Fail Of The Week: Silicone Molding That Won’t Cure


Mold making is a hacking skill we see pop up around here from time to time. But rarely do we hear about problems in the process, and they must happen. Here’s proof. This Fail of the Week focuses on [Michael’s] unfortunate experience with failed mold making due to uncured silicone around the master mold. It’s worse than it may sound, since he lost about a pound of silicone to the fail, and we’re unsure of whether he can even use the master again (how do you clean uncured silicone off of something?). Not to mention the time lost from setting up the pour and waiting 20 hours for it to cure.

Soon after the issue presented itself [Michael] started researching to see what had gone awry and noticed that the master should have been sealed with acrylic lacquer. This gave him the opportunity to test several different finishes before making a run at the full mold once again. He picked up a variety of the paint products he could find locally, used them to coat some scraps, and globbed on some silicone to see which worked the best.  He found a couple of different primers worked well, as did both glossy and matte acrylic coatings.

If you’ve never had a reason for mold making before, keep it in mind. You’d be surprised what kind of factory-production-type things can be pulled off by 3D printing a master, and casting a silicone mold of it.

2013-09-05-Hackaday-Fail-tips-tileFail of the Week is a Hackaday column which runs every Wednesday. Help keep the fun rolling by writing about your past failures and sending us a link to the story — or sending in links to fail write ups you find in your Internet travels.

60 thoughts on “Fail Of The Week: Silicone Molding That Won’t Cure

  1. One other technique I’ve seen used, is to paint thinner layers of silicone onto the master, allow them to partially cure, and build up 2 to 5 layers, and then fill out the mold cavity with the more rigid / thicker silicone. One of the advantages of silicone, is that it does stick to itself well.

    1. I used to use the layered method when using silicone. Since then I have been using Polyurea. It can be found at your local sherwin williams. It comes in a cartridge similar to a caulk tube. It is a 2 part component that mixes within the provided tip. It cures in about 15 minutes at any thickness, and reacts well to silicone or pertroleum release agents. I have been using this product for about 2 years now for decorative precast stone pieces. The time and hassle you save is well worth 12$ a tube. Hope this helps everyone!

    2. Kevin, don’t you mean silicone sticks to silicone very well? Which is why you need a good release agent when working to make 2 part silicone molds. It doesn’t stick to other surfaces well like vinyl, urethane, etc.

  2. I’ve had this same problem where the silicone wouldn’t cure for over a week. The mistake I made was buying silicone 2, instead of silicone 1. Always check the label, because that little difference in number ruins its ability to work in mold making.

  3. Unless you need your mould for food you can just as well use tin cure silicone. I am a big fan of the Smooth On – Stroke since it has thixotropic qualities. You paint it on and then you add thi-vex for the second and the third layer. The thi-vex will thicken the silicone so that it becomes trowelable. Also be carful with sulfur oil clay, this is a no-go. As i remember even non sulfur oil clay will inhibit platinum cure.

    “and we’re unsure of whether he can even use the master again (how do you clean uncured silicone off of something?)”

    Of course he can. Even IF the silicone adhered to the master, silicone will typically not adhere to anything but silicone :)

    1. I disagree. Other than the curing inhibition issue (which in reality is not that difficult to work around – I’ve made hundreds of moulds and never had a problem with it), platinum cure is far superior. Especially if planning to do any kind of volume of casts. It’s more tear resistant and you will get significantly more pulls from it than tin cure.

      Sulphur free oil based clay is fine – I use it all the time.

    2. You can buy silicone thixo agent to thicken any silicone mould making rubber. I use general purpose mould making RTV Silicone its cheaper. But cheaper than silicone is polyurethane mould making rubber both can be thickened another way instead of thixo. Using polyfibre. And for food stuff you are better to use food safe silicone.

  4. I was taught that it is sulpher that the silicone reacts with, and you need to be very careful about having any around your workspace. Your master being in contact with rubber could leave enough sulpher to inhibit the cure.

    Also, if your silicone is not curing properly, heating it might help – a sunny window or 50-75C oven. Keep an eye on the latter, and given the range of silicones, YMMV, but if you have an impending failure, it might be worth a crack.

  5. I used alcohol to clean it. I will have to give the vinegar a try, if this happens again some other time.
    The silicone cured, will not stick to almosts anything, but uncured it is a mess.
    I chose the platinum cure sorta clear 40. THis is a food grade platinum cure silicone that also is sort of clear. Then I can tell if the resin is in all of the fine details before I pressure cure the resin.
    I plan on using this for food molds, but my current project needed more silicone and I though that I would pay a bit higher for the longer library life.
    You can use the old pieces to fill when you pour a mold, but it really depends on the application. With what im working on, I cant reuse the old material. I will be keeping it around for future projects though. so not a complete waste of money.
    I have tin cure silicone as well, but I needed this specific kind for this project.
    You can use clay to create the filler for your two part mold. Just make sure it does not have sulfur. I use plasticine model clay. No inhibition. The brand is Sculpey and it is at almost evvery hobby shop. It smooths well and does not ever harden.

  6. Sounds like the retarder that slows the setting of the silicone is the issue here. In a university sculpture class a few years ago I learned that you can use silicone I (silicone 1 or one) that you can buy in a hardware store caulking tube to make silicone master moulds.

    The trick is to wash out the retarder before placing it around your object or master to be moulded. We would use a three gallon pail, fill it halfway with water and a capful/(good squeeze) of liquid dishwashing soap (sunlight brand). Then we would cut the nozzel off of the tube and pump the whole amount of silicone I into the soapy water.

    This is important! Acetic acid (is the retarder), makes the silicon smell similar to vinegar and why it cleans up so well using vinegar, is a mild irritant (skin) to most people. Wear hypoallergenic gloves for this stage. Long sleeve gloves are preferable. Make sure to wash your skin well if it comes in contact with the water and silicone in the process of washing out the retarder.

    Basically you gather up your squeezed out silicon and begin smooshing it together in the water like play dough and keep kneading and folding it over itself. The silicone starts out really gooey and sticky but eventually becomes more of a homogenous mass and tacky. That is when you pull it from the water, shake and remove as much water as possible, and form the silicone around your master. Form the silicone around your master at least 2-3cm thick uniformly. Make or place registration nubs or blocks on the exterior of your silicone. Leave to dry for about 30min to an hour, 2hrs if you want to be extra safe.

    find the best spots to set up a spout or spouts for pours, and your seam(s)
    Make a plaster hardshell to keep the form of your silicone as you do pours and so your pour comes out uniform and does not leak.

    1. If you want to make Silicone caulk moulds you can just add a few drops of Glycerin and Acrylic paint. This will distribute moisture throughout the mix. You need to make sure it’s well stirred though, the Acrylic paint helps – you just stir it until it’s a uniform colour.

    2. Thought I would chimen here for a minor clarification… What you described (“washing” the silicone) would no doubt work to make it cure faster, but technically you aren’t washing it, you’re adding water to the mix.

      Acitic acid doesn’t slow it down, it is actually made as a byproduct of the silicone curing, and what makes it cure is water and oxygen. By “washing” it, you are adding moisture throughout the sliocone which hastens the curing.

      As an alternative, you could also mix the silicone with a few drops of water or glycerin in a bowl, and then pour/scoop it into/onto a mold. If you want it to cure even faster, you can corn starch, baby powder or similar, to the silicone as a filler. As an example, I usually about 1 part cornstarch to 3 parts silicone by volume, then add a few drops of water. Stir into an even mix and it’ll cure fully in about 10 to 20 minutes, no matter how think it is.

    1. I’m using nitrile gloves. I wouldn’t dare use corn starch in this silicone. I know it works with the caulk you can modify for mold making. But that is to distribute oxygen to the silicone to cure it. otherwise caulk would take days to weeks to cure. These types of silicones are chemical reactions that occur after mixing two parts. Even if I completely sealed the mold material while curing, it would cure in the set amount of time it cures in.
      All I can say is, don’t use rustoleum satin paints. Find a satin acrylic. They recommend laquer, but flat acrylic laquer is not easy to find around me. Smooth on has a wonderful support team to answer questions. But they didn’t have a recommendation of a labeled product. Just acrylic laquer.

      1. Dammit my reply below got approved after yours was posted!

        Despite my reply, I suspected the gloves might be nitrile (the blue colour gives it away), so they definitely shouldn’t bother the silicone.

        What was the master part made from? Perhaps the sealing wasn’t as good as you thought?

      2. quote: “I wouldn’t dare use corn starch in this silicone.”

        If what I learned is true, silicone caulk cures under influence of moisture. Corn starch is just a medium to carry moisture into the mix. But if you wish to avoid using biologic materials like starch for the fear of them decaying under attack of microorganisms, then you could try using talc powder instead, which is a mineral (Magnesium Silicate, Mg3Si4O10(OH)2). Being a fine dust (as corn starch powder is), talc powder has great total surface area to hold moisture so it should serve the purpose fine. Additionally I guess those two OH groups could react with two acetic acid molecules in silicone caulk to produce two H2O (and one Mg3Si4O10(CH3COO)2) for even faster curing, but I am not a chemist.

    2. I reckon you’re on the right track. Based on his further testing the lacquer he used should be fine. The biggest difference I can see between the failure and test samples is that the test samples were not handled at all, so it’s very possible that the gloves caused contamination (latex contains sulphur).

  7. Things that inhibit two-part silicone curing. Latex. Anything with sulfur. Masking tape. Enamel paint.

    Tin cure silicone inhibits platinum cure, but not the other way around. If you don’t know which type you’re running through the meat grinder, only use it as filler for a tin cure mold.

    Condensation cure, the acetic acid evaporation type, will inhibit platinum cure. It’s also a gigantic no-no to use around anything electronic. The acid will corrode the circuitry. In other words, don’t fill your project box full of GE Silicone II to waterproof it. It shouldn’t be used on aluminum, steel, iron, copper etc unless the metal is first coated with something (like good paint) to resist the acetic acid.

    Condensation cure silicone can be used to repair rips and tears in tin or platinum cure molds. Smooth-On sells dinky little tubes of GE II at a huge rip-off price under the name “Sil-Poxy”. One opened, the little tube soon hardens. Just spend less on a tube of the GE stuff at a hardware store. I bought one tube of it to repair a mold, thinking it was something extra special, until I opened it and realized I got duped. :P No problems with anything else from Smooth-On.

    Crayola Modeling Clay (formerly called Clayola as a play on Crayola) works well with silicone.

    Art Chemical Products Klean Klay is most excellent for this kind of use. It also works great for plaster mold making, provided it’s coated with a waterproof parting agent. Well, poo! After many decades, looks like Art Chemical has closed its doors, but there’s still a large supply of their clay available. Dalchem in Australia is using the Klean Klay name, and there are several other similar no-sulfur oil clays.

    Silicone will dry out the clay and I know of no way to rejuvenate it. If you need to make the clay softer, get a cheap mug warmer and plonk an old saucer on it, then some clay. I got one for a couple of bucks at a thrift store.

    Pressure casting silicone molds is easy, inexpensive, requires no vacuum pump and will pretty much guarantee a good mold which will last a long time.

  8. If it was platinum cure silicone I have a list of things that cause inhibition…masking tape, latex gloves, anything vinyl, galvanized metal, polyurethane, rustoleum

    I clean everything with 99% isoprop alc before molding. Masters are always finished with automotive primer or clearcoat. A true lacquer for casting a clear part.

  9. Silicones are such versatile compounds. I have also had the never-cure failure, and learned that it’s safer, if you’re using household or home improvement silicone (really the wrong types, but passable for certain applications), to build a physical mold carrier and then use only a thin layer of silicone to complete your mold.

    If too thick, it may behave like a huge dam and never really harden inside.

    Success story using hardware store window caulk: I used paper towels held together by duct tape to make a <1" diameter cylinder about 5" long. Then I put silicone into it about 2/3 of the way up, and shoved a big crayon into the silicone.

    Exactly as I expected, the fluid level rose to accommodate the crayon. The silicone cured when I made my SECOND mold because I left significant gaps in the tape. The first one would have taken FOREVER to cure because it only had a thin layer at top to use for the process.

    I learned you can use silicone and an oven to cast your kids' leftover crayon bits into brand new crayons if you are careful. And if you use a thin enough mold, it's not so hard to remove the crayon without breaking it. You COULD use a mold in two interlocking pieces, but that adds complexity. I might do that if I were going to mold plastics with it.

    Good luck. I believe you can use the silicone you already have, and you have to make sure your molding material won't stick to the mold once set. Someone mentioned acrylic, which is one of many options — you have to know the chemical properties of your material at the temperatures you'll be using. Silicones are pretty trustworthy up to about 320C. For the crayons I was at like 63C.

  10. If it takes 20 hours to cure then you’re using the wrong silicone. Most good silicones for mold making are a 4 hour cure time or less. I often use a platinum cure silicone that has a
    40 minute cure time. Layering your silicone is just over complicated and adding solvents to it can cause the silicone not to cure at all. I’m a mold and model maker by trade. 

    1. Have you ever used smooth on products? This is the Detmold time for this specific type of silicone. It is actually 16 hours, but I left it for 20. It’s the right type for my application. This test was specifically for inhibition. Nothing other.

      1. Most of the silicone I use would be smooth on. Here in Ireland thats pretty much all you get. Platinum silicone can be inhibited by nearly anything. Tin cure are far more reliable and as long as theyre mixed fully will set no matter what. For most applications id use tin cure

        1. Hi
          I had bought some plaster which I was told was sulphur free but I am having problems with my sillicone. It does not cure especially where plaster has been in contact with it. Also what glue do you recommend I use when making glass box for mould to sit in. I had tried using glue gun but the glue hardens then without much movement falls off the glass. Any advice is much appreciated. I am using a food grade sillicone by the way.

  11. I’ve been having this issue with my 2 part platinum silicone. I’m not using any products with sulfur or latex to mix or store the silicone in…. but it’s remaining tacky after it sets. I’m thinking it’s the crosslinker I’m using that could be contaminating the mix. Too much crosslinker can actually retard the curing process (which has never made any sense to me, honestly).

    It’s either that or I swear, there’s sulfur in the air or something. It’s not an entirely crazy thought: I work across the street from a sugar factory and I’ve read that processing sugar can release sulfur into the air. I do mix my silicone with wooden mixing sticks; they COULD be treated with a sulfur, but I’m not sure.

      1. Is it very humid where you live or is your work area damp. Platinum cure silicone is trickier than tin cure. Have you tried tin cure or do you specificaly need platinum. For most applications tin cure will work just as well and is less likely to not cure and is usually much cheaper.

        1. It’s not humid this time of year- it being winter in Memphis. I’m working with a medical grade silicone, so I’m unable to use tin.

          I’m going to check the humidity- thanks!

          1. If you give me a product name or link to the product, I could look over the documents to see the proper mixing techniques. Is it volume or weight based measurements? If weight, is your scale off?

        2. It’s a 1:1 ratio. I’ve been using the same silicone for 2 years, and never had a problem until now. I thought of a new idea though!:

          I think it IS the weather. During shipment I think it got too cold (almost freezing maybe) and now the bonds aren’t fully linking as well as they should.

          1. I received my shipment when it was below zero for several days in Ohio. I was site it was bad. With the kind im using, it recommended bringing it to 60f before using and to mix the Components very well in case they separated during shipment.

    1. I use some wooden paint stir sticks, the ones about a foot long and 1″ wide. I spray them with several coats of clear lacquer and let that completely dry. Then the cured silicone peels off easily. On a humid day while sugar processing is going on and they’re stinking up the countryside, I could believe some sulfur could get onto your equipment. Keep your mixing sticks and cups/buckets clean and covered until you’re ready to use them, and don’t mix silicone when the sugar factory is going strong and the wind is blowing your way.

      A little too much catalyst won’t bother silicone. With some of them it will just speed up the cure and make it stiffer. Not enough catalyst will either take a much longer time to cure or it won’t cure at all as there won’t be enough to combine with all the silicone.

  12. i dont know if it has been mentioned, but you can clean off uncured silicone with lacquer solvent (not thinner). the only problem is if the item you are cleaning off is made of styrene or another very weak plastic it could damage or even melt the surface of your master. so basically test it in an inconspicuous spot. i personally wipe as much of with paper towels until its just sort of sticky, then clean it off. i have had only positive outcomes from using solvent on already cured resin from smooth-on. so if the master is resin it a good chance itl survive the cleaning process.


  13. I know this is an old thread, but I found it because I’m having a similar issue.
    I think I actually figured out why my silicone isn’t curing properly, but it gives rise to another question…

    For the following, I am using FormSil 25 from, which is a tin-cure silicone.

    OK, so I put on the first (detail) layer. After 24 hours, not really dry. After 48 hours, the thicker sections (where it pooled at the bottom) were cured, but the thin detail skin on the model was still really tacky.
    For the second layer I kept my silicone components nice and warm overnight and increased the amount of catalyst slightly. I was certain these tricks would help, but… pretty much the same results (well-cured thick sections, really tacky thin sections).
    I was feeling pretty stumped.
    For the third layer I mixed the same ratio of catalyst and added thixo. That layer is still curing, as I write this.
    HOWEVER…. as I was laying up that third layer, I got to thinking, and realized that I have been under-catalyzing. The directions for the Silicone state: “100 parts A to 5 parts B.” I do know that silicone is normally mixed by weight, but can also be mixed by volume. The instructions made no mention of weight vs. volume, neither did the website. Well, I assumed that when the term “parts” is used, that must mean volume… so I took great care to mix by volume. Well, I now think that was wrong. And since part Part A (catalyst) is lighter than Part B, I’m pretty certain I’ve been under-catalyzing by 20-30%.

    My question is: Will the third layer (which I have just applied, also with insufficient catalyst) and the 4th layer (which I will properly mix by weight)… will these layers help to cure the very thin sections of layers 1 and 2, by virtue of the fact that they are going on thicker? (I have read that thick sections cure better/faster, and my own experience with this project bears that out…)


  14. Well I can now answer my own question.
    It all worked out.
    Having those inner detail layers, which were really not curing properly on their own, covered by thicker sections did allow them to cure.
    So the mold has worked out in the end.

    1. What you experienced was called thin section inhibition of a 2 part, tin cure silicone, where the silicone will not cure in very thin sections (a couple of sheets of paper or less) but cures fine in thicker sections. This is caused by an over saturation of the silicone with moisture (water that is in the formulation itself + atmospheric humidity). As you saw, adding more layers resolved the problem – the easiest thing to do. Another way to resolve it is to limit the atmospheric moisture (cover it, put it in a room with a dehumidifier)

  15. Can anyone help me: I’ve been trying to paint a platnium silicone object, using the same platinum silicone as a pigment base to paint with. I know it’s not my pigment because it cures without problems in my mixing cups and on my tools. But on the object itself it just seems to stay tacky forever. And I’m not sure what I’m doing. The mixing cups and objects all remain in the same room, my studio, while I wait for the cure to set but for some reason its staying tacky on the object. And I clean my object with naptha and then acetone, as per a tutorial i watched…. any ideas? I do thin it with naptha which I know slows it down, but it will definitely firm up in my mixing cups without any problems but just take forever where I’ve painted…

    1. 1: the ratio of naptha to silicone mix might be too high.
      2: you should only apply silicone to silicone if it is within 1 week of curing- i once let a costume sit for over two weeks before applying more- it rejected te second layer and the first layer ended up tacking (mainly bc i couldnt completely remove all second layer i was adding.
      3: humidity is a huge factor, don’t where you live, but if it’s raining out- it could affect your curing as well.

      try to put a dehumidifier in the room with what you’re trying to cure.
      try to increase temperature of your curing room as well- sometimes this helps as well.

      1. Thanks although it still feels like a mystery to me:

        1: I considered this but again the pot where I mixed the naptha and silicone sets without problem (the leftover unused)

        2: less than a week old, also i read that you can remedy this by wet sanding with a fine grit sandpaper which I’ve done but still had problems. Although if this is the reason can you tell me why it rejects the second layer? Just curious, I can see why it might not adhere but why reject altogether and not cure?

        3: I live in the desert where there is super low humidity.

        So 2 seems like the most likely answer but still doesn’t make much sense to me because these aren’t that old… :/

  16. Hi Im not sure if anyone will still see this,I have made a mould from liquid latex that was a copy of a dental stone model of a hand,Im trying to pour rtv 25a platinum cure silicone into it but the outside is not setting? all sticky uncured even an hour after centre is set? any advice please,was using alginate but needed to copy a stone modified model,can i clean it somehow? could it be residue from the stone?

  17. HI All

    i hope some one answering me on that

    i maked a mold (layer method) after cured i want pour my fda silicone to make the shape i want but the mold i made gives me some oilly touch on the silicone and when i wipe it -it comes again after a while
    never gone

    and for that when i pour my fda silicone after sometime the oilly touch will comes again and making inhibits
    how i can stop the oil that comes from the silicone ( i mixed it very very well)

    Also when i brush my fda silicone on master shape ( master shape is made from silicone or pvc) the fda silicone never cure just the first 1mm not cure

  18. Possibly somewhere around posts by Steve, salec & mak, above. Got some stuff a while ago on ebay, it was red in color, powder in a plastic container, instructions to fill to top with water, shake well before each use. Mix 2-5 drops per 10grams/one squeeze of silicone. (Acetic cure RTV type silicone). The label : Green silicone : Silicone sealant hardener jell . Worked well, @ 3 drops around 5 or so minutes. Haven`t been able to find it anywhere since, nothing that really resembles it. Managed to contact the Seller a few years later, he said the chemist relocated & none left. Looking for a solve to the mystery. A few things have led me close to an “aha” moment, however, the powder mixes with water, the other things I have seen are already liquid, eg: glycerin, & other things do not mix with water ….. & the ratios 2-5 drops to 10grams{one squeeze of caulk gun}…. I`ve got about half a container left {~100ml} & used around 20 RTV silicone tubes, yeah, alot of molds but will be needing more, or something of the like. Commercial companies do not seem to have.

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