Cohesion Physics
Mechanical resistance defines the force required to pull apart the bonded layers within a multi-ply paperboard sheet before structural failure occurs. This inter-ply strength determines the capacity of a carton to withstand internal pressure or external stress during high-speed folding and filling operations. The measurement specifically identifies the perpendicular tensile load that splits the thickness of the material.
Mills maintain this integrity through controlled fibre refinement and the precise application of starch or synthetic bonding agents during the wet-end forming process.
Lamination Dynamics
Uniform density across the cross-section of the sheet dictates the degree of internal bonding. Lower density areas between plies result in weak zones that succumb to peeling forces when machines apply rapid mechanical tension. Operators inspect this property to prevent the surface coating from separating from the substrate under suction cups or vacuum handling heads.
Delamination occurs when the forces exerted during converting exceed the internal adhesive forces developed between the individual plies. Testing typically involves the use of specialized pull-testers to record the exact grammage force per unit area.
Production Boundaries
Machine speed increases often degrade the efficiency of layer formation and therefore lower the final bond values. Moisture content plays a secondary role by softening the internal bonds during subsequent printing cycles. Variations in recycled fibre quality introduce unpredictable performance outcomes because shorter fibres carry less bonding potential than virgin wood pulp.
Tight control over the drainage rate on the forming wire ensures the layers knit together before the sheet passes into the press section. High internal bond values offer the primary defence against sheet failure in demanding packaging applications.
