A high molecular weight polymer that can be shaped at some stage in its manufacture by the application of heat and pressure to give a product that is stable at normal temperature. A thermoplastic is a substance that can be softened by the application of heat so that it can be shaped and moulded indefinitely. A thermosetting plastic is not able to be softened by reheating due to a chemical change such as the formation of cross-links between chains of the polymerized molecules. PVC (polyvinyl chloride) is a thermoplastic that is often used for piping systems and is stronger and more rigid than most other thermo-plastic materials.