[Jewelry making - Article 00947] - Re: [Orchid] 2 Hours Casting

[Eddie Bell]

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Dear Ron,

>     Don't you think it is about time that we produced a casting unit
>     of this type? Dental labs have been using this technology for a
>     hundred years even though they are now using equipment that is
>     aimed more at high temperature, high production applications. If I
>     were burgled of my equipment, the first thing I would mourn would
>     be the loss of my ancient dental casting machine.

    Thanks for the nice comments about my last post; I am usually
    accused of being too tedious and esoteric but I keep on thinking
    that people really do want to know how to control the casting
    process, even if they haven't realized it yet. J Since you ask, I
    will give you my opinion (only worth what you are going to pay for
    it) about centrifugal casting. First, I am not intending to convert
    you or anyone else who has mastered the art of casting from the
    machinery you use and like to something else. To answer your
    question directly, I don't expect that we will see much development
    in centrifugal casting in the future except for platinum. Like you
    and many others, I started with a spring driven casting machine that
    I bought used from a retired dentist in the winter of 1960-61.
    Gradually as I learned more about the process I became convinced
    that centrifugal casting had some shortcomings. For one thing
    centrifugally cast metal is always turbulent and requires the metal
    to start at a higher temperature because a turbulent liquid loses
    heat faster than a non turbulent liquid. It can also entrain gas
    (another form of gas porosity not related to reaction gas).
    Centrifugal force can segregate gold from alloy to make the casting
    gold rich at the top and alloy rich at the other end and the high
    velocity erodes investment causing inclusions (usually mistaken for
    porosity). And if you get too much flux in the crucible you get flux
    inclusions. And anything floating on the surface of the melt can
    wind up in the casting. And if you cast more metal than the flask
    can holdwell you know what happens. 

    And now before I get a flood of posts I will say that in spite of
    all that you have made money with your centrifugal casting machine
    and I am happy for you. Out here in the west we say "if it is not
    broken don't fix it". But that was not my experience. Sure I made a
    lot of good castings but I also made a lot I had to fix or recast
    too. In short I was not satisfied with the consistency or the
    quality, I still am not but my standards are much higher today. 

    What we have learned so far is that we should be melting in a
    graphite crucible so copper oxide is reduced to pure copper and no
    flux is used. The metal is melted in a closed chamber filled with
    inert or neutral gas so no oxygen comes in contact with the metal
    when it is liquid. The crucible is bottom pour so the cleanest metal
    goes out first and any floating debris never leaves the crucible or
    is in the sprue button. The pour is less turbulent because only
    gravity is moving it and metal temperature is controlled with a
    thermocouple immersed in the melt. Pressure differential is achieved
    by vacuum assist and /or over pressure. The flask is enclosed under
    the crucible and purged of air and backfilled with nitrogen gas
    before the metal is cast. This includes the space between the bottom
    of the crucible and the top of the flask. Then the flask is held in
    the protective system until the metal has solidified. 

    There is more I could talk about but I don't want to sound like an
    advertisement, the point is that science and engineering has been
    applied to the casting process and this kind of equipment has been
    around long enough that it is proven to be worth the investment.
    There might be someone out in Orchid land who has made the investment
    to go from centrifugal casting to a closed bottom pour system who
    could tell us what their experience is. I would expect them to say
    that they have better castings that take less time to finish than
    they had when they cast with their centrifugal machine. I know that
    has been my experience.

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