Investigation on mechanism of ink peeling-off of sheet-fed offset printing on gloss coated paper

摘要

"Yukifuri" ("Snowfall") is a quality issue where white dots appear sporadically on the printed surface like snow-falling. At several printers, when gloss coated paper was sheet-fed and offset printed, snowfall appeared in overlaid part of printed surface at the upper part of the piled up printed sheets, even though it was on the level of several tens of sheets, and this became a problem. When such snowfall appears, the resulted products have to be disposed of. Accordingly, this problem is desired to be solved. Since we have no study case in the past, we investigated how to reproduce this phenomenon and tried to discover the mechanism how such snowfall is induced and studied the related factors. As the result of our study, we found that snowfall can be reproduced by creating 4 colors overlaid portion with a sheet-fed printer, and on top of this printed material a sheet of transferring paper is piled up together with a weight, which is rotated slowly rubbing the printed surface. It was found that at the stage when a solvent component of ink penetrates into the paper and ink tack becomes high, if it is rubbed applying a certain weight, the ink layer peels off at the interface with the coated layer, causing snowfall. This appeared on all brands of printing papers we tested this time, but the time of generating this phenomenon is different depending on the paper brands. In the case of coated paper, on which we observed appearing of snowfall within short period of time, it was found that the volume of fine pores in the coated layer is Jarge. We guess that when the printed material is piled up after printing and moved, the top portion of the stacked printed material is rubbed with slight movement caused by vibration and snowfall is generated. As the countermeasures, it is suggested to fix the printed material firmly. Actually, a printer successfully solved this issue using tools they devised.