2008 CD102 Double Image

[Kris]

Hello all. I am having an issue where we get a double image in the tail corners of the sheet. I do believe that we are having a problem with paper stretch that is leading to this, but the problem seems to only show up with heavy coverage on the first unit. It is also more noticeable if we have to push the ink hard to achieve proper density levels. Can an excess amount of ink going to the sheet cause this doubling effect when coupled with paper stretch. Any help with this is greatly appreciated. Thank you!!!

 

[junker1984]

Model year? Total impression count? Sheet weights that show up more? Sheet weights that show up less, if so. More info needed.

 

[turbotom1052]

Have you tried running enough sheets of a known good quality to determine if its the paper? Any time I have any sort of fit issue or doubling issue I like to throw in a lift of 1st quality stock. In the event that crappy paper is all you have available I would try testing with a sheet a little heavier. You will of course need to adjust your pressures to accommodate the test stock. Be sure to throw in a big enough lift to allow the press to come up to production speed and to get the tack settled down. 25 sheets for this test is just not gonna cut it. Id think you would need to run at least 100 to 200 sheets. Once the tack has settled down you will want to pull out at least 4 to 5 consecutive sheets and look at each of them in the tail corners. Doubling problems from crappy stock will usually show up with heavier coverage jobs in the tail corners of the printed sheet.

 

[Kris]

The press is a 2008 CD102 with about 250 million on it. We only run 60# C1S here and don't have anything heavier on hand to test with. I know that 60# is already a very light sheet and prone to stretching in the tail, but I seem to only get the doubling issue with dark, heavy coverage colors. When I flip through a finished stack you can see the doubling fade in and out, which had me thinking the ink from the first unit was offsetting onto the second unit blanket and then printing to the next sheet that may have a little bit more or less stretch. Not sure if this is a possibility or not. Thank you junker and turbotom for the response on this!

 

[turbotom1052]

Kris my guess, based on the 60lb CS1, is that your doing labels? Can you impose the labels so that the heavier coverage ones are closer to the gripper? Also seeing how you seem to only print on lightweight stock, I'm thinking that you can set up all your grippers a bit tighter to hold the sheet better. I do believe there is an across the board adjustment where you don't have to go gripper by gripper setting each individually. Junker can better advise on that.

 

[junker1984]

Common misconception, Tom. There has never been a means to "tighten" the gripper's hold on a sheet. The adjusting set screws on each gripper's holder is for timing of open/close only. Pressure is determined by the springs in the same holder assembly. If you change the set screws, you're only changing the opening and closing time of each gripper. Given the amount of impressions Kris has on his machine, given gripper/pad replacement hasn't already been done, it's long past time. This is a major undertaking, time and expense. First, Kris, be sure your pads are thoroughly cleaned with a wire brush. Second, closely examine the cam followers on impression and triple drum gripper bars. Keep in mind, "eh, not too bad" is not good enough. By examine, I mean visually and by feel. A real bad follower will show scoring on its running surface, but a slightly worn follower (which can cause variances in timing) may not show signs on the surface. Triple drum followers are easy to check, but you have to lift slightly with a pry bar on the levers of the impression cylinder gripper bars to raise the follower off the cam.

 

[Kris]

Turbotom - Yes we are printing labels. We do our best to put the heavy coverage near the gripper, but at times orders are large enough that the item goes across the whole sheet.
Junker - I will take a look at the gripper pads and cam followers and see if anything is noticeably worn. I just had another pressman bring two sheets for comparison as well. One was perfect on the whole sheet, put a new load in the feeder and all the sudden the double image was there. We switched that load out and the problem went away. Your tips to check are very helpful and I thank you very much for the help.

 

[turbotom1052]

Common misconception, Tom. There has never been a means to "tighten" the gripper's hold on a sheet. The adjusting set screws on each gripper's holder is for timing of open/close only. Pressure is determined by the springs in the same holder assembly. If you change the set screws, you're only changing the opening and closing time of each gripper. Given the amount of impressions Kris has on his machine, given gripper/pad replacement hasn't already been done, it's long past time. This is a major undertaking, time and expense. First, Kris, be sure your pads are thoroughly cleaned with a wire brush. Second, closely examine the cam followers on impression and triple drum gripper bars. Keep in mind, "eh, not too bad" is not good enough. By examine, I mean visually and by feel. A real bad follower will show scoring on its running surface, but a slightly worn follower (which can cause variances in timing) may not show signs on the surface. Triple drum followers are easy to check, but you have to lift slightly with a pry bar on the levers of the impression cylinder gripper bars to raise the follower off the cam.

My bad. I thought I remember there being such an adjustment.

 

[Kris]

Hey guys, we did find a cam follower that was in pretty bad shape in the first unit. Fingers crossed that our problems go away after this. Thanks again for all the help!

 

[turbotom1052]

Not to piss in your cheerios, but id think where this is one bad cam follower, there are others on the verge of going, especially considering the high impression count. If you need to get a tech in to do the job it might be prudent to do the entire press, if you can fit it in your budget.

 

[Kris]

Ha ha. We did take a look at the rest of them and they seem to be ok. It appears that for some reason, that one was not getting greased. I will definitely keep my eye on the rest though. The "budget" always seems to equal as minimal as possible to run. Understandable for the business, but can definitely make operating the press a nightmare.

 

[turbotom1052]

If the problem persists even after replacing cam followers Id look at trying to make as many adjustments in Pre Press as possible. It will require cooperation from the keyboard jockeys. They need to understand that their work habits need to be altered for the greater good. Pre press guys need to whenever possible image the problematic labels closer to the gripper, and allow the less register sensitive images to fall to the tail. This unfortunately is not always possible, because of all the dies that are already made. If these register sensitive labels MUST be imaged towards the tail of the sheet you have to find work arounds. On labels created from 4 color process, you can have prepress adjust their curves so that they pull out a bit of the C,M,Y and run more black in the rich black portions of a label. This will often help with the dimensional stability of the sheet. You could also try altering your sequence so that the colors with less ink on the tail images run down first. On 4 color process images with drop out type, you could have prepress make the typeface a little bolder, especially if the typeface has very fine serifs. You may get a bit of pushback on this from pre press, because they often want to just do things the way they always did things, but we are talking about the greater good here. On press you could also experiment with different blankets and different packings. You will probably want a blanket with as quick release as possible. You would probably also benefit from lower tack inks run in descending tack, especially on 60lb coated litho stock. Lastly I would say that you should look into the possibility that it might be more feasible to alter your layouts to run problematic jobs on a smaller sheet. When it comes to sheetfed labels perhaps instead of running the full size sheet, you can run a smaller sheet and just make an additional form, or alter your counts. Unless your running very long press runs where every square inch of the biggest sheet possible needs to be utilized, then its often best to run a smaller sheet and perhaps add a form or two. This smaller sheet although not utilizing all the possible real estate of the press, can often be made ready quicker, run at higher press speeds, and run with less potential for many press related problems.

 

[Kris]

Turbotom - These are all very good tips. Thank you. Our guy that lays out sheets is actually very open to changes if they are needed so I will definitely talk to him about these things. We recently made the change to a six color process so we run K C M Y in units 3 through 6 so spot colors are always ran in units 1 and 2 if we have to run them. We also have an SM74 here that I love running because we don't have the stretch issues like on the big presses and it definitely does set up a lot quicker. We are always utilizing every inch of the sheet and it's hard to justify not doing that for cost reasons.

 

[turbotom1052]

Its best to match the tool to the job. A CD is a great press for what its designed to do, and that is heavier stock printing. Its my feeling that when it comes to running lightweight stocks such as 60lb litho the better tool for the job is a Speedmaster. That doesn't help you much seeing how you already have the CD but I can offer this bit of info. A CD as its engineered will offer a larger volume of air than a Speedmaster. When your running the big lumber through the press, this higher air volume is very welcome. When it comes to sheet control during impression and transfer from unit to unit, this higher volume of air gets a little difficult to control at the low settings, which is what would be required when printing 60lb C1s. There also is the issue of the transfer shells. I don't remember what year it was that Heidelberg did away with the transfer shells but if your model year CD was so equipped then id suggest you install them if their not already in the press. I do seem to remember modifying one CD we ran with a large bleed off valve to dump a bunch of air right out of the big turbine style compressors. By bleeding off a decent amount of the compressors output BEFORE it got to the presses valves it allowed a finer level of control at the smaller valves. One thing you don't want is to have that sheet blowing all over the place as it transfers from too much air.

 

[Kris]

I will have to look into that. I believe we have a valve set up to bleed under the feed board.