Gas Inclusion?

Discussion in 'Castings, finishing/ repair/ and patina's' started by Ferrisbeu, Jul 13, 2021.

  1. Ferrisbeu

    Ferrisbeu Silver

    Im sure there are more knowlegable casters out there who can tell me why some of my larger casts get gas holes occasionally. I think it is over temp brass. My mold was at 1k degrees and my high temp gun died so I couldnt check it. any clues?

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  2. DavidF

    DavidF Administrator Staff Member Banner Member

    Looks more like it was caused by the metal shrinking back as it cooled..
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  3. Zapins

    Zapins Gold

    I agree. Shrinkage. See how the thin neck area is ok but around it it has pulled away from the thicker sections? This happens when the thin section freezes first and the larger sections are still liquid and cooling. They start shrinking and try pull liquid from surrounding areas but can't get any liquid from the already solid thin section so they tear in that area because they weren't able to pull liquid in to replace the shrinkage amount.

    You'll need to fix spruing so you feed the fat sections from a large sprue that can feed them and remain liquid until the big sections are filled and solidify. If the sprue solidifies before the fat section you'll get shrinkage defects. Sprue must solidify last.

    This is more of an issue in solid pieces than hollow casting because shrinkage is a percentage of the total thickness. So fatter pieces tend to have more shrinkage and therefore tear more easily.
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  4. Ferrisbeu

    Ferrisbeu Silver

    Awesome. My pour sprue is 3/4 inch and the cup is about 2 inches. I'm pouring from the base of the statue toward the head. I had one out of the 2 work fine. I think I kept pouring as it vented and the vents closed. Something to ponder. bigger vents and overflow pours might do it. I don't know how to do hollow casts.
  5. DavidF

    DavidF Administrator Staff Member Banner Member

    Bury a shrink Bob in the mold. Having a large pouring cup that is exposed to ambient air will hurt more than it helps. I did a video about this year's back..
  6. Zapins

    Zapins Gold

    Are you doing shell or pop for investment?

    Hollow isn't too hard. How are you making copies of the sculpture? Got a mold?
  7. Ferrisbeu

    Ferrisbeu Silver

    No idea what a Shrink Bob is.
    Sponge Bob, yes....Im self taught and dont watch many how-to videos.
    i made a mold of a 3d print I made and then pour hollow shell paraffin positives.
  8. Zapins

    Zapins Gold

    Do you have a welder?

    You can make hollow casts by spruing the hollow wax and leaving the bottom open. Then dunk it in the slurry so the slurry goes inside the opening at the bottom. Its pretty straight forward.

    Of you have a welder you can weld the bottom shut.

    1/8" to 1/4" is the goal wall thickness.
    Last edited: Jul 13, 2021
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  9. DavidF

    DavidF Administrator Staff Member Banner Member

    Ok, then take and bury sponge Bob in the mold (he'll burn out). Basically just a big block that the casting can feed from as it cools.
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