Mold Making Process
Vulcanized Rubber Molds
Strips of natural gum rubber are packed around a metal model and then placed in an aluminum frame. The packed frame is placed in a vulcanizer, which heats and pressurizes the mold. Once the proper time has elapsed, the mold frame is removed and allowed to cool. The mold is removed from the frame and the model is cut from the mold, leaving a negative impression of the piece.
RTV (Room Temperature Vulcanization) Molds
We make two kinds of RTVs: urethane and silicone. The process is essentially the same for both. The model is sprued and placed in a mold frame. The molding compound, which is generally in two parts, is mixed, vacuumed and then poured into a frame and around the piece. This is then subjected to a vacuum for a short period of time in order to remove any air bubbles. The mold is then left to cure, usually overnight. Once cured, the mold is cut and the model removed. With more complex or larger parts (e.g. large bracelets, small sculpture, etc.), we build a frame and pack the model partially in clay, pour RTV material around it and let it cure. We then remove the clay and pour the rest of the mold. If needed, we pour a plaster or hydrocal mother mold around the RTV mold to give it more stability.
Considerations for Selecting Mold Type
The choice of mold is often determined by the type of model to be molded, and by the issue of shrinkage. If the original model is metal, we often make a vulcanized mold, since the metal can tolerate both heat and pressure. If the original model is wax, organic material or anything else that cannot withstand the pressure and temperature of a vulcanized mold, RTV is the mold of choice.
If you want to minimize shrinkage, RTVs are the way to go. Shrinkage rates are somewhere between 0 to 3% in RTV molds, whereas vulcanized molds can shrink from 5 to 8%. There is, however, no way to accurately predict shrinkage rates, so if you’re concerned about ring sizing or bezel sized for pre-cut stones, RTVs are typically the better mold to use.
Vulcanized molds generally have a longer shelf-life, and can be used for thousands of injections. Some RTV compounds have a long shelf-life, but others last only a few years before they break down. We will be happy to work with you to help you determine the best molding compounds and processes for your pieces.
