The expanded polystyrene casting process uses a mold of sand packed around a polystyrene foam pattern that vaporizes when the molten metal is poured into the mold:

Question

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This describes the lost-foam casting process, where a polystyrene foam pattern is used to create a mold in sand, and then vaporizes when molten metal is poured in, creating a metal casting in the shape of the original foam pattern.

The Expanded Polystyrene Casting Process Uses a Mold of Sand