The role of material composition, fiber properties and deformation mechanisms in the deep drawing of paperboard

The aim of this paper is to contribute to an improved understanding of deformation mechanisms of paperboard in the deep drawing process with immediate compression which reaches the highest degree of formability. Experiments with different fiber types and material structures were conducted on a laboratory and pilot scale to determine the major material-related influences on the formability of paperboard. The different fiber type combinations were studied together with different additives. The results show that increased pore volume and fiber-to-fiber mobility significantly increase initial wrinkle height which is significant for the quality of the drawn three dimensional (3D) structure. A material structure consisting of a mixture of cellulose fibers and a low percentage of stiff regenerated cellulose or synthetic staple fibers in combination with alkyl ketene dimer or cationic wax as an additive leads to a wrinkle-free wall until a forming ratio (forming height divided by base diameter) of at least 0.22 is reached. Accordingly, the process can be broken down into three phases. In the first phase, the cellulosic structures are deformed or broken, and the material density is reduced due to fiber-to-fiber movement. Refining and wet pressing decrease pore volume, increase the number of bonds in the material, thereby obstructing fiber-to-fiber movement. In the second phase, wrinkles occur. The introduced bending stiffness index together with the tensile index provides an indication of the material behavior in these two phases. The third phase is a restructuring of the material structure and a fixation of wrinkles by creating new bonds in the limited wrinkle areas. The experiments indicate that refined fibers make significantly higher wrinkle strength possible and thus better fixation of the final 3D structure shape. Additives with low melting temperatures can also contribute to improved wrinkle strength even in a low mass percentage of 0.5 %.