How does the contraction of a metal during its cooling creates cavities?

Question

In the case of sand casting, the molten metal starts to show some internal and external cavities after it’s cooled down. How does this cooling down, which implies contraction, create these cavities?

In: 2

Answer 1

[Here’s a crappy diagram](https://i.imgur.com/IqV73eP.png). On the left, you see some black circles in a red box. If the circles shrink, they might all shrink in the same way, and you get the result in the top right, where the shrunken version is uniform. But what if some of the circles cling to the left side of the box as they shrink, and other cling to the right side? Now, although the circles are shrunken, the object as a whole hasn’t contracted very much and cavities have formed.