engleski

Modification of flexographic printing plate’s surface properties by variation of UVC finishing

Flexography is a printing technique mostly used in packaging printing. In the beginning, used mostly for printing on the corrugated board, this technique has developed and is applied today in printing on various packaging materials and printed electronics. Many parameters influence the quality of the final product in flexography: photopolymer material used for the printing plate production, quality of the file adjustment, type of the anilox roller, type of the base placed under the printing plate to regulate the pressure, properties of the printing ink and printing substrate, and a set of parameters associated with the printing plate production. The printing plate making process includes three exposures to UV light, and finishing with exposure to UVC wavelength. Finishing has to be performed in order to terminate the photopolymeriazation process. During the transfer of the printing ink from the printing plate to the printing substrate, surface properties of the printing plate highly influence the quality of the imprint. The aim of this paper was to determine the influence of flexographic CtP printing plate’s UVC finishing on its surface properties by means of contact angle, surface free energy and EDS analysis. Standard UVC finishing for this type of printing plate equals 4 minutes of irradiation, with central wavelength of 256 nm, 18 mW/cm2. In order to define the influence of different duration of UVC finishing on the surface characteristics of the printing plate, samples used in this research varied from UVC finishing duration of 1 minute to the extreme of 14 minutes. Contact angle was measured by means of Sessile drop method, and surface free energy was calculated applying OWRK method. Results have indicated that UVC finishing influences printing plate’s physico-chemical surface characteristics by changing the contact angle and the surface free energy parameters. This could result with negative influence on the process of the printing ink transfer from the printing plate to the printing substrate. Measurements indicated that prolonged UVC radiation results with lower contact angle, primarily of the polar probe liquid, with final outcome of increase of the total surface free energy. EDS analysis showed that the changes in contact angles of probe liquids and surface free energy could be caused by the increase of oxygen concentration in the surface layer of the printing plate. Therefore, duration and intensity of UVC finishing must be strictly adjusted and regularly monitored in order to avoid unwanted changes of the flexographic printing plate‘s surface properties, consequently its behaviour in the printing process.

flexography; photopolymer; CtP; UVC finishing; surface free energy; EDS

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