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Thread: Shrinkage question

  1. #41
    Senior Member Wolfcreek-Steve's Avatar
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    Quote Originally Posted by DavidF View Post
    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"
    You have a long thin part that is constrained at each end by bonded sand, something has to give, how can you avoid hot tear. This looks like a worst case scenario for hot tears
    What is that squeaking noise?

  2. #42
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    Quote Originally Posted by 71buickfreak View Post
    ....I have enough factored in to build 1 frame with a casting.....
    I get your reasoning behind cast v fabricated, but I think you need to step back and take a look at what it will cost if your 1 off casting doesn't work.
    Quote Originally Posted by 71buickfreak View Post
    This is a huge undertaking, and I am cautiously optimistic. Otherwise, I am throwing $1600 bucks in the trash....
    Lots of experienced casters here have thrown in there 2 bobs worth, mostly advising against casting it. I think you at the end of the day you need to think about what might happen if the casting fails. What then? Dont get me wrong, I would love to see this work. Its just the big BUT.

    Cheers Phil
    So, whats your Plan B?

  3. #43
    These comments are more from the pattern makers perspective but I had a three decade relationship with a friend that owned a foundry that did virtually all my castings for me. More recently when the foundry was sold, I built my own home foundry.

    They had a resin bonded air set line and in most cases, they would choose to do my one-off and small volume parts on that line. The reason being it was easy to do without a lot of rigging or special tooling. In most cases they would prefer a loose pattern with a follower. They would mold one half and then remove the follower, coat the cured resin mold and backside of the pattern with several coats of graphite and then make the other mold halve. The follower usually had alignment features included to self align the mold halves.

    They would cut the runners and gates afterward on the cured mold with a die grinder and burr, and sand paper.

    They cast many large parts on that line, large as in flasks that were 6-8 ft square and a 1- 1.5 ft deep. The ability to cast a shallow depth was one of the advantages to the air set line and their first pass success rate was high; not perfect.

    I definitely agree that hot tear is a concern. Besides how the part is fed it can also be a strong function of the shape of the part and how easily it can retract from the drafted portions of the mold. The resin will break down locally in the vicinity of the molten metal and allow some movement but not very deep into the mold. I would say if the foundry you are using has seen this part, and is experienced and well versed in with the resin bonded sand and one-offs, I bet they succeed......maybe not on first shot, but maybe so. How accurately the shrink is estimated may be a different matter but again depends on how easily you can tune the resulting part for fit.

    All I was saying in my previous post was I wouldn't consider doing the job for hundreds of dollars or even the price you subsequently mentione....at least not if the objective was to make money. It's a lot of development work and risk for no reward because it's a one-off and no chance for future income.

    I think a lot will depend on how they gate and feed the part, but again, if they've run a lot of parts experience will be their friend. The dimensional accuracy of parts cast in the resin bonded line were outstanding, provided they were left to cool in the mold. Consequently they would be soft castings but some alloys were much better.

    One thing that I never fully understood was they did shrink less than green/oil sand molded parts; not a lot less, but measurably less. There is the obvious reason, that being the rigid mold, but at some point the casting solidifies and must shrink per the metals coefficient of thermal expansion. The only thing I could come up with is the castings would directionally solidify and cool, while some portions were still being fed.

    Good luck and let us know how it goes.

    Best,
    Kelly
    My furnace build ----- I toil and fettle then foam turns to metal!

  4. #44
    Moderator DavidF's Avatar
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    Quote Originally Posted by Wolfcreek-Steve View Post
    You have a long thin part that is constrained at each end by bonded sand, something has to give, how can you avoid hot tear. This looks like a worst case scenario for hot tears
    You can add in a void into the mold to thin down the inner wall so that the mold can collapse in more easily. Also keep in mind that the heat of the melt will break down some of the binder as well.
    Really just need to pour the sucker and see what happens and if any adjustments to the mold will be necessary. Easier to learn from a fail than to speculate on the "what if" scenario. Hot tears can be welded up pretty easily anyways.
    A calm sea does not make a skilled sailor...
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  5. #45
    Moderator DavidF's Avatar
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    Watch this video. it shows how the mold is prepared. No hard ramming or jolt squeeze like in green sand casting.
    A calm sea does not make a skilled sailor...
    http://thehomefoundry.org

  6. #46
    Senior Member Wolfcreek-Steve's Avatar
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    Historically these windshields are cast in pieces and welded together.
    Go to Google images, type in Hallock windshield.
    Hallock-Windshields~~element52.jpeg Face_down.jpeg images.jpeg
    What is that squeaking noise?

  7. #47
    The long runs are roughly 1.5" square, there are only two areas where it is thicker than that, and that is 1.87" x .5" x 2". In my discussions with the foundry, they will be using multiple sprues at key points to accommodate the shrinkage, which in theory should reduce the potential for hot tears. It is still possible for it to happen, but in my research, the best method for casting this type part with a high dimensional tolerance and surface finish, resin sand is the only way to go.

    Besides that, the few shops in Oklahoma that still use green sand don't have the ability to cast something this large.

  8. #48
    Moderator DavidF's Avatar
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    Quote Originally Posted by 71buickfreak View Post
    The long runs are roughly 1.5" square, there are only two areas where it is thicker than that, and that is 1.87" x .5" x 2". In my discussions with the foundry, they will be using multiple sprues at key points to accommodate the shrinkage, which in theory should reduce the potential for hot tears. It is still possible for it to happen, but in my research, the best method for casting this type part with a high dimensional tolerance and surface finish, resin sand is the only way to go.

    Besides that, the few shops in Oklahoma that still use green sand don't have the ability to cast something this large.
    Any ETA on a pour date?
    A calm sea does not make a skilled sailor...
    http://thehomefoundry.org

  9. #49
    Quote Originally Posted by Wolfcreek-Steve View Post
    Historically these windshields are cast in pieces and welded together.
    Go to Google images, type in Hallock windshield.
    Hallock-Windshields~~element52.jpeg Face_down.jpeg images.jpeg
    Yes, many are cast in pieces. If I have to, I will do it that way, but the original frame was cast as one piece, so I am going for the one-piece option for now.

    - - - Updated - - -

    Quote Originally Posted by DavidF View Post
    Any ETA on a pour date?
    Delivering the pattern on Tuesday.

  10. #50
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    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.

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