Adhesion Control
Fibre consolidation during the formative stage of paper manufacture relies on the moisture content present when the web leaves the forming wire. Wet-couching occurs when excessive water volume at the transfer point weakens the sheet strength enough to cause structural tearing. Mills adjust vacuum levels on the couch roll to extract moisture before the sheet transfers to the press section.
Proper tension prevents the fibres from pulling apart under the mechanical stress of the pickup roll.
Operational Tolerance
Moisture gradients within the sheet govern the limits of safe transfer speeds on high speed machines. Wet-couching often results from a failure to evacuate water at a rate matching the increased machine velocity. Machine operators calibrate the suction boxes against the basis weight of the furnish to maintain a consistent dry line.
When the sheet holds too much water at the couch, the tensile force required to peel the paper off the wire exceeds the wet web strength. This state forces an immediate reduction in machine speed or an increase in the vacuum pressure applied to the underside of the wire. High levels of filler content or recycled fibre percentages require modified couch settings to prevent this failure mode.
Economic Impact
Reduced production output stems directly from the downtime necessitated by web breaks and subsequent clean up of the couch pit. Frequent rethreading of the machine creates significant waste in paper stock and chemical additives while preventing the attainment of target tonnage for a scheduled run. Efficiency in the forming section protects the long term stability of the downstream equipment by avoiding the mechanical shocks associated with sudden speed changes or emergency stops.
Consistent control of this phenomenon dictates the total daily yield of the paper mill.
