Dot gain problem with Man Roland 505 (need help)

[Hannes]

Dear All,

We are facing a problem with Man Roland 505 dot gain. The dot gain is too high with most of the jobs. With coated papers, sometimes it is up to 69% in 40% area and sometimes it is 55% in 40% area (which is quite okay). It depends on something, but on what? The dot gain can rise and fall over a few print jobs.
At first, the plates have been measured and there is no dot gain there (Agfa Amigo TS). The pressure between the cylinders has been measured and it seems to be okay also. The inks have been changed from Hostmann fast-drying to Vanson Premium. The fountain system has been washed several times and the solution has been changed from Huber Combifix to Fuji Acedin. Still no changes. Different blankets have been tried from Kinyo to Conti-air.
I've told the printers to use as less water as possible and to keep the densities:
Yellow - 1,30
Mag/cyan - 1,45
Black - 1,80
I have been checking the fountain measures all the time and the pH is about 5.0. Conductivity is around 1500 ms, but it's high because of the solution and water quality. We have water hardness of 10-12 dH over here. Fountain system is using isopronol for about 12%.

I've ran out of ideas and I really need some help with this case. Has someone had same kind of problem? Which are we missing here? What should we check?

Best regards,
Hannes

 

[Cornishpastythighs]

Yellow ink density seems a tad high we run about .95-1.00
Black density maybe a tad high too we run around 1.70 but that doesnt look like the issue.
We run pH buffered to around 4.2 and use an RO unit which ensures stable water for our fountain solution.
just a couple of questions
Does the dot gain follow any particular shift or operator?
Does the dot gain start off ok and get worse during the course of the print run or is it high right from the start?

 

[Hannes]

Thanks for answering. The dot gain does not depend on a shift or operator. With some print jobs the dot gain is worse from the beginning till the end. And with few jobs it is okay.
I think the pH of 4.8-5.3 is normal in sheetfed printing. And the densities are given by Van Son printing ink factory so it should work this way.

 

[yavo]

Did you check dot on the plate?
Between two result plates.

 

[gordo]

Thanks for answering. The dot gain does not depend on a shift or operator. With some print jobs the dot gain is worse from the beginning till the end. And with few jobs it is okay.
I think the pH of 4.8-5.3 is normal in sheetfed printing. And the densities are given by Van Son printing ink factory so it should work this way.

First of all, this is why your shop should have created a "Golden Reference" sheet see here: Quality In Print: The Golden Reference - part 1 of 2

And once you've sorted this problem then you should make a Golden Reference so that you are better prepared in the future. Also never change more than one thing in your process (e.g. inks and blankets). Change one thing (e.g. inks) and watch the impact on the process over time. Once you're confident that the one change is working, then change the second thing (e.g. blankets).

You've got to take a step by step systematic approach to solving this problem.

The first step is to make sure that the plates that go into the pressroom are consistent. If the plates are linear then 40% tone in the file should be a 40% dot on the plate. If you curve your plates then the 40% request in the file should be whatever the target dot on the plate is required. You should be able to confirm that your plates are correct by checking the quality control records from platemaking - assuming that your shop is QCing the plates and keeping records.

Next, dot gain is the relationship between a solid and a tone area. As solid ink density (SID) changes so does the measured dot gain.

Your shop has targets for SIDs (yes the Yellow is a touch high but with an instrument set to Status E it is not unreasonably high). How consistent are your SIDs through the run and from run to run. The concern is that your press operators might be overly adjusting their SIDs in an attempt to match the proof. I.e. the dot gain variation may be a symptom of a press that is not set up to align color to the proof. If the SIDs get too far off their optimum then the press can loose stability and dot gains will loose consistency through the run.

Make sure that all your instruments are set to the same Status ("E" if you're in Europe "T" if you're in the Americas).

Next you need to examine the solids and halftone dots under a loupe at a magnification of at least 25x. You should compare the dots on the plate to the same dots on the press work. Like so (plate on the left and press work on the right).

The dots on the press sheet should look very similar to the dots on the plate (evenly solid film of with a clear separation between ink and non-ink area and about the same size.). What you are looking for is whether and how the dots are distorted from job to job or during the press run (i.e. slur, dot doubling, over emulsification, attacked by fount solution, etc, etc) E.g. http://qualityinprint.blogspot.com/2008/12/slur.html and http://qualityinprint.blogspot.com/2008/12/doubling.html

If you do not already have one, buy a USB microscope, they are less than $75 and can provide an excellent reference as to how you are printing as well as to enable people to see what issues you may have. Every printshop should have one of these. Here's an eBay link: usb microscope | eBay

best, gordo

 

[Hannes]

Thank you, Gordo, for the very detailed answer. Yes, we have checked these things one by one, not all together at a time. We started with checking the plates and dot is perfectly consistent there. In 40% area the dot is 40%. Next was checking the cylinder pressures. Then we changed the blankets and in the end we switched to VanSon ink + Fuji fountain. None of these things has had a influence to the dot gain in the press. It comes and goes all the time. I have been trying to understand if it depends on different papers also. Densities we try to keep between the numbers I wrote in the first post.

 

[maxon]

Hannes, does it affect all colors ? And have you inspected the plates taken off the press after longer runs ?

 

[Hannes]

It effects all colours but not at a time. In some print jobs, for example, the dot gain is high with magenta and yellow, and with other jobs the dot gain is high with cyan and black. With a few jobs the dot gain is high with all CMYK. That is strange for me because I don't understand what it depends on...

 

[Kaoticor]

It effects all colours but not at a time. In some print jobs, for example, the dot gain is high with magenta and yellow, and with other jobs the dot gain is high with cyan and black. With a few jobs the dot gain is high with all CMYK. That is strange for me because I don't understand what it depends on...

Do you notice the shift in dot gain when you run at the same speed, or are some of your jobs being run at different press speed? We kept passing this over saying our jobs were running at the same speed so that wasn't causing any shifting, but when we really payed attention we found a significant shift happening at different speeds.

 

[Hannes]

I really haven't paid attention to it. That's a good idea. Could you please explain how the dot gain changed with different speeds?

Best regards,
Hannes

 

[Alois Senefelder]

Dot Gain and Speed

Dot Gain and Speed

Hello Hannes and fellow Lithographers,

Cause: Variation of Dwell Time in the Plate/Blanket/Impression Nips, as the cylinders rotate through the - Forces of Pressure

PDFs - I hope you will find of interest and value.


Regards, Alois

 

[Alois Senefelder]

Part # 2

Part # 2

Hello fellow Lithographers,

To continue ---

PDFs - "The effect of printing blankets on the rolling condition of printing cylinders"



Regards, Alois

 

[Hannes]

Thank you for the answer. At the moment, we haven't found solution. All I know is that when you print with the lowest densities then the dot gain is 15% to 20%. If you print normal densities or a little higher then the dot gain is 20% +

We will start compensating dot gain by decreasing dot on the printing plate.

 

[frank67]

Which fountain are you use from Fuji?

 

[Hannes]

Acedin D1991 - works everywhere else and I think it very good fount.
Before Fuji there was Hostmann's Combifix

 

[bikozak]

This is a good one Hannes. If you really did check all the things mentioned by Gordo above, then to me it sounds like your ink is not stable at the densities you are running. I say this because you see variation from job to job and color to color. I would be interested to know if the dot gain fluctuations coincide with heavy or light coverage between colors and jobs.

 

[Kaoticor]

I really haven't paid attention to it. That's a good idea. Could you please explain how the dot gain changed with different speeds?

Best regards,
Hannes

Sorry for the delay in responding, forgot about this thread. Here at our shop we notice shifts in dot gain in press speed. Some of the pdf's mentioned on here touch on a few of the contributing factors. One thing you can look out for (in addition to what the other posts mentioned) is press temperature and running speed.

Example - You are running the press at a slow speed and it has not heated up yet, and then you make your calibration curve on your plate. Later, you get in to a heavy production run and speed the press up and more heat is generated, affecting various conditions inside the press. Variables like dot gain can look completely different. We had to start making curves only after normal conditions were reached.

 

[BeauchampT]

Sorry for the delay in responding, forgot about this thread. Here at our shop we notice shifts in dot gain in press speed. Some of the pdf's mentioned on here touch on a few of the contributing factors. One thing you can look out for (in addition to what the other posts mentioned) is press temperature and running speed.

Example - You are running the press at a slow speed and it has not heated up yet, and then you make your calibration curve on your plate. Later, you get in to a heavy production run and speed the press up and more heat is generated, affecting various conditions inside the press. Variables like dot gain can look completely different. We had to start making curves only after normal conditions were reached.

A great deal of what Kaoticor mentions here relates to the effect of press conditions on ink properties. Differences in speed and temperature affect the viscosity, tack, rheology, etc. of the ink.

One question regarding the densities that you run on press - you say you run coated paper, so those densities don't sound totally out there to me, but...what sort of densitometer are you using (Status E, T, etc...)? Those values may not be an issue at all.

 

[PSchw]

You may have a registration issue on your press. Have the press checked with a special test form. I have a computer ssytem what measures you exact registration. You can contact me at any time via email [email protected].