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Old April 8th 08, 05:40 PM posted to rec.crafts.jewelry
Heinrich Butschal[_3_]
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Default Investment Castings problem -- surface defects caused by ...?

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Subject: Investment Castings problem -- surface defects caused by ...?
From: Heinrich Butschal
Date: Tue, 08 Apr 2008 15:14:46 +0200

Paul Wilson schrieb:
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


Hi Paul,

the surface is at some regions perfect. At the most regions I see the surface
of crystallised Silvermetal. So this effect comes if the fluid silver is
poured into the form and the heat is enough to stay some short time fluid.
When crystallisation is starting, the volume of the metal gets smaller and the
fluid metal shrinks from the surface away.
So this effect has really nothing to do with investing technique, this is a
problem of crystallisation short after the casting process.

Especially with thick moulds this problem is growing. The caster can reduce
this problem if the investment is lower in temperature and the casting
temperature of the silver is near to liquidus temperature.

Such castings are only possible if the caster use a specialized flask with
such optimised temperatures. This is a question of trial and error with
several of such coins ans also a question of costs.

Most of the industrial casters are not used to do that, for their customers
won´t pay the higher prices.

So You must cast Yourself or optimize the mould for the regular casting
process and regular casting temperatures. In that case this means, You have to
reduce the thickness of Your coin.

Mit freundlichem Gruß,
Heinrich Butschal
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