Structural Rupture
Defect formation during the folding of stiff paperboard materials often occurs when the outer fibers of the creased zone split under tensile stress. This score line fracturing diminishes the structural integrity of the carton, creating a rough, unsightly edge that weakens the package and ruins the printed graphics. The occurrence of this cracking sets the maximum bending angle and board thickness that can be used on a high-speed packaging line.
Mechanical Strain
Creasing uses a male rule to force the paperboard into a female die, deforming the internal structure to create a localized hinge. This operation intentionally shears the internal plies to allow the board to fold without rupturing the top liner. If the score line fracturing occurs, it means the internal ply separation was insufficient, forcing the outer liner to absorb all the bending strain.
Factors such as board dryness, excessive rule height or incorrect die width can prevent the smooth delamination required for a clean fold.
Board Elasticity
Carton converters maintain the moisture content of the paperboard between six and eight percent to prevent dry fiber breakage. High-quality multi-ply boards use long, chemical softwood fibers in the outer layers to provide the high tensile strength needed to resist score line fracturing. Evaluating this resistance involves using specialized bending stiffness testers that measure the force required to fold the crease.
