Cross Direction Crease Matrix Channel Geometry Tuning for Chemical Pulp
Cross direction creasing of chemical pulp requires expanding matrix channel width to 1.5 to 1.7 times board caliper plus rule width to prevent liner cracking.
Top Ply Rupture
Mechanical Failure
Breakage of the uppermost layer of a coated board during the folding process indicates a structural fault. Top ply rupture compromises the visual integrity and moisture resistance of the package. It usually occurs along the spine of a book or the edge of a carton.
Causal Factor
Inadequate moisture in the board makes the surface fibres brittle and prone to cracking under tension. Preventing top ply rupture requires that the strain on the outer liner remains below its elongation limit. Excessive ink coverage or a brittle varnish layer can also initiate the failure by reducing the flexibility of the surface.
Selecting a board with long chemical fibres in the top ply helps to mitigate this risk by providing better tensile strength.
Diagnostic Step
Verification of the fault often involves a dye penetration test to see if the crack reaches the underlying layers. Operators inspect top ply rupture under a loupe to determine if the break is a clean snap or a jagged tear. Adjusting the relative humidity of the storage area to approximately 50 percent often prevents the material from drying out.
If the issue persists, a reduction in the depth of the crease lowers the stress on the outer face.