Clear and reliable advice for working with silicone rubber

I’m sorry to say that the purpose of this short posting is not to ‘give it’ .. at least not yet .. rather, I’m turning the tables just for once and ‘asking for it’!

For example, if anyone can recommend sources of information they’ve relied upon in their practice I’d be interested, particularly in those that go into a little bit more detail or even those that are a bit more exploratory. I try to provide clear guidance here, but I haven’t really organized it all into one place yet, and I’m trying to get my facts ‘straighter’, and maybe pick up some more tips from others, before I do. The information provided by manufacturers should always be the first ‘ports of call’, and some suppliers do have some reliable ‘how to’s on their websites .. but maybe there are some I’ve missed along the way.

David Neat, mouldmaking and casting, Lukasil 429 and catalyst

I’d also be interested in what you have experienced regarding ‘shelf life’ i.e. how long you’ve continued using the components before the mix behaves differently to when you first got it? How did that compare to the supplier’s advice? Were you given a clear statement of shelf life in the first place, or did you have to hunt/gather to find it? Worst of all .. did you only find it written on the product once you’d got it? You see, it’s basically evident that suppliers or manufacturers show rather dubious behaviour when it comes to owning up to shelf life. Also, have you ever tried the ‘unthinkable’ i.e. tried a silicone rubber with a catalyst not intended for it? I’ve never had any catalyst going spare, and you probably haven’t either .. but if anyone has done, I’d be very interested in what happened?

David Neat, mouldmaking and casting, making a silicone mould in coloured layers

I recently did some tests which confirmed what I’d always suspected (or maybe read at some point in the past, because I see it is mentioned very occasionally) .. that however old the silicone ‘body’ might be, as long as it moves when you tip it, you can make it work by getting some new catalyst. I tested remainders of silicone which had been shoved in a corner waiting to be taken to the Recycling Centre, some going back more than 7 years .. and they all worked! Yes, they were different .. they were much more viscous than they should have been; the working time was much shorter, and they cured in no time at all! .. but still good for some things!

David Neat, mouldmaking and casting, making a skin mould on shaped styrofoam

When you’ve bought a silicone has it come with the essential facts i.e. not just the ratio of the two parts to mix, but the following: .. its weight in grams per cubic centimetre (good to know if you want to work out how much you’re buying in volume); its viscosity (useful when making a choice, because thinner ones could flow into detail better and leave fewer air bubbles); recommended cure time, etc. Has your supplier even said .. most importantly .. whether it is an ‘addition cure’ (platinum) or a ‘condensation cure’ (tin) silicone? This is important because whereas condensation cure silicones nowadays are fairly unaffected by anything, ‘addition cure’ silicones are a different matter. Or .. are they really? At times I’ve treated addition cure silicones really carelessly in the past without any problems. Again, any personal experiences would be welcome!

You see what I’ve done here? In asking the questions I’ve given information at the same time .. think of it as a standard teaching technique, though reversed in this case. Nevertheless .. I really do need other people’s help on this subject, so any facts you can contribute would be much appreciated!

9 thoughts on “Clear and reliable advice for working with silicone rubber

  1. I bought some Trylon liquid latex rubber online from Hobbycraft last week and disappointed that there were no instructions on how to use it.
    Looking at the Trylon website, there was nothing I could find neither. Very poor support

    • Thanks Steve! I think that was one of the things that prompted me to ask these questions .. the fact that ‘re-sellers’ such as Hobbycraft, B&Q, Greatart etc don’t take up the responsibility of informing, or .. just as importantly .. making the Material Safety Data sheets available. We see the same (perhaps more so) on Amazon, Ebay etc. This will get worse as they all get even bigger! In your case, with the liquid latex, it may also be something else perhaps .. that not a lot can be said about it in technical terms. Also, one brand of liquid latex can’t differ much from another, so any info about what to do with latex from any source will do in this case.

  2. I generally prefer a platinum cure rubber, having found one that is fast and reliable – except when it must come into contact with a sulphur based clay like plastilene, plastillin etc – where it won’t cure properly on the surface. There are ways round this, but it’s worth knowing. I find a platinum rubber gives a more reliable longer lived shiny finish when casting repeats of glass or other shiny items. The life of a mould can be important and I’ve found that platinum moulds last longer with better results than tin cure. I’ve never known the weight in grams per cubic centimetre, and it’s never occurred to me to ask – however the viscosity is important, especially if it cures quickly – the faster it goes off, the thinner you need it to be in order to reach all the details before it cures. Also I’d say shore hardness is important – sometimes you need a stiff mould and can then do without a jacket; other times you need a softer rubber that can peel away easily but definitely needs a jacket – so it’s worth learning about shore hardness and checking that before you decide which silicone to use.

    • Many thanks for this Rachel! Especially also for ‘that’ correction (you’re right, I won’t own up publicly .. known by heart but not always by brain!). Yes, for so long I’ve steered clear of using platinum silicones mainly because I had vague memories of things going pear-shaped when using 1:1 silicones (and it may not even have been their fault!). Recently I’ve really been coming round to them .. i.e. have been trying Polycraft ZA22 from (it’s really Zhermack ZA22) .. so thin that air bubbles hardly stand a chance, in spite of (as you mention) a very fast cure.

  3. I use polytek products–have really only used one of their PlatSil products.  (need to check which one–it’s the purple one).  ANd one of the easyflo 60 resin for casting.

    Would answers to all your questions really be helpful?  If so, I’ll dig and do my best ….. I’ve certainly benefited from all your research and experimentation?  But if you aren’t interested in Polytek, let me know.


    • Thanks Fran! Yes I have used Polytek products but can never remember which ones (and sometimes I never know in the first place, because of distributors re-selling them under different names). Sometimes the only clue is on the MSD sheet. But no .. don’t dig .. or rather, I’ll message if I’m wondering about anything Polytek! Thanks again!

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