Urethane Casting


MOCHI > The Workshop > Crafting > Crafting Techniques > Casting > Urethane Casting

WARNING: The usual safety warnings apply. Watch what other chemicals you expose yourself to! Daifuku Mochi has had a weird allergic reaction when doing casting under specific circumstances. [Hives suck. - Daifuku]

This is just an overview. Follow the manufacturer's directions for your particular casting resin.

Find or make your mold. A lot of things can be used for molds. Your mold needs to be slightly fleixble because the final product is going to be hard and solid. You might have to twist it or slam the mold against a hard surface to get the cast out. We've used commerical molds, things that weren't really designed to be molds, and made our own.

Prepare your work space. Once things have been mixed, you don't have time to dink around. Lay out all the things you need on a clean work surface. We've done things on an old cookie sheet lined with foil. If there are any spills, you can throw away the foil rather than try to pry hardened bits off of the cookie sheet or the furniture.

Prepare the mold. Make sure your mold is clean and dry. Most casting resin manufacturers also make a special release agent that will help stuff unmold. It's like using non-stick spray in cooking. Try not to use too much release agent or you'll get funny "bubbles" on the surface of your cast.

Pour your pre-polymer. We usually pour the pre-polymer into a disposable plastic cup. If there are things that you want to add (like color dye or other additives), carefully stir it into the pre-polymer. Try not to stir too fast or you'll get air bubbles.

Add the catalyst and pour it into the mold. Again, work carefully so you don't get air pockets. Let the excess mixture solidify in the disposable cup and throw it away later.

Wait for it to cure. This is usually the hardest part for the impatient members of MOCHI! Heat does speed up the chemical reaction, but you want to be careful not to heat things up too much. A sunny windowsill is often warm enough.

Unmold. This can be tricky. We've managed to ruin good casts because they got scraped or cracked when they were unmolded. If it's being resistant to unmolding, extend the curing time. The resin shrinks as it cures and it might unstick itself.