Newbie question on Dot Gain on a sheetfed

[mcf]

Hi All

Hope you don't mind a newbie question - we provide plates to customer with several Roland B1 presses. They are experiencing issues on press with the sharpness of the printed dot - the dot appears to be spreading too much. It only affects some jobs that have a 4 colur tint grey in them (screenshots from a manual) - and it is apparent as a 'mottling' in these gery areas. What should be a nice even grey appears patchy.

They think it's a plate issue and we have scrutinised our workflow and the plates in detail and have yet to see any inconsistency (though we have seen some differences in quality of the dot on plate when screen from our Apogee system compared to our Trueflow system).

On Black then run at a dot gain of about 24% (slightly lower on the other colours). My question is, is this likely to be a dot gain issue? Is there a best methodolgy for getting to the bottom of this and what factors would be at play on the press. I'll freely admit that I don't have the knowledge to tell them whether it's a press problem for sure and what can be done to fix it. They have done all the standard things like change blankets and they do control their press reasonably well.

Thanks

Mike

 

[janezl]

There might be issue with screening system (Agfa vs. Screen), or there is difference, how (maybe) different plates behave with water on the press?

Janez

 

[gordo]

Hi All
Hope you don't mind a newbie question - we provide plates to customer with several Roland B1 presses. They are experiencing issues on press with the sharpness of the printed dot - the dot appears to be spreading too much. It only affects some jobs that have a 4 colur tint grey in them (screenshots from a manual) - and it is apparent as a 'mottling' in these gery areas. What should be a nice even grey appears patchy.

Many things on press can effect the integrity of the printed dot as do some things in plate imaging/processing.

I would start by examining the dots on plate and the same dots as they appear in the presswork. You will need a at least a 25x, preferably a 50x or 75x microscope to do a proper evaluation. You can buy them quite inexpensively from Betascreen ( Pocket Microscopes )
If you have access to a quality scanner you can use it to document what the plate and presswork dots look like. This is explained here: Quality In Print: Your hidden microscope

The function of the press is to reproduce, with ink, the bitmaps on the plate as accurately as possible. You can see and example of the process here: Quality In Print: Dot Gain/TVI (Tone Value Increase) - part 1 of 4 Examine the solids (non-halftoned) areas and then compare the dots on plate with the same dots on the press sheet using high magnification to see how they've been distorted and get a good idea as to what the cause might be.

For example:

On the left are (FM) dots on the plate, center is solid black, at right are the FM dots on the press sheet. In this case the poor transfer of black ink to the press sheet (as evidenced by the solid) also appears more subtly in the screened halftone and, in this case, will show up as mottle on the press sheet.

Practice makes perfect - so the more you look at press sheets up close like this - good sheets and bad sheets - (and talk to press operators about what you see) the better able you will be to figure out where any problems might lie.

best, gordon p