Injection molding machines work by injecting molten thermoplastic into a two-part mold. The first injection molding machine was developed in 1872 by John Wesley Hyatt. This machine was used to mass produce items like buttons. The first screw injection molding machine, which is what modern machines are based on, was built in 1946 by James Watson Hendry.
The process works by melting plastic resin pellets inside a barrel. The barrel contains a screw with a shaft whose diameter increases along its length. As the screw rotates, the plastic is forced into an incrementally smaller area. This compression is the primary mechanism responsible for melting the plastic. The barrel is also heated to further aid the melting process. Once enough plastic to fill the mold has melted, the screw retracts. The screw is then pushed forward to force the plastic into the mold at very high pressures.
Injection molds are made from at least two parts. They 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. It can easily produce millions of parts per machine annually. Injection molding can create complex parts. It is used for everything from gears to garden chairs. Injection molding is better than Blow Molding for the high-volume production of complex plastic parts. To learn more, see our guide on the Molding Manufacturing Process.
A typical injection molding setup is shown in Figure 2: