The molder receives plastic resin in the form of small chopped pellets (usually 1/8 in. long). These are fed into the hopper of an injection molding machine, where they fall into an auger-type screw channel, which feeds the pellets forward inside the heated barrel. As the mass of plastic moves toward the front of the barrel, it is plasticized or melted. The screw is allowed to travel back until a sufficient quantity of molten plastic accumulates in front of the screw to fill the cavity in the mold. The screw is then pushed forward under high pressure to force the molten plastic through the machine nozzle into the closed mold. Once in the mold, the plastic flows through a distribution system called runners and then through gates into the part cavities. As soon as the plastic cools and solidifies in the mold cavity, the mold is opened and the part is removed.

The mold is usually heated or cooled to provide the proper temperature for plastic solidification. The mold also has some type of mechanical assist, called ejection, to help extract the part from the mold. While the part is cooling in the mold, the next shot is being plasticized within the barrel. The mold then closes and the process is repeated. Although this is a very elementary explanation of the injection molding process, it should be observed that the cooling time in the mold is usually the controlling factor in determining the production rate of the machine. An overly thick wall section, even in just a small portion of the part, can significantly lengthen the cooling time and hurt the overall economics of the part.