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  • Density difference gripper to tail

    What do you find is the average/acceptable difference in density from gripper to tail of sheet? The reason I'm asking is we moved our colorbars from the tail to the gripper but left some marks at the tail that I read with the spectro to check Lab colors of inks and we found that cyan was 21 points lower at the tail, magenta 17 points, but black and yellow was only 2-3 points lower.

    Is this normal or is something not right with cyan and magenta units?

    Thanks,
    Terry

  • #2
    Re: Density difference gripper to tail

    Prepper:
    The 21 points is pretty excessive. On a new press, data by my side, it can be 0.02-0.04, over 50 consecutive sheets. Press manufacturers will allow 0.05. The highest reading should be in the center of the sheet, suggesting these numbers came from a test form.... The oscillators can be adjusted to put the highest density near the center of the sheet.
    Reasons for the big point spread are certainly mechanical. If it's good for two of the colors and off for two others, then probably not the ink. Could be water feed. This leads back to rollers and rollers settings, blankets and blanket tension, back cylinder settings, gripper adjustments. Time for some maintenance. Yes, I could be overlooking a few things, you need to be there and see how things are done. By the way, the press above was an LS40.
    Yours truly,
    John Lind
    Cranberry Township, PA
    724-776-4718

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    • #3
      Re: Density difference gripper to tail

      you measure lab in points

      points of what
      what instrument do you have for measuring

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      • #4
        Re: Density difference gripper to tail

        No, don't read lab in points, the points I was referring to was density, I was just saying why I had some patches at the tail while the colorbar was at the gripper

        Thanks

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        • #5
          Re: Density difference gripper to tail

          Thanks John, that's what I was thinking since 2 colors were way off and 2 were okay. The next job on the press was some black solids and they found they couldn't print them without going thru units 2 and 3 and doing a thorough cleaning and roller setting. Guess what? That's the 2 units the cyan and magenta were in on the previous runs.

          Terry

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          • #6
            Re: Density difference gripper to tail

            sorry got confused when you said

            I read with the spectro to check Lab colors of inks

            i didn't realise you meant Density

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            • #7
              Re: Density difference gripper to tail

              Definitely this is a mechanical problem. Good you found it. Not all of us are measuring front - end of the sheet.
              Check your blanket and the under packing and make sure that the packing covers the whole cylinder. I have seen that sometimes the packing was too short and this will affect the pressure and the end of the sheet.

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              • #8
                Re: Density difference gripper to tail

                What type of press is it and please describe your ink train and condition of the plate and blanket cylinders. Its sounds more like ink roller setting.
                Test each print couple by printing a screen on each cylinder and check the dot condition from gripper to the tail.

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                • #9
                  Re: Density difference gripper to tail

                  It will probably have little to do with blanket and more to do with roller settings. Your not carrying enough ink in the roller train.
                  With blankets you would be having dot gain unless your using secastic

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                  • #10
                    Re: Density difference gripper to tail

                    It's a Heidelberg 72SP, running a 19" sheet on this particular job, on the gripper to tail density difference that was measured those 2 sets of patches were 17.75" apart. We haven't figured it out yet, going to put some patches around the cylinder in the middle of some work on the next job and see what we measure today.

                    I guess the size of the press would have been important to mention as this is a 4up press and not an 8up. Also, the job we measured this on didn't have much coverage where we were measuring the difference. Could that contribute to the big difference?

                    Thanks,
                    Terry

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                    • #11
                      Re: Density difference gripper to tail

                      If all roller settings are good move away from it. What does your overall blanket and packing mic at. and what do you mean by patch. do you have a mic that will gauge the blanket while still on the press if so check the blanket at gripper and tail. I have run into some of the larger format blankets being inconsistent in there
                      measurement from head to tail in the past.

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                      • #12
                        Re: Density difference gripper to tail

                        Try Change your oscillation start point. Open the the door on the drive side and there should be the oscillation control arm. Stop the machine when it at its lowest point. Open the bolt and inch the machine roung halt a turn and lock up the bolt. See if that helps. The film of ink from head to tail is controlled by oscillation. You can even set it so you get more at the tail than the head. I used to a heidelberg data sheet to tell the diffrience for all the numbers marked on the oscillation wheel.

                        Terry

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                        • #13
                          Re: Density difference gripper to tail

                          It sounds like an adjustment of the *ink charge* is needed, which is done on the drive side of most presses. What this setting does is sets the timing of when the ink ductor hits the fountain roller therefore.......(replenishing) the ink supply. You can set the timing point to anywhere along the plate you like......including the plate gap. We use this setting quite often. It coincides with the ink ocillation that Terry mentioned. Proper setting techniques should be found in your Operators manual or can be received from the press manufacturer.

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                          • #14
                            Re: Density difference gripper to tail

                            Thanks for all the great info and help, it really is great because we are a small in-house shop, no one here including pressman has ever worked in printing outside of our place, so no real world, round the clock, all kinds of jobs, experience.

                            Ok, some further info on this problem from a job ran yesterday. It seems magenta is high in the first 1-2 inches of the sheet and then drops off from there to about 10 points low in the middle (8" back) and 3-6 lower even than that at the tail, 19 inch sheet.

                            Magenta seems to stay about 10-16 points lower gripper to tail. The other colors KCY 2-6 points different at the most. Then we read a sheet later in the run and we have the same thing in black and cyan on that sheet, a 10-13 point difference gripper to middle of the sheet!

                            Now I've measured differences from the same run where it is the same, gripper to tail, in the black and cyan and then 10 points different from gripper to middle of sheet?

                            Are we just loading up at the gripper for some reason? Would it come and go? Could it be water balance? This was a 25,000 run, so not small but not all day either.

                            Any more input?

                            Thanks again!

                            Terry Walker

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                            • #15
                              Re: Density difference gripper to tail

                              First off, I don't think its even possible to be 21 points off from gripper to tail... unless something "large" in between the two points is taking the ink off. The only real test to determine how much is to print a blank form with a gripper color bar and a tail color bar.... maybe one in the middle for reference as well. Then see if its really that far off. All presses have a gripper to tail ink variation. Its determined by the ductor timing of the press, and the form roller circumferences and spacing. Now if someone has some rollers not touching in the ink train... and has basically incapacitated one or more of the form rollers by starving them of ink... it might be possible.

                              Also your "test" needs to be conducted one unit at a time, as the possibility exists that the unit is putting the ink down and successive units are back-trapping it off, or mottling the solid patches of the color bar. So when it mottles up... and you start reading the paper = lower ink density.

                              I have heard the argument for years that the colors bars belong on the tail... but for no real reason. Yes they may read slightly different on the tail than on the gripper, but it makes no difference as long as you determine what the standard densities are for your shop. If you run them the same density every time, you will be printing the same every time.

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