(1) Capable of being repeatedly softened by heat and hardened by cooling. (2) A material that will repeatedly soften when heated and harden when cooled. Typical of the thermoplastic family are the styrene polymers and copolymers, acrylics, cellulosics, polyethylenes, polypropylene, vinyls and nylons. (Plastics Engineering Handbook of The Society of the Plastics Industry, Inc., edited by Michael L. Berins, 1991).