What is Injection Molding?

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What is Injection Molding?

There are several processes in plastic manufacturing including extrusion, blow molding, rotational molding, thermo-forming, casting, 3D printing, and injection molding. While injection molding is one of the most common, the process used is determined by the design of the part.

Raw Material

  • It begins with the raw material in the form of small pellets. These pellets are fed into a horizontal heated steel tube, called the barrel, which has a rotating auger, called the screw, inside it. The pellets are drawn out of a hopper and into the barrel by the auger where it is melted by temperatures typically ranging from 4-6 hundred degrees Fahrenheit.


  • When the machine cycle is initiated, the screw plunges forward under high pressure and is forced out the nozzle on the end of the barrel into the injection mold.

Injection Mold

  • An injection mold is a custom piece of equipment made specifically for the part being produced. A negative void, the cavity, is machined into the center of this mold that represents the shape of the part. When the melted plastic enters the cavity, it fills the void and packs it full of plastic.


  • A cooling process much like that of an engine block using water channels then begins cooling the plastic, returning it to a solid state.


  • Once the plastic is re-solidified, the mold, which is made in two halves and bolted to two separate hydraulic operated plates, or platens, separates the mold in the middle, exposing the part(s). Finally, steel pins that are installed in the mold and are cut flush with the cavity surface are pushed forward, ejecting the part out of the mold.

Of course there are many exceptions and variations to this explanation, but this section hopefully gives you a basic understanding of this process.

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