On Sat, 1 Nov 2008 11:59:24 -0700 (PDT), Dar
wrote:

On Nov 1, 12:20*pm, "Ed Huntress" wrote:
"Dar" wrote in message

...
On Nov 1, 10:52 am, "Ed Huntress" wrote:





"Don Foreman" wrote in message

news

On Sat, 01 Nov 2008 11:07:44 -0500, Tim Wescott
wrote:

Do you know of any references on how to do it right? I've got a project
in mind (that probably needs to wait 10 years) that could be done quite
well with plaster cast aluminum, I think.

Google on investment casting. Jewellers do this routinely with high
melting point metals like gold, silver and even platinum.

The stuff to use is investment casting plaster.
http://www.gp.com/build/product.aspx?pid=5901

It can be found at jewellers' supply places. Once the mold is made,
it needs to be "burned out" or calcined at about 1200F long enough to
"burn out" the wax and also to drive off excess moisture.

With ally you'll probably need quite a tall sprue to get enough
pressure to get molten metal into the smaller internal features -- or,
use a centrifugal approach.

Just be aware that investment molds typically have much less of a venting
problem than cast plaster molds do, because they're a lot thinner. I
should
say "gas porosity" rather than venting, actually.

--
Ed Huntress- Hide quoted text -

- Show quoted text -
So, back to sand , green sand... *I've heard that a lot but never
seriously looked into it. Is
it mixed w/oil *, but with extra fine grain, or what?.

First, green sand is not mixed with oil. It's mixed with clay and water. It
is based on sharp, fine-grained sand, but your Googling should give you a
better idea than we could in a short message.

Regarding the oil, there are two types of molding sand that use oil. One is
the very popular Petrobond, which most users say is easier to use than green
sand. It's a combination of sand, motor oil and a processed clay that is
"organophilic," which means it absorbs oil rather than water.

Just to avoid confusion, this is NOT the same thing as the old-style
core-molding mixes that used linseed oil as a binder. Those materials had to
be baked before use, and the point of their formulation was to stand up
better to crushing forces that occur when a casting cools around a core.
There also are baked-sand molds as well as cores, but, today, they don't use
linseed. I'm not sure if they ever did.

I eschewed lost wax investment casting because I don't have the gear
anymore and don't
want to pay someone else for these molds, and plaster sounds like just
an extra bit too
much trouble . Oil sand I can do down and dirty *.

It's supposed to be really nice for hobby-scale operations.

googling green sand for now...

D- Hide quoted text -

- Show quoted text -

Interestingly, I found a green sand , Diamond Green Sand , and the
guy there said
I could mix a particular one, formulated for detail, with oil or
water . Now, to find
a place that will sell less than a truckload, lol !

D

http://www.budgetcastingsupply.com/
will sell you as little as 25 lb of Petrobond.