Structural Preservation
Mechanical preservation of paperboard volume during wet pressing determines the eventual bending stiffness of the finished sheet. High bulk retention ensures that the paperboard retains its thickness even under intense mechanical pressure. This property prevents the collapse of internal void volumes in the fibrous network.
Fiber networks with high initial springback behavior resist permanent deformation.
Dewatering Control
Process settings in the shoe press control the compression rate of the wet web. When shoe press dewatering is optimized, bulk retention increases because the hydraulic pressure carries a portion of the press load. This distribution of force limits the mechanical crushing of the fiber matrix.
Low nip pressure paired with extended dwell times yields the highest thickness preservation. Soft-cover backing rolls and specific felt designs distribute the compressive force evenly to avoid localized compaction. Water removal must happen without collapsing the fiber walls or compressing the springy springwood fibers.
Calendering Influence
Smoothness requirements often conflict with the goal of preserving board thickness during final finishing. A heated hard nip calender can reduce thickness while improving surface properties. Mill operators adjust moisture and temperature to localize deformation near the surface of the sheet.
This thermal gradient helps maintain the bulk of the internal layers.
