Thermosets harden in the hot mould by means of a chemical reaction in which the initial molecules are cross-linked through the application of pressure and heat to form macromolecules. As this process is irreversible, thermoset moulded parts cannot be melted down again as is the case with thermoplasts. Thermosets are almost exclusively processed with fillers and reinforcing materials, whereby the base material can range from granulates or rods through to dough-like polyester.

In contrast to thermoplasts, thermosets have certain distinct advantages such as high strength even at high temperatures, high heat distortion temperature, greater hardness and rigidity as well as good dimensional stability of the moulded parts. The material is thus predestined for the electrics industry, household appliances or machine parts.