If heat makes things expand why does plastic shrivel up when exposed to it?
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On the micro sclale plastic is made of tiny polymer chains like spaghetti. When it is shaped, it is often stressed to the point that these chains straighten out. Heating them makes them go all wiggly, and this makes the whole thing contract – but there are loads of ways to be wiggly vs. very few to be straight, so it tends to happen nonuniformly even if the plastic is uniformly heated. So the plastic *may* deform. Not all plastics do this – *crosslinked* plastics where the spaghetti are all welded together often behave quite differently.
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