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?
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[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.
As you correctly noted, the metal shrinks as it cools. The outside of the part will always cool before the center of the part. So. the outside will usually become a solid shell before the inside has completely solidified. As the inner material solidifies and shrinks, the outer shell either has to collapse in or voids will form. Some of both happen. Indentations in flat areas are common (showing the shell collapsed in as the inner material shrank). But voids occur too. By the time the last liquid freezes, the shell is often so rigid it doesn’t pull in so the only option is a void.
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