Characterizing Porous Absorption in Recycled Packaging Paper Grades
Characterizing porous absorption in recycled paper requires measuring dynamic capillary wicking and pore size distribution to control ink set times and glue bonds.
Mottle Formation
Visual Nonuniformity
Print density variation describes the irregular, blotchy appearance of an ink film across a solid or uniform halftone area on paper or paperboard. Mottle formation arises from localized variations in substrate absorptivity, coating structure, base sheet formation, or ink-water interactions during the printing sequence. The defect creates visible lightness and color fluctuations at spatial frequencies ranging from sub-millimeter scales up to several centimeters across the printed surface.
Image analysis systems quantify mottle by measuring standard deviations of gray-level values across discrete wavelet bands. The classification excludes macroscopic print defects such as streaks, pinholes, or repeating mechanical press chatter marks.
Absorption Disparity
Uneven base paper formation creates spatial density gradients that transfer variable compaction forces into the coating layer during calendering. High-density fiber flocs absorb less coating color, yielding thin, highly consolidated coating patches surrounded by thicker, porous coating zones. When printing inks contact this heterogeneous surface, oil and solvent vehicles drain at differential rates, creating localized variations in pigment concentration and surface gloss.
In multi-color offset printing, back-trap mottle develops when successive printing blankets lift variable amounts of previously printed ink from the substrate due to inconsistent ink tack development. Water-interference mottle occurs when dampening solution applied at early print stations absorbs unevenly across the paper coating, repelling ink films applied at subsequent stations. Flexographic printing on corrugated liners generates mottle when uneven flute contact and varying linerboard porosity disrupt uniform ink film split across the impression nip.
Formulation Control
Coating formulations containing structured calcium carbonate and optimized latex binder ratios provide uniform pore size distribution that suppresses localized absorbency differentials. Papermakers adjust wet-end refining, drainage profiles, and retention aid dosage to produce uniform fiber distribution in the base sheet.