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Dot shape for web presses

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  • Dot shape for web presses

    We're looking into trying something different in dot shape. Currently we're using a round dot shape, but would possibly like to try
    euclidean, or maybe a hybrid dot shape. This trial is prompted by a rash of problematic patterning issues as of late. Also, is anyone
    running their yellow at a different line screen than the rest of the colors, ie. KCM at 150 line and the yellow at 175 line??
    We're using Prinergy workflow, Harmony rip, Magnus VLF, and Kodak processing. Any help would be greatly appreciated.

  • #2
    Re: Dot shape for web presses

    We frequently output our yellow at a different line screen than the other three colors to prevent moire patterns. I've also read that some use stochastic screening for their yellow and conventional screens for the other inks with success.


    • #3
      Re: Dot shape for web presses

      It would help to understand what kind of patterning issues you are encountering, as I do not know what patterning would be caused by a round dot vs other dot shapes - and other dot shapes could cause other issues for you on press.
      I'm pretty certain that, if you are running 150 lpi, your yellow is about 162 lpi.
      Yellow is sometimes run FM/Staccato in order to avoid C/Y or M/Y moiré. Typically, with 150 lpi, it would be 40 micron Staccato.
      But again, understanding what kind of patterning you are experiencing would help to understand the cause and suggest a possible solution.

      best, gordo


      • #4
        Re: Dot shape for web presses

        The pattern I'm speaking about is probably a moire pattern. We've gone back into the history of the job
        to make sure the right angles were used. Now on one job the pattern was corrected on press as there
        seems to have been a problem with roll pressure and the issue was corrected on that particular job.
        I guess what I'm trying to do is e;liminate any and all prepress issues as to narrow this down and
        pinpoint the cause. As to different shape dots, sometimes a euclidean or hybrid dot which I believe
        has characteristics of both round and square, kind of a "rounded square". Kodak claims it helps with
        moires. We are moving towards fingerprinting our presses for FM printing in March whach also would be
        a great help. Thanx for replying.


        • #5
          Re: Dot shape for web presses

          The only screening related moiré that I'm aware of that might be affected by a change in roll pressure is wire-side web moiré. I.e. the faint patten of the screen on the wire side of the web when the paper was originally made clashing with one of the screens in the halftone when that side of the web is printed. Changing the shape of the dot will not affect that issue, since moiré is basically an interference pattern resulting from two patterns that have a similar angle and frequency. Since going from round dot to Euclidean or elliptical does not change the halftone's angle or frequency then the dot shape change shouldn't have any affect on moiré.

          best, gordo


          • #6
            Re: Dot shape for web presses

            The standard and most stable dot shape for the web press is round. A euclidian dot changes throughout the tone scale. starting out round, changing to eliptical, square at 50%, back to eliptical then to round in the shadow.
            The advantage of eliptical is that vignettes print smother than round, helps flesh tones as well. The disadvantage of eliptical is that the dot chain is perpendicular to the web direction, this causes the dot chain to "fill in" and increase the yellow gain and stability.
            Dan Remaley


            • #7
              Re: Dot shape for web presses

              I agree with Dan that the best dot shape for an AM screen for offset (web or sheetfed) in a CTP environment is Round. The choice of dot shape is important because it has an impact on the aesthetics of the halftone as well as its lithographic performance on press. This is especially true for lower frequency screens in the 85-175 lpi range.

              When dots grow in size from highlight to shadow there is a point at which the dots first touch each other. When that happens there is an effect called the “optical bump.” In a gradient blend this shows up as an artifact of a dark line in an otherwise smooth blend. Another issue is that, because paper moves through a press from printing unit to printing unit, presses are directional imaging devices which means that dots that are also directional can interact with the directionality of the press and cause artifacts. Directional issues on press include slur, doubling and tailing (slinging).

              Here are the basic dot shapes in use - every vendor will have these options with very subtle variations:

              Round dot (a.k.a. non-transforming dot) ; Dots are round through the tone range. They start as a point and simply grow equally in all directions through the tone scale.
              Benefits: Dot shape is the same for all screen angles and frequencies, optical bump is hidden in the shadows at the 75% tone, dot is non-directional so it is less affected by press problems. Excellent for computer-to-plate imaging because of the greater integrity of halftone imaging as well as the ease of dot gain compensation with tone reproduction curves.
              Issues: Not suited for film-imaged plates because the diamond shape that results at 75% and darker tones is very sensitive to dot gain and sudden loss of shadow detail.

              Euclidean dot: Round/Square/Round (a.k.a. transforming or composed dot) Dots begin round at 1% and transform to square at the 50%. Shadows from 50% to 99% are a negative image of the 1%-50% tone scale.
              Benefits: Dot shape, except for 50% tint, is the same for all screen angles and frequencies, dot is non-directional so it is less affected by press problems.
              Issues: optical bump occurs at 50% midtone tint - very visible in vignettes.

              Elliptical dot: Rounded corner diamond/elliptical shape
              Benefits: optical bump is moderated by being split into two – when the dots first touch at the long width at the 40% tint and then again at the short width at 60%.
              Issues: dot shape varies at different screen angles which can cause single color moiré and uneven dot gain. Dot is directional, at low lpi frequencies the “chaining” of the dots as two points touch can cause artifacts that appear as lines. Directional problems on press such as slur and doubling can cause strong tone and color shifts depending on the angle of orientation of the dots relative to the angle of the paper as it travels through the press.

              best, gordo


              • #8
                Re: Dot shape for web presses

                Amin to that, I wish I had learned such things long time ago myself... Thanks for sharing.


                • #9
                  Re: Dot shape for web presses

                  Nice! Eloquently understandable Gordon.

                  Yeah, round seems to work best in my experience. Most of the screening technologies I've been exposed to will vary the actual line screen and angle a hair from color to color so patterns more often occur from chance alignments with the inking systems and substrates than true moire of the dots. Also, that bump mentioned is very real. In wide web flexo we've actually seen the 75%, where the non printing shape is a 4 pointed star, carry more ink than the actual 100% patches. With wise curves you can manage it to advantage.

                  We've had some bad experiences manually manipulating line screens. Not that it doesn't achieve results but in the potential for other errors. One either has to tell the workflow to respect the application line rulings or send different colors through different workflows. In the first case it's easy to find out that there's all sorts of elements set to odd line rules that you never noticed. In the second, you just best be sure both flows are very identical except line rule. Sucks to have a black element cut out of one color because of a stray overprint setting difference.


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