This week, I embarked on a quest to make spheres of ice. I very nearly succeeded.
I first tried machining a mold out of wax in which to cast a urethane rubber mold for the ice. I made the mistake of attempting to machine it with the ball end mill, so there was a very leaky seam around the two halves of the mold, and the registration holes did not register.. Also, I neglected to set the “tolerance” setting on the toolpath generation program to 0, so this attempt did not produce spherical castings.
I had a little extra urethane left over, so I tried making a mold of my office key in it. I left the key in the rubber while it cured, and then pulled it out leaving a key-shaped cavity behind. I was surprised to find that the rubber was flexible enough to allow the teeth of the key to slide out without damaging the interior shape of the mold. I then cast another key in the mold out of some low-temperature metal alloy that I found in the lab. The copy came out nearly perfectly. Unfortunately, the metal was quite brittle and broke off in the lock when I tried using it.
Next, I went back to the drawing board with the ice spheres idea. I designed and machined two mold designs.
The first was a mold for another rubber mold, this time with the seam around the middle of the sphere so there would be less flashing due to the higher liquid pressure at the bottom of the mold. I also milled it with the flat end mill for sharp corners to minimize flashing. I cast urethane rubber, rubber-banded the halves together, filled the mold with water, and put it in the freezer.
The other design was a bit more complicated. I designed two halves of a mold with a platform for a ping pong ball in the cavity, so that when the rubber is poured in, it would flow around the ball, creating a thin-walled mold that the ping pong ball could pop out of, leaving a round cavity behind with perfectly smooth walls. It worked well, but I was unable to test the resulting mold with water because some of the rubber was stuck to the hot glue that I used to anchor the ball to the wax in the mold and tore when I was demolding it. When I get time, I will make another one and try freezing water in it.
The next morning, i took my mold out of the freezer to find a nice little ball of ice waiting for me inside of it! It was not perfectly spherical, however, as the rubber had allowed the ice to deform as the water froze and expanded. I would like to try making another mold out of less flexible material, like a harder rubber perhaps, and see if that would do a better job of restraining the deformation of the sphere as it is freezing.
