engleski

INFLUENCE OF THE PAPER COATING ON THE PRINTING PLATE’S PROPERTIES IN WEB OFFSET PRINTING

The main task of the dampening solution in the offset printing is oleofobisation of the free surfaces on the printing plate. Ink which is then applied binds only to the surfaces of the printing plate and with the damping solution, under the infl uence of pressure, through offset rubber sheet is transferred to the printing surface (paper tape). In addition to its primary role, damping solutions with the passage through the printing machine also maintains the pH value of the surfaces of the machine parts with which it comes into contact. With the proper emulsifying damping solution gives a proper ink viscosity, stickiness, gloss, drying contributory and allows standardization of colorimetric parameters of the ink on the print. Passing through the printing machine, surface of the paper and its coatings, because of the friction between the paper and offset cylinder and chemical dissolution, by the rotating machine parts based on the cross from the offset cylinder and comes to the printing plate. Ink and dampening solution with the presence of dissolved paper coatings particulate, fi llers, binders, grain of the paper, traces of pigments ... alter the properties required for good performance of the press. This phenomenon is inevitable, but at the optimum temperature of the printing press (30-35 0C) additives in the wetting solution (buffer, softeners, emulsifi ers ...), and additives in the ink can compensate for this phenomenon, so it does not signifi cantly affect the quality of the printing. The most important parameter that changes with the interaction of the paper coatings, ink and damping solution is the changes of rheological properties of dyes and stickiness itself. This phenomenon is especially pronounced if the temperature of the printing machine greatly deviates from the optimum (pronounced phenomenon in the summer months due to the increase in temperature of the machine, and much less frequently in winter months due to a lower temperature.) In the web offset printing the change of the appearance of sticking ink becomes more pronounced in systems that do not use iso-propyl alcohol in the damping solution, since the additional supplement do reduce surface tension, but do not cool parts of the printing machine. If the printing machine is warmed signifi cantly over the optimal temperature ink becomes more viscous and, less saturated with pigment. To maintain the colorimetric parameters the automatic systems for applying ink increase the drift. Because of the larger deposits and changing the chemical composition of ink on the offset rubber canvas and strengthen the cohesion and adhesion forces, which increases the pulling surface coatings. Covered papers incorporated in the ink, of which the most common chemical are calcium carbonate and cellulose fi bers, through the offset rubber sheet come into the contact with the printing plate. With the successive infl uence of pollution from paper into the ink tribological system becomes much more aggressive and leads to increased wasting of the printing plates.

paper coating; printing plate; web offset printing

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