Things You'll Need
Silicone release agent
Two-part urethane foam spray
Urethane foam comes in two forms. Soft urethane foam is flexible and used to make items such as car seats, soft props and foam mats. Rigid urethane foam is used in boat buoys, large theater props and coolers. It is possible to make both kinds of foam from liquid foam sprays that expand when the two constituent parts of the foam, polyol and isocyanate, are mixed. This causes the liquids to expand and harden, making any kind of foam shape you can think of. Use urethane-based paints to paint the foam after you have made it.
Choose a mold for the foam. Any non-porous mold will work, such as candle molds, prop molds or even molds you make yourself from non-porous materials, such as fiberglass or silicone. Prepare the mold by cleaning it with a degreasing cleaner and a soft cloth. Dry the mold after cleaning. Spray the mold with a silicone release agent to coat the sides and bottom of the mold and make it easier to remove the foam later.
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Put on a respirator mask, rubber gloves and eye protection. Work in a well-ventilated area and never breathe the vapors produced by the foam. These vapors are toxic and dangerous.
Mix the two parts of the urethane mixture in a disposable bucket. The mixture will be difficult to remove from the bucket once it has hardened, and a disposable bucket offers the easiest clean-up option. For soft foam, mix 28 parts of the "A" mixture with 72 parts of the "B" mixture. Measure the parts by weight. For rigid foam, mix 50 percent of the "A" mixture with 50 percent of the "B" mixture. Mix together until the two parts blend evenly and a uniform color is achieved.
Pour the foam quickly into the mold. The foam will start to expand and harden within a few minutes, so it is important to work with the mixture as quickly as possible.
Allow the foam to expand and cure overnight in a room that is at least 70 degrees Fahrenheit. If the room is cooler, the foam may not expand properly or cure at all. If you have curing problems, try placing the mold onto a surface that does not conduct heat well, such as wood, and keep the temperature in the room above 70 degrees F. Curing should occur within 24 hours of mixing.
Remove the foam from the mold by popping it out from the bottom. Use a pneumatic sanding disc or electric carving knife to further shape and cut the foam. Always wear a respirator mask when working with foam because it can release toxic fumes when sanded or cut.