‘lexicon’ definition
silicone short for ‘silicone rubber’, a synthetic rubber (wholly manufactured, unlike latex or alginate) favoured for its durability, flexibility and excellent detail reproduction. RTV (room temperature vulcanization) silicone rubbers are supplied as liquid part and catalyst which, when mixed together in a specific ratio, will harden or ‘cure’ at normal room temperature in up to 24hrs
longer summary
By far the most popular choice of mouldmaking material, especially for professional work where reliable results are needed. Infallible detail reproduction, easy application (block or skin), low shrinkage, durable and long-lasting moulds, accepts all casting materials (usually without release agent), range of types for different purposes (hardness, cure speed, resistance to heat etc.), usually only porous prototypes need special sealing/release, easy cleanup with white spirit. Not re-usable (but can be recycled, by granulating and adding to new mix). Working with silicone can be slow process (waiting for cure), limited shelf-life (usually 6mths), sensitivity of addition cure silicones to contamination. Mixing/working guide: differs greatly according to type but on average 5-10% catalyst addition by weight, working time varies from 5-40mins and curing time from 2-24hrs. Average cost: £22 per kg (addition cure silicones generally more)
Worklog notes
13/03/2016 Most silicones come as pourable but can be made into a thixotropic (thick, non-slump) paste by adding a special agent which has to be bought separately. Commonly only a very small amount of this needs to be mixed in, and always after the catalyst has been added. In the case of my preferred general-purpose silicone, Lukasil 429 from specialplasters.co.uk, the recommended dose is between 0.5 and 1 % and I dose this with a pipette because such small additions are difficult to weigh. Two things are worth noting specially .. firstly that the addition of a thixotropic agent can greatly reduce the pot-life (working time) and secondly that it’s a very good idea to test the effect of the agent first because this will vary .. sometimes a lot! .. with different batches of the same product, or with the age of the silicone or agent. So mainly for my own benefit I’m recording the test I did today for future reference.
0.5% thixo added to 50g catalysed silicone. Mix becomes quickly thixotropic, but still ‘butter’ spreadable. After 20mins very slightly tougher but still good for brushing. After 30mins first signs of rubber-like ‘pull back’ but very slight. After 45mins, too much ‘pull back’ to use as detail coat but ok as secondary coating.
14/03/2016 I’ve only used two addition cure silicones in my work so far (aka. ‘platinum cure’ silicones, characterized by 1:1 mixing of the two components and by increased sensitivity to contamination). Addition cure silicones often have special properties over the more regular condensation cure or ‘tin cure’ silicones available. I’ve used one occasionally, the T40 from Tiranti, because it’s very fast curing .. just a few minutes pot-life and ready for demoulding in 15-20mins! I bought it roughly 10 years ago .. and it still works! I tested it again the other day just out of interest .. still no change. Over the years I’ve relied on it being there, usually for emergency repairs to other moulds, since ‘silicone will stick to silicone’ regardless of which type they are. Although condensation cure silicones are one of the things supposed to inhibit addition cure silicones I haven’t noticed any problems with the T40 setting on it. I was surprised at first that the T40 still appeared to be functioning normally long after the recommended shelf-life .. in marked contrast to all the regular condensation cure silicones I use .. but I never expected to be still using it after 10 years!
But it’s much the same thing with the other addition cure silicone I sometimes use .. Platsil Gel 10, which is translucent and very soft-setting, commonly used for prosthetics. I’ve had this for about 5 years and although it takes a little longer to cure than it used to, it’s also still usable.
davidneat 
Hi David!
Just thought I’d share some notes on my experiments with low-cost pourable silicone for molds.
I managed to have great success making a two part silicone mold using GP silicone for building & decorating.
Many tubes silicones act differently when thinned but I have found that white general purpose silicone from Screwfix (No Nonsense Brand) works very well and only costs £2-3 a tube.
The recipe is GP silicone, white spirit, acrylic paint and cornflour. All very cheap and easily sourced. White spirit can supposedly be swapped out for any suitable filler.
Truthfully I still haven’t figured out the ratios but I have found it is easier to add as you mix to get the correct
I start by squirting out half a tube of silicone into a large cup and I gradually add spirit or thinner until it is thin enough to be able to be poured instead of ‘scooped’. (It requires a LOT more thinner than you would expect, to the point where the final mix feels like it is only 40% silicone.)
Once I am happy with the consistency, I add about a teaspoon of acrylic paint (the dirt cheap tubes from Poundland work) and a teaspoon of Corn flour.
I don’t know why, but those two ingredients will cause the silicone to start curing, whereas tubed silicone would typically cure by exposure to the air.
As soon as you add the paint and cornflour you need to mix them fast until the paint has blended and it is one uniform color and start pouring straight away as it will begin to thicken rather quickly.
I try to make the first layer thinner to capture all details and then pour a second layer that is much thicker to keep the thin layer in place.
I’m surprised how well it worked and I’ll definitely do it again.
Two things to note: when mixing it will stink of vinegar, this is normal but you may want to do it outside as it’s rather strong.
Due to the large quantity of spirit needed to thin the silicone and it’s evaporative properties, you may find that the silicone molds will actually shrink if you leave them in the open air. This can easily be remedied by wrapping your silicone moulds in plastic wrap when they’re not being used.
I hope this information is useful to someone 🙂
Hi Reece,
Thanks for this! Yes, have tried just with cheap bathroom silicone and cornflour (roughly 1:1 by volume) to make a sticky dough very like mashed potato .. sets very quickly, doesn’t shrink too much. Tried thinning with white spirit but, as you note, needed a lot .. and makes it shrink a lot. Still! .. not a bad thing to try if you just want a kind of ‘putty’ mould material that you can press around a very simple original to make a quick/cheap mould!
Hi David, I came across your site while looking for information on mold making with silicone rubber.
Thank you for sharing your expertise!
Two part silicone is expensive. Is it possible to use the 100% silicone caulking tubes you can buy at the hardware store to make a casting mold?
Im sure it would have to be thinned with something to make the silicone more fluid.
Have you tried using this? I look forward to your reply.
Hello Glenn,
Yes I’ve tried and .. although it’s not impossible, it’s a pain to do! Normally silicone bathroom sealant takes far to long to cure especially if it’s used in thickness more than, say, 5mm. But if you look online using the search words ‘Oomoo’ or ‘silicone cornflour’ you will find that it has been experimented with as a form of quick-setting rubber. It’s been tried very little as a mould material, as far as I know. The addition of cornflour will cause the sealant material to set pretty fast (I think 30mins-1hr although this depends on how much cornflour). What you get though is a very sticky paste that looks like mashed potato but feels like epoxy glue. The only way of making an impression from it is to clump it into a ball (wetting hands helps a lot) and pressing a form down into it. It can’t be made into a liquid .. I’ve tried that. The only dilutant is white spirit (mineral spirits) and I gave up having to add more-and-more (until there was a change) because (even using low odour white spirit) the air was getting disagreeable. If you try yourself let me know, but I’m guessing the resultant liquid material would either not set, be too weak or shrink too much .. probably all these!
Thank you very much David for your reply and consensus. Wishful thinking on my part.
I have another question regarding casting parts from silicone molds.
I will start a new thread.
I have had some success with silicone molds made from tubes and posted a comment on this page explaining the details and method. I hope it is useful!
Hello, I can’t find when your blog was posted, hopefully not so long ago and you are still around, thanks for helpful info on silicones, but I cant’t find info anywhere about how to bond silicone to other materials as polyester resin? I’m currently using some cloth on uncured silicone and then laminate on top, but is there any better way, as when I’m using cloth it is very dificult not to leave voids between cloth and silicone.
Hi Tomas,
What you’re doing is the only possible way! I would start with the polyester laminate; secure relatively thick but porous cloth onto this; then coat this with catalysed but unthickened silicone so that it infiltrates.