Vacuum pump for casting system - Ganoksin Orchid

Hi Linda,

kaierwo supply professional and honest service.

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.

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