Tag Archives: matthewDG

Silicone front

I’ve been holding back something; remember that I had to cut off two pieces which make the back of the head to get this mold to work? I need to make a quick box mold for these, because, if I’m smart, I will be mixing enough silicone material to fill my mold up with some leftover, and I’ll have this second mold to use it up in. So, the usual procedure. Make a cradle, model clay up to the parting line, and box it in.

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Next, I’m ready to pour the silicone blanket for the front side of the big mold.

It is especially important to apply mold release to all the surface of the clay model, and paste wax to the plaster shell that contains it; otherwise, the silicone rubber will stick tenaciously to both and make it impossible to part the mold later, destroying the work entirely. I use a spray-on product, Mann Ease-Release 200, which is hydrocarbons in ether base, that I get from Smooth-On, same place I get the silicone rubber (Mold Max 30) from . The paste wax is Briwax, a furniture wax with a lot of solvent of some kind like Toluene that vapors off quickly. I get it at a hardware store. USE RELEASE, over use it, and use it AGAIN. And if you can’t remember for sure, use it again. That’s where this high-end release is good; it doesn’t build up and alter the model’s surface like many other common household substances which are often use for release. I won’t even name them.

After that, I assemble the mold, and seal off the bottom with a flat clay piece, which is the pouring funnel area for the finished mold, to keep the silicone in. Wrap the mold seam with tape, to seal, and bind it up with a rubber strap, a cut up bicycle inner tube. They work great.

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Next, I mix the silicone. I never have pictures of this because I’m too busy working quickly before the material passes it’s pot life. Here’s the summary: Decide how much silicone you’ll need to fill the mold. This is hard. What I do, is weigh the clay blanket that I removed earlier. Based on a lot of experience, I make some calculations. 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. Think about this. The clay is heavier that the silicone rubber, but the volume is the same for both, so I’ll want this formula to figure how much less of the silicone by weight I’ll need to fill the same volume as the clay. I derived it from the product’s specification sheets. Believe it, I have a whole notebook of these numbers worked out for each mold I make. There is more: Realize also, the clay blanket doesn’t precisely define the volume of the mold. It is like a loose drape over the sculpture. By much trail and error, I have learn that the volume of clay is about one half the volume of the space the silicone needs to fill. So double the volume calculation and work from that. You want to have enough to pour the whole mold at once. You do not want to have to mix up extra while the first batch is setting-up. You can, but it is stupid. I have done it too often, trying to be frugal with the cost of material. Just mix up what you know will be more than you need, and have some other small molds going which you can pour the extra amount into at the moment. It is better to mix up too much and throw it away, than leave the material on the shelf getting old past its expiration date. For this mold, I am mixing double plus another one-half, or 250%, of the volume of the original clay blanket. This mold is intricate, so I’m also using a silicone thinner at 5% of the total in order to help it flow better into and throughout the mold.

You can find instructional videos on the web which explain the proper mixing and pouring of silicone rubber material.

It helps a lot to use a vacuum pump to depressurize the air bubbles out of the mix. You’ll get a smoother mold without the chance of small pin-bubbles marring the surface details.

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The silicone is poured in from the highest point of the mold. Various other high spots in the mold are vented to allow air to escape, so that the rubber can fill the mold completely. As each vent shows rubber flowing out of it, it is plugged with a bit of clay, so that the rubber will continue to fill higher and higher. There are eight vents to see in this picture.

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On the side, is the box mold for the head pieces.

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And some leftover. That’s good.

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It’s a lot. A lot of planning, a lot of work, a lot of time. Truth is, it stresses me out, doing this. Sometimes, I think I should find another way to live my life.

Back plaster

I boxed up the mold, preparing to pour the plaster for the back side. I just build this quickly using foam-core, hot glue, and tape, lots of tape. I seal the inner seam of the box with clay where it meets the front side plaster so there is no leaking out of the new plaster. I line the inner side of the box with packing tape to release the plaster. I added the sprue and vents for the silicone to eventually pour into, in the form of cardboard tubes and soda and cocktail straws, but that step is still a way off (the clay under the plaster will be removed and replaced with poured-in silicone, forming an inner blanket which is supported by the solid plaster outer mold). It’s a lot of details with not much to picture.

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Mix the plaster, something I’m not showing you, pour it on and shape it up. You can see some markings above which I use to estimate the thickness I want from the plaster.

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I’ll let this dry a few days, take away the box and smooth it out some more.

It is a relief to be at this point. I’m maybe halfway done, but some of the most critical planning and preparation steps are complete now, and if I’ve done this correctly, most of what follows will be clean straight-forward work without anxiety.