Mechanical Refining Impact on Multi Ply Boxboard Z Direction Tensile Development
Controlled low energy refining of middle ply mechanical pulp yields superior Z direction tensile development without caliper loss in multi ply boxboard.
Fines Migration
Fibre Loss
Unbonded short cellulose fragments escape the forming wire during wet web consolidation, depositing inside white water circuits or lodging within sheet voids. Fines migration describes this displacement of microscopic particulate matter through the internal channels of paper and paperboard during dewatering and drying stages. Suspended fragments travel past primary retention boundaries when chemical flocculation proves insufficient to lock them against longer structural strands.
Small particles migrate toward the wire side or accumulate near internal z-axis voids, altering porosity gradients across the cross section. Drainage elements apply hydrodynamic shear forces that mobilize unattached debris, pulling suspended matter downward through the accumulating mat or driving fines upward toward vacuum boxes. Papermakers adjust retention aid dosages and vacuum profiles to arrest particle transit before structural stratification compromises the converting performance of the finished stock.
Drainage Resistance
Hydrodynamic drag increases sharply when loose cellulosic fines clog interstitial pathways within the forming web. Permeability drops because migrating fragments accumulate at constrictions between longer structural fibres, creating a dense layer that impedes water flow. Press section felts experience rapid blinding when excessive unattached particulate matter transfers from the sheet surface under mechanical nip pressure.
Sheet consolidation halts prematurely if drainage impedance prevents moisture removal from reaching target dryness levels prior to calendar stacks. Operators monitor drainage rates continuously to detect shifting retention dynamics that signal improper flocculation chemistry inside the wet end.
Surface Uniformity
Localized concentrations of migrated particulate material create density differentials that ruin subsequent coating and printing operations. Coating color absorption accelerates where fines accumulation leaves open porous zones, causing mottle and uneven ink setting across offset grades. Sheet smoothness suffers when mobile fragments cluster at the top surface, producing micro-roughness that degrades high-speed print fidelity.
Converters measure Parker Print Surf values to verify that internal particle stabilization prevents surface dusting during rotary converting passes. Final sheet quality depends entirely on maintaining strict control over particulate mobility throughout the wet end of the paper machine.