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

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

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It’s not shriveled. It’s rapidly changing from solid to liquid and becoming more dense. Some of the material is also getting hot enough to turn into gas.

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.

Heat usually makes things expand because it’s making the molecules of the thing you’re heating more energetic so they spread out a tiny bit.
This isn’t always the case though, depending on how those molecules are arranged. Sometimes the molecules are already spread or stretched far apart, and heating them loosens them up enough to pull back together.

Plastic is made of really long molecules that look like chains (polymers). When we make stuff out of plastic we usually stretch and push those chains and mold them into the shape of whatever we’re trying to make because the chains are strong even when they’re spread over a wide thin area (like plastic bags or bottles).

When you heat up a plastic bag or bottle, those long chains loosen up and pull each other back together so the plastic seems to take up less space because it used to be spread out in the shape of a bag or bottle, but now it’s all blobbed together.

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.

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.