| Making Molds in Urethane Rubber |
Urethanes are a huge family of rubbers as well as plastics which are easy, economical, and quick to work with. Rubber elastomers and hard-plastic urethanes are extremely similar chemically, but the hard types have a plasticizer in their formula. It is easy to find urethanes which can be painted on and have great tensile strength, exceeding that of silicone rubber. Urethane rubber typically costs around $35 per gallon, a third of silicone's price, and used as a paint-on, goes a long way. But each material has advantages: silicones allow one to cast wax, polyester, and urethane resins without any release agent, while urethanes usually require a spray-on mold release of some sort, and never release as nicely. None is needed for plaster or concrete, (although it will extend the mold's useful life) or for wax when the mold is new. After casting resin into a urethane mold a while, the mold's surface grows harder and rougher. You'll have to replace it if a slick surface is vital. Also, be aware of heat buildup, the real enemy of this rubber; the sooner you demold, the longer your mold will last. Demolding too early may cause distortion, but this can be used to advantage- urethane plastic castings taken out at the "leather-hard" stage may be draped and twisted considerably, and soon take their final set retaining these deformations. These castings easily knit with armatures and inclusions, and with each other. So a half-hard, half-rubbery piece or a living hinge is easy to pull off. And speaking of pulling things off: don't leave a urethane casting in a mold overnight. Spray release won't prevent the masses from bonding over time. Where the paint-on urethanes shine is in creating large, cheap, extremely flexible sheet molds. I like to make thin reinforced plaster or polyester backups, often in several pieces. Say you're pouring waxes, and you have to work thin: to avoid distorting the part when demolding; the thin painted-on rubber mold is easy to peel from the cast wax. A thicker cast rubber mold won't peel away like that, and this means whatever you cast in a heavy rubber mold has to attain great early strength before demolding. Concrete and plaster edition castings go much faster when the mold can be recycled sooner. The pourable urethanes are advantageous when brushstrokes would harm your pattern; then pouring the rubber on is safer. When molding a bas-relief, the labor is cut to almost nothing with a poured mold. Another advantage of the urethanes over silicones is in incorporating fibers and foams. Some silicones won't knit with these inclusions (some will) but I often place woven fiberglass cloth or cheesecloth in my paint-on urethane molds at the places I know are wont to tear. And I place foam rubber pieces (also urethane,) cleverly cut and buttered with the rubber, into undercuts, so as to have easily-pulled-out collapsible areas. |
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