Urethane Rubber—Urethane rubber compounds are tougher than silicone rubber compounds, so your molds will last longer. Although not required, curing urethane rubber molds inside a vacuum chamber improves their clarity. By removing trapped air bubbles, the chamber prevents the finished mold from looking speckled.

Silicone Rubber—Silicone rubber compounds handle higher temperatures than urethane rubber compounds—up to 570° F. Silicone rubber also cures faster than urethane rubber, and it won't shrink as it cures, which means you won't need to guess at the final size of your mold. Because silicone rubber compounds trap air bubbles when mixed, they must be cured inside a vacuum chamber. The chamber removes the trapped air bubbles, strengthens the mold, and improves its clarity.

Flexible—Flexible compounds cure into soft, open-cell foam, which absorbs water, air, and gases. They are often used to cast gaskets, cushions, and other bendable objects.

Rigid—Rigid compounds form stiff, closed-cell foam, which prevents water and air from flowing through the part. Machinable, they’re dense enough to create prototype modeling blocks, so you can test setups without wearing down tools.

Aluminum Oxide—Aluminum oxide is high strength.

High-Purity Ceramic Alumina—High-purity ceramic alumina resists shrinking in casts, has a low level of contamination, and is a strong electrical insulator. It’s often used for constructing furnaces and handling liquid metal.

Silicon Carbide—Silicon carbide has a high heat transfer rate, making it ideal for use in metal casting molds. It has the added benefit of excellent erosion and oxidation resistance.

Silica—Silica withstands rapid changes in temperature without cracking and has a low rate of thermal expansion.

Zirconium Oxide—Zirconium oxide is often used in metal handling applications. In addition to its high temperature resistance, it’s also chemical resistant.