Online Detection of Moisture in Heatset Printing: The Role of Substrate Structure during Liquid Transfer

Liquid transfer in a heatset printing process to coated papers has been evaluated online. The porous coatings, applied in various combinations of single coating onto a fine paper substrate, together with selective particle size distributions containing calcium carbonate pigments were calendered under different conditions to establish a range of porosities and pore structures while keeping the formulation and hence the surface chemistry constant. The transfer of fountain solution to the papers was analyzed from unprinted areas (nonimage) at six different positions along the printing line, namely, between each printing unit and after the dryer section, using near-infrared absorption reflectometry. In this way, real-time analysis of the amount of fountain solution (defined as water content) transferred to the paper per printing unit as a function of physical paper surface characteristics has been achieved. The role of printing speed and fountain solution dosage level on water uptake by the various coated paper substrates has been investigated. It was concluded that the higher the speed, firstly, there is less compression of the surface roughness and, secondly, less time for the liquid to respond with respect to capillary forces, resulting in less liquid transfer.