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Mysteries of vacuum investment equipment


I’m searching for a vacuum pump to connect to a table for investing
flasks. Based on some reading it seems that the consensus is to use
those pumps that are normally used for clearing air conditioning
lines. These pumps usually have a handle on top and consist of an
electric motor connected in line to a square chamber. Rio Grande,
and others have examples.

The question is what properties are required to vacuum investment;
vacuum pumps seem to range in (cfm) cubic feet per minute of air
capacity; and the intake port sizes seem to vary. The “ultimate
vacuum” varies somewhat as well; with some measured in inches of
mercury and some in microns.

I didn’t want to buy a pump that is too small for the job or an
expensive pump that’s overkill for vacuuming flasks.

Vancouver Island


There have been recent threads about tweaking pump size, tubing and
fitting size etc. The pumps are measured in CFM, cubic feet per
minute, and evacuate considerable volume from air conditioning
systems. In comparison, the casting and investing volumes are quite
small. And even a small rated pump evacuates the small space in an
extremely short time. Even a small pump will evacuate the near 0
volume of a flask and hold that vacuum quite well. The same is true
for vacuuming investment or RTV where it takes longer because there a
larger volume that is still quite small compared to the A/C system
volumes. It is not rocket science.

In my view, the volume of an evacuated flask is measured in cubic
millimeters and is so small as to be almost negligible for a pump
rated in CFM. Time is critical in the vacuum drawn on the flask and
liquid metal but the time difference is very small as a result of the
pump vacuum rate in such a small evacuated volume. Time is not really
an issue with investment or RTV debubbling where there is a larger
but relatively small volume.

In my view, the quality of the meeting surfaces are more critical
than the time/pump size. That is, the pad/flask or the pad /chamber
edges must be clean and smooth and have no nicks or dings. The
rubber/silicone pads must be clean and scratch free. I started out
with a number 10 can for an investment vacuum chamber and it has done
quite well with a finely sanded edge and rubber pad. It actually
works better than the Plexiglas bell jar I used in a workshop metals
studio that had scratched and dinged edges . I just never got around
to buying a Plexiglas bell jar.

There is always a discussion of where to buy a pump. I bought a
large pump, 6 cfm, not because it was large but because it was cheap.
I went to an A/C supply house and bought it used for less than half
price and cheaper than a new smaller pump. It is mounted on the floor
in a dust proof box several feet away from the vacuum table surface.
When casting, It pegs the needle in an eyeblink even though it is not
in immediate proximity to the table. Another place to find used pumps
is a pawn shop that accepts tools and equipment. My suggestion is to
buy a cheap but available used pump and play with it. A used pump
from a reputable manufacturer could serve quite well. The usage in
casting is trivial compared to the heavy use in A/C maintenance

Ben A Harris


Basically, you are removing the bulk of the air from your investment
or water so you dont need a super low vacuum. The flow rate isnt
going to be an issue either with your set up, I use a glass or
plastic vacuum desiccator and stand the investment mix in it and pump
for a matter of a minute and have a simple pressure gauge that
measures pressure in inches of mercury. When you get to 29-30 that is
good enough. IF you are repumping television cathode ray tubes that
is another matter. Technique in debubbling is more important than
vacuum achieved. IF you use degassed water before you start you
shorten the time needed to degas the mix. This can be done quite
easily by using boiled water stored in a closed container. Careful
mixing of the investment, temperature control and very careful
pouring and gentle vibrating of the flask will get rid of more air
than the extra money spent on a go-faster pump. A 50litres/min pump
will be more than big enough. If you try too hard to get rid of every
bit of air you start to vapourise the water and that will condense in
the pump doing it no good and in the long term may cause considerable
harm. Make sure your pump has a ballast valve to purge the pump. An
ex-lab pump will probably cost you very little and are commonly seen
on ebay if you want to go a bit bigger but dont want to spend. Some
auto workshops have small pumps for carburettor setting, these will
do if economy is paramount as they can be had for very little money

Nick Royall

When you get to 29-30 that is good enough. 

The 29 or 30 on the gauge of most vacuum casting machines is just
reading the difference between chamber pressure and outside pressure.
In high altitude places where air pressure is less, a good vacuum
will read less on your gauge. For example, in Colorado Springs at
about 6800 feet altitude, a good vacuum will only show about 23 on
your gauge. At sea level, it would be 29-30.

The Jewelry Equipment Dr.