Dynamic Crease Stiffness Reduction Ratios under Ultra High Speed Packaging Line Strain Rates
Dynamic crease stiffness reduction ratios must be evaluated under operational strain rates of 1000°/s to 5000°/s to prevent high-speed cartoning line jams.
Cellulose Relaxation Time
Molecular Network
Paperboard fibers store and dissipate strain energy through transient rearrangement of hydrogen bonds within the amorphous regions of wood polymer networks. The characteristic period required for internal mechanical stress to decay to a fraction of its initial value under constant deformation is known as cellulose relaxation time. This physical property governs the rate of stress dissipation during die-cutting and high-speed carton erecting, and it ceases to govern mechanical behavior once the polymer matrix undergoes permanent structural fracture.
Viscoelastic Response
Converting machinery applies rapid displacement during score formation, forcing cellulose chains out of equilibrium. Short cellulose relaxation time allows the internal hydrogen bond network to reform in the displaced state before the carton blank exits the creasing station. Conversely, long relaxation times maintain high residual internal stress, which causes carton panels to resist folding or spring open on automated packaging lines.
Fiber origin, refining intensity and plasticizing additives directly modify the internal mobility of the polymer network. Elevated sheet moisture shortens the time constant by swelling the cell wall matrix and lubricating molecular movement. Temperature increases during calendering or pre-heating also accelerate bond exchange kinetics, altering the force needed to maintain a fold.
Rate Dependence
Rapid folding speeds exceed internal dissipation rates. High strain rates increase rigid resistance.