[Jewelry making - Article 00255] - Re: [Orchid] Vulcanized molds refuse to fill

[Mark Bingham]

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Jim,

    Every one of the suggestions before mine are spot on, illustrating
    how complex it really can be just to "shoot wax". Jenny especially,
    with her guidance about warm molds, helps nail down a special
    flow/chill issue. After you solve injection temperature and pressure,
    sprues and vents, hold times, release chemistry, flow dynamics *gasp*
    - note there's a company called Moldflow that exists just selling
    software to design plastics molds for best flow/fill and molded
    properties - you are left wondering why some of the very experienced
    respondents talk about squeezing the mold progressively as the wax
    cools. The reason is (mostly) that injection wax is a partially
    crystalline thermoplastic. You don't just suffer thermal contraction
    when the wax solidifies, you also suffer up to 14% volume reduction
    through the phase change that occurs! 

    If you cool it fast through the melt point, wax stays rather
    amorphous with properties that include a tad more flexibility, lower
    density (higher volume fill for a given weight of wax). Trouble is,
    it can only cool from the outside surfaces, there is no "microwave
    refrigerator" to cool it uniformly through thick sections. So fast
    cooling of thick wax sections can chill the outside wax skin into a
    rigid shell, allowing internal voids (even tears and cracks) to form
    when the inside wax solidifies and shrinks. If it's a thin molding,
    the outer skin can be pulled inwards by the cooling central wax,
    causing sinks in the final surface, near the thickest parts.
    Eleventh-hour squeezing a flattish mold distorts the thickness a
    little, but can prevent those voids, cracks and local sinks. Fast
    cooling generally means less need for follow-up injection, especially
    if you squeezed! 

    If you cool it very slow through the melt point, a better ordered
    crystal structure can form, which behaves more rigidly, packs the
    molecules tighter to a slightly higher density (less volume fill for
    a given weight of wax). Trouble is, you need follow-up injection to
    add wax volume because of the higher achieved density with slow
    transit of the melt point. That volume needs to reach where the wax
    is chilling LAST. And it ties up you and your mold longer. 

    Why am I telling you this? Because you should not feel bad that you
    sometimes have a troublesome design & process. It's awfully complex,
    it's most often hand-operated adding even a bit more variability. If
    you do all the right things, all the individual advice given before
    mine pushes you in the right direction. Mine can give the problem
    another nudge, but all this materials science won't stop wax casting
    being a slightly black art. Now where's that vial of powdered toad's
    liver? 

Mark
www.fourth-axis.com

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