Paperboard Anisotropic Hygrothermal Expansion Mechanics in Converting Operations
Paperboard expands five to seven times more across the grain than along it, requiring precise humidity control and pass timing to maintain converting register.
Tight Center Doming
Central Convexity
Convex elevation across the interior zone of a stacked paper or board pile creates an uneven, crown-shaped top surface. The distortion defined as tight center doming occurs when sheet perimeters lose moisture to dry surrounding air and shrink, while the dense pile core retains its original moisture and dimensions. Sword hygrometers placed in the stack reveal a lower equilibrium relative humidity at the edges compared to the central zone.
The scope covers pile height differentials generated by edge shrinkage, stopping where mechanical winding tight cores or pallet deck irregularities cause physical stack crowning.
Desorption Gradient
Moisture evaporation occurs along the outer borders of stacked sheets whenever the processing room atmosphere is drier than the delivered paperboard. This perimeter moisture loss forces cellulose fibres at the stack edges to undergo desorption and shrinkage along their cross-machine direction. Because the tightly packed centre of the pile prevents air circulation, the interior moisture content remains unchanged, preserving original sheet dimensions in the middle.
The shrunken, shortened outer edges pull inward and downward against the unshrunk center, forcing the excess central material to buckle upward into a dome shape. The height of the dome rises in proportion to the humidity differential, stack height, and sheet surface area. High-basis-weight paperboards exhibit greater structural resistance to this bending, yet develop high internal shear stresses that cause feeding friction between adjacent sheets.
Press Feeding
Vacuum feeder suckers on high-speed sheetfed presses struggle to seal against the sloping, convex surfaces of domed piles. The presence of tight center doming leads to frequent press trips, sheet skewing at the front lay guides, and double-sheet feeding errors. Blanking stations operating on domed stock suffer cut-to-print misregistration because sheets stretch unevenly over the raised central core during positioning.