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  • Dot gain on press

    My company has been printing a particular job on a standard 24 point coated SBS stock with our akiyama 40” 6 color press for many years. We have recently started printing this job (same stock) on our 50” KBA rapida 8 color without much success. A special green build in the file with only 9% yellow prints very very yellow on press. The plate curve (stochastic 25 micron) for the akiyama brings the 10% yellow dot down to a 7%. The KBA has it’s own profile and brings the 10% yellow down to 5%. This was not good enough to match. I had to reduce the yellow in this area to 3% ( I created a special reduction plate curve) to get the match. Why would the KBA gain so much (yellow only it seems) in this 10% range? Does anybody have any ideas? I know this is not really a prepress/plating issue, but it seems today that prepress needs to know more and more about the pressroom than ever before.

    Thanks in advance

  • #2
    Re: Dot gain on press

    Jeff,
    Is the fountain solution same on both presses? If it is, what is the pH, Conductivity and temperature of the press ready fountain solution on both presses? What is the temperature of the form rolers on both presses? From my experience, if one press or one job require more water on plates, you can have an over-emulsified ink on the rollers and this will inhibit ink transfer. The more ink/water is pushed on press, (there is apoint of diminishing return) the dot gain can be affected. In printing, less ink, lower film thickness will have higher tack at the nip and will print sharper( lower dot gains). I do not know whether KBA prints too full generally, I do not think so because I have a client who prints 20 micron every day of the year. They did struggle in the beginning, making many plates with different plate curves until the right ink and founatin solution and a color management program was instituted. You do need a strong ink, I was told.
    Is your problem on this one job only or is it difficult to match the proof on many jobs?
    GJ

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    • #3
      Re: Dot gain on press

      You asked:
      "Why would the KBA gain so much (yellow only it seems) in this 10% range? Does anybody have any ideas? I know this is not really a prepress/plating issue, but it seems today that prepress needs to know more and more about the pressroom than ever before."


      Follow a simple and logical plan to determine the cause of the problem or prepare to waste much time.

      1.) MEASURE the plate to verify that the proper curve has been applied. Be sure the plate meter is capable of measuring your stochastic screening, and is in calibration according to a traceable standard.

      2.) MEASURE the following parameters on press to verify that none of them are out of normal tolerances:
      - blanket packing
      - plate packing
      - determine total impression beyond "kiss impression"


      3.) EXAMINE the yellow print image under blue light (use a Beta Color Viewer with a 50x microscope)to check stochastic dot fidelity. They should not be doubled, slurred, or show signs of over-impresssion, ink emulsification, or any of the usual pressroom problems. If the pressroom doesn't have a Beta Color Viewer, then THEY can't see the problem either.

      Stand over the shoulder of the operator taking the measurements. You'll learn something if you're not familiar with it, or you'll keep the pressman honest if you are familiar.

      Please report back so that we all can learn from your experience.

      Regards,

      Larry Goldberg
      Technical Director
      Beta Industries
      larry@betascreen.com
      www.betascreen.com

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      • #4
        Re: Dot gain on press

        Larry,
        How do you measure blanket packet and plate packing? The press guys probably know, but it’s nice to get a third parties input.

        Jeff

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        • #5
          Re: Dot gain on press

          Check to see if they are running an under blanket on that machine. Instead of packing ?
          What does KBA say ? Call Doug Parker at KBA

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          • #6
            Re: Dot gain on press

            "How do you measure blanket packet and plate packing?"

            The only reliable method is to use a packing gauge. This is a mechanical device that is aligned parallel to the axis of the blanket or plate cylinder . It is first placed on the cylinder near the bearer and the dial indicator is zeroed when in contact with the bearer.

            It is then moved to the working area of the cylinder and the indicator is brought in contact with the blanket or plate. The DIFFERENCE in the readings is the squeeze contribution from that cylinder. Be careful about measuring on a window-paned blanket. Heidelberg makes a very good, three-gauge unit. There are several good one-gauge units. Don't poke holes in the blanket with a needle-type unit.

            Measuring the thickness of the blanket with a micrometer is not a good method because a properly installed blanket is torqued down, with a TORQUE WRENCH. It will change gauge in the first few minutes and a little more as it is run in, but it will then stabilize. Valid measurements with a mircometer require proper technique and a good feel, or a weighted, bench-mounted micrometer.

            Packing sheets are less compressible and can generally be accurately measured by all but the most ham-fisted operator.

            Regards,
            Larry

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