Laminar Resistance
Internal tangential stress generated between adjacent fibrous layers in multi-ply paperboard during bending deformation governs carton creasing, folding and rigidity. Under flexural loads, inter-ply shear transfers mechanical force across the internal interfaces that join bleached top liners, recycled center plies and bottom backings. This internal resistance establishes overall sheet stiffness by forcing distinct fiber strata to act as an integrated composite structure.
The measurement ceases to apply once internal delamination occurs, causing the multi-ply board to split into isolated, flexible layers.
Stress Mechanics
Bending forces applied to paperboard induce tension in the outer top ply and compression in the bottom ply, creating opposing horizontal shear forces along intermediate ply interfaces. Internal starches, micro-fibrillated cellulose additives and fiber entanglement generated on multi-ply cylinder or fourdrinier formers resist these interfacial shears. If the shear resistance between plies is too low, the plies slide past each other, leading to premature delamination and loss of structural bending stiffness.
If the inter-ply bond is overly rigid, the board fails to delaminate properly during scoring, causing deep surface tears and cracked outer plies along carton creases. Controlled micro-shear failure within interior plies is required during high-speed carton scoring to form a permanent hinge without rupturing visible print layers. Laboratory instruments quantify this property using Scott bond impact tests or z-directional tensile methods to verify ply adherence under sudden mechanical separation.
Converting Performance
Folding boxboard and solid bleached sulphate grades rely on tuned inter-ply adhesion to achieve reliable performance on automated packaging lines. High-speed folding carton formers exert rapid mechanical deflections that demand consistent shear compliance along pre-creased scorelines. Low inter-ply strength causes blisters and delamination during heat-sealing, blister packaging and high-temperature laminating processes.
Excessive bonding forces the outer coated liners to absorb all folding strains, leading to cracked ink films and exposed bare fibers along critical package corners. Board manufacturers adjust inter-ply starch spray volumes and refining degrees on individual plies to balance stiffness against creasability.
