Having problems with Surface Shrinkage

Discussion in 'Castings, finishing/ repair/ and patina's' started by ChasByf41, Dec 18, 2021.

  1. ChasByf41

    ChasByf41 Lead

    I am attempting to recreate some historical flag pole tops. They are cast in Aluminum. I am getting surface Shrinkage on the forward part of the spherical section. Using Petrabond sand and release powder. I was watching Andrew Martin's youtube video Foundry49 on Chrinov's law. Can anyone help me out with the proper way to gate, and riser my mold?

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  2. DavidF

    DavidF Administrator Staff Member Banner Member

    Looks like your sprue was the one at the bulb section???
    If so I would make this a riser, move it closer to the part, and make it a bit larger.
    Then use the one near the center of the part as your sprue, and eliminate the riser neat the point.
  3. Petee716

    Petee716 Gold Banner Member

    The last part to solidify will always be the one with the defect. As David said, the riser needs to be larger and as close as you can get to the bulb. Even touching if its practical. It's probably a good idea for it to be the last thing to fill. I would have the mold fill through the riser into the bulb so that area of sand gets the most heat as well. I would go well oversized on the riser because the bulb on the casting will stay hot for a couple reasons. One because its connected to the hot tapered neck, and the other because of it's mostly spherical shape. (A sphere has the least surface area of any shape of the same volume, thus it has the least heat loss). 20211218_195450.jpg

    Are you using a sand core for the pole end?

  4. Melterskelter

    Melterskelter Gold Banner Member

    My guess is that Pete’s last question is key. If your core had a bulb and stack on the top end so that the bulb area became thin-walled instead of a solid, your shrinkage problems would probably disappear. I doubt the other risers are needed and likely none would be needed at all if you just build out your core a bit more.

    BTW, your casting is looking very good. Nice pattern, core, and molding work. You’ve got a lot of the essential elements of casting working for you. How much casting have you previously done?

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  5. ChasByf41

    ChasByf41 Lead

    Thanks Pete, I will give it a try. Yes. the base has a hardened sand core in place. Last time I put a riser over the bulb and filled directly over. I still got a little shrinkage. I'
  6. ChasByf41

    ChasByf41 Lead

  7. ChasByf41

    ChasByf41 Lead

    Thanks David, I will give it a try.
  8. ChasByf41

    ChasByf41 Lead

    Thanks Denis. None what so ever. Just stared this about a year ago and have used You Tube to guide me along. Glad to have found this forum for additional an additional resource.

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