The injection unit of injection molding machine is much like an extruder. The injection unit melts the polymer resin and injects the polymer melt into the mold. It consists of a barrel that is fed from one end by a hopper containing a supply of plastic pellets. The unit may be: ram fed or screw fed.
The injection unit consists of a granulate hopper, cylinder, screw, nozzle, heating bands and hydraulic drives and serves the purpose of melting and injecting the moulding material.
The reciprocating-screw machine is the most common. This design uses the same barrel for melting and injection of plastic.
The alternative unit involves the use of separate barrels for plasticizing and injecting the polymer. This type is called a screw-preplasticizer machine or two-stage machine. Plastic pellets are fed from a hopper into the first stage, which uses a screw to drive the polymer forward and melt it. This barrel feeds a second barrel, which uses a plunger to inject the melt into the mold. Older machines used one plunger-driven barrel to melt and inject the plastic. These machines are referred to as plunger-type injection molding machines.
Screw position at the end of the dosage process; the plasticized material is in front of the screw tip. Screw position after the injection process; the plasticized material is injected into the mould. A material cushion is left in front of the screw for injection into the mould during the holding pressure phase.