by Hatemonger » Thu Apr 28, 2011 1:38 am
I'm a little confused about this vacuum/pressure thing. Up until reading this board, everything I saw about casting led me to believe that you vacuum the rubber, then pressure the resin. The idea of doing the same thing to both seems strange.
Maybe this is just old school instead of state-of-the-art, or maybe I misunderstood from the beginning, but I thought the process was:
1) Mix the rubber and degas under vacuum. This is mostly to draw out bubbles introduced during mixing. Once all the air bubbles surface and pop, you can stop the vacuum (should be only ~5-10 minutes depending on rubber)
2) Pour the mold. With a brushcoat for undercuts and small voids, and a careful pour, you should introduce little to no extra air.
3) Allow rubber to cure (24 hrs or whatever).
4) Mix resin. Since pot life on resins is typically much shorter (for quicker turnaround if nothing else), there is not usually time to vacuum.
5) Pour resin into mold, knowing there are inevitable bubbles in the mix.
6) Apply pressure to the filled mold, which shrinks bubbles down to (hopefully) near-invisible size.
7) Leave pressure on while resin cures. Since the cure time is quick, this is also a short process, and since the resin is harder, it will 'freeze' the bubbles in place after the pressure is relieved.
It seems to me that with pressurizing the rubber, you're just distorting the bubbles instead of removing them, and I would think if you don't re-pressurize to exactly the same PSI every time, you're going to get variations in how the bubbles appear (or don't) on the castings. So what am I missing?
- H8