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Thread: Liquid color issue

  1. #1
    Join Date
    Sep 2018
    Location
    Portland, OR
    Posts
    24

    Liquid color issue

    We have a new tool that was transfered into us. It's 48 cavity hot runner in acetal (Celcon M90) using a liquid color. We have the process dialed in to make parts at the center of the tolerance range for size (+/- .002).

    We are having a lot of issue getting good mixing with the liquid color. Shinning a light through the parts shows swirls of light and dark areas, and on several cavities the knit line has clear variation of light on one side, darker on the other. There seems to be a slight correlation between which cavities show this more than others, but it isn't consistent.

    We were feeding the color into the middle of the feed throat just below the loader and changed to using a copper tube to dispense it just above the screw. We moved from a larger diameter tube on the peristaltic pump to a smaller diameter. We tried with and without a .020" mesh filter nozzle body.

    Shot size: 230 g
    Color ratio: 1%
    Cycle time: 20 seconds
    Mold temp: 180 C
    GP screw, using about 30% of the barrel.

    Any advice for getting better mixing? We seemed to get slightly better results from high back pressures and low screw speeds but couldn't get the size in tolerance.

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

    Re: Liquid color issue

    You may not be able to get good mixing from the GP screw. If you read the Celcon processing guide (which I highly recommend you do), what you ideally need is a barrier screw with a high (3 – 3.5) compression ratio and a fairly short (6-7 flight) transition section. The question then is whether the volume/margin on the project will justify the additional cost.

  3. #3
    Join Date
    Aug 2019
    Posts
    25

    Re: Liquid color issue

    Along with what JayDub suggested about an alternative screw, we have contacted customers and convinced them of making the switch to dry color for uniformity. Just so much easier to work with and get the desired results using dry color rather than liquid. I'd weight the option and approach customer with potential resolutions.

  4. #4
    Join Date
    Aug 2011
    Posts
    571

    Re: Liquid color issue

    Good points above.

    For now, if you are getting good mixing with higher back pressures and screw speeds, then set the back pressures and the screw speeds to these higher values and then adjust your pack and hold pressures (may need to go lower to compensate for the increased back pressure). If this does not help, try the pack and hold times (are you separating pack and hold or is it just one time?). Next adjust melt temps.

    the best is to do a good DOE at the higher back pressures and temps.

    Let us know so that we can all learn.

    Thanks,
    Suhas

  5. #5
    Join Date
    Sep 2018
    Location
    Portland, OR
    Posts
    24

    Re: Liquid color issue

    Thanks for the input, I'll be doing a DoE soon. We have a smaller screw for the machine that is a barrier screw designed for acetal, but the supplier was not able to install it while they were here due to them missing (forgetting?) some parts.

    For an update, we tried a different color that is a standard concentrate in pellet form. This eliminated most of the color 'swirl' issues we were seeing. However we are still seeing some discoloration across the knit line in a few of the cavities. It is not always the same cavities but it does seem to more frequent in certain cavities. Our next step will be investigating the hot runner, and older system from Mold Masters.

  6. #6
    Join Date
    Sep 2018
    Location
    Portland, OR
    Posts
    24

    Re: Liquid color issue

    Another update, smaller barrier screw is installed. Mixing in the liquid color is very homogeneous but the knit line issue remains. Waiting to hear back from Mold Masters and hopefully get a technician out to visit from them.

  7. #7
    Join Date
    Aug 2019
    Posts
    25

    Re: Liquid color issue

    Thanks for the update Nikom. Curious to hear of the resolution from Mold Masters.

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