Structural Property
Physical mass per unit volume of a paper or board substrate after manufacture. Calculating apparent sheet density involves dividing the basis weight by the thickness measured under standard pressure. This value indicates how tightly the fibres are packed within the sheet.
A high value suggests a compressed structure with low porosity.
Porosity Impact
Internal compaction of the material dictates the resulting porosity and stiffness of the finished product. When apparent sheet density increases, the sheet typically becomes thinner and less rigid for the same weight. This change affects ink absorption because a denser surface limits the penetration of vehicles into the fibre network.
It also determines how much space a finished stack of cartons occupies during shipping and storage. Printers monitor this to predict how the substrate will respond to fountain solutions and pressure in the nip. This value remains constant only if the moisture content is stable.
Sourcing Boundary
Production specifications for folding boxboard often prioritize thickness over mass to ensure carton strength. Adjusting the apparent sheet density allows mills to meet stiffness targets without adding extra fibre. Mills control this through refining and wet end pressing.
Excessively high density can lead to cracking during the scoring process.
