Fibre Plasticity
Application of pressurized water vapour to a moving web of paper or board prepares the fibres for mechanical finishing. The steam shower conditioning increases the temperature and moisture content of the surface, making the cellulose and lignin more pliable. This step is usually located just before a calender nip to improve smoothness and gloss.
Bulk Maintenance
Heat from the steam reduces the force needed to compress the surface irregularities of the sheet. By only affecting the outer layers, the process preserves the bulk and stiffness of the internal fibre network. This allows for the production of high-quality folding boxboard that remains light but rigid.
If the steam is applied unevenly, it can cause moisture streaks that lead to uneven ink absorption. The conditioning system must be calibrated to match the speed of the machine and the grade of the paper. Controlled hydration ensures that the surface fibres reach the glass transition point without soaking the core.
Profile Correction
Modern systems use segmented nozzles to apply different amounts of steam across the width of the machine. This capability allows the mill to correct moisture variations and ensure a flat sheet. Controlled hydration improves the efficiency of the drying process and reduces the energy required for calendering.
