[Jewelry making - Article 01701] - Re: [Orchid] Understanding Casting Shrinkage

[bensteiger]

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Neil,

    I just returned from vacation and saw your detailed post, which took
    some work, thanks. 

    I think I understand what you are saying, and will try to reiterate
    some of the main points, hopefully correctly, to see if I get it. 

    On initial flow, the cooling action between the mold and metal will
    create an air gap, and thus a volumetric reduction for both inner and
    outer diameter surfaces. This form of shrinkage explains why it is
    sometimes necessary to create a bulge in a wax when a flat plane
    surface is desired in the final casting. 

    But there is also a second form of shrinkage occurring, that which
    happens when the metal crystallizes into a complete solid. It is for
    this type of shrinkage that inner and outer diameter surfaces can
    vary closer to or farther away from the mold walls. A combination of
    surface tension, gravity and orientation produce a "desire" of the
    molten metal to reform in a particular shape. Thus parts of the mold
    will be fighting the metal's force from cooling, while other parts of
    the mold will have an increased air gap and be unstressed by the
    casting. 

    If only the first type of shrinkage occurred, draft angles would not
    be required for release for multi-use die cast molds. But they are
    needed, which means that the crystallization shrinkage force is
    dominant over the lava-tube like volumetric shrinkage. 

    Predicting and compensating for this shrinkage with risers, ribs,
    draft angles, radii and all the other tricks you mentioned are what
    make casting an industrial science, a profession I now more fully
    appreciate (from a respectful distance). 

    I hope that isn't a mischaracterization of your words. 

Ben Steiger
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