Cast urethane is a two-part epoxy that cures into a hard plastic, similar to PVC. Because cast urethane can capture tiny details of a mold and has such a high tensile strength when hardened, it is often used to prototype machine parts and other components that will eventually be cast in metal or with injection-molded plastic. However, you can do your own urethane casting at home with molds you buy or make yourself.
Spray the insides of the mold with urethane mold release, and allow it to dry.
Add equal amounts of both parts of the casting resin together in a clean, dry container.
Mix the resin together, being careful to stir in all parts of the mixing container.
Pour the resin into the mold within three minutes of mixing it.
Allow the urethane to harden for at least 10 minutes before removing the mold.
Items cast in urethane can be painted with plastic-bonding paints, or you can add colorant as you're mixing the resin. You can buy kits to create your own silicone molds, so you can produce very hard, durable replicas of almost anything.
Once it's mixed, urethane resin hardens very quickly. Make sure your mold is ready to receive the urethane before you begin to mix the resin. Be sure everything that comes in contact with the urethane resin is completely clean and dry. Small amounts of moisture can cause bubbles to form in the urethane as it's hardening. Ony use urethane with silicone molds. Other kinds of molds -- such as plaster -- can hold trace amounts of water that can ruin the cast item.