A calender is a series of hard pressure rollers used to form or smooth a sheet of material such as paper or plastic film. In a principal paper application, the calender is located 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 make the paper smooth and glossy for printing and writing.
Calender section of a paper machine
The calender section of a paper machine consist of a calender and other equipment. The paper web is run between in order to further smooth it out, which also gives it a more uniform thickness. The pressure applied to the web by the rollers determines the finish of the paper, and there are three types of finish that the paper can have. The first is machine finish, and can range from a rough antique look to a smooth high quality finish. The second is called a supercalendered finish and is a higher degree for fine-screened halftone printing.
The third type of finish is called a plater finish, and whereas the first two types of finish are accomplished by the calender stack itself, a plater finish is obtained by placing cut sheets of paper between zinc or copper plates that are stacked together, then put under pressure and heating. A special finish such as a linen finish would be achieved by placing a piece of linen between the plate and the sheet of paper, or else an embossed steel roll might be used.
After calendering, the web has a moisture content of about 6% (depending on the furnish). It is wound onto a roll called a tambour, and stored for final cutting and shipping.
The word "calender" itself is a derivation of the word κύλινδρος kylindros, the Greek word that is also the source of the word "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.
A supercalender is a stack of calenders consisting of alternating steel and fiber-covered rolls through which paper is passed to increase its density, smoothness and gloss. It is similar to a calender except that alternate chilled cast iron and softer rolls are used. The rolls used to supercalender uncoated paper usually consist of cast iron and highly compressed paper, while the rolls used for coated paper are usually cast iron and highly compressed cotton. The finish produced varies according to the raw material used to make the paper and the pressure exerted on it, and ranges from the highest English finish to a highly glazed surface. Supercalendered papers are sometimes used for books containing fine line blocks or halftones because they print well from type and halftones, although for the latter they are not as good as coated paper.