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How-To: Make cheap castable silicone from caulk

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How-To:  Make cheap castable silicone from caulk

Simple, elegant garage casting hack here from Instructables user mikey77, who calls the stuff “Oogoo,” which is, I think, a portmanteau of “Oobleck” and “Sugru.”

If you’ve ever bought casting silicone, you know it can be pretty expensive. If, seeking a cost-saving alternative, you’ve ever experimented with making castings using the dirt-cheap silicone caulk they sell at the hardware store, you know that it doesn’t work very well. Large volumes set up very slowly, if at all.

Turns out, though, that mixing in some cornstarch accelerates the drying process; just how much depends on how much starch you add. The author recommends starting with a 1:1 mix. I’m not sure about his explanation that the process works because the hygroscopic starch carries moisture into the internal volume of the silicone, but in any case there are probably other additives that will accelerate the process as well as or better than cornstarch. You might even find one that’s not opaque and allows for translucent castings.

Also, on a topical note, a commenter on our recent post about choosing a casting silicone claims that hardware store silicone caulk can be thinned to a pourable consistency using hardware store xylenes. (Thanks for that, SKR!) Perhaps the two additives could be combined to make a fast-setting homebrew silicone that’s also of pourable consistency.


17 thoughts on “How-To: Make cheap castable silicone from caulk

  1. Daniel Morgan says:

    This post is awesome. Thanks Sean.

    -Daniel in Austin

  2. W. Aaron Waychoff says:

    I have successfully used glycerin with silicone caulk to get it to set up. While I haven’t experimented enough to suggest specific ratios of ingredients, I have found it doesn’t take much.

  3. greg says:

    ..and was very pleased that such a simple and inexpensive mixture is so capable.

    Cornflour is going to be a toolbox essential for me from now on.

    A quick summary of my results can be found here.

  4. mgainer says:

    I played this game a while ago; turns out that 1 to 2% water by weight will do just fine as an agent to make the caulk cure through.

    Also, re the comment about xylenes – you can use paint thinner or kerosene. This, direct from GE customer support. When you thin it with X% of solvent, you can count on X% of shrinkage after the fact. If you use enough to get the caulk to flow on its own, you can count on losing a pretty fair bit of strength. My solution was to use a thin coat of thinned silicone to pick up surface detail and prevent bubbles, and then back that up with a thicker coat of un-thinned.

  5. salec says:

    You are onto something with glycerin there. Wikipedia says glycerol (Glycerin) is hygroscopic, which is good for setting silicone caulk up, but I guess more research (or chemistry knowledge) is needed to establish which is the ideal water content in it for this.

    Also, glycerol reacts with acetic acid (which is released as silicone sets up), building esters. One of the esters of glycerol with acetic acid is glycerin triacetate, or triacetin, which is used as, among other things, plasticizer. Plasticizers are stuff added to mixtures to make them more fluid and in some cases more flexible after mixes set. It may be good or bad, depending on what you want to achieve. Perhaps adding some “aggregate” of plastics sawdust (or ground down plastics – like for recycling) into mixture would make it more rigid and tough, compensating for additional flexibility if it was unwanted.

    At present I don’t have silicone caulk to play with, but I wander what would happen if we used (some) baking soda in the mix? It should react with acetic acid by releasing gaseous CO2, which would induce foaming and expansion of the mix in mold.

    Well, there’s a lot of potential in this for play and fun!

  6. Iceman086 says:

    Could the silicon mix be used in a mould made of the same material as long as a release agent is used?

    If so, could it be painted onto something like Plasticine or Raku clay in order to make a durable and reusable cheap mould? I know that a tougher mother mould material (something like Shell Shock for example) would still be needed ontop of the silicon.

    Guessing Vaseline or something similar could be used as a release as long as its put on thin but is there something else that might be better?

    1. Sean Michael Ragan says:

      Generally, as long as an appropriate release is used, you can do homogeneous mold/object castings. Dunno about Vaseline, though, it seems like the silicone is generally soluble in organic hydrocarbons. I would get a mold release appropriate for silicone:

      Dunno what’s in it, however, so as for a DIY version I can’t be of much help. Possibly a dry release agent? More cornstarch? You might save time just doing a quick experiment, honestly. (Which we’d all love to hear about when you’re done, BTW. =])

      1. Iceman086 says:

        I am actually in talks with Reynolds right now to do a Moulding/Casting workshop at my college.

        I have had a Smooth-On rep tell me that Pam (and products like it) has been used by alot of folks as a release agent but he cautioned me to stay away from it as it can have inconsistent results. He actually recommended the exact product that you linked above and I have used it in a workshop that I took. It works really well.

        I am thinking about testing Pam (or the cheapest knockoff there of), Vaseline and Murphy’s Oil Soap (slightly diluted in water) as releases. These would be cheap and readily available alternatives to the Ease Release 200.

        I know that as long as one of these will act as a resist to the silicone then it should work. Hopefully I will be able to test it and have some results in a few days.

        Thank you above for their input on this topic!

        1. Iceman086 says:

          So I finally got around to testing this moulding and casting method out. I found out some cool and interesting things too.

          The first of which was ratios. I found that when making a mould you can use a 4 to 1 ratio of paint thinner to caulk (2 oz of caulk to 1/2 oz of paint thinner) to make a small mould using the bottom of a plastic cup. I added in about 3 drops (literally drops) of paint to the mix in order to help show the details of the mould. Once I had a consistent mix of paint thinner to caulk I added in about 1 tbs of corn starch as the hardening agent. This gave me about 5 to 10 minutes worth of pot time and fully set within 45 minutes.

          I gave it an hour and a half just to be sure before I mixed up some casting silicone. This time I used a ratio of 3/4 to 1 (silicone to paint thinner) in order to get a mix that was able to flow. You can go 1 to 2 (silicone to paint thinner) if you want a thinner mix that is pourable but its up to you. I added in my paint (again 3 drops but yellow this time) and mixed until the 3 parts were consistent. I then added in 1/2 tbs of corn starch to the mix as a hardener. After doing this though the mix thickened and I ended up having to spoon it out. This batch still worked how ever and I was able to get some very nice results. The pot time was about 15 to 20 minutes with this mix and the dry time was around 2 1/2 hours.

          If you want a casting consistency that is still pourable I would recommend using the same ratio of Silicone to Paint Thinner but you can reduce the amount of corn starch to 1/4 tbs. HOWEVER! Reducing the amount of corn starch will increase the set time.

          I tried 2 kinds of releases in my experiments. The first was Remington Gun Oil with Teflon. I was told that this would work as a release as it is oil based and has Teflon in it but wouldn’t dry out. Turns out that is false. The Gun Oil did not work as a release for the casting but it did work as a release for the bolt that I made a mould of. In the end I would recommend what is in the next paragraph, Vaseline!

          I ended up going back to the tried and true release that is as versatile as Duck Tape (though Gaffe tape is superior!), Vaseline. I put a thin layer inside the mould and was able to use it as a release when I poured in the casting. It has to be a consistent thin layer. If it is to thick then you will loose alot of your small details. You will also need to put some on ANYTHING that the casting material will touch in case of any over pour.

          The silicone casting WILL and I repeat, WILL stick to itself. It is a very important thing to remember to cover ANYTHING that you might get casting material on as it will stick to itself.

          In some of the posts above people talked about shrinkage. I didn’t see any noticeable shrinkage given the amount of paint thinner that I used.

          Let me know what you guys think!

          Sets to self

  7. jptrsn says:

    Found another use for this great material!

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  11. Allen Harrington says:

    This is very interesting, I had an idea for a business venture and this will work perfectly!

    1. Aaron Orr says:

      Al Harrington!? As in President and CEO of Al Harrington’s wacky waving inflatable arm-flailing tubeman emporium and warehouse in Weekapaug!? That’s awesome dude!

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I am descended from 5,000 generations of tool-using primates. Also, I went to college and stuff. I am a long-time contributor to MAKE magazine and My work has also appeared in ReadyMade, c't – Magazin für Computertechnik, and The Wall Street Journal.

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