Excessive dot gain (or other) - please give me the advice!

[DeltaE]

Dear Experts,

Please find in the attached pictures which I took the 40% and 80% dot area in the printed sheet.

The above picture is the reference that we had printed last month. The below picture is the printout today.
You can see the difference in the black and magenta color. The dotgain of black and magenta in the printout are higher than in the reference.
The cyan and yellow looks good.
Therefore the printing image looks very dark and more magenta than the reference.
We always control the printing plate, and the dot area of the printing plate is in our tolerance (1%).

I had checked blankets, thickness of blanket packing sheets, pH and % alcohol of dampening solution, used as little amount of dampening solution as possible.
This job is printed on the same machine, same ink, same dampening solution, same paper, same color sequence; I had checked and confirmed that no slurring or doubling in this machine.
I had washed the ink rollers carefully, used the original ink from the package and pre-inking again.

But I still can not fixed this!
Anyone has idea please give a advice! I am very appreciated!
Thanks in advanced!

Regards,
DeltaE

 

[cementary]

Did you use same paper thikness for both printouts?
what about impression settings - were they same?
are you sure that you use same ink? We've found out that our ink vendor sometimes change formulation without bothering to tell us about it.
to me it looks like your black and magenta inks are over emulsified in the second run

edit

oh, and what about temperature? Do you have an ir Pyrometer to quickly check actual temperature in ink fountain and on rollers?

 

[Andrei83]

Check the tempering of the inking unit. the temperature is 28-32. The new edition from the very beginning is darker? 40-80%? Or in the process of printing a print run?

 

[Andrei83]

Perhaps a dirty moisturizing solution, you need to try to drain and a new pour. Conductivity looked?

 

[yavo]

Did you check the dot shape. Looks like a little different. Check ctp developer chemical. it maybe be old.
And ink fluidity level sometimes be different.

 

[gordo]

SIDs are the first thing to check. Amazing that it took after so many suggestions it took this long for it to come up as a possibility :-( Another thing to check is the image content in line with the patches that were photographed. Also, do all the patches across the color bar look the same or is there variation across the width of the sheet. Also is this happening through the run? Or just at the beginning or end? How long ( number of impressions) is the run?

 

[turbotom1052]

Looks to me like you've got some distortion of the magenta dot. Also looks as if the densities of the magenta vary from the first printing to the current printing. As mentioned you need to compare SIDs from the first printing to the current printing. When doing this allow for app. 5 densitometer points difference to account for dry back. You also mention the same ink in your checklist. When you say the same ink, can we assume that on both printings the ink was loaded into the fountain from a fresh sealed can? I know its painful to dump the seemingly fine ink, left in the fountain at the end of a day, but if your goal is truly repeatable press results you need to eliminate this variable.

 

[Manprint]

It looks like fibers in 80% are you certain about same supplier/batch of paper?

 

[Alois Senefelder]

Hello DeltaE,


Dot Gain Problems !


Your printers seem to have a poor understanding of Lithographic Printing,


A first fundamental MUST DO -- GO and LOOK ............ A good indication of Ink/Water Balance is a "thin scum line" unbroken 1- 2 mm wide that forms

on the bend at the grip edge of the plate and extends the width of the plate.


Now, questions: a) the same plates were used for both print runs? b) were the plates "Gummed Up" between printings?

IMHO -- The Black 40% H/tone, show a sign of "low dampening level" -- look between the dots and theyshow "Ink Whiskers/Grains Forming" joining up the dots.

Magenta looks emulsified, 40% dots "Snow - flaky"

Also when posting pics of the Print Control Strip - include "Star/Slur Targets"


Regards, Alois

 

[DeltaE]

Did you use same paper thikness for both printouts?
what about impression settings - were they same?
are you sure that you use same ink? We've found out that our ink vendor sometimes change formulation without bothering to tell us about it.
to me it looks like your black and magenta inks are over emulsified in the second run

edit

oh, and what about temperature? Do you have an ir Pyrometer to quickly check actual temperature in ink fountain and on rollers?

cementary
I am sure that ink, paper is same as before.
I had washed the ink rollers, used the fresh ink, printed with amount of damping just above the scumming point.
About the temperature in ink fountain and inking rollers is very good point to check (that I forget) - thank you! ^^

 

[DeltaE]

gordo turbotom1052 Alois Senefelder Erik Nikkanen Andrei83 yavo Manprint

I am sure that we used same batch of ink, paper, same blankets and packing thickness as our standard.
I maintain the stardard of dampening solution with pH = 5.2, temp = 12 degree celcius, conductivity at that time was 1100 microSimen, IPA = 12%.
I printed with the amount of damping just above the scumming point (I can see the very thin ink line in the front edge of printing plate...)
There was not over-elmusification when I check the solid area by microscope; And of course, I printed with just enough amount of water. (it that right?)
Regards to the solid ink desity, we have to control the Lab of solid primary ink with delta E below 3 (so I think the solid density is not so much different between sample and reference)
I also check the plate before printing, in the black plate 80% --> 79.1% and 40% --> 34.3%; in the magenta plate 80% --> 80% and 40%-->35.2%; so the dot area in the plate is not so bad when compared to our standard 80% and 40% plus/minus 1%.

The print length of this job is 10000 pieces, I alway take the sample when machine is in stable condition.

gordo : yes, all the patches in the line looks the same as this patch. And I happens in all the time, no change from beginning to the end.
turbotom1052 : we just printed the reference in 6th december 2017, a month ago. We have enough ink to print every three month. I will double check again about batch number of ink. And this ink seems to be not so thin when manually checking it.

SIDs is very good way to check this machine. Also temperature of the inking rollers is very good advice for me (I forget this point ^^)
I will add the star pattern as Alois advice if I have chance.

Many thanks for your input! Have a good day!
I will come back if I found something!

Regards,
DeltaE

 

[DeltaE]

I do not know how to relate your range of DeltaE below 3, to the range of ink volume in the ink film.

If we would be talking about density, then a range of SID of +/- 0.05 density points can be a range of about +/- 7% of ink volume in the ink film. This means that the total range is about 14%. 14% difference in ink volume can be considered to be in tolerance based on the density target tolerance but a 14% difference in ink film thickness IMO is quite large and could result in differences in dot gain even though the SID would be in tolerance.

Some sources even allow a density tolerance of +/- 0.10 points, which would even be much higher in ink film difference.

I don't think using Lab values is wise for process control but that is what is done at some places. You might be thinking you are controlling ink film tightly but it may be that the DeltaE tolerance results in a wide range of allowable ink films, which could result in unacceptable dot gain ranges.

Just a thought.

Erik Nikkanen You are right! Lab is not the way for controlling the ink film thickness in the offset machine.

I had measured the SID of black and magenta again with function relative ink density (status T) then get the result:
Black: ref = 1.99, sample = 2.01
Magenta: ref = 1.25, sample = 1.32
From this measurement, I dont see any big difference of SID.

As I know, dot-area = density of dot-area / SID.
So if the SID is changed and still in the tolerance, the density of dot-area will be also changed linearly.
From my experience, the dot-area value (or dotgain) will not change so much if the SID is still in tolerance.

When I wrote this topic, I mentioned "excessive dotgain (other).." because I think dot-gain is only the result of something wrong.
Then when looking more times at the 80% area, I find something like the "fill-in" effect. The non-printing area is not kept clean as in the ref. In my opinion, I think there is something wrong with the combination of ink and dampening solution.
What do you think?

Thank you!
Regards,
DeltaE

 

[gordo]

I had measured the SID of black and magenta again with function relative ink density (status T) then get the result:
Black: ref = 1.99, sample = 2.01
Magenta: ref = 1.25, sample = 1.32
From this measurement, I dont see any big difference of SID.

Assuming your measuring device is correct, your K SID/ink film thickness is too high by about 20 points. Your M is to low by about 10 points - your ref by 20.

As I know, dot-area = density of dot-area / SID.
So if the SID is changed and still in the tolerance, the density of dot-area will be also changed linearly.
From my experience, the dot-area value (or dotgain) will not change so much if the SID is still in tolerance.

That's not the formula for dot gain.

When I wrote this topic, I mentioned "excessive dotgain (other).." because I think dot-gain is only the result of something wrong.

Dot gain is not the result of something wrong. It is inherent to the process. It is a process control metric not a target. Tone reproduction is the target.

Then when looking more times at the 80% area, I find something like the "fill-in" effect. The non-printing area is not kept clean as in the ref. In my opinion, I think there is something wrong with the combination of ink and dampening solution.

An AM halftone has an inherent ink/water balance conflict. It needs minimum water in the highlights to avoid washing out the dots but maximum water in the shadows to keep them from filling in.

 

[DeltaE]

Assuming your measuring device is correct, your K SID/ink film thickness is too high by about 20 points. Your M is to low by about 10 points - your ref by 20.



That's not the formula for dot gain.




Dot gain is not the result of something wrong. It is inherent to the process. It is a process control metric not a target. Tone reproduction is the target.



An AM halftone has an inherent ink/water balance conflict. It needs minimum water in the highlights to avoid washing out the dots but maximum water in the shadows to keep them from filling in.

gordo
Thank you! I have just received a basic lesson!
I mean excessive dot gain when 80% in the plate becomes 97-98% in the printout.
Yes, your point shows me an idea that I should check the dampening solution again; it seemed to be able to clean the hafttone area but not able to clean the shadow.

Erik Nikkanen
Thank you for your support!

 

[cementary]

A first fundamental MUST DO -- GO and LOOK ............ A good indication of Ink/Water Balance is a "thin scum line" unbroken 1- 2 mm wide that forms
on the bend at the grip edge of the plate and extends the width of the plate.
Regards, Alois

Alois, from my experience "scum line" has nothing to do with the dot distortion due to lithography troubles. We've always seen this scum line on plates. Our printmen cannot add that much water to prevent it appearing

 

[cementary]

I had measured the SID of black and magenta again with function relative ink density (status T) then get the result:
Black: ref = 1.99, sample = 2.01
Magenta: ref = 1.25, sample = 1.32

Funny, but we have same SID target for black (Status E, but it doesn't matter for black). gordo mentioned that it is too high, but for our ink we can't go that low, 'cause at 1.8 we'll see Lab 21 0 0.
This lead me to another thought — was ink takeoff the same over printsheet in both runs?
With our ink we have noticed long time ago that when we have a little actual ink area coverage over the sheet — we have pluggy shadows and distorted dot. Especially with longer runs, closer to the middle of the run.
On the contrary, with high area coverage we have clean shadows and less distorted dot.

So i wonder — were measurements the same with first 500-1000 sheets?

 

[Alois Senefelder]

Cementary,


I did not claim that the "Scum Line" had anything to do with "Dot Distortion" it is merely the first step in eliminating a variable input, put simply for you - "A very good first visual indication of Ink/Water Balance"

Regards, Alois

 

[Alois Senefelder]

DeltaE,

You did not answer my questions about the Plates!


Make and type of plate ? also I would look at the Chemistry of your F.S


Regards, Alois

 

[Alois Senefelder]

DeltaE,


Also check the Shore Hardness of the Plate Dampeners and Metering Rollers


Regards, Alois

 

[gordo]

Funny, but we have same SID target for black (Status E, but it doesn't matter for black). gordo mentioned that it is too high, but for our ink we can't go that low, 'cause at 1.8 we'll see Lab 21 0 0.

The specification is, AFAIK, that at K 1.70 (dry, non-polarized) you should have L 15, a 0, b 0

If you're not getting that then that suggests that:
A: your polarized instrument is affecting the reading
B: your instrument's calibration may be off