The thickness of the ink film on the rollers is a very important consideration in printing. Typical ink film thickness on a lithographic press is 0.2~0.4 mils (0.0002~0.0004 in., or 0.005~0.010 mm), depending on the ink color and printing sequence. The press operator should control the amount of ink being carried by the inking system for more precise control of ink density and color and to avoid problems related to excessively thick or thin ink films.
common problems resulting from an excessively thin film of ink are low color strength and color saturation, low gloss, incomplete coverage of paper, picking, and hickeys. Also, ink/water balance is difficult to achieve.
Common problems resulting from an excessively thick film of ink are increased ink consumption, ink setoff, drying problem, slight degradation of light colors, graininess of print due to irregular dot gain, excessive emulsification of ink, and low contrast in the shadows due to dot gain.
Print contrast is a mean of determining optimum inking levels during the pressrun. Print contrast is calculated from a solid ink patch and a 75% tint patch, according to the following formula:
Print contrast = (Ds-D75)/Ds x 100
where Ds is the density of a soled (including paper) and D75 is the density of a 75% tint (including paper). Some reflection densitometers perform print contrast calculations automatically.
During the pressrun, the press operator should adjust inking levels until the maximum print contrast level is obtained, because the higher the print contrast, the greater the number of tones that can be distinguished between the 75% tint level and solid ink density. As the press operator increases ink density by increasing ink feed, the print contrast will increase until a point is reached where further in creases in density will cause lower print contrast values. This is due to the excessive ink film thickness needed to raise density beyond this point. This causes dot gain to rise at an accelerated rate, and print contrast decreases. The aimpoints for maximum print contrast are best based on shop standards. Target print contrast values for various substrates are listed in the publication General Requirements for Applications in Commercial Offset Lithography (GRACoL) and are shown in Figure 3-9.