ABSTRACT
Since most engineering plastics are consumed in the injection-molding process, it would be helpful for those concerned with selecting the best candidate resins for their applications to understand the basics of the process. The purpose of the extruder barrel and screw as a unit is to convert the solid plastic pellet fed from the hopper into a homogenous melt that can be forced under high pressure into a closed mold. In the transition zone, the channel depth decreases until it ends at the metering zone, where the channel depth of the screw is the shallowest, normally 30—40% as deep as the feed zone. The barrel for a reciprocating screw machine must be able to contain at least 20,000 psi of injection pressure, which can be generated during operation of the machine. Pressure generated by the hydraulic motor is then diverted to force the screw forward, injecting the material into the mold cavity.
