Investigation of Effect of Printing Conditions on White Dots of UV Offset Printing

Abstract

The appearance of white dots (vessel picking) is one of the problems associated with offset printing. The complaint on white dots of UV offset printing was made against the commercial coated paper that its quality on surface strength was enough to printing with normal oil-based ink. We tried to establish an evaluation method to clarify the effect of printing conditions and to develop a solution to the white dots. As a result, we found that we should use high tack UV ink under low temperature, because the tack of UV ink is much higher than that of oil-based ink under low room temperature. We discovered that the printing speed is also affected by low temperature. The surface roughness of the rubber blanket for offset printing influenced the appearance of white dots, and we confirmed that this was caused by the difference of the paper from the blanket by observation with a printing tester. The grain of paper affected the white dots as well. We assumed this was due to the bonding strength of the vessels to pulp fibers, which showed anisotropic characteristics depending on the grain. We concluded that one of the most important control factors, which was liable to variation, was room temperature. Furthermore, it was thought to be effective to use a rough blanket and short grain paper.