Ink Film Thickness

Precise control of ink film thickness is a fundamental requirement for achieving high-quality lithographic printing. On a typical press, this thickness ranges from 0.2 to 0.4 mils (0.0002–0.0004 in., or 0.005–0.010 mm), a parameter that varies based on ink color and its position in the printing sequence. Maintaining this specification is essential for consistent ink density, accurate color reproduction, and avoiding a range of press-related issues.

An excessively thin ink film can lead to several problems: low color strength and saturation, reduced gloss, incomplete paper coverage, picking, and hickeys. It also complicates the delicate balance between ink and water on press.

Conversely, an excessively thick ink film introduces its own set of challenges. These include increased ink consumption, setoff, drying problems, slight degradation of light colors, graininess from irregular dot gain, excessive ink emulsification, and low shadow contrast due to dot gain.

Print contrast serves as a key metric for determining optimal inking levels during a pressrun. It is calculated using a solid ink patch and a 75% tint patch with the following formula:

Print contrast = (Ds-D75)/Ds x 100

In this formula, Ds represents the density of a solid (including paper) and D75 is the density of a 75% tint (including paper). Many modern reflection densitometers can perform this calculation automatically.

The press operator should adjust inking to achieve the maximum print contrast value. Higher print contrast indicates a greater number of distinguishable tones between the 75% tint and solid ink density. As ink feed increases, print contrast rises until a peak is reached; beyond this point, further increases in density require an excessively thick ink film, which accelerates dot gain and causes print contrast to decrease. Target values for maximum print contrast, which are best established as shop standards, are published in Offset Lithography (GRACoL) and displayed in the figure below.

Ongoing trends in automation and closed-loop color control systems further enhance the ability to maintain these specifications consistently. Additionally, a continued industry focus on sustainability drives the development of inks and processes that optimize coverage and reduce waste.