But you've brought up a couple of new questions...Why does
vacuum allow greater versatility in sprueing? How does sprueing
differ from that done for centrifugal casting? Are there other
differences? What are the limitations (if any) of vacuum casting?
Alice, allow me to disagree with David here. I personally find that
sprues are LESS critical with centrifuge casting. Heres the
differences as I see them:
With vacuum casting, you are not limited as to flask size. You can
do really big flasks, so long as your burnough kiln can hold them, and
your vac pump can pull a sufficient vacuum on them. Witn centrifuge
casting, your arm has a definate limit on the size of the flask.
with vacuum casting, the force on the metal driving it into the flask
is ONLY the weight of the column of metal, plus the relatively modest
14 psi atmospheric pressure bearing down on the sprue. That’s usually
only a couple psi or less, of pressure pushing the metal into the
mold, and in most cases, the plain old weight of the metal column
exceeds the atmospheric push. It’s a misconception that the vacuum
pulls the metal. It does not. It only removes the air whose pressure
and presence would otherwise oppose the flow of metal. Now, with a
larger flask, sprued as a tree, the weight of the column of metal
will be quite substantial, and exert a considrable amount of pressure.
with a smaller flask, it won’t.
By contrast, with a centrifuge, what you’re doing is to greatly
increase the effective weight of the column of metal. The force
driving the molten metal into the mold can be many, many times greater
with a centrifuge than with a vacuum casting setup. This has several
implications:
Centrifuge cast flasks fill faster, as metal is moving faster, and
they tend to fill more completely, more easily, for a given casting
temperature and sprue arrangement, with the result that substantially
lower flask temps can be used, and sprues can sometimes be somewhat
less substantial and still get a complete fill. This lower flask temp
means the metal will then solidify more quickly, giving you finer
grain size, and resulting in stronger, denser metal. So in theory, at
least with small flasks, you’ll get better castings with a centrifuge.
However, in practice, this sometimes isn’t always the case, since the
extra force on the metal, and stress on the flask, can work against
you. Investment is more prone to break down or fail when the metal is
slammed into the mold with a centrifuge, so you can get inclusions of
investment more easily, and mold breakdowns more easily with
centrifuge casting. Your investing technique must be better, and the
way you sprue the models must be a bit more careful to avoid thin
sections of investment that can easily break away. And your spruing
technique should also take into account that the metal will be moving
faster, so take care to avoid channeling the sprue into fragile
surfaces, right at sharp corners, or the like.
Personally, I’m set up to do both. With a small flask of say, one or
two rings, I’ll always cast it with the centrifuge. My results that
way are far superior and more reliable. but if I’m casing a tree with
a dozen or two rings, and pouring three or four ounces of metal, then
I’ll vacuum cast it. If I’ve got a large enough mass of metal, and a
tall enough sprue column, then I’ll get reliable good results (and
yes, virginia, it really is the height of the molten metal column
above the mold cavity that determines the pressure on the metal, NOT
the overall weight of the metal you’re pouring, not the width or mass
of the button (beyond it’s contribution to the height of the column),
or the diameter of the tree sprue. Think about that old measure of
barometric pressuRe: Inches of mercury…" Doesn’t rely on the
diameter of the column at all, just the height (and of course the
density of the metal itself), and the pressure indicated will be
accurate for any shape of column, despite changes in diameter or
directions it flows.)
Vacuum casting is easier to do, and generally safer. And there are
more places in the procedure where a mistake or inaccuracy in the
process won’t kill the casting. but when you can use a centrifuge for
a give flask or a vacuum setup, the centrifuge will generally give you
a better casting, so long as you’ve got your process controls down
right.
Peter Rowe