The expendable pattern is a foam that can be formed by compressing polystyrene beads into the desired shape and then bonding them together by heating. Alternatively, pattern shapes can be cut from sheets and assembled with glue. Sand is then packed around the pattern to form the mold. As the molten metal is poured into the mold, it replaces the pattern, which vaporizes. The compacted sand remains in place, and, upon solidification, the metal assumes the shape of the mold.

• A. The foam pattern remains in the final part
• B. The pattern is made of ceramic
• C. The foam pattern is vaporized and replaced by molten metal
• D. It uses a wax pattern within a sand mold

Answer: (C)

This item tests the lost foam (expendable pattern) casting concept. A foam pattern made of polystyrene is used inside a sand mold, and when molten metal is poured, the pattern vaporizes rather than remaining intact. The metal flows into the space left by the vaporized foam, and the sand mold stays in place, so the final part assumes the shape of the mold without the pattern itself. This is why the best description is that the foam pattern is vaporized and replaced by molten metal.

The other statements describe different processes: patterns that stay in the final part would not reflect the lost-foam behavior; a ceramic pattern implies a ceramic-pattern/mold system rather than a foam pattern in sand; and a wax pattern within a sand mold corresponds to lost-wax casting, which uses wax and typically a different mold construction.