Hi I have started casting recently
I have made my moulds with CS25 condensation cure silicone.
I’m making tyres for models i make, using PX60.
After 20 casts the silicon started to lose its release ability and I started having to spray a liquid wax release agent into the mould that helps but is still not as good as I’d like. But now it’s at 50 casts and the mould is totally Dried out and air bubbles are now a problem even when degassing and putting it into a pressure pot.

I’m thinking it might have something to do with the shape of the tyres as I have other simpler shaped tyres that have 50 casts but the rubber doesn’t stick but some are starting to change colour where the rubber touches the silicone now.

Is their a certain number of cast that it can do or has it got to do with my method?

You are correct in thinking there is a limited lifespan with silicone moulds.  20 pulls from the mould is typically about right for a complex shape with a PU or epoxy resins.   Some simpler shapes will be easier to demould and last longer as you have found.

What happens is that during the cure process, many resin systems will dry out some of the natural silicone oils in the mould.  Over time this will compromise the mould surface and eventually the mould will be too worn to use and/or stick to resin.   The effect depends on the resin system - some are more aggressive than others and also the shape.  There is an element of mechanical flexing to the wear as well hence why simple shapes which are easy to demould are likely to have a slightly longer mould life than complex shapes needing a lot of twisting and stretching of the rubber.

A technique that works well to improve mould life is to rub some pure silicone oil into the mould after each use to replace lost oils.  Also using some of the silicone release sprays helps by protecting the mould surface as well.

Warren Penalver
Easy Composites / Carbon Mods - Technical Support Assistant

Thank you warren. Is their another silicone that would suit what I’m doing better as i want to make 1000’s for casts of certain moulds ?

You might want to look at a rigid plastic mould or even billet tooling for that kind of quantity - assuming of course you can demould without the flex a silicone mould would make. 

Warren Penalver
Easy Composites / Carbon Mods - Technical Support Assistant