Degassing the silicone molding rubber worked great, so I thought it would be equally great degassing the casting resin. Well…no.

Degassing the mixed resin turned the compound into a froth that quickly overflowed the pour cup and made a mess inside the vacuum chamber; I was just lucky I’d put a paper plate down under the cup! I mixed another batch of resin in a larger cup and degassed it. More froth and it was quickly obvious that the larger pouring cup wasn’t large enough. Rather than let it overflow again, I played with the valve I use to release the vacuum, cracking it just enough to keep the froth inside the container…barely. The resin has a 20 minute pot life, so I degassed it for about 7 minutes, poured it into the molds, and degassed again for another 10 minutes, playing with the valve again to keep it from frothing entirely out of the molds.

24 hours later I demolded the parts and was quite taken aback with how many bubbles were in the castings; enough so that the castings got tossed.

Hmmm…back to the Internet and more research. That’s when I heard about pressure casting. Instead of degassing to get rid of bubbles, the resin-filled molds are exposed to pressure. A pressure pot is used for application of liquids, most often paint, using a spray gun. With some minor modifications, it can also be used to pressure cast resins.

Pressure pots can be expensive. Once again, Harbor Freight had something reasonably priced that looked like it would do the job; their item number is 66839 and purchase of it will get you one of these:

It’s a 2.5 gallon unit that’s rated for a maximum pressure of 60 psi.

Since I’m not going to be spraying anything, there were some parts to remove. The siphon tube inside the lid screws out easily and a 1/2″ NPT plug plugs the hole. I don’t need the pressure regulator that’s supplied. Instead I added a brass T-fitting threaded 1/4″ NPT. I added a 90 degree elbow to one end of the T, then a valve, and then a hose coupler to the valve. To the other end of the T I added the supplied pressure gauge. To keep things from leaking, all fittings were sealed using Teflon tape intended for that purpose:

Before I ran a casting through it, I wanted to test it first. I fired up my compressor and adjusted the regulator to 50 psi. With the valve on the pressure pot’s top closed, the hose got attached to the coupler. Per instructions (yes…I read them), I put the lid in place (I ended up adding alignment marks to the lid) and tightened the clamping screws hand tight and then slowly opened the valve. Nothing hissed or exploded but the safety valve let go at about 30 psi. I increased the pressure coming from the compressor’s regulator to 60 psi and adjusted the release valve on the pressure pot until it let go (that’s done by tightening the collar by the release ring if you need more pressure to make it pop, loosening it if you need less pressure). I closed the valve, removed the hose, and bled off the pressure coming from the compressor before readjusting the regulator back to 50 psi and hooked things back up and applied pressure. Once fully pressurized, I closed the valve, removed the hose, and brought the pot inside to see how many pounds of pressure it would lose over an hour’s time, which turned out to be about 14 psi/hour, so that meant when I used it to cast I would leave the pot connected to the compressor overnight making sure the compressor stayed turned on.

That’s what I did when I ran the first set of molds through the pressure casting process. (Put something flat on the bottom inside the pot because it’s dished and trying to get several molds to sit flat would be impossible without it.) I let the pot sit in the garage connected to the compressor overnight and under pressure. When I demolded the parts 99% of the bubbles were gone. Well, not gone but compressed so small that they can’t be seen…which is the intent of pressure casting.

Good enough!