It is usually slow and inefficient to mold thermoplastics using the compression molding techniques described above. In particular, it is necessary to cool a thermoplastic part before removing it from the mold, and this requires that the mass of metal making up the mold also be cooled and then reheated for each part. Plastic Injection Molding is a method of overcoming this inefficiency. Injection molding resembles transfer molding in that the liquefying of the resin and the regulating of its flow is carried out in a part of the apparatus that remains hot, while the shaping and cooling are carried out in a part that remains cool. In a reciprocating screw injection molding machine, material flows under gravity from the hopper onto a turning sc