d13f00l wrote:
I'm starting to get into metal casting, mostly aluminum for now. I've
tried a couple casting methods, but I'm still inconclusive as to which
I should use.
I've tried the lost foam method, It's rather annoying. It works well,
but I lose my form in the process. 
If the cast fails, I start over
at square 1. It's also no good for making duplicates of things.
I also today just tried plaster of paris. I formed a mold in a can I
lined with vegetable oil(to help the mold slide out when it's ready), 2
halves with pins in the top half to align the bottom half. I let it
dry for an hour or so, then tried to get it out. It slid out easily.
Too easily. I cracked the bottom 1/2 of the mold in two. I took my
form out of the mold, and tried to fix the bottom part of the mold by
pouring more plaster around the form filling the damaged part. I also
added pins into the undamaged part to hold the newly formed hack-fix.
I let it dry, it seemed to hold. I threw the mold in the oven at 500F
for a couple hours, I had made the plaster for the fix a little thin so
it could fill cracks, and that part was still rather damp. Oh well.
I threw the mold into my forge and let it cook at ~1800F for 10 minutes
or so. The entire mold was glowing red. Oops. Some fissures had
formed on both halves. I guess it wasn't a good idea to fire it. The
mold was very very dry, upon removing the bottom 1/2 the seam where I
had fixed it broke and it fell apart. Doh. I only bought a little
plaster, since I wanted to see how well it would work with casting
aluminum before I tried anything. I fired up my foundry and melted
some aluminum, and placed the top 1/2 of the mold(still fairly intact,
but fissured) onto a flat plane of sand. I put on some gloves and
goggles, and poured the aluminum into the sprue. I was expecting the
mold to shatter, hiss, or something. I've heard bad things about using
plaster of paris as a mold, since it retains water. Nothing happened,
the pour went perfectly. Unfortunatly, the mold wasnt divided at the
sprue, so I had to ruin the rest of the mold to get the cast out. It
came out very nice, smooth edges and no bubbles. I learned a few
things today, what to do and what not to do, but I'm still undecided as
to weather its a good idea to use plaster of paris as a mold. Does
anyone have any idea about the following questions?
1) If I divide the mold at the sprue, will the aluminum come out
cleanly so the mold can be reused? I'm leaning towards yes. The form
I used came out cleanly when making the original mold. Aluminum
shrinks when it cools, so if anything I'd imagine it coming out easier.
Generally speaking, shrinkage of the cast will make removal easier,
unless restricted by the mold. Plaster based molds are weaked by the
heat and cast removal, so are seldom reused.
2) How dry does the plaster have to be? This cast was fairly small,
maybe 3x4 inches. The pour did nothing to furthur damage the mold I
had made, despite it was already cracked from drying it in the forge.
I''m really hoping I can make a reusable mold.
What kind of plaster? Plaster without a refractory begins to weaken and
change when heated beyond 135 F. Adding a refractory such as silica,
will extend the range. There are commercially available plaster
compounds designed for this purpose, but are still considered usable for
only one cast. Compounds for aluminum are heated to 500 F, and for
bronze about 1100 F, as I recall.
I'm still wary of the cope and drag method. I don't really have access
to greensand readily, I could use used motor oil as a bonding agent for
the sand but I'd prefer not to breathe that in, plus, the method seems
a bit complicated, having to pack and smooth out the sand, as well as
pinning the cope and drag together. Also, when casts get too big, you
need additional supports in the cope.
It would be NICE if I could make plaster work right, it's very cheap
and hopefully reusable.
--
Billy Hiebert
HIEBERT SCULPTURE WORKS
Small Part Injection Molding
http://www.hieberts.com