Dynamic Z Direction Tensile Stress Propagation in Recycled Fibers during High Speed Creasing
High speed creasing generates Z-direction tensile stress waves that fracture short recycled fibers unless matrix clearance absorbs elastic strain energy.
Shear Strain Concentration
Localized Stress
Localization of shear stress within a narrow zone of a multi-ply paperboard sheet occurs during creasing and folding operations. This shear strain concentration arises because the outer plies are stretched while the inner plies are compressed. This internal yield is necessary to form a functional crease that can be folded easily without rupturing the surface liners.
Ply Delamination
Ply bond strength must be engineered to withstand normal handling while allowing for this controlled internal delamination during creasing. Starch spraying and refining are used at the wet end of the board machine to adjust the bonding between the layers. If the ply bond strength is too high, the board will fail to delaminate internally, causing the fold to crack and ruin the appearance of the box.
Conversely, low bond strength can cause the board to separate during printing. Finding the correct balance is a primary task of board machine operators.
Crease Performance
Evaluation of the creasing properties is performed by measuring the crease recovery force, which indicates how much resistance the fold offers to folding. When the localized shear strain is correctly developed, the crease stiffness is significantly reduced, allowing the carton to be folded with minimal force on automatic packaging lines.