[Jewelry making - Article 00768] - Re: [Orchid] Casting reheating investment molds - flask blow-out

[Kathy Johnson]

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>         If you try with the vacuum, as soon as you put the vacuum on,
>     listen for a popping sound. If you hear it, the flask has just blown
>     out. This will leave an open path for the molten metal to go
>     through the flask and either into the  vacuum line, flask chamber
>     or even vacuum pump. Most of the time the molten metal just gets to
>     the brass vacuum table fittings and freezes up. All in all a lot of
>     work. Not only lost metal but rebuilding a vacuum system can be
>     time consuming and expensive. 

    This happened to me a couple months ago. I had not re-heated the
    flask, but apparently the top layer of the investment wasn't thick
    enough, and the vacuum pulled it apart when I turned it on. I heard a
    brief metallic "thunk" noise as I turned on the vacuum machine. I had
    no idea what it was. So I poured 60 grams or so of molten sterling
    into the flask. The vacuum machine made a straining sound when I
    poured. I STILL didn't realize what had happened until I lifted the
    flask to put it aside to cool, and saw flames shooting up from the
    top of the vacuum machine! I sat the flask back on the machine to
    smother the flames, and grabbed the fire extinguisher. A few seconds
    of panic ensued--how could I handle a hot flask that needed to be
    moved, pull the pin on the extinguisher and shoot it at the machine
    all at once?  Fortunately when I moved the flask again, the flames
    were out--it was just the silicone rubber casting pad that had burnt
    a bit. 

    I put the flask on the cooling area and inspected the machine. I
    feared that something inside was still burning, but found nothing. I
    quenched the flask that broke, and found that the entire mold inside
    had been "lined" with silver--I had some really pretty sterling lace
    that was all hollow and crumbled when touched. I then left the shop
    for a while to go calm down. Later I found that the metal had gone
    through the vacuum lines and ended up in the clear plastic external
    filter cup. The inner metal filter was permanently welded to the
    cup, which had melted around the filter. And the plastic vacuum line
    leading to the filter had been shriveled up from the heat of the
    molten metal. A call to Swest produced a new filter and cup, and a
    trip to the hardware store got more pneumatic tubing to replace the
    damaged vacuum line. $25 in parts and the machine worked fine.
    (actually it worked before we replaced the parts--we found that we
    could swap around the external casting and investing filters, since
    only one is used at a time). I've thanked the deities that watch over
    idiots repeatedly since then that nothing worse happened to the
    machine. And now I listen very carefully for weird noises coming from
    the flasks every time I cast. 

--Kathy Johnson
Feathered Gems Jewelry
http://www.featheredgems.com

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