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Summary:
Although a number of paper properties are nowadays determined using measurement technology, printers are still not always in a position to reliably predict the quality of a print. This is due on the one hand to the interaction of paper, printing ink and printing-related parameters and, on the other hand, to the fact that paper properties cannot be measured under practice-oriented conditions in the laboratory. An important fundamental paper property that greatly influences print results is the surface structure of the paper. This quality is also termed smoothness (roughness). Nowadays, smoothness is usually determined in the paper industry using indirect measuring methods. In addition to smoothness, the measurement of printing smoothness is very important, i.e. the determination of the surface structure of the paper under converting conditions. Appropriate measuring instruments have been developed (Chapman tester, FOGRA contact area tester, Pira printing smoothness tester). A substantial drawback of these devices is that they only provide an integral measured value. An existing FOGRA contact area tester was therefore equipped with a high-precision distance measurement system and connected to a digital image processing system. The development of the surface structure as a function of paper deformation under load is available as the result of measurement. This technology was used to measure eight natural gravure papers (SC papers) that had been gravure printed in a large print shop. The results were evaluated by both visualization and image analysis. The results of smoothness measurement were correlated with the print evaluations. It was established that there is a strong relationship between surface structure, the deformation behaviour of the paper under load and the quality of the printed image. The best correlations were obtained for a newly defined parameter Grad(CA)p.