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I searched in many places and on Youtube and I found ways to make silicon mold putty from cheap silicone using ordinary household stuff and kitchen stuff.

But nowhere is a recipe on making silicon casting resin out of the cheapest everyday silicone.

Clearly, casting resin has a much better usage spectrum than molding putty and it is the way molds should be created. Ethylene-Acetate dissolves silicone and results in very long hardening times (weeks or months) and I suspect that it also could make the molds shrink a lot while hardening. Any ideas?

I am not dependent on kitchen materials and I can buy some chemicals like Ethylene-Acetate in the local painter's store.

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The problem with attempting this is the nature of the cheap silicone. Most store bought silicone is already polymerized before use. For example, the silicone caulk which can be purchased in home improvement areas is a finished product which has been infiltrated with solvent. These caulks contain no catalysts to facilitate reactions. Any curing is the result of solvent drying from the caulk and solidifying the resin.

If you were to attempt to further dissolve one of these materials with something like ethyl acetate (see here for a simplified list of solvents), it probably would not go well. All you would be doing is further impregnating the silicone with solvent. While this would decrease the viscosity, it would increase your drying time. Shrinking would occur during your casting, and I expect that you would also experience a very messy result, no matter the quantity of release agent you applied.

Consider a very simple silicone (aka polysiloxane). Two silanol molecules react by hydrolysis to form a disiloxane and a water molecule. In order to reverse the process and obtain your starting material, you would need to put the water back into the disiloxane to cleave it. This is energetically expensive.

Even the full polymerization process is not a spontaneous one. When you purchase a silicone casting resin from a supplier, you are actually buying a Part A composed of n-siloxane (where n is a small enough number to be a liquid) and a Part B composed of crosslinker and catalyst (this is often silicic acid and tin because they are inexpensive). So what you are actually doing is taking a short chain polymer and crosslinking it into a solid mass.

Adding silicic acid and tin to a store-bought silicone product such as caulk would probably cause a reaction. The result would probably be a very hard rubber with incompatible properties for casting. However, I have not attempted this. Furthermore, I am not sure silicic acid would be easily accessible to a home chemist.

Despite how fun it would be to synthesize your own silicone casting resin at home from everyday chemicals, it probably isn't feasible. There are some inexpensive options out there for resin casting, and I encourage you to go with those.

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  • $\begingroup$ many thanks this is a good explanation! to everybody: I seek inexpensive casting resin vendors, please add links or names here! thanks a lot, Gerald $\endgroup$
    – Gerald Trost
    Commented Oct 25, 2015 at 15:44
  • $\begingroup$ Glad I could help. I didn't want to put any sort of brand support in the answer. My current favorite is OOMOO 30 from Smooth-On. For molding projects that only needed a temporary rubber, I used Cast-A-Mold 25T. I found both through Amazon, although I have been making purchases directly from Smooth-On lately. For kitchen chemistry, I recommend something like the OOMOO which is mixed 1:1 by volume rather than the comparatively more complicated 10:1 by mass ratio. $\endgroup$
    – WesH
    Commented Oct 26, 2015 at 19:56

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