8.3 - Press conditions define heatset piling tendency

摘要

In heatset printing formation of piling on blanket non-image area is due to the interactions between all the materials used in printing; plate, blanket, ink, fountain solution and paper. A newly developed online imaging system for monitoring piling formation has shown that non-image piling occurs either around the halftone dots or all over the non-image area of the blanket depending on the paper type printed. The high-speed camera with microscope optics is a useful tool to visualize formation of piling on blanket and the changes induced by material modifications in the process. However, to be able to understand the transfer of ink and fountain solution onto paper, the amount of ink and water on the blanket should be measured. We approached piling by developing new online devices to follow the changes in ink pigment, ink binder and water amount on plate and blanket. The information from the measurements was utilized to monitor the piling formation on non-image area of blanket in the ink transfer unit. Online device Inka detects changes in ink pigment amount on plate and blanket. Online device Hippo follows water amount changes on plate and blanket and changes in ink binder amount on blanket. Water amount on plate increased sharply when fountain solution feed was increased, whereas a more gradual change in water amount was observed on blanket. In addition to fountain solution feed level, the ink emulsification properties also affect the water amount on non-image area of blanket, whereas the role of paper coating structure was insignificant. It is more favourable to use somewhat higher fountain solution feed than normally is run to prevent the ink attachment on the non-image area of blanket and to diminish piling tendency. Even if the piling looks like ink, it also contains paper fillers and fibers in the case of uncoated paper and coating pigments in the case of coated paper. After printing with coated paper the piling layer is more accumulated around the halftone dots and with uncoated paper the piling forms a uniform layer on the non-image area of the blanket. An interesting finding was that ink pigment and ink binder can separate and accumulate at different rate on blanket.