Processing: Injection Molding
molding is the primary method used for forming thermoplastic shapes. Resin is
fed into the hopper of a screw-type machine and is heated and melted in a screw
barrel. In modern machines the screw performs melting and transport functions
and serves as a plunger mechanism. When an appropriate amount of melted material
(often referred to as "shot" or "charge") is delivered to
the mold, screw rotation ceases and the screw moves forward to inject the material
into the mold and seal the opening. Once the resin has set, the mold is opened,
the part ejected, and the process repeated. This is referred to as a cycle.
The parts emerging
from the mold are generally finished, requiring no additional work. Details such
as ribs or screw threads can be formed during the molding process. Metal parts,
such as screw driver blades, may be placed in the mold and bonded to the injected
plastic material - a procedure known as insertion molding.
usually requires both heat for melting the plastic resin and cold water chilling
for the mold (in the case of thermoplastic resins) and the hydraulic system.
There are three
specialized subsets of injection molding:
(CIM) / Sandwich Molding - injects one plastic as a skin and a compatible
plastic as the core of a part into the same mold.
Common Applications: typewriter keys, foam core automotive components with hard
plastic outer shells
Injection Molding (MIM) - combines dissimilar materials, even non-plastics,
in the same mold resulting in inseparable components.
Common Applications: glass encased in clear plastic, ball joints which combine
a plastic socket with a stainless-steel ball
Injection Molding (RIM) - mixes reactive resin components just prior
to their insertion into the mold. Once mixed, a rapid chemical reaction sets the
Common Applications: hundreds of automotive parts including pickup truck beds