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.