Ways to clean an intricate aluminum casting without a blaster?

Discussion in 'Castings, finishing/ repair/ and patina's' started by Chassy, Apr 29, 2022.

  1. Chassy

    Chassy Copper

    This is my first lost foam aluminum casting. Any recommendations on ways to get in to clean all of the narrow nooks and crannies without spending a few hundred dollars on a blaster and air compressor? I can only get so far with a wire brush & tube brush. I'm thinking maybe a vibratory tumbler, but that opens up the question of the type of grit to use. I see ground glass, glass bead, aluminum oxide and soda, all of which have pretty similar descriptions. Any suggestions?

    I'll be doing a deeper dive on finishing techniques in a few months, but am looking for something easy and good enough while I'm still working on learning to build patterns and pour. However, I do know that I'll need a cleaning technique that will work for removing a thin layer of drywall residue, which I'll be using to coat my next foam pattern (and probably all going forward.)

    Thanks!

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    Last edited: Apr 29, 2022
    Chassy, Apr 29, 2022
    #1
  2. Mark's castings

    Mark's castings Gold

    You could try some caustic soda solution and then some aluminium wheel cleaner with hydrofluoric acid in it which attacks silicates including the silicon in the aluminium to give a white-silver appearance. Hydrofluoric acid is poisonous in addition to being acidic so the concentration is kept to below 10% or so. Do not get it on your skin at all as people have died from insignificant spills of the concentrated form. I don't know how true it is that concentrations below 15% are less dangerous but I'm very careful about using it. the only treatment is local injections of calcium gluconate to try and neutralize before it strips the calcium in your body (the initial spills do not hurt and people have ignored them). I remember one guy where I worked cleaned the work ute aluminium tray with the 15% stuff and complained his hands ached for two weeks afterwards after using the hose and splashing it all over himself while rinsing it off.

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    Last edited: Apr 29, 2022
    Mark's castings, Apr 29, 2022
    #2
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  3. Zapins

    Zapins Gold

    I'd check Facebook marketplace or craigs list for a cheap setup or find a friend with one. If you're gonna do casting it helps to have the right tools. Otherwise its pretty rough work by hand.
     
    Zapins, Apr 29, 2022
    #3
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  4. jkahn

    jkahn Copper

    Before I got a water blaster I used to use my ultrasonic by putting the casting in a beaker with fluid, in the cleaner letting it go for a while.
     
    jkahn, Apr 30, 2022
    #4
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  5. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    Blasting is bar none the fastest and most effective, but, they do require a fairly substantial air compressor and if you have neither, I think you are probably right that a vibratory or barrel tumbler is next best. Barrel tumblers are very easy to build but painfully slow. You'll need small sand-like media to get in the nooks and crannies which will make it even slower, and the tumbling may blunt edges of your castings. Vibratory is faster but a more difficult build. Real vibratory media can be expensive, like $5+/lb.

    Mark's suggestion on chemically removing may be an option if you don't mind having those chemicals around in the quantity to do the job. -Pick your poison.

    You might get most of the way there with a pressure washer like the car wash, but the carbon on the surface of lost foam castings can be quite stubborn without abrasives.

    Wire wheel works well on flat non-intricate parts.

    Quenching the casting in a water bucket will blow off the vast majority of the plaster/refractory coating but will cling in sharp deep crevices. Compressed air will remove almost all the rest while wet. Pressure washer will do it too.

    I have a blast cabinet but also have a rectangular tub vibratory build in the works. It's designed for a specific purpose and the tub is a little over 1 ft3. That's 50lbs/$250 of 1/4" triangular plastic media. I'll eventually need three grades of media to do the finishes I'm after. I'd like larger, but $1000 per media charge isn't attractive.

    For barrel tumblers most people us a 5 gallon bucket with paddles on the inside, then placed on a roller like a washing machine tub. Things like pea gravel and sand can serve as abrasive. I've seen people use cement mixers too.

    They all make one heck of a racket.

    You just need to decide how much and what type of casting you will do. Hand finishing them is laborious versus a 5-minute trip to the blast cabinet. If you will be an infrequent caster, befriend someone with a blast cabinet. A 12-pack is much less hassle and expense......then you can help him drink it.

    Best,
    Kellly
     
    Al2O3, Apr 30, 2022
    #5
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  6. Chassy

    Chassy Copper

    Thanks for the ideas. A steel wire brush, Simple Green and elbow grease got a lot more of the residue off. However, I'm not sure if the Simple Green really did much to add to the action of the wire brush. I also tried brass and nylon brushes of various types and sizes. They got into most of the tight areas, but didn't do much to get rid of the carbon. For kicks, I also tried brake cleaner, but it's really for grease and residue and didn't do much. As automotive things go, I'm thinking that carburetor cleaner may be worth a try since it's made to get rid of carbon deposits. Similarly, maybe oven cleaner. But my next step is pressure washer, assuming it's still working. It's been a couple of years since I used it.

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    Chassy, May 1, 2022
    #6
  7. Mark's castings

    Mark's castings Gold

    It's getting there......

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    Mark's castings, May 2, 2022
    #7
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  8. Petee716

    Petee716 Gold Banner Member

    I got the impression from your OP that there was no coating on this one. That will make a huge difference. After you’ve done some coated pieces this one may not even make the grade just because of the sand surface finish. Once the pattern is coated, the sand will be out of the picture and the finish of the foam pattern itself will be exactly what you see in the casting. That won’t eliminate the need for cleaning, but a smoother finish will give you a helluva head start. Pouring colder should also give you a smoother finish as well but I wouldn’t know the lower limits for foam casting. I’m sure Kelly has done that testing.

    Pete
     
    Petee716, May 2, 2022
    #8
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  9. Melterskelter

    Melterskelter Gold Banner Member

    Another cleaning method that could be tried is to tumble it using fairly small media that would get into the crevices fairly well.

    Denis
     
    Melterskelter, May 2, 2022
    #9
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  10. Chassy

    Chassy Copper

    I'm curious - I think my molten aluminum was on the hot side. Is there something in the casting that made it appear so though? I'm thinking that hotter metal = slower solidification of the skin = more sand incursion.
     
    Chassy, May 2, 2022
    #10
  11. Zapins

    Zapins Gold

    Hotter metal is also more fluid and forces into small gaps between cracks and burns the sand onto surface and contracts more when it cools so you get more rips and tears.
     
    Zapins, May 2, 2022
    #11
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  12. Petee716

    Petee716 Gold Banner Member

    There wouldn’t be any way for me to tell that the metal was too hot in this casting (this observation is coming from an amateur). Your uncoated foam pattern cast in loose sand has given a surface finish that pretty much masks porosity or other defects you might see in overheated metal. I think Zapins observations above are exactly right, but I doubt it makes much difference if your patterns are uncoated. I just know that, in general, lower temperatures will give a smoother surface finish than overheated metal.

    Pete
     
    Petee716, May 2, 2022
    #12
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  13. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    As with conventional sand casting, required pour temperatures for lost foam castings can vary depending upon the nature of the part. Thin walled parts typically need higher pour temps than thick sectioned castings.

    In general, lost foam typically requires 100-150F higher metal temps than conventional open cavity sand casting. This is because it takes some additional energy to melt and evaporate the foam, but also because mold fill times are longer since the velocity of the metal front is paced by the evaporation rate of the foam, not just gravity and feed system cross section, and that allows more heat loss to the mold during the pour. Lower molten metal velocities can be an advantage for lost foam.

    When you add a refractory coating to your patterns, fill times may increase a bit because part of the purpose of the coating is to control the rate the evaporated foam escapes into the mold. It's also an additional insulation layer and will modestly reduce heat loss to the mold media. On small patterns this may be hard to notice. Besides replicating the pattern finish and preventing sand inclusions, the coating will also noticeably increase the stiffness of the pattern when dried which is helpful when molding, but, it will also dramatically increase the weight when wet, and on larger more fragile patterns, this can introduce the risk of distorting or breaking the pattern. I have threads on dip coating. Dip or brush, each have there place and pros/cons.

    My pour temps are typically 1350-1450F. Though I have certainly seen folds and various other casting flaws, I almost never see shrink defects in my lost foam castings. Part of this is because my castings tend to be fairly thin and uniform wall thickness and are top gated. Top gating would be very poor practice in most conventional open cavity castings because the metal would turbulently tumble into the mold creating many opportunities for casting defects, but in lost foam, the metal front just sits on top the foam pattern and (tranquilly if done properly) consumes it from top to bottom of the mold. The side benefit of top gating is the feed pressure increases as the metal approaches the bottom of the mold which is helpful since this is the coldest metal. Then you get great directional solidification from bottom up. This is probably the single biggest reason I rarely see shrink defects.

    The addition of an offset pouring cup is also very helpful in maintaining a tranquil pour.

    You should take the guess work out of pour temp and build or buy yourself a contact pyrometer to measure metal temp. This can be done inexpensively and there are a number of how to/source examples here on the forum.

    You had asked what kind of abrasive media is best. That can be personal preference. I use fine glass beads. Aluminum oxide is more durable and lasts a little longer. Coarser grits mean duller finish and greyer color for aluminum. The opposite for fine grits. Steel shot produces a shinier surface because it peens instead of abrades. (Fine) Wire wheel also tends to be shiny.

    If you want to remove carbon deposits, try oven cleaner. It will probably etch the aluminum too depending upon brand and exposure time.

    Best,
    Kelly
     
    Al2O3, May 3, 2022
    #13
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  14. Nick Lazenby

    Nick Lazenby Copper

    I’ve had some intermittent success using carbine burrs on my rotary tool. It’s tedious and a little rough but if you follow it with a quick acid “pickle”, the results are generally satisfactory. I’m mainly working with copper alloys but I’m sure aluminum would work.

    Speaking of rotary finishing; has anyone played around with a magnetic polisher? Basically a spinning disc with mounted magnets under a container filled with ferrous media? I have some strong magnets laying around and it seems easier to make since you won’t be fussing around with any RPM reductions (basically any motor should work)
     
    Nick Lazenby, May 29, 2022
    #14
  15. Smoking Shoe

    Smoking Shoe Silver

    YouTube has some useful/inspirational videos. One such found pretty easily:

     
    Smoking Shoe, May 30, 2022
    #15
  16. Zapins

    Zapins Gold

    Ya I made one several years back. It works well for silver castings. Never tried it for bronze. It is not abrasive. It just burnished what you put into it. It retains tiny fine details.

    Here is my build thread: https://forum.rocktumblinghobby.com/thread/78478/
     
    Zapins, May 30, 2022
    #16
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