Directional Orientation
Structural disparity between the machine direction and the cross-machine direction arises from the way suspended papermaking fibers align on the forming wire. This characteristic of fiber anisotropy establishes that the physical properties of the paperboard vary according to the angle of measurement. The cellulose fibers tend to align parallel to the direction of the moving wire during wet-end drainage.
Physical Property
Mechanical tests reveal that tensile strength is higher along the direction of fiber alignment than across it. Conversely, the board exhibits greater tear resistance when the tear propagates perpendicular to the majority of the fibers. Understanding this directional nature is necessary for designing corrugated boxes where the flutes must stand vertically to support stacking loads.
Engineers specify different paper grades based on these oriented strength profiles. Sheet formation adjustments at the headbox can alter the ratio of alignment to suit specific needs.
Printing Performance
Feeding sheets through a press requires careful control over the dimensional stability of the substrate. Because fiber anisotropy affects how the sheet expands when exposed to moisture from offset lithography inks, misregistration can occur if the alignment is inconsistent. Stable runs depend on maintaining a uniform fiber distribution across the entire web width.
