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Thread: Shrinkage question

  1. #51
    Moderator DavidF's Avatar
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    Quote Originally Posted by cjcaster View Post
    I have seen a number of automotive engine blocks cast in online videos using bound sand, and there are no hot tear issues, even though some of these blocks are quite long.

    It depends on how many risers you use, with the intent that the riser is the last thing that solidifies, and the frame is allowed to solidify while drawing molten metal from the risers.
    unfortunately the metal is still contracting after it has cooled to where it can no longer draw from the risers. Thats the point when hot tares can occur.
    A calm sea does not make a skilled sailor....

  2. #52
    Senior Member cjcaster's Avatar
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    I am no "hot tear" expert, and not a casting expert either, so take all this with a grain of salt.

    Some research online indicates that hot tear can be a complex phenomena.
    The articles get pretty deep in science and technical terms.

    Here is one paragraph that I think supports what I was saying, but I can't be entirely sure:

    On the mesoscopic and microscopic level, the important factor is believed to be the feeding of the solid phase with the liquid.

    The source of the article is here:
    http://www.totalmateria.com/page.asp...ite=ktn&NM=204

    The article also states that locations of hot tears can often be predicted, and some examples are shown.

    My guess is that the reason old engines were designed with a constant thickness of the walls of the parts is to promote even cooling and solidification, and to prevent hot tears.


    This article mentions keeping your pour temperature as low as possible:
    http://www.unitedpmr.com/hot_tearing.php

    and I have noticed that my parts have far fewer defects and a much better surface finish (with aluminum) if I keep the pour temperature as low as possible while still being able to fill the mold without cold shuts.

    I have noticed hot tearing on some parts when I let the aluminum get too hot.
    I have re-poured the same part at a lower temperature and the problem was eliminated.
    I typically pour aluminum around 1350 F; slightly higher for a large complex mold, and slightly lower for a simple small mold.


    Here is a quote:
    Hot tearing occurs as a result of tensile strains arising in the “mushy zone” of the casting in regions without sufficient feeding during solidification. The right conditions can occur from (1) casting design; normal contraction
    of thinner sections causing deformation or restraint of thicker sections which cool more slowly, or (2) bulk resistance of the mold and core, which prevents contraction of a casting, or (3) the rigging design, especially if feeding is marginal, by providing additional restraints or due to the high thermal gradients that are often needed to achieve directional solidification. In a complexshaped casting, tearing may occur as a result of the combined effect of the aforementioned causes [1].


    from this article:
    http://user.engineering.uiowa.edu/~b...gemcwasp09.pdf


    Only one way to really find out if hot tearing will occur, and that is to pour the part and see what happens.
    I would say that without the right risering, and the right gating, you would be highly likely to have problems.
    But I do think this part could be cast successfully using bound sand, but I have no scientific basis for that; its just my hunch after having used bound sand for several years.


    Edit:
    I am not finding any articles specifically related to casting long thin parts, but I did find this information from Chapter XI "Causes and Cures for Common Casting Defects" in the Navy Foundry Manual:

    "Hot tears and hot cracks are both caused by improper design that does not provide adequate fillets at the junction of sections or joint sections of different thicknesses without providing a gradual change in section size by tapering. Inadequate fillets (sharp corners) produces planes of weaknesses at the junctions of the sections and cause failure at these points.
    Failure from improper joining of heavy and thin sections is caused by the early solidification of the thin section before the heavy section has solidified. The contraction of the thin section produces a stress which is greater than the strength of the partially solidified heavy section."
    Smarter, not harder.

  3. #53
    Fingers crossed! Try to bring us a video of the pour. They should let you watch. Tell them we made you! lol
    Jason
    Visit me: WWW.HandcraftedLanterns.com
    "Sometimes by losing a battle you find a new way to win the war"
    -- Donald Trump --

  4. #54
    Moderator DavidF's Avatar
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    (2) bulk resistance of the mold and core, which prevents contraction of a casting
    This is the concern i have with this particular casting. It is a very long casting and will continue to contract long after the risers stop feeding. The frame does have a pretty thick cross section though, so maybe my concerns are unwarranted. Im just going to wait and see the finished casting at this point
    A calm sea does not make a skilled sailor....

  5. #55
    Quote Originally Posted by DavidF View Post
    This is the concern i have with this particular casting. It is a very long casting and will continue to contract long after the risers stop feeding. The frame does have a pretty thick cross section though, so maybe my concerns are unwarranted. Im just going to wait and see the finished casting at this point
    Yeah, me too! I wish I could be there for the pour, but chances are, I won't be able to be there. I have 4 months to finish this SEMA car...

  6. #56
    Moderator DavidF's Avatar
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    Quote Originally Posted by 71buickfreak View Post
    Yeah, me too! I wish I could be there for the pour, but chances are, I won't be able to be there. I have 4 months to finish this SEMA car...
    Hope you stick with us after the casting. Many of us are car guys as well. If you can get a pick of the mold though, that would be great!
    A calm sea does not make a skilled sailor....

  7. #57
    I planned on asking for pics if I can't be there.

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