In my long quest to stop bubbles, I made a dangerous little pressure pot from 4" ABS sewer pipe. It consists of a flat end cap, glued to about 2 feet of pipe, glued to a FPT cleanout, with an MPT cover with a little brass air fitting threaded in.
Despite ABS being made for negligible-pressure sewer use, it actually hydrotested to 125PSI without exploding, much to my surprise! The fat spiral thread starts to leak above 50psi, but I can keep the internal pressure at 90psi without working my air compressor too hard.
Inside, I put my little AK FCG casting mold, carefully reinforced with fiberglass mesh along the weak spots:
I mixed up some of my old cans of Freeman 1080 urethane
, which after sitting on my cool dark shelf for 8 months still seems to work exactly like before--except the white pigment settled out, leaving cool clear plastic! As before, I mixed in about 10% chopped glass fibers (50% chopped glass is a bit stronger, but too viscous to gravity-fill my complex mold). I vacuum-degassed the mix, poured it into the mold, degassed one final time, and then let it cure at 90psi in my new pressure pot.
Here's the resulting transparent plastic FCG set, fresh from the mold. There are a few bubbles where the reinforcing fiberglass held some air, and I think the steel inserts produce a few bubbles too, but overall there are way fewer bubbles than without the pressure cure step.
Those with sharp eyes will notice I actually used a Dremel to carve in a few places on the mold, to fatten up weak areas like the back of the hammer and side of the trigger hooks.
The resulting parts are quite pretty--they're nicely clear, and you usually can't see the cast-in fiberglass mesh, except at certain angles it glints in the light. (Word of the day: Chatoyance
.) It's tough to capture on the camera!
Here are the new parts installed on my AK, where they seem to function fine. I'm trying to hold off on the live fire test until they cure for a few more days, but now I'm excited about casting again!
Assuming these parts survive for some reasonable number of shots, I'm planning on using up most of my remaining urethane resin to crank 'em out. I've basically standardized on the AK FCG for my projects lately (I have an AR lower designed using AK parts), and these are Alaska-made and weigh 85% less than steel!