a b s t r a c tA laboratory creasing device to capture the most important properties of a commercial rotary creasing tool was designed. Finite element analysis of the creasing of a multiply paperboard in the laboratory crease device was presented. The multiply paperboard was modeled as a multilayered structure with cohesive softening interface model connecting the paperboard plies. The paperboard plies were modeled by an anisotropic elastic-plastic material model. The purpose of the analysis of the laboratory creasing device was to present material models that represent paperboard, and to investigate how well the analysis captured the multiply paperboard behavior during laboratory creasing. And to increase the understanding of what multiply paperboard properties that influence the laboratory crease operation. The result of the simulations showed very good correlations with the experimental obtained results. The results indicated that the paperboard properties that have the most influence is the out-of-plane shear, out-of-plane compression and the friction between the laboratory creasing device and the paperboard.
The localized deformation patterns developed during in-plane compression and folding of paperboard have been studied in this work. X-ray post-mortem images reveal that cellulose fibres have been reoriented along localized bands in both the compression and folding tests. In folding, the paperboard typically fails on the side where the compressive stresses exists and wrinkles are formed. The in-plane compression test is however difficult to perform because of the slender geometry of the paperboard. A common technique to determine the compression strength is to use the so-called short-span compression test (SCT). In the SCT, a paperboard with a free length of 0.7 mm is compressed. Another technique to measure the compression strength is the long edge test where the motion of the paperboard is constrained on the top and bottom to prevent buckling. A continuum model that previously has been proposed by the authors is further developed and utilized to predict the occurrence of the localized bands. It is shown that the in-plane strength in compression for paperboard can be correlated to the mechanical behaviour in folding. By tuning the in-plane yield parameters to the SCT response, it is shown that the global response in folding can be predicted. The simulations are able to predict the formation of wrinkles, and the deformation field is in agreement with the measured deformation pattern. The model predicts an unstable material response associated with localized deformation into bands in both the SCT and folding. the machine direction (MD), and the transverse direction to MD is known as the cross direction (CD). The failure stress in ZD is typically two orders of magnitude smaller than the failure stress in MD, while the failure stress in CD is about two to three times lower than MD. To obtain a low weight, paperboard is commonly produced as a sandwiched structure, with stronger mechanical properties in the outer-plies (top and bottom) and weaker properties in the middle. Measurements and simulations have been performed for a single-ply board in this work.Good foldability implies minimum spring back and absence of cracks along fold lines, cf. Cavlin. 1 Because of the bending state present during folding, in-plane compression strength has been attributed for being the dominant factor affecting the foldability of paperboard. The SCT value multiplied with the thickness is shown to be correlated to the maximum bending moment by, for example, Edholm. 4 However, later investigations have confirmed that the out-of-plane shear is an important mechanism to consider during converting procedures, cf. Nygårds et al., 5 Beex and Peerlings, 6 and Borgqvist et al. 7 The in-plane compression strength is difficult to measure because of that structural instabilities (buckling) easily are triggered as a result of the slender geometry of the paperboard. To overcome the difficulties associated with the structural stability in compression tests, short-span length can be used to prevent the buckling. An alternative experimental method is ba...
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