Thermal Contraction
Physical behavior of wood fibers as they contract when exposed to elevated temperatures during drying governs the final flatness of a paperboard sheet. This system of thermal shrinkage mechanics explains how heat drives off bound moisture, causing the individual cellulose fibers to shrink in diameter and length. The tension of the web prevents this shrinkage in the machine direction, leaving the cross direction to experience the majority of the movement.
Drying Response
Drying sections of a paper machine apply heat to evaporate water from the wet fiber slurry. As moisture levels decrease, hydrogen bonds form between the closely packed fibers, locking in the stresses generated by the thermal shrinkage. If the temperature is applied unevenly across the width of the web, the resulting variations in shrinkage will cause localized sheet tension and warp.
Mill operators adjust the dry profiles to balance these internal forces across the multi-ply board. This thermal response determines the dimensional stability of the board during later hot-melt gluing or lamination processes.
Converting Stability
Processing lines that employ high-temperature drying or UV-curing systems can re-activate these dried-in thermal stresses. Understanding thermal shrinkage mechanics helps converters predict how sheets will behave when subjected to flash heating on the press. Stable substrates resist the tendency to curl when exiting the dryer, ensuring clean stacks and reliable feeding.
