Under pressure

I've been doing some work on casting lately. When I first started casting it was a revelation, but then I realised that I was getting tiny little air pockets in the corners of many of my castings.

To get rid of these took very careful mixing, careful pouring, careful prodding in corners of the mold with toothpicks, and then a bucket load of luck. Out of about 20 castings, only 4 came out with no noticable air pockets in the corners; a rather poor success rate.

I did a bit of thinking and decided there were two ways to get rid of bubbles:

• More luck, or,
• Using a vacuum to remove any trapped air from inside the resin before casting.

I tried using a vacuum to degas the resin; it ended badly. I poured the mixed resin and poured it into a syringe, then blocked the end and pulled the plunger back. This created a semi-vacuum in the syringe, and sure enough I could see the air bubbles getting very large and popping. Then all the resin started foaming, and then in about 10 seconds flat the entire syringe of resin set solid in a very exothermic reaction! Ouch!

Back to the drawing board. Turns out most casting resin doesn't appreciate vacuums. Doing some research however I discovered a thing called a "pressure pot". It seems there is a third way of degassing: by placing the resin under pressure, any air bubbles inside are forced to shrink. To look at it another way, imagine you are on a long haul international flight and half way through you take a sip from your water bottle. You put the cap back on and put it back in your bag. When you get home you get out your water bottle and see that it's shrunk in on itself! It's the same principle. At high altitude you fill the water bottle with low pressure air (since planes are pressurised at less than 1 atmosphere), but when you get down to sea level, the air pressure is much higher.

So I set about making a pressure pot.

First ingredient is a pressure pot. Duh. I found one on TradeMe that was for spray paint. Slightly different principle I believe, but same equipment. It'd had a hard life so I gave it a good clean and chopped off the outlet tube that stuck down into the pot. I bought a couple of brass blanks for blocking the extra air tubes, which left:

• An inlet hose
• Air pressure gauge
• A safety valve
• A small bleeder valve

I did some tests on my compressor but it leaked pretty badly. I tried some grease on the threads, but it still leaked a lot, and now there was grease all over the garage! I bought some thread tape and tried that... still leaked a bit. I tried a LOT of thread tape and finally got an almost air tight seal!

So now the casting process:

1) Mix up resin. I have some very accurate wee scales that measure down to 0.01g, or single drops of resin. As soon as you start mixing you have about 3 minutes before it sets solid.

2) Pour into molds. I have the molds already sitting in position. I mounted the pressure pot horizontally on my WorkMate, and made a little platform to support my castings. This way they aren't disturbed as the pot is put on. At this stage I flex the molds and look for any large air holes.

3) Place waxed paper on top, and add weights. The waxed paper doesn't stick to the resin and gives a nice smooth rear surface.

4) Screw on pot.

5) Connect air compressor. I have a quick-connect fitting which makes it very quick. Watch and listen as the tank quickly gets up to 20psi. If I had a better seal I could go higher, but I find 20psi does a pretty good job.

After 15+ minutes the resin has set enough to be removed.

There is only one problem with the pressure pot method: it will find every flaw in your mold! When I made the silicone molds, I didn't have a pressure pot, and so there are very small air bubbles here and there in the mold. Normal room pressure casting doesn't find these flaws, but high pressure casting does. But that's ok, it's a lot easier to carefully remove material than it is to add a square corner back again!

And so far I'm having about a 90% success rate. That is far better than before. Once I have a decent set I will make them into a larger block, then make a high pressure silicone mold from that. Then with a larger block I can pressure cast that and within a couple of days I can have enough pieces for an entire building!