Ganoksin was of great help for me in the last couple of years. It’s
one of the best resources of the web and certainly it’s members are
very qualified.
I have a ring which gives me a big headache lately and would be
really happy if somebody can give me some pointers
Now, the facts:
18k White Gold (12.5% Palladium/ 12.5% Silver)
Kerr SatinCast 20 (5 hour burnout cycle as recommend by Kerr)
Flask Temperature at casting 650C
Centrifugal Casting (with oxigen/propane torch)
As you can see in the photo there are holes in the bottom half part
of the ring and the casting did not fill up correctly. Sprue has
holes as well. The upper half is completely smooth as it should be.
My modifications for the next casting cycle are shorter and thicker
sprue and increase of the centrifugal casting speed. I am guessing
that the material cooled down to quickly resulting in the holes.
Now, after researching the web, the other reason for the holes could
be gas porosity and/or the reaction with the investment…
As you can see, I am a bit in the dark here. If somebody had some
similar issues or has some ideas, I would be grateful.
Kai
PS: If this has been discussed “to death” or it is not the correct
way to ask in this mailgroup, I apologize
You are on the right track with the shorter wider sprue, also I would
not use Satin Cast with Pd White as it is too hot a metal for it. I
use a high temp dental platinum investment for Pd White and I have
used flask temps of 800C for thin section Pd White pieces. I know
some folks use gypsum bonded investments for it but I have better
luck with the platinum investment.
Large casting require a lower flask temperature of 800-950F. Small
detailed pieces require a higher flask temperature of about
900-1150F. Most porosity is caused by overheating the metal or by
poorly designed or insufficient spruing.
Split spruing to reduce “hot spots” and porosity. Make your sprue,
“V” like.
Flask temperatures vary, depending on the size of the pieces being
cast and how much fine detail they contain. Large plain pieces
require a lower flask temperature, 800-950F, while pieces with fine
detail or filigree require higher flask temperatures, 900-1150F.
Another rule of thumb is flasks used in vacuum/vacuum assist casting
should be 75-100F hotter than when casting the same pieces in a
centrifugal machine.
Those “holes” look like flux pits to us. Your casting looks like you
used too much flux, too large a gauge of sprue wax, and your casting
temp is kinda high. We use about a 1/2 inch sprue and cast at 1000 F.
I would definitely agree that they appear to be flux pits. Were the
pits bright before you pickled the piece? Or was there flux in the
pits? Large hemispheric pits such as pictured are a first indication
to me that the problem is too much flux.
The advice about cooler flask temps for larger castings is also right
on in my experience as is Jim Binnion’s advice about phosohate bonded
investments for paladium white golds.
I have a ring which gives me a big headache lately and would be
really happy if somebody can give me some pointers
This looks like it might be a combination of things. What first
caught my eye was the sprue itself. The sprue shouldn’t get smaller
toward the model…make it consistant in size and smooth. When the
sprue gets smaller toward the model… the metal has a tendancy to
spray into the model rather than flow. The pits & porosity may have
been caused from the lack of metal. From just looking at the end of
the sprue… away from the ring, it doesn’t look like there was a
sprue “Button”… in other words, exccess metal… If this was the
case, It is possible that could have caused the craters. The
model…and sprue need to be sealed off from any outside air and the
button I mention does this job. As the metal inside the investment
cools it actually sucks more metal off this button… keeping the
seal tight and feeding the casting the required metal. This helps to
prevent shrinkage of the casting as well.
I usually weigh out the required metal plus 15% to get a button.
There are other things that could be at play… metal too hot, flask
too hot when thrown…although part of the ring appears to be ok.
Was the metal at least 50% fresh casting grain and did you use a
flux?? Saying no to either of these could have caused what I see.
… I lean toward the lack of metal and perhaps the shape of the
sprue to a small degree, especially when I look at the bottom of the
ring. This seems to be a really large sprue too… hard to tell in
an image for sure. Two smaller sprues will nearly always work better
than one big honkin sprue.
Off subject but to save you a lot of work… on models designed with
detail all the way around on the outside, consider spruing from the
inside of the ring… saves TONS of work.
I know others will chime in and give perhaps more insight.
There are other things that could be at play... metal too hot,
flask too hot when thrown...although part of the ring appears to be
ok. Was the metal at least 50% fresh casting grain and did you use
a flux?? Saying no to either of these could have caused what I see.
....... I lean toward the lack of metal and perhaps the shape of
the sprue to a small degree, especially when I look at the bottom
of the ring. This seems to be a really large sprue too... hard to
tell in an image for sure. Two smaller sprues will nearly always
work better than one big honkin sprue.
If you have not cast palladium white gold you will not understand
the issues. It casting temp is way hotter than almost any other
jewelry metal except platinum. It is also not very fluid when it is
molten so you need big, short sprues and high flask temperatures or
it will freeze and you get no fills. This is why I recommended higher
flask temperatures and larger sprues and platinum investment.
Thank you James, Steve, Jo, Ivan, Andrew and Dan (I hope I did not
forget anybody) for your detailed answers. I am currently casting the
rings in silver to make some rubber molds, I do not want to carve the
wax model again
For my next white gold run, I will definitely use a shorter/wide
sprue, calculating the weight for the “button”.
I will use less flux (borax), unfortunately I did not see any flux
remains (might have come off when removed from the tube) Temperature
wise I think I am ok. Probably the metal temp was not right, I wish I
would have a high temperature thermocouple, mine stops at 1000C :(. I
use a carbon rod to stir the liquid metal and check on the viscosity.
The white gold had this gooey “feel” compared to Sterling Silver
(which is more water-like).
