Calculating Surface Temperature Limits during High Speed Thermal Paperboard Embossing
Maximum surface temperature limits during high-speed paperboard embossing are bounded by latex binder scorching and steam-induced Z-direction blistering.
Transient Heat Conduction
Thermal Transfer
Heat transfer through a paper web changes over time as the material passes through different temperature zones on the paper machine. The rate of transient heat conduction determines how quickly the core of the sheet reaches the temperature required to activate starch or binders during drying and calendering. This process depends on both the thermal conductivity of the fibres and the moisture content of the board.
Operational Mechanics
Calenders apply high temperature and pressure to the paperboard surface to smooth it and control its density. Under these conditions, transient heat conduction governs the temperature gradient across the sheet’s thickness during the brief moment of nip contact. A steep gradient allows the surface fibres to be smoothed without collapsing the bulk of the sheet, preserving the board’s stiffness.
This mechanical preservation is essential for folding cartons, which require high bending resistance to withstand the forces of automated filling lines.
Variables Control
Moisture acts as a primary conductor within the porous structure of the paperboard. Maximizing transient heat conduction during thermal processes requires precise control of the moisture profile and machine speed. This control ensures that the board receives sufficient heat to achieve the desired finish without causing thermal damage to the surface fibers or drying the board excessively.