Structural Strength
Flexural resistance perpendicular to the sheet plane determines the load-bearing capacity of a paperboard carton under compression. This mechanical property, known as out-of-plane stiffness, enables a carton to resist bulging and deformation when stacked on a pallet or handled by a consumer. It is determined by both the thickness of the board and the elastic modulus of the individual fibre plies.
Multi-ply designs maximize this resistance by placing long, high-modulus chemical fibres in the outer layers and bulkier, high-yield mechanical pulp in the core layer. Such strategic distribution of fibres mimics an engineering I-beam, providing the high stiffness needed for modern packaging. By optimizing this property, converter lines can reduce the overall basis weight of the board without sacrificing the strength of the boxes.
Board Converting
Folding endurance and creasing behavior depend on the anisotropic nature of the fibrous network. During high-speed cartoning, the out-of-plane stiffness of the panel prevents the carton from bowing out under the pressure of the filling nozzles. Boards with low stiffness will bulge during the filling step, leading to issues with the top-flap sealing and causing jam-ups on the packaging line.
Consistent stiffness is required for automated packaging equipment.
Testing Method
Standard bending tests evaluate the force required to deflect a specimen through a specific angle. Instruments measuring the out-of-plane stiffness use a two-point or three-point bending setup to apply a controlled load to the board strip. The deflection is measured with a high-precision sensor, and the resulting stiffness is reported in millinewtons.
This test is critical for qualifying board shipments before printing.
