Coated Folding Boxboard Crease Quality Audit Procedures
Crease audits verify internal mechanical ply shear through moment ratios between thirty and fifty percent without top coating fracture under dye staining.
Crow-Footing
Fracture Morphology
Branching micro-fissures spreading in multi-directional patterns across coated paper and board surfaces define a specific film rupture defect encountered in cured ink and overprint varnish layers. Known within pressrooms as crow-footing, this structural coating breakdown resembles tiny bird footprints etched into the continuous polymer matrix. Pressroom engineers identify this failure when multi-axial tensile stresses during drying exceed the tensile strength of the curing ink film.
The defect remains confined to heavily coated paperboard and printed fine papers subjected to radiative curing, ceasing at unprinted base board boundaries where coating cohesion does not apply.
Curing Imbalance
Imbalanced drying rates between surface skins and underlying liquid vehicle layers drive the localized micro-fracturing of graphic films. When high-intensity ultraviolet radiation or excessive hot air strikes a wet ink film, the top boundary polymerizes rapidly into a rigid skin while wet solvents remain trapped underneath. Subsequent vehicle outgassing or continued crosslinking shrinkage strains the brittle skin beyond its yield limit, rupturing the continuous surface into branching fractures.
Cold paperboard surfaces worsen this failure by retarding vehicle absorption into the base coating. Maintaining balanced photoinitiator ratios and regulating curing lamp intensities prevents premature skin formation during the run.
Substrate Interaction
Absorbent board substrates pull fluid ink vehicles downward, leaving pigments and binders stranded at the surface to exacerbate film cracking. Flexible base stocks aggravate film failure when carton blanks flex across converting cylinders. Severe crow-footing diminishes gloss levels and breaches barrier integrity on coated folding cartons.