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  • HPS Moire

    We are a flexo shop using Harlequin Precision Screening (HPS) and I am experiencing a 1/C moire mostly on 1 particular angle. I've played with the settings and I can get the moire to move but I can't get rid of it.
    Our angles are
    7.5 C
    67.5 M
    82.5 Y
    37.5 K
    The pattern appears mostly in the 67.5 angle. I've switched colors and it follows the color. Is there a different screen set I can use that will eliminate this or is there a HPS adjustment that will help?

    Any ideas would be appreciated.

  • #2
    Do you have your HPS options to optimize for a screen angle set that includes 67.5?

    Comment


    • #3
      Is your anilox cell count at least four times the line screen ruling you're using?
      (It is possible for an anilox roll screen pattern to be a source of screen interference. Note that the Yellow frequency may be slightly higher than the C, M, and K frequencies).
      And I assume you are using a round dot?

      best, gordon p
      Last edited by gordo; 05-20-2010, 01:30 PM.

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      • #4
        My HPS options are set to optimize 67.5.
        If I adjust the angle or frequency accuracy, the pattern moves around. If I up the line screen the pattern goes away somewhat but 150lpi is too fine for some of our flexo screens.
        Gordo - the pattern shows up on the screened proofs AND plates, so it's not the anilox interference.

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        • #5
          oh, and yes - round dot. I'm not sure what (if any) different dot shapes are ideal for flexo either.

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          • #6
            Originally posted by bwillstead View Post
            oh, and yes - round dot. I'm not sure what (if any) different dot shapes are ideal for flexo either.
            If you abide by FIRST (Flexographic Image Reproduction Specifications and Tolerances) you have no choice - it's round dot. To quote from the specifications "Failure to use round dots is cause for rejection."

            The reason I asked was because it is very, very, rare for a round dot screen to manifest single channel moiré (which is what I believe you mean by "1/C moire"). Unlike other dot shapes the round dot shape is identical for all screen angles. So if you have single channel moiré in one color you should have it appear in all channels. The fact that you see it in only one channel is bizarre. I'm not sure how you would see this problem on a flexo plate - most of the ones I've seen are pretty transparent.

            Moiré happens when one pattern harmonically beats with another pattern. So you need two patterns overlaying one another for it to occur.

            The usual way that happens are:

            1) One or two extra pixels occurring on the edge of a halftone dot. If those extra pixels repeat themselves with a regular frequency that harmonically beats with the halftone screen itself, then you'll see single channel moiré. Angling screens by 7.5 degrees (as you've done) adds "noise" to the screen pattern effectively eliminating single channel moiré. Round dots typically do not have those extra pixels occurring on the edge of a halftone dot and hence do not suffer from single channel moiré.

            2) Prescreened images/tones at a different dpi (resolution) will create single channel moiré. I.e. a one bit screen tone at 2400 dpi imaged on a 2540 dpi device will create single channel moiré. (Depending on your proofing device it is possible to see a moiré there that won't be visible in the presswork).

            3) Halftone screen clash with the anilox roller cell pattern (effectively a screen) - but you've eliminated that possibility.

            Have you tried using a screen frequency that is an even divisor of your output device's dpi? E.g. if your output is 2400 dpi then try screens of 150 lpi, or 120 lpi?

            perplexed gordon p

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            • #7
              Is it possible that the imagery itself has a pattern that conflicts with the Rip angles? Fabric for example, or PRE-screened as Gordo suggested?

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              • #8
                I am sending a 1-bit tiff to an EFI server so I am color managing a prescreened file. Even though it's not being re-screened, I can see where that may create a pattern. However, When that same tiff is sent to our platemaker as-is, we get the same pattern. It is mostly showing up in the 30-60% simple screens (not so much in photos).

                Now, when I change the line screen to an even divisor as Gordo suggested, it helps. However, the pressmen have complained about some of the 150 lpi screens because they plug easier in the transition dots. So, I need to stay at 133.

                Is there a different setting in the HPS that I can use in the 'frequency accuracy' that would throw my lpi to 133.333 so it is evenly divisible by 2400?
                Anyone hear of issues with elliptical dots and flexo?

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                • #9
                  Originally posted by bwillstead View Post
                  I am sending a 1-bit tiff to an EFI server so I am color managing a prescreened file. Even though it's not being re-screened, I can see where that may create a pattern. However, When that same tiff is sent to our platemaker as-is, we get the same pattern. It is mostly showing up in the 30-60% simple screens (not so much in photos).

                  Now, when I change the line screen to an even divisor as Gordo suggested, it helps. However, the pressmen have complained about some of the 150 lpi screens because they plug easier in the transition dots. So, I need to stay at 133.

                  Is there a different setting in the HPS that I can use in the 'frequency accuracy' that would throw my lpi to 133.333 so it is evenly divisible by 2400?
                  Anyone hear of issues with elliptical dots and flexo?
                  Unless there's something very different about the EFI workflow, I don't know how you can color manage a 1 bit tiff since it has no grey levels to change. The 1 bit would have to be descreened first. That could cause issues like moiré.
                  You won't see the moiré in images because there are enough tone transitions to break up the pattern. You will see single channel moiré in flat tone areas.
                  Try 120 lpi.
                  You can always try elliptical dots, however, you will no longer be in specification with FIRST. As a result, your customer could reject the job. Elliptical dots will probably not eliminate single channel moiré. They may even make it worse. You may also have to adjust your bump curves and UV exposure. Your dots will not be as stable because elliptical dots are directional, hence the shoulder supporting the dots will provide more or less support depending on the direction of the dot relative to the direction of the press.

                  gordon p

                  Comment


                  • #10
                    Originally posted by gordo View Post
                    Unless there's something very different about the EFI workflow, I don't know how you can color manage a 1 bit tiff since it has no grey levels to change. The 1 bit would have to be descreened first. That could cause issues like moiré.


                    gordon p

                    The EFI will weaken the dots in a 1-bit tiff and adjust the inks inside those dots based on our profiles. It does a great job of matching and you can still see the same dot pattern as the plates for those clients who still want to see dots in their proof.
                    I've tried using 120 lpi this morning on a few troublesome jobs and it worked pretty well. Thanks for the input, I guess I have a few more options now and will just have to look at my issues on a case-by-case basis.

                    Comment


                    • #11
                      Dots, yes, but not the same dots on press

                      Originally posted by bwillstead View Post
                      The EFI will weaken the dots in a 1-bit tiff and adjust the inks inside those dots based on our profiles. It does a great job of matching and you can still see the same dot pattern as the plates for those clients who still want to see dots in their proof..
                      It may be that this is not an issue for your customers, but we have a few flexo customers really want to see the same dots on the proof as they will eventually have on the printed work - for these customers, we suggest Star Proof. It also offers a few other useful tools - for example, you can test traps by throwing it out of register and simulate dot loss / dot gain.

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                      Simi Valley California

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