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.
Micro Fracture
Material Defect
Structural failures in paper fibers and coatings occur at the sub-millimeter scale under mechanical stress. When a micro fracture develops in the barrier coating of a paper cup or liquid packaging board, it permits liquid or grease to penetrate the underlying paper. This localized failure can compromise the structural integrity of the package, which leads to leaks and package failure.
Folding Performance
Converting processes that involve sharp folds or scoring can induce these tiny cracks in the outer layer of the board. The severity of the damage depends on the stiffness of the fiber board, the flexibility of the polymer coating and the speed of the folder-gluer. Packaging converters measure the resistance of the board to this cracking to select the correct grades for heavy folding applications.
Moisture Defense
Barrier properties must remain continuous across the folded edges to protect the product inside. When these tiny cracks form, they create pathways for water vapor to enter, which decreases the shelf life of food products. By adjusting the moisture content of the board during converting, manufacturers can minimize the risk of fracturing and ensure a seal.
This quality control measure is especially important for frozen food packaging where the board must withstand both cold temperatures and mechanical transport. Proper scoring geometries and tool alignment help maintain the integrity of the barrier layer through these harsh conditions.