Calender

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 separately 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, as well as of a consistent thickness for capacitors that use paper as their dielectric membrane.

The calender section of a paper machine consists 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 1836, Edwin M. Chaffee, of the Roxbury India Rubber Company, patented a four-roll calender to make rubber sheet. Chaffee worked with Charles Goodyear with the intention to "produce a sheet of rubber laminated to a fabric base". Calenders were also used for paper and fabrics long before later applications for thermoplastics. With the expansion of the rubber industry the design of calenders grew as well, so when PVC was introduced the machinery was already capable of processing it into film. As recorded in an overview on the history of the development of calenders, "There was development in both Germany and the United States and probably the first successful calendering of PVC was in 1935 in Germany, where in the previous year the Hermann Berstorff Company of Hanover designed the first calender specifically to process this plastic".

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