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Kang

silicone caulk as cheap mold material?

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Has anyone else tried making silicone molds using cheap caulking?  I did a forum search, found nothing.

 

You can't use it straight out of the tube, that much I know from personal experimentation with some HistArts assemblies I needed a bunch of a few years back.  It won't dry/cure/harden properly in thick layers within any reasonable kind of timeframe.  The mold I tried to make just about turned itself into a pretzel overnight and still had some big gooey sections for weeks afterwards...  I have since learned that this is all because of additives they put into caulking to keep it from setting up other than when it is squozen out in a thin bead... or something. 

 

But there are ways around this; I have found instructions for several methods.  The simplest one I read about (not one I have been able to find a good online tutorial on, but I was able to ask a question or two from a guy who tried it) was to dump the whole tube (use a 100% silicone type) into a bucket of water with some dish soap, then knead the caulking underwater for a while to rinse out whatever is inhibiting it from curing.  Gloves may or may not be necessary to keep it from sticking to your hands.  I think the inhibitor might be acetic acid, due to caulking's vinegary smell.  Other methods mention that mixing water into the caulking one way or another actually catalyzes the curing reaction, as opposed to rinsing out whatever stops it from curing, not sure which version is the truth.  Other methods involve thinning the silicone with mineral spirits so it can be painted on or poured more easily (may cause the mold to shrink after a while though, or else maybe that will happen no matter what?), and/or mixing with glycerine or adding corn starch etc.  I think the corn starch is supposed to work by carrying atmospheric moisture in has absorbed into the silicone to catalyze curing.  The one guy I have discussed this a little bit with online (a folk artist who goes by "Metal By Nevin") had used the knead-in-soapy-water method to mold some frozen dead fish to be cast (in resin I believe) for use in a museum exhibit (historical fishing village diorama) somewhere in Alberta where he used to work, and he said the molds worked well and could be re-used without trouble.  He's the one whoe explanation involved rinsing out the curing inhibitor as opposed to adding a catalyst; perhaps he had the exlanation wrong. He also told me he used the same method on his own time to mold one of his hands, then cast a wax hand to use for making a metal hand-shaped soap dish via lost wax investment casting.  But I figured it might also be helpful or of interest to this crowd...

 

Seems a real shame if museums can't afford the right products for making museum-quality exhibits.  But, on the other hand, if using cheap off-the-shelf molding materials can yield museum-quality results, then that could be a real game-changer!

 

I'm obviously not tyring to write a tutorial here or anything, just wondering if anyone here has tried it - if so, please post more info (maybe write a tutorial here or something?  :)) - have I got the details or chemistry wrong?  Which method did you use and how well did it work?  Can it be used for small scale things like minis/bits/terrain?  And so on.  I don't want to waste my time researching which of the many options is the best way to do this if someone here already knows, or if that is still going to give far worse results than using the "real" products made for mold-making.   But even then there I suppose there could still be some small jobs where it might be a satisfactory as well as an economical choice.  Always good to have a few backup techniques in one's arsenal.

 

Hopefully this will inspire someone to give it a try who might not have known this was possible.  There are plenty of tutorials for these methods online as it happens; the tricky part is finding some consensus as to which of the various methods and recipes works best/easiest.  If it works well enough, it might be a good alternative to dropping big bucks on RTV silicone mold-making materials, at least for small jobs or one-off molds that might only get used once or twice.  I will try it one day myself, but it's not at the top of my list.  I would use the knead in water method just because I know someone who's actually tried it.  But if one of the other methods is better for some reason, that's something I would love to learn more about...

 

{sigh} another novella,  apologies.  I just can't seem to stop myself rambling on and on some days...

 

Kang

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I've personally done quite a few molds and some casting using silicone caulk.  For one thing, it's seemed to me that you need to use silicone I, as opposed to silicone II.  Not sure what the difference between them is as they're both 100% silicone...

 

When I've done it I've mostly used the 1:1 ratio of silicone and cornstarch (eventually with the idea that moisture helps the curing process).  My first few molds i did use mineral spirits in the mix as well--maybe a few teaspoons per half cup of silicone/cornstarch.  Those molds turned out beautifully....and then shrank upwards of 10% in the span of a couple of months.  I had not begun storing molds in the freezer either, which supposedly slows the shrinkage.

 

Haven't done any lately, but it seems that just skipping the spirits--which are solely to thin the mixture to make it pourable--causes less shrinkage.

 

I've wanted to try the under-water method.  The idea I took from it is that shrinkage occurs as the curing agent (yeah, probably some derivative of acetic acid) is lost to the atmosphere.  I'm clearing up some work space currently in order to run more experiments like this though and will let you know the results.

 

Edit after seeing Bruunwald's post:  I use this type of mold for slimes, oozes, rocks, etc--things with shallow/soft details.  It probably would lose a lot of detail if you tried it with, for instance, a miniature.

Edited by BLZeebub
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I've been making RTV silicone molds for several years now, but recently caught a couple of YouTube videos where people were instructing how to use caulk as a cheap alternative.

 

Frankly, the mess, time required for full setting (a week+), and rather horrible results when cast (loss of detail, difficulty de-molding) make this a non-viable alternative for me. The time and effort required to make it work, for the limited results, in no way offsets the cost of using materials that are actually created for the purpose.

 

Two-part RTV is simply not as expensive as you might think. It may seem so if you're just judging the cost by itself. You have to consider what you actual get for your money. TAP plastics can sell you enough material to make two molds of the size of a regular Hirst Arts mold, for the price of only one of those molds. That's actually a lot of material. You can get two good sized scatter terrain pieces molded with that, or dozens of small ones. You could make four or five molds of 8+ 25mm or 30mm custom minis bases with that.

 

There are also re-usable alternatives, if you are just out to make very small duplicates of bits. CMON markets a product called Instant Mold, which is a waxy material that you heat, press your bits into, then it hardens and you can duplicate your pieces with green stuff or similar until you are blue in the face. When you get bored, you can heat it up again and mold other pieces with it.

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When I worked in the Canterbury Museum's display department back in the late '80s, we were making a lot of large moulds of all sorts of things (bones, mostly), and making them entirely out of RTV was cost-prohibitive. What we did was skin each piece in RTV for maximum detail retention, and once cured, we smoothed on layers of silicon caulk smooshed through burlap sacking as a reinforcing material. A couple of layers of caulk and burlap was generally enough to make a reasonably firm, strong mould, and the caulk is one of the very few things that will actually stick to RTV. The rubber skin-moulds were further supported in a plaster matrix for casting. 

 

It's a process that takes a few days; the caulk takes time to cure with each new layer. But it does make a very strong and relatively cheap large mould.

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Thanks for the feedback. I may try this sometime and make some simple caulk molds just for fun just because I like to DIY, and like it enough that I am willing to endure a bit of hassle if it ends up working somewhat in the end.  But it is always a juggling act; trading reliability for affordability and a little more fiddling around is how it usually seems to end up, if it goes well at all.  There certainly are cases where shelling out for the commercial product in question instead of taking the homemade approach is definitely worth the extra cost.  If I do try this, I won't expect it to outperform commercially available molding goo. 

 

The guy I mentioned who did this to make those museum fishes and a wax copy of his hand said he was able to pull a washed caulk mold that held fingerprint level detail, but obviously fish in general are down at the smooth end of the scale for animals, and the finest details on a life-sized hand don't have as much deep (relatively speaking) relief as say the folds in a cloak on a mini, or even chainmail links.  I guess that's why sometimes the mineral spirits are used to thin out the caulking, so that a more detailed mold can be made. 

 

I guess in a pinch you could use that mineral oil method to mold more intricate things as long as you were only going to use the mold once or a very few times, in the very near future (before it shrinks).  Versus using the presumably less shrinky 'knead underwater' method which might work better for less intricate molds that may last longer and aren't so prone to shrinking...

 

Kang

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