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Vacuum pump for casting system


#1

My husband is gagging at the cost of the replacement vacuum pumps
offered for casting systems. Rather than paying $350 - 400 from a
jewelry supply company, he insists he can purchase a similar unit
from Harbor Freight for $150 and still get the job done. I’m
generally clueless about mechanical things, but do know that with
aircraft, autos, etc., the parts are specifically made to fit into a
certain space. Not an issue with a vacuum table. I’m also all about
saving money, but is there something else that hubby isn’t aware of?

Linda


#2

Linda- I have used two homemade vacuum pumps in my career. Both
worked well. Tim and I still use our homemade one to this day. We
just took a vacuum pump and attached it to a home made casting
platform and bought a bell jar. It’s made us plenty of money over the
years and cost maybe $100.00 to build.

Have fun and make lots of jewelry.
Jo Haemer
www.timothywgreen.com


#3

Hi Linda,

I’m coming from the ‘serious vacuum pump’ end of the world. $3-400
pumps are the cheap ones. Serious pumps are 3-4 thousand dollars,
or more.

God knows what Harbor Freight has, but I wouldn’t trust it for more
than getting me through an emergency.

So, for casting, the pump you need depends on what you’re doing.

For degassing investment, you need reasonable speed, and a pretty
good ultimate vacuum. On the order of 3-5 CFM (cubic feet per minute)
and at very least 28"Hg. (28 inches of mercury, on a standard
hardware store type vacuum gage.) It’d be nice if you could get to
29"Hg, but the important thing is that if you put a coffee cup of
water in your bell jar, and hit the pump, the water in the cup
"boils" in under a minute. If you can do that, you’re good for basic
investment degassing.

For vacuum casting, you need a very fast pump, with an ultimate
vacuum that doesn’t really matter that much. It’s all about moving
as much volume through the system, as fast as you can. Which is why
the vac-u-cast rigs tend to use (relatively) cheap vane pumps, as
opposed to the rotary pumps that give better ultimate vacuum. Vane
pumps are faster, and cheaper. (Essentially mutant shopvacs. In
fact, my college shop did some pretty good vacuum casting with a
big shop vac.) I don’t know the specs on what you need "officially"
for that, I’ve got big rotaries, and with casting, it’s really “as
big as you can, as fast as you can” The reason being is that the
bell-jar for investing tends to seal pretty well, and you’ve got
plenty of time to get it sealed down. Casting flasks? Not so much,
the casting systems tend to leak like a sieve. Which is why you
need lots of speed on the pump, to overcome the leaks, and give you
some hope at a decent negative pressure in the mold. (Nevermind that
investment is designed to be air permeable. That’s why vacuum
casting works in the first place. But it is one huge leak, all in
itself.)

One other word of advice: The older vac-u-cast rigs, the ones that
use a flat plate with a hole in it as the casting table, had a trap
under the hole to catch any metal that blew through the bottom of
the flask. Preventing molten metal from getting into the vac line,
and then into your pump, likely destroying it. Sometime in the last
20 years, they stopped building the trap into them. It’s just a hose
fitting, leading straight into a plastic vac line, positioned
directly on top of the pump. So if you lose metal into it, not only
does it get sucked into the pump, it then burns through the line and
drips all over the pump motor. If you have a flat-table style
casting system, and you don’t have a trap under it, add one. They
can be made from black iron plumbing parts pretty easily. (Under
$20. Lose just one ounce of silver into it, and it’s paid for,
because you can easily salvage the silver out of the trap, rather
than trying to chip the grains out of your pump motor. (Ask me how I
know this…))

(The short form is that you buy a 1/2" thd “T” joint, a short bit of
1/2" thd pipe, a 1/2" end cap, a 1/2" thd nipple, and whatever size
hose nipple you need to get your vacuum line onto. Put the hose
nipple on the side joint of the “T”, add the short bit of pipe to
the “bottom” of the T, close the bottom of the pipe with the end
cap, and then put the thread nipple into the other side. Make a
larger hole in the center of the vacuum plate, and thread the nipple
into it. Thus modified, if metal blows out the bottom of the flask,
it drops into the bottom of the pipe, where it sits until you
unscrew things and fish it out. The vacuum line pulls out the side,
half way up the wall, where the metal can’t get at it unless you
really screw up. (By blowing out a really large casting.))

(1/2" pipe is actually about 3/4" internal diameter. No idea where
the size rating comes from, but it’s a lot larger than it sounds.)

One last word of advice, if you end up re-plumbing your casting
system. Weird as it seems, with vacuum systems, you want pretty
large bore pipes. Short, but large-ish diameters. Last time I
rebuilt one, I used 1" ID tubing. Shortest runs I could manage, but
decent bore size. With any pump fast enough to be worth using, the
pump will clear most of the air out of the lines in the first few
seconds. After that, the throughput of the system has more to do
with how it moves low density gasses. Low density gas moves better
through big pipes. Weird, but many things about vacuum systems are
weird.

Regards,
Brian


#4

Anything that comes from Harbor Freight is crap, that doesn’t mean
you can’t get some good use out of it but you must recognize it is
crap and be willing to shrug your shoulders and go buy a new one when
the HF model dies, you pays your money and takes your chances. We
abuse vacuum pumps by pumping water vapor (investing) and dust
(casting) so it may be cheaper in the long run to buy the HF version.
But make sure you are comparing apples to apples the critical specs
are ultimate vacuum rated in inches of mercury or sometimes in Tor or
microns and pumping speed in CFM or liters per minute. So make sure
it is being replaced with a similar unit.

James Binnion
James Binnion Metal Arts


#5

Very interesting, thank you.

do you knwo a brand name offering vacuum casting with fast pump?

thks best
nathalie


#6

The vacuum pumps used for this stuff are originally made for the
heating/ventilation/air conditioning industry. The only practical
difference is that the ones sold for the art industry often have a
filter on the outlet side to keep the pump oil from getting all over
stuff, and that’s a standard device that can be moved from pump to
pump.

Ron Charlotte
Gainesville, FL


#7

I bought my vacuum pump from an HVAC supply wholesaler. They had
several very good used pumps. They selloil by the gallon cheap.
Their customers are airconditioner repair men who use the pumps to
dry out A/C systems before replacing the freon

For a bell jar I use a number 10 can with the rim polished

Ben


#8

Thank you, everyone, for your thoughts on the Harbor Freight vacuum
pump for our casting set-up. I do realize that HF items are low end,
and probably won’t stand up to repeated use. But hubby is pretty
handy, and understands the intricacy of such things - something I
don’t fully comprehend.

The pump he came home with does meet the same specs that the
original pump had - 3 cfm, and producing 29" of mercury. Have not
yet had the opportunity to set it up on the table and insure that it
will provide the level of suction it claims.

I’d like to particularly thank Brian for his in-depth explanation of
degassing and casting, and especially on the suggestion of the
"metal trap" between the casting table and the pump. It was easy to
grasp and should be easy for hubby to implement.

I was also interested to read about the various home-made apparati
that people are using in lieu of “store bought” systems. It’s great
to know there are still free spirits among us, with plenty of
ingenuity to spare.

Love this list!
Linda


#9

Hi Linda,

The Harbor Freight vacuum pump either will not perform well or it
will not last long (or both!). I have been looking after vacuum
pumps for more than 20 years. My favorite brand is Alcatel. You can
turn one on today, and check it again in three years, and it is
still running strong (15 to 30 mTorr).

Another favorite brand, more suited to your application, is the
Welch. We have two model 1374s, and two 1397s that have been around
for as long as I can remember. The welch pumps have all been rebuilt
once with longer service than most of our other pumps. We use
Leybold DryVacs and Leybold TriVacs, Leybold D60ACs and D30s, Edwards
8s, 80s, and 275s, Vacubrands, and Stokes. That list doesn’t include
the cryo pumps, diffusion pumps, and turbo-mole cular pumps. There
are itty bitty pumps hidden everywhere, including a turbo molecular
pump the size of a beer can (another favorite).

Do a search on “BioPharma Exchange”. In those auction listings you
will find the above pumps being sold for $300 to $400. Some probably
retailed for the $3,000 or $4000 previously mentioned in this
thread. The risk is that they probably need to be rebuilt. Sometimes
it is that they are being replaced by low contamination dry pumps.
The price of dry pumps has come down a lot, and they are much better
for clean processes than oil pumps. Another reason for their low
price is that they are very heavy! You will get hit by freight
charges if the auction is not close enough for you to pick it up
yourself.

You could also check your local university asset management office.
I have three Alcatels and two Edwards pumps from contract
maintenance agreements that are surplus. I haven’t had the need to
rebuild them and put them back into service. If I need the space,
they will regretably be sent to the asset ffice. I have seen a lot
of Welch pumps set aside for new pumps, too many! I’m certain you
could find one, and be set up for a long time! Keep the pump dry!

Jeff