On Wed, 09 Apr 2008 18:26:01 -0700, in rec.crafts.jewelry Paul Wilson
wrote:


Thanks for your insightful comments. I too was drawn to the dendritic
structure on the metal surface. I thought it held a clue, but what?

Another way to think about it is this: The metal will always crystalize as if
solidifies, but the only way you can see the physical shape of that crystal
structure in relief on the casting surface, is if the remaining liquid, as the
crystals form, is drained or pulled back away from the crystals by and the
surface by the shrinkage of the cooling metal. What you need to be happening
is for the surface layer of metal to be the first to solidify when it hits the
surface, and solidification to then proceed from the surface inwards, as well as
from the bottom up towards the sprue. If the metal that first hits the mold
surface freezes up before the insides do, then it will conform to the mold
surface without additional texture, and subsequent shrinkage would then be
either pulling more metal from the sprue feed, or once that's blocked, would
simply be creating voids / porosity in the interior of the mass of metal. If
you're almost where it needs to be, the surface can freeze, but shrinkage pulls
that formed surface back or out of shape before the solidified layer gets thick
enough to support itself. This would lead to those few areas where it seemed
small sections of otherwise properly conforming surface bits were displaced,
cracked, etc, or to situations where an entire cast surface may end up being
bowed in, even when the mold is not. As I said in the first post, the solution
is whatever will give you better progressive solidification from the end back
towards the sprue, and from the surface towards the interior, rather than the
whole mass solidifying all at once. Key to that is a cooler mold temperature,
so the surface layer chills more quickly. One other thing you might try along
with a lower burnout kiln temp (and thus mold temp) at casting time, is before
putting the mold in the machine, let it sit a minute or so, sprue hole down
against an insulating surface, and the back end up in the air, and thus cooling
in the air. You could accentuate the resulting difference in temperature
between the two ends even more by playing a torch flame directly on the sprue
hole area for a bit, warming up the investment in that area, before casting.
Again, the idea is to promote the metal at the end of the mold chilling faster
than the metal at the sprue and button area. This idea comes actually more
from a useful trick when casting larger flasks with a bunch of rings sprued to a
central thicker tree trunk style sprue. The flast is pulled from the oven a
hundred degrees or so higher than the actual recommended casting temp, but
allowed to simply sit for a few minutes, sprue hole down, before casting. The
steel flask and outer layer of the mold starts to chill, while the interior core
of the mold, where the sprue is, doesn't so much. So the rings extending from
that sprue are now in a temperature gradient, which substantially improves
progressive solidification, helping with both filling and porosity problems.
Your large coin is a little harder to do that with, since the symmetry of the
thing doesn't so nicely match the way a flask cools, but you get the idea, I'm
sure.

It turns out only 3 of the 9 medallions (all different) had bad
surface issues, so I think we're close. We do, in fact, use a
centrifugal process (an old Jelenko Thermotrol).

Nice old machines.


Most of my waxes are too thin to cast, so this project presents other
issues it seems!

It's difficult to get something too thin to cast that would still have been
servicable in metal. Especially if using a centrifuge. But sometimes it takes
playing with extensive sprue feeds, and surprisingly high flask temps. I've
done things which were literally filligree delicate, but casting with a flask
temp left right at the 1350 high point from burnout. Way too hot for most
things, but the items I'm thinking of, in 18K yellow gold, needed that, and came
out fine when cast at those temps. Many of those shapes were not much more than
a third of a millimeter in thickness, with even more delicate surface features
(edge textures, etc)

Think about it this way. If you used an investment material that could
withstand the temperatures at which your metal actually melts, and you cast your
metal into a mold that started out above the metal's melting point, the metal
would have all the time it wanted to fill the mold completely, and so long as
your details weren't so thin that surface tension itself kept the metal out (and
the force of the centrifuge could overcome that), everything would fill. Then
you'd have to simply cool the mold. Shrinkage might present problems, but with
really thin sections, since shrinkage is a percentage of the thickness, chances
are you'd not notice it much unless you mixed very thin sections with
substantially thicker ones...

cheers

Peter