Ply Adhesion
Internal fiber separation forces determine how multi-layered paperboard resists delamination under the mechanical stress of high-speed converting operations. Inter-ply bond strength defines the threshold of energy required to separate adjacent paperboard layers in a Z-directional test configuration. Paper machines establish this property during the wet end forming stage through adequate stock refining and optimal press section consolidation.
Insufficient hydraulic pressure at the wet presses leaves micro-voids between the plies, which directly causes premature failure during subsequent offset printing or die-cutting.
Failure Mechanics
Tensile stresses perpendicular to the plane of the sheet occur constantly when viscous printing inks tack onto the paperboard surface. Inter-ply bond strength dictates whether the substrate holds together or splits apart under the pulling force of high-tack inks running through lithographic units. Lighter cylinder weights reduce overall caliper but require higher specific bonding energy to prevent internal ruptures during carton blank separation.
Converters evaluate this Z-directional resistance before running high-speed folding carton lines to avoid catastrophic delamination during automated creasing and gluing.
Conversion Limits
Edge-wicking vulnerabilities multiply rapidly whenever internal separation forces drop below established converting thresholds. Inter-ply bond strength prevents moisture from penetrating the exposed edges of liquid packaging board during prolonged cold storage environments. Caliper variations across the web width alter the local consolidation pressure, creating weak zones where the internal layers separate prematurely.
Mill operators monitor Scott-type internal bond values continuously to keep the manufacturing process within strict tolerances required for leak-proof carton production.
