Evaluation of Optical Formation Measurements on Printing Papers and How They Can Be Used to Predict Print Quality on Uncoated SC Papers

Different paper grades have been used to evaluate the optical formation measurements for newsprint and SC paper. Formation of the SC papers has been measured both before and after supercalendering to see how it affects the optical measurements. The optical measurements have been compared to β-radiation measurements and the results showed that optical measurements give a fair estimation of the formation on unbleached newsprint and uncalendered SC paper. The results also showed that due to differences in light-scattering, optical formation of papers with different grammages and brightnesses cannot be compared. After the formation was measured, both optically and by β-radiation, the SC paper sheets were printed in a Prüfbau gravure press to investigate whether there is any correlation between formation and and print quality. The print quality aspects which were taken into consideration were print mottle, print density and missing dots. The results show that optical formation measurements can be used to predict the print mottle of uncoated SC papers. A strong correlation was found for print mottle and formation in the 1–4 mm wavelength scale which is within the wavelength range that affects the visual perception of print quality. Similarities in correlations between print quality and formation before respectively after supercalendering make it possible to assume that formation measurements before supercalendering give fair estimations of formation after supercalendering. Unlike mottle, no correlation was found between sheet formation and missing dots, which is one of the most crucial print quality aspects in gravure printing. It is likely that missing dots are caused by smaller-scale properties and can be predicted by porosity measurements. Utvärdering av optiska formationsmätningar på tidningsoch journalpapper och hur de kan användas för att förutse tryckkvaliteten på obestrukna SC-papper