Monitoring heatset performance to control accumulation

The aim of this work was to clarify the mechanisms of accumulation build-up on heatset web offset press. The influence of press settings on accumulation tendency was also studied. On-line microscopic imaging system was used to monitor the state of the printing plate and blanket during printing. Images were taken from non-image, halftone and image areas. Press performance was followed during three different full-scale printings. According to results accumulation on blanket starts from plate conditions. Small ink particles detected on the non-image areas of the plate are transferred on the blanket surface causing accumulation build-up. Filamentation of ink emulsion at the nip exit gathers ink at the trailing edges of the image areas or even slightly over the edges. This leads to enrichment of the inked material at the trailing edges of the image areas and around the halftone dots on blanket surface. Nip pressure further enforces ink towards the edges of the image areas. The accumulated layer could be up to ten micrometres thick containing elements both from ink and paper. This can be especially harmful around the halftone dots as it can reduce the size of the printed dot during long print runs. Based on these findings mechanisms of heatset accumulation on ink transfer unit was clarified. On-line microscopic images from printing plate and blanket show that press settings have clear effect on accumulation tendency on blanket. Furthermore, the on-line imaging provides means to solve problematic accumulation related runnability issues of on-going offset process.