One of the reasons that
injection molding is so popular is that it creates lightweight parts,
of specific color, with built in surface texturing, in a single
operation of the injector.
The plastic injection
molding manufacturing process can
be broken down into six steps.
- It all starts with
plastic pellets or powders (Plastics are also available in sheet,
plate, rod, and tubing form).
- These plastic pellets
or powder or granules are fed by a vibrating hopper into the heated
cylinder where it forms a melt.
- The melt is then pushed
into a split die mold through a nozzle by using a hydraulic plunger
or a rotating and reciprocating screw as shown in the figure below.
- As soon as the pressure
in the mold increases, the plunger or screw stop but continue
to apply higher pressure for the melt to fully fill the mold cavity
giving the part good dimensional stability and tolerance.
- The clamping force
is applied hydraulically or electrically opposite to the nozzle.
At this stage, the mold is held and allowed to cool. For thermoplastics,
the cooling off period is 5 to 60 seconds, however for thermosets,
it could be several minutes.
- Next, the female and
male portion of the mold are separated and the part is ejected
either by a built in ejector pins or a human worker.
above are two types of plastic injection molding machines: (a)
Plunger Injection Moling and (b) Reciprocating rotating screw