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Thread: core deflection

  1. #1
    Join Date
    Nov 2018
    Posts
    1

    Exclamation core deflection

    Hello Everyone,
    looking for some advice from all you process wizards out there. i have a 16 cavity mold making 3" long pipettes. The mold is hot to cold and has 2 gates on each tip. Issue we are having is core deflection. my process guy has tried everything and he is still not able to provide 16 good tips. best he can do is 10 and the balance are deflected so bad they are shutting off on one side. we are using a 35 melt polypro material., we have also tried a 50 melt polypro but still had similliar results.
    mold is 2 quadrants of 8
    could this be a venting issue? any suggestions will be helpful.

  2. #2
    Join Date
    Feb 2016
    Location
    San Francisco Bay Area, CA
    Posts
    140

    Re: core deflection

    Are you seeing actual core deflection or tip runnout?
    You can verify this by checking the wall thickness along the circumference of the part, from base to distal end. If wall thickness is relatively consistent ( +/- 0.003" or so) then it's likely runnout. If wall thickness varies greatly around the circumference it is likely core deflection.

  3. #3
    Join Date
    Aug 2014
    Location
    Michigan
    Posts
    70

    Re: core deflection

    A couple of questions, is it cavity specific (always the same 10 cavities good, or different all the time)? What is the diameter of the core? You are going from a hot drop to a cold drop so this may not apply here but just to be thorough, has any flow grouping, flow balance or shear histories been studied on this mold? Have ALL of the gates been measured with gauge pins?

    And a couple thoughts, a general industry rule is the length of the core should be equal to or less than 5*Core_Diameter to prevent deflection. Polypro is a semicrystalline material so your best chance of lowering viscosity is going to be through shear (faster rotation on recovery, faster injection speeds and small orifice nozzles (I know the last 2 sound counter-intuitive as most of the time these 2 will also increase pressure but not always)).

  4. #4
    Join Date
    Jul 2012
    Location
    Belle Vernon, PA
    Posts
    119

    Re: core deflection

    Definitely check wall thickness first. Also, as Husky said, same cavity or various? I'm assuming that this part is cored all the way though (as in, both ends of the part have an opening)? If so, how are the cores shutting off against the other mold half? We have a tool that we run with 1" Dia x 9" cores, and they have a boss sticking out past the end of the core that seats in the other mold half, that's how we are able to get around end deflection.

    I'd also start by not packing the parts as much and see if you still have an issue. Also check the physical cores to see if they're possibly bent.

    Just some ideas.

  5. #5
    Join Date
    Aug 2011
    Posts
    560

    Re: core deflection

    In the past, I had a similar issue. Investigating further I found that it was the packing pressure and time that influenced the core shift.
    Makes a LOT of sense if you think about it!
    Suhas

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