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Thermosetting plastics (thermosets) are polymer materials that cure, through the addition of energy, to a stronger form. The energy may be in the form of heat (generally above 200 degrees Celsius), through a chemical reaction (two-part epoxy, for example), or irradiation.

Thermoset materials are usually liquid or malleable prior to curing, and designed to be molded into their final form, or used as adhesives.

The curing process transforms the resin into a plastic or rubber by a cross-linking process. Energy and/or catalysts are added that cause the molecular chains to react at chemically active sites (unsaturated or epoxy sites, for example), linking into a rigid, 3-D structure. The cross-linking process forms a molecule with a larger molecular weight, resulting in a material with a higher melting point. During the reaction, when the molecular weight has increased to a point so that the melting point is higher than the surrounding ambient temperature, the material forms into a solid material. Subsequent uncontrolled reheating of the material results in reaching the decomposition temperature before the melting point is obtained. A thermoset material cannot be melted and re-shaped after it is cured.

Thermoset materials are generally stronger than thermoplastic materials due to this 3-D network of bonds, and are also better suited to high-temperature applications up to the decomposition temperature of the material. They do not lend themselves to recycling like thermoplastics, which can be melted and re-molded.

Some examples of Thermosets are:

• Vulcanized rubber
• Bakelite, a phenol-formaldehyde resin (used in electrical insulators and plastic wear)
• Urea-formaldehyde foam (used in plywood, particleboard and medium-density fibreboard)
• Melamine resin (used on worktop surfaces)
• Polyester resin (used in glass-reinforced plastics/fibreglass (GRP))
• Epoxy resin (used as an adhesive and in fibre reinforced plastics such as glass reinforced plastic and graphite-reinforced plastic)
• Polyimides used in printed circuit boards and in body parts of modern airplanes
• Reactive injection molding (used for objects like milk bottle crates)
• Extrusion molding (used for making pipes, threads of fabric and insulation for electrical cables
• Compression molding (used to shape most thermosetting plastics)
• Spin casting (used for producing fishing lures and jigs, gaming miniatures, figurines, emblems as well as production and replacement parts)

• Thermoplastic
• Vulcanization
• Fusion bonded epoxy coating
• Plastic