It was noted that certain polymers would soften upon heating， and could then be made to flow when a stress was applied. When cooled again， they would reversibly regain their solid or rubbery nature. These polymers are known as thermoplastics. By analogy， ice and solder， though not polymers， behave as thermoplastics.
Thermoplastic polymers are characterized by softening upon heating and hardening by cooling. Since the giant molecules of these materials have no strong bonds between the individual molecules， they can be softened by heat and remolded over and over again. This is an advantage in molding processes such as extrusion or injection where scrap or rejected products can be reground and molded again. Some of the thermoplastic materials will burn freely when exposed to an open flame while others of this group will not suooort combustion.
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