Machine Direction Fiber Orientation Mechanics in Packaging Board
Jet-to-wire velocity ratios govern machine-direction fiber orientation, setting board anisotropy ratios that dictate box compression and score crack resistance.
Headbox Shear Rate
Hydrodynamic Velocity
Fluid dynamic parameters inside papermaking stock delivery systems govern fiber floc breakup and velocity gradients before suspension jet discharge onto forming fabrics. On the wet end of a paper machine, headbox shear rate measures the velocity differential across adjacent liquid layers in the headbox nozzle chamber. Fluid flow models calculate velocity gradients based on stock throughput and internal tube bank dimensions.
High velocity gradients disrupt cellulose fibre entanglement, maintaining uniform dispersion of suspended solids within dilute papermaking stock. Proper shear energy prevents fiber flocs from reforming prior to jet landing on the wire. Operating at target shear rates ensures consistent basis weight distribution across the entire machine width.
Dispersion Uniformity
Formation quality in fine paper and packaging grades depends directly on controlled stock shear before wire deposition. When headbox shear rate drops below threshold limits, cellulose fibers re-flocculate into dense clusters that create cloudy formation and localized mass variations in the finished sheet. Excess shear introduces hydraulic instability, causing jet spraying and air entrainment that disrupts sheet structure.
Mill operators adjust internal pressure attenuators and micro-slice actuators to stabilize shear across the nozzle exit.
Turbulence Boundary
Calculations lose physical relevance when stock consistency exceeds two percent solid content. High solids concentrations transition suspension rheology from Newtonian fluid behavior to non-Newtonian plug flow, invalidating standard shear gradient equations.