QMDI Plus doubling problem

[ronrogers1234]

Seems when we have heavy coverage we experience really bad doubling of the black which obviously effects registration and color. It has happened on 80# gloss text, 100# Gloss text, 12pt C2S so it does not seem to matter which paper. Also we have tried short grain text as well as long grain cover versus long grain text and short grain cover and have not seen a difference. The doubling is not all the way to the tail.
If we back way off on pressure (losing image to paper) the problem goes away. My pressman thinks it has to be grippers. We have checked all grippers and it seems they are holding fine but maybe during impression it is pulling out.
We are packing our blankets .016" for a total of .093".
Our pressman has been running DI's for 3-years and is comfortable setting the feeder properly and is stumped?

 

[Paul Cavanaugh]

There is a post that covers this subject you may want to look over if you already have not: http://www.colorprintingforum.com/direct-imaging-offset-press/slurring-color-shift-2978.html

Keep in mind bearer height on the QMDI blanket cylinder is .090. If you pack too much over that you will cause premature plate wear.

The buckle control for sheet advance can effect doubling as well as the sheet hold down brushes in the feeder. I have seen these brushes worn so badly there are only a few hairs left on them. I have also seen them so full of blanket wash from people over filling the tray they remain bent in a permanent position no even touch the paper.

Check these items and see if it makes a difference.

 

[Primadonna]

Since there is no transfer cylinders on this press you can rule out gripper problems since the sheet is held by the same gripper through the printing process. If you had a gripper problem on one gripper, you wouldn't see movement on the other units/colors but just on every 4th sheet, not just in the black. If you are not overpacked, then check your plates. But one question. Did this problem develop after a plate change? I always say go back to the last thing that you changed, if so, retighten the plates and try again.

Good luck and keep us posted.

 

[mrheidelberg]

Since there is no transfer cylinders on this press you can rule out gripper problems since the sheet is held by the same gripper through the printing process. If you had a gripper problem on one gripper, you wouldn't see movement on the other units/colors but just on every 4th sheet, not just in the black. If you are not overpacked, then check your plates. But one question. Did this problem develop after a plate change? I always say go back to the last thing that you changed, if so, retighten the plates and try again.

Good luck and keep us posted.

What would happen if there was a substance on the gripper pad and the sheet was slowly being pulled out as it was passing through each of the separate colours due to maybe differing ink tack on the blankets giving a slightly different fit which could then give a different colour ?

 

[Primadonna]

What would happen if there was a substance on the gripper pad and the sheet was slowly being pulled out as it was passing through each of the separate colours due to maybe differing ink tack on the blankets giving a slightly different fit which could then give a different colour ?

Again if this is the case you would only see the double on that particular gripper with debris on that pad, not the other 3 grippers saying they do not have debris in the exact same place, which is highly unlikely. If its every sheet, there is an issue elsewhere.

Lets assume here that they have not changed ink so rule out the tack. But he did say on heavy coverage, so no issues on medium to light coverage????? I have seen tack do other things, not double. Did you change the blanket(s). Check to see if the new blanket is the same caliper thickness. If its not and you are packing it the same as the rest, this may be your problem. Check if there is any build up of ink on the impression cylinder or print a smaller sheet to see if it goes away.

Doubling is caused when the sheet moves as it prints in the later units. So you will see black, cyan ans magenta double, never yellow because yellow doesn't transfer to another blanket like the other colors do.
Try this next time. Print blk, mag and yellow then check the double. Then print blk, cyan and check for the double. Then print blk and yellow and check the double. This will do a process of elimination to see what unit is giving you the problem.

I run a set of color bars that have a slur bar and double marks built into the CMYK bar that I constantly monitor for quality control.

Good Luck!

 

[Paul Cavanaugh]

Primadonna is correct in his assessment for troubleshooting doubling, it is amazing how many people confuse slur with doubling. You must be printing at least 2 units to see doubling. Having slur bars as well as doubling targets in the color bar are important to track these types of issues.