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Thread: Shrinkage question

  1. #31
    OK so I spent a few hours with the foundry and they are going to do the job. The trick, however, is that they don't know what is going to happen with the center dimension in terms of shrinkage. They have not done a part quite like this before.

    They use resin bond sand, so there will be a hard surface on the interior of the window frame. That should in theory preclude it from shrinking much along the horizontal lengths. I suspect the shrinkage will occur along the outer edges. It could pull away from the interior edge as well. I need to grow the part for shrink, so I need some ideas. If this were a green sand mold, the part could shrink along the length, but the bonded sand is like concrete, I don't think it will shrink there. I have included a couple of pics, one with my thoughts on where it will shrink, and one non-marked pic so you guys can mark it up if you like. I appreciate your thoughts. I have one shot at this, I don't get a second chance. I seriously do not think it will shrink on the horizontal, there isn't anywhere for it go.

    Frame.jpg

    Frame with mark up.jpg

  2. #32
    Quote Originally Posted by jagboy69 View Post
    Then get this guy to build it for you in Aluminum. Pay the money and have Marcel Delay's shop do it. You should have heard of him. The 41' Continental cabrio is a nice car, love the front, hope you do something cool with the other end. I'm afraid if you cast this thing, it might look like it came off an MGB! Most people associate cast with CHEAP! Agreed they don't know what is involved with casting a trinket, but they automatically assume you're turning these out in cheap quantities in the not so nice part of town.

    Marcel's  
    Auto Body Shop
    Address: 1621 Commerce St, Corona, CA 92880
    Phone (951) 371-2290
    I appreciate your suggestion, but Marcels would be even more expensive than my guy. I can't do 6k, or even 3k for the frame, that is too much for my budget. I have enough factored in to build 1 frame with a casting. It will certainly not look like an MG...

    While many people may associate cast with cheap, they don't when it comes to custom parts. The actual estimate for the project is $1100 for the casting itself, and about $500 for the setup including the custom flask they have to build, along with the hand gating.

    This is a huge undertaking, and I am cautiously optimistic. Otherwise, I am throwing $1600 bucks in the trash... Hence the need for educated estimates on where I need to grow this thing.

  3. #33
    Senior Member
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    That is going to be one beautiful piece.
    My understanding for a dimensionally correct piece, you have to use a shrink rule when designing the pattern. They come in various rates for each metal.
    Aluminum is 3/16" per foot.

    http://www.ebay.com/itm/Starrett-24-...IAAOSwxu5ZIupI

    Good luck with this.
    Bones

  4. #34
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    With a 'hard' mould you risk hot tears in the metal as it tries to shrink, could take some very clever gating and bobs to get this one right.

    Keep us updated, would be very interested to see how the foundry handles this one.

  5. #35
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    Aluminium is a "hot short" metal. Weak and crumbly at elevated temps. The ablity to collapse during the freezing of the metal is an important characteristic of casting sand (normally more a consideration for core sands). If that resin bonded is too strong to collapse, then you risk your frame is coming out of the mould in peices.

  6. #36
    Senior Member Wolfcreek-Steve's Avatar
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    I would never try to mold that in resin bonded sand, but if the foundry plans to do it that way, one pretty much has to let them try. (On their dime) Don't let them destroy your pattern and don't pay for anything until you have a good/usable part
    What is that squeaking noise?

  7. #37
    And what is your reason for not using resin sand? Every surface has a minimum of 2 degrees of draft, with the exception of the cowl surface, which is being layed up for a follower to be removed after the front surface side of the mold is created.

  8. #38
    Moderator DavidF's Avatar
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    Nothing wrong with resin bonded sand provided you have the proper draft. Pretty much standard commercial foundry stuff now a days.... I think its chocolate cake time..
    A calm sea does not make a skilled sailor....

  9. #39
    Quote Originally Posted by DavidF View Post
    Nothing wrong with resin bonded sand provided you have the proper draft. Pretty much standard commercial foundry stuff now a days.... I think its chocolate cake time..
    That is my thought, and resin bond typically yields better dimensional tolerances than green sand.

  10. #40
    Moderator DavidF's Avatar
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    Quote Originally Posted by 71buickfreak View Post
    That is my thought, and resin bond typically yields better dimensional tolerances than green sand.
    Same dimensional tolerances, just does not require the molding pressure green sand does. The metal shrink rate is consistent dependent on the alloy. If the metal can not break down the mold as it cools then you get hot tearing. Its kind like the laws of gravity....or simply "shit happens"
    A calm sea does not make a skilled sailor....

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