Structural Compactness
Volumetric mass concentration of the interior fiber layer in multi-ply paperboard determines the bending stiffness and thickness yield of the finished packaging sheet. Achieving an optimal core density requires a careful balance between fiber bulk and sheet consolidation during the wet pressing phase of manufacture. This internal property directly influences the compression resistance of cartonboard grades.
Board Thickness
Caliper retention during sheet formation depends on maintaining a low weight-to-volume ratio in the middle ply. Lower core density preserves the separation between the outer high-density plies, which acts like an I-beam to resist bending forces. Mechanical pulps or high-bulk chemithermomechanical pulps are commonly chosen for the center layer to ensure a voluminous structure.
When the board passes through the calender rolls, the pressure must be controlled to prevent crushing this porous middle zone. This preserves the bending resistance while keeping the overall basis weight low.
Compression Yield
Resistance of the packaging material to creasing and scoring depends on the internal shear strength of this central layer. If the core density falls below the threshold required for internal fiber bonding, the plies may separate during converting. Such delamination reduces the structural integrity of the carton during folding.
Higher density provides better resistance to ply separation but increases the overall weight of the board without adding equivalent stiffness.
