Layer Assembly
Laminated paperboard structure comprises several distinct layers of chemical and mechanical pulp pressed together while wet. A multi-ply fbb combines bleached chemical pulp on the outer plies for a smooth print surface with high-yield mechanical pulp in the middle plies to maximize bulk. This structure provides high bending stiffness at a lower basis weight than single-ply boards of equivalent thickness.
Bulk Optimization
Mechanical fibers in the center layer maintain their volume under compression, which increases the thickness of the board without adding unnecessary weight. The strength of the multi-ply fbb lies in this separation of the outer chemical pulp layers, which act like the flanges of an I-beam when the board is bent. This mechanical separation allows the material to resist bending stresses during carton erection and filling.
Printing Performance
Bleached chemical pulp on the top layer offers a uniform surface that takes clay coatings well for high-resolution graphics. On high-speed offset lithography presses, the multi-ply fbb must resist surface picking and delamination caused by highly viscous inks. This resistance is achieved by optimizing the starch application and wet-end pressing, which ensures that the adhesion between the plies remains stronger than the tack of the ink.
Converting plants monitor the ply bond strength of these multi-ply grades to prevent bubbling during subsequent hot-foil stamping and UV varnishing processes.
