Is there any logic to define the amount of water needed for a particular job.
Is there any correlation between the image ratio and water quantity.
I am sure this should have been discussed many times. But water setting is still being left to the individual's skil and the irony is water setting plays critical role for all the other automatic settings which control colour.
A.Sathishkumar
Water settings is a difficult issue.
For one thing, the setting and the actual delivery of water are most likely not directly related. Normally one does not have a positive feed for water on presses except for some newspaper presses that have spray dampeners.
Also the path of water out of the system is not totally controlled and is via water in the printed ink, water printed on the non image area of the paper and evaporation off rollers etc.
Water consumption is not linear as it would be with ink. At low speeds evaporation as a relatively larger affect than the water printed out on the paper. At higher speeds this relationship changes so that the water printed on the paper increases more than evaporation.
For these reasons, on presses that have water curves, some control curves will have an immediate jump in the curve at low speed then have a more or less linear relationship with press speed as the press goes to higher speeds.
I would say that there is no strong relationship with water consumption and image ratio. If there was a really strong relationship, the process would need to feed water into the press as it does ink by means of zones. Some researchers felt this was needed but basically one does not see that on offset presses. The process works quite fine with a relatively even supply of water across the press. Although some press operators find it useful to put water stops in certain locations for specific problems.
So yes adjustments of water are required. This is what the operator normally does but there has been research done to try to automate this. One method is to measure the acoustic noise for the nips and use that to modulate the water feed.
On a Chambon press that I was familiar with they had a very interesting concept. On their modified Dahlgren dampener, they had a drive motor that drove the dampening rollers but it did not drive them with a fixed speed. Shear forces from the slip nip between the dampening unit and the first form also helped rotate the dampening system. If the roller train had less water, that would cause the shear force to increase and that in turn caused the dampening system to increase and add more water. Also if there was too much water on the roller train, the shear force would drop and slow down the dampening system and therefore supply less water. So within a small range, it was self correcting. The operators loved this simple system. We were running EB inks.
There are potentially many ways to make water control easier for the operator. Density control is an ink feed control issue.
