How long do you pull a vacuum on it?We cast Flexane urethane in disposable 3D printed molds at work, for overmolding underwater cable joints and whatever.
Print the mold out of PLA with thin walls (1-2mm) and coat it with a suitable mold release first. Cast your part and let it fully cure, then heat the thing up in an oven to 100C to soften the PLA, put on some Ove-Gloves (seriously) and peel the PLA off by hand.
Also, pull a vacuum on your urethane after you mix it, but before you pour it, to get the air bubbles out of it. It'll "boil" when you do this and possibly make a mess, look out. About 1/3c of mixed Flexane 80 will almost overflow a standard red solo cup.
The urethane I ordered only has a working life of ~10 minutes, so I can either pressure-cast it, or run the risk of not being able to pull vacuum long enough to boil out the water.