Online detection of moisture in heatset printing: Liquid transfer to printed image and non-image areas

In the present study, near-infrared sensors are used to investigate moisture variations on paper in the heatset web offset printing process. The moisture content was determined from both non-image and image printed areas at multiple locations along the printing line. The amount of water based liquid (fountain solution) transferred to the two studied areas as well as the internal moisture content of the paper was investigated in terms of near-infrared absorbance response. In order to achieve absolute moisture content estimates, the sensors were calibrated for the respective measurement regions. It was found that the moisture content was higher in the areas with printing ink compared to those without. The heatset drying had a stronger moisture drying effect on the non-image areas. The tests were performed in both a laboratory and a full-scale heatset offset printing environment.