RTV molding and urethane castings

Often called urethane casting, the RTV molding process has been used to create models that put the "function" in the RP industry's phrase "fit, form and function." People have been using RTV molding (room temperature vulcanization or silicone rubber molding) for years to fill the void between prototype and production. RTV molding is an effective and inexpensive way to crate multiple copies of a master prototype part. The resulting copies are cast using polyurethane, silicone or wax. Silicone molds are capable of reproducing remarkable detail, and because of the flexibility of silicone, draft angles, inserts, and undercuts are not as critical as in other molding processes. In recent years, there has been vast improvement in urethane casting technology.

How Does RTV Work?
The RTV mold is created from a master pattern, typically an RP model. The pattern is used to create a silicone rubber mold. The silicone is poured around the pattern to produce a core and cavity of a mold. The RP model is then removed, leaving a negative image into which polyurethane is cast. These molds can produce up to 20 parts per mold, and they can be made as family or multi-cavity molds.

When is RTV Molding useful for rapid prototyping?
RTV is very helpful when you need a spec-like material for multiple sets, in a short timeframe, for less than a traditional tool would cost.

Materials
The materials that can be cast in a rubber mold are two-part polyurethane, silicone rubber and wax. Typically urethane is used due to its many material properties. A prototype material can be chosen to match the hardness of your production material, from 10 shore A to 80 shore D, giving the prototype the same "feel" as a production part. Today's urethanes can withstand heat up to 220 degrees F. The silicone materials can be shot in shore A durometers for prototyping. Waxes can be shot for investment casting patterns.

Colors and Textures
To prevent colors from chipping or flaking off, parts can be cast in color instead of painted. These colors can be matched by a Pantone number or a color chip. White is more difficult to match, but depending on the material, it can be done. Texture is added to the SLA patterns so when the mold is created, with the texture being transferred directly to the mold. Different textures are available, and multiple textures can be placed on the same part for cosmetic purposes.

Over Molding
RTV works wonderfully for insert molding or over molding. Two patterns are produced, one with the over mold and one without the over mold. Then, two rubber molds are produced, and the urethane is cast. The first part is cast into the rubber mold without the over mold. That part is then pulled from the mold and inserted into the rubber mold with the over mold. Then the over mold is cast around the existing cast part. Once that part is pulled, it has both materials cast to complete the two-part over mold.

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