Press Dot Gain help please!!!!!

[contrakid]

Hi all.

I am struggling with large dot gain on press nearly all the time.

I am not a press minder i am prepress and like to think i know what i'm doing although i have my doubts about our minders and my MD is not technical.

We our running a 2006 Heidelberg XL105 sheet fed press. Fuji LH-PJE plates on a Supra Setter 105. Printing mainly brochures / leaflets corp magazines etc.

What i would like to know is what size dot are you putting down on plate to give you ISO dot gains on press for paper types 1 & 2 ISO 60.

To keep it simple lets say just for a 40% and 80% tint

The problem i am getting all the time on press is my dot gain is reading around 26% on a 40% tint in job.

In job = 40%
On Plate = 32%
Measured on Press = 66%

Would you say 32% on plate is somewhere around the standard for this sort of setup to give an ISO standard dot gain of around 14% on 40% (54% on press) or am i to sharp or to full?

On press yesterday they could not get a customers house colour to match proofs 100% Y 31% C. visually compared to a process tint book the were getting more like 100Y 50C. On plate measured 25%. I even got them to take pressure off on the press, run the ink weight down to 1.1D on the cyan and check blankets tightness and ink train lines and still on press my 40% on colour bar was reading 20% dot gain at 1.1D cyan!

I did not think it poss to make a 25% dot on plate into a 50% on press.

I must admit the paper was not great, 90gm matt art but even so!

What could they being doing? I'm sure my plates are within spec but they keep point the finger at me.

Any feed back would be much appreciated

Cheers

Simon

[Cornishpastythighs]

You should get Heidelberg in to conduct a press fingerprint. I would have to assume this was done at the time of installation/commissioning and someone signed off on the press as being ok. It sounds as though your process has gone out of control. If your plate/pre press system is ok then you have to look at the other inputs into the machine. Man-Methods-Materials. Dot gain that high on a fairly new press providing its not slur or double could be the result of excessive ink/water emulsification. How do your solids look, are they full of tiny white voids(water in ink) Have you checked the shape of the dots to eliminate mechanical issues. Next you should have your fountain solution supplier check out the dampening system for correct dosage/temp/cleanliness.
Next have your inking roller supplier come in and evaluate the inking rollers for settings,Temperatures, durometer and wash up procedures.
Have your ink supplier evaluate a press sheet and make recommendations. Have Heidelberg in to do a operator skill assesment on your machine minders, do all your minders have the same dot gain or is there one that seems to have much better results.
This may sound like a long winded answer to your question but unless its as easy as tightening a blanket to cure the issue I dont see any other way to truly track down the root cause and attack it.
Good luck

[Kaoticor]

Do you use a calibration curving program? If your dot gain is consistently high, it is as simple as collecting a trend and calibrating to match ISO specs. Also, make sure you are basing your dot gains according to the paper type you are using, and the numbers recommended by ISO. I know that goes without saying, but can't tell you how many times our prepress has mixed up coated / uncoated values in processing.

[PDeuth]

Dot gain has a number of important independent variables. The surface finish and absorbancy of the paper has the largest effect. "Squeeze" or impression pressure is a large contributer. The amount of ink, the ink film thickness, on the plate is important. So is the water balance on press. All these independent variables need to be known and controlled.

To control ink film thickness use the solid ink patches on your control bars, as well as the 75% tints. Too much ink is to be avoided, as is too little. Start with industry standard readings of SID and go from there.

To control "squeeze" get a packing gauges, both off-press and on-press, and use them. Keep your packing and pressures under control.

Ink/water balance is tricky. There are no dedicated targets for controlling balance. Back off the water until scumming occurs, then add a bit more water; look for sharpening in the 25% dot, or check to see if you're able to print a 2% dot.

Know your paper. Have seperate curves or profiles for different papers. Start small here; one for coated and one for un-coated. The main concern it to be consistent. Dot gain is a fact of life whenever pressure and liquid inks are used. There's nothing wrong, or right, about dot gain. Dot gain (TVI) is easily compensated for in prepress today. The key is to run your presses consistently, using the best set-up for the press/paper/job/environment.

The variables I listed are the important ones. There are, perhaps, another 20 that are important. No matter the number, some are more important than others. It remains a matter of control.

[Servicetech]

Dot gain has a number of important independent variables. The surface finish and absorbancy of the paper has the largest effect. "Squeeze" or impression pressure is a large contributer. The amount of ink, the ink film thickness, on the plate is important. So is the water balance on press. All these independent variables need to be known and controlled.

To control ink film thickness use the solid ink patches on your control bars, as well as the 75% tints. Too much ink is to be avoided, as is too little. Start with industry standard readings of SID and go from there.

To control "squeeze" get a packing gauges, both off-press and on-press, and use them. Keep your packing and pressures under control.

Ink/water balance is tricky. There are no dedicated targets for controlling balance. Back off the water until scumming occurs, then add a bit more water; look for sharpening in the 25% dot, or check to see if you're able to print a 2% dot.

Know your paper. Have seperate curves or profiles for different papers. Start small here; one for coated and one for un-coated. The main concern it to be consistent. Dot gain is a fact of life whenever pressure and liquid inks are used. There's nothing wrong, or right, about dot gain. Dot gain (TVI) is easily compensated for in prepress today. The key is to run your presses consistently, using the best set-up for the press/paper/job/environment.

The variables I listed are the important ones. There are, perhaps, another 20 that are important. No matter the number, some are more important than others. It remains a matter of control.

I agree, there are many variables here, I was on a service call once where they were complaining about unacceptable dot gain. It turned out to be dampening issues (print quality), I reset all the dampeners and replaced a few of the worst rollers and voila! Stuff happens I guess, this printer had been in business for more than 50 years.

[contrakid]

Thanks fore all the replies guys, some interesting stuff.

I had a minder run some dot gain test plates for me today and Magenta unit was within 1% of ISO Paper Types 1 & 2 dot gains figures but cyan unit had huge dot gain. Same curve on both plates.

Had minder wash down by hand both units blankets and check blanket was tight and put next job on and
dot gain problem has gone for now!

Now when i asked the other minder to do this 2 days ago when we had same problem it did not change anything!

I'm confused. Dot gains now on mag and cyan 40% is around 12%

[BeauchampT]

Well, each unit is going to have different dot gains - ink manufacture makes a large difference.

Why not just do your own fingerprint test? Use a flat curve (no adjustment) on a set of plates, run it off to the density that gives you the right Lab values to match ISO. Get some data, and make a plate curve adjustment (accepting that each plate will have its own curve). If you don't have the ability to make independent curves for each plate, well, that's another story.

As to a wash down of the unit 'fixing' the problem - that does point to an ink/water balance issue being involved; perhaps a bad batch of ink, perhaps running an overemulsified unit and needing to start from scratch.

Hope it works out for you.

[Lukas Engqvist]

Blankets usually need to be tightened after they run a bit, but if it's loose blankets you should pick it up as slurring or doubling. Do you have slur marks on your press sheet for quick deformation analysis? (Concentric circles)

[contrakid]

Well, each unit is going to have different dot gains - ink manufacture makes a large difference.

Why not just do your own fingerprint test? Use a flat curve (no adjustment) on a set of plates, run it off to the density that gives you the right Lab values to match ISO. Get some data, and make a plate curve adjustment (accepting that each plate will have its own curve). If you don't have the ability to make independent curves for each plate, well, that's another story.

Hope it works out for you.

Yes i can do a dot gain curve for each unit but i refuse do change the cyan curve when one day its perfect and the next it has 26% gain. I have a different curve for Black and Yellow and same curve for Cyan and Magenta.

Thanks anyway but i'm sure its finger problems with the minder not doing his job properly just hard trying to pin it down when your not a minder yourself although how some of our minders get the job when they can't even seem to read a colour bar i really don't know.

Should a prepress guy have to explain to a minder what the little squares with thin lines running across them a different angles means when one is darker than another! Them new presses our great but some of the minders just seem to reply on the axis control to do all the work for them and never look at the colour bar to make sure them have actually got the machine running correctly in the first place.

[contrakid]

As to a wash down of the unit 'fixing' the problem - that does point to an ink/water balance issue being involved; perhaps a bad batch of ink, perhaps running an overemulsified unit and needing to start from scratch.

Hope it works out for you.

Thanks for this info can you explain a bit more about an overemulsified unit and how this could happen etc. or point in direction of something i could read.

Cheers