"The first form roller will tend to have the highest percentage. This is due to the fact that the image area on the plate has been starved of ink due to the printing of ink to the blanket."
I know that you know a lot about this Erik, but that statement is ambiguous and perhaps misleading. You seem to be commenting on the first form roller relative to the others in relation to the demand put on them by the plate in each revolution. But what about commenting on what the ink train design brings to each form roller regardless of the image on plate?
Again, I know that you know this, but you must explain it here, or give a link to a source where it is explained. Otherwise this does not advance the original poster's understanding of the issue.
Al
Hi Al, I will try to clarify but as I said, it is complicated.
One can not ignore the plate because it affects how the ink comes down the roller train. Ink in the roller train does not flow in the sense of a river. It splits. The vast majority of the time, the ink is not moving but sits on the surface of rollers. Ink is transferred from one roller to another or from the form roller to the plate by means of ink film splitting.
In the simplest of models, one uses a 50% 50% split. In reality this may not be the case but it is suitable for the model. This means that at every nip there is an equation. The ink film on one roller surface plus the ink film on the second roller surface is added in the nip and then the total splits into two ink films that are the same.
So for any roller train, one needs to calculate the ink film thicknesses on all the roller surfaces in the roller train. One does this by setting up all the nip equations. Something like a + b = 2c, etc. where a, b, c, d, e, f, etc represent the ink film thicknesses on the rollers.
One then solves the simultaneous equations to determine the values for a, b c, etc. There can be more than 15 equations with 15 unknowns. One could solve it by hand but it is easier to use a math program.
This method is good for calculating the ink distribution for 100% coverage. From this calculation then one determines the percentages that each form roller provides.
For coverage that is almost zero, it is fairly easy to determine the ink films by hand and from that calculation one also gets the percentage that each form roller provides.
The percentages that the form rollers provide to the plate are not the same at 100% coverage and at almost a 0% coverage. The calculations at 100% and 0% result in a specific value but between 100% and almost 0%, the percentage values are not specific but are averages due to the way ink is taken of the form rollers as stated before.
I have run computer simulations of how the plate is inked when there are image patches on the plates and the result is that the ink film on the plate is different on each cycle of the plate. This shows that the plate is inked unevenly by the forms. The range of unevenness depends on the design of the roller train.
The percentages that the form rollers provide is not what is important. What is important is the variation in ink film on those form rollers. That is a press design issue.
Hope that helps but as I said, it is complicated.
