Bubbles

Discussion in 'Sand Casting' started by KDM, May 6, 2021.

  1. KDM

    KDM Copper

    I'm casting aluminium in a home made greensand.
    The recipe was 10% bentonite (cat litter) and the rest building sand.
    The part I'm trying to cast is about 5in by 3 and about 3/4in tall.
    I have an inlet, several small vent holes and one large vent hole at the opposite corner of the piece.
    The mould keeps its shape really well, but when I pour, the aluminium bubbles like mad and the resultant part usually has one or more big bubbles of air in.

    What could I be doing wrong?
    Wish I'd had the foresight to photograph it before cutting it up and re-melting it in disgust!

    I thought the sand might be too moist, but any less and it doesn't stick well. I hear people say "pour on through, even when you think it's full". I'm not sure I have that bit right.
    Does there need to be a head of liquid aluminium pressuring the liquid down into the mould?
    Is the big outlet hole a mistake? (I vaguely recall having better results wit just the small vent holes.)

    I've done this loads of times and the success rate is maybe 10%. When i get it right, I'm not sure why!

    Thanks.
     
  2. Chazza

    Chazza Silver

    Yes, a head of metal is ideal.

    Sounds as if there is too much water in your sand and the bubbles you see is steam escaping.

    Try a drier mix,

    Cheers Charlie
     
  3. Tobho Mott

    Tobho Mott Gold Banner Member

    If you have 10% bentonite and your sand is properly mulled, it should not need as much water as it sounds like you have in yours. Maybe it just needs more mulling. Pictures of the molds and parts would probably help you get a more reliable diagnosis.

    Jeff
     
  4. Petee716

    Petee716 Gold Banner Member

    Proper bentonite clay is super fine, like flour or talcum powder. Thoroughly mixed dry with the sand and then moisture added. The clay's primary purpose is to make the sand stick together by film coating, basically encapsulating, each grain of sand. If it's too coarsely ground the sand wont stick together properly and has probably resulted in overwatering.
    Just an idea.

    Pete
     
  5. Billy Elmore

    Billy Elmore Silver

    There are a lot of defects that can be described as bubbles and most of them have to do with moisture which is generally caused by excess moisture in the sand, but could still be shrink or gas. Gas being the least likely due to the description of vents. If you have a bad chemistry of aluminum..get too hot...cook too long...might see shrink or similar defects....if you have an excess of gas during melt or improper venting you may see gas absorption in the melt....impurities in the melt can often be described as bubbles so make sure you clean the melt before pouring....and finally...I dont think it is as common in home foundries as it is in industrial but you can not mull the sand enough and develop clay balls in the sand that will form perfectly round gas bubbles on the surface of your casting. Im sure I am missing some but one of these are most likely your culprit and my first guess is too much moisture in the sand.
     

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