Registration Error from bad Cylinder Geometry?

[chriscozi]

Hi All,

Looking for some knowledgable feedback. Mitsubishi 6 color.
Old press operator (OPO) retired after 6 months training New Press Operator (NPO)
NPO says 'Yellow is always off register. OPO always split the difference.'
Hmmmm. Two diagonal corners can fit.
Plates are good. Checked tifs at imager and they are exact.
GM says 'Not the press.'
We are trying to diagnose by switching units/plates etc.
I say cylinder geometry and or plate mounting.

Thoughts? Comments?

 

[Cornishpastythighs]

I assume yellow is last down colour. Has the problem been there forever, any changes to blanket, plate, packing suppliers?

 

[chriscozi]

I assume yellow is last down colour. Has the problem been there forever, any changes to blanket, plate, packing suppliers?

Yup. Last of six possible.
Always been that way per new operator.

 

[SheetSqueezer]

Curious on a 6/C press, why you’re not using units 2-5 for KCMY? Especially if this is an ongoing issue. That’s practically SOP at most shops I’ve been at and would also present an easy fix.

 

[chriscozi]

Curious on a 6/C press, why you’re not using units 2-5 for KCMY? Especially if this is an ongoing issue. That’s practically SOP at most shops I’ve been at and would also present an easy fix.

Press Operator said 'worse.'

"We are trying to diagnose by switching units/plates etc."

(See original post)

 

[SheetSqueezer]

Press Operator said 'worse.'

(See original post)

It may take a couple of hours to diagnose correctly but worth it to troubleshoot the issue; if all 6 units are functional, I suggest pulling a grid test on all of them together to see where the press is at. Make sure all units are identically packed on all cylinders, and that all impression cylinders start out buried and then come off that setting incrementally, together. This part is especially important as they are the ones that are always moving from thickness adjustments, job to job, and if they become out of sync, it’s not unusual for one or more of them to create a consistent image distortion.

 

[chriscozi]

It may take a couple of hours to diagnose correctly but worth it to troubleshoot the issue; if all 6 units are functional, I suggest pulling a grid test on all of them together to see where the press is at. Make sure all units are identically packed on all cylinders, and that all impression cylinders start out buried and then come off that setting incrementally, together. This part is especially important as they are the ones that are always moving from thickness adjustments, job to job, and if they become out of sync, it’s not unusual for one or more of them to create a consistent image distortion.

All units are packed indentically (new).
Impression cylinders buried??

 

[Alois Senefelder]

Hello, chriscozi and SheetSq.

I agree - use the "Grid Test"

Clarification re Printing Press Cylinder Config. - The Impression Cyl. on most modern Presses run in FIXED Bearings any
adjustment is set via the Blanket Cyl !!!

Also, have a qualified Press Engineer - Check the Backlash Gears

 

[SteveSuffRIT]

When you say the register is off, can you be more specific?
It's always yellow (last down) but is it always shorter or longer around the cylinder or across the cylinder? I assume paper is grain long (sheetfed), ~60# coated and flat, not wavy or tight edged.

 

[Alois Senefelder]

Hello SheetSQ.
Clarification again! the Blanket cylinder adjustment moves toward or away from the Impression Cylinder
which does not have Bearers!

Regards, Alois

 

[Cornishpastythighs]

Most presses have fixed bearers on plate/blanket cylinders and adjustment for squeeze is done via changes in packing. There are adjustments however for the Pre-Load for the bearers usually checked by using a foil strip. This should only be done by a qualified technician
To allow for different thickness of stock the Impression cylinder is moved closer or farther from the blanket cylinder. Has anyone used a packing quage to check correct squeezes

 

[Alois Senefelder]

Cornish, I'm surprised at your answer! and SheetSQ

Enlightenment !!!!!!! Read the text on the PDF

Regards, Alois

 

[Cornishpastythighs]

Cornish, I'm surprised at your answer! and SheetSQ

Enlightenment !!!!!!! Read the text on the PDF

Regards, Alois

Always willing to learn Alois , what press make does this PDF refer to, some presses have moveble blanket cylinders others move the impression, of course all press blanket cylinders must have some movement to allow for installation of blankets and plates when off impression
cheers

 

[Alois Senefelder]

Hello Cornish, The PDF is from the M.A.N press manual, a challenge for you, find any modern printing
presses that move the impression cylinder, if you do - I will enter a Cornish Pasty eating competition!

Regards, Alois

 

[Cornishpastythighs]

Hello Cornish, The PDF is from the M.A.N press manual, a challenge for you, find any modern printing
presses that move the impression cylinder, if you do - I will enter a Cornish Pasty eating competition!

Regards, Alois

I think maybe a Komori Chambon still moves the impression cylinders for different stock thicknesses. Just in case I am baking you a special treat Cheers

 

[chriscozi]

When you say the register is off, can you be more specific?
It's always yellow (last down) but is it always shorter or longer around the cylinder or across the cylinder? I assume paper is grain long (sheetfed), ~60# coated and flat, not wavy or tight edged.

One corner of the yellow at the tail.
Across (small) AND longer.
Yes long grain and stable sheets.
Confirming to you on Tuesday Wednesday as it's a holiday here.

 

[turbotom1052]

Yup. Last of six possible.
Always been that way per new operator.

Chris... I ran that very press a number of years ago and I can tell you that problem did NOT exist. You are saying that the press is packed to specs on all units with new blankets, but what you're not saying is if the plate and blanket measures up to the correct height with a packing gauge???

 

[chriscozi]

Chris... I ran that very press a number of years ago and I can tell you that problem did NOT exist. You are saying that the press is packed to specs on all units with new blankets, but what you're not saying is if the plate and blanket measures up to the correct height with a packing gauge???

According to the operator the packing on all blankets and plates is new and of the correct height.
Ok - the update went like this:
1. "Exercise" the impression.
2. Mount grid plates in correct units.
3. Note poor fit in the MAGENTA unit. (hmmmm)
4. Switch yellow-magenta plates, poor fit stays in the magenta unit.

So this gives the exercise some real data points.
It is not the plates ie the problem stayed in the unit. (As prepress expected)
It is not the yellow unit (operator misgivings aside . . . ) ie the problem moved to the magenta unit.
So the operator is left with a gripper issue, they believe.
They are now in the middle of checking all transfers etc.

Sound about right?

 

[turbotom1052]

According to the operator the packing on all blankets and plates is new and of the correct height.
Ok - the update went like this:
1. "Exercise" the impression.
2. Mount grid plates in correct units.
3. Note poor fit in the MAGENTA unit. (hmmmm)
4. Switch yellow-magenta plates, poor fit stays in the magenta unit.

So this gives the exercise some real data points.
It is not the plates ie the problem stayed in the unit. (As prepress expected)
It is not the yellow unit (operator misgivings aside . . . ) ie the problem moved to the magenta unit.
So the operator is left with a gripper issue, they believe.
They are now in the middle of checking all transfers etc.

Sound about right?

"Chris..."the correct height" measurement can be arrived at in multiple ways. Some may say that if you just add the blanket thickness thats marked on the back of the blanket, along with the thickness of all the paper packing, and just subtract that from the blanket cylinder undercut you will arrive at the blanket height over bearer. Problem with that method is that very often the imprinted blanket thickness and imprinted packing sheet thicknesses are not always accurate. Then there are others that in the absence of a packing gauge will measure the entire stack and just subtract the cylinder height below bearer to arrive at blanket height. This method would be more accurate but still fall a bit short if you're trying to troubleshoot a problem. It's been quite a few years so I dont remember if there was a packing gauge in the shop. A packing gauge for the purpose of troubleshooting would be the way to go. This would allow accurate measurement of both the plate height and blanket height above bearer. The total squeeze between plate and blanket should be .004" when using compressible blankets as measured with gauge. It doesn't matter much just how you get there, but more that you do get there, and that it's the same between all units. To measure .002 above bearer on the plate, and .002 above bearer on the blanket will work as well as any other combination within reason. I would say that your theory on gripper settings would be where my thoughts would go first. You could do a little test to prove this out. Run enough blank sheets to get the press up to its operating speed with all of the units turned off except what you suspect as the problem unit. You must run enough paper to get back to proper ink water balance. I would say at least 50 sheets. Then take then single color printed sheets and run them through the press a second time. This will tell if that unit registers to itself. You need to be sure that there is no headstop bounce or side guide misregister on both passes. If the suspected unit registers to itself, and it's established that there is no headstop bounce or side guiding problem, it would be pretty safe to say that the the grippers are not the problem. Keep us posted. Feel free to call if you need further assistance. Mark should have my number.