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Thread: Help!, Warped walls with PP

  1. #1

    Help!, Warped walls with PP

    Hello,

    I have a part, about 2" x 3" x 2", and if you see the picture, the walls are warped in. This was not the case with the original "tool shop parts" but nothing I have tried has had any affect on this, other than lots of cool time, which puts us well over quoted time.
    (The tool shop parts, from China, did have some warp, but is at least 50% less than ours.)

    The TooL Shop process says it was at quoted cycle time, and has common process parameters, which I did try to match , some of it was difficult to understand, .... I do think I was 15 F cooler on barrel temps.......... so I thought I would poll the experts and see if I missed something.

    Material - Low flow. mineral filled, homopolymer PP
    4 cavities (all look the same with respect to warpage)
    shot size capacity of barrel - 85% (yes, it's high, could that be the issue?)
    Barrel temps, all 420F
    Water, (everywhere!, but ended up at 80F/80F)
    Pack PSI ( high and low)
    Fill Time (fast and slow, but ended up at 1.6 sec)

    So, I believe the warpage is just natural shrinkage of PP, and I would expect the results seen, but the Tool Shop parts have thrown me a giant monkey wrench!

    Thx!,
    Joel

    2 pics attached, one is showing the warpage in question, the other is showing where the gate is located. The gate is a tunnel gate.
    Attached Thumbnails Attached Thumbnails gate pic.JPG   Walls Warped.JPG  

  2. #2
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    Re: Help!, Warped walls with PP

    Couple things come to mind

    1) warpage comparison at 10% of gate seal time vs 120% of gate seal time (ex: 0.2s vs 6s) - to understand if the added pack pressure is resulting in stress in the part, or if you're packing too much volume near the gate, resulting in less shrink near the gate (higher packed volume) and more shrink at end-of-fill (less packed volume) *This especially with that mineral filled PP. *** But be mindful when you run Long Pack time to proportionally decrease cooling time to keep both cycles the exact same!

    2) cold mold temp and longer cooling time will help 'quench' the plastic into steel position - but be mindful of parts warping later on in warmer temps
    Cooling time study: 5s cooling, 10s cooling, 15s cooling etc. Ultimately you will eventually get a part that comes out straight even if you have to go to 2 minutes cooling time

    3) Increase back pressure and slow fill time to get as much plastic volume at end-of-fill as possible

    4) and of course uniform cooling for fastest cycle times

  3. #3
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    Re: Help!, Warped walls with PP

    Run the A-side 10-20*F hotter then the moving half. That will relieve some of the stress that is causing the part to collapse inward.
    Rick.

  4. #4

    Re: Help!, Warped walls with PP

    Thx for the feedback, this one has got me puzzled

    I did try differential water and the parts looked the same.... I did think it would help, but nope.

    Chris, could you please explain the "this is especially true with mineral filled PP" part. Thx!

    Also wondering if it would produce better results in a larger screw/smaller shot capacity (?)

    Thx tremendously for your feedback!
    Joel

  5. #5
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    Re: Help!, Warped walls with PP

    Quote Originally Posted by Joel JS View Post
    Chris, could you please explain the "This especially with that mineral filled PP. Thx!
    "Low Flow" mineral filled PP - suggesting there may be an increased pressure delta from gate to end-of-fill contributing to warpage. At least that's how my brain works.

    Difficult to say for sure if shot capacity could have anything to do with it!
    Running a larger screw (larger diameter) and larger nozzle ID would help maintain plastic pressure throughout the system.. If your diameters are too small (screw, nozzle, sprue/runner, gate size) you could be running the system too lean increasing pressure change even more. However if your machine has sufficient pressure and your recovery/fill/cushion data looks consistent you're probably fine. What is your gaze size btw? Wouldn't be a bad idea to bring that 85% capacity down - seems residence time would be rather short depending on the cycle time.

    I mean I'd image some warpage with that part for sure.. Even those small radii where the sidewalls connect to the body are probably contributing to warpage.
    Aside from having perfect specific mold temperature distribution around your part to help warpage, really you just need to decrease shrink in every possible way.. Lower mold temps (go even colder if you can), lower melt temp, packing the snot out of it, increasing back pressure, decompress just enough to allow you the check-ring to set. Trying a profiled pack and hold - a lot of pressure at first then ramping down to gate seal

    Or just run longer cooling time and have it re-quoted, or get a fixture to put parts on after dropping from the mold

    How do you know the original tool sampling wasn't running with a longer cooling time?

    Let us know how it goes!

  6. #6

    Re: Help!, Warped walls with PP

    Thx Chris for the response,

    The tool shop process says 29 seconds cycle time, we must add 10 seconds to that to come close to their parts in warpage, but more like 17 seconds!....... I suspect foul play, yet the tool shop was never pressured to produce parts at 29 seconds without warpage, so there's no reason to lie......(???).......As Charlie Brown would say.....aaauuuggg!!!

    I ran parts at Chiller/Chiller, but they looked the same, so I decided to leave it at Tower/ tower,.... I will re-read your post again later, and maybe give it another try.

    Thx again for the reply!!!
    Joel

  7. #7
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    Re: Help!, Warped walls with PP

    Are they warped right out of the mold or after they sit for a few minutes?
    I still say the outside steel that forms the legs needs to be hotter to allow the molded in stress to move them outward. With mineral filled resin I think a cold mold is wrong. I’d warm the entire mold up and especially that forming the outside of the legs.

    Rick

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    Re: Help!, Warped walls with PP

    BTW did you check parts to see if they ran them with CFA or blowing agent in them? That would alleviate some of the warpage.
    Rick.

  9. #9
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    Re: Help!, Warped walls with PP

    It is mainly a part design problem.
    To get good parts, you meed to have a good part design, the right material, a good mold design, the right machine and then it is the process. These are the 5 pillars.
    All the best. The solutions and the discussions above are great! Keep going guys and thank you for your contributions.
    Suhas

  10. #10
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    Re: Help!, Warped walls with PP

    Any colorant involved?

    KOM brent

  11. #11
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    Re: Help!, Warped walls with PP

    Suhas is absolutely right, that design sucks. Processing that warp out is going to be a daily struggle if you can do it at all. I would recommend you change the design if possible (I am sure it is not), so next, I would add windage if you can and if you can't go with a cooling fixture provided your part isn't going to see higher temps in the field that would cause it to warp back.

    This is exactly why part designers, mold designers, and process engineers need to have a party, I mean a meeting during the design phase to keep everyone in the loop and to help weed out designs that will cause major issues later on.

    If all else fails, blame the mold designer!

    Good luck with this one.

    Husky

  12. #12
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    Re: Help!, Warped walls with PP

    Just one more thought,

    Your running PP which is semi-crystalline so you will have orientation affecting warp. Do you know what size nozzle tip the previous molders were using? The smaller the nozzle tip diameter the greater the shear and orientation, the larger the nozzle tip diameter the lower the shear and orientation. Its an easy theory to test.

    Also, do you know if the previous molders used any tricks like cooling fixtures?

    That's all I gots,

    Husky

  13. #13
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    Re: Help!, Warped walls with PP

    Husky,I bet they made some sort of cooling jig or inserts that they snapped into the parts as they sat on the table! Also did we ever find out where the water lines are at in the tool? Maybe some creative water lines or even a blowing agent could get you running better parts.
    Rick

  14. #14

    Re: Help!, Warped walls with PP

    Thanks to everyone for their replies!

    We are actually waiting to hear back from our customer...... the parts may be acceptable with the warp as they are a throw away protective covering!.....

    But if it is not, I will try some of these ideas above.

    FYI

    - Precolored Grey
    - I can bend the parts (opposite the warp direction) a little when they come out and they end up much flatter
    - Not sure nozzle tip they used.
    - They say they ran the same the material, it looks the same for whatever that's worth.

    Thx, Joel

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