Aluminum inserts for a fancy shoebox?

Discussion in 'Sand Casting' started by Tobho Mott, Dec 1, 2021.

  1. Tobho Mott

    Tobho Mott Gold Banner Member

    Made these for a local guy with a laser cutter who is building a fancy display shoebox for his customer. I never heard of fancy display shoeboxes before, but I don't get out much these days.

    They will be inset into the top and the underside of the box's lid.

    20211129_094304_copy_520x638.jpg

    He provided the 3d pattern file, I supplied the printing and a coat of bondo. Had to sand them smooth due to this ugly black spot at the end of a "flow line", which I am not sure how to prevent on thin flat castings like this. But I think they look better sanded anyhow.

    20211121_121727_HDR_copy_1040x520.jpg

    Jeff
     
    Last edited: Dec 1, 2021
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  2. Petee716

    Petee716 Gold Banner Member

    How did you gate them? How thick?
     
  3. Tobho Mott

    Tobho Mott Gold Banner Member

    I gated into the back side at the toe on these ones. I picked the toe hoping that area was narrow enough for the metal flow not to split, after the first attempt gating into the back of the foot-hole came out like this:

    20211121_121816_HDR_copy_1040x520.jpg

    Alas, same defect but differently placed.

    The shoes are about 4mm thick before sanding. The gate pattern I used is actually a tiny bit thicker than that, but supposedly modeled to go with the sprue pattern I used. I'll try to get measurements of the gating patterns posted here later. I used SwDweeb's Campbell-esque 3d printed sprue, runner, spin trap and gating models for this. I realize that doesn't get much love here. I tried them because the 3d files are available and because standard runner and gate patterns are appealing to me. The math that went into their design is supposed to slow the metal down enough to allow laminar filling and prevent any splashing inside the feed system or cavity for copes about as tall as us home gamers usually use, but it did not prevent the metal front from splitting then rejoining like this. Maybe I should have tilted the mold uphill from the sprue/gate end a little more than I did... I saw the same thing happen to my castings before I started using those patterns for gating too.

    Olfoundryman pointed out a similar black spot on one of his flask side castings and blamed it on worn out greensand. But I don't think I've used my petrobond that much yet...

    Jeff
     
  4. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    What did you use for parting compound? Any chance it's caused by a clump of that or maybe a clay or binder clump in the sand?

    Best,
    Kelly
     
  5. Petee716

    Petee716 Gold Banner Member

    Can you see the defect line on the back too?
     
  6. Tobho Mott

    Tobho Mott Gold Banner Member

    Yup. It's on both sides. I've recently started using diatomaceous earth for parting, but I've had molds parted with talc and chalkline dust come out the same. In any case, I was really careful to blow all the excess parting out, I'm positive the molds were really clean.

    Edit - no clumps of clay or anything like that in the sand either that I have noticed - end edit

    When the casting isn't just a featurless flat surface to look at like these are, these spots don't really stand out too much. But here it was just an eyesore. They seem to form on both sides of the casting at the highest spots on the drag (like the eye sockets of the skull ashtray I made a few years ago), and/or where fronts of molten metal meet each other as seen here.

    Jeff
     
    Last edited: Dec 1, 2021
  7. Castings look great!, that defect looks like what we call "Sand burn" although who know what the correct term is. I get it from organic particles in the sand burning into the metal as I'm using beach sand with the occasional tiny bit of leaf or wood in the mix. I'm hoping to experiment with some sort of spray or brush-on coating for the sand.
     
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  8. Petee716

    Petee716 Gold Banner Member

    I’m a lot more inclined to point to interruption in the pace of the pour. I’m working on some more of those 12x12x5/16 thick trivets I posted before. I think I’ve got the gating and tilt angle strategy pretty well sorted and I’m still getting the kind of defects Jeff is showing. My next pour should be this weekend and I’m going to try to focus on keeping my pouring strong and steady. I think even a nearly imperceptible inconsistency might be enough to make a difference.

    Pete
     
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  9. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    ....or even a small splinter of wood from a flask in the sand.

    Best,
    Kelly
     
  10. Tobho Mott

    Tobho Mott Gold Banner Member

    Hmm, I'm sure I kept the sprue full and taking metal as fast as it could through these pours. I might have some video footage of that but I'll have to go through a bunch of stuff to find it.

    As far as flask wood splinters in my sand, or course there are some. But I have trouble seeing why these would always and only cause the black drossy spots to coincide with high points on the drag and the ends of flow lines that lead back to gates, ie. places where molten metal seems to have met up with molten metal.

    I'll try to get some pictures that show this later today, once work calms down.

    I really appreciate the suggestions and ideas, guys.

    Jeff
     
  11. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    If you have tilted your mold, how do the points shown become the high points?

    It may just be entrained air that contains products of decomposed Petrobond binder. Will wait to see gating system but if you were gated in the point of the toe, with a tilted mold, and tranquil filling system, sort of hard to see why you'd have divergent metal flows.

    BTW, I like the Campbellesque spin trap and gating system, but you're right, opinions vary here. Did you vent the trap? Interested to see where it is positioned wrt height of tilted mold and gating system.

    Best,
    Kelly
     
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  12. FishbonzWV

    FishbonzWV Silver Banner Member

    Those look very clean Jeff. Good job on controlling the furnace atmosphere.
     
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  13. Tobho Mott

    Tobho Mott Gold Banner Member

    Here's the high points version. Smelko greensand and a different sort of gating system:

    Capture+_2021-12-02-10-20-26_copy_365x270.png

    And (toe option) gating for one of the shoes. Spin trap was vented. Maybe I need to tilt the mold a bit more... I just eyeballed it, I'll at least use a level next time to make sure I don't just think it's tilted the right way:

    Capture+_2021-12-02-10-07-46_copy_344x242.png

    Capture+_2021-12-02-10-08-13_copy_344x243.png

    Looking at it now I think maybe since the shoe is thinner than the gate pattern, it sped up the flow and kinda squirted into the mold cavity anyway. Not to mention it is going into a narrower section in this toe-gating case as well. But that was not true with the foothole-gating option.

    Jeff
     
  14. Tobho Mott

    Tobho Mott Gold Banner Member

    I can see how that might cause a similar defect. But I'm not sure that is what happened here... I found the footage of the toe-gated shoe pours. 2nd and 3rd molds are the shoes. I'm on my phone right now so it's hard to be 100% sure from the footage, but the part where I overpour the 3rd mold and spill aluminum on my flask alignment pin and pouring deck has triggered a memory of me thinking, "Shit. Well at least I kept the sprues full." :D



    Edit - good luck with the pour this weekend Pete!

    Jeff
     
  15. Tobho Mott

    Tobho Mott Gold Banner Member

    Found it. Martin discusses reasons why black spots appear at 3:25 here:



    To sum up, he mentions air bubbles floating through the casting as one reason, and molten metal fronts slamming together as another, but he also says that old greensand with a lot of fine facing sand already mixed into it over time makes this effect worse or more likely.

    I don't use any facing sand but petrobond with my petrobond. But I do sometimes wonder if parting dust buildup in my sand might have an adverse effect some day. However I also recall seeing black spots when my petrobond was still bright red, fresh and innocent.

    In any case, I think I'm going to start using some of the fresh petrobond I've been keeping in reserve as facing sand, to see if it helps reduce the appearance of these spots.

    Jeff
     
  16. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    Martin pretty much touched on everything that's been mentioned, but per his dialogue regarding old sand having a higher facing sand content and a greater propensity for producing such flaws, I would think that also means a less permeable mold, which in turn means any entrained air/gases persist longer, which would give more time for more gases to evolve into the bubble and for oxides to form.

    For you, maybe your PB is pretty fine and perhaps a lighter ram up would help. ? -Just thinking out loud.

    Best,
    Kelly
     
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  17. Tobho Mott

    Tobho Mott Gold Banner Member

    It's entirely possible I've simply been ramming too hard all this time. All I really have had to go by is people's written descriptions here, on AA, in books, and whatever I can pick up from watching videos...

    Jeff
     
  18. I see the same effect in aluminium cast in resin sand too, it's rare with kiln dried pure graded silica sand and a lot more common in plain old beach sand that was washed and sieved but had fine pieces of seashell and other things like mineral sands in it.
     
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