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HELP! Mottled images on press

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  • HELP! Mottled images on press

    I was wondering what the possible cause of mottled half-tones on press could be. Plates are good, blankets are fresh, is there something else we should check? Iim hoping to get a few possible causes to throw at the pressmen so they can do a process of elimination on it. Any help is greatly appreciated. Thanks!
    Ernie

  • #2
    Re: HELP! Mottled images on press

    I would put non-uniform paper surface as the primary cause, followed by ink/water balance or improper pressure, worn form rollers or blankets. This issue is sometimes more prevelant with higher linescreen...a least here anyway. Could be a courser screen masks the issue though.

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    • #3
      Re: HELP! Mottled images on press

      Thanks for the reply! We are already running at 133. I will tell the guys about the other possible causes and hopefully we can narrow down the cause.

      Also is it possible that the PH of the fountain solution could cause a screen issue?

      Edited by: Ernie Lail on Dec 13, 2007 2:56 PM
      Ernie

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      • #4
        Re: HELP! Mottled images on press

        what paper are you using? Try running some gloss paper through and see if you still get it, if you do, its your fountain solution, get them to adjust the mixture, your getting to much watery ink from the rollers.

        Edited by: max on Dec 14, 2007 8:38 AM

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        • #5
          Re: HELP! Mottled images on press

          Im already printing on 100# CS2. Adjust the mixture? Fountain solution mixture? How so? I dont know much about the fountain solution mixture, please enlighten me. Thanks.

          Edited by: Ernie Lail on Dec 14, 2007 10:09 AM
          Ernie

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          • #6
            Re: HELP! Mottled images on press

            It is usually the paper - as someone has suggested changing it is the first step.

            It could also be the sequence you are running.

            In the UK we have traditionally run KMCY

            because if you run KCMY then deep blues may mottle.

            The reason is that when running KCMYthe cyan solid goes down first and the Magenta tint sits on top. when what you want is the
            magenta tint to be printed on the paper first and the cyan going over the top.

            Hope it helps
            Peter

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            • #7
              Re: HELP! Mottled images on press

              Ernie
              I like the Peter Arnel approach the best. Could you tell us WHAT is mottling? Is it the blue sky, which would be back trap mottle, like Peter suggests, or is it a light screen area, in which case, it could be piling. Is there any build up on the blanket where the mottle occurs?
              John Lind
              Cranberry Township PA
              724-776-4718

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              • #8
                Re: HELP! Mottled images on press

                Hi Ernie,
                I had this problem about 10 years ago on a heatset web job. It was the paper. It was a part-recycled stock and weeks after they identified artefacts left in the paper.
                The paper company sent techs is, they tried changing the ink sequence, they tried the fountain solution, made new plates and finally the printer said 'enough - we're changing the paper.' Paper supplier would still not admit it was their paper even when job printed perfectly on different (Stora) stock.

                Hope that helps.

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                • #9
                  Re: HELP! Mottled images on press

                  The job is a 2 color christmas card, the background is mostly solid PMS, but there are screen graphics within the solid (same PMS as solid @ 70% & 85% screens) that are mottled. The pressman said that the plate and the blanket looked fine, the problem seemed to develop once it hit the sheet. The plate is on the last unit so i guess that would eliminate back trap. Also ran some 1 color (Black) jobs on the uncoated side of 16pt C1S board wth med-light coverage with same problems in about a 50 or 60% screen @ 133 lpi. Ran the same christmas card, same ink, same stock on a ryobi 3302 @ 150 lpi and it looked fine. The press thats having the problem is a 2 color 36" H'berg sordz. We are calling in a mechanic to have a look a the press itself. I appreciate all the input!

                  Edited by: Ernie Lail on Dec 17, 2007 8:19 AM

                  Edited by: Ernie Lail on Dec 17, 2007 8:20 AM

                  Edited by: Ernie Lail on Dec 17, 2007 8:20 AM
                  Ernie

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                  • #10
                    Re: HELP! Mottled images on press

                    Seperate your screens from your solids. You may be plugging the screens while trying to maintain a good solid. You can let your screen plate work as a bump plate for your solids to clear up any hickies or paper deficiencies. Put a 60% screen of the same color solid under the solid areas and keep your other screens as they were. The pressman can then maintain balance of inks.
                    As the others said, paper is a main cause of this problem, Topkote and Carolina being two which I have seen to be a problem. The coating seems to be the problem.

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                    • #11
                      Re: HELP! Mottled images on press

                      what about the black back that doesnt have much coverage thats plugging? Making a separate plate may fix the situation but not the problem. We ran the same stock on a smaller press and didnt have those problems with mottled screens. So, i dont think it would be the paper. Logically speaking.
                      Ernie

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                      • #12
                        Re: HELP! Mottled images on press

                        Ernie:
                        You've eliminated a lot of variables. I think that mechanic will find a case of slurring or doubling on that press, since it went away on another press. Is the blanket torqued to 45 ft lbs? Do the dots look round or slurred? You're on the right track.
                        John Lind
                        Cranberry Township, PA
                        724-776-4718

                        Comment


                        • #13
                          Re: HELP! Mottled images on press

                          Yes the blankets are tight, i will let you guys know what the mechanic says. Hes been here a couple hours, but i havent heard anything so far.
                          Ernie

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                          • #14
                            Re: HELP! Mottled images on press

                            Mechanic said it was a combination of being overpacked and too much bearer pressure. They also said the plate was bad and that it was maybe a chemical problem, but the dots looked fine on the plate and blanket. We are using Agfas Azura plates, so theres no chemicals involved other than a gum bath before going to press. Made another plate and my GM made them run it, turns out the one of the cylinders had more pressure on one side as well. They fixed that and now the screens arent mottled. Could that be the cause of the mottled screens? Doesnt sound logical to me. But anyways... i appreciate all the great advice

                            BTW can anyone suggest a good mechanic in the charlotte, NC area?

                            Edited by: Ernie Lail on Dec 19, 2007 8:00 AM

                            Edited by: Ernie Lail on Dec 19, 2007 8:06 AM
                            Ernie

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