Volumetric Ratio
Dimensional thickness of a paper or paperboard sheet relative to its grammage defines the physical space a specific mass of material occupies. Apparent bulk provides a measure of how much volume a printer or converter receives for every kilogram of substrate purchased. It is calculated by dividing the caliper of the sheet, measured in microns, by its basis weight.
Fibre Architecture
Mechanical pulping processes generally produce fibres that retain their stiff, cylindrical shape to create a more porous internal structure. This arrangement allows for higher apparent bulk compared to chemical pulping, where the removal of lignin results in flatter fibres that pack tightly together. High-yield pulps are preferred for applications like book printing or cereal cartons.
Calendering rollers can be adjusted at the mill to compress the sheet and reduce this value when a smoother finish is required for high-quality graphics.
Shipping Efficiency
Material stiffness increases at an exponential rate with height, meaning a high apparent bulk allows a lighter board to provide the same structural integrity as a heavier, denser grade. This relationship enables the production of lightweight packaging that maintains its shape under load. Using bulkier grades reduces the total tonnage of fibre required for a production run.
