Viscoelastic Creep Compliance and Interfacial Fracture Energy in Paperboard Laminates
Creep compliance dictates long-term carton deformation, while interfacial fracture energy governs delamination under score bending and pallet loads.
Hygroscopic Expansion Strain
Dimensional Alteration
Cellulose fibres absorb and release atmospheric water vapour, which causes the paper structure to change its physical size as relative humidity fluctuates. This physical movement is measured as hygroscopic expansion strain and occurs unevenly across different board directions. It remains a major challenge in multi-pass offset lithography.
Fibre Response
Internal swelling of individual fibres translates to macroscopic sheet movement, which is several times larger in the cross-machine direction than in the machine direction. The hygroscopic expansion strain affects paperboard plates by causing curl and cockling. Moisture uptake during aqueous coating processes can also trigger this dimensional shift.
Controlling the moisture content during storage prevents these shape changes before the board reaches the printing press.
Registry Control
Print registration errors occur when sheet dimensions shift between successive color stations. High precision print runs demand absolute stability, so sheets must be acclimated to the press room atmosphere before processing. When the hygroscopic expansion strain is too high, the paperboard stretches between printing units, which results in misaligned ink layers and blurred text.
Printers monitor both relative humidity and the equilibrium moisture content of the pile to mitigate this issue. Using board grades with high dimensional stability reduces the risk of alignment failure on complex packaging jobs.