Cellular Anatomy
Wide, barrel-shaped water-conducting cells form continuous fluid transport channels throughout the xylem tissue of angiosperm hardwood trees. In papermaking substrates, vessel elements represent large, short cellular structures with open ends and thin cell walls derived from deciduous tree species such as eucalyptus, birch, maple and acacia. These anatomical structures differ markedly from elongated, slender libriform wood fibres that provide structural strength to hardwood sheets.
Because vessel elements possess broad surface areas and limited microfibril entanglement, they do not collapse or conform easily during pressing and drying stages on the paper machine. Consequently, they adhere weakly to adjacent fibres on the finished sheet surface.
Offset Picking
High-tack printing inks and rubber offset blankets exert intense mechanical peeling forces that pull loosely bonded surface cells from paperboard top liners. During multi-colour sheetfed offset lithography, vessel elements positioned at the sheet surface lift out of the paper matrix under ink tack tension, creating blank unprinted voids known as vessel picking defects. The extracted cell adheres to the offset blanket cylinder, collecting subsequent ink and printing a repeating white ring or halo defect across thousands of subsequent carton impressions.
The problem intensifies when running fast-curing inks, high press speeds or high-viscosity UV varnishes on packaging board made with coarse tropical hardwood furnishes.
Surface Stabilization
Papermakers suppress picking defects by optimizing mechanical refining, wet-end chemical retention and surface sizing application. Refining cuts and fibrillates large cells, improving their hydrogen bonding capability with surrounding fibres in the base sheet. Modern board mills apply cooked starch solutions or synthetic surface sizing polymers via size presses or film coaters to glue vessel elements firmly onto the outer fibrous network.
Pigment coating formulations containing styrene-butadiene or acrylic binders further seal the surface, preventing ink tack from dislodging cells during demanding flexographic and offset print runs. Converting specifications for folding carton board establish surface pick resistance thresholds using IGT printability testers to verify that hardwood top liners withstand target press speeds without picking.
