Dot Gain and Density shift from G7 Targets (Web Press)

[BKR]

We are a running a 6-unit heatset half-web press. We completed the G7 qualification in February of this year. For approximately 6-weeks after the qualification, we were able to match proofs by running at the target solid ink densities we established during the G7 process. We had very low make-ready waste during this time and the pressmen were able to set the density targets in the closed-loop color system and know that the color would match the proof.

At some point, we began to drift from the density targets we established with the G7 curves. It is evident in both conventional and FM work.

[Nailing down the exact point in time has been difficult because we ran several film based jobs and jobs on uncoated papers around the time this issue started. 98% of our work is done at 175 LS on gloss or dull paper.]

Looking at the data, the variance is in the Black, Magenta, and Yellow. The Cyan is printing right where it is expected to print.

Targets
Density Dot Gain
K 1.63 23-25
C 1.35 16-19
M 1.30 18-21
Y 0.92 17-20

Actual (Average density across the sheet from 40+ jobs)
Density Dot Gain
K 1.56 28.3
C 1.36 17.9
M 1.20 23.1
Y 0.90 25.3

My theory is that there is excessive dot gain in the K, M, & Y. In order to keep the gray balance and match proofs, the pressmen are reducing the density targets. This provides us with acceptable color, but I am losing the efficiencies of having a calibrated press that runs at consistent density to match proofs.

I am looking for potential root causes of this problem and I would like your help.

I am inclined to believe the problems lies in the pressroom rather than Pre-Press. My ink is the most likely suspect and I am working with my vendor to analyze the ink.

Do you suspect there could be anything in pre-press that could cause this type of problem? If it were in all the colors, I would think a pre-press issue would be a more likely cause.

I do not believe it is blankets, fountain solution, packing, roller settings or temperatures as these items have been changed or are common to all printing units. We have run the inks in different units and have had the same results.

Any help would be greatly appreciated.

Thanks,

Brian

 

[Greg Imhoff]

Color Drift is a real issue everyday. Static targets are best at the time established and so SPC collection tools help to monitor and report. Regarding color drift does the data record the paper types and LAB value data sets?

 

[arfamoe]

Are you 100% certain your issue does not lie in the realm of Blankets, packings, ink/water balance, slurring/doubling(mechanical) or other such pressroom devils, have you tried another ink supplier to validate your suspicion about the ink. Can you measure your plates to verify they are not causing the issue.

 

[Dr Who]

Solid Ink Density is one of the biggest factors on press for increases in TVI/Dotgain and the fact that you are reducing SID's and getting increased Dotgain points to something being seriously out of whack. As you pointed out the increase in TVI across the majority of units suggests consumables or pre-press. To eliminate pre-press measure the dots at specific ranges on the plates with a dot meter if they are within spec then its the press room. Stating the obvious but always look back at anything you have changed: New Paper types, plates, blankets, founts , temperatures in units and chilled water, anything. If nothing changed then have the ink independantly analysed, if in the UK Leeds university is a good place to start with this as they have excellent facilities and are looking to get involved with industry, if this seems too much keep everything the same and trial a new quality ink, always remember the rep works for the ink company and is selling you something. If all of this fails to identify the problem have the closed loop colour system camera looked at to ensure correct calibration and that the illumination is correct