[Jewelry making - Article 00995] - Re: [Orchid] Curing investment under pressure for casting

[Peter W . Rowe]

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>     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,
    not help. 

Hope that helps.

Peter
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