A calender is a series of hard pressure rollers used to finish or smooth a sheet of material such as
paper,
textiles,
rubber, or plastics. Calender rolls are also used to form some types of
plastic films and to apply coatings. Some calender rolls are heated or cooled as needed. Calenders are sometimes misspelled ''calendars''.
Etymology
The word "calender" itself is a derivation of the word κύλινδρος ''kylindros'', the Greek word that is also the source of the word "cylinder".
History
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.
Chaffee worked with
Charles Goodyear
Charles Goodyear (December 29, 1800 – July 1, 1860) was an American self-taught chemist and manufacturing engineer who developed vulcanized rubber, for which he received patent number 3633 from the United States Patent Office on June 15, 1844.
...
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 Hannover designed the first calender specifically to process this plastic".
In the past, for paper, sheets were worked on with a polished
hammer or pressed between polished
metal
A metal (from Greek μέταλλον ''métallon'', "mine, quarry, metal") is a material that, when freshly prepared, polished, or fractured, shows a lustrous appearance, and conducts electricity and heat relatively well. Metals are typica ...
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 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
Cast iron is a class of iron–carbon alloys with a carbon content more than 2%. Its usefulness derives from its relatively low melting temperature. The alloy constituents affect its color when fractured: white cast iron has carbide impuriti ...
or
steel, and soft rollers coated with
polymeric 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.
Calendering paper
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
Printing is a process for mass reproducing text and images using a master form or template. The earliest non-paper products involving printing include cylinder seals and objects such as the Cyrus Cylinder and the Cylinders of Nabonidus. The e ...
and
writing, as well as of a consistent thickness for
capacitor
A capacitor is a device that stores electrical energy in an electric field by virtue of accumulating electric charges on two close surfaces insulated from each other. It is a passivity (engineering), passive electronic component with two termi ...
s 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
A machine is a physical system using power to apply forces and control movement to perform an action. The term is commonly applied to artificial devices, such as those employing engines or motors, but also to natural biological macromolecules ...
, or
MF Paper
MF or mf may refer to:
Biology
* Mossy fiber (disambiguation), in neuroscience
* Mycosis fungoides, a type of skin disease
* Myelofibrosis, a chronic clonal malignant disease
* Microfibril rosettes, the sites of cellulose microfibril synthesis in ...
and it can range from a rough, matte (non glossy) look, to a smooth, high-quality finish.
The second is called a
supercalendered finish
A calender is a series of hard pressure rollers used to finish or smooth a sheet of material such as paper, textiles, rubber, or plastics. Calender rolls are also used to form some types of plastic films and to apply coatings. Some calende ...
, or
MG Paper
MG, Mg, or mg and variants may refer to:
Organizations
* MG Cars, an automotive marque of the now defunct MG Car Company
* MG Motor, a present-day car manufacturing company
* MG JW Automobile, a Pakistani automobile manufacturer
* Champion Air ( ...
(Machine Glazed) which is glossy/glazed and suitable for high-degree, fine-screened
halftone
Halftone is the reprographic technique that simulates continuous-tone imagery through the use of dots, varying either in size or in spacing, thus generating a gradient-like effect.Campbell, Alastair. The Designer's Lexicon. ©2000 Chronicle, ...
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 finish). It is wound onto a roll called a ''tambour'', and stored for final cutting and shipping.
Supercalender
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
Cast iron is a class of iron–carbon alloys with a carbon content more than 2%. Its usefulness derives from its relatively low melting temperature. The alloy constituents affect its color when fractured: white cast iron has carbide impuriti ...
and highly compressed paper, while the rolls used for
coated paper
Coated paper (also known as enamel paper, gloss paper, and thin paper) is paper that has been coated by a mixture of materials or a polymer to impart certain qualities to the paper, including weight, surface gloss, smoothness, or reduced ink absor ...
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
halftone
Halftone is the reprographic technique that simulates continuous-tone imagery through the use of dots, varying either in size or in spacing, thus generating a gradient-like effect.Campbell, Alastair. The Designer's Lexicon. ©2000 Chronicle, ...
s because they print well from type and halftones, although for the latter they are not as good as coated paper.
Calendering textiles
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 as
moire to produce its watered effect and also on
cambric
Cambric or batiste, is a fine dense cloth. It is a lightweight plain-weave fabric, originally from the commune of Cambrai (in present-day northern France), woven greige (neither bleached nor dyed), then bleached, piece-dyed, and often ...
and some types of
sateen
Sateen is a fabric made using a satin weave structure, but made with spun yarns instead of filament.
The sheen and softer feel of sateen is produced through the satin weave structure. Warp yarns are floated over weft yarns, for example four ove ...
s.
Other materials
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
Vinyl may refer to:
Chemistry
* Polyvinyl chloride (PVC), a particular vinyl polymer
* Vinyl cation, a type of carbocation
* Vinyl group, a broad class of organic molecules in chemistry
* Vinyl polymer, a group of polymers derived from vinyl m ...
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.
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.
See also
*
Glassine
*
Paper mill
*
Roller mill
Roller mills are mills that use cylindrical rollers, either in opposing pairs or against flat plates, to crush or grind various materials, such as grain, ore, gravel, plastic, and others. Roller grain mills are an alternative to traditional ...
*
Rolling (metalworking)
In metalworking, rolling is a metal forming process in which metal stock is passed through one or more pairs of rolls to reduce the thickness, to make the thickness uniform, and/or to impart a desired mechanical property. The concept is simil ...
*
Two roll rubber mill
References
General references
*Westerlund Leslie. C. " How to Make Smooth Papermaking Technology" ;Westerlund Eco Services; Rockingham; W.Australia. 2008.
* Hawkins, William E, The Plastic Film and Foil Web Handling Guide CRC Press 2003
*Jenkins, W. A., and Osborn, K. R. Plastic Films: Technology and Packaging Applications, CRC Press 1992
* Yam, K. L., "Encyclopedia of Packaging Technology", John Wiley & Sons, 2009,
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Papermaking
Industrial equipment
Plastics