Spotted/mottled background print result

[petegiar]

Hello,

This is my first post. I do prepress production for the publisher of the Carmel Gallery Guide. The publisher had a problem on a press check with a gray background that printed very mottled. The printer said it had nothing to do with the prepress or image, but was a press issue.

Can someone tell me what might cause this?

The form was 40" x 28" run on a Heidelberg duplex sheetfed press.
The area where this problem occurs is on the gripper side, in the corner.
Total ink coverage is less than 300% total ink density. Dark areas show more of the spotting.
Gray area ink values are approx. 77% cyan, 67% magenta, 57% yellow, & 60% black.
Our ad page size is 4" x 9".

When I inspect the form it is slightly out of register in the affected corner (1/2 point), but this doesn't look like a registration issue.

Attachment is an attempt to show the spotting, but image quality is poor.

By the way, we love our printer, but would have liked to get a better result for this particular image.

Thanks in advance,
Pete

 

[Boneck]

It sure looks like the paper grain to me...

 

[gordo]

It does look like mottle.
Left normal -right mottle:

Mottle can be caused by the paper (but then it would show up elsewhere]
It usually results from the degree of emulsification (which effectively reduces the ink's transference) or the presence of excessive numbers of relatively large water droplets. The printed film is effectively broken up or interspersed by a series of lighter spots which correspond to the droplets of water which were present as the ink film was laid down. Too much water can also bring about mottle. If the problem is mild, then some over emulsification may occur but not necessarily enough for classic emulsification symptoms to show themselves.
Mottle can also be caused by too low an impression cylinder setting (or if localized - uneven impression cylinder pressure).
To properly diagnose the problem you'd have to look at other aspects of the presswork (halftone dot integrity, eveness of solids etc.).

 

[Deleted member 16349]

It usually results from the degree of emulsification (which effectively reduces the ink's transference) or the presence of excessive numbers of relatively large water droplets. .

I would say that that is a myth. Over emulsification does not prevent the transfer of ink. This is based on the tests I have done with positive ink feed. It would be hard for other observers to see this relationship since conventional presses, that do not have positive ink feed, seem to show excess water as being a factor that reduces ink transfer. It is a wrong conclusion. On conventional presses, less ink is transferred in the print due to the roller train being starved of ink at the ink feed due to the excess water on the roller train and not due to some conditions at the paper surface. But I know many will disagree with this but that is what IMO happens.

 

[gordo]

Erik, what are your thoughts regarding the OP's problem?

 

[Deleted member 16349]

Erik, what are your thoughts regarding the OP's problem?

It is hard to tell from such a small print sample and it could be many things. I wonder if loose blankets or smashed blankets in that area might be the problem. Maybe also variations in wet trapping due to all CMYK screens being printed with some misregister.

I wonder what kind of screens are used there. Is it AM or FM?

Also the print of hands don't look so clear either. Don't know if that would be related. Just an observation.

 

[Alois Senefelder]

Not a Myth !

Not a Myth !

Erik and fellow Lithographers,


Mottling - a PDF that I hope you will find of interest and value.


Regards, Alois

 

[Deleted member 16349]

Erik and fellow Lithographers,


Mottling - a PDF that I hope you will find of interest and value.


Regards, Alois

I think you are basically wrong. Do you have any experience with a press that has positive ink feed? There is a need to test these ideas on equipment that can control variables.

So for 100 years, the printing industry makes tests on equipment that can not control the input variables, such as ink feed and people think they have an understanding of how the process works. I don't think so.

 

[Foster]

I have seen this at our shop, a few times. We will hang new packing and blankets, to see if it helps the mottling look. It usally gets better, but sometimes dont completly go away. Then we look for a better grade of paper, and thats when we see a big difference.

 

[keith1]

Can't tell from the image posted but a mottled appearance can most often be traced back to pressure. Start with the easiest - blanket packing and roller pressures. If it's just one small area it could very well be that the blanket is smashed & needs replacing.

Keith

 

[D Ink Man]

Would you like to fix this with INK perhaps?

Many printers often times do.

D

 

[turbotom1052]

Have you tried a different ink set? Or a different ink sequence? Looks to me like it could be a trapping issue. Even if the ink set your using was always good to you, on occasion a bad batch will get through where the tack, on one of the inks are off. One by one try turning off the impression on a single color, and try to pinpoint the offending color. Once you've established what color is causing the problem you can remedy the trapping issue by adjusting that inks tack either up or down as needed.

 

[Alois Senefelder]

Image Area

Image Area

Hello Petegiar,


In order to help you we need the following information.

1) What is the sheet size? 2) where is this 4 x 9 image on the sheet? 3) Grip Edge/ Leave Edge or Centre

and is this on the Work Side or Gear Side of the press sheet?


If possible post a GOOD pic. of the printed press sheet, indicating the said image.


Regards, Alois

 

[Jim Klann]

Otherwise known as backtrap mottle ! Usually seen on a 5 color or more and usually a cyan / magenta issue. It's an ink issue involving tack and ink film thickness (tack & ink film thickness work hand in hand). Unfortunatley it shows up during make ready and who wants to tell the boss you have to wash up, mess around with the ink and hope you can fix it while the customer is standing there wondering what it is that you don't know how to fix

You really have to love this biz to enjoy it !

 

[Richard]

blanket to impression, another click (s) increase

 

[BeauchampT]

Leaving Erik's usually fascinating comments on the nature of ink aside.....

If it is one corner of the print, probably a low blanket - mottle with heavy coverages is a very typical pressure problem that has nothing to do with emulsification.

Paper is also an option, however, you could rule that out if a similar image is being printed on anther part of the sheet without any mottle. Quick test would have been for the OP to run the image backwards (change the problem image to another corner of the sheet by remounting the plates differently).