And... on to a Mold That Matters. To the left and below is a sculpt that will ultimately be a base for a fursuit style head, with the first coat of rubber thinly brushed on. (The "halo" around its head is part of the mold, not part of the sculpt.)
This mold is made of silicone rubber, not urethane rubber as was my last mold. I want to cast resin into this mold, and urethane resin and urethane rubber, I've been told, do not Play Nice together. Among other things, a Runaway Exothermic Reaction may occur, which generates a lot of heat and results in the mold and the cast being irretrievably fused together. Not good.
More specifically, this mold is made of *platinum* cure silicone rubber, not *tin* cure silicone rubber. The former is much more expensive than the latter, but supposedly molds made with it will last a lot longer as well.
Platinum cure rubber is also supposed to be more prone to reacting with the model material. Sealing the model is strongly recommended (the very nice tech support at Smooth-On recommended using Krylon Crystal Clear Acrylic Sealer) and a small test on an inconspicuous area is also recommended. Silicone reacts with some kinds of wood, latex (wear vinyl gloves, not latex gloves, when using it) and most famously, sulfur. For this reason any modelling clay used to sculpt a model needs to be sulfur-free.
Both silicone and urethane brush on molds need to be applied in layers, usually around four, to a total thickness of a quarter inch. One beauty of silicone rubber, as opposed to urethane rubber, is that its' possible to apply additional coats anytime after the most recent coat is no longer sticky to the touch. In contrast, once one layer of urethane rubber is no longer tacky, another layer will no longer stick, and once a section of a urethane rubber mold is started it must be finished. Stress!!! Working with the silicone is much more relaxed and easygoing on one's nerves.
To the left, the sculpt with the second coat of rubber brushed on. I've lain it down on its back to minimize the rubber "boogers" dripping off its nose that I got with the first coat. I've also dyed it a lovely shade of pink with Smooth On's Sil Pig, so I can tell more easily that I've covered the orange first coat thoroughly.
In between coats, I mixed rubber thickened with Thi-Vex and trowelled it into undercuts, eye sockets, etc and built up a cut-seam ridge using a popsicle stick as an applicator. That's another advantage of silicone. Thi-Vex is relatively benign, while Cab-O-Sil, the thickener for urethane, is made of finely ground glass. It blows around very easily and you really, really don't want to breathe it.
To the right, the rubber all but finished. The last step was to make keys to help the rubber mold register with the Plasti-Paste mother mold. I did this by mixing up a small batch of rubber, splatting it down between two paint sticks and then cutting it up into squares when it had cured. I then stuck the squares onto the mold using more thickened rubber.
Here I also learned that Smooth On's instructional videos do sometimes leave steps out of the process that would be useful to know. The particular video I'd been watching over and over again did not show keys at all, and I assumed for a long time it was due to some special property of the Plasti-Paste mother mold material.
To the left, getting ready to apply the Plasti Paste mother mold. I've trimmed the edge of the rubber, applied a dividing wall made of Klean Klay and covered the wall with tinfoil. I then applied Sonite Wax and Ease Release over the whole thing.
There's another difference between urethane and silicone mold rubbers- they each need their own kind of release. The universal release spray used with urethane is made with silicone- if it's used with silicone rubber, it will act like a glue instead!
To the right and below right, half the Plasti Paste mother mold applied.
The chief advantage of a Plasti Paste mother mold over the traditional plaster is its much lighter weight. I was planning on hand-slushing resin in this mold, which means holding it and turning it for fifteen minutes while the resin sets up. Something not easily done with a 40# plus plaster mold!
I was dreading using this material as the instructions said it was very sticky, set very fast, and got very hot, but all this turned out to be manageable. The trick was to learn to mix only what could be applied in ten minutes. A saving grace was that, like silicone, additional coats of Plasti Paste could be applied after previous coats are fully cured. Whatever didn't get covered in one pass would get covered in the next, no big panic. It should be built up to be about 1/4" thick; in larger molds such as this one it should be more like 1/2" thick.
As an additional bonus, it wasn't as stinky as I feared it would be either.
The instructions also said to smooth the Plasti Paste down with wet paper towels or denatured alcohol before it was fully cured to avoid "an aggressive surface". And when they say "aggressive surface", they mean it. This is one downside of this material- even smoothed down, it is fairly "toothy". I nick my fingers on the edges routinely, and rip my rubber gloves while casting. I need to find heavier gloves!
To the left, the clay dividing wall removed, under the careful supervision of the Feline Assistance Department.
Again I applied tinfoil over the Plasti Paste, and applied Sonite Wax and the release spray over that. Supposedly the tinfoil keeps the two Plasti Paste halves from sticking together, and helps to mark the parting line when separating them. The downside of the tinfoil is that it very badly wants to stick to the uncured Plasti Paste and pull away from the cured material, making a gap in between the two halves. Fortunately it did not turn out to be at all critical, especially since the halves were bolted together.
Below, holes drilled through the two mold halves so that they later can be bolted together. And then, time to remove the mold from the sculpt- the moment of truth!