[Jewelry making - Article 00297] - Re: expansion/shrinkage

[Redray1]

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In a message dated 97-04-13 05:43:16 EDT, you write:

>     but sand castings would shrink also. Is anybody out
>     there able to verify the one or other opinion from his/her experience?

Markus

    Almost all types of metals will shrink when cast. It actually has nothing
    to do with the casting process but more to do with the melting process.
    When you increase the level of energy in a piece of metal with heat it
    causes the outer layers of the elements electron shells to become unstable
    and expand. This causes them to bond more easily with other elements. A
    prime example is silver will bond with copper to form sterling silver when
    heated. It will also bond with anything else with a few extra electrons to
    spare such as sulpher and oxygen. This gives you porosity after repeated
    heatings. But back to my point. This energy causes the metal to expand and
    it will stay that way until it is allowed to cool. When you pour the metal
    into a mold it is going to shrink. The rate of shrinkage is more pronounced
    the thicker the piece is. For instance your shrinkage rate might be as much
    as 8%. If the section in question is 1 mm thick the shrinkage will only be
    8% of 1mm or .08mm. If the area is 4mm thick then the shrinkage could be as
    much as .32mm. The ways to combat this situation is to ensure enough of a
    button to feed the piece and heep the button molten while the casting
    itself cools. This allows the casting to draw upon the molten metal above
    to fill any void created by shrinkage. This works best with vacuum casting.
    If the button cools to rapidly it can actually draw some of the metal back
    out of your mold in rare cases. All metal will shrink as it cools with very
    few exceptions for some of the rare earth elements, and one of the
    radioactive isotopes. RED


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