RTV Molding & Urethane Casting
Rapid Manufacturing in urethane & Silicone
RTV Molding is a rapid manufacturing and rapid prototyping technology, and urethane casting often refers to RTV (room temperature vulcanization) molding of which it is a subset. The three most common materials used to produce RTV molded parts are polyurethanes, silicone, and wax. Polyurethane parts created using RTV molding have a high level of detail and excellent surface finish, making them an excellent choice where production-like parts are needed.
RTV molded parts are often used to fill the void between prototypes and production, and are an inexpensive way of to produce a small set of parts from a single master prototype part. While rapid prototypes are more cost effective for individual prototypes, urethane casting can offer cost savings when half a dozen or more prototypes are required.
When is RTV Molding useful?
RTV molding is most effective when you need a short run of parts that closely match the appearance of production parts, and/or when you need parts that have many of the functional characteristics of production materials. It is also a faster process than the typical production of molded parts, so with RTV molding you can have parts in hand in as few as two weeks.
There can be a small cost savings over rapid prototyping for as few as six copies of a part, but RTV molding is usually most cost effective when at least a dozen copies are made. For single prototypes, rapid prototyping is the better way to go.
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Polyurethane is a polymer resin widely used in RTV molding because of its many material qualities. Its hardness can range from 10 shore A to 80 shore D, so polyurethane can simulate many production materials, from hard plastics, to soft rubbers. Urethanes are the mainstay of RTV molding because of their ability to closely match partsâ€™ production material characteristics. Modern urethanes can also withstand heat up to 220F, making them functional in a wide range of environments.
Silicones can be used in casting as well. They are available in a range of shore A durometers and are effective for making the softest parts. Wax can also be used for investment casting patterns.
Colors and Textures
Rather than painting urethane parts after they have been made, as is usually necessary for rapid prototypes, dyes can be mixed directly into urethane resins before casting. That prevents colors from flaking or chipping and gives parts a production â€œfeelâ€. Colors can be matched by Pantone number or color chip. Both opaque and transparent parts are possible. Textures can be included in high resolution SLA or PolyJet masters to to add details that will give finished urethane parts a production appearance.
How does it work?
Producing cast urethane parts starts with the creation of a master pattern. SLA rapid prototypes are typically used for the patterns, but SLS and Polyjet prototypes work as well. To make the mold, silicone rubber is poured around the pattern. Once the silicone has set the pattern is removed, leaving a negative image which polyurethane can then be cast into.
Silicone molds can usually produce up to 20 parts. Family or multi-cavity molds can also extend their usefulness. Because silicone molds are flexible, they tend to be more forgiving, and slides are usually not required for minor undercuts.
Insert or over molding is a snap with silicone molds. To make over molded parts two patterns are made, one with the over mold, and one without. Then the two molds are made from the masters, and the urethane parts are cast. First, the base part is cast using the mold without the over mold. Then, the part is moved into the second mold, so that the second casting can be poured over it. Using this process, production-like over molded parts can be made quickly and at an affordable price.