An instrumental determination of the effect of sheet formation on the printability of uncoated fine paper

Sheet formation has an important effect on print quality. For manufacturers of printing grades, this relationship is important as they try to ensure the best possible printability for their product. In the present work, we determined, for uncoated fine paper, the effect of formation on offset print mottle. We examined a set of 89 North American commercial uncoated fine papers, ranging in grammage from 72 to 104 g/m2, coming from different mills and different companies. The sheets were printed on a Heidelberg four-color Speedmaster sheet-fed offset press operated under commercial conditions. The formation of the sheet was measured with the PaperPerFect formation analyzer. Print mottle was determined using a wavelet-based instrumental technique. Both formation and print mottle were found to vary significantly within the 89-sheet set. Formation and print mottle results were then correlated. The highest correlation was found for the 5-8 mm scale of formation, with a coefficient of determination (R2) of almost 0.5. Other scales of formation had little or no effect on print mottle. Thus formation at this scale explained about half of the print mottle variability of our data set and was therefore the most important single factor affecting print mottle. Papermakers should improve formation at this specific 5-8 mm scale in order to improve printability of uncoated paper for this printing process. Improving formation at other scales will have little or no effect.