| Using Silicone Rubber |
The best rubber for general-purpose sculptural mold-making is called 2-part liquid RTV (Room Temperature Vulcanizing).silicone. This comes in two basic varieties; tin-cure and platinum -cure. The tin cure type is cheaper and more forgiving of impurities in the model and release agents, the platinum cure type lasts longer and can be tougher. It also comes in various hardnesses, measured on the Shore D scale with a "durometer", a hardness meter which pushes a tiny ball bearing into a surface, measuring the force required. A reading of 20 is very soft, 80 is very hard.The more undercuts you must mold, the softer the silicone you must use, to yank it out of difficult places. Being softer, it elongates and deforms easier. When softer silicone is used, it is less able to hold itself in place, and more likely to sag as gravity pulls. So softer rubber molds must be made somewhat thicker to avoid collapse. I prefer using a product called G.I. 1000 from Silicones Incorporated (see listing of suppliers at the end of this article). If you've never done this before, I strongly recommend hiring an experienced moldmaker to assist you and walk you through the process. There are a lot of fine points to the moldmaking process that are hard to convey in a brief written article, which can nevertheless make the difference between success and failure. Besides the silicone rubber and its standard catalyst, you will need to buy two additives: the first is a "thixotropic" agent to mix with the silicone. This agent makes the silicone thicken so that it can be laid up by hand in thicker coats without running off. The second is "ultrafast catalyst" which helps the action of the thixotrope and causes the rubber to cure rapidly enough to apply several coats in a day. You may also need to get a bag of glass microspheres, or silica-based thickeners called Cabosil or Aerosil. These are used to further thicken the rubber to a paste-like consistency. If you use them, be sure to wear a dust mask- the particles are very light and will float everywhere as you try to mix them in- and they aren't something you want to breathe. Mix the rubber in HDPE plastic buckets, or use disposable plastic cups if they are flat on the bottom. For the thin material, stirring with a stick is sufficient; propeller mixers held in portable drills help with the thicker material. To avoid sticky spots in the mold, mix in one container, then scrape out the contents into a clean container and remix- this avoids pickup of uncatalyzed rubber. Weigh the rubber and catalyst carefully using a precision scale- the rubber needs every bit of its 5% or 10% by weight, but not too much. Your sculpture may need to be sealed so that the silicone won't sink into the surface. If in any doubt, do a test on a small section to determine if it tends to stick. If the answer is yes, then use either mold soap or a spray release that is NOT SILICONE-BASED - teflon spray will work. Again, if you are molding something precious, test a small area to check for bad effects- sometimes the release will discolor marble, or ruin the finish of a carved wood piece. To get the model out of the mold, as well as to release the finished castings, you either have to make your mold in sections, or cut it out afterwards. To do the former, set up oil-clay walls along the parting line, making sure to use a clay that you've proved compatible with the rubber you're using. Carve some "keys" in the clay wall to insure registration, and apply the rubber to this side only, stripping off the clay when it is done, then release the edge with soap or another agent, and apply the next section. For a cut mold, paint the entire model with rubber at the same time, but build up some ridges at least 1/2" wide and equivalently thick along the parting line. Cut down these ridges with a wiggling stroke when the mold is done (shell and all) to extract the model, but don't cut any more than necessary- less seam is best. Design your shell to push the cut edges together when the mold is clamped shut. It helps if you have a vacuum pump and chamber to eliminate air bubbles from the mixed rubber prior to applying the first coats. But if it is brushed on thin, the flowing material will tend to eliminate them without this. Be aware that high temperature and humidity accelerate the curing of rubber- do not use so much ultrafast catalyst that the material seizes up before you can get it smoothly applied Your first (face) coat needs to be finely applied and not very thick. Take the time neccessary to eliminate air bubbles. You must wait for this coat to fully cure before proceeding, or you will find yourself pulling it off with your brush as you try to add rubber. The second (face) coat can be thickened as much as the agent will allow (usually 5% by weight is the maximum amount of liquid thixotrope you should use- any additional thickening is by adding powders). You must wait for this coat to fully cure as well. |
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