If heat makes things expand why does plastic shrivel up when exposed to it?
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Both things can be true at the same time. Things can expand, and have their internal structure break down. Water for example expands as it gets heated up and when it freezes, for different reasons.
Keep in mind that plastic is forced to the shape it is in during manufacturing. Depending on method.
However your example of how plastics behave heated apply only to some plastics, not all. Namely plastics that are formed and shaped from molten state.
The plastic packet that shrinks wjen you heat it, is only the shape it is because during manufacturing it was forced to stay in the shape it is as it cooled , and after certain temperature it hardened. When you melt it, it returns to “original shape” as in to a shapeless blob, and this experiences heat expansion.
So if you melt plastic to a nonstick surface as it cools you’ll notice it peels itself off and together.
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