[Jewelry making - Article 00935] - Re: [Orchid] Vacuum casting seal problem

[Peter W . Rowe]

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>     Changing pads hasn't helped.  The machine is pulling a vacuum
>     (I've tested it every time), but I just can't get the flasks to
>     seal. 

    check that the bottom edges of your flasks have not become too rough
    and nicked to seal with the gasket, and be sure also, that you're not
    filling the flasks quite level with the bottom edge.  You need that
    edge to be at least slightly higher than the level of the investment,
    so the flask is resting on that edge, not the investment.  What's
    sometimes gotten me is that though the investment can be the right
    level, it sometimes crawls up the wax web a little, and after
    burnout, the holes around where the web used to be are slight peaks
    of investment that can rise slightly above the rim of the flask. 
    Just scrape those off before casting, if you didn't notice it before
    burnout. 

    Also, take a fine scouring pad (like scotch brite) to the metal
    surface that the gasket pad rests on,  to be sure there's no
    corrosion or some thin deposits or build ups that might keep the pad
    from properly sealing.  And if the pad doesn't well seal to the
    surface, you can also lightly oil (vac pump oil) that surface to the
    pad seals better. 

    And, in checking your vacuum pump, how are you checking it?  same
    table/connections as the casting one?  Some machines that cast on
    one surface and invest on another with a valve to choose between
    might have a fine pump and do just fine investing, yet have some leak
    in the connection to the other table, and then have trouble pulling a
    good vacuum on one side only.  I know that sounds simplistic, but
    it's sometimes easy to overlook the most obvious things while
    pursuing the most esoteric possibilities... 

HTH
Peter

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