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
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)
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.