Aluminium lost foam fail

Discussion in 'Lost foam casting' started by Mark's castings, Apr 9, 2020.

  1. Just a quick post to show the failed aluminium lost foam castings that I'd attempted on Monday. I have some big ball bearings bought for cheap at an auction that I'm going to use for a plough disc bowl sand mixer. The plan is to have the inner rotating tube bolted to the horizontal plough disc and should give a three inch hole to empty the sand into a bucket below. So the bearings are angular contact 7300 series with 190mm O.D. and 90mm bore.

    So for both pours I was downwind of the fireball and had to interrupt the pours twice to shift out of the flames.

    The runner was coated in plaster of paris and worked quite well, it's long to allow plenty of sand weight on top.
    lost foam fail 2.JPG

    lost foam fail 1.JPG

    Ran out of metal on the second pour after interrupting pour twice.
    lost foam fail 4.JPG

    lost foam fail 3.JPG
    Last edited: Apr 9, 2020
  2. Melterskelter

    Melterskelter Silver

    That’ll buff out. ;-)

    How well I know the feeling of disappointment when digging out a casting full of hope and then you get the first glimpse of a defect. Uggh. It would probably offend your sensibilities, but would either casting actually work?

  3. Well the smaller tube will have the mixed sand falling through it, so if it went through the holes in the casting it'd end up inside a 7318 ball bearing. The bearings are massive overkill but I bought two lots of large bearings at an auction for $150: I wasn't going to hang around and made a stupid low offer and got a call days later telling me to come and collect them. I'm going round to Pete's tomorrow and whip up a resin sand mould of the two pieces with a quick and dirty pattern for the outside and a split pipe to make the core for both of them.

    I was trying multiple dumb things: melted the styrofoam out of the pouring cup with thinners and that may have melted too far into the main pattern. The sprue had a plaster coating but the main styrofoam body just has paper masking tape over it to keep the sand out of the glued joins and their tiny gaps. The styrofoam was horizontal and there were air gaps under the flange from the motor vibrated sand: if I'd angled the pieces that would have cured that fault.

    So in hindsight, do a full plaster coating, no masking tape, angle the pattern in the sand and don't squirt thinners anywhere.
    Last edited: Apr 9, 2020
  4. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    Maybe I should add a wind sock to my set of mandatory pre-pour preparations.....:rolleyes:

    Interrupted pour = mold collapse.
    1. Can't see what you had for a pouring cup but given the volume of metal in those parts it may have been difficult to keep up after the classic lost foam pause without a decent size pouring cup to provide that buffer.
    2. I fight to insure no smoke/flame/burp back but in some cases in one-offs, it's just unavoidable and to be expected. A small fan or compressed air stream pointed at the top of the cup helps ability to see through smoke/flame.
    3. That's a pretty decent wall thickness part so somewhat lower surface area to volume ratio than most of my parts which means more gas to expel. I'd make sure to apply the PoP no thicker than needed to cover the color of your foam and allow it to be as permeable as possible.
    4. I'd still position the cylinder a little canted as opposed to vertical. That's really only for the horizontal surface under the flange. Your part didn't seem to suffer much/any sand destabilization under the overhang.
    All of the above said, I bet it would have been successful with an uninterrupted pour. Never give in. You must try again!

  5. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    Was posting while you were.

    :eek: ....and I'm not at all a fan of hollowing patterns to reduce foam nor foam pouring cups. Polystyrene Foam burns in air but melts and turns to vapor in the absence of O2. A freestanding cup with structure allows the sprue to become covered quickly and start a more tranquil progression of the molten metal front. Pouring on a lot of foam in open air is just fuel for chaos and cup turbulence, and sight obscuring flame and smoke.

    A side note, thermosetting plastics/foams like urethane, will not melt. They only burn and that is why they are poor lost foam pattern materials.



    .....and stand upwind with a fan pointed at the pouring cup!

  6. Of all the weird things I did, the masking tape actually seemed to work well and the casting replicates the tape lines. Gas production aside, the tape charred to a fragile carbon coating that hosed off the casting.
  7. Jason

    Jason Gold Banner Member

    ahhh, you can weld that up. :(
    Mark's castings likes this.
  8. Melterskelter

    Melterskelter Silver

    Mark's castings likes this.
  9. Jason

    Jason Gold Banner Member

    Yup. I see a bearing.
  10. Yeah sorry about that, I was on another aircraft forum and was trying to manually insert photos of a tiger moth my mum used to fly when she was learning aerobatics. That forum is set up badly and I thought I could cut and paste some code from here to embed a pic.
    Here's Mum in the early 1960's going out for a flight:


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