Step 3: Seal the original part to prevent adhesion of the liquid silicone to the part and to obtain an easy release of the casting. Just like in casting composites, apply a release agent to all surfaces where the possibility of adhesion exists and is not desired. The corbel was made from high density foam and spray-painted with a primer. While technically “sealed” by the primer, we took the additional step of adding a release agent to the part. If your original part is raw wood, plaster, concrete or any porous surface, apply a coat of water-based paint or varnish first to seal the original part, then apply a release agent to the part.
Step 4: Assemble the walls around the part. Walls need to be about one inch higher than the highest point of your part. On small parts, one-half inch is high enough, but on larger parts, add thickness to support the final cast part. Here again, we used laminate countertop pieces to complete the framework of our “box.” All laminate parts were coated with a release agent after assembly as an added precaution against unwanted adhesion to the frame. The frame is held together with readily available C-clamps. You can also use screws, hot glue or any other non-permanent mechanical means to fasten the box sides together.
Step 5: Seal the inside seams with clay to prevent the liquid silicone from leaking out before it cures. Also, use clay to seal the outside of the box seams to prevent hydrostatic pressure leaks. After the frame is sealed, apply one final coat of mold release to all parts—the inside of the frame and the original part.
Step 6: Mix the two parts of silicone rubber. It’s a simple “Part A/Part B” system, much like mixing resin and catalyst. In the case of TinSil® 80-30, the ratio is one part A to 10 parts B, by weight. You’ll need a digital scale to weigh your portions. The mixing ratios and formulas are on the containers, and the recipe is easy to follow. Polytek has a unique approach to selling its two-part system: The company gives you just the right amount of Part A to fully mix with Part B. (Imagine buying a drum of resin and getting just the right amount of catalyst to harden the whole drum, at a given ratio, with no waste.) There are guidelines for how much mix you’ll need to fill your frame, and its based on cubic square inches of volume (L x W x H), less the approximate size of the part. It’s “X” amount of liquid silicone per square inch. Different silicone rubber products have different ratios due to varying density.