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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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  • I was compiling questions for something else and the key words picked up this one. It's probably long overtaken by events. The 2-part silicone peels off a lot of materials even without mold release. The construction silicone is designed as an adhesive or caulk, so it's designed to bond with stuff. If you stick with the type that smells like vinegar, the corn starch suggested by Congenital Optimist can help if you use it right. It speeds the cure of that type of silicone and gets it to cure throughout, even if it's pretty thick. It does thicken it or turn it into a putty. (cont'd)
    – fixer1234 ♦
    Oct 31, 2022 at 10:55
  • A little bit will help it cure without affecting the flexibility too much. The more you add, the faster it cures, and the stiffer it gets (too stiff makes it harder to use as a mold). With the starch, it will go through a phase when it starts to cure where it's like a putty that isn't very sticky. At that point, you press it around the object like clay, and you may not even need mold release for many materials. There's a lot of online advice for how much starch to add, and how it affects the working and curing time.
    – fixer1234 ♦
    Oct 31, 2022 at 10:55

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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, 2018 at 12:37
  • To the vaseline, so it is spreads more evenly.
    – Congenital Optimist
    Nov 17, 2018 at 16:38

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