Why does shrink wrap shrink when heated, instead of expand?

Question

This question is based on the assumption that most materials expand when heated. Why does shrink wrap do the opposite?

In: 52

Answer 1

Shrink wrap will actually expand when heated, but only as much as other plastic products do, which isn’t much.

The shrinking isn’t from thermal contraction it’s from the polymer chains pulling together.

From something like this

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to this

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Which is why once it’s shrunk it is thicker.