Screening Retention
The long fiber fraction designates the percentage of cellulosic material remaining on a designated sieve mesh during Bauer McNett classification of a virgin kraft pulp sample. Refining intensity alters the long fiber fraction directly by shortening soft and hardwood elements before paper machine wet end deposition. Lower numerical values on the drainage tester scale indicate excessive mechanical work during stock preparation.
Tensile strength and tear resistance depend upon retaining adequate morphological length within the refined furnish matrix. Finished linerboard grades demand specific retention thresholds to prevent internal bonding failure during corrugated box conversion.
Drainage Resistance
Drainage kinetics slow down when the long fiber fraction increases beyond targeted mill parameters because larger elements form porous networks that impede water removal on the Fourdrinier wire. Vacuum boxes must apply higher suction pressures to dewater bulky furnishes adequately before pressing stages occur. Board stiffness rises proportionally with the retention of unbroken cellular walls during pulp preparation.
Papermakers adjust refiner load downward whenever drainage times exceed acceptable limits on the wet end control screen.
Tensile Tradeoff
Tensile strength diminishes when refiners preserve an excessive long fiber fraction because inter-fiber bonding sites decrease per unit mass of sheet structure. Bending resistance improves significantly under identical grammage targets because rigid cellular networks create higher moment of inertia within corrugated fluting media. Converters balance these competing physical properties by blending short eucalyptus pulps with unrefined long pine furnishes prior to headbox injection.
Sheet porosity follows the same mechanical adjustment because open formation permits higher air permeability values across finished packaging substrates.
