The goal of this project was to clarify the phenomenon of copying in sheet-fed offset printing. The project was performed during 2003-2004 at VTT. Other participants of the project were Graphic Industry Research Foundation, M-real (paper maker), Sun Chemical (ink maker) and three printing plants: Otavan Kirjapaino Oy, Lönnberg Painot Oy and WS Bookwell Ltd. The research work contained two printings, material analysis from ink and paper, print in laboratory and correlation analysis of the measured data. The printing tests were performed in two printing plants with commercial inks and papers. The samples were taken during these printing tests. From smearing two different phenomena were studied: smearing in edge cutting and copying. Edge cutting deviates from the copying by shorter time (1-3 days compared to weeks-months) delay after printing but also in the way that it happens from the paper surface to another caused by huge even (in xy-plane) pressure. In the copying the pressure is very peaky. Copying can be predicted with the drawing test and smearing in edge cutting with the rub-off test. In the first printing the main variable was paper. In this test series there were four 130-135 g/m2 papers and one 80 g/m2 paper. The 80 g/m2 paper had 15-40% more copying than higher grammage papers. The difference was not seen in xy-plane methods. In copying there is z-direction pressure through the paper, which lead the higher grammage (and probably higher bulk and compressibility) papers to have less copying. The higher roughness of paper in these silky grades seems to decrease the copying but increase the smearing in edge cutting. In the other test series the variable was ink. In this printing the copying level and also its variation was less. So the paper influenced more to the copying than the ink in this study. From ink properties only the higher ink drier content decreased the copying (long time after the printing). With higher vegetable oil content the copying was not less as supposed but on the other way around. In copying there is higher pigment/binder ratio than in the printing where it has copied from, so pigment transfers more easily than the binder.
|Series||VTT Processes. Project Report|
- sheet-fed offset printing