Poured urethane foam is a common material for casting floating objects like duck decoys.
Pourable urethane foam is an excellent choice when you want to cast lightweight shapes. It is particularly good for casting objects that must float, such as duck decoys. One important property of urethane foam is that it expands after it is mixed. For this reason, you'll need to use pourable urethane with a two-part, rigid mold that can withstand the pressure of the expanding foam. Two-part molds take a little more doing, but the resultant mold will allow you to make dozens of urethane castings.
Instructions
1. Build an open-top box with scrap wood. The box should be just large enough to hold the model, with about an inch of space around all sides. The box should also be deep enough so you can cover the model with about an inch of plaster.
2. Fill half the box with modeling clay, pressing it down into the box and smoothing the surface.
3. Lay the model on its side in the box and press it into the clay so that half the model is above the clay and half is in it.
4. Cover the exposed surface of the clay and the model with a thin layer of petroleum jelly.
5. Mix five parts of the gypsum cement with two parts of water, or according to manufacturer's instructions. Allow the water to soak into the cement for five minutes, and then mix for five minutes with a stirring stick or mechanical mixer.
6. Pour the cement into the box, up to just under the upper edge. Tap the box to make sure any trapped air bubbles escape.
7. Allow the cement to thoroughly harden (24 hours, or according to manufacturer's directions). Invert the box and remove the model from the hardened mold.
8. Place the hardened mold back into the box, with the molded side up, and put the model back into the mold. Cover the exposed area of the mold and the model with a thin coat of petroleum jelly. Mix another batch of gypsum cement and pour it over the model and mold in the box.
9. Allow the gypsum cement to completely harden, then invert the box and remove the mold. The two halves of the mold should come cleanly away from the model.
10. Spray the inside of both halves of the mold with a polyurethane sealer to seal any pores in the cement.
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