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Old April 8th 08, 03:43 AM posted to rec.crafts.jewelry
Peter W.. Rowe,
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Posts: 355
Default Investment Castings problem -- surface defects caused by ...?

On Mon, 07 Apr 2008 19:17:11 -0700, in rec.crafts.jewelry Paul Wilson
wrote:

Folks,
I sent some very nice waxes off to be cast in sterling, but a few came
back with surface defects. My casting guy says it's because the cast
pieces were "too heavy." I think it was a problem with the investing
technique.

What is your diagnosis? I posted pictures at
www.eliasbing.com/surface_defects.html.


Thanks,
Paul


I'm not sure, but I think I see what appear to be two things going on. One
seems like surface displacements, fissures, or the like. That might be too wet
an investment, incorrect water temp or mixing time, or some other factor giving
rise to water marks or small cracks, etc. But I'm not sure I'm seeing that.
Those marks could be just more of my second observation...

The main thing it looks like, unless these were CAD milled or grown waxes and
that texture really was on the wax, is that the texture looks sort of like
dentritic crystal structures. Now, the metal always has that structure anyway,
to a degree. Crystaline, at any rate. But usually it doesn't manifest itself
at the surface as a texture unless the metal is cooled too slowly, allowing that
crystal growth, and it can suggest metal starvation too, from too small a sprue
structure. If these things are heavy medallions fed with just one edge sprue,
there's no way that would have been enough metal flow to properly fill the mold
quickly, and that could give you that. If that's the situation, then your
caster's statement that the items were too heavy is correct, but incomplete.
Complete would be "too heavy for the size sprues used". And if the item is that
heavy (and three and a half ounces is heavy), then it's also very possible the
mold was too hot, again allowing too slow a cooling / solidification time.
Something that heavy is almost an ingot in bulk, and the mold would have needed
to be much cooler than what would be appropriate for, say, the typical ring.
Depending on casting method, a ring might get cast with the flask at anywhere
from 700 to 1000F. A three ounce heavy medallion, though, I'd guess would be
better off cast around 400 to 600F, maybe considerably less depending on the
thickness.

There's another clue in the photos too. Note that the dendrite structure is
most visible on the head, where the medallion is thicker. The edges, with the
lettering, have a better surface. Some of those areas seem just fine. That
supports the thought that the center, being heavier, solidified last, and being
metal starved (because the rim silidified first, and now the sealed off center
section is solidifying, plus it's solidifying too slowly, means that as it
cools, metal shrinkage drains still molten metal from the surface towards the
inside, leaving the dendrite structure visible. Again, it's a combination of
mold temperature, weight of the item, and insufficient metal feed (spruing)

Things that will help:

if you can find a way to cast this via centrifuge, rather than vacuum assist
pour, the higher force on the metal will help a lot, plus allow you to use an
even cooler mold. But I'm guessing this is too large for most people's
centrifugal casting machines?

Barring that, a cooler mold, and much heftier and short sprues, with a bigger
button, so there's an increased chance the molten button will still be able to
feed metal to the core of the thing as the core solidifies.

If you can set up a temperature gradient, so the bottom of the mold is
significantly cooler than the middle, and the middle is significantly cooler
than the top edge or wherever sprues enter, and the sprues and button are
hottest yet, then you'll have a better chance of getting progressive
solidification, which is what you need to avoid the shrinkage and cooling
problems that I think you've encountered here.

Hope that helps.

Peter
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