Pressure Caster (again)

In my experiments (before I bought an air conditioning service 5 CFM
vacuum pump) the trick was to keep the metal fluid and hot as long as
possible. It can cool very quickly, especially if compressed air is
blowing over it, as the pressure bulkhead is engaged over the flask.
Doing this quickly is crucial=21. I tried this years ago, and quickly
adopted a cheap venturi vacuum converter (compressed air in, vacuum
out for =2413.99 from Harbor Freight). It worked better as this rig
could apply vacuum slightly before poaring rather than applying
pressure while the metal cools.


Alex=40nospambobotek.net

On Wed, 27 Aug 2003 04:38:24 GMT, mike72903=40aol.comnojunk (Mike72903)
wrote:

I posted here a couple of weeks ago looking for information to build an
inexpensive pressure caster. Well the caster got designed and built but=
I have
experienced considerable difficulty getting proper castings. I've tried=
many
combinations of sprue size (16 guage and larger), air pressures (1 to 15=
psi)
air pressure rise (start low and increase) and investment configuration =
where
the metal is melted (not sure what that's called). Nothing worked at al=
l. The
best I could get was a partial casting, generally just enough to fill th=
e sprue
and sometimes a little or the pattern. The pattern was always the same,=
two
circles cut from =BD inch hollow tubes with the two circles melted toget=
her to
form a figure '8' and requiring about 5 grams of sterling plus some for =
the
button. The sprue was typically 3/8 to =BD inch long. One common thing =
about all
the attempts was the time from the kiln (1250 degrees F.) to the casting=

attempt. This time was as short as possible, it being my assumption tha=
t a
hotter investment would facilitate flow of the metal into the cavity. F=
inally,
in desperation, I attempted a casting allowing the small (about 1.5 inch=
by 3
inch) burned out flask several minutes to cool. Not sure what the temper=
ature
was but it was just to the point where a wooden match applied to the met=
al
flask would not ignite from the heat. To my surprise the attempt was
successful. So, was this just luck or is it correct to allow a burned o=
ut
flask time to cool? I have read and studied Tim McCreight's =22Practic=
al
Casting=22 as well as other books about casting and find little specific=
info
about this. I have no other casting experience or any kind to draw on I=
f
anyone who has done pressure or steam casting has any advice I would cer=
tainly
appreciate it. Regards to all, Mike in Arkansas.

Mike72903 wrote:
I posted here a couple of weeks ago looking for information to build
an inexpensive pressure caster. Well the caster got designed and
built but I have experienced considerable difficulty getting proper
castings. I've tried many combinations of sprue size (16 guage and
larger), air pressures (1 to 15 psi) air pressure rise (start low and
increase) and investment configuration where the metal is melted (not
sure what that's called). Nothing worked at all. The best I could
get was a partial casting, generally just enough to fill the sprue
and sometimes a little or the pattern.

There are some possible problems:
1. The burning cycle could be too short too cold. So sometimes the smut
which comes from burning out wax is not totally removed. So it closes
the porosity of investment and the ais can´t get out. Aditionally the smut
get partially changed to CO2 and H2O by the heat of the metal. So this
gas tries to press the metal out of the form.
If this is so You will find the surface of Your castings partially black and
also the investment partially black and strong smelling, when opening
under water.
2. The metal could be too cold, only melted on the surface and inside
like porridge. It takes some time and heat until the metal is compleatly
melt.
3. The way of air pressure is not steered from center gauge, to the end of
form. Sometimes pressure passes the region between flask and investment
faster and so they build a counter pressure to the center.
Put some wax sprues in the backside of the flask wich help to steer
the presssure.


Good luck
Heinrich
--
Heinrich Butschal
Gutachten, Projektmanagement
www.butschal.de