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High temperature casting instructions


#1

I’ve been having some casting failures lately, and need some
guidance. I’m casting very thin pieces - 1/2 mm with some spots even
slightly thinner. I’ve tried sterling with palladium, which didn’t
work too well. Most of

the pieces filled (not all), but the resulting pieces cracked. I’m
sure I over heated the metal - maybe had the furnace too hot too. I
also tried 18K yellow, and that worked a bit better, but not
satisfactory. They were rather rough and some of the sections didn’t
fill either. What flask temperature and what melting temp do I need
for such thin pieces? I know I can go back to my CAD files and make
them thicker or build in some sprue channels but I feel that I don’t
have to do that. Any advice will be very much appreciated.


#2

Believe it or not you may have better luck with a lower flask
temperature. When you heat investment above a certain temperature it
begins to break down and release sulfur dioxide gas which can
prevent thin parts from filling because the gas is generated faster
than it can pass through the investment. So by reducing the flask
temperature you can reduce the gas generation rate and can increase
the possibility of filling. The temperature that the investment
breaks down at varies depending on how much carbon is left in the
investment from burnout but it is well below the temperature of most
molten jewelry metals. The sulfur dioxide is also one of the sources
of gas porosity in castings.

James Binnion
James Binnion Metal Arts


#3

Hi Rose,

It seems to me that you are working at the limits of our casting
technology. My experience is still limited but for what it’s worth
this is what I have found anecdotal as it is.

Thin sections of casting are always difficult to fill. Measures to
deal with this include having a somewhat higher flask temperature
when casting. I usually cast at 450C but with something with thin
sections I take it a little higher - say 475-500 or when the kiln
reaches 450 cast the flask with thin sections because its core
temperature would be higher.

Adequate sprueing is particularly important for thin castings using
such means as supplementary sprues to vulnerable sections and trying
to ensure as smooth as possible flow path for the metal from the
button to the casting and to provide a resivoir of molten metal to
feed the shrinkage of thick sections… I would rather have to clean
up after a few more sprues than have to throw the whole casting into
the crucible.

I have generally found it is more difficult to cast thin items using
vacuum casting compared to centrifugal casting.

The issue might also be burnout. It is good to give flasks with thin
sections a longer soak time at 650-700C to ensure sufficient oxygen
reaches thin sections to achieve complete wax combustion.

The metal also makes a difference. I have found less problems
casting thin items in gold, even 9K, than.925 silver. I think it is a
combination of the higher density and greater fluidity of molten gold
compared with silver. This seems to be consistent with your
experience as well. In making this statement I must point out that I
have not cast a lot of gold for others and all in a centrifugal
caster and all of my own castings are in silver and occasionally
bronze using a vacuum.

Finally I have found in my own work that sometimes I just have to
make sections of my patterns a little thicker to get reliable
castings. In silver I normally stick to a 1mm minimum thickness
except around the perimeter. I expect I could go thinner in gold.

All the best
Jen


#4

I am casting sterling silver filigree. Most of my pieces have aprox
a 1 mm framework either around them or running through them
somewhere, and filigree wires which are generallly 1x 0.5 mm, at
least in the originals. I do the standard burnout, but on the cooling
side, I stop at 1150 deg F for casting. I am using vacuum casting,
however. I used to do centrifugal, and that did not require as much
upward temperature adjustment. I have an electromelt, and usually
have the temperature of my sterling at between 1900 and 1920 for
filigree. If you do heavier work at this temperature, it does create
porosity, but the filigree type work has to be hot enough to fill as
it cools so quickly in such small dimensions.

Other things to consider:

Your design…you always have to think of the flow of your
silver…can it get from the main sprue to the end of your design
easily or does it have to turn back and go against the flow. just
remember the old expression :go with the flow…when it comes to the
design…

is your tree set up correctly? check the angle on your pieces you
are casting to take advantage of either the vacuum or centrifugal
forces at work.

If you are vacuum casting, is there too much plaster between the
ends of your pieces and the vacuum? you need just enough plaster
between the ends of your pieces in the mold, and the vacuum that the
mold will not blow out, but not so much that it impedes the vacuum.

If you are using vacuum, I strongly recommend using Wax web liners
for the flasks. This allows your vacuum to reach up into the sides of
the flask, allowing stronger pull on the pieces lower in the trees.

Again with vacuum, leave an air space when you invest. I think I got
this from Tim McCreight’s book, Practical Casting, but it said if
you leave a space of up to about 1/4 inch on your flask, it allows
for a more even pull than if your top it off, and then the pull is
only on the center of the flask.

Size does matter! I do much better with my 3x5 inch flasks than my
6x4 inch flasks with my vacuum casting of filigree. It takes longer
for me to get the flask to ‘grip’ to the pad and every second counts
when you have to cast extra hot! In addition, the bigger the flask,
the more plaster…and you have to keep that distance from the tips of
your pieces in mind.

If this helps, I photographed some of the wax trees and finished
castings a while back so you can see how they are set up
http://www.ganoksin.com/gnkurl/ep7z6d

I didn’t photograph the flasks, but you usually don’t want more than
half an inch- an inch between the top of your tree and the end of the
flask.

Hope that helps
Jeanne
Jeannius.com


#5

You didn’t give us much to go on Rose, for instance are you using
centrifugal or vacuum? What flask temp are you using? How long are
these pieces and how much of them is made up of thin sections? How
many pieces in a flask and what gauge sprue wire are you using? Are
you sure you are treeing correctly?

Whenever possible, gate so that the metal goes from the thickest
portion (ideally the same or slightly smaller cross section as the
sprue) and ends in the thinnest portion. Going from thick to thin and
back to thick can cause problems that will make you to want to pull
your hair out.

Rough castings can definitely indicate too high a flask temperature.
A yellow ring (sulfur) around the button is a good indication that
something got too hot, but it can have other causes as well. Jim is
right about the investment breaking down at too high a flask
temperature, but if you are trying it at 1000 F, that’s not too high,
in fact that would probably be too low for almost all of the white
gold alloys we use. Most 18K white alloys need to be pretty high, I
use around 1150 to 1350 (the metal manufacturer’s recommended flask
temp, also the investment manufacturer’s recommended max temp),
yellow is significantly lower, 900 or so, maybe 950 with thin cross
sections. Check the websites of both your metal and investment
manufacturers for recommended and max flask temps.

I always try to use the lowest metal temperature I can get away
with. As soon as it stops sticking, I throw (or pour) it. Vacuum or
centrifugal, silver or gold, white or yellow, macht nichts. If there
is potential for problems, e.g. porosity on a thick piece or
non-fills in thin pieces, I change the flask temp and casting method,
never the metal temp. Raising the metal temperature while casting has
about the same effect as it does when trying to get solder to flow.
If it doesn’t want to flow and the temperature is right, there’s
something else wrong. Getting the metal hotter seldom fixes the
problem, it usually only creates more problems.

I think if you are using vacuum, something as simple as switching
over to centrifugal for the thin pieces might be all you need to do.
High G forces seem to work much better at forcing the metal into tiny
and/or thin areas better than gravity. All vacuum really does is get
the air out of the way (think of trying to fill a glass globe with
water by pouring it through a tiny tube - the air stops it from going
in), 1 G of gravity is what really does the work of pulling the metal
into the flask. I don’t know what the G forces are on a spring driven
centrifugal caster, but it’s one heck of a lot more than 1 G, plenty
to push the air out of the way through the investment.

The cracking you mentioned can be caused by over-heating the metal,
but it’s more likely to be the result of quenching too soon. For
white metals I always let it sit for at least ten minutes. If I have
time, I let all flasks, regardless of the metal, sit until I can pick
them up with a bare hand. The less shock the metal receives while
completely locked in the investment the better. This is extremely
important for platinum, thermal (or even mechanical) shock shortly
after casting is what causes most cracking and brittleness in
platinum castings, assuming good metal and proper casting hygiene is
used.

I have no scientific backing for any of this, just many thousands of
castings over the years. Hope this helps!

Dave Phelps


#6
The cracking you mentioned can be caused by over-heating the
metal, but it's more likely to be the result of quenching too soon.
For white metals I always let it sit for at least ten minutes. If I
have time, I let all flasks, regardless of the metal, sit until I
can pick them up with a bare hand. The less shock the metal
receives while completely locked in the investment the better. 

With silver such cracking can be hot tearing caused by too high a
flask temperature. There are often multiple possibilities for for a
failure. Everyone who has responded has offered good for
the OP that is based on their experience and processes. It will
probably take some experimenting and analysis to find and fix the
problems.

James Binnion
James Binnion Metal Arts


#7

Thank you for all the casting advice. I’m still having trouble.
Today I did a casting in 18K yellow with disappointing results.
Better than before but not good.

Lettering areas didn’t completely fill, logo and fine details were
mushy and sloppy looking, and the surfaces, well if it were a baby’s
bottom I would describe it as a pimply rash.

I did not have any cracking like I did earlier with the palladium
sterling, and I’ve tried different temps.

Today I had a flask temp of 1150 with a melt temp of 1900. When I
tried the palladium sterling both temps were 100 degrees higher than
that.

My pieces aren’t thick and thin, they’re evenly thin, at about 1/2
mm. They are well sprued and properly treed. I know that’s extremely
thin, but I’ve had it done with good results before with crisp logo
and details. I do vacuum casting and have an electromelt. I use
graphite gaskets and draw an immediate vacuum.

It seems that the no-fill and the rough surfaces are two different
problems. The incomplete fills are probably caused by too low temps
I would imagine, and I may have to go back and add some thickness to
those areas; but the rough surfaces I don’t know. I’m using a 12 hour
burnout cycle with a high of 1290, and using the standard Satin Cast
investment.

I used a 4X6 perforated flask this time, and I probably did just
what you warned against Jeanne, and that is to top it off with too
much investment, because just a few days ago I did have a blowout
(first time that’s ever happened to me), so this time I probably-over
compensated.

But still, in the past I’ve always had smooth castings if I start
with smooth waxes, but not now. This is getting a bit demoralizing,
with one failure after another. I’m in a real slump here but I don’t
want to give up and outsource it when I know I can do it.

Thanks again for sharing your knowledge and expertise.