Standard Crease Depth and Rule Height Calculation for Multi-Ply Boxboard
Calculate crease rule height by subtracting compressed board caliper from cutting rule baseline, sizing matrix width to rule thickness plus 1.5 to 1.8 times board thickness.
Fiber Shear Strain
Internal Distortion
Mechanical deformation within the paperboard sheet occurs when parallel layers slide past each other during bending. Paperboard experiences fiber shear strain when it is subjected to the intense pressure of a creasing rule, which forces the internal layers to separate and slide. This controlled internal damage is necessary to create a clean, durable fold without tearing the surface of the board.
Structural Rupture
Delamination occurs within the sheet as the shear forces exceed the strength of the internal fibre bonds. This localized failure allows the board to bend along the crease like a hinge, rather than cracking on the outside. If the internal bond strength of the board is too high, the layers cannot separate, which forces the outer liner to stretch and crack during folding.
This failure can ruin the appearance of the package and expose the interior fibres to moisture, which weakens the entire structure of the box.
Converting Optimization
Adjusting the width and depth of the creasing channel controls the amount of strain introduced to the board. High-quality multi-ply boards are engineered to have a lower internal bond strength to facilitate this internal shear during creasing. Testing the creasing performance on a sample press ensures that the board achieves the correct level of internal delamination before long production runs begin.