[Jewelry making - Article 00528] - Re: [Orchid] Bronze casting porosity

[MWKohlleppel]

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    G'Day Carlos. What you have described is classic shrinkage porosity. 

    One question firstly, What type of bronze are using? 

    I will assume that it is a type of Silicon Bronze, and the response
    will be for that type of material. The burnout cycle is ok and the
    flask temperature should be 490 Deg C. 

    Now here's your problem your sprue is way too long, what is actually
    happening in simple terms is that the metal in the sprue is
    solidifying before the object has had time to solidify. As your
    casting is solidifying it requires a supply of liquid metal (from
    the button) if this supply is not there then what happens is the
    highest melting point metal starts to solidify and forms the basic
    crystal structure which in essence under the Microscope looks like a
    Christmas tree, any way when that element has solidified then the
    lower melting point alloy will fill the spaces in between the
    branches but if that supply is cut off  then the result will be just
    as you described. So by keeping the sprue short and large you are
    ensuring that as the casting is cooling it has an uninterrupted
    supply of metal. Cut your sprue length to 1/4 to 1/3 of its present
    length. 

    If it is silicon Bronze you may quench after the red colour has gone
    out of the button. 

    I have tried to simplify the answer as I could spend a week
    describing the Solidification Process of various alloys. 

Regards Michael W Kohlleppel
Art Tech Castings Australia
Investmentcast AT aol.com


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