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Casting defects - subsurface pits

Wise people;

I wonder if any of you have ever had castings that turned out like
this: my casts are mostly ok but I get fair sized subsurface pits
that appear when polishing. Is this a gaseous problem or the metal
freezing or investment getting into the metal? Any ideas especially
if you’ve had this problem before would be helpful

John F.

John

I am not sure about what is causing the problems you are having but
sounds sort of like incomplete burnout and water vapor possibly.
Make sure the flask is snowy white at the sprue opening before you
cast and when you look down into the sprue opening with the flask at
full 1350 degree heat the reddish white glow should go all the way
down the sprue. To me, they almost take on a transparency… Try
soaking at the 1350 degree temperature for an extra hour or flip the
flask up with the sprue opening to the top for about a half hour at
full heat (1350) Remember that moisture rises as steam so by
inverting the flask sprue opening up you let the final moisture
escape more readily. also try letting the 300-350 degree soak go on
for an extra 30-60 minutes… If it is moisture retention then this
should solve the problem. Wonder what size flask you are using? Also
scape the top of the flask level before beginning the burnout by
using a sharp knife.A glaze forms on the top of the flask during the
first part of the drying (you can sometimes see it) which is more
dense than the rest of the investment and tends to prevent the escape
of moisture through the top of the flask. If you scrape the glaze off
it facilitates the evaporation of the remaining water upward and out
which lessens stress on the walls of the pattern cavity due to steam
being forced to escape though them instead of through the top of the
flask…

Regards,
Roger

John,

You wrote, "fair sized subsurface pits that appear when polishing."
May to be a problem from over heating the metal. Could be included
gas. The shape of the pore will give away the cause - if the pore is
spherical with a more or less smooth surface, it’s gas. If the pore
has a jagged appearance and has all these little spears or dendrites
sticking into it, maybe there was not enough molten metal available
to crystallize into a dense volume. i.e., the metal froze before it
could feed that area where the pores occur. If the pore is block
shaped, perhaps some investment broke off and got trapped in the
casting.

Where are the pores in relation to the gate (sprue)? Are they evenly
distributed throughout the piece, just below the surface? Are they
clustered around the gate? Do you have pictures of the problem?

Chuck from Wiseacre Asheville. :slight_smile:

Subsurface pits can be caused by the metal/flask temperature
combination being too hot. A too hot combination will normally
produce round pits inside the metal.

Some time ago I read in a jewelry book that the sprue button
surface can tell you a lot about the temperature combination. If
the button top surface is concave the combination was too cold. If
the surface is convex the temperature combination was too hot.

When I first started casting I was told that my flask temperature of
850 degrees was too cold and that I should use a flask temperature
of at least 1000 degrees. I tried that and ended up with a sprue
button that blew up like a sponge. The casting has subsurface
porosity.

Broken off investment can become trapped under the surface of the
metal. If this happens the pits/voids will have a shape to them
and will usually be larger than porosity.

Incomplete burn out normally leaves carbon on the surface of the
mold which cause pits in the surface of the metal.

Based on your comment “my casts are mostly ok but I get fair sized
subsurface pits that appear when polishing.” I would suggest you
lower the metal or flask temperature. I vacuum cast with a mold
temperature of 860 degrees with a sterling melt temp of around 1830
degrees. Those temperatures work for me.

To anyone interested I have a very simple process that prevents
fire scale from forming when vacuum casting. I have produced a
paper with sketches and photographs on the subject which I would be
happy to share with anyone interested in eliminating fire scale on
sterling castings.

The process costs next to nothing and is very simple. A little bit
of wax is placed on a solder pad under the hot flask and on the
sprue button. Place a larger empty flask around the casting flask
and add a solder pad on top of the cover flask. The wax attempting
to burn around the casting flask inside the cover flask produced a
reducing atmosphere. No oxygen is left to combine with the copper in
the silver to form firescale.

This simple process eliminates fire scale on vacuum cast parts.
Sterling castings will come out of the quench water clean and
silver colored, not black.

Lee Epperson

Could be flux inclusion ,try using less flux or no flux at all .You
did not state which metal you are having problems with ,is it all of
them?

Regards
David Sheard
England

r

End of forwarded message

Hi John F., Vac or cetrifugal? Gold, sterling, bronze or other?
Induction, resistance or torch melt? Piece type, size and shape?
Flask temp? We need more info to help you.

John, J.A.Henkel Co., Inc., Moldmaking Casting Finishing, Producing
Solutions For Jewelry Artists

A lot can be learned from the size, shape and concentration of the
pits. Check the archives.

Andy cooperman

Hello John,

It could be bubbles in your wax which burst when you vacuum your
flask/investment. They burst open and fill with investment leaving
little holes, sometimes just below or on the surface. If in doubt, I
will not vacuum something I suspect might have a bubble. I will just
use a vibrating table instead and alot of de-bubblizer.

Hope this helps!
Mary Linford
Blue Star Wax Carving

John

Your problem with your castings is several things.

1: The flask temperature that I use is 1000 F. when It comes out of
the oven.

2: The temp of The Sterling is By color. When the ,molten sterling
gets a mirrow look it is ready to cast.

3: In spruing small objects such as rings or small pieces I use 1/8"
sprue with a 3/16" wax ball just above the button. This takes the
heat away from the part that is castand reduces the process of
purosity. If the metal is too hot when cast you will have purosity.
If the part you are casting weighs more than the button you will have
problems. I hope this will help solve some of your problems.

4: You will have to keep experminting with these systems and finall
you will get it.

Yours
Mark

John,

I usually use a 2 1/2 inch flask. Today I was using a 2 inch flask.
Normally, I can get up to a half dozen rings into such a flask.
Today I was casting links for a tennis bracelet. Don’t do much
sterling. any silver I use is restricted to models or prototypes.

As for porosity, I still get some, but I have found that what I do
get I can remove with a burnisher. Usually a rotary burnisher
running on the flexible shaft.

A couple of things that seem to be of major importance in preventing
porosity in the first place. Flask temperature: I usually try to cast
yellow gold at about 900 degrees F. White gold a little hotter.
Heavy pieces a little cooler. Lighter pieces a little hotter. Of
equal importance is spruing. Perhaps more so. I try to use as heavy a
sprue as I am willing to cut with a jewelers saw. Some people say
that it should be as heavy as the heaviest part of the casting.
Probably good advice, but I haven’t gone this far. One thing to
remember is that as the casting cools, it will contract. If there is
a heavy enough sprue, the sprue can continue to feed metal to the
casting. Another thing that I can’t prove, but I believe is so is
that turbulance will affect the quality of a casting. This means
anyplace where metal makes a particularly violent turn or is pinched
through a small sprue, there will be problems.

So in a nutshell, I look for temperature, cooling effects and
turbulence.

I’m sure a few other people can add a lot.

Campbell

All those other things being true also, you might try something which
works for gravity casting and regarding spruing. This is done for
pieces much larger than jewelry, but principle the same. Your main
sprue (gate) has feeders from it which will fill your piece, diameter
being at least equal to your thickest part, gate is greater thickness
than feeders. Gate too is extended straight down beyond the model.
Feeders come off from the gate in the middle of the gate length. If
your space is tight, you can form the gate shaped like a “J” so that
the gate wraps around your model within the flask and the feeders
would come off the J in the middle. Theory being that by dropping
your metal into gate first leaves much of dross there and
eliminating turbulence. Essentially metal “fills” the piece rather
than being forced into investment mold directly. This method has
gravity as impetus behind the metal rather than centrifugal force,
and used for casting bronze sculpture. But I have always used it for
spruing up my jewelry pieces with never a problem with porosity.

Hope that helps.
Susan

    Subsurface pits can be caused by the metal/flask temperature
combination  being too hot. A too hot combination will normally
produce round pits inside the metal. 

We have had similar problems casting 18K and our advice from the
metalurgist at United Precious Metals was that we were probably not
hot enough. He said that if you are used to 14K it is a fairly common
mistake. Flask and metal hotter for higher karats.

Stephen Walker

Lee

    To anyone interested I have a very simple process that 
prevents fire scale from forming when vacuum casting. I have 
produced a paper with sketches and photographs on the subject which
I would be happy to share with anyone interested in eliminating
fire scale on sterling  castings. 

Could we have more details on the process.

Thanks
Charles Friedman DDS
Caster of small functional Sculptures