Optimizing Score Line Geometry for Low-Fibre-Length Recycled Substrates
Low-fibre recycled board requires widening female matrix channels to 1.8 times caliper and reducing penetration depth to prevent top-liner rupture.
Shear Band Formation
Deformation Mode
Localized material yielding under concentrated shear stresses creates narrow zones of intense plastic deformation within paper and paperboard structures. The localized physical mechanism called shear band formation develops during creasing and top-to-bottom carton compression loading. This deformation mode concentrates strain into distinct narrow bands, allowing cellulose fiber layers to delaminate cleanly during creasing without tearing outer linerboard surfaces.
It governs the crease formation and structural integrity of folding boxboard during packaging converting operations. The occurrence of this microstructural strain process stops when material loading remains within the linear elastic response regime of the fiber network.
Microstructural Strain
Applying mechanical compressive forces during creasing forces the internal ply structure of paperboard to yield in shear rather than tensile failure. During shear band formation, hydrogen bonds between adjacent fibers rupture along narrow angled planes, inducing localized delamination between board plies. This internal delamination reduces the flexural rigidity along the creased line while preserving the tensile strength of the outer surface liners.
Packaging converters depend on this controlled internal failure to achieve precise 90-degree panel folds without cracking decorative print coatings or face papers. Incorrect moisture content or excessive starch application stiffens internal plies, inhibiting shear band generation and causing surface cracking along carton edges. Microscopic inspection of creased board cross-sections reveals distinct shear planes that confirm proper crease matrix geometry and punch penetration depth.
Structural Failure
Inadequate delamination along crease lines leads to excessive spring-back force on automated folding gluer lines. Suppressing internal shear deformation forces outer linerboard layers into tensile rupture during box assembly.