Chemi-Thermomechanical Pulp Core Optimization for Low Grammage Boxboard Manufacturing
Optimizing CTMP core sulfonation and bulk yields lower grammage boxboard that maintains flexural rigidity while reducing landed carton cost.
Moisture-Gradient Calendering
Surface Density
Controlled compression of a paper web occurs when moisture content is varied across the cross-machine direction prior to entering the nip. Moisture-gradient calendering functions by locally manipulating the compressibility of fibres to correct caliper non-uniformity across the width of the sheet. Selective application of water or steam allows mills to target specific zones of the substrate, hardening the surface in areas of high bulk while softening others for increased density.
Mechanical Action
Precision spray headers or steam boxes deliver precise moisture profiles to the web before it reaches the steel rolls of the calendar stack. These moisture variations create differences in fibre plasticity because dryer zones offer higher resistance to compressive forces while wetter fibres undergo greater densification. Heat transfer from the rolls accelerates evaporation in moist zones, which further fixes the local density and surface finish.
Adjusting these moisture inputs provides a method to manage profile defects such as baggy edges or soft spots that remain persistent during dry finishing. Continuous monitoring of the finished caliper allows the control system to shift moisture levels dynamically to maintain uniform thickness.
Production Constraint
Moisture-gradient calendering imposes limits on the total moisture pickup allowed to avoid sheet breaks or excessive drying energy requirements in the subsequent sections of the machine. The effective range of this process remains tied to the initial sheet moisture and the dwell time under the nip pressure. High moisture levels improve the smoothness and gloss of coated papers, but excessive water can lead to fibre swelling and reduced tensile strength if the drying capacity is exceeded.
Successful implementation requires synchronization between the profiling equipment and the roll loading to ensure that the target smoothness profile corresponds with the desired dimensional stability of the final product.