Monday, March 29, 2004
Handy Facts on Molds
Sculptor Joel Haas offers some handy facts on making molds for miniature wargames figures. He also has handy facts on sculpting epoxy. A few handy facts on moulds (or molds, in the USA) I used to make Room Temperature Vulcanizing rubber gravity casting molds. Properly made and used, I found them to be the practical equal of any Industrially Vulcanized spin casting molds. I started off using a Dow Corning product, but quickly changed over to using Silicone, Inc’s GI 311. The Silicones, Inc product is made only an hour and a half drive west of where I live and the company was small enough I could talk at length to knowledgeable people for advice on the product. Molds wear out. The rubber on your car tires (Industrially vulcanized rubber) wears out. The reason is the same—heat and friction. Over time, heat cooks the rubber to a crumbly state and the friction of the road or pulling a figure out of a mold, tears away the crumbly, cooked rubber and, eventually, a new mold must be made. A lot of factors can vary the length of time a mold remains usable—the type of rubber, the type of catalyst used to harden the rubber, the actual design of the mold, the casting temperature of the mold, and the skill of the caster. Some people use a vacuum flask to draw air bubbles out of the RTV mixture to keep air bubbles from being trapped against the master figure during mold making. If that happens, you can come up with a casting that has random “metal grapefruits” coming off its body. I never had a problem with that, and saved the cost of and hassle of a vacuum casting machine by simply brushing on a thin layer of RTV rubber and letting the thin layer next to the master cure before casting the rest of the rubber. Since RTV will stick to itself, this is not a problem. In fact, this is an advantage. If a mold tears, you can simply mix up a bit of RTV and use it to glue the mould back together; and, if there is an air bubble trapped in an inconvenient spot, you can simply mix a small amount and, suing a toothpick, fill in the air bubble hole. You can save even more money this way—I would chop up old molds and mix the small pieces into the backing layer to save on buying fresh mold rubber. Most RTVs offer a regular and an ultra fast cure catalyst. The regular cure needs 8 to 24 hours to set up. The fast cure can set up in as little as 5 minutes. I buy the regular cure stuff and just a small extra bottle of the ultra fast. The more catalyst added to mold rubber the quicker the mold will become brittle. The more catalyst added, the stiffer the rubber, too. Finally, the ultra fast cure is so fast that it almost guarantees air bubbles being trapped next to the master figure. I use the quick cure catalyst as a way of patching air bubbles in molds, or of making quick molds of dolls or armatures. Some companies sell a very high end RTV rubber with a platinum based catalyst curing agent. I use the regular tin based curing agent and have not really found that for drop casting figures on the kitchen stove top that the extra cost of a platinum cure mold was worth the money. I keep the completed molds in plastic bags, labeled with a felt tip pen because after you make more than about a dozen molds, they start to get hard to sort out, so it’s easiest to keep them each in their own bag. Joel Haas, sculptor 3215 Merriman Ave Raleigh, NC 27607 919-828-8836 studio 919-828-8829 home 919-754-9118 fax http://artistsregister.com/artists/NC4 or http://www.joelhaasstudio.com
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I made another test mold yesterday and used some talcum powder I had purchased a while ago. It worked better than the cream. I’ll have to give the vaseline a try. I use it sometimes for lubricating sculpting tools to keep the greenstuff from sticking.
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