Silicone back

I’m doing the second side, the back side, of silicone mold. This repeats the steps done for the first, front, side. Of course, the mold is turned-over now. Apply a liberal amount of release to all the surfaces of the two halves, the one with the model sitting in the silicone of the front side, and the other, back side plaster. Where silicone will meet silicone there must be a barrier of release or else the two halves will bond permanently. It is surprisingly easy to neglect this for some reason, when one is pre-occupied with so many details to consider. I apply paste wax (Briwax) to the plaster, several coats, and a spray type release I’ve mentioned before, which is hydrocarbons in ether, or something. Works fantastic. I’ll say it again: APPLY A LIBERAL AMOUNT OF RELEASE TO ALL SURFACES. If you can’t remember clearly applying release, then do it again.

Assemble the plasters, strap it together and again, cover the feet/pour spouts area, sealing it tightly so there will be no leaks.

a_siliconeback

And that box mold I’m doing on the side? I added some pour spouts there too, and applied release. I’ll pour the second half right on top of the first, with whatever’s left over in the bucket from the big mold.

Here’s the mold, with the silicone poured in. Like before, there are vent holes at the high spots which let air escape the mold as the silicone fills it. The clay plugs the holes when the silicone oozes out. I have again mixed up 250% of the volume of the clay blanket removed in the last step. It follows the formula: The weight of clay times 1.06 equals the volume cubic inches of the clay. The volume cubic inches times .68 equals the weight of the Mold Max 30 silicone product that I’ll need. The rational about qualities to mix up are all in the earlier post “Silicone front”. On that first side, it turns out I had a good bit of silicone material left over, even after the box mold was poured. Based on weighing the leftover amount, just doubling the volume of the blanket would have been enough. And although I know this, I’m still going to use 250% volume, to be safe. It is a large batch, as much as I can get into the vacuum tank at one time.

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Enough to fill up the box too.

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After curing for 24 hours, I open the foot-cover to see. It’s all good, but do notice that 10 grams of material has leaked into the left spout. The seal wasn’t perfect.

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Split the mold open carefully, slowly, gently.

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The mold, by chance, opened with the first side facing out. This side hasn’t seen the light before, and it’s our first look at it, but it isn’t the side we just poured. It looks good without any air gaps at the high spots – all the venting worked properly.

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I want to free the other side. I invert the whole thing and suspend it on some cups to let it, sort of, ease its own way out, for a couple of hours.

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Then I gently start to pry it away from the plaster shell…

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..until it is worked loose.

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Interesting. See the air gap there on the belly of the figure? Although there had been oozing silicone out of that vent hole before it was plugged, there must have been some subsidence of the material while it was curing, leaving that void.

o_siliconeback

I think it may be the same amount of material which found its way leaking into the foot-funnel. Or perhaps, because I forgot to seal the edges around the plaster half with the blue tape, air got into the mold and allowed the subsidence to happen. Otherwise, the clay plug should have secured vacuum to hold the silicone up. Who knows, but in this case it is easy to fix. I’ll re-assemble the mold and pour some silicone directly down the vent hole and fill the void. Since I will not apply any release to this area, the new materials will bond tightly to what’s there.

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After 24 hours I open the mold and see that the repair is good.

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