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