Lost Foam Defects: The Dreaded Sand Float!

Discussion in 'Lost foam casting' started by Tobho Mott, Jun 19, 2018.

  1. Tobho Mott

    Tobho Mott Gold Banner Member

    Got an order for a couple of my lost foam skull belt buckles, one in aluminum and one in bronze.

    First time I've cast silicon bronze or done lost foam casting with anything other than aluminum...

    I was going to cast silicon bronze last weekend anyhow for a couple of reasons: 1) I have a bunch of it and have never cast it before; 2) I needed to test my new sand; 3) need a bronze buckle, so may as well use bronze and kill 2 birds with one stone.

    I set up 2 patterns so I could get a bronze buckle of my own. Plus a sand mold, which I've already posted about.

    See my 2017 Halloween casting contest entry thread and videos for more details on how I make the patterns. As I mentioned there, I tried cutting the belt loop into the foam instead of the casting this (next) time. Pic taken before I added the drinking straws which mold the belt hooks...

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    First one I poured, I saw the surface of the sand puff up and knew the casting was boned. Second one came out great...

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    Successfully cast belt loop and hideous sand-floated example:

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    (Anybody got a use for a bunch of left side only skull bookends? I don't know what else to do with the leftover right halves of the foam skulls... :D)
    More pix of the defective casting, note how the belt loop floated on one side but not the other?!

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    Pic of the whole unsightly blob:

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    I used a slightly shorter sprue than normal and I guess I did not vibrate the sand into place tightly enough. Guess I underestimated the pressure that this heavy (vs. aluminum; I almost could not believe how heavy a full #12 crucible was!) bronze would have behind it too; a brick laid on top of the sand for some exrta weight probably would have saved it. Seeing the first one fail therefore stopping pouring a little sooner saved the second.

    Jeff
     
  2. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    As you noted, when you cast the heavier alloys you are vastly more susceptible to float, especially with unbound sand. Weighting the top of the sand might do it but a vertical orientation of the skull with the sprue pictured above likely would as well because the majority of projected area of the part faces the walls of the container which contains/restrains the sand (presume it was cast in a bucket, correct?) with a small amount of projected area looking upward.

    The intake manifold lid I'm on working on right now is an extreme example of this. I'm going to pour it on edge instead of flat. -Thread coming.

    Best,
    Kelly
     
  3. Tobho Mott

    Tobho Mott Gold Banner Member

    Yeah setting it up more vertically would have been another good idea that could have prevented this. Just didn't occur to me - other than carving the loop out of the foam sprue this time, I went with the same setup that had worked for me every single time before (all with aluminum). These don't have all that much horizontal footprint like your heat sinks, so for this one to float on me came as a big surprise, should have seen it coming probably. Wish it was easier to see into the soup can through all the fire to know when to stop pouring! Well, at least now we have some sand float documentation on here, right? o_O:D

    Jeff
     
  4. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    When I was casting the lead counterweights for my large furnace I tried a couple experiments with lost foam and lead because I expected the worse from the high head pressure. Very easy to float the sand and also move the sand. One foot of lead height produces a lot of head and those molds would need to be very well packed and contained on all sides. I opted for an open face ingot mold.

    Best,
    Kelly
     
  5. Last cast I did when someone was here watching I overpoured the first bucket so much I only got a little metal in the second bucket. Just enough to fill the pattern, it turns out.:cool: They still think I know what I'm doing.:rolleyes: Wish I had the balls to pour short and still fill the pattern.

    On floating, can you pour through a 45# weight? Maybe laid on a disc of plywood?
     
  6. Tobho Mott

    Tobho Mott Gold Banner Member

    There's an idea! Could work, I bet.

    These patterns had hollow sprues, I'm thinking the actual buckles must have filled almost immediately and the rest of the pour in both cases was really only for my own peace of mind. :D Impossible to see through all the smoke and fire though, so's to confirm they're full. I like to fill up my soup cans T least halfway to be extra sure, but here this led to a failure. Live and learn...

    Jeff
     
  7. Jason

    Jason Gold

    Bronze chases really easily, but takes lots of time if you want to save the piece.
    If it doesn't work on edge, it's time to get you running shell for bronze. That is, if you want to keep screwing with bronze/foam, shell the foam and watch it burn out cleanly as the shell vitrifies. It will handle the weight of your bronze without the sand issues. I wish shipping was cheaper to you, I'd mail you some slurry and silicas.
    The alternative and more expensive way would be using solid ultravest investment.

    Isn't a bronze belt buckle going to be heavy? Don't really want to see your pants falling down???
    pullyourbritchesup.jpg
     
    Rtsquirrel likes this.
  8. Tobho Mott

    Tobho Mott Gold Banner Member

    I hollowed out the back of the foam skull slices to reduce the weight a bit. I'll let you know once I've cut the sprue off!

    To be honest, I only broke out the foam skulls this time because somebody asked me to make one of these buckles after seeing an aluminum one. Was planning to pour bronze that weekend anyhow to test out the new sand so asked if they'd rather pay extra to have a bronze one... They asked for one of each, no accounting for taste I guess! :D And so, here I am messing with bronze and foam. Probably not gonna be a regular combo. But... It takes me all of 10 minutes plus hot glue gun warmup time to make a buckle pattern like this, so as infrequently as it happens, it's pretty hard to turn down a few bucks to pour one of these before the ingots while I'm at it.

    I would like to try ceramic shell and investment and all different types of casting at some point, but I don't have a kiln and people keep beating me to the free ones on kijiji by mere moments whenever they pop up. :(

    Jeff
     
  9. Tobho Mott

    Tobho Mott Gold Banner Member

    Well, the one I kept for myself weighs about 380g, or a little over 13oz. Hefty. Heavy enough to pull the belt out of my pants if I'm walking around with it undone for some reason, but not when standing still doing... Whatever one does while standing still with their belt undone, which nobody wants to hear about.

    Still pix are already above, but I finally got the video edited. Here's a better look at a couple of bizarre sand float defects:



    Jeff
     
  10. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    Jeff, a per my earlier comment, here's a couple of suggestions that become more important for lost foam pours with high density metals and/or high projected surface area pattern orientations.
    1. Turn the pattern on edge so it has the smallest cross sectional area visible as you look at it from the top.
    2. Bury it deeper in the flask, use a longer sprue, and make sure it is well vibrated/packed. Float occurs because the hydrostatic pressure of the molten metal working on the exposed/projected area of the pattern produces a lifting force greater than the weight of the sand resting on top the pattern. Burying the pattern deeper in the flask only helps because the sand is actually packed and to a degree interlocked together, not just resting on top the pattern. If it was the latter, every inch of additional sprue depth just adds more lifting pressure which is already greater than every additional inch of sand.
    3. If you don't want to change anything, OIF's suggestion of a weighted plywood board on top the surface of the sand with a pouring hole is simplest. In my rig, I could also use vacuum and a film layer on top the flask. This generates an additional couple pounds per square inch of retention force on top of the mold.

    I take it you used your fine media with no coating for the cast. Is that correct? I was just wondering how mud would have tolerated the bronze casting temp.

    Best,
    Kelly
     
  11. It looked to me that he had his foam coated with something. I've got a lost foam brass pour coming up. I'll use mud.
     
  12. Tobho Mott

    Tobho Mott Gold Banner Member

    Nope, no coating, I'm a recalcitrant bare-dogger who doesn't like sitting around watching mud dry. Used the same glass bead medium as always for my LF sand; some of it did melt a bit and stick to the casting, but I chipped that all off easily enough with a little light tappy-tap-tapping.

    To be clear, these were the same castings I used as an example in the OP, not new ones; I'm only posting about them again because now I finally completed the video editing and it seems like the video is a good visual aid for this thread.

    Great tips for helping to avoid this guys, I would have tried some of what Kelly suggested earlier in the first place before the one that floated was even poured, particularly burying the pattern with a more vertical orientation, if I hadn't cast several of these before and never floated the sand. Only tricky thing about that is the angle of the belt loop sprue and the drinking straw hook, but I can think of a couple ways around those being a problem. If I have to see a lost foam defect, I would actually choose sand float, it's almost not even upsetting when it happens, because the outcome is invariably super freaky looking, as though Scotty messed up the controls on his transporter beam or something. (Went with a Star Trek analogy so you guys won't think I'm a nerd... really it's more like a wizard failed on his mishap die roll and botched the teleportation spell.)

    But those other non-floated buckles were all done in aluminum previously; as discussed, this was really a case of the extra density of the bronze, which I had failed to account for, coming into play. 2nd and 3rd attempts came out fine when I simply stopped pouring sooner instead of waiting to see the molten metal come up to the surface through the drinking straw like it did in my 2017 Halloween contest entry; you do kind of get a feel for how much metal actually needs to be poured after a few of these guys. There's no real need to fill the soup can all the way up, doing that with aluminum buckles just saves a bit of space in the ingot mold is all. I did vibrate the mold more (gave the buckst a couple extra kicks) and added some weights on top of the sand for the third one FWIW, since by then I had already seen the first one fail. I doubt those couple chunks of broken bricks actually held anything down much; surely it was stopping the pour sooner that really made a difference.

    Jeff
     
  13. Rtsquirrel

    Rtsquirrel Silver

    Caught the video on YouTube early yesterday (4th like), thats one tough looking belt buckle. Gave me an idea for a friends clothing store.
     
    Last edited: Aug 30, 2018
  14. Tobho Mott

    Tobho Mott Gold Banner Member

    It happened again, I seem to average sand float about half the time when I'm casting lost foam in bronze.

    Hollowed out this foam bunny rabbit for one of my dollar store style foam castings, put a foam bridge across the opening on the bottom to sprue into and to drill a mounting hole in once it was cast.

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    The bottom part of the bunny (topmost as buried) floated, so I lost his chest and feet, as did the loose sand core inside the hollow area.

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    Not sure if it's easy to tell but the bottom 3/4 of what should be the bottom of that core is solid bronze, other than some deep shrink holes. Although it still has the styrofoam bead texture that should have appeared way deeper down.

    I have a theory. The bunny's styrofoam body might have insulated the core from my amazing bucket vibration technique of kicking and tapping the bucket a whole bunch of times, causing it not to get compacted so well, resulting in the core float that made the casting barely even hollow. Then all that sand from the loose core getting pushed upwards by the bronze might have helped the top part of the mold float, where the little bridge and his feet and chest were. Maybe using real sand instead of this glass bead stuff would help avoid float with denser metals too, I'm not sure. (Not having to chip a layer of melted glass of the casting sure would be nice too, but that never happens with aluminum).

    I think with a little practice I can learn to stop pouring lost foam bronze before the sand floats. Normally it happens if I keep pouring for too long after the soup can stops draining, but this was the exception, I stopped pouring when I noticed sand spilling out of the bucket, but the sprue still drained out down beneath the top of the sand! Hence the theory, to explain the early extra floatiness.

    Capture+_2020-04-06-07-50-20_copy_664x439.png

    Already redid it in aluminum, and it came out great, but I'll put that one in the dollar store foam patterns thread.

    Jeff
     
  15. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    Float is more common with high density alloys. I have done lead lost foam!
    1. Don't allow the cup to protrude much above the mold surface. Bury it nearly to the top. If the bottom of the cup is just barely below the surface only a little filling of the cup above the sand surface will produce enough head pressure to displace the sand and cause a run out, and that erosion further destabilizes the sand and it all erupts.
    2. Bury your pattern deeper. Bronze will produce nearly 3x the sprue pressure of aluminum for a given depth. If the sand is not well packed, it acts more like a fluid so the pressure on the pattern/mold cavity from the weight of the sand and sprue pressure of the molten bronze are both hydrostatic and increase in the same proportion with depth, so burying deeper would not appear help.........but well packed sand doesn't act like a fluid. It acts more like a bonded, and will stay in place much better if vibed.
    3. Use the densest mold media you can. -Practical matter here. Silicon Carbide grit woul dbe good. Aluminum Oxide Close. Silica and is cheap......cheap wins. Glass beads are light and low refractory........not so good fro bronze.

    Hollowing the pattern was wise but you'd be far better off spruing into the bottom edge (base of the bunny) than running the sprue down inside the hollowed pattern.

    -My 2 Cents. You must try again!!!

    Best,
    Kelly
     
  16. Tobho Mott

    Tobho Mott Gold Banner Member

    Thanks Kelly. Good tip about pushing the soup can down deeper. The (aluminum) second attempt is already done, see here, sprued up like you suggest. :D

    Was't sure how the density of the glass bead stacked up, thanks again. Been meaning to check out the reputedly nice fine (cheap too) "hydro sand" from a local quarry for its greensand potential for a while now anyhow; if it lives up to the hype, it may finally replace the glass media.

    Edit - I've also been working on a sand casting pattern for an air powered flask vibrator that could maybe be attached to a sand bucket too. I think that covers almost everything you mentioned. As for burying the pattern deeper, yup, if I can't do that next time I try bronze, I'll put some weights on top of the sand instead and start shopping for deeper buckets. ;)

    Jeff
     
    Last edited: Apr 8, 2020
  17. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    Cut the bottom out of a bucket and stack them if needed.

    Best,
    Kelly
     
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