26 th -29 th September 2007, Zadar, Croatia Original scientific paper INK TRAPPING IN HYBRID PRINTING TECHNOLOGY I. Zjaki, I. Berti, S. Jamnicki Faculty of Graphic Arts, University of Zagreb, Croatia Abstract: In this paper the authors are comparing prints made with combination of offset and flexo inks used in hybrid printing technology. A good image reproduction is determined by appropriate trapping. Poor trapping means that superimposed inks are improperly laid down on the previously printed colours, causing poor colour balance and poor overall appearance. Hybrid press systems use combination of different printing units therefore it is possible to implement a flexo unit in an offset line and vice versa. Different printing units have advantages of their own and by combining them into one production line the quality of a print job can be increased. Flexo printing process uses low viscosity inks and offset printing process uses paste, viscous inks. According to that, the rheology and drying characteristics of each type of ink are different. The aim of this study was to investigate how two different types of inks would interact one with another when printed in different conditions. Key words: ink trapping, hybrid printing, flexo ink offset ink 1. INTRODUCTION The recent demands of the market impose using the combination of different printing techniques for achieving the desired high quality graphic products. Because of that the hybrid printing systems have been developed. Hybrid printing systems use the combination of different printing techniques in one production line. They can be composed of different conventional printing techniques, and that is achieved by combining the offset, the flexo or the gravure printing units in one printing system. In addition, the digital printing units can be combined with one of the conventional printing units (Kipphan H., 2001). Among the hybrid printing systems which combine the conventional printing techniques, those connecting the offset and flexo printing are mostly used. With the combination of these two techniques it is possible to positively influence the quality of the output graphic product. Each technology has its advantages which, when connected in one production line, can improve the quality of prints (D’Heureuse W., Kipphan H., 2001). In printing packaging, there is the growing need for special inks and coatings which are not easily printed using the standard offset printing technique. The combination of the offset and flexo units in one printing system can be the solution for high quality packaging and label printing. In such systems the flexo unit is usually located at the last position and it is most often used for application of coatings. The attached flexo unit can also be used for application of inks producing special effects (spot or metallic inks). Occasionally, the flexo unit can be located at first position, as in case of printing on aluminium foil for the application of base ink layer (Stadler P., et al. 2002). The characteristics of prints produced by hybrid printing technique combining the offset and flexo printing processes have been studied in this work. 2. EXPERIMENTAL 2.1. Problem description The prints made by combination of the offset and flexo inks have been studied in this work. The print quality characteristic chosen for the evaluation of hybrid printing techniques was trapping value of various ink combinations. Trapping has been defined as the ability (or inability) of the printed ink to accept the next layer of the printing ink (Zjaki I., 2007). Good acceptance of ink on the existing ink depends on rheology (viscosity and tackiness) of ink which is overprinted, on the ink film thickness and on the printing sequence of the inks. The hybrid printing system, consisting of offset and flexo printing units, was simulated by overprinting the inks of different rheological characteristics. Laboratory prints were produced by overprinting the uniform layer of the flexo ink on offset ink and vice versa. With this procedure, the secondary colours (blue, green and red) were reproduced and measured. -161-