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Investment Castings problem -- surface defects caused by ...?



 
 
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  #1  
Old April 8th 08, 03:17 AM posted to rec.crafts.jewelry
Paul WIlson
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Posts: 8
Default Investment Castings problem -- surface defects caused by ...?

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
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  #2  
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
  #3  
Old April 8th 08, 05:40 PM posted to rec.crafts.jewelry
Heinrich Butschal[_3_]
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Posts: 21
Default Investment Castings problem -- surface defects caused by ...?

To:
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
--
Schmuck Gutachter und Schmuckverkauf http://www.butschal.de
Schmuck nach Maß anfertigen http://www.meister-atelier.de
Firmengeschenke und Ehrennadeln http://www.goldschmiede-meister.com
Schmuckmanufaktur http://www.schmuckfabrik.de
Schmuck gut verkaufen und günstig kaufen http://www.schmuck-boerse.com
  #4  
Old April 10th 08, 02:26 AM posted to rec.crafts.jewelry
Paul WIlson
external usenet poster
 
Posts: 8
Default Investment Castings problem -- surface defects caused by ...?

To:
Subject: Investment Castings problem -- surface defects caused by ...?
From: Paul Wilson
Date: Wed, 9 Apr 2008 16:47:37 -0700 (PDT)

On Apr 7, 9:43*pm, "Peter W.. Rowe,"
wrote:
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


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?
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).

Most of my waxes are too thin to cast, so this project presents other
issues it seems!
  #5  
Old April 10th 08, 02:49 AM posted to rec.crafts.jewelry
Peter W.. Rowe,
external usenet poster
 
Posts: 355
Default Investment Castings problem -- surface defects caused by ...?

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
  #6  
Old April 18th 08, 04:38 PM posted to rec.crafts.jewelry
Séimí mac Liam
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Posts: 48
Default Investment Castings problem -- surface defects caused by ...?

Paul Wilson wrote in
:

www.eliasbing.com/surface_defects.html


Why aren't they being die struck, if I may ask?

--
Saint Séimí mac Liam
Carriagemaker to the court of Queen Maeve
Prophet of The Great Tagger
Canonized December '99

  #7  
Old April 20th 08, 07:01 PM posted to rec.crafts.jewelry
Heinrich Butschal[_3_]
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Posts: 21
Default Investment Castings problem -- surface defects caused by ...?

Séimí mac Liam schrieb:
Paul Wilson wrote in
:

www.eliasbing.com/surface_defects.html


Why aren't they being die struck, if I may ask?

Very often this won´t help.
More important is the thickness of the casting sprue and the mold/metal
temperatures.
Here you find a ppt presentation. At slide no 7, there is a picture of a
casted silver coin underneath a casted part of a watchband-clip. The surface
is completely even nearly like the original coin.
http://butschal.de/download/Karlsruh...Bu tschal.ppt


Mit freundlichem Gruß,
Heinrich Butschal
--
Schmuck Gutachter und Schmuckverkauf http://www.butschal.de
Schmuck nach Maß anfertigen http://www.meister-atelier.de
Firmengeschenke und Ehrennadeln http://www.goldschmiede-meister.com
Schmuckmanufaktur http://www.schmuckfabrik.de
Schmuck gut verkaufen und günstig kaufen http://www.schmuck-boerse.com
 




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