The final operation on a papermaking machine, performed to impart to paper a desired finish and to increase the surface smoothness of a paper web. The calender usually consists of a stack of highly-polished steel rollers. As the paper web snakes through them, the paper is compressed and surface inconsistencies are smoothed out. The degree of calendering depends on the desired level of surface smoothness and gloss. The degree of calendering improves some paper qualities at the expense of others, however. Although increased calendering increases the apparent density, gloss, ink holdout, and smoothness of the paper, it has a deleterious effect on the brightness, compressibility, ink absorbency, opacity, porosity, stiffness, and thickness of the paper. The end-use requirements of the paper are of prime consideration when deciding the degree of calendering. Additional gloss and smoothness are often achieved using off-machine supercalendering equipment.
Certain paper defects are generated by calendering. See Calender Cuts, Calender Spots, and Calender-Blackened Spots/Streaks.
Calendering is also used in the forming of thermoplastic films used as printing substrates. In this process, the plastic resin is heated to a gelatinous mass and run through heated rollers which, like paper that has been calendered, imparts to the plastic film a glossy, smooth surface.