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Thread: Flow Lines?

  1. #1
    Join Date
    Mar 2019
    Location
    Nebraska
    Posts
    26

    Flow Lines?

    Hello all! It has been too long since I have been on to post any questions/responses.

    Today I was taking pictures of PET tubes with extra back lighting for our quality department and they immediately started questioning me about "flow lines" on the part. The parts all pass inspection criteria and are acceptable, however, they want to know why.

    I told them that I would get back to them after talking with my team (since I am still a newbie at work and didn't want to misspeak).

    My team said that they are flow lines and we always have had them, so I wanted to get other opinions/explanations. I have also noticed these lines before that were slightly discolored and looked a little degraded, but my team said it comes and goes as well.

    I am inserting the image to the text and also attaching so that you can see the lines. The lines are very hard to see, so I traced the lines as well as lightening the image in hopes of making it stand out.

    Click image for larger version. 

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  2. #2
    Join Date
    Aug 2014
    Location
    Saint Paul, MN
    Posts
    157

    Re: Flow Lines?

    The images are too small for my old eyes to make out much detail, but the defect doesn’t look like what I would call a flow line, it seems to be more related to gate blush.

    Can you give us more detail about the mold – cavitation, runner style (hot/cold), gates, etc.? Also, you could run a fill progression to see where the cavity just begins to fill, then compare that to the pressure curve – if you see a dip, there may be a hesitation at the flow front causing the flow line.

    That wouldn’t necessarily explain the degraded appearance though, so I’ll take a wild shot in the dark. Degraded material coming off the gate – usually with a smoky appearance – can be associated with valve-gated hot runners. In that case, raising the gate zone temperature by a few degrees (5 or 10 F) will generally show an improvement.

  3. #3
    Join Date
    Dec 2016
    Posts
    117

    Re: Flow Lines?

    Hi,

    Well, my old eyes can't see em very well either,....... guessing these are heat related "flow lines,"........ also guessing these are direct gated hot drops??? ...... if so take an actual temperature inside these drops with a mini probe if possible (use hot gloves), I have seen many times the temps are much higher than what you see it showing. I would also minimize suck back so it doesn't pull in too much air prior to shooting again, but has enough so you don't get any drool.

    Send the group more information as necessary and I am sure they will help you.

    Joel

  4. #4
    Join Date
    Feb 2016
    Location
    San Francisco Bay Area, CA
    Posts
    191

    Re: Flow Lines?

    Are you drying the material? - Same dryer used every time it runs? Over drying will cause degradation and can cause flow lines

  5. #5
    Join Date
    Mar 2019
    Location
    Nebraska
    Posts
    26

    Re: Flow Lines?

    Sorry for the delay! We are running a 96 Cavity Valve Gated Hot Runner and material is from one dryer and has a minimum of 4 hours of drying. We tested the material for moisture and we were acceptable.


    Thanks JayDub for the wild shot in the dark and to check the temperature of the tips Joel.

    Next time the press is down and/or they see tubes with these marks I will try your suggestions.


    We are running a 96 Cavity Valve Gated Hot Runner
    Material is from one dryer and has a minimum of 4 hours of drying

  6. #6
    Join Date
    Feb 2016
    Location
    San Francisco Bay Area, CA
    Posts
    191

    Re: Flow Lines?

    would look into machine down time and throughput of your dryer. PET can degrade easily if dried too long

  7. #7
    Join Date
    Aug 2019
    Posts
    4

    Re: Flow Lines?

    I would say looking at the image that this is more of a heat check issue than a true flow line this happen when the surface of the tool gets to hot . Are the core steel ? how hot does the surface of the core run when in cycle ? I could imagine that water does not run up to the tip of the core . Look at the pictures are the lines only on the inside of part ? Do you see them on first shots ? I would try to lengthen the open cycle time to allow the tip of the core to cool before mold closes and see if that helps .

    Also when valve gating if you have your hold time to long and the plastic starts to freeze at the surface when your valve gate closes is like a second inject when valve gate closes . So maybe try to slow the valve gate closing speed and see if the flow lines change .

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