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

It doesn’t shrink, it just changes shape. The volume of the material remains the same after “shrinking”. It’s just a bit thicker.

If you put a water drop on a waxed surface, the drop forms a bead. If you try to spread out that bead and freeze it, you get a flat sheet of ice. If you melt it again, the molecules’ mutual attraction causes the sheet of ice to contract from the edges to form a bead of liquid water again, because this is the lowest energy state with the lowest internal stresses. The same principle applies to plastic – when you heat it, it “relaxes” into a more stable shape, typically conforming to whatever it’s wrapped around as it contracts and gets thicker.