What really happens in the nips of modern calenders?

For most paper grades, calendering is the papermaker's last opportunity to substantially alter the properties of the paper and its end-use performance. Extensive research towards the end of the 20(th) century has provided some good insights into the factors governing the performance of hard-nip calenders, quantitative design relationships, and the factors limiting this approach. These insights provided strong incentive for the development of soft-nip calendering, and the use of high calendering temperatures. The majority of new calenders being installed around the world now apply soft-nip technology. A review of these developments is presented, leading from the quantitative relationships governing hard-nip calenders to a qualitative view of what happens to uncoated and coated paper in the nips of modem soft-nip calenders. The factors affecting replication of the smooth roll surface onto the paper surfaces is discussed. In soft-nip calendering, the deformation of the roll cover has to be considered, and how it interacts with the paper. In calendering of coated paper, the replication pressure and shear forces at the surface of the coating are also affected by the underlying paper structure. New techniques for detailed analysis of paper surface structure will provide the key to verifying the current hypotheses about what really happens in the nips of modem calenders.