3. It 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
4. 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
5. 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
6. 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
7. 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
8. 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
9. 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
10. 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
11. Advantages
More uniform melting
Improved mixing or additives and dispersion
throughout the resin
Lower injection pressure
Fewer stresses in the part
Faster total cycle
12. 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
13. 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.
14. Obviously, the mold must be cooled in order for the
production to take place. Because of the heat capacity,
inexpensiveness, and availability of water, water is used
as the primary cooling agent. To cool the mold, water
can be channeled through the mold to account for quick
cooling times. Usually a colder mold is more efficient
because this allows for faster cycle times. However, this
is not always true because crystalline materials require
the opposite: a warmer mold and lengthier cycle time.
15. 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)
16. 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).
18. Injection molding is one of the most important
processes for plastics and it has a very wide list
of kinds of products it can produce, which makes
it very versatile.
