When classified by chemical structure, there are two generally recognized classes of plastic materials: Thermosets, having cross-linked molecular chains, and Thermoplastics, which are made up of linear molecular chains. Thermoset polymers require a two-stage polymerization process. The first is done by the material supplier, which results in a linear chain polymer with partially reacted portions. The second is done by the molder, who controls final cross-linking. Short chains with many cross-links form rigid thermosets, while longer chains with fewer cross-links form more flexible thermosets. With all thermosets, the polymerization is permanent and irreversible. Thermoplastic polymers require no further chemical processing before molding. There are two types of thermoplastic polymers: Crystalline and Amorphous. The pyramid graphic located here identifies many of our common thermoplastic materials.