Hygral Stress Dynamics and Edge Waving Mechanics in Recycled Paperboard Skids
Hygral edge waving in recycled paperboard skids results from perimeter moisture absorption driving compressive buckling against a dry, rigid core.
Hygral Stress
Moisture Tension
Internal mechanical forces develop inside paper networks when moisture content changes induce localized swelling or shrinkage in constrained fibers. Material physics defines hygral stress as the internal tension or compression created within a paper web when moisture-induced dimensional changes are resisted by mechanical constraints. Unbalanced stresses across sheet layers produce dimensional distortions known as curl and wave edges.
Expansion Process
Cellulose fibers swell preferentially in diameter rather than length when absorbing water vapor, causing anisotropic dimensional changes across machine and cross directions. Coated boards experience differential moisture movement because surface coatings impede vapor transport while raw back sides respond rapidly to ambient humidity shifts. As constrained paper fibers expand against neighboring layers, internal compressive stresses build within the wetter zones while tensile stresses form in drier zones.
High heat in web drying sections accelerates moisture loss, creating severe moisture gradients through the sheet thickness. Subsequent storage in non-conditioned environments triggers moisture re-absorption, relaxing or accentuating internal stress fields. Converting processes suffer from register errors and feeder jams when asymmetric hygral stress causes sheet flat distortion.
Equilibrium Boundary
Moisture conditioning standardizes test samples at fifty percent relative humidity to relieve transient internal stress gradients before physical testing. Exceeding the elastic limit of the web during moisture transitions causes permanent mechanical deformation and persistent board curl. Hygral stress determines dimensional stability during printing and converting operations across variable climate conditions.
Modeling Hygral Stress Profiles and Edge Waving in High-Bulk Recycled Cartonboard Skids
Modeling transient moisture gradients and hygral stress in high-bulk recycled cartonboard skids establishes barrier wrap limits to prevent press edge waving