The Molding Mystery

I’ve played with silicone molding a couple times before, but this is the first two-part silicone mold that I made. There were some size limitations on the project, but it was otherwise pretty open-ended.

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It actually took me a while to find something to mold, and then I remembered these little pigs that I had laying around from testing my 3D printer. As is evident from the above image, this print went particularly poorly, as the top of the pig wasn’t completely filled, and the print layers are very obvious. If I tried to mold this, the silicone would seep in everywhere and would be disastrous.

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So the solution was J.B. Weld! I originally wanted to use Bondo, but I didn’t have any on hand, and the Bondo would have been pretty porous after sanding. I knew J.B. Weld sands pretty well, so I just used that instead. The finish was really lumpy, so more had to be done after it fully cured.

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After some 120 and 400 grit sandpaper, the piggy was pretty smooth, and all the holes were filled in.

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Before building forming the mold, I made some sketches to get an idea of where to part the mold and place the sprue.

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Piggy in mud! The first step was to bury half of the pig up to the parting line in clay. It’s a little tilted here to minimize the undercutting. The silicone is pretty flexible, so some undercutting near the parting line and near-vertical walls are acceptable.

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I used the end of a sharpie to poke some divots into the surface of the clay. These are for aligning the two halves of the mold later. Using some more clay, I sealed the edges of the foam core box built up around the clay block. It was sort of like caulking, but way more relenting. The next step was to mix and degas the silicon mixture and pour into the top of the box.

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After waiting 24 hours, I pulled everything out, removed the clay, and inverted the silicone block. I then placed a straw flush with the feet of the pig and built up some clay around it for the sprue.

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After both sides of the silicone mold were completed, I could remove the original pig and the straw. Next, I poured in the polyurethane casting resin. Ir seemed to come out pretty well. With some sanding and filing, and I was able to remove the sprue that was also cast. There are some bubbles on the feet, which I probably could have gotten rid of if I had degassed the part after pouring the resin.

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That’s it! The parting line is visible on the cast part, but it’s not too noticeable.

And as a mechanical engineer, I needed to get the orthogonal views of the pig. Thanks for checking this out!

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