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I'm trying to make some home-made molds out of two-part casting silicone.

I mix both components in equal proportions, pour them into a container with the item that I'm trying to cast (A resin model of prop for a dolls house) and leave them overnight to set in a warm dry place.

The silicone sets perfectly everywhere except where it is touching the resin blanks. It's firm, has an even color and texture, and has no bubbles in it. Textbook perfect.

Where it touches the original it is slimy and sticky, and there are multiple air bubbles. It's like there is a thin layer of slime only where the two materials are in contact with each other. There and only there.

I've cast from this resin earlier this week, and from the same bottles of casting silicone, and everything was great. And it's set everywhere except where the two materials touch so I don't think that I didn't mix the silicon properly.

I've been using this exact same brand of resin and silicone for a couple of years now and this is the first time that I've had this problem.

At first, I thought that I'd simply used too much mold release spray on the original, so I tried it again without it, but I had the same problem.

After the first failure I cleaned different sets of blanks using three different methods to determine if there was an external contaminant involved and tried the casting process again..

  1. Detergent scrub
  2. Alcohol scrub
  3. Alkali scrub

Exact same problem all three times.

I'm using Elegoo gray resin (one-part, photosensitive UV cure), and Limino molding silicone. I've successfully made two casts using the same bottles less than a week ago.

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    Are you using platinum cure silicone? What type of resin are the originals made of?
    – fixer1234
    Oct 1, 2022 at 9:27
  • The Resin is Elegoo Gray photosensitive resin., Limino two part silicon. I've used both to successfully make molds last week, so I know that the products are good. I've been using both products in combination for 2-3 years now Oct 1, 2022 at 10:11
  • I would question the formulation of the resin. There's nothing to suggest that the silicone product would change, but the resin might. As a test, consider to apply a coating of epoxy or similar sealing substance to the resin object, which would confirm surface contamination reacting with the silicone.
    – fred_dot_u
    Oct 3, 2022 at 22:28
  • How long did the resin cure? If it's grey UV resin, I would cure it for far longer than suggested to make sure the UV light can penetrate through the grey outer layers. It sounds like some uncured resin might seep out.
    – Elmy
    Oct 4, 2022 at 9:44
  • I hardened it for 3 minutes under uv and 2 days on the windowsill, which has always been enough in the past. I'm making doll house furniture so we're only talking about things between 3 and 10mm thick. Oct 4, 2022 at 13:51

2 Answers 2

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Testing demonstrated that high heat triggered the symptom, leading to a solution of avoiding the heat. The self-answer speculates on the underlying mechanism, which if correct, suggests that this is a dependable solution. Room temperature curing appears to avoid the problem, but the suggested reason isn't characteristic of silicone.

Ordinarily, the "why" wouldn't matter as long as the solution works. However, there is an alternate explanation for the symptom. If correct, it suggests that the silicone and resin aren't incompatible. Their working together may rely a bit on meticulous care and luck. If that's the case, using them together could lead to occasional failures of the same kind with a room temperature cure.

According to the manufacturer, the Limino silicone uses a platinum cure chemistry. Many materials inhibit the cure of this chemistry; see this list from Dow. It takes only trace amounts to inhibit the cure. If the resin contains any of these substances, or produces any during its cure, it will be in the master. Cleaning the surface won't affect what is inside the master.

Even fully-cured resin can exude materials. That happens faster at higher temperatures. If you clean the surface, mold it, and the silicone cures before the master exudes any of it, you're home free. But if trace amounts migrate to the resin surface before the silicone cures, it would inhibit the cure.

That seems like a plausible explanation for why the silicone that is in contact with the resin has a film of uncured material when cured in a hot place. This can be verified as follows.

Clean the resin master and leave it in the same hot location for longer than the silicone was there (you're just determining whether or not anything migrates to the surface, so the longer you leave it there, the more likely you'll see it; you can repeat the test later with shorter times to calibrate the operational window). Then apply some of the silicone and cure it at room temperature. See if the problem is replicated.

If it is, it means the resin and silicone aren't compatible. You can get away with using them together as long as conditions are within certain limits, but you're always at risk of wasting expensive silicone if the cleaning isn't good enough, or the cure takes too long, or the temperature is higher than normal, etc.

If you determine that the materials aren't compatible, you might consider choosing a different solution. Since you're probably locked into the resin (it could be that any resin for that machine would have similar issues), an obvious choice would be to use a silicone with a different curing chemistry. Other options could be the subject of another question.

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  • I ruled out contamination through a series of intensive scrubs using a degreasing agent, a strong alkali and s high percentage alcohol. Ruled out the resin itself, as this was a constant rather than a variable and had been present during multiple previous successes. Once heat was removed as a variable the problem was resolved. Returning it returns the problem. Thus by process of illimitation heat is the cause. Jan 26, 2023 at 21:27
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    As described in the answer, cleaning the surface has no bearing on internal material that migrates out. Also the Limino silicone is platinum cure according to the manufacturer. Sorry you're missing the point.
    – fixer1234
    Jan 26, 2023 at 22:38
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The problem turned out to be the high temperatures.

Raising it sufficiently above the manufacturer's recommended level was "cooking" the silicon through heat rather than curing it through a chemical reaction.

Also, the resin blanks were cold, and were a poor conductor of heat, so the areas that it occupied deep inside the mold cured at a slower rate than the rest of the mold. Hence the slime was uncured silicon mold material.

The solution was to leave the entire mold to cure at room temperature for a longer period of time to allow it to cure evenly, and to ensure that the blanks weren't cold.

This has been confirmed through repetition as being the nature of the problem that was being experienced.

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  • "The silicone sets perfectly everywhere except where it is touching the resin original. It's firm, has an even color and texture, and has no bubbles in it. Textbook perfect....Where it touches the original it is slimy and sticky, and there are multiple air bubbles. It's like there is a thin layer of slime only where the two materials are in contact with each other. There and only there." So it isn't the heat. In general, answers shouldn't be guesses or speculation.
    – fixer1234
    Oct 18, 2022 at 1:45
  • I made more molds under different conditions using both the same, and different, resin masters. I also cleaned the previous masters with various chemicals to remove potential surface contaminating. The molds that succeeded were the ones cured in a cooler location. The ones that failed the most were cured in a hotter location. Oct 21, 2022 at 16:27
  • In that scenario, I would suspect the resin master before the silicone. If the silicone was directly affected by the heat, it wouldn't show up only at the contact with the master. And if something about the cured master inhibits the silicone, temperature shouldn't make a difference. But even after cleaning the surface, heat could cause anything in the resin that wasn't fully-cured (or a byproduct) to migrate out. The slime could be material migrating out of the master, or perhaps what migrates out inhibits the silicone cure at the surface. (cont'd)
    – fixer1234
    Oct 21, 2022 at 17:35
  • Tests: 1. After you clean the slime from the mold, does the clean mold surface look like a reasonably good impression of the surface? If the silicone cure is being inhibited, the silicone would have the general shape but no surface detail. 2. Does the slime feel like either the uncured silicone or the uncured resin? If it feels like the uncured silicone, the silicone cure might be inhibited. If it feels like the uncured resin, the resin cure isn't getting all of the internal resin and some is migrating out when heated. (cont'd)
    – fixer1234
    Oct 21, 2022 at 17:35
  • If it feels like neither, it could be byproducts of the cure migrating out. 3. If the slime was byproducts of the silicone cure leaching out in the heat, it would be everywhere on the silicone exterior. That narrows it down to the resin masters. Make a fresh resin master from the same material and cure it in the usual way. Then clean the outside. Put it in the same hot location for the same amount of time or longer to make any effect easier to see since it can happen repeatedly with the same master. Does anything at all appear on the surface (residue, liquid, sheen, etc.)? (cont'd)
    – fixer1234
    Oct 21, 2022 at 17:36

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