Thermosetting, or thermoset, plastics are rigid and set in a one-time pattern when they come out of the plastic injection mold. They cannot by recycled or melted for further use. Examples of thermoset plastics are automobile tires, appliance cases, electrical and electronic circuit boards, and electrical switches.

John Wesley Hyatt developed the first plastic injection mold, and the injection molding process, in 1868. He successfully injected hot, liquid celluloid, often called the first thermoplastic, into a split-die mold by the plunger method, a process that was little changed until James Hendry built the first screw plastic injection mold in 1946. Today, nearly all plastic injection molding uses the screw method.

To inject liquid plastic into a mold using the screw method, the screw below the molten plastic hopper is moved back to allow the material to flow into the mold. The screw assembly is then advanced to close the injection entry, using a non-return assembly to keep the material from backing up into the screw area. After allowing enough time for the plastic to cool, the mold is opened and the part removed, either by hand or by machine. The mold is then closed, the screw opened, and it is prepared to receive more material.

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