Compression molding is a method of molding used for molding thermoset plastic resin. Thermoset plastic resin are liquid at room temperature, but solidify upon heating, and once solidified, they remain permanently solid. The hardening is considered to be due to a chemical reaction called cross-linking.
Typical thermoset plastic resin are phenol resin, melamine resin, urea resin, epoxy resin, etc.

The compression molding method is a classical method of molding which has been in use ever since plastic resins were invented.
Male and female molds are prepared, the molding resin is loaded inside the female mold, the male mold is closed, and the set of molds is heated thereby solidifying the plastic resin and preparing the molded article.
At the time of closing the male mold, excess resin overflows from the parting surface and becomes burrs. These burrs are removed after molding. (Fig. 1)

Compression molding is being used even now for food utensils such as plates, bowls, etc., electrical insulating parts, decorative buttons, etc.
Since thermoplastic materials cannot be used in environments where they are heated or where resistance to chemicals is required, thermoset plastic resin are used in such situations.

The mold structures of the three types:
1. Positive type mold
2. Flash type mold
3. Semi-positive type mold.
The semi-positive type mold is used most often.