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HOW TO MAKE A TWO-PART RUBBER MOLD OF A COMPLICATED MODEL You
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2 4 5 6) Cut the clay so it is about ¾ inch away from the model, using straight line cuts. Construct a foam board “wall” around the clay. The foam board should be about ½ inch higher than the highest part of the model. The foam board is tacked to the pallet board with hot melt glue. The clay is pressed to the foam board to seal any gap between the foam board and clay. See page 39 of the Castcraft Moldmaking Guide for more information about making foam board forms.
7) Here is one way to determine how much rubber to mix to make the first half of the mold: Cover the model with plastic wrap. Try to get the plastic wrap to follow the curves of the model as much as possible. Fill the form with water from a measuring cup, and make note of how much water it takes. Mix a little more rubber than this, because the plastic wrap probably won’t follow every curve of your model.
8) Use a release agent on the clay and the model, so the moldmaking rubber does not stick. Most companies that sell moldmaking rubber also sell release agents. Check your mold for strength, and reinforce if necessary. Here we added some craft sticks as braces, hot-melt glued to the top edge of the foam board.
9) Mix and pour the rubber for the first half of the mold. It is good technique to pour the rubber into the lowest part of the mold and let the rubber slowly rise up over the model. This ensures that the air will be pushed up and out. If you just dump liquid rubber over your model, there is a chance that an air bubble will be trapped and create a void in your mold.
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