Internal Stress
Variations in water content through the thickness or across the width of a paper web affect its dimensional stability and finishing potential. The moisture gradients drive how the sheet responds to heat in the calender and how it curls during the printing process. This distribution must be managed to ensure the board remains flat and workable.
Dimensional Control
Uneven drying causes the fibres on one side of the sheet to shrink more than the other. When the paper is stored in a warehouse or fed into a press, these internal stresses release and cause the sheet to twist. Applying steam to the dry side of the web helps to balance the moisture and relieve the tension.
High-speed converting lines require extremely flat board to avoid jams and misfeeds. The tension between the top and bottom layers must be neutralized to prevent post-print curling. Controlled re-wetting systems provide the precision needed to fix these structural imbalances.
Finish Optimization
Selective application of water to the surface layers allows for better smoothing without densifying the core. If the moisture is too high, the paper becomes weak and may break under the tension of the machine. A mill uses infrared sensors to monitor the profile and adjust the drying cylinders accordingly.
