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Low dot gain condition on press need help

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  • Low dot gain condition on press need help

    I need some help, first off I am not a pressman but a long time prepress guy and I am trying to help my pressroom manager troubleshoot a issue on a 5 month old Mitsubishi sheet feed press. We noticed a few weeks ago that we were starting to have some issues matching our Swop calibrated proofs on press (18% dot gain built in) where as they had been beautiful before. I started pulling sheets and measuring dot gain and noticed that the whole press was falling off over the last 4 weeks.

    I took a sheet that ran 4 weeks ago and the Solid Magenta was ran at 1.41 and had a dot gain of 15% which is a bit clean as it normally was about 17% and then yesterday we had a job that ran that was on the same stock same everything and now with a Solid Magenta density of 1.48 had a dot gain of 9%. This is happening across the whole press so I know it can't just be one unit that is say underpacked or has bad rollers. We know the plates that went on press were fine as we only run linear plates and we read every plate as a q.c. measure.

    Any ideas where to start, could it be that the rollers were not set properly? Or need to re-reset as they are tightening up. The press is ran 24-5 so it already has a good amount of impressions but I can't believe that it would need new rollers already. Fountain solution? Conductivity?

    Any ideas would be greatly appreciated.


  • #2
    Re: Low dot gain condition on press need help

    New presses nearly always need rollers reset - so have a look at one tower and see what is happening -thats the first step- a call to Mitsi woud not do any harm either -I always log calls to engineers first and then try and find solution



    • #3
      Re: Low dot gain condition on press need help


      I get sharpening of the dot when the blankets harden off. Sometimes this is only after a few weeks with the same blanket on. Maby due to a bit harsh a chemical in the autowashers. if it is this then it should be an easy fix, Second is a long shot, but check chiller rollers temp, too low could cause ink tack to be too high causing less dot gain. i run between 25-27 degrees C


      Edited by: Paul Green on Apr 10, 2008 6:16 PM


      • #4
        Re: Low dot gain condition on press need help

        Did anything change on the press? Ink, Fount, Blankets, packing, roller temp. Also, did you check your plates before they went on press, to make sure the correct curve was applied. You could also check for calcium build up on the blankets and rollers. It may not be visible so use a product that is designed to remove it.



        • #5
          Re: Low dot gain condition on press need help

          Another thought...make sure you are checking your plates before they go to press with a "dottie" or similar densitometer designed to read plates. Check three areas - 25%; 50% and 75% areas...not just the 50%.

          Also, check the very beginning of your runs to be sure you are starting out that light. Your plates may actually be sharpening on the run due to any number of problems.

          Best of luck.


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