Overtime: The eternal conflict - ink/water balance - the tale of the tones

[gordo]

An AM/XM halftone screen has a built-in conflicting ink/water balance requirement on press. The highlight dot and quarter tone range from 1-35% requires minimal water and maximum ink in order to prevent those dots from being washed away. The three-quarter tone range from 65-99% requires the opposite - a larger volume of water in order to prevent the shadow dots from filling in and disappearing. On the other hand, the mid-tone range from 35-65% is more of a balance between ink and water.

Halftone dots and the tone range they represent are affected differently by the condition of the ink on press - assuming of course, that the plate, press, and chemistry are set up correctly. Unfortunately, if the press operator attempts to fix tone reproduction in some areas, that built-in difference in ink/water requirement can exaggerate the inherent conflict and cause problems in other parts of the tone range.

1 - 1-35% This tone range is primarily affected by the body/viscosity of the ink. If the body is too soft the highlight area will print too full which may cause the press operator to decrease solid ink density in order to reduce the dot size. Alternatively the fountain solution may over-emulsify this tone range causing poor ink transfer and loss of highlight detail. If the ink body is too heavy the dot may print too sharp causing the press operator to increase the density or blanket pressure.

2 - 35-65% This tone range is primarily affected by the strength (pigment load) of the ink. If the ink is too weak the press operator will increase solid ink density which will cause increased dot gain and result in presswork that appears too full. If the ink is too strong the midtones may print too light. Also, the strength of the ink also impacts how well the inks trap, which in turn affects the color gamut the press should be able to achieve. Varying the strength and stiffness of the ink to achieve good tone reproduction in presswork is a method press operators, who don't have good communication with prepress, often employ. It's almost always better to use tone reproduction curves applied in plate imaging than to modify inks.

3 - 65-95% This tone range is most strongly affected by mechanically induced dot gain or chemistry issues i.e. (poor ink water balance). If the tone range from 1-65% is evenly balanced then excessive gain in the shadow tones is usually caused by running excessive water, too much blanket pressure, and/or mechanical slur.

 

[SteveSuffRIT]

The most common method for determining ink/water balance is to see if the background of the paper is clean or dirty at the gripper/lead edge. This is a macro, unaided view with no magnifying glass.
Most press operators during make-ready drop the water feed until they see the dirty edge than slightly raise it to print clean again. That's the water setting they will use for the rest of the run.
There is a technical difference between dry/catch up (ink/water balance), tint/tone (over-emulsification/ink water pickup), and scum (plate desensitization/fountain solution etch-gum chemistry).
It's never been clearly explained to me why the lead edge of the plate always dries up dirty first????