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Thread: Recovery Issues at Start-Up

  1. #1
    Join Date
    Aug 2019
    Posts
    1

    Recovery Issues at Start-Up

    Our company has ~40 machines and we mostly mold PBTs and Nylons. On some of the PBTs and Nylons it takes up to a half hour to get the material to feed into the screw and purge properly. Once the material starts purging well we have no other process issues once in cycle. Weve tried raising the rear zone temperatures, hotter feed throat, high/low manual BP, high/low RPMs, pulling the screw back then trying to extrude, starve feeding material, and forcing the material down into the feed section using a piece of wood (seems to work the best but we cannot do this on modern machines as we cannot get to the feed throat due to the large purge shields on the machine).

    Some info on our machines:
    Barrel sizing: 22mm to 40mm
    All electric Roboshots
    GP screws

    This does not happen for every Nylon or PBT, it seems to be just a few grades. Its not a terrible problem, just annoying and you end up wasting a good bit of time and material trying to get the machine to purge. When were having this issue and pull all the material out to look at the screw it is never wrapped and in good condition. Any ideas?

  2. #2
    Join Date
    Aug 2014
    Location
    Saint Paul, MN
    Posts
    153

    Re: Recovery Issues at Start-Up

    Can you tell whether the material is getting to the screw or hanging up in the throat? If the latter it could be coming out of the drier too hot and soft and bridging. You could try drying at a lower temperature for longer. A technique that is supposed to help with screw feed is to have grooves cut in the back end of the barrel (essentially right under the feed throat), but I don’t have personal experience with it.

  3. #3
    Join Date
    Feb 2016
    Location
    San Francisco Bay Area, CA
    Posts
    188

    Re: Recovery Issues at Start-Up

    Interesting - it's fine once it's running though. So what would cause dosage issues when screw is empty vs screw is full?
    The material enters the screw same way in either condition. So that sort of eliminates any potential issues above the screw, right?
    You mentioned most solutions that came to mind.
    I might try a colder (like cold) rear barrel temp and profile up to a hot temp to the nozzle
    if that doesn't work try the other way around

    Tried with high and low back pressure? or try to simulate what the screw would do in fully auto?
    block the nozzle (with cold slug in sprue maybe?), use back pressure extrude the screw. I might try pulling screw back and trying to extrude in different stages as well; feed zone, mixing zone.

    let us know

  4. #4
    Join Date
    Aug 2011
    Location
    Upstate of South Carolina
    Posts
    628

    Re: Recovery Issues at Start-Up

    Have you confirmed all is correct inside injection unit? Maybe wear or pellets stuck to the screw after shutting down. How do you shut these machines down and for how long? Run them completely empty one shutdown and see what you get.
    I’ve seen this with incorrect barrel temperatures (t/c wrong/damaged) and when the cold screw prevention times were too low. Is there a torque setting on charging drive? Maybe running high speed versus high torque.
    Could there be a setting in machines that causes low torque and screw RPM on restarts or a set idle timer? I’ve seen that on several new machines.
    Rick

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