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Thread: Shrinkage question

  1. #1

    Shrinkage question

    frame top view.jpg

    Long story short- I am having a windshield frame cast and I am trying to calculate shrinkage. The pattern was designed on the car in actual finished dimensions. I can't really go back and add width to the overall length. I deliberately designed the long horizontal runs thin with shrinkage in mind. A friend of mine is concerned the overall pattern could shrink 3/4-1" over the length when cast. Here are the specs:

    Aluminum

    top length 52", roughly 3/4-1" deep, 2" height

    bottom length 56", roughly 3/4-1" height, 2-4" deep

    The verticals are thicker and wider, but is it 15" high, I can live with .25" in vertical shrink, but I can't have it shrink that much horizontally.

    I would appreciate your input.

    frame pass side.jpg

    frame driver side.jpg
    Last edited by 71buickfreak; 06-12-2017 at 02:06 AM.

  2. #2
    Senior Member Robert's Avatar
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    You did not specify but I assume you are talking about aluminum. 1.5% shrink would be typical. So for your pattern you could expect 3/4 to 1" shrink in the length. I think your friend is on track. Could you cut the pattern on the straight parts and add length?
    Robert
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  3. #3
    Yes, aluminum.

    No, I can't add it in. There is too much potential for altering the lines. Even though it is fairly thin, you think it would shrink that much? It has to follow the curve of the cowl, and there two vertitcal legs that line up with the a pillar, hence the concern of shrink and how much. The legs will be welded in after casting, so i have a little room, but not 3/4. I could live with .25.

    Is there a better material that might shrink less? I do a lot of polyurethane casting, so shrinkage is not something i deal with often.

  4. #4
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    Welcome to the forum. You're definitely going to get contraction no matter what you do. You didn't mention the metal but I'm assuming aluminum. I usually estimate 1/8 per ft. It will not only reduce your length and width but will also reduce your windshield cavity dimensions. Other than modifying the pattern I don't think there's any way around it.

    Pete

  5. #5
    Do you think the foundry will be able to give me a precise shrink rate? If I am to modify the tool, it has to be within an 1/8" or so.

  6. #6
    Im not meaning to be obtuse, i just wasnt expecting that much shrinkage, and widening it that much will be very difficult. And if i widen it too much, that creates a whole other problem.

    I appreciate your input.

  7. #7

  8. #8
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    I use 1.5% also, and that is generally what I get.

  9. #9
    Does shrinkage vary by the method of casting used????? I know all metal shrinks as it cools, but suppose this thing was cast lost wax ceramic shell. While pricey, I can't see it shrinking an entire inch! If it did, the shell would shatter as the aluminum cooled. Penny for your thoughts?
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  10. #10
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    Either the shell will fail or the casting will be flawed one way or another regardless of the method used. Here's an interesting read. It pretty much presents the factors right at the beginning.
    http://nvlpubs.nist.gov/nistpubs/jre...8n1p37_A2b.pdf


    Pete

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