Calender

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For the system of referencing dates, see calendar.

The calender is a series of rolls in a stack, at the end of a paper machine (on-line) or separate from it (off-line). The purpose of a calender is to smooth out the paper for enabling printing and writing on it, and to increase the gloss on the paper surface. It is understood to be a process of using pressure for embossing a smooth surface on the still rough paper surface.

Formerly, the paper sheets were worked on with a polished hammer or pressed between polished metal sheets in a press. With the continuously operating paper machine it became a process of rolling the paper web. The nip pressure can be reduced by heating the rolls and/or moistening the paper surface. This helps to keep the bulk and the stiffness of the paper web which is beneficial for its further use.

Modern calenders have hard and heated rolls made from chilled cast iron or – in a few cases – steel, and rolls with "soft" covers of polymeric composites. Thus the working nip becomes wider and the specific pressure on the paper more even.

The hypothesis of "micro friction" in the nip contributing to the smoothening process is false. With a microscope, fine scratches in the roll surface have been found to be replicated on the calendered paper. This is contradictory to the assumption of a relative movement between the roll and the paper surface.