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How to Vulcanize Black Rubber Molds for White Metal Casting
Rubber Molds Vulcanizing requirements
1. The Vulcanizing time and temperature is very important. The temperature should be 310 - 315 degrees F (154 - 157 degrees C). The time should be 15 minutes for each 6mm of thickness. Which is the equivalent to about 1 hour total for a 25mm thick mold.
2. Gradually increase the pressure during the first 9-10 minutes and then maximize pressure at 10 minutes. Regarding maximum pressure, use whatever maximum pressure you normally use. If the rubber does not fill into every small space then increase the maximum pressure at the end an additional 5% above your normal maximum pressure.
A good method used by many is to drill 2mm holes in the side of the mold frame (one 2mm hole every 60-70 mm around the side). This allows some of the rubber to "bleed" out of the frame like black spaghetti. The purpose is to allow the rubber to move into the fine details of your design and also to allow any air inside to escape.
3. The mold should be about 60-65 Shore A hardness if vulcanized correctly.
Source: Bob Romanoff