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.
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
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!
Flagship Models Inc.
" 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
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.
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.
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
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?
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.
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
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.
: 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
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.
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-.
Flagship Models Inc.