There are several reasons I use perforated flasks. First of all some of the
flask temperatures I use (for thin bronze pieces) is 1200 degrees F. There
is a high temperature gasket available that works for flasks above 1100
degrees F (it also works for cooler flasks too). I have used the same one
for around 5 years (it is still in perfect condition) and have absolutely
no complaints. Second, with perforated flasks I know my castings will not
fail because of improper sealing and lack of draw through the cavities.
It sounds to me that most problems associated with vacuum casting is from
the use of less than desirable equipment and processes. The important
factors are, sufficient vacuum (i.e. a large pump helps), and proper
sealing of the flask to the chamber (high temperature gasket and perforated
flasks for optimal results). I know that perforated flasks are expensive,
but they work very well.
Many times I have left flasks fully invested for a couple of weeks with no
problems. As I have stated before I steam dewax all of my flasks before
burnout. The steaming process fully rehydrates the plaster again. During
the initial burnout I allow the flasks to soak for at least 2 hours at
precisely 200 degrees F. (This is where a computerized kiln controller
helps). This allows the moisture to be driven out without boiling (212
degrees F), which could cause pieces of plaster to blow out of the mold.
For steam dewaxing I use a hot plate and one of those enameled canning pans
with the wire rack in the bottom.
Some of the best advice I have received has come from companies that want
to sell me something. In the casting business this has come
from the refiner whose interest is selling me more casting grain. I will
not buy more casting grain if my castings are no good. Therefore it is to
the advantage of the superior refiner to help casters attain good results
so they will buy more casting grain. Please note I said refiner, not a
reseller. Selling equipment and using equipment are two completely
different things. However, many people who work in the jewelry supply
industry have worked for years previously in jewelry manufacturing and can
offer some very good advice.
I agree with Winston Harness about building sprue trees. I do it just like
you see it in the books. It works, and I get twice as many pieces cast at
one time compared to other spruing methods.
I also use an electric mixer to mix the investment. This assures proper