The calender is a series of hard pressure rollers at the end of a papermaking process (on-line). Those that are used separate from the process (off-line) are also called “supercalenders”. The purpose of a calender is to smooth out the paper for printing and writing on it, and to increase the gloss on the surface.
The word “calender” itself is a derivation of the word kylindros, the Greek word for “cylinder”.
In the past, 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 part of the process of rolling the paper (in this case also called web paper). The pressure between the rollers, 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 web paper which is beneficial for its later use.
Modern calenders have “hard” heated rollers made from chilled cast iron or steel, and “soft” rollers coated with polymeric composites. This widens the working nip and distributes the specific pressure on the paper more evenly.
Calenders can also be applied to materials other than paper when a smooth, flat surface is desirable, such as cotton, linens, silks, and various man-made fabrics and polymers such as vinyl and ABS polymer sheets, and to a lesser extent HDPE, polypropylene and polystyrene.
The calender is also an important processing machine in the rubber industries, especially in the manufacture of tires, where it is used for the inner layer and fabric layer.
^ Merriam-Webster definition
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