I am interested in buying a vacuum casting machine. The choices have
confused the heck out of me. I would like to be able to cast perforated
flasks but I’m not sure if I really need that high-tech of a machine.
Nothing high tech about it. Most of the supply companies carry small
combination investing/casting machines in the $900-$1050 price range
which come with a vacuum chamber and adapter plates for both
perforated and solid flasks. They even come with a 3 1/2" x 4"
perforated flask to get you going. The chambers will generally
accomodate up to 5"x7" perforated flasks.
The drawback to these machines, however, is the size of the investing
table (only support a 9" or 10" bell jar) which limits your ability
to do larger batches should your demand for higher production grow.
Also, the vacuum pumps supplied with these machines are generally
3CFM or 5CFM HVAC type pumps, the kid used for evacuating refrigerant
lines. This will also impede you in your ability to quickly draw a
high vacuum, and it is also advisable to have separate pumps for
investing and casting. Investing jiggle tables are quite simple and
inexpensive to build yourself.
For casting perforated and/or solid flasks, I recommend a Kerr-Cast
unit, which is simply a metal cabinet with the vacuum chamber,
adapter plates, crucible and handle, flask and rubber base, tongs, and
rubber gaskets. Attatch this to a VERY powerful high quality 2 stage,
direct drive vacuum pump ( Busch, Ulvac, etc.) with an inlet filter,
gauge and leak valve, and you are good to go.
The aforementioned HVAC type pumps are fine for investing. Go with a
5CFM 2 stage pump for quicker draw. The same pumps the jewelry supply
companies sell for $500+ can be purchased from McMaster-Carr for
$375.
There are drawbacks to perforated flasks. They do take up more room
in the oven because of the flange. I do not find this a good reason to
not use them however. I simply do more castings.
They are more expensive than solid flasks, but will far outlive them.
I have been using the same 50 perforated flasks for 5 years, and
expect to get almost as many more years of use from them.
For many years I used solid flasks, and although less expensive and
allowing greater oven capacity, I can find little else which is
advantageous about them. They are generally much thinner gauge steel
than perfs, and you will ultimately spend about the same to replace
them a third time as you would have spent on one set of perforated
flasks to begin with.
From the vacuum standpoint, you are drawing through a little hole in
the center of the casting platen with a solid flask, essentially
drawing your vacuum straight down through the center of the tree, AWAY
from your pieces. With perfs you draw down and OUT in all directions.
There are several “vacuum enhancer” products out there, such as wax
webs, flask liners etc. which claim to redirect the vacuum to the
outer wall of the flask as with a perforated flask. I have not found
this to be the case. They add to the cost of each flask and can slip
out of place during handling and investing and cause vacuum failures
or blowouts.
The last couple of years I used solid flasks I abandoned the use of
such “enhancers” and had no ill effects as a result. The fact that
these products claim to make a solid flask work as well as a
perforated flask should tell you that a perforated flask is better.
Whichever style of flasks you decide to go with, remember…once you
have a good seal and a full vacuum, the rest is ALL temperature. And
with good knowledge, you can make any tool work for you.
Mark Moretti
Fredericksburg, VA