Re: Just What This Forum Needs: Another Ironman Build
Today I finished the stone molds so I can slipcast the inner knee and elbow pieces. For those of you unfamiliar with the process, the mold is made using Ultracal 30, a gypsum product similar to Plaster of Paris (but much much better for this application). When the mold has cured, the sculpt is removed, the mold is reassembled, and liquid latex is poured into the mold. Since the Ultracal has a very porous surface, it wicks moisture out of the latex. Then the excess is poured back into the bucket and the mold is set up to air dry overnight. This process is repeated a couple of times to build up thickness for the latex parts. Then they're pulled out of the mold.
Here's a picture of both sculpts prepped for molding:
Because I'll need a two-part mold, I used water-based clay to make a parting wall. Since the sculpt is done in oil-based clay, there's no risk of the parting wall bonding to the clay itself. When the first half of the mold cures, the parting wall can be peeled off and any remaining residue can be removed with a soft brush and clean water. Here's what the molds look like with the parting wall built up:
The groove that's sculpted into the water-based clay will provide registration later to ensure that the two halves of the molds are properly aligned.
Once the parting wall is set up, it's time to layer on the Ultracal:
I start by layering on a couple of batches of mixed Ultracal to build a bit of thickness and capture all of the surface details on the sculpt. Because the Ultracal is a bit brittle by itself, once the first layer has firmed up it's a good idea to cut some strips of burlap, soak them in your next batch of Ultracal, and use them like bandages to build up a bit of composite strength. Think of it much like laying up fiberglass, but instead of glass and resin you're using burlap and plaster.
I didn't get any pictures of me actually laying up the burlap layers (my camera doesn't like being filled with gunk) but here's a shot of the first half laid up and peeking past parting wall:
Once the first half of the mold is cured, it's time to remove the parting wall:
Before laying up the second half, it's imperative to coat the mating surface with a release agent. In this case I'm using petroleum jelly:
The clay tabs that are sticking out the sides will leave holes along the parting line so I can pry the molds apart with a pair of screwdrivers later. You'll notice that they don't reach all the way to the inside of the mold. If they did, the latex would leak out during the slipcasting process.
The second half of the mold is laid up just like the first half:
Once the second half has cured, it's time to split the mold open and remove the sculpt:
Here's both molds out in the sun so they can dry out a bit more:
So that's where they sit right now. I will probably hold off until tomorrow morning before I decide to pour in the rubber. That way the molds will be nice and dry before I begin slipcasting.
In other news, I laid up the second half of the mothermolds for the thighs:
So I should have a pair of them pulled by this time tomorrow.
Stay tuned...
