Mold Positive: CAM -> Plaster -> Slip Cast

 Prep for CAM:

 

    Getting things prepped for CNC, I opted to do two pass on the foam with a tool change inbetween from the 1/4" end mill to the 1/8" end mill. The first pass I used the pocket clearing toolpath generation just to get most of the foam cut, as the 1/8" end mill was too short to clear the foam efficiently. I had accidentally set the origin of the stock to one of the bottom corners instead of the top, and so when I zeroed the CNC machine on the top of the foam stock, the machine attempted to cut the space above the foam. To fix this, instead of trying to rework the original file with a new origin, I simply zeroed the machines X and Y axes on the top corner of the stock, and then moved the tool head to an empty spot on the CNC bed and zeroed the Z axis against it. This worked perfectly.

    For the tool change, I had marked previously a spot on the stock to be my origin for zeroing the tool, and so after the tool pass, I zeroed the XY there, and then the Z on the CNC bed again. For the finishing pass I used the Parallel pass tool path. Fusion gave me a warning about part of the 1/8" shank colliding with the stock, but I adjusted it to be minimal contact, and after cutting, couldn't tell that there was any collisions.

 

Plaster Pouring:

    I did my best to clean out as much of the foam bits that remained, and then brushed on petroleum jelly for the mold release. I had also setup my mold so that there was enough wall left in the foam to be self contained and not need to use cottle boards.

    I mixed and poured the plaster, but while agitating it, lots of water started to settle out, so I likely didn't mix it for long enough before pouring it. I wicked away the water with paper towel, and fortunately the plaster still cured properly.

    Getting the plaster out was somewhat tricky, but gently flexing the foam mold helped. Unfortunately, the plaster pulled out lots of loose foam bits that I wasn't able to remove, leaving lots of tiny pockets in the plaster.

 

Slip Casting:

 

    The plaster mold took a while to dry, as I was using the foam mold to store it. Once dry, I did a first pour of the slip, filling to just the lip of the cavity and had to let it dry for over an hour, forgetting that I should have washed the petroleum jelly off first. After it was dry enough, it released fairly easily, pulling many of the foam bits out with it. I tossed this first cast into the reclaim, and then waited a while for the plaster to dry before doing the second casting. I poured about the same amount of slip as before, and waited about 45 minutes for it to dry enough, checking periodically. It released slightly less easily, but I suspect that it was from leaving for too long to dry. As expected, there were several little bumps across the surface where the foam bits were, and I attempted to smooth some of them, but there were too many, and the texture from the plaster mold differed too much from the parts I had smoothed, so I left it as is, hoping the glaze would help hide the imperfections.

Firing:

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    After being fired to cone 6, there was some warping, and I believe it was due to taking it out of the mold, because it bowed slightly as it came out. This could probably be avoided by either compressing it while it's still in the mold, or by using slab clay to press into the mold instead of using slip. Overall, I'm satisfied with the outcome, and certainly feel like I have a better understanding of how the materials work both with and against each other,  and I can foresee this knowledge being useful for future projects.