Thermoset plastics, or thermoset composites, are synthetic materials that strengthen when heated, but cannot successfully be remolded or reheated after initial heat-forming or molding. After thermosets are molded, the resulting parts are highly resistant to high operating temperatures, corrosion, as well as resistant to certain chemicals, oils, and fluids. The combined chemical and temperature resistance lends molded thermosets to be used in a variety of aggressive and challenging environments from electrical applications to automotive powertrain and transmission components to products with outdoor exposure. Implementing a thermoset molding process to make product or component allows end parts and assemblies to remain dimensionally and chemically stable against aggressive elements including exposure to moisture, high temperatures and chemicals.
Examples of thermoset materials include Bulk Molding Compound (BMC), Phenolic, Epoxy, Diallyl Phthalate (DAP). Bulk Molding Compounds are also known as unsaturated polyesters and vinyl esters. Thermoset materials may be processed by injection, compression, insert, transfer, and injection-compression molding similar to thermoplastics such as Nylon or ABS.
Understanding Thermoset Plastics
Due to its chemical composition and material property benefits, molded thermosets are used in a variety of end-markets and applications, especially for parts exposed to challenging and aggressive environments. Thermosets will not degrade with exposure to certain chemicals, oils, nor automotive fluids. In electrical applications and assemblies, thermoset housings protect internal components with superior dielectric strength and electrical insulation. For outdoor applications, components remain durable even in wide-ranging weather conditions like heavy sunlight, rain, sleet or snow. Whether a product application is installed in Alaska or Arizona, thermosets remain a preferred material choice of engineers and designers for challenging applications requiring heat-stable parts or protection for internal components. Cost-wise, thermosets are competitively priced against similarly-performing thermoplastics, with cost per lb. generally ranging between $1-$3.50 per lb. for thermoset molding compounds.
In scientific terms, a thermoset monomer must have three or more reactive ends, with its molecular chains crosslinking in three dimensions. Post-molding, thermosets have virtually all molecules interconnected with strong, permanent, physical bonds which are not heat reversible. Thermoplastic monomers on the other hand, only have two reactive ends for linear chain growth.
In layman terms, molding or curing a thermoset is like cooking an egg or baking a cake. Once an egg is dropped into boiling water and cooked, it cannot revert to its original physical state as a yoke. Similarly, after thermosets are molded, it cannot be ground up and reprocessed or remolded.