Polypropylene Surface Film Stress Response under Folder Gluer Dynamic Tension
Dynamic tension on folder-gluers strains polypropylene film across score radii, where excessive speed triggers stress whitening and adhesive shear failure.
Stress Whitening
Material Strain
A localized change in color from printed or clear polymer-coated surfaces to an opaque white occurs when the material is subjected to mechanical forces or tight bends. This aesthetic defect is known as stress whitening, and it occurs when plastic films and heavy polymer coatings on folding cartons are folded or stretched. It arises primarily around the crease lines and scored corners of rigid boxes during assembly.
This phenomenon represents a physical change in the material structure rather than a loss of printed ink or chemical bleaching, and it degrades the finish of high-end cartons.
Void Formation
Microscopic cracks and cavitation voids develop within the polymer structure when tensile stresses exceed the elastic limit of the material. These tiny pockets scatter light in all directions, making the affected region appear white to the observer. High density polyethylene and polypropylene coatings are especially susceptible to this deformation when converted at low temperatures.
Packaging Appearance
Visual quality in retail environments suffers when dark printed packaging displays white lines along its folded edges. The defect can be avoided by incorporating flexible polymers or by heating the crease lines during the folding stage. This preserves the color intensity of the printed graphics across the life of the package.