engineering fundamentals Silicone Rubber Molding (SRM, RTV Molding)
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Highlights for All SRM Methodologies
  • Mold Fabrication Time: From pattern to first part in 3-15 days.
  • Material and Quantities of Parts Produced from a Mold in Table Below:
Part Material Number of Shots
Epoxy 15 - 30
Polyurethane 10 - 50
Polyurea 15 - 60
Investment Casting Wax 50 - 300+
Silicone Rubber 15 - 100
Low Melt Metal Alloy 15 - 90
Polyurethane Foam 5 - 250
(These are estimates only for a part of average complexity with a typical material such as urethane. More detail leads to faster degradation.)
 
RTV Silicone Rubber Molding Process Description
Room Temperature Vulcanization (RTV) rubber molds are the least expensive and fastest way to create about a dozen prototype parts. As shown in the figure below, even with undercuts, the part is able to be removed through distortion of the mold. The figure illustrates a small toy locomotive molded with urethane.
 
Urethane casting material is commonly used for product prototypes. Urethane can be formulated to have material properties similar to elastomers or ABS. Urethane can be machined, sanded, glued, and painted. Urethane can have material properties, color, and surface texture very similar to that of production materials.
 
  1. An RP pattern is used to create a master male pattern as shown below. The RP technique most often used is the SLA process.
  2. The SLA part is made slightly larger (e.g. 0.003 in/in) than the final product since this is what shrinkage the final part will see.
  3. The SLA part is sanded to a suitable cosmetic finish and can be sealed.
  4. The master pattern is fitted with a sprue and gate and then surrounded by a parting surface which establishes the parting line for the mold (see the illustration). Alternatively, the cured mold can be cut carefully to form the parting line.

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  6. The assembly is fixtured in a vat, and liquid RTV is then poured over the pattern and parting surface combination as shown below.

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  8. Once cured, the RTV is removed from the vat and separated from the pattern and parting line surface to yield the two halves of the molding tool. RTV air-cures, so that the cure time depends on the geometry, the RTV type, and the environment. Cure time can range from .5 to 40 hours. Aging of the mold after cure for up to three days can improve mold life.
  9. Thermoset resin such as urethane is poured or injected into the mold, as shown below.

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  11. Finally, the finished part is removed from the mold. Undercuts are overcome by distorting the mold, which springs back to its original shape as long as distortion is not too severe.

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  13. The part must then be post-processed by trimming any flash, as shown below, and possibly sanding. The gate and sprue must also be removed.

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