Kenny hit the nail on the head. Resins that cure slower are usually softer and less brittle. Yes, the downside to epoxy is the 1-3 day cure to demold but most of them still require 5-10 days to fully cure and not be flexible. Many use it because it sticks to everything but the downside is it's flexibility and tendency to yellow quicker than most others. That's only a problem if you clear cast. I've never had Alumilite Urethane "Clear" or "Clear Slow" resin be brittle. Are you heating it after you demold it. Excessive heat makes resin brittle. Like Kenny, I use polyester "Silmar 41" for snakes and feathers, and "Clear or Clear Slow" for most everything else. If you think urethane is brittle, turn Polyester Resin. You'll feel better. Lol