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The plastic material is very suspicious. We have cast many
stereolithography parts and have found that is is not so much
the time the patterns are held at high temperature; it is the
temperature that they reach to vacuate the ash. We recommend
that you try a standard ramp up in your kiln, and then, go to at
least 985 C and hold it for at least one hour. Ramp down to your
scheduled flask casting temperature and hold in accordance with
your standard procedures. If the problem is due to ash from the
plastic, this will resolve the situation. Hope this helps! Tim

Tim, If you heat a sulphate bonded investment to 985C won’t that
destroy the investment? What happens to the ash at the higher
temp? Is it the increased convection in the mold that forces the
ash out? I had trouble with plastic when I started casting it,
but found that a longer soak at 760C works well. The normal
maximum temp recommended by manufacturers of sulphate bonded
investment is 732C. Some can be pushed higher but I found that
Kerr Satincast (what most jewelers use) will not hold up.