Optimerad wellpapp för minimala ränder vid flexografisk tryckning

Quality issues within flexographic post-printing of corrugated board have been prevalent in the industry since the printing method's adaptation. In particular, the print defect called "striping", which is classified as a noticeable difference in the print that aligns with the corrugation of the fluted structure of the board, is the most common defect observed for this method. Although quantitative and qualitative studies have been employed in the past, it is still unclear exactly what aspects of the printing system affect the degree of striping. This thesis aims to build upon current knowledge in this research field, and employ a simulation-based optimization of the corrugated board components and aspects of the printing system, in order to minimize striping using the Finite Element Method. The focus of the simulation lied in the mechanical relation between the involved components, particularly the optimization of material properties and geometrical variables. Since previous research implied that the degree of striping could be directly correlated with the contact pressure variation in the printing nip, this measure became the main output in the model. A parameter sensitivity study was conducted, to which it was found that parameters related to the printing plate and the corrugated board top liner had the strongest influence on the contact pressure variation. Parameters that had a significant influence on the output were included in the optimization process. Multi-objective optimization was employed with the contact pressure variation set to be minimized, while the material usage was minimized as well. This procedure gave a set of optimal solutions, which formed a Pareto frontier. A separate FE-model was used to compare the structural stability of the solutions. From the results, it could be seen that most optimal solutions involved focusing on increasing the stiffness and thickness of the top liner, along with decreasing the thickness of the bottom liner. Depending on whether minimizing striping or the material usage of the corrugated board is in focus, different fluting thicknesses and stiffnesses can be used. When investigating the structural stability of the optimal solutions, the critical load needed for the corrugated board structure to buckle was significantly reduced due to a thinner bottom liner being used. The Pareto-optimal solutions in conjunction with the sensitivity analysis offer valuable insight into the printing system's behavior and key influencing parameters. In addition to this, the analysis of the optimal solutions impact on the structural stability highlights potential weaknesses in solely focusing on print quality. These findings could act as practical recommendations for industrial implementation.