I just noted that the IPMS-USA Journal has an article titled 'Build your own vacuum chamber'. Can anyone tell me more about it? What's the intended use? I recently bought a vacuum pump, and I'm planning to build a vacuum chamber, for RTV and resin casting.
Rob
My models:
It's probably cheaper and far safer to buy a polycarbonate vacuum dessicator from a lab supply house.
A vaccuum chamber is intended to "de-gas" RTV (Room Temperature Vulcanizing) rubber that molds are made from. The secret to making a good mold is to eliminate the air in the rubber. The vacuum chamber via the vaccuum pump removes about 30 atmospheres of pressure causing the air in the RTV to expand and rise to the top thus "de-gassing" the rubber.
Vaccuum chambers aren't cheap. The chamber I use in my business is a bell jar that sits on a base with a special gasket to keep the negative pressure. It cost over $600.00 new, but I was fortunate to find a used one. That doesn't include the pump (another $250.00 to $400.00). Now you know why every modeler doesn't have one. When you build yours, make certain the top allows you to view inside. This is important so you can observe the rubber's progress during the de-gasing process.
Now you need to make yourself a PRESSURE pot and you'll be ready to cast like a pro!
Rusty White Flagship Models Inc. flagshipmodels.com
" the vaccuum pump removes about 30 atmospheres of pressure causing
There's only 1 atmosphere (14.7 pounds/square inch) to begin with. Removing 30 atmospheres would be like pouring 30 pounds of sugar out of a 1 pound bag.
Val Kraut
Rob,
You don't need to build a vacuum chamber. You can get one at Sears for $100.00. It is a syphon tank spray setup. You just delete one of the openings and put a vacuum gauge in one and a valve in the remaining one and you are ready to go. very safe too.
Do Sears export?
Be carefull if you make a pressure pot. that is a potential BOMB anytime you pressurize over 20psi the vessel can EXPLODE. Make sure that there is a pressure relief valve set a a minimum of 80 PSI and the vessel that you are using is rated three time that.
Polycarbonate vacuum dessicators are just as good, much lighter and a hell of a lot cheaper. Mine ran $120 from VWR.
How can a vacuum pump remove more than ONE atmosphere? One atmsphere =
1 Bar = 14.7 psi.Even if the units were psi instead of atmospheres, ambient pressure is only 14.7.
That's an excellent suggestion, I never considered them! Thanks for the tip.
Rob
My models:
I've always done my RTV and resin casting with vacuum only. It seems all American casters use pressure pots for casting. I have yet to find a European caster who uses that technique! Not that I know all of them though.. My airbrush compressor goes to 5 or 6 bars (75-85 psi), is that good enough?
Rob
My models:
Apart from the excellent suggestions made in the replies, can anyone tell me what the IPMS-USA magazine article is about? I'm in the Netherlands, so I have little or no chance to lay my hands on it. Is it worth chasing down for that article?
Rob
My models:
Theoretically it can't but nobody is even using the correct units here. Atmospheres and bar are units of pressure, not vacuum. The cheaper pumps are rated in "inches of water" which is how many inches of water they can draw up a specific diameter tube (if you're lucky these pumps will pull the equivalnet of 25"Hg). Next come pumps rated in "inches of mercury", same idea. A theoretical "perfect vacuum" is 29.92"Hg (generally these pumps will be rated to 27-28"Hg). Then you get into the $1000 or more pumps for lab use and at about that price point you start seeing them rated in millitorr, the more you pay the closer to a "perfect" vacuum you can pull, assuming your vacuum chamber and seals can take it. To get even close to "perfect" you'll spend tens of thousands of dollars and end up a huge chamber, hardline piping and multistage pumps with turbos, cryos and ion gages.
You're welcome.
40-65 PSI is good enough. What you need is a tank compressor that can dump all that pressure into the pressure pot in one shot. Of course the pressure pot is also important, don't cheap out when dealing with pressurized containers.
The usual workaround is degassing RTV rubber with vacuum to have crisp molds and pressurizing the resin cast to make air bubbles collapse in the resin mass until the resin is cured, resulting in compact and flawless finished parts. That's the most common method, apart from free air casting, I've seen used in Italy, Europe.
Rob de Bie wrote: : : Apart from the excellent suggestions made in the replies, can anyone tell : me what the IPMS-USA magazine article is about? I'm in the Netherlands, so : I have little or no chance to lay my hands on it. Is it worth chasing down : for that article? : The long time editor of the IPMS Journal, for the IPMS/USA organization, claims to have been forced out of his position.
That was page 1, and in color. Pages 2 and 3 were a center fold section showing the 100 or so issues that he was the editor of. Also in color.
The gent involved did not exactly cover himself in honor and glory in his parting shot, I am sorry to say.
Really, a tempest in a teapot, unless you happen to be in his shoes.
Bruce
Yes, you need a pressure tank compressor because the resin must sit under pressure for a good 30 minutes or longer for the resin to completely cure. A small tank (non oil bath) or diaphram compressor will burn out pretty fast running constantly for half an hour. They just aren't made for that kind of extended use. BTW, the bigger the tank the better. For a single pot I would think a 20gal tank would suffice. Just remember the smaller the tank the more often the compressor must kick on to maintain the pressure.
You don't need to vac the resin, just the RTV. For small molds vaccuuming really isn't necessary. Before I got my set up, I would spray the master mold release and PAINT the RTV into the small detail areas, then follow up with filling the mold.
Rusty White Flagship Models Inc.
You can also build one from a pressure cooker. I had two converted from pressure cookers I bought at Ace Hardware. You still have to add the quick connect, the pressure gauge (that's on my airline coming from the wall), and bleeder valve. They both work very nicely. However, my pressure cookers have a saftey hole in the rim of the cooker which will blow the gasket if it rises above 40lbs per sq inch. I blocked mine off with thin sheet metal made from an Xacto saw. I can now pressure it up as I need. However, 40lbs is enough. The whole setup costs around $120.00 +or-.
Rusty White Flagship Models Inc. flagshipmodels.com
I think you mean 30"Hg ,ie: 30 inches of mercury not atmospheres
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