Deformation dynamics in creasing and folding of board

Laboratory creasing and folding experiments, with dynamics similar to that of real production, were carried out to study cracking mechanisms of coated folding box boards. The study included three double-coated board grades with varied strength properties and thicknesses. The board deformation was followed with high-speed CCD imaging to determine the local strain rates together with the accumulated volume change and delamination of the board. The measured strain rates were compared with theoretically estimated creep rates. During creasing, the shear strain near the edge of the ruler dominated the deformation. However, there was a large variation in strain rates and thus stresses depending on the creasing geometry and board grade. Two main deformation modes were separated depending on the relative groove width compared to the board thickness. Small strain rates during creasing led generally to large strain rates during folding. Thus, the creasing geometry should be chosen carefully according to the board grade, with the goal of medium-level maximal strain rates (and stresses) near the surface coating in both operations.