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I'm currently experimenting with making silicone molds for casting. I'm not interested in the commertially available molds and want to make my own. For financial reasons I use the cartridges of transparent silicone commonly available in home construction stores.

In my first experiment I wrapped a plastic bead in silicone to make a mold. The silicone took on the color of the bead, so it clearly reacted and melded with the plastic. I was unable to remove the bead from the silicone and threw the whole thing away.

Next I carved an object out of candle wax and covered that in silicone. When the silicone was cured, the object was covered by a thin layer of very white wax, almost like a frosting. After removing the object from the mold the white residue stayed behind and the planes of the object were not cleanly molded and full of imperfections.

Then I tried a special "casting putty". It is not polymer clay like Fimo, but a putty designed to seal gaps in molds before casting. It gets soft and malleable at body temperature and hardens when cooled down. Again, the silicone melded with the putty. I haven't tried polymer clay because I noticed that I couldn't make the planes of the putty object as smoth and shiny as I wanted.

I wasn't expecting silicone to react and meld with materials, least of all candle wax.

Is there a material that can be easily cut and carved into clean, shiny planes but that does not react with silicone? Or do I have to coat my objects in another material to protect them from the silicone?

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The reactions you noticed are quite certainly not the silicone, but the by-product/reaction-starter of the hardening silicone, an acid. This acid can usually just escape as vapor and is what makes the acidic smell after using silicone. There are different versions of home construction silicone, that use different chemical pathways for hardening and therefore have different by-poducts.

You may want to try something that has a tin based reaction starter (2 component system) or a 1-component system based on amines (smells like fish, not like acid, while hardening).

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Cover the object in petroleum jelly (aka Vaseline) first, you may also try adding paint thinner to it, to make the coating smoother.

Using different types of construction silicone may give different results. Some of these contain glue and other components that make the cured silicone stick better to different surfaces. So I guess the simplest and cheapest neutral silicone is the best.

EDIT: some say that adding the corn starch to silicone makes the molds dry faster. Not sure if it makes the silicone less liquid though.

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  • Do you mean adding paint thinner to the Vaseline or to the silicone? – Elmy Nov 16 '18 at 12:37
  • To the vaseline, so it is spreads more evenly. – Congenital Optimist Nov 17 '18 at 16:38

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