I made my vacuum chamber, or pot more out of necessity than anything else. When making a mould out of silicon, you can add silicon oil to thin it slightly, but even with that, by the time both that the hardener has been added and stired in, there is a huge amount of tiny air bubbles trapped inside it.You can leave the pot to stand, as well as tap it on the table top and this does help the air to rise, but to remove almost all of it, a vacuum chamber is needed to do it right.So I did search eBay for a cheap vacuum pump, but the words cheap and vacuum pump just don't go together. It was then I came across an article about negative and positive pressures, and it seemed it was far easier to keep air from getting in, when forming a vacuum, than pressurising it and it getting out.While putting a container under high pressure needs a very solid container and burst proof seals, a vacuum chamber just needed to be fairy study, with a good seal as all the forces are pulling the lid closed tighter, increasing the seal.It was at this point I decided with nothing to loose to have a go at making one.
So this is the finished product. Itís a mix and match of parts I had and bits I had to buy. Luckily I had a vac-man pump I got from work years ago as it was going in the rubbish, and I thought it was to good to throw away, so kept it. Ten years later it found a use.The length of rubber hose is petrol pipe I had over form another job, but in this picture itís very long, but since this shot was taken, I have sincecut it down to five inches and a higher vacuum pressure is now obtainable. With the long length under high vacuum pressure there was too much air in the hose to depressurise before drawing from the pot, hence lower vacuum pressure.The container itself is a kitchen storage jar off eBay. I got four for a fiver so have three different size ones spare. It has thick seamless stainless steel sides, and a double wall lid. It also has a nice wide silicon seal, and study sprung clasp.The other bit I had to buy was a one way valve. So I took a chance and bought a non return valve used for fish pond air pumps. It was a risky buy and if it didn't work I was going to seven pounds out of pocket, but it worked just fine.
So this is a close up of the one way non return valve as used inline with all big fish pond air pumps. It came with two hose connectors, but I only needed one, as the other end was bolting straight into the pot.As I have air coupling for my spray guns and compressor, I always have spares just in case, and had just the bolt for the job. Quite simply it needed to be long enough to go from the inside to screw up and hold the valve tightly against the pot. It was actually a tad long to start with, so I took the end down a fraction at a time on my lathe, till it was just the right length.I used a metal washer and rubber washer each side to make sure the hole was sealed, and PTFE tape round all the threads, to make sure no air could get in.Although the valve is meant for stopping water siphoning back down the air line in the event of the air pump failing when used in a fish pond, its actually a very well made valve, and holds the negative pressure for around an hour I found by chance while testing it.
This is the valve to let the air back in. At first I thought I would just pop the lid when done, but thatís when I found out it holds the pressure for around an hour.I depressurise the pot I tried to open it, but the lid was held fast. I therefore left it until the lid popped open on its own which took around an hour.As this would be no good when degassing the silicon, I had to add a valve to let the air back in. As I didn't have anything at hand in the workshop, my dad came to the rescue suggesting a car tire valve, which at first I didn't think would work, but then it dawned on me to strip off the rubber, and turn it round.Taking the rubber off is a job in itself, and then I had to machine down the valve so I could get hold of the brass pin bit to let the air in.As the valve is fitted in reverse to how it would normally be used, the higher the vacuum pressure inside, the great the force pulling the valve close.The end results worked fine and with a pull on the little brass bit, the air rushes back in releasing the lid.
Not being a very big container, I can only mix around 300ml of silicon at a time, but as most of the things I make and cast are small, itís fine.So this is the pot in action. Unlike a proper vacuum or degassing chamber, this is manual so it needs re-pumping every few minutes to keep a good negitive pressure, but I have found ten minutes under pressure, and the silicon is almost bubble free.I experimented at first by preparing the silicon how I used to, by letting it stand and tapping the pot against a table, till it looked bubble free, then put it in the vacuum pot to see how much air was left. I found the letting it stand and tapping it, gets out a tiny fraction of the trapped air, nowhere near as much as I thought came out.Once under pressure the first thing you find out is it rises to almost twice its volume due to trapped air. At first small bubbles come to the surface, then it almost starts to bubble like its boiling, where the bigger bubble rise from deeper down.The less air trapped, the less it rises till you depressurise it and it rises no more, and only tiny bubbles are seen. These actually disappear when itís at room pressure they are that small.
So there you have how I made my own vacuum or degassing chamber. Not the most high tech version around, but for the home caster its more than adequate. Since making it and degassing the silicon I have noticed an improvement in the finish of the silicon moulds, which can only mean better casts from them. So for under £15 its been well worth the effort.