Rio Grande was founded more than 60 years ago by my father, Saul Bell. A jeweler for more than 80 years, Saul was involved in every imaginable aspect of the jewelry industry, including manufacturing, diamond-setting, watch-making, wholesaling and retailing. He loved to share his vast knowledge of the jewelry-making process. My brothers, sisters and I still benefit daily from his knowledge and wisdom, and I hope to pass on some of what I have learned from him and from other master jewelers through this series of blog posts.
There are many opinions about how to make a good rubber mold, and I guess one more—mine—won’t hurt.
So here is my recipe for an excellent natural rubber mold (because “good” isn’t good enough).
For a mold to be excellent, it has to be free of internal stress. This means that, after it is cut, the top and bottom halves each lie flat (not concave or convex) and, when they’re put together, they lie perfectly parallel to each other (with no gaps along the cut line), without any pressure on them. Here is how I do it and why.
We all remember that only gases can be compressed and that rubber, like most materials, expands when it is heated. So, even though the process is called “compression molding,” the increase in volume that occurs when rubber is heated has to go somewhere. If the mold is in a closed vulcanizing press, the “something” that gives is usually the vulcanizer.
We see the result of this in the amount of flash, where the rubber was forced between the mold frame and the press platen. When this flash is heavy, the surface of the mold is likely to be concave. When a mold plate is placed on a concave mold, the pressure of the wax can deform the rubber up to the plate and the wax pattern will be a bit heavier than it should be.
I pack a mold so there is as little air as possible around the model, then I over-pack the mold with about half a sheet of rubber to make sure the mold is not under-packed. My natural rubber mold frames are vented with 1/8″ holes centered on all sides. Anyone who has ever packed a rubber mold knows that cold rubber makes a good plug, so don’t expect the vent holes to vent rubber until the rubber is in a liquid state—and it doesn’t become liquid until it reaches the vulcanization temperature. Even in a preheated vulcanizer, it takes time for the heat to penetrate to the interior of the mold. To allow for this, I place the packed mold in a pre-heated press and lower the top platen so that it makes good contact with the top of the mold, allowing heat to transfer well but leaving lots of room for the upward expansion of the rubber. I set a timer for 1.2 minutes per 1/8″ of mold thickness (a 3/4-inch thick mold would be preheated for 7.2 minutes).
When time is up, I press the mold as tight as I can all at once—I see rubber spaghetti extruding from the vents and I usually hear some air bubbles pop. If the platen temperature and total process time are correct, you should get an excellent mold every time.
You may have heard that tire manufacturers don’t vent tire molds, and it’s true, they don’t. The reason is that the un-vulcanized rubber tire pre-form is placed in a metal mold that controls the outside surface geometry of the tire. Then they inflate a bladder inside the tire, using steam to provide both pressure and heat. The steam, being a gas, can be compressed and therefore allows the rubber to expand without pressure relief vents. When I was a teen, I hung out at Alba Tire Company, where they recapped old tires. They preheated the rubber to vulcanization temperature before putting it in the press so that it was already expanded and wouldn’t deform the tire or need a vent. Packing a jewelry mold with 300-degree rubber would work too, no vents needed . . . just kidding.
Cleverly used, vents significantly improve your chances for achieving an excellent mold—every time.