Changes in the surface roughness of aluminium oxide (non-printing) areas on offset printing plate depending on number of imprints

A printing and non-printing surface structure of the offset printing forms are key factors in maintaining the conventionaloffset printing. The main characteristics of the surface are: physical and chemical structure, surface tensionand surface roughness. Surface topography is one of the critical factors which could cause the instability inthe quality performance and the durability of the printing forms. During the printing process these characteristicschange and directly influence the print. Performed investigations are based on the fact that the changes in physicalchemicalproperties of the non-printing areas as well as the changes in the surface micro structure of the printingforms directly influence the quality of the reproduction. In this paper, the behaviour of non-printing elements of athermal CTP plate developed in a fresh developer and used for a print run of 123.000, 177.000 and 300.000 copieson a web offset heat-set printing press has been investigated. Investigation made in a paper showed that high depthof focus SEM (Scanning Electron Microscope) can provide detailed topographical information about the surface,but cannot provide quantitative topographical information. Due to this fact, the printing forms have been also observedand analysed before and after the print run by a mechanical stylus profilometer. The investigations revealthe appearance of the physical changes as well as the geometrical ones. As a result of these changes, the influenceof a fountain solution on the printing form may also vary, as well as the balance between the ink and the fountainsolution during the printing process.