Ink-paper interaction, a new analysis for the control of back trap mottling

Back trap mottle is caused by non-uniform ink-paper interactions and setting during printing, resulting in non-homogeneous ink-transfer to the subsequent station(s) in the printing press. In this paper a method is discussed to reduce back trap mottle for uncalendered multiple blade coated papers. Pointed out is that forgiven paper-roughness and process-conditions (drying-conditions, coat-weight etc.) iris possible to reduce back trap mottle by adjusting the surface properties of absorption, porosity and surface-energy in a way that the film-split plane occurs more towards the direction of the opposing blanket roll. In this case less back-transfer of ink from the paper to the offset blanket will occur, resulting in less sensitivity for back trap mottle. To reduce the sensitivity, surface properties should be adapted such that the adhesive strength between paper and ink exceeds the cohesion within the ink-film. In this situation, adopting a faster ink-setting will actually reduce the back-transfer of ink during printing which results in a decrease of the sensitivity for back trap mottle. On the other hand, if the adhesive strength between paper and ink is low, it will demand slower ink setting to reduce the sensitivity for back trap mottle, which is attended with a risk for set-off in modern high speed presses. Print density measurements on the static "pull-off" areas of tack test stripes (Gane and Seyler, Tappi Coating Conference Proceedings, 1994) have been used to judge the strength of the adhesion between paper and ink in relation to the tackifying cohesion forces within the ink.