Environmental Engineering Reference
In-Depth Information
4.3 PROCESSING METHODS FOR COMMON
THERMOPLASTICS
4.3.1 Injection Molding
This is the most popular processing technique employed by industry and is
similar to metal die casting. Injection molding machines essentially consists
of two parts: a heated barrel and a cooled two-part cavity mold that opens
to allow removal of the product. The barrel carries a long reciprocating
screw that transfers the plastic pellets from the hopper at the base of the
screw forward toward the mold. The barrel and screw are heated to melt
the plastic resin into a highly viscous liquid. A large amount of mechanical
power is needed to melt, transfer, and compress the plastic in the barrel.
Once sufficient quantity of melted plastic (or the “shot”) is transported into
the nozzle area in front of the screw, the screw itself acts like a piston to
ram it into the cavity mold. The ramming pressure is held just long enough
for the plastic melt in the mold to consolidate, cool, and solidify before
the screw now rotates back to position itself for another cycle. The mold is
rapidlycooled(usuallywithchilledwater)tosolidifythemeltintoaproduct.
When the mold opens steel pins built into it push out the formed now cold
part. The mold recloses and snugly fits against the gate through which the
melt is delivered, in anticipation of the next charge of melt to be delivered
into it.
Injection molding machines are rated according to the clamping force
available to keep the mold closed against the ramming pressure of the melt
driven by the screw mechanism.
Figure 4.5
illustrates the basic parts of an
injection molding machine, and
Figure 4.6
shows an actual machine and
some typical molded products. With multicavity molds, some of the melt
is inevitably used up to fill the runner areas of the mold. However, after
recovering the product, the runner waste is recycled.
