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A number of parameters are involved to characterize printing quality such as density, gray scale in black and white printing, dot quality, mottling, and color location. The so-called mottling is especially significant. In the recent printing and paper forum within the framework of an international research project 24 paper types have thrown light on the mottling phenomenon in sheet offset printing machines. Based on a comprehensive evaluation of these presentations the discussion covers the influence of the paper on the development of mottling as a function of the raw material and manufacturing technology with consideration of the ink-water combinations. Topics include introduction; mottling as a printing phenomenon; selection of paper and printing trial (paper selection and paper properties; printing layout and printing trial); practical printing evaluation of the mottling effects (in general; pattern analysis printing evaluation); influence of the printing layout variants on the mottling effect (high quality coated offset papers (HGOP); and diverse other offset papers (DWOP). Various types of mottling include single ink; back trap; water interference; screen; and wet ink trap mottling. The 24 paper types studied were divided into high quality coated offset (18 papers) and other diverse offset (6 papers). The HGOP group consisted of papers of 115, 135, and 150 gm/sq m and chosen from matte, semi-matte, silk matte, glossy silk, and glossy. The DWOP group included 2 heatset (one matte, one glossy) papers, 2 pigmented papers with different specific volume, a coated recycling paper, and an uncoated paperboard. Printing was achieved on 8,000 sheets/hr five color sheet offset machines with use of strip rubber blanket. The mottling was characterized by a power spectrum analysis (PSA) index for an FFT of 512 pixels that corresponds to a measurement surface of 32.5 by 32.5 mm. Thereby the mottling increased with increasing PSA. The complex mottling relationships are described and graphically illustrated. Source

The product quality achievable in offset printing is determined by a number of influencing variables. Paper can be envisaged to be a carrier or substrate that exhibits ghosting during the printing of smooth, ultrawhite grades. When ghosting occurs, it manifests itself as a function of the base paper and coating layer after a short or medium production time. The factors that are of decisive importance for the print result, however, are the numerous interactions that are related to or occur during the printing process. The influence of paper has not yet been quantified successfully owing to the complex relationships that occur. Knowledge about exactly which interactions occur during ink transfer is therefore still limited and thus the prerequisites for a solution to offset printing problems like ghosting and vanishing dots are still lacking. A European research project has been launched to study paper-related influences on ghosting in detail. The studies are based on a printing trial: 18 coated heat set web offset papers (50 up to 100g/m2) were printed under virtual identical conditions using a "ghosting" form, until ghosting effects became visible. Ghosting was evaluated both visually and analytically (tool development). The different papers differed significantly in the intensity of ghosting effects. It will be demonstrated that ghosting is caused by an area reduction in printed dots by as much as 10% on the ghosting side. It will also be demonstrated that ghosting is dependent on the printed image on the non-ghosting side. The print results form the basis for the analysis of possible interactions between ghosting and important paper properties, first results of which will be presented as well. Source

An integral measurement parameter is predominantly still used today in the paper industry for the assessment of the surface structure of paper and cardboard. It is generated by indirect means with the so-called air flow measuring procedure and allows no comprehensive characterization of the surface. In the framework of a research project, two optical measurement principles were closely examined with respect to their suitability for the topographical assessment of paper and cardboard based on different product groups. Potentials and limits of the optical topographical measurement of paper will be worked out based on extensive metrological examinations. A bridge to the conventional air flow measuring procedure will be established. Source

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