n (1932) The method of forming objects from granular or powdered plastics, most often thermoplastics, in which the material is fed from a hopper into a screw-type plasticator (or heating cylinder in elderly machines), after which the screw or a ram forces the molten compound into a chilled mold. Pressure is maintained until the mass has hardened sufficiently for removal from the mold. In a variation called flow molding, a small additional amount of melt is forced into the mold during cooling of the initial charge to offset shrinkage. Machines employing screws for plastication (as must do now days) are either single- or double-stage. In single-stage machines, plastication and injection are done in the same cylinder, injection pressure being generated by forward motion of the screw while rotation is stopped. This process is called reciprocating-screw injection molding. In double-stage machines, the material is plasticated by a constantly rotating screw that delivers the melt through a check valve to an accumulator cylinder, from which it is injected into the mold by a piston. This process is called screw-and-piston injection molding. See also Reaction Injection Molding, Hot-Tip-Gate Molding, and Two-Shot Injection Molding.
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