Im casting a magazine for a paintball pistol using a two part mold made with CS2 silicone rubber and Fast Cast Polyurethane Casting Resin. The problem is that fast cast isn´t very strong. These magazines needs to be strong enough to be dropped down on a concrete floor. I have tried that with the fast cast mags and they just shattered. 

I find that Epoxy casting resin is much stronger than polyester. It may be strong enough for what you want or can perhaps be improved with a suitable filler.

Brian.

Fast Cast isn't ideal for that application sadly.  There isn't much you can add to it to make it less brittle. You could experiment with adding carbon fibres into the mould then pouring the resin around it.

I wouldn't recommend putting loose carbon into the resin itself, as when you mix it, it goes like custard and won't pour.

We don't currently stock any stronger PU casting resins. It is something we may look at in the future if there was sufficient demand and we could find a decent product.

Warren Penalver
Easy Composites / Carbon Mods - Technical Support Assistant

I made a 16mm diameter x 300mm rod from clear epoxy casting resin as a test. It bounces when dropped and is slightly flexible. I also used it for casting some 28mm soldiers which were very strong where as the fast cast PU resin was very brittle. I would rather not say where I got the resin from as it may be unfair to Easy Composites, unless Warren says I can, as they have nothing suitable.

Brian.