Results 1 to 11 of 11

Thread: Unstable cushion

  1. #1
    Join Date
    May 2014
    Posts
    11

    Unstable cushion

    We have been chasing cushion variation on a 612 nylon job so I am reaching out for ideas or things I may be overlooking.

    Its a 96 cavity, 12-drop, hot to cold runner mold running in a 165T Roboshot. Two other machines are running the same setup with a nice, repeatable cushion. The screw and NRV have been replaced, tried increased decompression, 100% virgin, and the barrel was measured to be ok, but maybe close to out of spec.

    What I find strange is how the switchover pressure does not vary that much, but the cushion is all over the place. No complaints from quality about the parts, either.

    Has anyone experienced anything like this?

    Thanks!
    Attached Thumbnails Attached Thumbnails 450 cushion.jpg  

  2. #2
    Join Date
    Sep 2017
    Posts
    8

    Re: Unstable cushion

    Good evening! - I am sure youíve already checked but I got to ask anyway because itís one of the basics! Are you leaking at the nozzle? I have seen a variation in cushion on nylon jobs due to drooling out of the drops as well since nylon requires a lot of heat. What kind of nozzle tip are you using? Is it made for nylon? They make nozzle tips with a reverse taper specifically for nylon to combat the drooling if this is a problem.

    Is the manifold leaking inside the mold? Have you verified the actual position of the screw is reading true? Is your back pressure consistent? If your back pressure varies so will the volume. I would definitely check the recovery side of things first.

  3. #3
    Join Date
    May 2014
    Posts
    11

    Re: Unstable cushion

    Those things are looking good. No nozzle leakage or drool (hot runner with no nozzle retract). Backpressure and screw position should be consistent since they are servo controlled, but I will verify. I'm trying to hook up an eDart today to compare with the other machines. I'm leaning towards the barrel being worn, but its really strange that the peak and transfer psi are barely changing with such huge swings in cushion.

  4. #4
    Join Date
    Sep 2017
    Posts
    8

    Re: Unstable cushion

    The eDart should show you a lot of your issues. We also have RJG at our facility and theyíre really good for data. What kind of check ring are you using currently? Is it like a 3 piece slider or a ball check? If all checked good itís gotta be like you said, a worn barrel or a bad check ring which I know you just said youíve install a new screw and check ring. Something doesnít add up. Now Iím curious what the issue is lol.

  5. #5
    Join Date
    Sep 2019
    Posts
    4

    Re: Unstable cushion

    What size of a cushion are you looking for? Set up the Backflow monitor and the screw end Pos. This will tell you if the problem is at the check ring or the feed Zone. Sounds like there's a problem in the feed zone of the barrel.

  6. #6
    Join Date
    Sep 2019
    Posts
    4

    Re: Unstable cushion

    The back flow will tell you how much material is getting by the check ring. Compare to the other presses running the same process.

  7. #7
    Join Date
    May 2014
    Posts
    11

    Re: Unstable cushion

    I am not exactly sure how to monitor back flow.

    We just put on a new 4-piece check ring today. The cushion range is reduced, but not yet where it should be.

  8. #8
    Join Date
    Oct 2019
    Location
    Harrisburg, PA
    Posts
    5

    Re: Unstable cushion

    Cushion should be +/- .03in. An easy test would be to move your whole process back, add an inch or two to all of your positions, and see if it's more consistent. That will indicate if your barrel is worn. Bozzelli mentions some other things in this article. https://www.ptonline.com/articles/ho...lve-is-leaking

  9. #9
    Join Date
    Feb 2016
    Location
    San Francisco Bay Area, CA
    Posts
    195

    Re: Unstable cushion

    Decompression as well as wet material comes to mind.

    Also if your nozzle ID is too small your cushion will bounce - especially with hot-runner tool. Difficult to get consistent pressure/times/speeds through your a tight nozzle, through the hot-runner, into your (small?) gates, and then into your full cavities.

    See how much injection pressure you use at the nozzle by doing an air-shot -
    or if you want to be more thorough you can do a pressure loss study to see where all pressure is going through your system

  10. #10
    Join Date
    May 2014
    Posts
    11

    Re: Unstable cushion

    We finally fixed the variation with a new screw and barrel.

  11. #11
    Join Date
    Aug 2011
    Posts
    587

    Re: Unstable cushion

    Thanks for letting us know Filiberto. This helps us learn!
    Thanks,
    Suhas

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •