Creating Multi-Cavity Molds

When looking to create more than a few reproductions of a model, most toolmakers will create multi-cavity molds, like the one shown here. We’re going to demonstrate the making of a simple six-cavity open mold, although this same concept can be applied to two-part molds, clear molds and even composite tooling.

We start off by machining our model. For this example, we’re using Freeman’s machinable wax because of its ease of machining and excellent surface finish. Our machined block looks like this. From here, we trim the model down to a smaller size, which will enable us to fit six models in a 6 by 8 inch mold.

Here is a single cavity mold created from our first model. We could use this mold to cast five additional models, but instead, we chose to run our machining program five more times to create our six master models.

Our mold frame is large enough to ensure at least 3/4 of an inch of silicone rubber will surround our models.

Next, we position our models on the mold board to make sure they are properly centered. Then we apply Plast-Econ modeling clay to the edges of each model to secure them onto the mold board and make sure the rubber won’t flow underneath.

Next, we remove the mold frame and trim the excess clay from around the edges. And then we apply a little wax release on the mold bottom. The wax release will remove any of the remaining excess clay.

With the mold frame reattached, we begin to pour our Rhodia V-340 silicone rubber. We pour in a narrow stream which enables the material to easily flow around detailed edges and into cavities. Unlike some other materials, V-340 will not entrap air in our cavities. Instead, it flows to the bottom on its own, quickly releasing any bubbles and allowing more material to fill the remainder of the cavities.

The next day, we remove the mold board, revealing the bottom sides of our six models. Each model is then removed easily, revealing our six-cavity mold.

After cleaning up some of the flashing, our mold is ready for casting.

Here you see two six-cavity molds being filled with our Freeman 1090 Clear Polyurethane. The next morning, we have 12 perfect clear castings and the mold is ready to go to work again.