Hey, remember these guys? These are my finished master patterns for my enamel settings. I designed the steel sheet cutouts and sculpted rims onto them using epoxy putty, with the aim of creating molds from them and casting pewter onto the cutouts. Here's how that went.
I ended up having to nix the game piece in pewter for the moment due to the size of the base; it ends up absurdly heavy and too wide for most game boards. At some point I'll have to figure out how to accurately gauge the weight shift between the sculpting material and the metal casting. For the time being, I'll be making the game piece from resin. Casting it in an epoxy resin requires the wide base due to how lightweight it is, but I will narrow the base and hollow it out slightly for a pewter cast, similar to the footprint of a chess piece. However, I will make alterations to a resin cast rather than to the direct sculpture because physical pieces don't have an undo function.
The orangey-red material is uncured silicone rubber for the pewter mold. I carved cavities in the silicone so that the metal inserts lie flush against the mold and the parting lines are where I want them rather than cutting across the design. Once the pieces were in place and the rubber was laid down, the whole thing went into a mold form and vulcanized: heated to about 325 F under thousands of pounds of pressure for a couple hours. During this process the silicone basically liquifies and captures all of the fine detail of the piece, then cures into its solid state. A release agent put onto the mold before vulcanization allows the mold to be split apart along the lines chosen during molding.
Here you can see the color difference caused by the heat of vulcanization: the browner pieces have been heated while the greenest piece is the original color. Most epoxy putties can be vulcanized several times, so multiple molds can be made from the same sculpture. The molds themselves can be used multiple times, as opposed to lost wax casting techniques that have a 1:1 wax sculpture to finished casting result.
After the rubber is cured, gates are cut to allow the molten metal to flow into the cavities. There is a good bit of trial and error involved, and the gates and air vents are altered until you get a consistently good cast. Because the metal is cast at about 450-550 degrees F (depending on alloy), it only takes a few minutes to cast and cool, making the process pretty quick. Of course, I was greatly helped by having Peter Stachowiak of Perth Pewter overseeing and helping me; he's got 30+ years of moldmaking experience and knows all the tricks. For the record, this is really a very simple mold, but it's my first. And hey, it worked!
After getting all of the cavities to cast correctly, I put down the steel inserts in their places. Then I crossed my fingers and cast again. The pewter flowed into the cavities and through the holes in the rims of the steel pieces, locking in securely. Because the steel melts and solders at over a thousand degrees higher than the pewter, the two are bonded together not chemically but mechanically. It's the equivalent of bolting two pieces of material together rather than welding them.
Here's the pieces all cleaned up and with patina darkening the low-lying areas. The images below show how the magnet-set enamel clips in and out of place.
And here, in the spirit of exhaustive completeness, is the resin molding for the game piece. The is the first test of the mold, prior to venting, using a simple 2-part room-curing silicone. After pouring the silicone around the green and letting it cure, I had to slice it open to remove it. This is opposed to the vulcanized mold, which had the two halves predetermined prior to vulcanization. Once I had the original cut out I poured in a casting epoxy, which is the clear version you see here. I'll cut down and resculpt one of these as necessary, and possibly do a pewter version. But in the mean time, I'll be making some colored plastic versions. Fun stuff!