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Homemade vacuum casting pump

Jewelry Discussion
#1

Hi everybody

I am new here. I am thinking of doing some casting. Since spring
centrifugal casting machines are expensive here (I live in Poland)) I
am thinking about vacuum casting. I could not find any vacuum
casting table but as I see these are really simple things to build.
But I have a question about vacuum pump power. How strong it have to
be to do some small castings(ring, pendant or maybe someting else -
generaly small items). I have found not expensive small pump(about
90$ - its a lot of money) but I do not know will it be sufficient.
Its parameters aRe: 220V/50Hz, 1-stage, 1/4 HP, End vacuum 5Pa/0.05
mbar, performance 42l, 1300 rpm. I would be glad if someone could
tell me if this one could do the job ecause I dont have more money
now.

Some of you will ask why you not try steam casting. I have tried it
with partial success. 6 tries - 2 success. I would like to do vacuum
casting because of it simplicity and high success ratio(at least I
have heard that)…

So, will that kind of pump will handle in casting small items?

Thanks for help,
Mark

#2

mark - any air pump, compressor pump, car engine motorcyle lawn
mower gas powered washing machine can provide vaccum if you connect
them to an electric motor. the pumpor engine must move the air it
depends which side of pump you attach the hose on the pushing side
you get compressed air on the pulling side you get vaccum the bigger
the pump or motor like a car engine or motorcycle or lawn mower
engine the more air you can move pulling or pushing … i built my
first vaccum casting machine from a refrigerator compressor.

the pump was not as strong as i would like it to have been but…
i had enough success with it untill i could buy a real vaccum pump .
what is great about refigeration pumps is that they are already
built with the motor inside just be carefull to have someone take
the freon out so it does not go into the atmosphere i sppose if you
put two or three together or get one really large one you can get
good results. the part i had the most trouble with was the investing
it takes 29" of vaccum at least to get the job done and really get
the air out especially from a fine pattern on a wax model.

make sure to look for a vaccum gage i found one i use now at a car
swap meet in a bucket of old parts for $1. it is nice to see someone
like you trying to experiment and build thier way to success,

good luck
goo

#3

Thank you for your reply

As I calculated Standart atmosphere pressure is about 101500 Pa and
it equals to about 30.0546609893 inces off mercury. That pump is
listed earlier have end pressure at about 5-10 Pa so it can pull out
about 101490 pa and thatt is 30.051699939. so it can pull about 30
inches of mercury. Calculations based on

http://www.sengpielaudio.com/calculator-pressureuni.htm

I have mailed pump manufacture and they says that it can pull abou
29-30 inches of mercury. So will it handle casting small flasks and
degassing small ammounts of investment? My flasks are about 5x6 or
5x7 cm.

Thanks, Mark

#4 
I have mailed pump manufacture and they says that it can pull abou
29-30 inches of mercury. So will it handle casting small flasks
and degassing small ammounts of investment? My flasks are about 5x6
or 5x7 cm.

It is not the absolute vacuum that is the limitation but the volume
per unit time or liters per minute that are going to be an issue
with a small pump. Ideally you want at least 80 liters per minute of
pumping speed.

James Binnion
James Binnion Metal Arts

#5

Mark

That pump will work just fine. You will need to keep the size of
your bell jar for investing as small as possible due to the small but
it is more than adequate for casting.

James Binnion
James Binnion Metal Arts

#6

Mark,

I have played with fridge compressors (good for 27") and standard AC
vac pumps. Fridge compressors are free to cheap but not the best. A
refrigeration vac pump is much better.The best I have seen with my AC
after rebuilding all seals and clean oil at 550’ above sea level was
29.25 and a bit. A refrigeration vac pump will probably serve you
well (~300$), just don’t believe all of the sales hype. They lie :slight_smile:

jeffD
Demand Designs
Analog/Digital Modelling & Goldsmithing
http://www.gmavt.net/~jdemand

#7

i had forgotten one soloution i used to offset the volume issue of
litres p/ minute on smaller pumps. you can attach a tank in line to
create a "volume of vaccum ", i used an old freon container that i
adpted with an extra valve so as to have inlet and outlet. if you
start the pump before you are ready to cast or invest and pull the
air out of the tank then when ready you can place the flask or bell
jar on the pad and release the valve the pumps ability will be
assited by the extra volume. a large volume of vaccum in a chamber
will offset the inability of the pump and get you where you want to
be in time to make things happen, i tried it it works ! - goo

#8

When we service casting machine vacuum pumps, we make sure they are
capable of 23 “inches of mercury”. Our shop is at 7000 feet altitude
making 23 the max. At sea level, 29 is achievable. The 0-30 vacuum
gauge is for convenience in operating the machine and is not precise
enough to measure the level of vacuum needed for casting purposes.
We do a final test of a serviced vacuum pump with a micron gauge
which is a measure of absolute pressure (not dependent on atmosphere
pressure or barometric pressure). Each pump must pull down to 50
microns. Refrigeration and air conditioning service vacuum pumps are
capable of this and are what is usually in a casting machine. A
small pump will take longer to do the job, as long as it can pull a
good enough vacuum.

John
The Jewelry Equipment Dr.

#9

Another way of dealing with a small output vacuum pump is to us a
tank between the pump & where the vacuum is needed, with a valve in
the line. This way the vacuum can be in effect accumulated and a
greater volume is available when needed for a short period of time.
This works just the same as the tank on your air compressor, but in
reverse.

Mark Chapman

#10

We don’t vacuum cast but we vacuum invest with a dual-purpose vacuum
caster… The gauge tops out at 30 inHg, and it runs at 26 most of
the time…FWIW

I wanted to say that I’ve been wanting to expand my knowlege of
vacuum in general for a few reasons, and this brought that out.
There are some articles in Wikipedia that are especially complete
and accesable, and the section on vacuum is one of those (as is
particle physics, BTW). Much more than this thread needs, but for
any who are interested, it’s pretty good…

http://www.donivanandmaggiora.com

#11

Thank you for your all replies.

That pump I am talking about is used in air conditioning systems.

City where I am living is about 20 m abouve sea level so its not
much.

When I will buy that pump I will try degassing inviestment in small
chamber, and if it will not work I will use additional tank just as
you said.

Thank you for all it really helped to make up my mind
about buying it.

Cheers, Mark

#12

jeff - every body knows a frig compressor is not the best choice for
a vaccum pump what the guy was asking for and what i was offering is,
what would you do? when you are poor and do not have money, the next
best choice could be a frig compressor. with an inline volume tank.
then when he makes enough money he can go out and solve his problems
with some of that money

goo

#13

Hi Gang,

the next best choice could be a frig compressor. with an inline
volume tank.

One other thing to consider if you’re using a refrigeration
compressor that’s been made in the last 20 years is condensation. The
valves used in these compressors are usually reed type valves & they
don’t do very well when subjected to moisture that exists in the air
in most parts of the world. It’s a good idea to add a dehumidifier
between the compressor & the vacuum chamber. This will reduce the
amount of moisture that gets into the compressor. You can get
dehumidifiers form places that sell refrigeration tools, compressors
etc. In that business they’re typically called ‘dryers’.

It is also a good idea to add a ‘drain valve’ to your vacuum tank to
be able to drain any accumulated moisture (water).

Dave

#14

I used a fridge compressor or two for years. They aren’t great but
much better than nothing when you have no money. One lasted for a
decade even after I sucked investment into it. A real refergeration
pump now with a drier/filter to cover dumb mistakes but I don’t think
it makes any difference. I still get water in the oil. A drain in the
accumator tank is a good idea, I did rust through through the bottom
of a free freon tank. ( now where is that hissing sound commimg from
:slight_smile:

jeffD
Demand Designs
Analog/Digital Modelling & Goldsmithing

#15

Has anyone considered the use of a standard tap aspirator? An aspirat
or is a device that uses the venturi forces of flowing water from a
tap to draw a vacuum. Properly installed, a good quality aspirator,
available from most lab supply companies will quickly draw a vacuum
equivalent to the approximate vapor pressure of water at the
temperature of the water flowing through it. With typical tap water
temps in the high 60’s to low 70’s here, I was able to draw a vacuum
in excess of 27" to 28" Hg here. An aspirator seems to have the
potentialbenefit of both being inexpensive and having no moving
parts. If the water in the investment is slightly warmer than what
comes out of your tap, (plaster reactions being exothermic, it should
be), then thewater will boil.

Once the boiling point is reached, little further is to be gained as
the water vapor itself should keep you from getting to significantly
lower pressures.

I will admit not using one for jewelry work as I hand forge rather
than cast my work, but I have used one for de-gassing water and
scientific instruments (pressure transducer assemblies) that
required complete de-gassing for maximum accuracy and responsiveness.
This process was quite sufficient to maintain our lab’s national
bureau of standards class “a” traceability for scientific
measurements. I have done casting in the past and had enough hit and
miss to understand how folks might be risk-adverse. If you have a
system that works consistently, by all means use it. However, once I
decide to move somewhat in the direction of casting, I expectto
givethis a try.

Bill Carlie
Night Heron Studios

#16

The two objectives in using a vacuum pump for plaster degassing are

1: To get it to boil so as to remove the air, a tap aspirator will
do this quite effectively.

2: To complete the neccessary mixings, vacuumings and pourings
inside (usually) 9 1/4 minutes.

A large tank can take quite a time to empty, this is why you need a
large pump. The crucial factor is the amount of spare space between
the flask(s) and the tank. If you can cut this down you will get to a
hard vacuum much faster. A big flask in a tank only just bigger than
it gets to boil quite fast.

The acid test I learnt was to boil a cup of water at 20 deg C in 45
seconds, if it will do that all is OK. I have a big pump and tank cos
it’s easier to do everything in one system.

You can make quite good vacuum tanks out of HEAVY duty drainage pipe
with a steel plate glued on the bottom. Cylinders are very strong.
Make sure you make the perspex lid thick enough, mine to go on a 12"
tank is 25mm thick, you can still see it dome in slightly. The maths
at 14lb /sq inch are frightening!

I had a customer who was worried about it being strong enough, he
wanted to test it to overpressure, I found it hard to explain that
you couldn’t go past a vacuum.

regards Tim Blades.

#17 
Has anyone considered the use of a standard tap aspirator? An
aspirat or is a device that uses the venturi forces of flowing
water from a tap to draw a vacuum. Properly installed, a good
quality aspirator, available from most lab supply companies will
quickly draw a vacuum equivalent to the approximate vapor pressure
of water at the temperature of the water flowing through it. With
typical tap water temps in the high 60's to low 70's here, I was
able to draw a vacuum in excess of 27" to 28" Hg here. An aspirator
seems to have the potentialbenefit of both being inexpensive and
having no moving parts. If the water in the investment is slightly
warmer than what comes out of your tap, (plaster reactions being
exothermic, it should be), then thewater will boil.

I tried one of these “tap aspirators” (also called a “water eductor”)
some years back, hoping to save the money that a vacuum pump would
cost. While it did draw some vacuum, it was not enough to boil my
investment at room temperature. In fact the pieces came out looking
worse than if I’d simply mixed and poured the investment without
doing anything further. It seems that if you get almost - but not
quite - to the boiling point, you actually form bubbles in the
investment that are not scoured off by the boiling action. In the
end, I still had to buy a vacuum pump, so I ended up losing money,
not to mention the huge amount of water down the drain…

Andrew Werby
www.computersculpture.com