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I've been making moulds using an off the shelf two part silicone compound. I've made a mistake with one mould and while it's solid the surface is still tacky after 24 hours.

Is there anything that I can do to salvage this mould, or should I regard it as a complete loss, throw it away and start again?

The mold is supposed to be for casting using a 2 party epoxy resin. I'd prefer to salvage it if possible as it used about a 1/4 of my supply of silicone.

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If it's still tacky after 24 hours, that's not a good sign. If it feels solid, you can try cleaning it to see what happens.

At least some types of uncured silicone mix well with limonene, a solvent made from citrus peels that smells like oranges (use pure limonene, not a cleaner or other product that contains it as an ingredient). The stuff isn't cheap (best price is currently around $20 per quart in the US, so you might spend more trying to clean the mold than the value of the silicone if you succeed, which isn't guaranteed). If you can't get limonene, you can try mineral spirits, but that doesn't work nearly as well.

Put the mold in a sealable container, using standoffs or fishing line to suspend the mold with all faces exposed. The best container material for limonene is PETE, but polyethylene or polypropylene will work for the short time they will be in contact with the solvent. Polyethylene or polypropylene may temporarily swell, and the limonene will eventually start to pass through it, so don't store it long term in those plastics. An airtight metal container will also work.

Immerse the mold in the solvent for about a day. The mold may swell. The solvent will penetrate the silicone to some depth and leach out some of the subsurface uncured silicone. Then wash the mold well in soapy water.

The limonene will be contaminated with silicone, so the only potential reuse for it would be recleaning the mold if more uncured resin leaches out over time, or cleaning future silicone disasters.

Let the mold sit for several days until there is no more orange smell and no signs of swelling.

If the surface is still tacky, the mold is a lost cause. If this process cleans the surface well, keep in mind that the silicone didn't cure properly, so it may not perform well.

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  • Strangely, the parts of the mould that were touching the box used to contain it seem OK, it's the top that was exposed to the air that is really tacky. Are there any non-chemical alternatives, heating, cooling, exposure to UV light? Would a coating of mould making silicon likely help?
    – Aaargh Zombies
    Commented Sep 25, 2021 at 12:12
  • @AaarghZombies,, if the tackiness is uncured silicone, there is nothing that will make it cure at this point (temperature, UV, etc.). However, curing everywhere but the top isn't characteristic of that. When you say you made a mistake, are you just assuming it was something you did, or are you aware of something specific that could have affected it? Did you use a mold release? If there was too much, perhaps some excess floated on top of the silicone. It's hard to guess whether you could seal it in with a coating of silicone; it depends on what the tackiness is and how deep it goes. (cont'd)
    – fixer1234
    Commented Sep 25, 2021 at 16:33
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    I would try cleaning that surface first. Instead of immersing the mold in solvent, try wiping the tacky face with a paper towel wetted with mineral spirits, or even just washing it with soapy water. You could also try rubbing the tacky face with talcum powder. The tacky material may stick to the powder and you can "erase" it, or it may leave a dusting of the dry powder stuck to the silicone so the surface isn't tacky.
    – fixer1234
    Commented Sep 25, 2021 at 16:33
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You should wash the mold in hot water to vulcanize it a bit and then dry it with a heat gun or hair dryer on the inside, very carefully. After that, if you have a tube of Silpoxy for mold repairs, take a brush or use your finger if you don't mind peeling off goo and apply 2 layers sort of like greasing the mold with vaseline, but make certain to apply thinly and gently so you keep detail. You might have to go to a third coat. As it dries you might notice it seems to release water.

Also, if you want to try the limonene method of cure without spending 20 bucks, get a bottle of rubbing alcohol and a couple of oranges. Peel the top thin layer off the oranges with a peeler and put thin orange peels in alcohol in an old glass jar, use peels of 2 oranges and 1 cup alcohol and put it in a place that is warm. It will smell very orangey in a few days, and you can use it to try that method without spending a fortune.

Wet paper towels with the orange alcohol and pack the inside of the mold. Check it and rinse it out in a few days. The decoction also works using vodka and at least you will have the orange form of limoncello to drink if you make enough-some to soak the mold and some to drink. Don't drink what you used on the bad mold.

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  • In the end, for what I was doing (making doll house furniture), I found it simpler to shred the silicon and use it as filler for new molds, once I'd figured out what I was doing wrong. You're answer might help others though. I tried using alcohol but the surface of the molds was too badly pitted to be fixable.
    – Aaargh Zombies
    Commented Mar 24, 2023 at 8:24
  • The limonene information is interesting. Does soaking the orange peel actually get some limonene into the alcohol, enough to make it better than just the alcohol? Also, I've heard of using limonene to clean uncured silicone, but never heard about a "limonene method of cure". Can you point me to something about that so I can learn more about it? I did a quick search and didn't see anything.
    – Dolly
    Commented Mar 25, 2023 at 6:48
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sprinkle the uncured sticky surface with powered zinc oxide.

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