Maybe that is because you don't need a button. A button is a
total waste of metal, take a look at the physics. It does not
increase pressure on the casting and by the time the button is
filling the casting is already solidified so it does not provide
shrink filling ability.
How long does it actually take for silver to solidify after pouring?
(vacuum casting, electromelt for melting) I’m mostly self-taught,
having learned only a little on how to use the equipment from the
woman from whom it was bought. She told us to run the vacuum until
the visible metal in the casting, button or tree, begins to darken,
to make sure the metal is solidified. I suspect it solidifies much
faster than that, but of course I can’t see it to tell. What’s the
actual time frame for the metal to freeze after pouring?
What's the actual time frame for the metal to freeze after
pouring?
It depends on the volume of metal, as well as the temperature of the
mold and the metal when poured. But a reasonable guess would
generally be in the area of 5 to 10 seconds, max. Usually less.
When the visible sprue button has solidified, the metal inside the
mold generally will have done so too. And the vacuum can be turned
off sooner than you being absolutely sure the metal has solidified.
it’s not like the metal will run back out of the mold cavity if the
vacuum is turned off. The vacuum doesn’t hold the metal, or even
initially pull the metal, into the mold. It only removes the air
that would otherwise impede the metal’s flow into the mold. It’s
mostly just gravity that makes the metal flow into the mold, and that
hasn’t gone away just because the vacuum has been turned off. Note
that you DO want to let the metal and flask cool to some degree
before you break it out or quench it, since doing that when the
metal is still too hot can cause cracking of the metal.
I normally remove the flask from the vacuum pump immediately after
pouring the metal. The button is usually still molten when I remove
the flask from the vacuum table. I let the flask cool for between 6
to 15 minutes depending on the amount of metal poured.
On the first large castings I did I quenched too soon and had jagged
holes in the casting. I assume the holes were caused by metal that
had not solidified and blew up when quenched. I no longer have that
problem when I let the large castings cool for 15 minutes.
Peter is correct. The vacuum pulls air out of the mold. It does
not pull the metal into the mold. Gravity pulls the metal into the
mold. I cast lar ge pieces of pottery. I try to sprue the items so
that gravity pulls the metal down against the design in the mold
rather that have the metal forced upward into the design.
Button button who has the button. I have seen small buttons shrink
into the flask leaving a concave button with a hole in it. I believe
that indica tes that the metal in the sprue button may still be pulled
into the mold before it freezes. I would never pour a casting with
having a substantial sprue button.
As I have always said the casting art is not consistent between
casters. What works for one may not work for others. Generalizations
about casting without getting down into the nitty gritty of each cast
may not give the best results on all castings.
What's the actual time frame for the metal to freeze after
pouring?
It depends on the volume of metal, as well as the temperature
of the mold and the metal when poured. But a reasonable guess
would generally be in the area of 5 to 10 seconds, max. Usually
less.
It is more dependant on the cross section of the metal at a
particular point in the casting than the total mass of metal cast.
The Metal freezes at varying points in the casting at varying times.
While the button may remain molten up to tens of seconds after
casting, the models may solidify in milliseconds it all depends on
their cross section, temperature of the investment, temperature of
the metal.
Jim Binnion
James Binnion Metal Arts
Phone (360) 756-6550
Toll Free (877) 408 7287
Fax (360) 756-2160
@James_Binnion
Member of the Better Business Bureau
The vacuum pulls air out of the mold. It does not pull the metal
into the mold. Gravity pulls the metal into the mold.
I’m having a lot of trouble understanding this. Are you telling me
that if I had a mold where the air could get out another way, I could
just pour the metal in and it would fill the mold due to
gravitational forces alone? I don’t think so… doesn’t metal have a
high surface tension and tend to stay in a ball? Isn’t that why the
vacuum or the centrifuge or sling work? I know that gravity casting
works with cuttlebone or charcoal, but usually not with the detail
you can get with vacuum or centrifugal casting and the sprue area has
to be so large. What am I missing?
Think Sand Casting and foundry work. The metal is poured into
cavities and the air vacates by means of risers, vents. My hobby is
antique steam powered machinery. I am always awed by what was cast in
a sand pit and the metal poured down the holes. Think of casting
flywheels 40 feet in diameter, Locomotive frames 50 or more feet
long, all done in one piece and in one pour with only gravity to
assist and vents to get the air out of the way. The ship I served on
(USS John F. Kennedy CVA-67) had bronze propellers 25 feet in
diameter, sand cast. Much of the casting done around the world is
done by gravity alone.
There is a book “Contemporary Southeast Asian Arts and Crafts” 1977,
Thelma R. Newman. The section on metalworking is an eye opener for
what you can do with how little in tooling. Why there is not a
pyrometer or vacuum pump in sight and the air feed for the furnace
are pumps made of bamboo.
What I may be trying to say here is that we need to be careful that
our technology does not blind us. I do need to say that I want to
know the “why it does what it does”. “Just because” is never good
enough even though that may be the only answer we can find at the
moment.