Plastic Injection Molding

aboundingdriprockUrban and Civil

Nov 29, 2013 (3 years and 11 months ago)

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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 material
s


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




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)

Plastic Injection Molding


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



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

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

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

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

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


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

Molds

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 5
º.

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



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

Shapes