Why are my home-made silicone molds coming out slimy where the silicone touches the thing that I'm casting?

Question

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.

Answer 1

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.

Answer 2

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.