To: atm{at}shore.net
From: Dennis Rech 
Reply-To: Dennis Rech 


At 07:23 PM 12/22/02 -0800, Richard Schwartz wrote:
>Have you actually made molds for aluminum from dental stone?   Will it take
>the heat?  Will it reject the electrons the aluminum will try to push onto
>it in an effort to becom aluminum oxide?
>
>Hmm...  melting aluminum?  I guess I could do that in my kiln...   Any
>special tricks to melting down cans for this purpose?  Every time I tried to
>melt a can, I got aluminum oxide, not metal.

Hi Richard,
I cannot tell a lie (not exactly true), I have never used Dental Stone to
make an aluminum mold. I have, however, used many different types of
plaster to make molds. Usually I add about 50% white silica sand to it just
because I once read that it was the right thing to do. Any sort of
refractory material should do for aluminum molding. The secret is to let it
cure at room temperature, then put it into a kiln and add heat until the
mold is the temperature of the metal that you are going to pour into it.
Any sort of plaster contains a lot of chemically held water and as soon as
you raise its temperature 1000 degrees, its going to release a lot of
steam. If the molten stuff doesn't blow out the spew, it will likely
contain all sorts of voids from the expanding gases. there is a goodly
chance the mold will also split open and the contents will run down into
your shoes.
If you properly heat the mold, there is no water to be released and things
come out pretty good.

Really, if you are planning to do lost wax casting you should just buy a
proper refractory slurry such as Satin Cast and skip brewing your own.

There's way too much surface area on a can for it to melt properly. Ingots
are the way to go. Also, your kiln probably heats the aluminum too slowly,
giving it lots of time to oxidize. You have discovered an excellent way to
make dross though.

Thick chunks of casting alloy heated quickly gives the best pours.

Using plaster for a mold probably makes less sense than sand casting. My
personal favorite method is to use Petrobond and a standard flask and
pattern setup. Just like all the books show. Petrobond is a commercial
product that is a very find sand mixed with whatever ingredients that it
take to do it right. It has the consistency of cookie dough and beats
trying to make your own crude mixtures. If you are doing you own first
pours there will be lots going slightly astray and you might as well
eliminate the sand variable. Petrobond cost about $50 for a 75 pound bag in
Seattle. You will probably need two bags and it will last many dozens of
pours.

I buy 33 pound ingots of casting alloy at a dollar a pound. Usually type
A356. It pours wonderfully well, but will be a little soft for fine
machining unless it is heat treated. It will work fine as a grinding or
polishing lap as cast. You will probably burn 3 gallons of propane for 20
minutes to melt 33 pounds of aluminum.  I get two 16 inch polishing tools
out of an ingot and a half.

IF you are already set up for casting, there is a  significant savings to
casting vs. machining from plate stock. One inch thick 6061-T6 aluminum
plate cost $3 a pound here. It took a 100 pound piece to make two 18 inch
polishing tools this week. About 60% of that was scrap made by turning
squares to rounds.  $50 for casting vs $300 for machining.
The labor is about a push.
Molds are made out of wood in the same lathed method that we machine.

Commercial foundries charge about $3 a pound for castings if you provide the pattern.
Its really the way to go.
I still think tile tools are a great deal. Is there an Optics Workshop this
year? If so, hope to see you there again. Dennis

--- BBBS/NT v4.00 MP