Tensile Strain Measurement in Recycled Paperboard Grades
Recycled paperboard strain measurement requires direct optical extensometry under ISO 1924-3 to isolate true sheet elongation from clamp slippage.
Fold Cracking
Surface Rupture
Structural failure of the outer plies of paperboard occurs when a carton is folded ninety or one hundred and eighty degrees during gluing or packing. This unsightly defect, termed fold cracking, arises when the tensile forces acting on the outside of the bend exceed the elongation limit of the fibers or the clay coating. It is particularly common in high-density, heavily coated boards where the outer layers cannot stretch enough to accommodate the bending radius.
The resulting rupture exposes the grey or brown recycled fibers beneath the clean white surface, spoiling the appearance of the printed design.
Moisture Influence
Substrate moisture content is highly influential in preventing this failure because dry fibers are brittle and break easily. When the board is kept within the recommended range of forty-five to fifty-five percent relative humidity, the fibers remain pliable and can slide past each other during creasing. This pliability distributes the bending stress more evenly through the board, significantly reducing the occurrence of fold cracking during high-speed folding.
Mitigation Strategy
Printers often use surface coatings or water-based varnishes to increase the elasticity of the surface. Scoring and creasing rules must also be adjusted to ensure that the board is pre-crushed before folding, reducing the tension on the outer ply.