casting jewelry...making wax models (part two)

"Tree" of wax models on a rubber base

Now that I have a mold, it's time to try wax injection to make multiples. I'll show you the photo of the end goal first, so you can see what I'm shooting for.

This is a "tree" made out of the smaller wax models. It will be then embedded in a heat-resistant plaster, the wax melted out, and molten metal sucked into the cavity left by the molten wax. This s the traditional lost wax casting you might have heard of.

Want to know why your dentist charges so much for a crown? It's because your dental crowns and bridges are made by this process. When the dentist makes a mold of your mouth with that rubbery stuff, they use it to create a mold that can create a wax that is then used to cast the cap.

Manual wax injection machine with mold

So here's the process. I made the original silicone molds in the last blog post, then used a sharp knife to split the molds (another YouTube video). Wavy cuts keep the mold aligned correctly.

Now it's time to fill them with wax. I have an injection wax unit that I recently purchased, and all I knew about it was that I have to put some wax in it, turn it on, and voila! Well, it was a little more complicated that that...

It ends up you have to pump up the unit, then push the mold onto a nozzle. Sounds easy enough! Well, the reality is that the pressure level is more important than I thought—and instructions say "keep the pressure between 3 and 15 psi." Well, which is it? It ends up you just have to experiment. Something about the temperature of the wax and correct psi that will make a clean model.

Completed wax model in the mold

I also found that putting the rubber mold between two pieces of acrylic helped make a better wax model. When I didn't do that, wax oozed between the cuts in the mold, and made a really messy model. I turned the pressure down, and everything worked better. However, holding the acrylic plates hurt my hands, and a shooting pain went through the top part of my hand. Of course the problem sent me off looking for something online to hold the mold. I couldn't possibly be the first person on earth to have this issue?

Sixty dollars and fifteen minutes later, a mold holder was on its way from Rio Grande Jewelry Supply. I love the internet.

Silicone mold with cornstarch dusting

I also noticed that some of the waxes had places where the mold wasn't filling completely. I also saw something that I'm familiar with from working with glass, and also ironically from working at Apple years ago. Apple had a VERY expensive Cray computer that simulated mold pours of the plastic casing of Apple's computers. It would show where the molten plastic would cool while pouring, showing what is known as "chill marks." Those are undesirable, by the way.

It ends up my waxes were doing the same thing. A quick post on Metalsmiths Coffehouse on Facebook provided another suggestion: dust the mold with cornstarch. Another YouTube video suggested cutting vents in the rubber to allow air to escape.

The mold to the left has already been injected, as you can tell from the red staining from the wax. The cornstarch dusting and vents worked perfectly.

I got back to work making waxes. I didn't realize until later that night that some of them had small holes or were incomplete. I might need a new pair of glasses! So I threw those back in the injector tank and made more. It ends up that making extras is a GOOD IDEA.

The end result, a BUNCH of my new little pendant clasps, ready to be attached to the tree.

 Wax models from both silicone molds, ready to be attached to a tree.

Wax models from both silicone molds, ready to be attached to a tree.