If you follow my work, you know that I use my resin 3D printers a lot to produce soft silicone molds or pieces (original blog post and a follow-up). Resin printers can create precise and detailed patterns. You can quickly prototype, cast miniatures, dices, chocolate molds… There are plenty of uses for soft silicone molds. However, some silicones play well with resin-printed patterns, and some don’t. In this post, I will explain to you which silicones cause the trouble, why we care, and also, how to prevent the cure inhibition. The recipe I give you is surprisingly easy and doesn’t require any special equipment. It actually outperforms existing commercial solutions (e.g., Inhibit X) both in terms of price and performance.

Types of Silicone Rubber

When we talk about using silicone rubber for making a mold, there are two types: tin-cure and platinum-cure silicones.

The tin-cure silicones (often called condensation) are silicones that cure just fine in resin-printed molds. I’ve been using these silicones to manufacture my resin spouts and make all my molds. They cure by mixing in a small amount of curing agent (usually 2-3 % by weight). The curing agent contains salts of heavy metals that trigger the curing reaction. Once you mix it, it will happily cure. No matter if it is underwater, in a resin printer mold, if it is cold or hot. What matters is how much curing agent you add. If you add more, it cures faster, however, the resulting mechanical properties are worse (the rubber is more fragile and can be easily torn).

The second type is platinum-cure silicones. These silicones are often mixed in a 1:1 ratio of both compounds. Sometimes, they are mixed in a 10:1 ratio. This type of silicone is very sensitive to the curing environment. It doesn’t cure if it is in contact with: resin printed mold, tin-cure silicone, sulfur, latex, acetone, garlic, chlorine, duct tape, and more (see, e.g., this page for an extensive list). The inhibition, also often called “poisoning the silicone”, is permanent. The silicone will never cure in the presence of the substance. The near surrounding (from 0.1 mm to a couple of millimeters) of the poisonous instance just stays liquid and never cures. It doesn’t even turn into a gel.

You are probably wondering – why would anyone use platinum-cured silicones when they are so much trouble to work with? Their properties are far superior to tin-cure silicones. And if you use some kind of commercially made silicone equipment, it is probably platinum-cure silicone. To compare the properties, I will use Lukopren 5221 (tin-cure) and GMS A30 (platinum-cure).

First, they are much stronger (2 MPa vs 9 MPa), they are also much harder to tear, and often have a much longer elongation at break (150 % vs 500 %). When we talk about mold-making, platinum-cured silicones have often immeasurable shrinkage (though GMS A30 is an exception), tin-cured silicones usually shrink up to 2 % by volume. Platinum-cured silicones also do not degrade under UV light and also they don’t degrade with time. Since they are mixed in a 1:1 ratio, a static-mixing nozzle can be used to mix them on the fly. Platinum-cure silicones also often have much lower viscosity.

So there are a lot of reasons to use them, however, how to use them with resin-printed molds?

What Causes the Inhibition and How to Prevent It?

Let’s start with a disclaimer ­– I am no chemist and I understand it far, far worse than anything else. If I say something wrong or misleading here, please, let me know in the comments! I am happy to learn and fix the text accordingly.

When you google silicone cure-inhibition or poisoning, you find a lot of Reddit threads, YouTube videos, and texts that suggest the only solution – coat your printed model in some kind of varnish. And be sure it is not a varnish that also poisons silicone. This solution, is, however, unacceptable to me. I use my molds to make precise and detailed parts. Putting a varnish or paint is not only laborious, but it also changes the dimension significantly. It also gets stuck in low spots and removes details. There are users trying to find another recipe but without much luck.

When you try even harder or you have access to scientific journals, you can find several scientific papers about the topic. The most interesting one (and also the only one publically available) is PDMS Curing Inhibition on 3D-Printed Molds: Why? Also, How to Avoid It? This paper claims that the cause of the inhibition is the photoinitiator in the resin. Therefore, if you want to prevent the cure inhibition, you have to get rid of it (at least from the surface). Note that I have heard from multiple sources (including some manufacturers) that the source of the inhibition is not the photoinitiator but traces of sulfur in the resin that are left from the manufacturing process. I have no way of finding where is the truth.

Nevertheless, the paper above suggests two recipes: the first one is to cure for 4 hours and then keep the printed pieces heated to 60°C for 48 hours, or keep the printed piece at 120°C for 2-8 hours. This is rather extreme and has several downsides. The first method works only on half of the resins they tried (and also none of them is for LCD printers); the second method is unusable for large molds as 120°C if far beyond softening of most resins (most resins soften around 60°C). The authors don’t mind as they design molds for microfluidics, which are tiny and beefy. However, for large molds, we are out of luck (unless we use Siraya Tech Sculp or Sculp Ultra – but I haven’t tested if this method works due to its difficulty to execute).

There is also a chemistry blog that claims to solve the problem by long washing time. How long? They succeed after 48 hours of washing in a stirred bath of IPA or ethanol. And also they always use a virgin bath and change it 2 times during the procedure. This is not only expensive (we need a fresh bath every time), and time-consuming but also presents a problem with large molds as most resins soften when soaked in IPA for extensive periods of time.

Those were the DIY-feasible paths. There are also papers that suggest oxygen plasma treating, coating in metal using magnetron vapor spattering, and more. Unless you are Ben from Applied Science, this is not the way to go.

Therefore, I dig into a lot of experiments and tweaking. But I finally found a relatively simple and cheap recipe to post-process the molds in order to prevent the cure inhibition. If you want to learn more about this path, don’t leave after learning the recipe; there’s a section showing all approaches and dead ends I tried.

My Recipe Fore Platinum-cure Silicone Inhibition

The recipe I present I tested with most of the Siraya Tech Resins (Fast, Sculp, Tenacious, Blu, Fast + Tenacious Mixtures, Simple). I print the parts standardly and then:

• I wash them properly in two baths of IPA (just like I always do): first is a dirty bath to wash most of the resin, then a clean bath. I use Mercury X and each cycle is about 10 minutes.
• Then I do the third round of cleaning – this time in soap water. I rinse the prints properly. However, I had plenty of success without this step. It just ensures there are no uncured resin residua.
• Then there are two recipes:
  • For any models (requires a coating solution):
    • I cure the pieces for 30 minutes in Mercury X underwater. Without water, the process doesn’t work.
    • I quickly dip dried and cured pieces into a 1% solution of polymethyl methacrylate (PMMA) in acetone. Once they dry, I repeat the dip. If you don’t know what PMMA is, then it is acrylic, often called plexiglass. If you have a complex model, I suggest even using a less concentrated solution with more dips so you prevent residue deposition on the surface where the acetone got stuck.
  • For thick-walled models (requires no extra equipment):
    • I cure the pieces for 30 minutes in Mercury X underwater.
    • I let them sit for at least 6 hours in the water
    • I change the water and cure them for another 30 minutes.
    • No coating in PMMA is necessary.

That’s it. Now your printed parts are ready to be used for casting platinum-cured silicone!

Why this recipe? I noticed, that there has to be some interesting reaction (that I don’t fully understand) when curing underwater that prevents poisoning of the silicone. Air oxygen inhibits the curing reaction of the resin (which is hopefully a well-known fact). Originally, I thought that underwater curing is effective as it prevents oxygen from reaching the surface of the prints. Therefore, no oxygen can reach the surface and the curing reaction, that disarms the photoinitiator, happens also on the surface. However, I no longer think so. I tried using glycerol instead of water (which has the advantage that it is not soaked by resin). The prints were cured nicely (much better than on-air), but they still poisoned the silicone. Actually, after consultation with a few people, I think that the photoinitiator (or its residual products) actually dissolves in water when illuminated with UV, and thus, it is washed away. I suspect this is so also as the water after curing gets a weird rubbery and unpleasant smell.

Why are there two paths for thin and thick-walled models? It seems that completely washing the photoinitiator takes some time. However, resins soak water (see details) therefore, when you have a thin-walled model, it soaks the moisture quickly, softens, and gets deformed. Thick-walled models don’t lose structural integrity, therefore we can wash them properly. For thin-walled models, we try to eliminate as much of the photoinitiator as possible. For the rest, we try to seal it in a thin coating of PMMA that should prevent any photoinitiator from leaking to the surface. I had no success only with PMMA. My guess for that is as follows: The PMMA coating solution dissolves the photoinitiator on the surface and mixes with it. Therefore, if no elimination is performed, we get the photoinitiator (or its residua) all over the surface. However, if we first remove it from the surface, we can seal the surface. Without sealing, it seems that over time the photoinitiator (or its residua) leaks to the surface again. However, if we apply the long enough washing, this seems to be no longer the case. The PMMA creates an immeasurably thin coating and the printed pieces get a nice shiny surface.

To prepare the coating solution; you take a piece of (preferably clear) plexiglass and dissolve it in acetone. It takes about 1 hour to dissolve a piece. Note that there are better solvents for plexiglass (e.g, dichloromethane), but they are much harder to obtain.

Results

The results are more than satisfactory. I tested my recipe with the GMS A series of silicones and the SmoothOn Mold Star series. Post-processing adds about 45 minutes/5 hours (based on the path you choose) compared to traditional post-processing, which is bearable. In the coating paths, the patterns never go through any stress, therefore, they preserve dimensional stability. Some stress might be added in the slow path since the resin soaks some water. However, the coating is PMMA is easy to do and it does not disturb the surface in any way. You can actually see the individual voxels of the model imprinted in the silicone! This is a huge improvement over spray painting.

The process is, however, not 100% reliable. There are some free variables that I haven’t been able to identify and eliminate. Roughly in 1 out of 20 attempts, the silicone gets poisoned. If this happens, I rerun the process, and then I have so far succeeded. Therefore, there is some aspect of the process I do not have under control and I am not aware of its impact. Possibly ambient temperature? Humidity? I am not able to say at the moment. For this reason, I always try the silicone on a small part of the model whenever I have a new mold. Cleaning uncured silicone is really annoying.

The treatment also seems to be persistent. I have over 60 pulls from some of my molds without any need to repeat the process. Also, molds treated with PMMA do not need any mold release. The silicone does not stick to them.

The process doesn’t need any special equipment or hard-to-get or expensive chemicals.

One interesting finding is that PMMA seems to bond well to the surface and gives it a nice finish. It also seems to improve the nail-scratch resistance somewhat. Note that these are only my impressions during the experiments, I haven’t explored this aspect of the coating yet and I have no hard data on this.

Hey, and What About InhibitX? Or Release Agents?

In one of my previous blog posts, I linked to Joshua’s blog post about preventing cure-inhibition. His solution was to use InhibitX. In the process of finding a working recipe, I also got a bottle of it. It is a pretty expensive substance (and also hard to get) – 300 ml costs about 70 €. You don’t use much of it, but still, pretty darn expensive (especially when you spill it).

InhibitX works, but it does not work on its own. Joshua heat treats the printed parts before soaking them in InhibitX. I also found that without heating them (though; I do not do it as aggressively as Joshua), InhibitX doesn’t work. However, InhibitX has one annoying issue. The silicone sticks really well to surfaces treated by it. Therefore, if you don’t apply a release agent, or don’t apply it properly, you can not only get a ruined casting but probably also a ruined mold. The silicone is really hard to get off. This is where the PMMA coating really shines – the silicone doesn’t bond.

This brings us to the usage of release agents. I have found that paraffine or vax-based release agents don’t prevent poisoning the silicone (e.g., Easy Release 200, Luporen separátor, and others). What somewhat works are PVA-based separators. However, they are really hard to apply correctly without leaving visible trace marks. PVA is also hard to apply on resin molds as it doesn’t soak the surface. What helps a little is to add a drop of soap to it. The PVA-based release agents also have a strong tendency to fill the low spots, and therefore lose details. I also stumbled upon problems with sharp edges – the separator did not stick there and the sharp edges caused inhibition. Lastly, you have to wash and reapply the coating before every use. Not worth the time and effort in my opinion.

PS: There are some people claiming that InhibitX is just naptha (according to the safety sheet). However, I don’t believe this is true. Naptha is only a solvent for the active ingredient, which is probably harmless, and therefore, is not listed on the safety sheet. I base my observation on the fact that the bottle says “shake well before use” (if it was only naptha, why mix it?) and the liquid also has some brown tint. Also, once it dries on a surface, it leaves brownish stains on the surface. I also did experiments with naptha and other solvents showing they do not prevent the inhibition.

The Path Towards the Recipe

The path toward finding the recipe was much longer than I anticipated. I had so many partial successes, that, however, have been shown to be dead ends. usually, the procedure worked for flat surfaces, but still, it caused inhibition in sharp inner corners.

I used a triangular staircase model for my test (see photos below). This model is small enough to not waste too much material and it also contains details and inner corners. The internal corners showed to be crucial for the experiments as they cause most of the troubles. Some methods worked well on external surfaces, however, the internal corners suffered.

First, I experimented with curing the models. I tried curing them for a long period of time, using summer sunlight, curing them warm, and cold. I even cured them locally with a high-power LED. I also tried various wavelengths from 460-304 nm. None of it gave promising results.

I tried various cleaning methods. From soap, ultrasonic cleaner, and various solvents (ethanol, acetone, xylene, naptha, …). Also, none of them seem to be 100% working.

Washing the prints in sodium hydroxide gave interesting results. Based on the length of washing, the cure inhibition was either more-less prevented or started again. It seems like in short washing cycles (10 % solution for 10-15 minutes) it gets rid of the photoinitiator. However, washing it longer again starts poisoning the reaction again. This might be due to the fact that NaOH damages the resin (as confirmed by the guys from Siraya Tech) and therefore, more poisonous compounds are brought to the surface. Interestingly enough, washing in a 20 % solution of citric acid has a similar effect.

Conclusion

Although I showed you the relatively simple procedure for preventing the cure inhibition, it is still some extra work and it is up to you to judge if it is worth it. If you make a mold for a single or only a few pulls, it might be worth it to just use tin-cure silicone. They don’t require any post-processing (they even cure in just cleaned and cured mold) and work just fine – after all, I used them for the last two years for all the mold making.

However, if you require a long-lasting or extremely strong mold, the process is definitely worth it. The search for the recipe was after all started after I released my resin tank cleaning kits. If you don’t know them, check the video below:

So far, I made them from tin-cured silicones. However, starting January 2022, all my spouts are made using high-quality platinum-cured silicone. Therefore, the spouts and spatulas should last indefinitely. This is also why the kits were unavailable for a few weeks – I was switching the process. Also note that I extended those kits for new printers: Mars 1 and Mars 3. You can check them out in my store.

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