Week 6 -- Casting
This week we're tackling molding and casting. For my model, I stumbled upon NASA's great archive of 3D models and decided to make a sapce shuttle. This decision came given the constraints we have with the milling process. We have a 3-axis milling machine, which limits us to having any models with interior features. I also decided to make a 1-sided mold, which further constained me to choose a model that did not have significant 'bottom' features.
I milled my board using the Modela miller with a .125" ball-tip bit. The roughing and finishing runs estimated that they would take 4.5 hours to complete, but everything was finished in about 3 hours. The Modela wax shavings were much larger than I had expected, but the machine didn't seem to mind - plowing right through all the excess wax. I did vacuum the excess every hour or so, just so that I could visually check that everything was going smoothly.
For molding we used the Oomoo silicone rubber kit. I measured the two components out by weight, and followed the prescribed ratio of 1A:1.3B. I mixed and shook the two parts for about 10 minutes after combined. I attempted to remove as many air bubbles as possible by shaking and dropping the cup on the table. When pouring, I also tried to keep the stream fine as an attempt to remove any additional bubbles. I followed this procedure until the entire surface of my model was covered, and then poured a bit more quickly, assuming that air bubbles within the cast - but not on the surface- wouldn't be much of a concern. Once the entire mold was filled, I shook it for another 10-15 minutes, trying to bang out any remaining air pockets. After an hour or so, the top layer of the mold was solid, but still a bit tacky. After 2.5 hours, the mold felt much more solid, but I gave it another couple hours just to be safe.
I began casting with the drystone. The mixing went well, but when demolding, the wings and tailwing snapped off. They were too thin and the material too brittle. I could imagine being sufficiently delicate that this doesn't happen on a second run - but I opted to try a harder casting material. I used the HydroStone Super X for the second run. I also put tape around the edge of my cast so that I could have a thicker top layer to begin with. This run came out a bit better - the wings still intact. However, the tail still snapped on when (delicately) trying to pull it out. My hunch is that the aspect ratio of the tail combined with it's thickness just makes it a very tricky thing to reproduce.