Study on the quality of reproduction of graphical linear micro-images using electrophotographic printing

Abstract

Today’s market of visual graphic communication includes many various advertising, publishing and packaging products printed on different materials. Many products are printed using conventional printing technologies such as offset printing or flexography, in addition to digital ones like Electrophotography and Ink Jet. The graphic images on prints often contain extremely fine graphic details, such as micro-lines or micro-text elements, which are part of various identification, security markings, codes, etc. It is a very important task to optimally model these micro-images on printed graphic communication products at the design stage, in order to ensure their brightness, legibility and geometric dimensions meet the requirements, not only immediately after printing, but also, for example, after a certain period during the life cycle of the printed product after exposure to the sunlight spectrum, or to other mechanical or environmental effects. This paper presents a methodology and results for the assessment of the quality of the reproduction of fine image details, i.e. the micro-lines of different widths arranged on the print in different directions. A specially developed original digital press quality control wedge with monochromatic micro-lines, microtext and screen dots bars of CMYK colours was applied to assess the accuracy of micro-image reproduction. In this part of the study, the wedge fields with positive micro-lines arranged in different directions individually and in groups were analysed. The width of the micro-lines was measured under the microscope to assess the accuracy of the dimensional reproduction and the deviations from the nominal value. To assess the influence of different parameters of the system “printing press-paper-inks” on the reproduction of image microelements, the prints for this part of the study were printed using electrophotographic printing techniques on different dry-toner printing presses on different types of paper. The width and deviations of the micro-lines on the print were measured along the print direction, perpendicular and at 45˚ to the print direction. Measurements of the reproduction accuracy of monochrome micro-line images, printed on dry-toner electrophotographic presses, showed that the accuracy of micro-line reproduction depends not only on the printing system, its resolution and the characteristics of the paper, but also on the direction of the positioning of the micro-lines on the printed sheet. The obtained results allow us to compare the capability of digital printing systems to reproduce linear micro-images on visual graphic communication products of any size and geometric orientation, to select optimal systems for printing specific products, and to model the layout of micro-images on products at the design stage, by assessing the orientation of the micro-images on the printed sheet.