Calender

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This article is about a device for smoothing paper. For the method of timekeeping, see calendar. A calender (or kalandar) is also an order of dervishes in Turkey and Persia.

The calender is a series of hard pressure rollers, at the end of a papermaking process (on-line) or separate from it (off-line), which is also called supercalender. 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.

The word “calender” itself is a derivation of the word cylindrus, the Latin word for “cylinder”.

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.

[edit] Other materials

Calenders can also be applied to materials other than paper in which a smooth, flat surface is desirable, such as cotton, many linens and silks, and various man-made fabrics and polymers - mostly for vinyl and ABS polymer sheets but also, to a lesser extent, to HDPE, Polypropylene , and Polystyrene.