Casting and MoldingMachining a positive:
I designed a simple part that was a bunch of towers rising to different heights from a square block of ground. I initially attempted machining my part with the Modela, but ran into problems using the proprietary software and Windows. The print job would get stuck in the buffer and could not be deleted without going to extreme measures. Also, the job on the modela, though it was only about 1"x1"x1" in volume, was slated to take 2.5 hours. Instead, I decided to use the shopbot to do the job. The result was not perfect because I only used a roughing pass and the 3D tool path software did not like that the spacing between my features was exactly 1/8" (the diameter of the endmill I was using). The shopbot was much faster however, completing my part in about 8 minutes. Full, detailed tutorial on the Tools and Tutorials page.
I planned to cast the Hydrastone gypsum-cement mix into my wax mold to create another mold into which I could cast metal. The Hydrastone does not need much water mixed with the powder and had a consistency of a non-Newtonian shear-thickening fluid. It poured into the mold in clumps but quickly flowed to fill all the cavities with accuracy. On impulse, I quiickly created a mold out of some modeling clay that I had brought to increase the capacity of the wax mold. I made a shape with a wooden depressor and filled that too. The castings set up within an hour in a toaster oven at 175F.
I immediately removed the clay from around the second casting. The Hydrastone had filled it excellently, but it seemed to have absorbed some of the oils from the clay. I cut most of the wax away from the first casting and then stuck it in a toaster oven at about 350F. The wax melted slowly (about 1 hour), but the temperature may have been too high because the Hydrastone developed a small crack diagonally across its length. It's also possible that it had not reached its full strength, as I put it in the second oven nearly immediately after I had taken it from the first.
I closed the crack with epoxy (the mold was still integral) and went ahead with the Ostalloy casting. This alloy is a low melting point combination of bismuth, lead, and tin. Pouring the liquid metal into the mold was very straightforward. After it hardened completely in about 15 minutes, I began to break away the Hydrastone mold. The sides came easily enough, due to the crack, but the center cuts took a bit of 'jackhammering' with a centerpunch.