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Thread: Liquid color issue

  1. #1
    Join Date
    Sep 2018
    Location
    Portland, OR
    Posts
    27

    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
    159

    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
    28

    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
    572

    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
    27

    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
    27

    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
    28

    Re: Liquid color issue

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

  8. #8
    Join Date
    Sep 2018
    Location
    Portland, OR
    Posts
    27

    Re: Liquid color issue

    A small update, we ran another tool that is a cold runner with the same shot size but different parts. No color issues were observed with this mold, but some processing changes were made that will hopefully translate back to the tool that was having issues.

    After talking to Reiloy we are using a pretty severe reverse heat profile (410 front zone, 460 rear zone).
    We installed a mixing nozzle body with four twist elements from PPE.
    We have started mixing the liquid color by essentially rolling it around the shop before use.

    Some additional steps we tried that did not help:
    We tried using the smaller screw that was designed for mixing acetal and saw worse performance, possibly due to a short residence time of about 35 seconds.
    We varied backpressue from 300 to 5000 psi and screw speed from 30 to 300 rpm.
    Attempted various ways of feeding the liquid color into the material path and premixing the color with the pellets.
    Mold Masters has not provided us with any technical support but has sent us all drawings of the hot runner system.

  9. #9
    Join Date
    Jan 2020
    Posts
    7

    Re: Liquid color issue

    If it had dark to light spots in both types of colorant my next question is does it have deep ribs or small or thin wall flow areas across the parts if so shooting slower or raising mold (steel) temps up. Iíve seen colorant do this from how it flows across the steel with both pellets and liquid colorants both of witch were fixed with a different shot speed normally slower and a higher steel temp. I had almost an exact same issue with a pellet we tried at the screw to fix it going so high on back pressure (with slower speeds) and saw improvement but never fixed it for more than an hour(also we bridged the feed throat) I did a doe involving steel temps and found it wasnít that it wasnít mixing properly at the screw but as we shot it the flow across the face of the tool was more of the issue

  10. #10
    Join Date
    Nov 2013
    Posts
    23

    Re: Liquid color issue

    You can mix the liquid color with raw material outside inside a batch mixer. then load to machine by pipe or manually.
    It is a temporary and messy solution but, mostly solves our mixing issues.

  11. #11
    Join Date
    Sep 2018
    Location
    Portland, OR
    Posts
    27

    Re: Liquid color issue

    We got a PPE mixing nozzle with four elements and that helped substantially. The parts are small with constant wall thickness, hollow cylinders about 1.5" long.

  12. #12

    Re: Liquid color issue

    I dident read the whole thing sound like gassing issues try slowing down injection openn vent drop cut off 4 speed change slow down screw rpm and heat mold and tool worth a try work with solid colarents i imagine agents causing problem

  13. #13

    Re: Liquid color issue

    Slow down screw feed I imagine u need a mixing screw but we get around it with solid colouring on GP screws open vent its gas from agents people often call it a short shot when can't fill it or haze not venting hope this helps

  14. #14

    Re: Liquid color issue

    Alcryin tpu santoprenene acrylic pc ultem poly pro crastin and thermo set

  15. #15

    Re: Liquid color issue

    More back pressure smaller shot pis it off only way 2 find out try it stay out of the box

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