A Sandcasting Primer
I Keep my sand in a rolling box under the molding bench. It holds up to 500 pounds of sand, which is plenty for my current needs. Petrobond always works better when it's warm, so in cool weather I put a small electric heater in the box and put a piece of foam insulation over the top. Another way to do it is to have some heat lamps over your sand pile.
If you can't find a muller to use, you can buy Petrobond premixed, which would be perfectly fine for a small amount, but which might get a bit expensive if you want a lot of sand. There are other types of sand - some use water instead of oil, but I don't have much experience with them. One nice thing about Petrobond is that you don't have to re-mull it, once it has been made. One not so nice thing about Petrobond - it kind of stinks.
The Tools: The
process can be done with a bare minimum of equipment, most of which can
be made in the studio.
Flasks: A flask is a pair of interlocking frames that are used
to contain the mold. One side is traditionally called the cope and the
other the drag. In a proper foundry, flasks are generally made of metal,
but for the small scale operation, perfectly satisfactory flasks can be
made of wood. Yes, Wood! They get a little burnt here and there,
but they can actually last pretty long.
The
Furnace: A furnace is what makes the heat for melting metal. I constructed
this one from lightweight firebrick, cut to fit and banded into an octagon
with fiberfrax (a refractory fiber) gaskets to seal between the bricks
(not absolutely necessary but nice). The "flamethrower" part
of the furnace began with a salvaged electric leaf blower. Then I made
a mixer of steel pipe, with a replaceable pipe nipple for the business
end.
#4 crucible in the furnace with aluminum ready to pour
#10 crucible in furnace with a recycled leaf blower
Crucible Handling
Tools: When the crucible is glowing hot, with a load of molten metal
in it, one has to be able to pick it up out of the furnace and pour the
metal into the molds. A crucible gets a little soft at these temperatures,
so you can't just grab the edge of it with some big channel locks - you
might be left holding a piece of the crucible's rim with hot metal all
over the floor!
Much better to have some special tongs that go around the crucible and
hold it firmly from several contact points. Again, I made my own, but
you can buy them, too. The configuration varies a lot with the size of
the crucible. A small crucible like a #4 or #10 can easily be handled
by one person with some big plierlike tongs. But say a #40 crucible requires
large 2 person tongs to lift the crucible out of the furnace, and a seperate
pouring ring for filling molds.
The
Crucible: The crucible is what the metal is melted in. They are made
of graphite clay ,silicon carbide and some other materials. I haven't
tried making my own yet (I
know some people who make them out of steel),
so I buy them. If one is careful with them, they'll last a long time.
I blew one up! .
Parting Dust. Is used to keep the sand from sticking to the pattern and to keep the two sides of the mold from sticking together.
Rams, Slicks, Sandworking Tools: You can buy these. Or make them
all yourself, as I have done in this example.
Patterns: We can't really have a discussion of sandcasting without
talking about patterns. A pattern is what the "original" is
called. It can be made from anything rigid enough to withstand being rammed
in sand. Patterns can be made from any combination of wood, cardboard,
plastic, metal, masonite, bondo, plaster... Since a sand mold is not flexible,
the pattern has to be able to slip out without hanging in the mold. This
means that undercuts need to be eliminated, and the features of the pattern
need to be made with good draught angles. For art casting, we have a little
more lattitude, and it may be acceptible for some sand to stick in the
pattern (for example in crevices in a natural object).
Making a mold:
I'm going to start with the most simple situation for making a mold.
That would be a pattern with a flat back that can be laid on the surface
of the bench.
We can simply dust the pattern with parting dust, set one half of the
flask over the pattern, and proceed to ram the first side of the mold.
It should be noted
that a pattern that will slip out easily should be placed on a board so
that the first side of the mold can be flipped while keeping the pattern
in.
The
Next Level of Complexity would be a pattern which has a hollow back, but
which still lays flat against the table. A packed sand fill helps keep
the pattern from deforming during ramming.
An even more complicated
pattern has an irregular back which must be placed in a sand bed. In this
method, one side of the flask is packed with sand, and the pattern is
pressed in. Then the parting line is defined by packing and smoothing
the surface with a sand slick.
A sandcast project
The Mold ready to put together
The drag has been rammed and flipped, and the follow board has been removed, leaving the pattern in the drag. Note the plywood runner pattern placed alongside the louver and the smaller pattern on the uppet right. It's all dusted and ready put the top side of the mold (called the cope) on and ram the second side of the mold
Left: A louver casting. Note this one has a hand cut runner.
Right: Castings from the cored mold illustrated below.
...Except for clamping!!!
Always clamp, molten metal can exert enough pressure to lift a mold up and shoot out the side.
Coming up:
getting a surface shell from an object using a pair of matched flasks.
The gooey Bondo patternmaking technique.
Pouring metal Definitely
should be wearing safety glasses here, but then I have been known to cast
barefoot!