Characterization of paperboard for creasing and folding

To form a package with desired properties, paperboard is creased and folded. Creasing creates delamination cracks which during folding cause the plies to buckle and form a permanent corner. The delamination cracks facilitate folding but reduce the load-bearing capacity of the packaging. The processes must therefore be controlled. The result is affected both by the constituent material properties and by the actual creasing and folding processes. A good understanding of these processes create opportunities to resource-efficiently create packaging that meets high standards. Evaluation and development of processes and paperboard is facilitated by models and simulation with correct material properties. Material data for delamination have been difficult to obtain because the test methods are often unstable. In recent years, together with Tetra Pak, we have developed a new experimental method for paperboard which we originally developed for pressure-sensitive adhesives. The results look promising but some critical properties are not fully captured in the models. This leads to the question: What mechanical properties of paperboard are lacking in the current models to better capture the creasing and folding processes in simulations?

In this project, we want to investigate two factors that probably have a substantial influence: 1) Effects of in-plane stresses in the paperboard. During creasing, the paperboard is exposed to combined tensile and shear loading. How the loads interact and how this affects subsequent folding is currently unknown; 2) Volume increase of the paperboard during delamination. From previous studies of cracking in other materials under shear load, volume increase has been observed. For adhesive layers, its influence has been quantified. Whether similar mechanisms are effective for paperboard has not yet been investigated. However, the possible impact on the processes is significant and not taken into account in existing models. With this project, there are good opportunities to improve the research environment and to achieve good co-production. Karlstad University has equipment and experience from the current experimental methods to derive material data. Örebro University has experience of paperboard and has experimental equipment (micro-CT) that can be used to study the microstructure of the paperboard. Through this, it is possible to connect the micromechanics with the material data. BillerudKorsnäs and Holmen, which both produce paperboard, assist with materials and material knowledge as well as expertise in modeling and simulation of the processes. Tetra Pak has experience of simulations of the processes and thus the result can be directly implemented in a model of the processes. Through collaboration, it is possible to connect the structure of the paperboard with its material properties in a way that is relevant to the industrial process in a scientifically interesting way.