Uncoated paper surface for coldset web offset printing : set-off studies

摘要

This study examines set-off in newspaper printing and its relation to newsprint properties and other printing parameters, consisting of a review of the literature related to the subject, a theoretical discussion and a set-off process analysis. Laboratory printing tests with laboratory-made paper and ink samples and commercial newsprint and newsink samples were carried out. In addition, the surface of newsprint, the structure of the offset blanket, the ink penetration and its location on the paper surface, and set-off prints were studied with microscopic methods. Ink transfer and ink setting and the situations where set-off is created were also examined. Main targets for this work were: to define the optimum surface characteristics for uncoated newsprint in newspaper printing with the coldset web offset (CSWO) method, and to gain a better insight into the set-off phenomenon.In CSWO printing, the compressibility and conformability of the blanket and the compressibility of the newsprint surface improve the ink coverage. After the printing nip the ink film splits only in the area where it has been in contact with the newsprint surface. In single-colour printing, the penetration of ink that is caused by the pressure in the printing nip is insignificant. In contrast, in multi-colour printing, the ink layer printed later pushes the previously printed ink deeper into the voids of the paper surface.Ink setting decreases set-off. It is the result of solvent separation from the ink layer. Ink setting is defined by the change of the set-off during the delay time. Filler loading improves ink setting. Set-off can be reduced by using less ink, higher printing nip pressure, lower pressure in the folder and in other set-off situations, and chemical pulp containing less compressible DIP-based newsprint.The most important properties of uncoated newsprint for set-off are: optimised roughness in relation to print quality and set-off, low surface compressibility, high specific surface area, good absorption ability of the fines area and uniform formation (no high-density calendering spots).