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Thread: Help! Witness Mark around Gate

  1. #1
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    Jun 2018
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    Help! Witness Mark around Gate

    Hi Guys,

    I have just went into production with my first injection mold. It is a 2 cavity mold with each cavity having 2 gates (gates are 2mm diameter).

    3 of the gates are perfect and leave no witness mark however 1 gate leaves a large circle around the gate (see the 'big circle' in photo attached). Sometimes this gate leaves barely any witness mark at all however the big circle does appear a lot which is leading to way to many rejected parts. Does anyone know how to fix? The molders have tried changing the temperature on the hot controller however we are still getting a lot of rejects. They also took apart the mold, tested the hot runners and said they appear to be working.


    Material being used is LG TR557 which is a transparent ABS.


    Thank you for any help, feeling a bit lost at the moment as the mold cost so much and even after trying for 3 days we still have not fixed the issue.

    Sean
    Attached Thumbnails Attached Thumbnails Bad Witness Mark.jpg  

  2. #2
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    Re: Help! Witness Mark around Gate

    Do you have a change in speed or to hold pressure at that point in the process? Might be the flow is pausing causing the mark. What type of machine? I know I ran some Sumitomos that had a function that was supposed to balance cavities out by pausing between 1st and 2nd stages. It caused a mark in the part where the flow front paused on our clear parts. Had to turn it off (no real way just a work around).

  3. #3
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    Re: Help! Witness Mark around Gate

    Quote Originally Posted by iautry1973 View Post
    Do you have a change in speed or to hold pressure at that point in the process? Might be the flow is pausing causing the mark. What type of machine? I know I ran some Sumitomos that had a function that was supposed to balance cavities out by pausing between 1st and 2nd stages. It caused a mark in the part where the flow front paused on our clear parts. Had to turn it off (no real way just a work around).
    Not that I am aware off. I will ask the manufacturer once he opens.

    By changing the temperature of the hot runner on the 'bad' gate, we have been able to fix the problem at that location. On the flip side, it now appears at a different gate.

    The molder now reckons it is due to the nozzle (attachment 2) that connects the end of the screw in the machine (attachment 1) to the mold. He thinks this could be too long resulting in a cold spot.

    Machine is a Hiatan 450 tonne. Realistically the part could be made on a 350tonne however the dimensions of the mold are too large. The total shot weight of the mold is 460grams (1lb).

    Thanks for reply,
    Attached Thumbnails Attached Thumbnails 2.jpg   1.jpg  

  4. #4
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    Re: Help! Witness Mark around Gate

    When running clear resins you must use a full internal tapered nozzle body and tip. That negates cold slugs and dead spots. Nozzle length looks ok to me providing you have two heater bands and a tge thermocouple near the tip by mold.
    I think your actual melt and manifold temps are a little low for the resin. Clear parts and resins can be difficult.
    Rick.

  5. #5
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    Re: Help! Witness Mark around Gate

    Quote Originally Posted by rickbatey View Post
    When running clear resins you must use a full internal tapered nozzle body and tip. That negates cold slugs and dead spots. Nozzle length looks ok to me providing you have two heater bands and a tge thermocouple near the tip by mold.
    I think your actual melt and manifold temps are a little low for the resin. Clear parts and resins can be difficult.
    Rick.

    So the molders suggestion worked. We shortened the nozzle, put a heater around it and polished its inside (that's what I am told anyway), and then went on to have 7 hours of solid production today with not one bad gate witness mark. Very very happy!!

    A new problem did arise however. This issue looks like an array of scratches (see photo 1, 2 & 3) that are on approx 15% of units made. Most of the time it shows up on just one cavity usually in the same spot each time, however occasionally it appears in other locations also. Any ideas? At first I thought it would have being due to the resin not being dry enough however as the issue is pretty location specific I no longer think that this is the case. I also saw the material drying in a hopper.

    Thanks a lot for both of your help. Very happy to fix the gate issue as I thought I would have to invest in new hot runners.

    Sean

    P.s. good to know about the internal tapered nozzle. I will keep an eye on this is the gate issue presents itself again. I will also start keeping a better record of the machine temperatures.
    Attached Thumbnails Attached Thumbnails 1.jpg   2.jpg   3.jpg  

  6. #6
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    Re: Help! Witness Mark around Gate

    Looks like some moisture streaks to me.

  7. #7
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    Re: Help! Witness Mark around Gate

    If it was moist material would the issue still be pretty location specific? Not sure how we could try the material any more than we have as it was in the hopper for a few hours.

  8. #8

    Re: Help! Witness Mark around Gate

    Quote Originally Posted by SeanD View Post
    If it was moist material would the issue still be pretty location specific? Not sure how we could try the material any more than we have as it was in the hopper for a few hours.
    Hi,
    Try to run two dryers if you have in company, without adding material in one who is on machine. I did same for my problem to be sure that material is always dry.

    Nemanja.

  9. #9
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    Re: Help! Witness Mark around Gate

    Quote Originally Posted by nemanja View Post
    Hi,
    Try to run two dryers if you have in company, without adding material in one who is on machine. I did same for my problem to be sure that material is always dry.

    Nemanja.
    Hi Nemanja,

    So do you mean have one hopper on the ground that is full with material that is in the process of being dried with a pipe transferring material from this hopper to the empty hopper which is on the machine? Material should be transferred to the hopper on the machine at a slow enough rate so that the machine hopper has barely any material in it?


    Not sure if I got it right!

  10. #10
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    Re: Help! Witness Mark around Gate

    If the defect is location-specific, it may be shear splay caused by the flow front hitting a geometric feature too fast. Play with the velocity profile to slow the flow front down before it hits the responsible feature. You can speed up again once past it. It may take some trial and error to identify which feature is responsible, but you can start by slowing the whole fill down.

  11. #11
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    Re: Help! Witness Mark around Gate

    JayDub, would putting in a vent help?

  12. #12

    Re: Help! Witness Mark around Gate

    Quote Originally Posted by SeanD View Post
    Hi Nemanja,

    So do you mean have one hopper on the ground that is full with material that is in the process of being dried with a pipe transferring material from this hopper to the empty hopper which is on the machine? Material should be transferred to the hopper on the machine at a slow enough rate so that the machine hopper has barely any material in it?


    Not sure if I got it right!
    For example, im using one hopper with dry material and dont add any new material till is close to empty, in meanwhile on separate from that machine i have one more dryer who is drying material. When machine is close to empty i just add already dry material, and wet material im adding to dryer who is separate from machine. Hope is more clear situation.
    Also is hopper on machine drying material? If material is to long out of dryer it will pick up moisture for sure (have one of this products-1ccm per shot). There we change material from hopper as soon as we see problem, and we solve it in that way.

  13. #13
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    Re: Help! Witness Mark around Gate

    Quote Originally Posted by JayDub View Post
    If the defect is location-specific, it may be shear splay caused by the flow front hitting a geometric feature too fast. Play with the velocity profile to slow the flow front down before it hits the responsible feature. You can speed up again once past it. It may take some trial and error to identify which feature is responsible, but you can start by slowing the whole fill down.
    Agree with this.

    An interesting study you can do is to run fill-only parts at 98% and step down to 90% , 80%, 70%, 60% ... etc etc.
    Looking at these parts will give you a good idea of how your melt is flowing so you can try to pin-point your defect and slow this section down, among other possibilities (cavity balance, pressure loss).

    Or maybe the defect you're seeing is contamination from the previous run. Small pellets slowly getting into the melt every now and then. They'll stick around in funny areas in the hopper if it's not cleaned thoroughly.

  14. #14
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    Re: Help! Witness Mark around Gate

    Okay I get you now!
    We have the material in a hopper being dried that is connected by a hoover to the hopper on the machine. This means material is dried for about 3 hours before it goes into the hopper on the machine.

    Alternating 2 hoppers like you do sounds better but 2 hoppers aren't available at the moment.

    I have looked at a mold flow analysis that I previously got done and think the material is hitting a wall and bouncing back slightly to create turbulence and hence the markings.

    I have run into quite a lot of trouble with this manufacturer and so am going to take manufacturing somewhere else for the next order. What spares would I need for the mold? Mold is 4 gate hot runner.

    Thanks guys. Production run again tomorrow so will talk to them about changing injection pressures.

  15. #15
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    Re: Help! Witness Mark around Gate

    You need to slow the injection rate PRIOR to the melt front contacting the feature that is creating the melt flow defect. Then you can increase the fill rate past this area. Other possible root causes are melt front hesitation, melt front splitting at a feature or back filling of the melt into this area. Each one has a separate fix.
    Rick.

  16. #16
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    Re: Help! Witness Mark around Gate

    Quote Originally Posted by rickbatey View Post
    You need to slow the injection rate PRIOR to the melt front contacting the feature that is creating the melt flow defect. Then you can increase the fill rate past this area. Other possible root causes are melt front hesitation, melt front splitting at a feature or back filling of the melt into this area. Each one has a separate fix.
    Rick.
    Okay, so previously I said issue was location specific. As it turns out it is location specific for certain periods of time. One hour no issue exists. Another hour it exists in a certain location. The following day it happens in a different location.

    The molder is now saying that it is due to the operator rubbing the unit of the mold when he is taking it out of the machine. This leads to some material staying on the mold which acts as a cold spot. Does that seem realistic to you?

  17. #17
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    Re: Help! Witness Mark around Gate

    I doubt an operator is causing that unless they’re putting lotion on their hands. Since it moves after some time two more possibilities: 1-Hot spots in tool due to poor cooling or insufficient cooling. 2-Residence time of melt in barrel. You’re consuming either a processing aid, FR or UV agent. Careful adjustment of the barrel settings should stop this as long as the job is too small for the barrel on the press. One last random thought; wrong compression ratio on press for resin.
    Rick

  18. #18
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    Re: Help! Witness Mark around Gate

    turn up the barrel heats and increase back pressure

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