The injection molding of thermoplastics can be subdivided into a several stages. At the plasticity stage, the feed unit operates pretty much as an extruder, melting and homogenizing the material in the screw/barrel system. The screw however is allowed to retract, to make room for the molten material reservoir. At the injection state, the screw is used as a ram for the rapid transfer of the molten material from the reservoir to the cavity between the two halves of the closed mold. Since the mold is kept at a temperature below the solidification temperature of the material, it is essential to inject the molten material rapidly ensure complete filling of the cavity. A high holding packing pressure is normally exerted, to partially compensate for the thermal contraction of the material upon cooling. After the cooling stage, the mold can be opened and the solid product removed.