<p>The problem turned out to be the high temperatures.</p>
<p>Raising it sufficiently above the manufacturer's recommended level was &quot;cooking&quot; the silicon through heat rather than curing it through a chemical reaction.</p>
<p>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.</p>
<p>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.</p>
<p>This has been confirmed through repetition as being the nature of the problem that was being experienced.</p>

CC BY-SA 4.0
2022-10-18T01:45:18.670Z
"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
CC BY-SA 4.0
2022-10-21T16:27:08.550Z
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.
Aaargh Zombies
CC BY-SA 4.0
2022-10-21T17:35:01.853Z
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
CC BY-SA 4.0
2022-10-21T17:35:47.717Z
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
CC BY-SA 4.0
2022-10-21T17:36:35.343Z
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
CC BY-SA 4.0
2022-10-21T17:37:32.727Z
If the surface gets slimy, there's your source. If there's any other change in feel or appearance, whatever that is might be affecting the silicone. 4. Make a fresh master but cure it just enough so the outside shell retains its shape and surface detail but there is uncured or partially cured resin internally. Clean it and repeat test 3 with it. 5. Test the uncured or partially cured resin with the silicone to see what effect, if any, it has on the cure. Like test 4, make a fresh master and give it just enough cure so the outside retains its shape and detail (cont'd)
fixer1234
CC BY-SA 4.0
2022-10-21T17:38:30.423Z
but there is uncured or partially cured resin internally. Make a mold of that with the silicone, following the same procedure with the hot location. If the slime problem is the same or worse, it points to uncured resin. If it's less, it points to a cure byproduct. 6. mix up some silicone and pour a layer in the bottom of a small container like a Dixie Cup. Put a drop of the uncured resin on top in the middle. Let the silicone fully cure. Clean off the resin spot. If the surface is smooth, the uncured resin doesn't inhibit the silicone. If the surface was affected in that spot it does. (cont'd)
fixer1234
CC BY-SA 4.0
2022-10-21T17:55:04.213Z
7. Constrain some of the resin in a 3D shape and give it just enough cure to create a shell that retains its shape and surface detail, but is internally largely uncured. Make a silicone mold of that following the same procedure. Carefully demold it, clean up the master, inspect it. Has its surface been at all degraded? That could point to something in the uncured silicone affecting the resin. It's also possible that heat directly affects the resin. If test 3 or 4 produced any symptoms like warping or slumping when the master wasn't being constrained by the silicone, it would point to that.
fixer1234
CC BY-SA 4.0
2022-10-22T13:51:40.920Z
The fact remains, I used the same masters and the same silicon, but cured at a lower heat, and everything went as expected. The mold made at the higher heat was slimy.
Aaargh Zombies
CC BY-SA 4.0
2022-10-22T14:15:53.657Z
OK. That suggests that heat is affecting something. The other characteristics suggest that the something is the master rather than the silicone. It's also possible material leaching out of the master when it's heated could be affecting the silicone. There's still a lot of possibilities within that of what's actually happening and how it's causing the problem. The 7 tests should give you a pretty definitive answer, but there's a good chance you'll have the answer before you run every one. Could increase the chance of that by starting with the tests you think are most likely to ID the problem.
fixer1234
CC BY-SA 4.0
2022-10-17T12:00:21.073Z
2023-01-27T16:09:10.800Z
2023-01-27T16:09:10.800Z
Aaargh Zombies
<p>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.</p>
<p>Ordinarily, the &quot;why&quot; 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.</p>
<p>According to the manufacturer, the Limino silicone uses a platinum cure chemistry. Many materials inhibit the cure of this chemistry; see this <a href="https://www.dow.com/en-us/faq/platinum-catalyzed-silicone-inhibitors.html" rel="nofollow noreferrer">list from Dow</a>. 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.</p>
<p>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.</p>
<p>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.</p>
<p>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.</p>
<p>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.</p>
<p>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.</p>

CC BY-SA 4.0
2023-01-26T21:27:27.623Z
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.
Aaargh Zombies
CC BY-SA 4.0
2023-01-26T22:38:47.130Z
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
CC BY-SA 4.0
2023-01-26T07:32:38.877Z
2023-01-26T22:58:10.717Z
2023-01-26T22:58:10.717Z
fixer1234
<p>I'm trying to make some home-made molds out of two-part casting silicone.</p>
<p>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.</p>
<p>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.</p>
<p>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.</p>
<p>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.</p>
<p>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.</p>
<p>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.</p>
<p>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..</p>
<ol>
<li>Detergent scrub</li>
<li>Alcohol scrub</li>
<li>Alkali scrub</li>
</ol>
<p>Exact same problem all three times.</p>
<p>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.</p>

CC BY-SA 4.0
2022-10-01T09:27:35.107Z
Are you using platinum cure silicone? What type of resin are the originals made of?
fixer1234
CC BY-SA 4.0
2022-10-01T10:11:52.793Z
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
Aaargh Zombies
CC BY-SA 4.0
2022-10-03T22:28:54.893Z
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
CC BY-SA 4.0
2022-10-04T09:44:01.457Z
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
CC BY-SA 4.0
2022-10-04T13:51:24.207Z
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.
Aaargh Zombies
CC BY-SA 4.0
2022-10-15T05:03:29.817Z
Try buying an entirely different product, and use that. Some products are just not very good/are old on the shelf...
user3025
CC BY-SA 4.0
2023-01-22T08:40:36.023Z
A lot of things will inhibit platinum cure silicone, which Limino is: https://www.dow.com/en-us/faq/platinum-catalyzed-silicone-inhibitors.html. Might be worth checking whether the resin contains any of the listed compounds or produces them during curing. Even if the resin is cured, all it takes is trace amounts of residue that might be driven out of the cured resin by the heat.
fixer1234
CC BY-SA 4.0
2023-01-25T19:48:12.870Z
@fixer1234, I've already stablished that heat was the problem. Once I allowed the silicon to cure away from the heat source no more problems. That was months ago now, so repetition demonstrates that this is the correct answer.
Aaargh Zombies
CC BY-SA 4.0
2023-01-25T20:22:01.673Z
Looks like we're kind of talking across each other. Yeah, it looks like heat was what triggered the problem, but heat in a range that a human can tolerate wouldn't **directly** account for the symptom (like cooking the silicone, or a cold resin master inhibiting cure). Platinum cure silicone is notorious for being sensitive to a lot of stuff that can inhibit its cure, even in trace amounts. If the resin contains any of those compounds, or its curing produces them, the master could exude trace amounts when heated. That would account for why it didn't happen in the absence of heat. (cont'd)
fixer1234
CC BY-SA 4.0
2023-01-25T20:22:10.103Z
This is just a new piece of information you could investigate that might point to a different solution; it might allow you to use heat, if you wanted, with masters made from that resin (like coating with a thin varnish to seal the surface). Anyway, this was an interesting question to post.
fixer1234
<casting><silicone-mold>
Why are my home-made silicone molds coming out slimy where the silicone touches the thing that I'm casting?
Aaargh Zombies
# Why are my home-made silicone molds coming out slimy where the silicone touches the thing that I'm casting?

(Asked by: username_0 on 2022-10-01)

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.

**Tags:** casting, silicone-mold

> **Comments:**
> **username_1**: Are you using platinum cure silicone? What type of resin are the originals made of?  
> **username_0**: 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  
> **username_2**: 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.  
> **username_3**: 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.  
> **username_0**: 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.  
> **username_4**: Try buying an entirely different product, and use that. Some products are just not very good/are old on the shelf...  
> **username_1**: A lot of things will inhibit platinum cure silicone, which Limino is: https://www.dow.com/en-us/faq/platinum-catalyzed-silicone-inhibitors.html. Might be worth checking whether the resin contains any of the listed compounds or produces them during curing. Even if the resin is cured, all it takes is trace amounts of residue that might be driven out of the cured resin by the heat.  
> **username_0**: @username_1, I've already stablished that heat was the problem. Once I allowed the silicon to cure away from the heat source no more problems. That was months ago now, so repetition demonstrates that this is the correct answer.  
> **username_1**: Looks like we're kind of talking across each other. Yeah, it looks like heat was what triggered the problem, but heat in a range that a human can tolerate wouldn't **directly** account for the symptom (like cooking the silicone, or a cold resin master inhibiting cure). Platinum cure silicone is notorious for being sensitive to a lot of stuff that can inhibit its cure, even in trace amounts. If the resin contains any of those compounds, or its curing produces them, the master could exude trace amounts when heated. That would account for why it didn't happen in the absence of heat. (cont'd)  
> **username_1**: This is just a new piece of information you could investigate that might point to a different solution; it might allow you to use heat, if you wanted, with masters made from that resin (like coating with a thin varnish to seal the surface). Anyway, this was an interesting question to post.  

---

## 2 answers

### Answer by username_0 on 2022-10-17. Score: -1 | Accepted answer

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.

> **Comments:**
> **username_1**: "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.  
> **username_0**: 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.  
> **username_1**: 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)  
> **username_1**: 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)  
> **username_1**: 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)  
> **username_1**: If the surface gets slimy, there's your source. If there's any other change in feel or appearance, whatever that is might be affecting the silicone. 4. Make a fresh master but cure it just enough so the outside shell retains its shape and surface detail but there is uncured or partially cured resin internally. Clean it and repeat test 3 with it. 5. Test the uncured or partially cured resin with the silicone to see what effect, if any, it has on the cure. Like test 4, make a fresh master and give it just enough cure so the outside retains its shape and detail (cont'd)  
> **username_1**: but there is uncured or partially cured resin internally. Make a mold of that with the silicone, following the same procedure with the hot location. If the slime problem is the same or worse, it points to uncured resin. If it's less, it points to a cure byproduct. 6. mix up some silicone and pour a layer in the bottom of a small container like a Dixie Cup. Put a drop of the uncured resin on top in the middle. Let the silicone fully cure. Clean off the resin spot. If the surface is smooth, the uncured resin doesn't inhibit the silicone. If the surface was affected in that spot it does. (cont'd)  
> **username_1**: 7. Constrain some of the resin in a 3D shape and give it just enough cure to create a shell that retains its shape and surface detail, but is internally largely uncured. Make a silicone mold of that following the same procedure. Carefully demold it, clean up the master, inspect it. Has its surface been at all degraded? That could point to something in the uncured silicone affecting the resin. It's also possible that heat directly affects the resin. If test 3 or 4 produced any symptoms like warping or slumping when the master wasn't being constrained by the silicone, it would point to that.  
> **username_0**: The fact remains, I used the same masters and the same silicon, but cured at a lower heat, and everything went as expected. The mold made at the higher heat was slimy.  
> **username_1**: OK. That suggests that heat is affecting something. The other characteristics suggest that the something is the master rather than the silicone. It's also possible material leaching out of the master when it's heated could be affecting the silicone. There's still a lot of possibilities within that of what's actually happening and how it's causing the problem. The 7 tests should give you a pretty definitive answer, but there's a good chance you'll have the answer before you run every one. Could increase the chance of that by starting with the tests you think are most likely to ID the problem.  

---

### Answer by username_1 on 2023-01-26. Score: 0

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](https://www.dow.com/en-us/faq/platinum-catalyzed-silicone-inhibitors.html). 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.

> **Comments:**
> **username_0**: 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.  
> **username_1**: 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.  

---

