Casting defects on two identical castings

Discussion in 'Lost foam casting' started by Maker, Jul 7, 2024.

  1. Maker

    Maker Copper

    Hello, I'm Dietmar from Germany.

    Today I cast aluminum with lost foam for the first time and I have the same error with two identical castings. As can be seen in the photos, the casting is interrupted.

    I think I placed the sprues wrong, right?

    The model has partly very thin walls, see photos.

    I let the drywall coating dry for 3 days and used dry quartz sand and shook it very well.

    I would be happy to hear your opinions...

    greeting

    Dietmar
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  2. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    Hi Dietmar.
    • Those are pretty obvious cold shuts and occur where one would expect...in the thin sections. Just increase the pouring temperature. I'd think 760C should do it.
    • What are you using to measure pour temperature?
    • With the proper pour temp, the feed system should be more than adequate, and some could probably even be removed.
    Other factors could be your alloy, foam density, coating thickness......but I suspect it's just the pour temp.

    Best,
    Kelly
     
  3. Maker

    Maker Copper

    Hello Kelly,

    many thanks for the quick reply.

    I am a bloody beginner and am always learning more about metal casting ... most of your videos!

    I measured the temperature of the melt with a standard infrared temperature meter, which showed me 700 ° G (1,300 ° F) for this casting.

    This temperature has so far been optimal for my sand casting (see photos, Alloy-cast with very fine structures details).

    Unfortunately, I can't measure in the melt because I don't have the right measuring device. I'll have to build a device for that soon.

    For the foam casting I use the same alloy as for the sand casting, it is melted alloy car cylinder head.

    So I'll try again with a higher temperature.

    I still have one question: which part of the feed system do you think could still be removed?

    Best,

    Dietmar

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  4. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    Yes, but lost foam casting will requre higher pouring temp than an open cavity sand casting for several reasons. It takes some energy to evaporate the foam pattern but moreover, the evaporation rate of the pattern is what dictates the rate at which the mold fills which is always slower than in a conventional open cavity sand casting. Than means the melt has more time to lose temperature to the mold surface. Very thick, less permeable, coatings aggravate this because it further slows the rate at which gas can be expelled to the mold, and further increases pouring/mold filling duration.

    It can be very difficult to get accurate readings with IR thermometers. The reflectivity (emissivity) of the molten metal surface varies a lot and other surfaces in the furnace and crucible can create a lot of interference and error. Still if you feel you get consistent results with IR thermometer, just use your method at a higher indicated temperature, but I recommend a k-type TC in a protective sheath for best accuracy.

    Comment 1: This center section should help insure complete fill but it probably complicates degating. It could potentially be removed or alternatively, keep it, and remove the outer two contact points. But how you presently have it gated would probably yield lower possible pour temp.
    Comment 2: I'd keep this part of the feed system, but you can probably cut down the contact area for easier degating.

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    One should always strive for lowest possible pour temp for best potential (aluminum) metal quality. The pour temp just cannot be lower than possible to fil the casting :)

    Best,
    Kelly
     
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  5. Maker

    Maker Copper

    Do you mean that (left illustration)?
    Or would the casting process in the illustration on the right be optimal?
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    Best

    Dietmar
     
  6. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    Yes, or as shown on the right but keep the white foam that connects to the bottom of the casting like shown on the left image.

    Either of these or your original method would work at higher pour temp.

    Best,
    Kelly
     
  7. Maker

    Maker Copper

    Unfortunately, the second attempt with the improvements to the sprues suggested by Kelly were not successful.

    I increased the pouring temperature to 760 degrees and added soda (for degassing) and borax (for better flow) to the melt.

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    Is it possible that a collision occurs as the melt flows and a cold skin forms?
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    In my opinion, the problem should not be with the coat, because the coat still adheres well to the model even after casting.
    Only a small amount of coat residue was found in the sieved sand
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    What else do I need to improve so that the casting is successful?

    Best

    Dietmar
     
  8. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    A long response. Despite all said below, I think with my processes and materials it would still successfully cast at 1400-1450F. I use A356 alloy. Sometimes it can be difficult to pinpoint the process differences that cause failure.

    Yes. It's still presents as a cold shut failure simply meaning that is froze before filling completely. The rounded and shiny edges at the interruption are confirming features. Increasing pour temp is the usual solution but, in this case, the heat required is being driven by the casting design, and that design is pushing you into uncomfortably higher temp ranges for aluminum. The problem with the design is the thinnest, highest surface area, portion of the part is the furthest from the gating/feed metal.

    Plastic molders would call the point where streams meet knit lines. I do occasionally get defects at knit lines (collision points as you say) and sometimes they contain oxides.

    Another contributing factor in lost foam casting is the presence of more vapor where two streams meet causes a larger amount of vapor, which takes longer to vent, and allows more time for heat loss and freezing. 4mm is getting to be pretty thin for (any type) of sand casting.

    Possible areas of improvements:
    1. Further increase pour temp. If it is not a critically stressed part, this is the easiest solution, but metal quality may suffer.
    2. Increase minimum wall thickness from 4 to 6mm
    3. Don't increase wall thickness and add an additional feed stream at the point of failure. You'd prefer a single feed direction for metal flow, but it isn't possible with a part of this design and the selected pour temperature.
    4. Also, possibly add a "bob" to the bottom of the cylinder at the point of failure to promote flow through this problem area. This probably won't help with one or more of the above suggestions, but it can move the metal and possible defects where the streams meet out of the casting.
    Other possible contributing factors and questions:
    1. Casting stock: Even though it is an automotive cylinder head, it may not be a great alloy for this casting. It may not have high fluidity aggravating the other factors. Some automotive parts are selected for other qualities, such as having sufficient properties without post process heat treating. The melt point is also not well known.
    2. Is the yellow foam polystyrene or polyurethane? What is the foam density? Higher density foams (>1.5lb/ft3) will create more gas to vent aggravating the failure mode.
    3. I'm still suspect of the pour temp accuracy. I've never been able to get IR and contact measurements to agree for molten metal measurement. But maybe yours is better suited. I don't know. But if not, this becomes an investigation based upon false information, and is merely due to cold pour temp.
    I like the uniformity and quality of the coating dip coated part.

    Best,
    Kelly

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  9. Maker

    Maker Copper

    Hello Kelly,

    thank you for your valuable suggestions, they help me to think further!

    I do believe that this alloy has good flow properties, because even very narrow webs were filled cleanly during sand casting (see photo). But with foam pouring, that maybe not enough flow?
    IMG_20240709_181549.jpg


    To foam quality: I use this polystyrene (thickness 60 mm) , see data sheet. It's has higher (double) density as yours, right?
    Datenblatt Bachl XPS 300C-SF.JPG


    I'll take up your idea...if I forego this large hole when casting, there would be a much larger (hotter) amount of material in the critical lower area. Am I thinking correctly?
    I have to cut these holes to the correct size on the milling machine anyway, as well as the front hole.


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    And if I supply this point with a 3rd connection, that should work, I think...
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    Best

    Dietmar
     
  10. Tobho Mott

    Tobho Mott Administrator Staff Member Banner Member

    I didn't think borax worked as a flux for aluminum castings. I could be wrong. Anyone?

    I've read it's used as a flux for copper alloys sometimes, but I won't pretend I understand all the chemistry that's at work.

    Jeff
     
  11. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    If the webs are only .085"/2.2mm (not 4mm!), I think that explains the problem. You can fill the mold much faster in open cavity sand casting because the fill rate in LF is dictated by the rate foam can be evaporated, which typically results in a significantly slower fill rate, thus more exposure to the cold mold and heat loss. When I cast this .063" thick casting I had to using vacuum assist and pour at a silly high temp of 1700F.

    Saucer Cup & Spoon – The Lost Foam Edition | The Home Foundry

    Yes, for sure.

    The foam density of 31-34kg/m3 is 1.9-2.1lb/ft3. That is higher density. I will not use higher than 1.5lb/ft3. So although not far out of range, may be a contributor to slower fill rate.

    I blew right by that. Agree, never heard of Borax with aluminum. I doubt it's soluble in aluminum because little is except H2. I know there is a popular YT caster that suggests such, and don't want to reignite that debate, but I don't think soda is an effective degasser. Not sure that has anything to do with the cold shut problem though.

    Best,
    Kelly
     
  12. Maker

    Maker Copper

    you are right, borax for alloy is total nonsense, I made a mistake!

    Today I made the 3rd attempt to improve the sprues
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    Then came the exciting moment when unpacking the cast...that looked good.

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    You can see that the melt was very hot from the burnt coat...but it worked. The sand has also become dark and damp from the evaporated polystyrene.


    That looks really good, I'm really happy. Now the machine work begins

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    I have another question about how to get the surface of the polystyrene model smoother. Sanding with sandpaper? Grid?

    By the way, the part is for a French 1920's auxiliary motor bicycle, see photo red marking. It is the holder on the handlebars.
    I have already cast the green part (Gasolin-Tank mount) using the sand casting process, see report above.

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    I have one last question: could this part also be made using sand casting? It's not yet clear to me how I would have to attach the required cores, so I still have no idea.

    Thank you again for your support from this forum!

    Best

    Dietmar
     

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  13. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    I typically use 220grit, but the finish can usually be improved up to 320 grit......both modestly used and worn.

    Yes, but as you have noted, it would require a complex parting line and core prints. That is one of the strengths of lost foam as neither of those two subjects are issues. With the hollow holes filled in, I doubt whether the bottom gating would have been necessary.....doesnt hurt anything but does require some more degating work. I will rarely try to core a hole less than say .75" ID, especially if it must be post machined anyway.

    Just sieve out and discard the chunks and wet sand. You want your sand/mold media to be very dry so it moves easily when vibrated. You may have a little more debris since your foam is somewhat denser. Because it is more dense, it is likely stronger, and will machine and sand better.

    I'd still recommend you make yourself a contact pyrometer. Accurate metal temp is important capability.

    Congratulations. It looks like a worthy project.

    Best,
    Kelly
     

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