Internal Resistance
Internal bonding forces represent the strength of multi-ply paperboard when subjected to opposing lateral movements. The inter-ply shear stress of a material defines the limit at which the individual layers of a sheet separate during bending or printing. Numerical bonding data is critical for solid bleached sulfate and recycled boards where several distinct mats are couched together during manufacture.
Production Failure
Heavy ink tack or substantial pressure in packaging machinery exert force on the internal structure of the board. When the inter-ply shear stress exceeds the strength of the hydrogen bonds between fibres, the sheet splits and causes machine jams or visual defects. Internal separation often occurs at the interface between the top liner and the filler plies.
Maintaining starch application at the wet end prevents these structural breakdowns.
Strength Measurement
Dynamic testing involves applying a rapid impact to a sample to measure the energy absorbed during rupture. A low inter-ply shear stress reading suggests poor adhesion between layers and may result in the face of the board peeling away during the stripping phase of die cutting. Scott bond tests provide a numerical value in joules per square meter to grade this property.
High internal strength allows the substrate to withstand the forces of high speed offset presses.
