On Tue, 13 Jan 2009 08:42:55 -0800 (PST), "
wrote:

On Jan 13, 1:00*am, Stealth Pilot
wrote:
On Mon, 12 Jan 2009 22:38:54 -0600, "Maxwell" #$$9#@%%%.^^^ wrote:

when the aluminium was poured into the mold a guyser of molten
aluminium ejected to the ceiling of the workshop. *this seemed to
continue for some time. an examination of the mould later showed it to
be totally empty. I dont know how dry you need to get it but I gave it
a fair go at getting a bone dry mould.

Stealth Pilot

Your experience pretty much matches mine. I was able to get dry
enough plaster to make castings without porosity, but it took WAY to
long to dry to that point. My first pour too resulted in a steam
geyser and I would have sworn I baked that thing long enough. I was
also having fill problems. One plaster cast per go made the
experimentation too time consuming. I gave up and went back to sand.
I was trying to make fined valve covers for my1/2VW and was having
trouble getting molds in sand to stay together while puling the
pattern. At that time I didn't have a muller and could not get oil
based sand to work at all - thus the try at plaster. I finally
managed to modify my sand formula and got satisfactory results. A
touch of sugar did the trick.

The fins on the valve covers were enough of a challenge that I won't
be trying to do any head casting in sand, oil or bentonite based.
IMHO buying/building a muller is worth the time and trouble as the oil
based sand is good stuff. The humidity in my part of the world is
such that an uncovered covered heap of sand will dry out too much in a
matter of hours. Oil doesn't and for that reason alone I made the
switch.

Just in case you don't have the formula, even though I'm sure you
do...........................

http://www.foundry.ray-vin.com/k-bond/k-bond.htm
http://users.hal-pc.org/~lwhill/sandsystems.html

=======================
Leon McAtee

priceless references.
I've never tried oilsand just a seemingly continuous tweaking of my
bentonite based mix getting it better and better.

maxwells investment reference gives little actual detail but they must
do some cunning stuff with the additives that they allude to in their
plaster. they've certainly developed the process way beyond what I've
achieved.
Stealth Pilot