What is mottling (print cloudiness)?

The term ‘mottling’, also known as print image cloudiness, describes the uneven, patchy or cloudy deposition of ink in solid-area images.

Mottling is caused by uneven ink repulsion and splitting, resulting in patchy ink deposition during multi-colour wet-on-wet offset printing. Partially uneven transfer of the ink into the paper surface or the paper’s grain structure means that the wet ink is retained to varying degrees on the paper surface in the next printing unit, and consequently transfers varying amounts of ink to the subsequent blanket (re-distribution).

This process repeats and intensifies across the subsequent printing units. This results in locally varying ink thicknesses during the printing process; and as offset ink is translucent and incorporates the white paper background into the colour impression, colour mottling becomes visible to the naked eye.

Spread across the surface, the uneven distribution of the printing ink is perceived as a hazy print image. As paper-related mottling becomes visible due to the back-splitting of the printing ink, it is advisable to print the critical colour – usually cyan – separately without back-splitting. If the cyan then prints smoothly, the printing parameters are correct and the paper should be replaced. In some cases, the printing process or used rubber blankets may also be considered as possible causes.