As I was searching for another casting project to hone my skills on, my eyes fell upon an injection-molded 3D puzzle cube made by Proto Labs. The cube had nine plastic parts that all fit together to make the cube. It fit the bill for an interesting casting project, so I started by planning out how each piece would be oriented in the casting flask. I wanted to be able to pull them straight out of the sand without having undercuts. The plastic parts were hollow with one side missing, so I had to put tape over the missing side so that I would cast solid aluminum parts.
After all the pieces were taped, I laid them in a circle in my casting flask. I positioned them close together so that the aluminum would only have to travel a short distance to fill the mold. Then, I dusted the parts in baby powder and sprinkled on casting sand, packing as I went. To make the other half of the mold, I first flipped the initial mold half over and then cut away sand that had gotten under the yellow piece. The yellow piece was tricky because it had an overhang/undercut in all orientations. Thus, I had to cut away the sand under the overhang so that I could later pull the piece straight out of the mold.
With the first half complete, I dusted everything with baby powder again and then packed sand into the second half of the mold, which fit on the first half with wooden pegs. When both casting flasks were finished, I pried the halves apart and carefully teased the pieces free from the sand. I then cut a sprue and gate system to feed metal to the pieces. The sprue and gate system should have rounded corners as much as possible to minimize sand erosion by molten aluminum. I did not make any vents for the casting, as the pieces were fairly small.
Outside, I melted a full crucible of aluminum and heated it for three minutes after the last piece of scrap melted to bring it up to pouring temperature. Then, I sprinkled sodium carbonate and sodium chloride on in a 50/50 ratio and scraped off the dross/flux mix. I poured the shiny liquid aluminum down the sprue, and half an hour later, I broke open the mold to reveal a casting that looked somewhat like an ancient Mayan symbol.
Using a hacksaw with a metal blade, I cut each piece free from its gate and began filing. Injection molding is quite different from sand casting, so the pieces weren't designed to be sand cast, and they were fairly rough straight from the mold. My strategy for filing them was to file two pieces until they fit together and then file additional pieces to fit the previously filed ones. I began with the orange and red pieces and ended with the green piece, since it holds the cube together. I found that filing the edges and corners of the pieces greatly improved the cube's appearance, so I did that, using a Dremel tool to get in the hard-to-reach corners. In total, I filed for 12 hours. It was quite awful.
When all the pieces fit, I sanded them with progressively finer grits to remove the scratch marks left from filing and sanding. I finished by sanding everything with a worn-out sanding sponge, which gave the cube a nice shine. When looked at from a corner, the cube appears to be a 3D maze, which I think is pretty neat. Although the cube was a lot of work and took an obscene amount of filing to finish, it was well worth it in the end. Nothing worthwhile is ever easy... still, I'm never doing this much filing again!