One of the most popular tooling applications for RP is the production of room temperature vulcanizing (RTV) silicone rubber molds.

Silicone is a versatile material (although somewhat expensive) that can be molded around a master pattern to produce a cavity. With the advent of rapid prototyping techniques, master patterns are often an RP model. Silicone rubber molds are used to produce urethane or epoxy prototypes.

   The process of making a rubber mold consists of making a master pattern, finishing the pattern to the desired appearance and casting RTV silicone rubber around the pattern to form the mold. Using the transparent material the model is suspended within a box and silicone rubber is poured to fully surround the model. After the silicone rubber has solidified, the parting line is cut with a scalpel and the model removed, leaving the required cavity.
It is then possible to mold two-part thermoset materials within the cavity. One of the most popular is polyurethane, which is available in a variety of mechanical properties and can mimic the mechanical and thermal properties of elastomers, ABS, nylon and other popular thermoplastics. Polyurethane is usually poured into the silicone rubber cavity under vacuum to avoid air bubbles in the molded component. The silicone rubber tool will generally produce about 20 polyurethane parts before it begins to deteriorate. This will depend on the amount of detail in the tool and the type of polyurethane being molded. Flexible polyurethanes require longer post cure times within the mold, which is placed in the oven at 149x F (65x C). This prolonged contact dries out the surface of the silicone rubber and renders it more brittle. Once this occurs, fine detail on the inner surface of the mold starts to break off and subsequent molded parts reflect this loss.

  Silicone rubber tooling provides fast, inexpensive molds, excellent part cosmetics, and the option of using multiple materials. The process is suitable for small or large parts. The primary weakness of the process is that the properties of the urethane materials are different from those of the thermoplastic materials used in production. Due to material cost and labor demands, individual part prices are relatively high.

  Even with its limitations, silicone rubber tooling can be used as a production process. Bastech (Dayton, OH) currently uses silicone rubber tooling to make an instrument case that has high cosmetic requirements, including texture, but very little strength requirements. In this project, the customer required only 100 parts per year. Measuring 350 x 300 x 20 mm (14 x 12 x 2 inches), the case would have required a significant investment in hard tooling.

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