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Plastics Processing: Injection Molding

Injection 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.

This process 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:

Coinjection (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

Multicomponent 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

Reaction Injection Molding (RIM) - mixes reactive resin components just prior to their insertion into the mold. Once mixed, a rapid chemical reaction sets the plastic.
Common Applications: hundreds of automotive parts including pickup truck beds