Hygroexpansion of Newsprint as a Result of Water Absorption in a Printing Press

A method has been found to investigate lateral dimensional changes in paper webs. The GFL Litho Tester using wet offset in the first printing unit and dry offset in the second unit. The print consists of solely of two register marks carefully aligned between the two printing units. The discrepancy between the register marks after the second printing is a measure of the lateral expansion. This is quantified using a microscope and camera to project the register marks on a video screen where the misregister can be measured with an ordinary ruler. The degree of misregister can be assessed with an accuracy of plus or minus 0.018mm, which is sufficient to establish and quantify differences between newsprint from different paper machines and between rolls from the same machine. Tests have been carried out with seven different paper rolls and significant differences have been established. The expansion recorded was between 0.11 and 0.25 mm over a 250mm wide measurement area i.e. almost 0.1%. In a full scale press with a web width of 1500mm, this means an expansion of 0.66 - 1.5 mm which gives an unacceptable large register error. A register error of 0.1mm can be seen by the naked eye. be measured with an ordinary ruler. The degree of misregister can be assessed with an accuracy of plus or minus 0.018mm, which is sufficient to establish and quantify differences between newsprint from different paper machines and between rolls from the same machine. Tests have been carried out with seven different paper rolls and significant differences have been established. The expansion recorded was between 0.11 and 0.25 mm over a 250mm wide measurement area i.e. almost 0.1%. In a full scale press with a web width of 1500mm, this means an expansion of 0.66 - 1.5 mm which gives an unacceptable large register error. A register error of 0.1mm can be seen by the naked eye.