When you need a smooth and polished finish, or you want to make exact copies of an existing injection molded part, casting resin parts in silicone molds is an excellent option. [Eric Strebel] has ...

I made a catalyzed silicone rubber mold using a Coyote Skull and then poured ... where thin and delicate so I used modeling clay to make those parts thicker prior to creating the mold.

Pretty soon he was using silicone rubber to make two-piece, high-quality molds of lenses, good enough to try a few casting shots with epoxy resin. [Zach] ran into a few problems along the way ...