The influence of wet pressing and precalendering on the structure and properties of double-coated woodfree board

摘要

Certain aspects of the interaction between the wet pressing and the precalendering of board have been investigated. In a full-scale trial on board machine 1 at the Iggesund mill, a board grade was produced using three different wet press settings; a reference setting (4-nip), a setting with reduced load at the felted nips and a setting with the third (single felted) nip open. The board line load when the board was precalendered was dependent on the wet press settings. The board was coated at two blade stations and the final bending stiffness and surface smoothness were evaluated. The wet pressing conditions had a clear influence on these properties of the coated board and also on the relationship between them. Differences in the structure of the sheets produced with the three different settings could be observed. In some cases the precalendering operation was observed to increase these structural differences, this emphasises the complexity of multi-step unit processes. The results from this study imply that, for different multi-step processes, the optimal sheet structure from the press section may depend on interactions between the various papermaking unit processes. In this study opening the third nip gave the highest bending stiffness and also the best calender response, whereas the most favourable relationship between smoothness and stiffness was obtained using the 4-nip setting, without reduced pressure.