Structural Separation
Internal bond strength defines the mechanical resistance of multi-ply paperboard against interior ply separation during converting operations. Paperboard delamination describes structural failure where adjacent fiber layers split apart parallel to the sheet surface under shear or tensile stress. Controlled delamination is necessary during scoring, while uncontrolled splitting destroys carton rigidity.
Interlaminar Shear
Multi-ply paperboard is produced by joining wet fiber webs on multi-cylinder or multi-gap formers. Paperboard delamination occurs when internal z-directional tensile stress or interlaminar shear forces exceed the mechanical bonding strength between plies. During carton scoring and creasing, intentional micro-delamination along central plies allows the board to bend without fracturing outer printable liners.
However, weak inter-ply bonding causes unwanted peeling during high-speed gluing and die-cutting of finished boxes. Refinement of recycled fibers, starch addition at ply interfaces, and couch press nip pressures dictate internal bonding strength. Standard testing protocols like TAPPI T569 measure internal bond strength using z-directional impact energy units.
Rupture Limit
Mechanical testing quantifies internal cohesion across multi-ply fiber structures. Paperboard delamination analysis does not apply to single-ply paper grades, plastic films, or aluminum foil laminates. Evaluation stops after total structural separation occurs across all internal fiber plies.
Testing results become invalid when sheet moisture content exceeds fifteen percent.
