Creating A Rubber Mold
Written by Tom Weishaar, C.M.B.J.
1. Recently this custom ring design came to my bench with the request that it be made in platinum and a rubber mold be created in case our store ever wanted to make a second ring. The new ring was to be made for a 1.5 carat center stone and also to be set with 24 small melee diamonds in the shank and center head. Creating high quality rubber molds can be a bit tricky. I thought you might like to see how I go about it.
2. I started the job by making this sterling silver master model according to the design specifications. Then I soldered a three millimeter brass sprue rod onto to the bottom of the shank. I like to make my models 10% heavier than the finished ring so as to allow for finishing. Also some of the intricate details seen in the drawing will be fabricated and applied later so as to achieve the cleanest possible look in the finished ring.
3. All master models are first polished, then go through a three step plating process prior to molding. The first step is to copper plate the model and the sprue. Copper adheres well to both the silver and the brass sprue rod, and prepares the model for the next step. I like to think of copper plating as priming a wood board before painting.
4. The second step is to nickel plate the model. Nickel is used because it provides a very tough surface that bonds to the copper and prevents metal oxides from contaminating the rubber mold. Rubber molds that are contaminated with metal oxides will stick to your wax injections making intricate wax patterns difficult to release. If you don’t know how to tell whether your molds are contaminated, just check for dark areas in the rubber around the where the models had been. Dark, blotchy rubber is usually the result of copper oxides.
5. Finally the model and the sprue are rhodium plated. Rhodium creates a wonderful surface that will not adhere to the rubber. A surface this smooth in you model ensures that injection wax will flow easily into your mold and release without sticking.
More detailed information on plating techniques is available in “The Midas Guide to Plating” available through Rio Grande (1-800-545-6566).
6. The master model of my ring measures 8 millimeters across. I’ve chosen a 1 inch mold frame which will leave a little over 8 mm of rubber on each side of the model. I recommend a 1 inch thick mold because it makes for a strong mold that I can cut deep keys into. I like to use Castaldo’s, No-Shrink Pink®, rubber for all my master molds. Since No-Shrink Pink® is a bit more expensive rubber, I often create a sandwich mold using Castaldo’s, Gold Label®, rubber for the outside layers of the mold. A sandwich mold saves a bit of cost and the gold color rubber serves as a device to show me how deep I’m making my cuts.
Creating a Rubber Mold continued
