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Thread: core deflection

  1. #1
    Join Date
    Nov 2018
    Posts
    4

    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
    148

    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
    71

    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
    121

    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

  6. #6
    Join Date
    Nov 2018
    Posts
    4

    Re: core deflection

    Thanks for the info, our core pins are piloted into a button on the a-side of the mold. but it is still deflecting? the dia is 1/4 dia x 6" with a .060 wall thickness. Could this be a venting issue?

  7. #7
    Join Date
    Nov 2018
    Posts
    4

    Re: core deflection

    Thanks, i will try using a smaller nozzle and generate as much shear as i can with screw speed and smaller nozzle. Inj speed is at max. would a higher flow polypro help? currently we are using a 35 melt polypro.

  8. #8
    Join Date
    Nov 2018
    Posts
    4

    Re: core deflection

    It is definitely core deflection. It deflects so much that it will make a hole in the side of the part. Wall thickness varies a lot around the circumference of the part, especially down towards the end of fill.

  9. #9
    Join Date
    Aug 2014
    Location
    Saint Paul, MN
    Posts
    142

    Re: core deflection

    Quote Originally Posted by dantesoro View Post
    our core pins are piloted into a button on the a-side of the mold.
    Probably a stupid question, but are you sure the core pins are in clearance at the distal end? It wouldn't take much interference to cause significant deflection on mold close.

  10. #10
    Join Date
    Feb 2016
    Location
    San Francisco Bay Area, CA
    Posts
    148

    Re: core deflection

    Seems odd it would deflect that much since it's dual gated. Maybe steel alignment issue. See if you can determine the orientation of deflection and reference it to the gate or to specific cluster area.
    How all cavity deflection is orientated will give you a great deal of information if all cavities in the mold are orientated the same way it could be mold alignment issue, or machine platens are not parallel. etc.

    To check if deflection is injection related, make 95% fill-only parts (or even 70% full) at a very slow injection speed and apply NO pack pressure.
    Since you're not 'forcing' the plastic in the cavity and applying no pack pressure, the cores should not deflect.
    Do this and incrementally increase fill speeds and note if cores are deflecting.
    If cores are still good then add light pack pressure and short pack time - keep increasing if necessary and determine when/if they begin to deflect.

    That said tip molds are very difficult - I've worked with them more times than I would've liked to :P venting does play a big role but I would first investigate the above so you can rule it out.
    There are a number of tool shops I've worked with that have tried and failed to make good tip molds..
    The best I know of are by a Swiss company Tanner molds. Some cool stuff. If you ever get a new tip mold I'd recommend them

    here are some cool videos guys running Tanner Molds - they seem to be pretty big in Europe on this stuff.
    They're running under 5 second cycle times; At previous company we had them around 7s-8s usually limited by machine capabilities
    https://www.youtube.com/watch?v=uDRJofeMlfo
    https://www.youtube.com/watch?v=fZ19sM7-KQw
    https://www.youtube.com/watch?v=HpDLrJfG4Pw

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