I looked at the Oomoo safety data sheet (available here) before I poured Oomoo into my mold (looking at the bottle's hazard warnings reminded me to do this). As someone who likes to collect all-inclusive information, this was kind of cool. Included some things I did not expect (but which make sense in hindsight), such as fire fighting measures and transportation regulations.
So, as a continuation of the corn stool saga started two weeks ago, I realized that molding and casting would be a great way to make a corn-like texture. However, my stool of OBS is already pretty heavy as is, and I knew that adding a stone facade would only increase the weight.
But what if the facade pieces were hollow? Then I would decrease the total density of the stool but still have attachable pieces! Thus, this week, I attempted to perform rotary casting. Though EDS has no formal in-house rotary casting system, Anthony told me it would be interesting to try rotary casting by hand (ie., just spinning the mold over and over again until mostly set); he was right, and it was a good experience.
Anthony advised me to take some rubber bands and a piece of acrylic to cover my open-facing mold (if I had had some foresight, I probably should have made a two-piece mold--oh well), then turn the mold over and over again in various ways to coat all the sides. I also decided to use some mold release on the acrylic just because I got a little nervous about anything sticking to the acrylic after setting (why? Oomoo can bend to pop things out, acrylic cannot); I probably didn't need it though.
The first corn piece I cast unfortunately broke; since the hydrostone didn't stick as well to the acrylic as it did to the Oomoo, the flat side of the piece (the side on the acrylic) was very thin and the corn piece broke both when I poked the flat side (everyone got a good laugh out of that one). I then casted it a second time, making sure to fill the mold with more material and keep the piece on the flat side longer. This time, it didn't break, though, and Anthony confirmed that it still was (at least partially) hollow by shining a flashlight "through" the piece.
For future consideration, Anthony also suggested just making a simple apparatus that had wheels and would hold the mold in place, so that rather than just turning the mold over and over again by hand, one could just roll the mold-holding cart back and forth for an easier and more even rotary casting. This sounds like a very fun idea for the future.
In total, the process this week was: 1) design initial mold and mill it into wax, 2) pour Ooomoo into initial mold to create rubber final mold, and 3) use available materials (including hydrostone, drystone, and leftover plaster of paris) to pour into final mold and create final casted part.
So, I showed up the day I had to mill without having finished modelling my part. After tussling with trying to get FreeCAD on my computer earlier in the day, I finally gave up and caved into getting a student Autodesk account. Tired of downloading things, I gave the browser version of Fusion 360 a try and it worked like a charm.
Both Anthony and I got a bit confused on how to make organic corn-like shapes in such an engineering-focused modelling program, but he suggested using the [??? I forget what this was called, but it makes less rigid shapes] tool to make more organic features. I eventually settled for something simple but corn-ish enough to be recognizable as such. After fixing some mistakes, adding milling properties, and looking at the simulated milling results, we generated the necessary GCode and took to milling.
So, we milled wax for the initial mold. It took longer than the software had predicted, and unfortunately what seemed to be a simple mill ended up being a botched job. The mill, likely missing steps in the Z direction, milled all the way through the block of wax and into the sacrificial wood below. Thus, we had to mill again, and this time, used a slower feed rate and [I forget the real name for this--the spinning of the spindle] rate. This time, everything came out fine.