Create lightweight molds with these two-part compounds. They expand as foam after they’re poured to fill every cavity. The denser the foam, the less it will expand. Before casting, apply petroleum jelly (not included) to prevent sticking, stretching, and tearing. It won't cause the foam to collapse as it cures.

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.