Low Dot Gain. Again

[danremaley]

Ok, so we've bumped into it again:
We have 4 ManRoland 900 (4 sections, no coater)
All presses are ± same year same condition and same configuration.
We use same: blankets, plates, ink (with target densities), fountain solution (with IPA 10%), ro water other printing chemicals, temperature settings for technotrans.
All 4 presses print mostly same on uncoated stock.
But on coated one of them has low dot gain on 3 (M) and 4 (Y) sections always and somtimes on 2 (C) section.
Drop is around 6-8%. 3 presses print with 14-18% gain, but this machine prints 6-10%
It's just it - we print, see low gain, take plates off, mount them on another press and see normal dot gain.
What i can't get my head around is if i make excess packing, like plus 0.1-0.2 and also make maximum possible impresson on impression cylinder i don's see difference in dot gain, while it probably should be.
Any thoughts, recommendations what to check also?

P.S. we had a serviceman for full shift, he also checked everithing and his conclusion was like "this is how this press work" ¯\_(ツ)_/¯

 

[Cornishpastythighs]

Yes as long as your press prints consistantly at the same dot gain whether it be sharp or whatever then you can build curves around that.

 

[gordo]

[SNIP] While 14-22% mid-tone gain may be an acceptable industry average, it is not that unusual to see less. And I say personally, IMO, the less gain, the better

A sidebar....
The standards people (e.g. idealliance) perpetuate a misunderstanding about offset printing and that conditions how they base their standards and educate printers.

Dot gain is not a target for the press. Dot gain is actually a process control metric. Dot gain is something you monitor through the process from original file to final presswork. Monitoring dot gain helps the printer to diagnose when and where the production process fails.
The target is tone reproduction. E.g. I want the 50% in my file to measure 72% on my press sheet irrespective of what the dot gain has to be to achieve that target final tone.

Solid Ink Density is not a color target. The press does not print color. The press operator does not "come up to color". Solid Ink density is actually an indirect measurement of Ink film thickness. Mechanically, ink film thickness is a critical factor in the offset printing process. Irrespective of the hue of the Ink being used, if the Ink film thickness does not meet a certain target then the offset process will fail.

 

[gordo]

Just a general thought.

Remember that the measuring instrument you are using knows nothing about halftone dots. It measures a relationship between two tone patches (solid and screened) and the paper tone. it then uses a mathmatical formula** to determine a tone percentage.

Because its a ratio of tones - not actual dot size, it's possible to have a change in one of the tones affecting your "dot gain" measurement which is not related to the size of the actual dot. For example, if the 1% to 25% tones have lost some density due to slight over-emulsification that does not affect the darker tones where there is a thicker layer of ink.

If you try to correct this "dot gain" with a plate curve that affects the size of the dots on the plate then you are effectively trying to compensate for an ink/water balance problem by changing tone values on the plate.

Doesn't make sense.

That's why you need a microscope to see what's happening with your ink laydown to help you interpret your dot gain measurements.

It's also why any activities like building plate curves or G7 implementations should always start with a thorough audit of the press condition - right down to the microscopic level.


** E.G. Murry-Davies, Yule-Nielsen, Clapper-Yule, Huntsman, etc.

 

[SheetSqueezer]

While I don’t doubt for a minute that they all may carry a portable microscope of some sort with them, I don’t know if I’ve ever seen a G-7 certification technician actually use one (I could be wrong). They usually end up completing the entire G-7 calibration process with nothing other than a 10-12x loupe, a computer and software program(s), an advanced handheld densitometer, and the plastic slide rule used to help scan the color patches on the pulled press test sheets.

 

[gordo]

Correct.

It's like a doctor providing you with a drug prescription without first examining you.

I was on the idealliance G7 committee and attended about 5 of their test press runs. I was the only one that had actual press experience. At one of the test runs at a commercial printshop the committee was having a lot of trouble getting the L*a*b* values to conform to the specification. While the group was speculating about the possible causes, I went over to the press and saw this:

When I told the group that the Ink sequence was wrong, and probably the cause of the problem, I was forcefully told that the Ink sequence was irrelevant when it came to the color. For that reason (and a related few others) I resigned from the committee. Things may have changed since then.

From a different pressrun

The below image arev the black printer dots on the operator side:

and the dots on the same sheet but on the gear side:

A difference that may not be visible with a 10x loupe.
Is this a problem? Should this press sheet be used for measurements?

My point is that the more info you can gather about the actual press condition before you start characterizing, building curves etc. the better the path forward will be and the greater your chance of success.

 

[SheetSqueezer]

Correct.

It's like a doctor providing you with a drug prescription without first examining you….

What you showed in your post I’m guessing is probably the unusual and maybe not really the norm at most shops - although TBH I really only really know about the situations/setups I’ve actually seen for myself. BTW none of which compare to what you’ve shown above, lol.

Most certifications I’ve seen at least get into the ballpark as far as correctly fingerprinting a press, before the dot manipulation begins. And then they usually end up being at least satisfactory for color matching after that - some very much so…. With that being said I also know of a couple of times where, after a while, the newly generated G-7 curves were decidedly undesirable for everyday color matching and ended up eventually being discarded entirely, for curves that consistently produce(d) more accurate results.

 

[cementary]

Just to rule out some software setting in the densitometer/spectro, are you using the same device to measure, not the brand or model but the same device?

Well of course not. Each press equipped with it’s own grapho-metronic fm19. But if we measure press sheet from one press on other - readings almost same.

 

[cementary]

Could you post some photo micrographs of your dots at 30 or 40% of the 4 press sheets?

In system brunner stripe we have only 50%
This is 14 dot gain on 50% patch

An this is 8% on 50%

 

[cementary]

May I have answer that your issue happens with all thickness of paper or "Uncoated - thick and Coated - thin paper"?

No, any uncoated paper 65-160 gsm prints quite same (regarding dot gain)

 

[cementary]

How about roller settings - Ink Form Stripes, Water Form Stripe?
Is your oscillation time the same? Or do you even run with your oscillation rollers on?
If everything is identical regarding the consumables you use and it’s isolated to 1 machine, compare settings and operator methods.

Ink/Water stripes are cheked on regular basis. Oscillators always on, also bridge roller and delta mode always on too.
Operator was same last time we checked between two presses.

 

[cementary]

Some presses just print sharper. Beyond the consumables (which matter a lot) it’s all related to the original mechanical build and current condition of an individual press … and it’s also why every machine has its own unique fingerprint. While 14-22% mid-tone gain may be an acceptable industry average, it is not that unusual to see less. And I say personally, IMO, the less gain, the better.

I would agree, but say 2 years ago all 4 presses printed almost same

 

[cementary]

@danremaley i'm not sure what you meant by that.
we have system brunner's option ok-balance on all presses and use system brunner's strip

 

[gordo]

In system brunner stripe we have only 50%
This is 14 dot gain on 50% patch
View attachment 293221

An this is 8% on 50%

View attachment 293222

Thanks for posting those pics. Superficially they look the same (great dots BTW)

Normally I would take the photos under lab conditions but using you photos. I tried making the backgrounds the same densitiy and then plotted their densities in 3D - effectively sowing the relative ink fim thickness.

This is how their ink densities compare (8% on left - 14% on right.

Not sure if that's enough of a difference to account for the different dot gains - but it is an interesting difference.

 

[cementary]

Not sure if that's enough of a difference to account for the different dot gains - but it is an interesting difference.

Oh well i completely forgot to use imagemagic. Thank you for that. BTW you pics of comparing am/fm ink film thickness are literally the thing we use to explain how it works
technically it should look like on the right there are a lot of water in the ink, but i think it's difference due to many light reflections on the right side picture

 

[gordo]

Oh well i completely forgot to use imagemagic. Thank you for that. BTW you pics of comparing am/fm ink film thickness are literally the thing we use to explain how it works
technically it should look like on the right there are a lot of water in the ink, but i think it's difference due to many light reflections on the right side picture

Yeah. That's why I said that normally I would take the photos under lab conditions, meaning controlled so that both samples were photographed in the same lighting - like we did for the AM/FM comparisons.