Transcript Plastic Injection Molding - Universiti Sains Malaysia

Plastic Injection Molding
Injection Molding
3 major functional units; injection, mold, clamping
Plastic Injection Molding
• is a manufacturing technique for making parts
from thermoplastic and thermoset materials
• 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 pressure into a
mold, which is the inverse of the desired shape.
• The mold is made from metal, usually either
steel or aluminium
• widely used for manufacturing a variety of parts,
from the smallest component to entire body
panels of cars
Plastic Injection Molding
• 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
performance
– 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)
• The most commonly used thermoplastic
materials are;
– polystyrene (low-cost, lacking the strength and
longevity of other materials)
– ABS or acrylonitrile butadiene styrene (a co-polymer
or mixture of compounds used for everything from
Lego parts to electronics housings)
– nylon (chemically resistant, heat-resistant, tough and
flexible - used for combs)
– polypropylene (tough and flexible - used for
containers
– 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 plasticiser).
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
additives)-Injection
molding act as mixer
– However, due to limited
size of barrel, mixing
capability is poor
Injection Unit
• 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
molding;
– 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
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
Reciprocating Screw Injection
Molding Machine
• 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 or 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
screw
• 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
• Advantages
– More uniform melting
– Improved mixing or additives and dispersion
throughout the resin
– Lower injection pressure
– Fewer stresses in the part
– Faster total cycle
Ram Injection- 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
Molds
• 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
Molds
• 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
mold
• 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
Runners
• 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 5º.
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
Gates
• 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
line
• 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
automatic
• 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-section
• Used for very large parts where a
reduction in flow would disturb
the resin’s flow pattern and might
result inadequate flow into the
cavity
Fan Gate
• Made by reducing the
thickness and not the
diameter of the runner
channel as it goes into the
cavity
• 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
Cavities
• 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
molded
• Resin with low melt viscosity is required;
so that the flow through runner, gate,
cavity – easily done with minimum
injection pressure
• Resin with injection molding grades have
low molecular weight and narrow
molecular weight distribution
Shapes
• Hollow parts can be created by allowing
the moveable plate to protrude into the
cavity of the stationary plate
Shapes
• 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
desired
Shapes
• 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
closed
• The core pin seals against the surface of the
moveable plate, prevent flow of resin into the
area