Hi Jill,
I have found that most problems with casting are the result of
poor thermal management during the casting process. As metal
cools from a liquid to a solid, it undergoes about a 5% volume
change. As this shrinkage occurs, metal must flow into the area
that is becoming solid. Large pits are usually the results of
flow blockage that traps liquid metal. Once a segment of the cast
has been isolated from liquid metal flow, it will continue to
shrink. Since new liquid metal cannot be supplied, the shrinkage
results in pit formation.
Think about what happens as ice freezes. Ice is one of those
unique materials that expands upon turning from liquid to solid.
We all know how much force can be generated as the ice freezes. I
have seen ice generate enough force to fracture quarter inch
steel piping. The same forces are at work when a casting cools,
and the metal changes its state from a liquid to a solid.
The Japanese railroad companies were the first to recognize the
importance of the cooling process during casting. Their
engineers modeled the thermal cool-down of steel railroad wheels.
The common practice at foundries then was to pour molten metal
from a sprue directly into a wheel. The top part of the wheel
would be the last place to cool and would result in steel that
was more porous. Porous metal is a bad thing for railroad wheels.
The more porous metal wears faster, resulting in a short wheel
life. (No one likes an oval railroad wheel). They cured this
problem by redesigning the sprue to provide a reservoir that
could supply fresh molten metal while acting as a heat
reservoir.
There’s currently considerable research within the engineering
community on how to numerically model the casting process.
Automobile companies must cast engine blocks, and airplane
companies must cast jet engine parts by the millions. The
numerical models include fluid-flow analysis, thermal transport
between the flask and the fluid, and complex material models that
predict how the metal will behave under phase change. These
models currently require some of the world’s largest computers
and require many hours of computer time. If you really want to
use one these models, it is also good to have one or two spare
rocket scientists around to help with the simulation.
By now, you are thinking “I don’t have the world’s largest
computer, or any spare rocket scientists.” You do, however, have
experience and intuition to rely on. Any time that I get a porous
casting, I always think, "what’s the last place to cool ?“
Usually, when you think about the casting in this light, the big
"DUH” will occur, and you will be able to correct your problem.
The next time that you get a poor, pitted cast, ask the question:
could these pits be in the last place to cool?
Remember back to those big men’s rings that looked great until
the final polish. Just as you were starting to get a good shine,
you began to see those pesky little spherical pits. They are
always just inside the surface, waiting for you to remove the
outer skin and expose them.
Try this experiment. Cast a simple sphere with the sprue in the
center of a flask. Saw the sphere in half. Polish the surface,
and then examine the surface under a microscope. You should see
small voids near the center of the sphere.
Out-gassing of the plaster cannot form the pits located just
under the surface. Instead, they are formed as the metal shrinks.
Tensile stresses in the liquid become great enough to cavatate
the liquid and form nice spherical bubbles that are trapped in
the freezing metal. Sometimes, metal is kind of mushy, and the
shrinkage process will be able to pull chunks of material from
the surface. That’s bad.
Here are some things to remember. The rate of cooling is
proportional to the surface area to volume ratio. A sphere will
have the least surface area for any given volume. A plate,
however, will have a very high surface area compared to its
volume. Inside corners will always be hot spots.
There are several things you can do to help control the cooling.
Layout the wax and sprue such that the important design is
nearer the edge of the cylinder. You can also use wax vents to
accelerate the cooling process. A piece of metal inserted in the
plaster (a thermal shunt) near the wax will also locally
accelerate the cooling process. Remember that the flask has a
natural thermal gradient with the center portion of the flask
being the hottest. Exploit this.
For some complex shapes, I form with wax a molten metal
reservoir using a shape that looks like a cherry. This spherical
cavity will be able to supply fresh metal and will sacrifice
itself by being the last place to cool.
Another technique that can be used to gain an understanding of
the solid freezing process is to use a wax injector with a clear
rubber mold. The wax that I use changes color and transparency as
it freezes. By looking at which parts of the wax freeze first, I
get in idea of how the molten metal will freeze.
Cooling isn’t everything. The alloy can also play a major role.
Most casting alloys contain an agent that will help with the flow
properties of the molten metal. Unfortunately for 0.95 silver, as
the metal cools, the copper can precipitate into small crystals
of pure copper surrounded by liquid silver. This mixture forms
what is known as a mushy zone. This mushy zone can choke the flow
in the casting process. Ingredients such as silica can reduce the
size in the mushy zone and improve the casting process. The one
drawback is that these ingredients make the metals hard to work.
Because these added ingredients can affect the annealing process,
you never want to use casting alloy for rolling.
If you really want to minimize the risks for your castings, try
pure metal. Pure silver or gold will have a much narrower mushy
zone.
For me, the proper placement and attachment of sprues, combined
with the placement of the wax within the flask, are the most
important factors in achieving a successful cast. Proper control
of the burnout process is also important to achieve a good cast.
A flask that is too hot or too cool will cause all kinds of
trouble for any casting. It’s got to be “juuuust” right.
Stephen and Nancy Attaway