Interfacial Shear Strain and Delamination Dynamics in Thermal Plastic Board Lamination
Interfacial shear strain from thermal mismatch drives delamination; controlling cooling gradients and dyne levels prevents bond failure at crease lines.
Residual Stress
Thermal Curl
Mechanical constraint during web drying creates residual stress across laminated cartonboard layers, forcing finished sheets to distort spontaneously when humidity shifts on the converting floor. Permanent internal tension locks into the cellulose matrix during the high heat phase of Yankee cylinder drying, establishing a silent mechanical gradient between top liner and back liner. Differential drying rates across the caliper develop unequal shrinkage forces that remain trapped after cooling, holding potential energy within the flat blank until atmospheric moisture releases the bond.
Converters measure this locked-in load through standardized curl testing, quantifying radius changes against known temperature benchmarks to predict how a pallet will behave during high speed die cutting and automated folding.
Moisture Reversal
Ambient humidity variations trigger dimensional changes when trapped mechanical forces encounter moisture ingress, driving edge curl or center buckling along the production line. Cellulose fibres absorb water vapor faster on unvarnished reverse sides than on polymer coated exterior surfaces, unbalancing the stored tension profile established during reeling. Expanding lower layers push against constrained top laminates, transforming locked mechanical stress into visible distortion that jams feed mechanisms in high speed gluing machinery.
Operators mitigate board deformation by conditioning stock in controlled environments prior to printing, neutralizing atmospheric shifts that otherwise release stored mechanical energy mid run.
Structural Threshold
Safe conversion limits depend entirely on maintaining internal forces below the fracture limit of the bonding adhesive joining paperboard plies together. Excessive mechanical tension from improper calender nip pressure breaks fiber-to-fiber bonds inside the multi-ply sheet, causing internal delamination during subsequent hot foil stamping or UV varnishing passes. Mill laboratories evaluate ply bond strength using pin adhesion tests, verifying that internal layers withstand the violent release of locked energy during aggressive shearing operations.
Exceeding this critical threshold initiates localized blistering across the printed surface, ruining structural integrity before secondary packaging assembly begins.