Polyester is rarely used for cast pieces, though. In large quantities, it gives off so much heat as it cures that it tends to ruin the mold it's in, as well as distort itself. Too much in one batch can start an exotherm and catch fire. (No, really- I wanted to see it for myself, and mixed up about a half a quart and left it in a paper cup. It started on fire and made a horrible mess.) Usually, "cast resin" refers to urethane-based casting resins, which are the white or bone colored resins previously spoken of. Urethanes are very strong and durable.
