Polymer Film Micro Fracture Mechanisms under High Strain Shear in Folding Boxboard Creases
High strain shear during boxboard creasing causes polymer film micro fractures when local elongation exceeds film yield capacity along the outer bend radius.
Bend Angle
Deflection Range
Instruments measuring mechanical stiffness record the angular displacement of paperboard required to determine flexural strength. Standardized testing protocols define bend angle as the specific arc through which a clamped substrate sample is rotated during a stiffness evaluation. Most laboratory procedures specify a fifteen-degree deflection to record bending resistance, whereas creasing performance tests often evaluate ninety-degree rotations.
Measurements cease once the designated angular target or structural failure is reached.
Board Resistance
Bending force varies directly with the degree of rotation until internal fiber networks yield under compressive and tensile stresses. Laboratory instruments applying a bend angle measure the peak resistance force exerted by the paperboard strip at specified angular intervals. Heavyweight multi-ply boards exhibit rapid force escalation during the initial five degrees of deflection, followed by internal ply separation as displacement continues.
Uncoated recycled paperboard reaches maximum resistance at lower angular values than bleached virgin kraft fibers due to differences in fiber length and bond density. Test equipment uses load cells to continuously capture force data throughout the complete arc of movement. Deviations from prescribed angular speeds alter the measured force values, requiring motor-driven test fixtures to maintain a constant angular velocity of five degrees per second.
High-speed cartoning machines subject blanks to rapid bending motions that alter the physical response observed during slow laboratory testing.
Folding Behavior
Converting operations depend on controlled structural deformation during high-speed carton erecting and closing operations. A precisely calibrated bend angle reveals the point at which permanent scoreline deformation replaces elastic bending. Inconsistent bending force at target angles causes carton jams and misaligned flaps during automatic filling operations.
Material suppliers supply force displacement profiles to confirm that scorelines flex without rupturing outer linerboard fibers.