I’ve been trying to cast parts for trombone mouthpieces. They have a fine thread, and the casting epoxy I’m using right now doesn’t set hard enough to hold the threads. They tend to deform and the parts separate. The original parts were made of bakelite, which is very hard and tough. Is there an epoxy resin (or even a polyester resin) that has the same hardness and durability of bakelite?