Mechanical Shear
Angular displacement of parallel internal planes within a material occurs when parallel forces are applied in opposite directions across its surface. In paperboard processing, shear deformation is the primary mechanism that allows the multi-ply board to delaminate internally along a crease line without tearing the outer liners. This controlled internal rupture is essential for creating a flexible hinge.
Substrate Response
During the creasing action, the sheet is forced into the female matrix, which causes the individual plies to slide past each other. This relative movement is enabled by the low internal bond strength of the middle plies, which are designed to fail under shear stress. When this occurs, the bending stiffness of the board at that specific point is permanently reduced.
Processing Failure
If the paperboard has too high an internal bond strength, the material will resist this internal sliding and the stress will be transferred to the surface layers instead. This resistance can lead to surface cracking or liner splitting on the outside of the fold, destroying both the barrier properties and the appearance of the carton. Therefore, board manufacturers must carefully balance the z-directional tensile strength to ensure that it allows for clean shear deformation while maintaining overall structural stability of the container during transport and handling.
