Web release in heatset web offset printing

In offset printing unit the interaction between ink, blanket and paper surface defines adhesion of the web on the blanket surface. After ink transfer paper web follows the blanket surface at the nip exit until it is released due web tension. The adhesion of the web depends on the surface properties of paper, printing material combinations, the layout and press settings. In general, shorter follow-up of the web on blanket surface predicts easier release thus longer follow-up can lead to higher release angle. The purpose of this study was to clarify the factors (material and process parameters) controlling paper web's release from the blanket. The release of six various paper grades was followed. Other studied variables were the release property of blankets and the influence of ink tack on web release. Furthermore, during printing the effect of fountain solution feed and nip pressure on web release was examined. The release force between printing materials was first studied in laboratory and the verification was carried out in full-scale heatset printings. On printing press the release of the web was measured using release angle measurement arrangements. Furthermore, an acoustic signal was gathered from ink film splitting to describe the web release. According to results, paper grade determines the level of web release angle. The results indicate that the smooth paper surface induces strong adhesion on the blanket which leads to higher release angle. Furthermore, the surface structure of the paper and the speed of ink absorption affect web release. In addition, it was noted that small amount of accumulated material on blanket surface easies the web release. According to studies, the release angle of the web can be lowered by selecting easy release blanket, lower ink tack, higher fountain solution feed and lower nip pressure