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One of the reasons that injection molding is so popular is that it creates lightweight parts, of specific color, with built in surface texturing, in a single operation of the injector.

The plastic injection molding manufacturing process can be broken down into six steps.

1. It all starts with plastic pellets or powders (Plastics are also available in sheet, plate, rod, and tubing form).

2. These plastic pellets or powder or granules are fed by a vibrating hopper into the heated cylinder where it forms a melt.

3. The melt is then pushed into a split die mold through a nozzle by using a hydraulic plunger or a rotating and reciprocating screw as shown in the figure below.

4. As soon as the pressure in the mold increases, the plunger or screw stop but continue to apply higher pressure for the melt to fully fill the mold cavity giving the part good dimensional stability and tolerance.

5. The clamping force is applied hydraulically or electrically opposite to the nozzle. At this stage, the mold is held and allowed to cool. For thermoplastics, the cooling off period is 5 to 60 seconds, however for thermosets, it could be several minutes.

6. Next, the female and male portion of the mold are separated and the part is ejected either by a built in ejector pins or a human worker.

Shown above are two types of plastic injection molding machines: (a) Plunger Injection Moling and (b) Reciprocating rotating screw Injection molding