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Ideal vacuum pump specs


ive been looking at some great photos of home made vacuum casting set
ups,with the casting chamber made from a plumbers flange,& large
metal pipe,& furnace from fire bricks with propane burner etc & i’m
interested in giving one a go, but before i contemplate jumping in at
the deep end i’d like to weigh up specs & likely costs, problems
etc. i’m also going to get some casting books so i dont have to keep
bothering people with basic questions. can anybody advise me on
ideal vacuum pump specs (does it depend on the size of the casting
chamber),& likely cost? is the power of the vacuum pump important?
(ive seen fridge vacuum pumps used) also is a kiln a must have piece
of equipment for vacuum casting?

thanks in advance for any help

Hi Joel,

you use your vacuum to do two different operations.

  1. To degass the liquid investment, this has to be done in a limited
    time and down to a vacuum that will boil water at 20deg C. The size
    of pump is a relationship with the size of tank. Ie bigger tank,
    bigger pump. Or more accurately in relation to the amount of spare
    space in your tank when the flasks are in it. The speed of evacuation
    will drop as the oil gets dirty or as miniscule leaks develop.

  2. To cast, you don’t need a hard vacuum to cast ( in fact I just
    did an excellent cast with a sudden leak when a seal slipped with a
    vacuum of probably only 1/2 qn atmosphere(0.5 Bar).

But you are always going to do both so you need a powerful high
vacuum one.

I have a couple of systems, the smaller one has a cyindrical tank 8"
in daimeter and 14" deep, I use a 56 litre per minute pump which just
about copes. You can always play about with bits of plastic pipe,
tubes are very strong and you cannot exceed one atmosphere, plumbing
fittings are cheap and can easily be sealed with silicone caulking,
just make sure that your pespex top is thick enough, for an 8"
cylinder 1/2" to 5/8" is sufficient.

I have a big reservoir for larger casting that is a piece of plastic
drainpipe about 14" in diameter, with an inch thick perspex lid, I
never ever bothered to glue the bottom on, a piece of sheet
stainlesss steel and a rubber seal and the vacuum holds it together.

The old fashioned rotary oil filled pumps are a great workhorse, you
need to put a lot of oil in them but they are very forgiving of water
and contamination( plaster dust). The more modern rotary vane ones
don’t like water in the oil and need to be run for a long time after
you have degassed otherwise they wreck the bearings (expensive!!)

regards Tim.

Just a note on vacuum pumps used in casting systems in the jewelry
industry. Most of them use a pump and motor unit designed for the
HVACR industry (Servicing air conditioners and refrigerators). They
are available at Grainger and Johnstone (in most larger cities) and
used ones are available at pawn shops.

The Jewelry Equipment Dr.

1 Like

In 20+ years of HVACR service here is what I’ve observed about good
vacum pump practices.

Use the largest piping connections practical. Use only vacum rated
hoses or piping, some tubing will collapse internally restricting
the airway.

Even a small pump could be used to provide a surge of vacum if a
tank is evacuated and released suddenly through a full port ball
valve. If the oil ever appears “milky” it is contaminated with
moisture. Flush it out and even flush out the new replacement oil at
least once. When draining the oil you can run the pump and use your
hand to block off the exhaust, thus forcing extra sludge out from
the bottem of the sump. (This from a J/B rep).

Also the seals can sometimes be rejuenated, by changing the oil,
running the pump overnight, followed by another oil change. This can
reseat the seals, wearing them in.

The pump will pull the deepest vacum after the oil has been purged
of moisture etc. by running valved off with the gas ballast open for
a 1/2 hour or so. I always valve off the pump and leave on but with
the gas ballast open; when I am done. This clears noncondensables,
moisture etc. form the oil. (Even fresh oil needs this to run at
peak perfromance.) Some leading brand names; J/B, Ritchie Yellow
Jacket, Thermal are made for the HVACR market. Welch makes for the
scientific market and is good but generally is more expensive.

Dan Wellman

thanks for all the help. so what should i be looking for regarding
cfm or suction power ona pump for a small investing chamber &
casting chamber? would a fridge vacuum pump be up to it? i’m in
thailand so it may be interesting shopping for this,what with the
language barrier.

thanks again

You neeed to empty the tank of air in 45 seconds, so it will boil a
cup of water at 25 deg C. There are formulae, it depends on the size
of your investing chamber, How many cubic inches Pi X Radius X
Radius X Height (Where is the squared button on my keyboard?)


The ideal pump would be 5 cfm for a small to medium sized bell jar
to achieve the vacuum at a rapid enough speed for proper degassing of
the investment. For casting only using solid flasks 3 cfm or even
less will do. But for perforated flask casting you will need the 5
cfm at a minimum or a very large tank to act as a vacuum reservoir.
Now with that said, when I was first starting out I used an
aspirator pump ( to invest and
cast. The one I have is similar to this one here

It will pull close to 29 in Hg vacuum if cold water is used and the
water pressure is 35 psi or greater. It is a little slow but if you
use the smallest bell jar you can make and small solid flasks it
works well enough for the small studio. You can build a bell jar
from a piece of heavy wall steel pipe and a couple of 1/2" thick
acrylic or lexan sheets and some sheet rubber to make the seals.


James Binnion
James Binnion Metal Arts


Jattrading, I am using 2 pumps pulling about 6 cfm each to get good
results your pump should pull at least 15 micron at sea level. I
would recommend looking for used lab pumps. Mine are Hitachi, but
Edwards or Alcatel are other names to look for. I found mine online
1 from Ebay 1 from a google search for vacuum pumps. ( It just
happend that a fellow in CA had one for sale so on good faith I sent
him a check and he sent me the pump)

Best of luck, Sam

your pump should pull at least 15 micron at sea level 

As someone who routinely using a J/B Micron guage (made by Hastings)
I seriously doubt that you attaining a vacum level of 15 microns
(approx 15 millitorrs).To even get the pump alone below 50 microns
requires pristine oil and can take hours as the oil is purged of
contaminations. (Even new oil must be purged to attain that level of
vacum). I have seen cases where the oil in a refrigeration system has
been overheated and it has “cracked”. The pump overfilled with oil
because it was vaporizing the oil out at several hundred microns.
Water, even as ice, will sublimate out at approx. 1500 microns. I use
a vacum hold test of 800 microns or less for 15 minutes as proof of
decontamination (this is the system with the pump valved off). (This
is the reccomendation of Trane.) Perhaps you were confusing 15 inches
of mercury with microns? There are approx. 30 inches of mercury in an
absolute vacum and 25400 microns in each inch of mercury. Even the
manufactors of ultra low temp (-80C) free zers don’t expect a vacum
of 15 microns to decontaminate their refrigeration systems.

What is most important is large diameter piping, the least amount of
restrictions, and uncontaminated oil. It doesn’t matter haw many CFM
your vacum pump is if your piping (or vacum table) is the limiting
factor. The piping is usually the limiting factor in how many Cubic
Feet / Minute (CFM) you pull out of the system. (Are the vacum
tables rated for CFM?) That is the basis behind my statement that a
smaller pump could be used to pull down a storage tank which would
then have a large, full port ball valve opened quickly to your vacum
table, flask, etc. I think that all said and done, 6 cfm should be
plenty of capacity. I kind of doubt that the volume of the evacuated
area is 6 cubic feet. 1 cubic foot would be evacuated in 10 seconds.
Is this fast enough to fill the casting before the metal stops
filling the mold? At least this should give a rough estimate of the
capacity needed. Volume of the chamber divided by the time the metal
is liquid converted to CFM.

Perhaps I need to understand the vacum casting process better. Is
the flask put on the vacum table, followed by the liquid metal
poured in. Then covered quickly and evacuated? Or is the common
practice to melt the metal while in an evacuated chamber etc. Isn’t
it the force of the atmosphere rushing to fill the vacum which pushs
the metal into the mold? Maybe Daniel Grandi of Racecar Jewelry will
be so kind as to fill me in on vacum casting as commonly practiced.
Vacum pumps I know.

Anticipating the Formula One this weekend, as I sit out in the front
yard; 1/2 mile from turn one. I like the crowds and the atmosphere
they bring to Speedway.

Dan Wellman

Dan, I had written 15 micron based on some reading I have done in
reaserching the making of my own vacuum casting machine. Sorry if I
am wrong, however I was informed that in order to get water to
release quickly the air from the mixing of the investemt within the
11 minutes of time one has from start of mixing to finish,the pumps
must pull down fast enough to do the job.

I have a vaccum system sold by Gesswien Co. that worked fine on
small flasks 4 by 2 1/2 inch but it struggled to do more than that
(it is 3 cfm). I found that a good system for larger flasks needs
more than 6 cfm and it should pull 29 inches of mercury ( which I
thought was 15 micron) and it needs to reach this level with in 30
seconds if not less. My 12 cfm system will evacuate ( bubbleing the
investment) a medium size bell jar in less than 10 seconds more than
fast enough.

In vacuuming a solid or perforated flask the vacuum must pull through
the investment plaster strong enough to pull the molten metal into
the mold before it returns to a solid, So cfm is impotant, you melt
your metal and pour it into the open end of your flask under vacuum,
the timing,metal & flask (mold) temp.are key to success.

Dan Thank you for your regarding Microns.

All the best, Sam