Offset Print Quality of Uncoated Mechanical Printing Papers: Results of a Commercial Heatset Offset Trial

Twenty-four uncoated mechanical printing papers, from newsprint to SCA, were printed by the heatset offset process. Correlations between subjective ranking and paper properties were much better on the top side of the samples. Among the paper surface factors examined, the total length of fibre exposed on the paper surface, or the speed bumps, gave the highest correlation with the subjective ranking. In general, correlations between ranking and paper properties were much poorer on the bottom side of the sheet. When the same samples were printed by gravure, all on the same side of the press, the bottom side correlations were less good. Therefore, top to bottom differences in correlations are an inherent paper structural problem which should be addressed in future work. Among print properties, only ink trapping gave a good correlation with subjective judgements. Optical and confocal microscopy showed the detrimental effects of surface pits and coarse surface fibres on ink transfer, halftone dot structure, and print quality.