We played around with all his toys; Boy, what a day! But on my way
home I kept thinking "pressure pot". Anybody has experiences with
pp's in jewellery casting? Or general insights and opinions?
He’s right, of course, in principal. Just as vacuuming the
investment causes bubbles to expand, and thus rise to the surface,
later pressure on the investment will cause any remaining tiny
bubbles to become tinier. Given the precision fit dental castings
must have, this added slight improvement may be worth the trouble,
but for most jewelry casters, the vacuum process is enough.
By the way, did you happen to ask what sort of metal and investment
he’s using? Many dental alloys are quite different from jewelry
metals, often melting much higher, and needing investments that take
higher temps than do normal jewelry investments. Also, keep in mind
that dental casting often requires an exacting control to eliminate
any shrinkage between wax and final casting, so things fit right.
This is quite different from jewelry casting, where models are
usually just made large enough to compensate for casting shrinkage.
The dentists can use slightly different higher expansion investments
to handle this problem as well. The combination of different, high
expansion and/or high temperature investments may well change the
whole story about getting bubbles out with just vacuum, so his
process, though it may look the same in terms of investing, may
indeed require the pressurized cure cycle for best results. Also,
keep in mind the different shapes/geometries of what we cast, versus
what the dentists cast. Most dental casting are fairly uniform in
shape from one casting to another (crowns, bridges, they’re all teeth
shaped and sized, which is usually smaller than jewelry castings),
which might allow more refining of the process to optimize it for the
things being cast. Dental casting is usually done with smaller
flasks, for example. These various factors might mean a denser
investment would be a benefit for them. With a lot of jewelry
casting, though, larger metal volumes and often more delicate and
varied geometries can mean that gas permiability through the
investment becomes important to get a good fill. After all, if you
want denser investment, use less water, and that’s what you get, but
when you do that, you may find problems in casting. While denser
investment might help sometimes, at other times it would hinder us,
Hope that helps.