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Anyone use a hand-operated vacuum pump?
Daverham -
Monday, February 22, 2010 - 11:29am
Casting casting | investment | vacuum Hi there, folks. I've been using home-made, ramshackle equipment for years and it's working OK. I'm trying to step it up. I have never used vacuum to remove bubbles from my investment (Small-scale jewelry casting), and I think it's time to start. I've had a few bubbles, luckily, not TOO bad over the years, but I don't want ANY. I'm too old for this bubble nonsense anymore. My options are: 1. Buy a super expensive vacuum pump and bell jar (bah!). 2. Make my own out of a fridge compressor. This sounds pretty good - anybody know if a mini-fridge compressor is strong enough? 3. New idea: I read that you need maybe 25-30 inches of vacuum ("mercury") to get the air out of your investment. Well, there are hand-operated vacuum pumps (for automotive use) available that can pull about 25 inches. Why not? Anybody know if this would work? Might be nice to minimize all the "gear" sitting around in my shop - and these things are only about $35! Anybody ever used one for this? Like this: http://www.amazon.com/OEM-25136-Vacuum-Pump/dp/B000CMDPBM/ref=pd_sbs_auto_1  25 to 30 inches of vacuum?
Rich Waugh -
Monday, February 22, 2010 - 11:16pm
25 to 30 inches of vacuum? Where you gonna get 30"? To achieve 30" (mercury) vacuum, or "0 Torr" is going to require at a minimum a two stage rotary vane pump or a diffusion pump. Both of which are very expensive machines. 25" or mercury is "low vacuum" and can be attained by a simple aspirator on an air or water line, a hand pump, or that rededicated refrigerator compressor. I've used vacuum "debubblizing" with pretty good success over the years, though not perfection. Good enough for me, though. I like the notion of pressurizing the bubbles to shrink them, too. Never tried it, but might someday. Vacuuming has a finite limit to the available pressure differential (14.7 psi), but pressurization doesn't have that limit. With a suitable pressure vessel you should be easily able to achieve a differential ten times that, resulting in shrinking the bubbles to something below the limits of detection. That would take more than a pressure cooker, though not a lot more. The devil is in the details, though. :) If you live where water is plentiful and cheap, I'd suggest a simple water aspirator. Shouldn't cost more than ten bucks tops, as it is one machined piece with no moving parts. Just a simple venturi in a water fitting. If water is an issue, and you have compressed air, you can do the same thing with an air aspirator. These are sold for auto air conditioning system purging and cost about twenty bucks, The same principle as the water aspirator, just using air. Either of these pumps will pull a vacuum of around 28" of mercury or a bit more. Sufficient unto the purpose, anyway. Rich  So... conclusion is that 25"
Daverham -
Tuesday, February 23, 2010 - 12:19pm
So... conclusion is that 25" of vacuum is good enough? Last night I purchased a used, working refrigerator for $10 and have already removed the compressor. I've heard that system works, so I'm going to give it a try - I'll report back with the results. Thanks for the info. I'm looking into the aspirator system too, mostly out of curiosity. I've seen venturi systems, but not involving water, that I know of. I made it and it works! A
Daverham -
Sunday, February 28, 2010 - 9:28pm
I made it and it works! A vacuum de-bubbler for $13 - thats right. $13! - 1 used mini-fridge from Craigslist: $10 That tiny little compressor pulled a really strong vacuum. I physically could not lift that vase from the rubber mat, no matter how hard I tried - even after 15 minutes with the thing unplugged! Since then, I added a little vacuum breaker valve in the copper line, under the table. Duh. I have thought of that in advance, but then got excited and forgot. I nabbed the compressor out of this little fridge: Vases and rubber gasket: Detail of the platform on top: The completed unit: Not the best shot - but it sucked tons of bubbles. I was so excited I forgot to take pictures until they were mostly gone already:  Home-made vacuum machine
Argyll -
Monday, March 22, 2010 - 10:44am
Greetings, Alright! Glad to have
Daverham -
Monday, April 12, 2010 - 2:32pm
Alright! Glad to have contributed to your decision to join the forum - the more people the better. Welcome.  Different uses for vac pumps
Daedalus -
Tuesday, April 13, 2010 - 8:39am
You folks have probably already thought of all this but don`t overlook the other uses you can put that vac pump to. In theory there is no difference between theory and practice. Nice post -= Very helpful
Moldamaniac -
Wednesday, April 21, 2010 - 4:47pm
Most appreciated for the info. I've been pondering the idea of knocking one of these up myself. You make it look quite simple.  very cool
B.J. Severtson -
Wednesday, April 21, 2010 - 6:08pm
Just a couple of suggestions. Rio Grande sells replacement plastic bell jars, They aren't that expensive. They have the ability to bounce off the floor. When filling your flask, actually before filling, wrap a duct tape flask extension onto the flask. Fill the entire flask into the duct taped extension, vacuum as normal, let the flask dry, trim off excess investment. the process avoids short pours, avoids (Boiling)investment from getting near the model, and allows you to use all of the flask. Consider putting springs under the table pad area. Consider using a coil of copper tubing as the connecting plumbing. The coil will give you time to react if you need to shut off the pump, before investment tries to go through the pump. Definately on the Duct tape
docone31 -
Wednesday, April 28, 2010 - 5:48pm
Definately on the Duct tape on the flask during vaccuming! It really helps get a complete debubblize. In the beginning was the Floyd, and the Floyd was with him. Jim. I hope you understand
tigervan -
Thursday, April 29, 2010 - 2:26pm
Jim. Yep, that's how it works.
Rich Waugh -
Thursday, April 29, 2010 - 4:53pm
Yep, that's how it works. The water flows through the aspirator and out the other end. The barb connects to a hole located just beyond the neck of the venturi in the aspirator where the diameter begins to increase again and sucks in air - it doesn't spew out water due to the difference in pressure from one side of the venturi to the other. The way it works is based on Bernoulli's principle that says when velocity drops, pressure drops also. So the aspirator has say, a 1" inlet for the water that tapers down to 1/2" and then tapers back up to 1" at the outlet. The vacuum hose connects to hole in the aspirator that is located lust downstream of the necked-down area (venturi). When the water flows through the aspirator it is restricted by the venturi and the pressure increases behind that point and the velocity through the venturi is raised. As the water comes through the venturi the volume of the aspirator is again greater so the water slows down and the pressure drops, inducing a vacuum in the air line. Hope this helps you to understand how it works - Rich vacuum pumps
visitor -
Thursday, May 20, 2010 - 10:43am
if it helps you can out more about vacuum pumps at the bcas website, it's short for British Compressed Air Association. Hope it helps. |
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ArtMetal
Investment and vacuum
bpfink / Actually they also sell real cheap used large pressure cookers as well as presurized metal paint containers that are about 3 gallon or so in size with good seals and clamp on tops and safety release outlets. I've got both and used both for just that. One little change though. I have a real electric vacuum pump and tied it to an old large oxygen tank as a holding unit to be able to vacuum down before throwing a line to it and sucking it out. It worked fairly well but had to let the plaster set up or still found some bubbles reform that never raised to the surface... or worse that raised up to a holding level still in the mold and against a caught section or pocket of wax.
Then I tried a variation and instead of using a vacuum, I used the pressure cooker with its safety release pressure valve set at just 25 lb. max. and attached an air compressor line and shut off valve to it instead.
It had a controlled feed into it so the release valve would always be able to pop off more than the intake could give but from what I found I got better results not expanding the gas in the mold and wanting it to rise, but by compressing the gas left in and minimizing it's bubble size. Then also found that by compressing it, releasing it, and repeating 2 or 3 times before leaving it in the compressed state for a few hours until the plaster had set, I got the best results of all. It is easier to let an air compressor line maintain a pressure than it was for me to keep a vacuum pump running to maintain a vacuum. Do take care though since both systems are playing with a container that can be very dangerous if pushed to limits.