Acalender is a series of hard pressure rollers used to finish a sheet of material such aspaper,textiles,rubber, or plastics. Calender rolls are also used to form some types ofplastic films and to apply coatings.[1] Some calender rolls are heated or cooled as needed.[2] Calenders are sometimes misspelledcalendars.
The word "calender" itself is a derivation of the word κύλινδροςkylindros, the Greek word that is also the source of the word "cylinder".[3]
Calender mills for pressingserge were apparently introduced to the Netherlands by Flemish refugees from theEighty Years' War in the 16th and 17th centuries.[4]
In eighteenth century China, workers called "calenderers" in the silk- and cotton-cloth trades used heavy rollers to press and finish cloth.
In 1836, Edwin M. Chaffee, of the Roxbury India Rubber Company, patented a four-roll calender to make rubber sheet.[5] Chaffee worked withCharles Goodyear with the intention to "produce a sheet of rubber laminated to a fabric base".[6] 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.[6] 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 Hannover designed the first calender specifically to process this plastic".[6]
In the past, for paper, sheets were worked on with a polishedhammer or pressed between polishedmetal 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).[7] The pressure between the rollers, the "nip pressure", can be reduced by heating the rolls 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 chilledcast iron orsteel, and soft rollers coated withpolymeric composites. The soft roller is slightly non-cylindrical, tapered in diameter toward both ends, to widen the working nip and distribute the specific pressure on the paper more evenly.
In a principal paper application, the calender is located at the end of apapermaking process (on-line). Those that are used separately from the process (off-line) are also calledsupercalenders. The purpose of a calender is to make thepaper smooth and glossy forprinting andwriting, as well as of a consistent thickness forcapacitors that use paper as theirdielectric membrane.
The calender section of apaper machine consists of a calender and other equipment. Thepaper 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, in three basic types:
After calendering, the web has a moisture content of about 6% (depending on the finish). It is wound onto a roll called atambour, and stored for final cutting and shipping.
Asupercalender is a stack of calenders consisting of alternatingsteel- andfiber-covered rolls through whichpaper is passed to increase itsdensity, 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 ofcast iron and highly compressed paper, while the rolls used forcoated 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 orhalftones because they print well from type and halftones, although for the latter they are not as good as coated paper.
Calendering is a finishing process used on cloth and fabrics. A calender is employed, usually to smooth, coat, or thin a material.
With textiles, fabric is passed under rollers at high temperatures and pressures. Calendering is used on fabrics such asmoire to produce its watered effect and also oncambric and some types ofsateens.
Various man-made fabrics made from synthetic materials are also calendered.
Calenders can also be applied to materials other than paper and textiles when a smooth, flat surface is desirable.
Polymers such asvinyl andABS polymer sheets, and to a lesser extentHDPE,polypropylene andpolystyrene, are calendered.
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.
Calendering can also be used for polishing, or making uniform, coatings applied to substrates- an older use was in polishing magnetic tapes, for which the contact roller rotates much faster than the web speed. More recently, it is used in the production of certain types of secondary battery cells (such as spirally-wound or prismatic lithium-ion cells) to achieve uniform thickness of electrode material coatings on current collector foils.
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