If heat makes things expand why does plastic shrivel up when exposed to it?

Question

If heat makes things expand why does plastic shrivel up when exposed to it?

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Answer 1

What’s happening here is that the plastic has stresses from how it’s shaped during manufacturing that are getting released when you heat it. It’s sort of like stretching a rubber band then freezing it. When heated back up the plastic is able to contract again.

When the plastic shrivels it’s getting shorter, but it also gets thicker. When we talk about heat making things expand it’s the overall volume that increases slightly when warm.