I was speaking with Mark last night and as he is planning to try his hand at pressure casting, he wanted to see some pictures of my setup. In anticipation for the inevitable flood of further questions I will receive from both him and anyone else interested, I decided to write up a bit of a quick guide to pressure casting equipment. Enjoy!
I'll start off with some pictures of the workshop and equipment. As some of you are aware, I'm back living with my folks for 6 months until I go back for my last semester of uni. Fortunately I was able to commandeer a section of my Dad's garage.
Okay, so now that's out of the way, let's talk about the pressure pot some more.
Mine is a 10L unit I picked up from eBay about a year ago. It came with a spray gun and a bunch of fittings and crap, including the black hose I use as a vacuum line. You want to take your chamber to at least 40PSI and I prefer 50PSI for my casting. Some hard core people go to 80PSI but I wouldn't want to do it with this type of tank and from what I've read anything over 50PSI makes negligible difference anyway.
In any case the safety release valve that came with the pot was only rated to 40PSI (even though the pot itself is rated to 100). This is a simple mechanism that will automatically release if the pressure reaches the release pressure, thus preventing any accidents from pressurising the pot too high. I broke the safety valve trying to get it off (when rearranging the fittings) so I had to buy a new one for $25. They're much more expensive than they look! I wanted to operate at 50PSI but the only one I could get was 60PSI, which is fine.
The pot is designed for spraying paint and thus had a big metal suction tube that goes from the lid down to the bottom of the chamber. The first thing I did was remove that and the other fittings. Whilst I did this next bit over the course of a year, it's still an arduous task.
Depending on the moulding and casting procedures you're planning to use you can get away with a simpler fitting setup. Because of my use of single part split moulds I need to sometimes apply a vacuum and then immediately the 50PSI pressure to the mould before the resin cures, and thus needed the pictured elaborate setup.
Just a quick recap of the basic theory of pressure casting. When you mix the resin (stirring it) you introduce small air bubbles. You also get bubbles as you pour the resin into the mould. These bubbles will collect on surfaces of the cavity, causing holes and loss of detail in the cast part. To remove these bubbles, the mould is placed into a pressure pot immediately after pouring the resin. By applying >40PSI the bubbles are shrunk down to nothingness and then once the resin is set, they are unable to grow again.
But unfortunately, the solution isn't this simple. What we don't notice is that the silicone mould is also is full of little air bubbles. When the mould is placed under pressure the bubbles near the surface of the model imprint shrink. This in turn causes little spiky shapes to appear on the surface of the cast part. In the worst case these bubbles in the silicone will actually pop, filling with resin. As the part is demoulded the resin bubbles tear away the silicone and the mould gets pretty stuffed.
The next solution then, is to ensure that the mould has no bubbles. This can be done in two ways:
- Place the mixed silicone under a vacuum to suck out all of the bubbles, before pouring into the mould box at room pressure.
- Place the mould box and everything into the pressure pot after pouring the silicone and leave it at 40PSI of pressure until the silicone is set (can be over 12 hours).
Pressure Moulding and Casting
Okay, this is the technique Jon is using and it seems to work well. His parts are all solid so he doesn't have to worry about implosion. You make your mould box as usual and clay up the master (or not, depending on the type of mould you're doing). Mix up the silicone and pour into the mould box (still at a slow speed as usual). One the silicone is poured the whole lot is placed into the pressure pot. Apply 40PSI (or 50 if you plan to cast at that pressure) and leave it under pressure for the entire time it takes for the silicone to set.
The following picture shows the simple pressure pot setup required for this:
Because there's only pressure being applied the connections are pretty simple. The only thing that may give you some grief if matching the different fitting types. If you can, try to work with mainly 1/4" BSP fittings - there are the de-facto standard for air fittings like this. The pressure pot had a couple of weirdo sizes (some metric) but fortunately came with adaptors to 1/4" and 5/8".
Vacuum Moulding and Pressure/Vacuum+Pressure Casting
I was lucky enough to score a vacuum pump for free (although it did require rebuilding and doesn't pull as nice a vacuum as is ideal) so I bit the bullet and went this route. I'm very happy with it. Many of my large parts are hollow and after a mishap trying pressure moulding a year ago I won't be doing that again!
I haven't yet done a pretty picture for this one, but this is as shown on the actual photos of my pressure pot. It's not that there's anything especially difficult about it but you need a few extra valves and things to make it work.
The main thing to worry about is the fact that you can't apply pressure to a vacuum gauge or vacuum to a pressure gauge. Well, you can but it's not exactly good for them...
For this reason you need extra valves for switching in and out the appropriate pressure gauge for your activity. I wanted to get a 3-way valve to select the gauge but they were disgustingly expensive and I compromised with a much cheaper option.
So that about sums it up for now. Ask away if you have any questions or I've been a bit vague. This isn't supposed to be a moulding/casting tutorial but more just a guide to the practical hardware aspects.