There are two main types of plastics:
When thermoplastics are heated the polymer chains that make them up are given enough heat energy to vibrate themselves into line and so they shrink back to their orginal form.
Thermoset plastics are different. When they are originally heated, they form cross-links between chains through side branches. If they are subsequently heated they do not soften like thermoplastics, they form extra cross-links or burn.
