Injection molding works by injecting molten thermoplastic into a mold cavity. The first injection molding machine was developed in 1872 by John Wesley Hyatt. In 1946, James Watson Hendry built the first screw injection molding machine, which is what modern machines are based on.
Injection molding melts plastic resin pellets inside a barrel. The barrel contains a screw with a shaft whose diameter increases along its length. As the screw rotates, plastic is forced into an incrementally smaller volume. This compression is the primary mechanism responsible for melting the plastic. The barrel is also heated to supply additional heat. Once enough plastic to fill the mold has melted, the screw retracts, then is pushed forward to force the plastic into the mold at very high pressures.
Molds are made from at least two parts and are held together with hydraulic rams so that the high-pressure plastic injected into the mold cannot escape from the parting line. The mold has cooling channels that cool the part enough so that it can be ejected from the mold without damaging it. Injection molding can achieve high production volumes unmatched by any other plastic processing technique. Compression molding, on the other hand, is better suited to medium-volume production. For more information, see our guide on what is injection molding.
A simple injection molding setup is shown in Figure 1:
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