I was thinking of a more conventional offset press - just with full-sized forms. Wouldn't some oscillators even out the ink distribution? Why is an anilox roller used in the pan on these devices? Why not a normal ductor roller?
By adding more rollers into the train, do you introduce too much ink into the ink train - more than can be pulled out?
Rich,
You are asking a lot of questions in a few lines and the answers are complicated. I will try to explain what I can.
First let me discuss ink distribution in the roller train. I am talking about the ink films on different rollers. The ink films vary vertically up and down the ink train and they vary horizontally across the ink train. This is due to differences in the ink required for the image being printed, basically the coverage and due to the geometry of the roller train configuration. Many people assume that the ink film at the form rollers is uniform for high and low coverage but this is also not true and depends on roller train configuration. This is too difficult to describe why this is but when one does calculations it shows this difference. Calculations are also only educated guesses and can not model every influence but it gives an idea.
Mechanical ghosting patterns are generated at the plate and form roller region. Ink comes off the form roller and goes to the plate. This can be in patterns that do not completely get smoothed out during one revolution of the form roller. Multiple form rollers help to even out that ink film on the plate but this is not always successful. Adding ghosting rollers to the last form roller helps because it gives an extra hit to help smooth the ink film on that roller. This action of a roller smoothing out the ink film on another roller, I call random averaging because it picks up thick ink films and deposits ink into thin ink films on the form roller.
Oscillating (vibratory) rollers in contact with the form rollers provide some help with moving ink laterally to reduce ink starved areas. The problem with oscillating rollers is that their capacity to move ink laterally is very low. You can see this is you do a simple test on press. Put some extra ink in one specific location about a 1/2" wide. Run the roller train and carefully look at the thicker ink film. You can see it because it splits with a very rough ink film. The oscillating roller might move laterally almost 2 inches but if you look carefully the thick region of ink does not follow the oscillating roller but tends to move only slightly in one location. The reason for this is that the stroke length is not what is critical for lateral ink movement but the speed of the roller moving lateral is. The thick ink film just splits and moves back relative to the movement of the oscillator roller. One rotation of the oscillator roller might move laterally only a fraction of a mm. It is a very interesting thing to observe but it is important to understand.
Anyhow in the upper roller train you do not want to even out the ink distribution laterally. There the oscillation should only be for avoiding ribbing. An uneven ink film distribution is required for the ink requirements of different image coverage.
In the Anicolor type devices, the anilox roller is used because it tends to provide a surface with an even ink film which is continually in contact with the form roller and it is going at press speed. A ductor roller can not do this.
More rollers in an ink train provides more ink storage but the rate of ink transport through a high ink storage or a low ink storage ink train is the same. Ink transport depends on the ink feed rate to meet the needs of the printed image.
Higher ink storage ink trains in general are slower to respond and therefore can cause more waste as one goes through the transient from one density level to the desired level.
I hope I understood and answered some of your questions.
i have done 2 decks in the 40" i run in the lst few weeks, and they were a bastard of a job!!
