Mechanical Refining Impact on Multi Ply Boxboard Z Direction Tensile Development
Controlled low energy refining of middle ply mechanical pulp yields superior Z direction tensile development without caliper loss in multi ply boxboard.
Wet Press Consolidation
Mechanical Compaction
Density increases in the paper web occur through the application of mechanical pressure while the fibers are still saturated with water. The process of wet press consolidation reduces the bulk of the sheet and forces the cellulose strands into closer proximity to encourage hydrogen bonding. The stage is the final opportunity to adjust the internal structure before thermal drying begins.
Water Removal
Roll nips apply intense force to the moving web to squeeze liquid into a permeable felt or a perforated suction roll. Efficient wet press consolidation lowers the energy demand of the dryer section because removing water mechanically is more cost-effective than using steam. As the pressure increases, the fiber network collapses and fills the voids that would otherwise be occupied by air.
The surface becomes smoother and less porous, which improves the hold-out of coatings and inks applied in later stages. Operators must balance the nip pressure to maximize water removal without crushing the delicate fiber structure. High-performance shoe presses extend the time the web spends under pressure to achieve higher dryness levels without damaging the sheet.
This control stabilizes the web.
Sheet Integrity
Resulting thickness and opacity depend heavily on the intensity of the pressing action. If the pressure is too low, the finished board will be too bulky and lack the surface strength needed for high-speed printing. Measuring the caliper at the end of the press section provides an early measure of final quality.