Rolls in Paper and Board Machines : Nip Design in Calenders

6.2.4.2.5 Nip Design in Calenders
The functions of covers or coatings in soft nip calenders are twofold. The hard heated roll in the calender is either a chilled iron roll with a hard wear-resistant surface, or it is coated for even more improved wear resistance. The surface of the hard roll needs to be extremely smooth, as the roll surface is imprinted onto the paper surface. Since thermal energy must be transferred to the paper through the coating, its thermal conductivity and thickness are important.

The soft elastic roll presses the paper web against the heated roll. This cover must exhibit a certain local deformation ability in order to at least partly follow the “microscale” topography of the paper web in the calender nip. The compression modulus of the cover is of similar magnitude to that of the paper. The local de¬formation of the cover ensures that the paper is not just calibrated in the nip, but that the thinner areas of the web are calendered as well. Thus soft nip calendering results in better printability compared to hard nips for most printing processes.

6.2.4.2.6 Other Applications
In winding, reeling and cutting operations the covers mainly have to support the paper without affecting the paper surface negatively. Soft, sometimes compressible covers are used, providing lower shear stresses in the nip compared to incompress¬ible elastomeric covers.

6.2.4.2.7 Application Overview
The functions of roll covers and coatings are various and significantly influence paper quality as well as the runnability of a paper machine. This explains the need for a variety of materials which enables one to have custom-designed covers and coatings for all applications. Each of the major materials has its unique advan¬tages, nevertheless an overlap of the different cover materials exists for many ap¬plications. Table 6.1 gives an overview of the materials presently used as roll covers and coatings, their main properties, as well as their major applications in paper machines. In the following sections these materials are described in more detail.

Future development of covers and coatings must take into account ever increas¬ing production speeds as well as the demand for improved runnability of paper machines. Here reliability, predictable operation (grinding) intervals, predictable lifetime and safety aspects in the case of damage are the main goals. The main issues in paper technology are innovations in coating and calendering to give im¬proved printability of the paper.