Mechanical Extraction
Dewatering zone in a papermaking machine operates by applying mechanical pressure to the wet web before it enters the dryer section. This specific mechanical extraction occurs at the couch press nip, where the newly formed sheet passes between a perforated suction roll and a solid press roll. Water is squeezed from the fibrous network into the felt or suction holes, raising the dryness of the sheet to approximately twenty percent.
This action ensures that the wet web possesses sufficient wet web strength to cross the open draw without tearing. Proper adjustment of the suction zone within the couch press nip prevents rewetting as the sheet emerges from the contact area.
Pressure Dynamics
Hydraulic pressure rises rapidly as the wet paperboard enters the contact area between the rolls. The couch press nip creates a high-intensity pressure profile that must be managed to prevent sheet crushing. If the pressure gradient is too steep, water moves faster than the structure can accommodate, causing localized disruption of the fibre network.
Moisture Gradient
Moisture content is distributed unevenly through the thickness of the paperboard during the transition. The exit from the couch press nip relies on the vacuum box of the suction roll to hold the water and prevent rewetting of the paperboard sheet. Proper felt selection and tension help to maintain dry sheet properties after this point.
