Internal Separation
Mechanical failure within the layers of a multi ply board occurs when the internal bonding strength is lower than the applied stress. In cases of fiber network delamination, the sheet splits into two or more distinct layers because the forces of a converting process exceed the cohesion of the fibers. This often happens during the rapid exit from a printing nip where tacky ink pulls on the surface.
It is a catastrophic failure that destroys the structural integrity of the packaging.
Bond Strength
Testing for internal bond strength measures the energy required to pull the board apart in the Z direction. When fiber network delamination occurs, it usually points to a weakness at the interface where two different pulp types meet. Starch or other bonding agents are often used between plies to prevent this separation.
If the moisture content is too high, the bonds weaken and the risk of layer separation increases.
Processing Damage
High pressure creasing or folding can also initiate a split in the internal structure. If the crease is too deep, it breaks the bonds between the plies and allows the board to bulge or crack. This effect is particularly problematic in heavy weight boards used for luxury packaging.
Proper moisture management and tool calibration are the primary defenses against this structural breakdown.
