylinder parts

The ring gate covers the
entire top of the cylinder
part so that the resin flow
is downward into the wall
of the part


Are actual molding locations

Resin enter the cavities through gate, fills
the cavities, and cools to form the solid.
The parts are ejected and finished

Cavities are the heart of the molding
process, and must be precisely prepared

The shape of the cavities determines the
shape of the part

Materials & Product Consideration

Almost all thermoplastic can be injection

Resin with low melt viscosity is required;
so that the flow through runner, gate,

easily done with minimum
injection pressure

Resin with injection molding grades have
low molecular weight and narrow
molecular weight distribution


Hollow parts can be created by allowing
the moveable plate to protrude into the
cavity of the stationary plate

Threads can be placed on the inside of a
part by using a core pin that is inserted
into the cavity where the threads are


A hollow part with a hole on the side is even
more complicated (the core pin is used)

The core pin slide into position after the mold is

The core pin seals against the surface of the
moveable plate, prevent flow of resin into the