Internal Density Distribution
The variation in density across the thickness of a multi-ply paperboard determines its mechanical performance and stiffness. A substrate density gradient represents this engineered difference in fiber packing between the dense outer plies and the high-bulk inner plies. This structural design mimics an I-beam, providing maximum resistance to bending with minimum material usage.
Deformation and Stiffness
High-density outer plies, made from highly refined chemical pulp, resist tensile and compressive forces when the board is bent. This substrate density gradient ensures that the bulkier mechanical pulp core in the center carries less density, keeping the overall sheet light and stiff. If the density were uniform across the sheet, the board would require much more fiber to achieve the same bending resistance.
This distribution of fiber types and densities is controlled on the forming section of the paper machine by managing the multi-ply headbox.
Process Measurement Routine
Papermakers use micro-slicing and X-ray density profiling to analyze how the fiber weight is distributed throughout the sheet thickness. Maintaining this engineered profile is critical for ensuring consistent performance during die-cutting and creasing. If the outer plies are too low in density, they may crack or tear during folding, compromising the structural integrity of the carton.
Conversely, if the core is too dense, the board loses its stiffness and fails to protect the packaged contents during transit.
