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Aspirators & LPG tanks for vacuum casting


#1

Hello, ive been looking around for a vacuum pump for lost wax
casting, but as i’m located in asia with no long term residence at
the moment, & a beginner, i’d like to look into other vacuum options,
hopefully that i wont mind leaving behind when i move.

As far as simple aspirators go, do you think that a water fed one
could bring a medium sized lpg, bbq type tank down to 29inhg, & the
tank could then be used as a supply for vacuuming investment & the
casting? Or would i be better doing it, feeding the aspirator with
compressed air & a tyre inflator, with gauges in the top of the
tank? Would a very small inexpensive tyre compressor be able to do
this, or do you need a high volume of air?

Ive already got the metal tubing for the debubble chamber which is
11in x 6in diameter.

Thanks for any help in advance.

Joel.


#2

Hi Joel,

the aspirator will work, but not fast enough. I think the way to go
is to cast with the type of casting machine that is like a bicycle
pump in reverse, you pull the handle to give you vacuum at the last
second. To vacuum, cut down on the spare space around your flask,
make a tank that the flask only just fits into, it is the spare air
you are sucking out that takes the time. Or steam cast, Keith Edwards
Lost wax casting of Jewellery is good and basic.

regards Tim B.


#3

Hello Joel,

Here is some advice and a caution.

If what you need is a tank of emptiness, then you might consider
doing what i did when I needed the same for a vacuum-forming machine.
I’ve never done vacuum casting, so I may be unaware of the special
requirements of that particular art, but then I never knew anything
about vacuum pumps either until I needed one - so what to do? I had
long since exhausted (no pun intended) my department’s equipment
budget so I couldn’t go out and buy something as exotic as a vacuum
pump. What i did was convert an old air compressor we had lying
around.

I detached the compressor’s output pipe from the storage tank and let
the compressor discharge to the atmosphere instead of pushing air
into the tank. I took the compressor’s intake pipe which normally
sucked from the ambient atmosphere and attached it to the storage
tank. It took me a day or two trying to think up any unanticipated
problems; would the compressor destroy itself? Would the tank crumple
into scrap metal? Would we all be sucked into the seventh dimension?
I couldn’t see why it wouldn’t work so I finally screwed up my
courage. It worked like a charm. I attached a pipe with a valve to
the tank’s outlet and ran it to where I needed the vacuum. When I
needed suction, I opened the valve. Bingo! I believe the contraption
still works after all these years.

I have a question for you - Most ordinary compressors have an
integral storage tank so why are you are considering using an LPG
tank for holding your vacuum? This worries me. My concern with that
is that if there is any propane (or whatever) gas left in the tank
when you first start to evacuate it, then you might be creating a
danger of fire or explosion by sucking a flammable vapour through a
compressor not designed for that purpose. The compressor’s diesel-
like action might be enough to ignite it or there is the possibility
of sparks caused by friction or static electricity. Admittedly these
disasters are probable relatively unlikely, but it’d only take once
to convert your compressor to shrapnel. I’d be more comfortable using
the tank which comes with the compressor or, if you need to supply a
tank of your own, use one which has never held any flammable gas or
liquids.

Finally - My vacuum-forming process was one which could be played
pretty much by ear. But it may be critical for your process that you
need a specific degree of vacuum. In that case you will need to buy a
simple vacuum gauge which you can insert in the system - between the
tank and the valve - so you can know how many psi or inches of vacuum
you have created in the tank. My sense of your query is that you will
not be doing this on an industrial level and that your budget is
limited. You can simply watch the gauge and switch off the compressor
when the desired vacuum is indicated on the gauge. If your needs are
more extensive then I’m sure you can get a vacuum gauge much like the
pressure gauges which come with compressors - one which includes a
regulator or limit switch to automatically turn the machine on and
off at pre-determined settings.

Good luck with this and all your aspirations!

Marty in Victoria BC, a place which does not suck, but it does blow
quite a bit, especially down by the water.