Directional Distribution
Fiber layout on the paper machine refers to the spatial distribution of pulp fibers as they settle on the moving forming fabric. The pulp furnish orientation is established within the first fraction of a second of drainage, as the jet from the headbox meets the wire. Engineers adjust the jet-to-wire speed ratio to either increase or decrease the degree of fiber parallelism.
Sheet Anisotropy
Paper exhibits different physical properties depending on the direction of testing due to the alignment of its internal structure. When the pulp furnish orientation is highly aligned with the machine direction, the sheet has high tensile strength in that direction but is prone to tearing along the fiber lines. This anisotropy can cause difficulties in carton converting, where folds must be made both with and across the primary fiber direction.
Runnability Limit
Multi-ply packaging grades require balanced fiber layouts to prevent warping during printing and laminating. A poorly managed pulp furnish orientation causes differential shrinkage, which results in diagonal curl when the paper is exposed to changes in relative humidity. This curling can stop a printing press by causing sheet-feeding failures or double-sheet registration errors.
Laboratory testing of tensile stiffness orientation helps papermakers adjust headbox recirculating valves and slice openings to maintain a balanced profile across the entire deckle.
