Creasing Rule Selection for Coated Folding Boxboard Conversion
Selecting steel creasing rules and matrix channels for coated FBB requires matching board caliper and CTMP core shear limits to prevent surface cracking.
Top Coat Cracking
Surface Rupture
Flexural strain along score channels during carton folding breaks microscopic mineral coating layers applied to paperboard surfaces. Physical fractures appearing in pigmented surface coatings are identified as top coat cracking. High bending forces extend outer coating surfaces beyond ultimate elongation limits, causing brittle mineral latex matrices to split open.
Microscopic fractures expose underlying grey or unbleached fiber layers along folded carton edges, compromising visual appearance. Binder ratios and pigment particle sizes dictate the flexibility of mineral coatings under mechanical strain. The defect classification applies to coated paperboard and coated containerboard packaging, ceasing to apply to uncoated kraft papers or flexible plastic films.
Coating Elasticity
Pigment formulations containing high proportions of coarse calcium carbonate exhibit lower strain-to-break values than formulations utilizing fine kaolin clay. Synthetic binder ratios directly influence coating elasticity, where low binder levels create rigid, brittle surface layers. Excessive coating coat weight concentrates mechanical stress during folding, accelerating surface micro-fractures along score lines.
Print Defect
Edge fracturing exposes raw underlying fibers, resulting in severe ink flaking and fiber show-through on printed folds. High ink coverage across creased areas worsens visual defect visibility by contrasting dark print backgrounds against white exposed coating fractures. Packaging converters adjust creasing rule dimensions and increase binder content in coating formulations to reduce surface fracturing during high-speed folding.
Maintaining proper relative humidity during storage keeps fiber webs flexible and reduces top coat rupture during carton erection.