Sheet Density
Physical paper structure values derived from single-sheet grammage divided by thickness define mass distribution across sheet thickness under standardized contact pressure. The measurement of apparent density ISO 534 establishes structural compactness for paper and board substrates under static mechanical load. Testing requires calculating the quotient of grammage and single-sheet thickness measured under a uniform static load of one hundred kilopascals.
Calculated density values govern sheet stiffness and fluid penetration rate during coating operations. Lower values indicate a porous network with high bulk, whereas elevated values mark dense fiber packing. The value stops applying when multi-sheet bulking thickness procedures are substituted or when corrugated fluting structures alter deadweight volume calculations.
Caliper Ratio
Fiber compression during calendering directly alters internal void space and sheet compactness. High mechanical pressure reduces void space while raising overall apparent density ISO 534 without changing total basis weight. Light calendering preserves sheet stiffness, whereas heavy calendering reduces bulk to smooth surface topography.
High density improves surface print smoothness. Lower density maintains bending resistance per unit weight.
Converting Limit
Rigid boxboard converting requires balancing creasing performance against structural stiffness requirements. When apparent density ISO 534 exceeds critical thresholds, creasing dies fracture surface fibers rather than forming clean hinge lines. Under-calendered board with low density folds cleanly but lacks the structural rigidity needed for high-stacking cartons.
Corrugating plants specify distinct density bands for linerboard to prevent scoreline cracking during high-speed converting.
