I recently made a plaque for my garage out of solid silicon bronze.
I created a form in Fusion 360, making sure to give any vertical surfaces a draft angle of 5 degrees (maybe I’ll do a little more next time!) The CAD software can even generate the estimated MASS of the finished item, useful for knowing how much metal to melt.
The 3D print is tightly packed in sand which has a little moisture and clay mixed in – just enough that it holds shape when squeezed in a fist.
Casting uses a special box which is open of both the top and bottom called a flask. The two halves of it are called a Cope and Drag. The bottom half of the box is set over a board. Then the pattern is set in place, face up. I sprinkled in sand “riddling” it through a wire mesh screen. This makes sure there’s no lumps, just fine even sand.
The sand gets tightly rammed down in place, and then the box is flipped over, revealing the BACK of the pattern. This is covered with a separating agent, such as talcum powder. Then the other half of the box goes in place and gets filled and rammed with sand.
Next, the mold is split and space for the molten metal is carved into the sand. Then the original pattern is removed and the mold reassembled.
I heated silicon bronze (mostly scrap copper, and a bit of silicon and manganese) in a propane powered furnace. Once liquid, the vessel of metal is lifted out with special tongs and set into a pouring shank. I then pour the bronze into the mold, and pour out any extra into ingot molds.
Once cooled to solid, the metal is removed from the sand, and quenched in water to get back down to handling temperature. Any metal other than the casting itself is removed with a Sawzall, grinder, or bolt-cutter. It can then be sanded, wire-wheeled, or polished.
Since this was a sign with raised lettering, I sprayed on a coat of black gloss paint. After it dried, I sanded the surface until I was happy with the shine and texture.
I also drilled and counter-sunk the corners of the sign for screws.
While this is a nice decoration for my garage, it was really just an experiment and PRACTICE in making such a thing. My father passed away a few years ago, and I always liked those “In Memory of” bronze plaques. I’d like to make one for him, but as always, I need practice first!
When I get to making that, you’ll be the first to know!
-Ben Nelson
