Brian Chapman asked:,
> I've been 2D/3D CAD drawing a TT-scale loco and am nearing
> a point where I'll begin CNC cutting and resin casting parts (casting
> for the first time . . . gulp). May I ask what resin you use for your
> rolling stock projects? (I do have a vacuum setup and will heat cure
> under pressure.) Thanks for the help.
This is a conversation that needs to take place off-list. But as a general comment, anyone just getting started in resin casting should practice with the cheapest resin they can find. You'll waste a lot of resin just learning how to work with it. Sort of the same advice given to those learning to use an air brush - practice on old Blue Box shells, not your $600 brass engine or the superdetailed resin kit that took 40 hours to build.
Look for a room-temperature-cure urethane with a mixed viscosity of around 300 cps, a pot life of 4 to 7 minutes, and a (cured) hardness of 72 to 80 Shore D. Quoted demold times are misleading for our purposes - the ones the manufacturers call out are based on masses of material, like 1" cubes. For such a specimen the exothermic heat generated by the curing reaction is nicely contained within the mass and it will "rock up" in a hurry. But our parts tend to be thinner and they don't cure as quickly. My resin is spec'd at 45 to 60 minutes demold time, and that would be fine if I were casting bricks. But I use 4 hours because I don't want sides, doors and ends that look like they were designed by Salvador Dali.
It's a messy business, and the only way to learn is to jump in and try. Once you get the mechanics down you'll be better equipped to make intelligent decisions about better (and more expensive) resins.