Packing a Rubber Mold

Talk Much time and skill is taken to make a jewelry model. Carving a wax model, fabricating, or a combination of both is the most time-consuming part of making jewelry. After the master model is created, a rubber mold is made to reproduce that model many times over at just pennies per copy.

This month we will make a rubber mold of a pendant mounting using Castaldo natural mold rubber with a rubber mold vulcanizer. The pendant mounting that is to be used is seen in photo 1 after it has been soldered to a mold sprue. Photo 2 shows everything needed to pack a rubber mold: frame, plates, mold rubber and a tweezer.

When choosing a mold frame, make sure it is thick enough to have at least 1/8” to 3/16” on each side of the model you are molding as seen in photo 3. If the frame is not thick enough, there is a good chance of puncturing the side of the mold while cutting it. Before packing a mold, be sure to scrub your hands and fingers. Dirt, oil, and grease can prevent rubber sheets from fusing together during the vulcanizing process.


Mold rubber has protective sheets that keep the rubber from adhering to other rubber that it is stored with, and it keeps the rubber clean until it is to be used. Peel the protective sheets from the rubber as in photo 4 and lay the rubber into the mold frame.

With the mold frame thickness I chose, I need 1/4” of rubber on both sides of the pendant. This means I need 2 slices of rubber in the frame, a center sheet of rubber and then another two sheets on the top to fill the mold frame. For the center slice of rubber I lay a piece of rubber flat on a plate or clean bench top, and push the model into the rubber leaving an impression in the rubber which is my guide for cutting a 20Jewelers Inc. January 2009 January 2009 Jewelers Inc. 21 relief area for the pendant to fit into (Photo 5). I use a scissor to cut this shape from the rubber slice.

Packing the hollow areas of the pendant in photo 6 is very important. This ensures the rubber material fills all areas around and in the pendant so an accurate mold of the model is made leaving no voids in the mold.

You will notice in photo 7 that the mold is packed half way and the pendant is pushed into the relief that we had cut out of the center slice of rubber. Notice that the rubber is packed tightly around the pendant with very little air gaps.

After packing the model in rubber, there may be scrap rubber pieces. Instead of throwing away that expensive rubber, I spread the scraps on the top and bottom of the mold for a little extra fill.

The mold frame, two mold plates (top and bottom), and the mold are then put into a preheated vulcanizer, photo 8. I set my vulcanizer at 307 degrees F for the rubber I use. By turning the top wheel, the vulcanizer compresses and heats the rubber until it is cured.

Castaldo mold rubber suggests a vulcanizing time of 7 minutes per slice of rubber. This mold has 6 slices including the scraps of rubber added to the top and bottom surfaces for a total of 42 minutes. It is best to lightly tighten the vulcanizer to soften the rubber for 5 minutes before tightening all the way. This prevents bending of the model.

The vulcanized rubber mold is ready to be cut in photo 9.

Next month, I will show how to cut this mold to easily remove an injected wax model.

Until then, keep your nose to the bench pin.

Steve Inlow

Steven Inlow is a JA Certified Master Bench Jeweler. He has been designing, manufacturing and repairing jewelry for over 30 years. He owns and operates Steven Inlow Designs, a fine jewelry manufacturing and special order shop.

This article is republished here by permission of the publisher of Jewelers Inc. and the author, Steven Inlow.