The first logical place to look for a cause for this problems is
flask temperature. If you've been having consistently good results
for the most part, I'd assume you're covering these issues. So what
may be happening is that the occasional problems arise when you've
got a larger flask than usual and it's not getting enough time to
equalize to the ideal casting temperature. And, of course, you want
that kiln calibrated. I like to cast white golds as cold as I can get
the mold to fill, usually around 850-900 degrees for vacuum.
Centrifugal might allow colder that that, depending on the article.
Many assume sprueing is the paramount issue. That will be true if
you're porosity is mostly of a shrinkage variety, and it will occur
in thick sections. But what most people don't understand is that,
beyond a certain point, the entire casting is a sprue of sorts.
But I'm going to take a guess here. I think it's all a design issue.
I also think what you are seeing is not actually gas porosity, it's
very fine shrinkage porosity. Why? Because it's in the thick
sections. The thin sections are cooling and drawing metal from the
thick sections. If it's truly a gas porosity problem, you'd probably
see an attendant dark color or rough surface or both. And you're
flasks, when quenched, will smell sulpherous.
Why do I say this is a design issue? Because if you design an
article that has abrupt transitions from thin areas to thick ones,
you have designed in a problem. I get wonderful results with my own
waxes in white gold, but then, I don't set myself up for problems. I
get problems when I try and break my own rules. But I have clients
who have designs with thick to thin shifts, and no matter what I do,
either the thin sections don't fill, or the thick one's have
porosity, or I have to do stupid pet tricks with the sprue system,
which leads to the next possibility. Read on...
White gold, especially the higher nickel alloys (the nice white,
stiff ones), doesn't like to be re-melted. Yellow has it's limits
too, but it's more forgiving. Each time you melt white gold, you're
changing the nature of the metal, burning off zinc, dissolving in
oxides, etc. Common opinion says you can use up to 50% sprue metal on
a casting, but with white gold, I like to stay below 30%. So, if
you've designed objects which require massive weight in the elaborate
gates and sprues, you'll be tempted to re-use all that metal. So, try
my approach, which is...
It's ALL about design. Everything, every bit of it, start to finish.
Anything with a wall thickness above 2 millimeters is going to be
trouble. Keep those thicker areas away from the walls of the flask,
and away from other waxes in the flask, so that more than one area
together don't get to form a "hot zone". And, the higher the nickel
content, the more trouble. If it's X-1, well, good luck with that
stuff. Also, am I nuts here or is there really a difference in
"density" between vacuum and centrifugal? Can molten metal actually
be compacted any more that it naturally crystallizes when it
solidifies? If that were true, don't you think we'd have some cases
of specialty metals that were used in industry such as "super dense
cast steel" for armaments?
David L. Huffman