Thermosetting plastics (thermosets) refer to a variety of 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 (e.g., two-part epoxy), or irradiation. Thermoset materials are usually liquid, powder, 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 cross-linking. Energy and catalysts are added that cause the molecular chains to link into a rigid, 3-D structure. A thermoset material cannot be melted and re-molded after it is cured.

Thermoset materials are generally stronger than thermoplastic materials, and are also better suited to high-temperature applications. They do not lend themselves to recycling like thermoplastics, which can be melted and re-molded.

Examples

• Natural 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 (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)

Methods used to mould thermosets

• Injection Moulding (used for objects like milk bottle crates)
• Extrusion Moulding (used for making pipes, threads of fabric and insulation for electrical cables)
• Calendering (used for making large sheets of plastic)
• Compression Molding (used to shape most thermosetting plastics)
• Blow moulding (used for bottles)