Calender
Encyclopedia
The calender is a series of hard pressure rollers used to form or smooth a sheet of material. In a principal 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
.
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 perhaps 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.
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.
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 halftone
s because they print well from type and halftones, although for the latter they are not as good as coated paper.
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 is also used in the production of certain types of secondary battery cells (such as spirally-wound or prismatic Lithium-ion cells) where it is used to achieve uniform thickness of electrode material coatings on current collector foils.
Papermaking
Papermaking is the process of making paper, a substance which is used universally today for writing and packaging.In papermaking a dilute suspension of fibres in water is drained through a screen, so that a mat of randomly interwoven fibres is laid down. Water is removed from this mat of fibres by...
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
Paper
Paper is a thin material mainly used for writing upon, printing upon, drawing or for packaging. It is produced by pressing together moist fibers, typically cellulose pulp derived from wood, rags or grasses, and drying them into flexible sheets....
smooth and glossy for printing
Printing
Printing is a process for reproducing text and image, typically with ink on paper using a printing press. It is often carried out as a large-scale industrial process, and is an essential part of publishing and transaction printing....
and writing
Writing
Writing is the representation of language in a textual medium through the use of a set of signs or symbols . It is distinguished from illustration, such as cave drawing and painting, and non-symbolic preservation of language via non-textual media, such as magnetic tape audio.Writing most likely...
.
Calender section of a paper machine
The calender section of a paper machine consist of a calender and other equipment the paperPaper
Paper is a thin material mainly used for writing upon, printing upon, drawing or for packaging. It is produced by pressing together moist fibers, typically cellulose pulp derived from wood, rags or grasses, and drying them into flexible sheets....
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
Halftone
Halftone is the reprographic technique that simulates continuous tone imagery through the use of dots, varying either in size, in shape or in spacing...
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
Zinc
Zinc , or spelter , is a metallic chemical element; it has the symbol Zn and atomic number 30. It is the first element in group 12 of the periodic table. Zinc is, in some respects, chemically similar to magnesium, because its ion is of similar size and its only common oxidation state is +2...
or copper
Copper
Copper is a chemical element with the symbol Cu and atomic number 29. It is a ductile metal with very high thermal and electrical conductivity. Pure copper is soft and malleable; an exposed surface has a reddish-orange tarnish...
plates that are stacked together, then put under pressure and perhaps heating. A special finish such as a linen finish would be achieved by placing a piece of linen
Linen
Linen is a textile made from the fibers of the flax plant, Linum usitatissimum. Linen is labor-intensive to manufacture, but when it is made into garments, it is valued for its exceptional coolness and freshness in hot weather....
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.
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 the past, the paper sheets were worked on with a polished hammerHammer
A hammer is a tool meant to deliver an impact to an object. The most common uses are for driving nails, fitting parts, forging metal and breaking up objects. Hammers are often designed for a specific purpose, and vary widely in their shape and structure. The usual features are a handle and a head,...
or pressed between polished metal
Metal
A metal , is an element, compound, or alloy that is a good conductor of both electricity and heat. Metals are usually malleable and shiny, that is they reflect most of incident light...
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
Cast iron
Cast iron is derived from pig iron, and while it usually refers to gray iron, it also identifies a large group of ferrous alloys which solidify with a eutectic. The color of a fractured surface can be used to identify an alloy. White cast iron is named after its white surface when fractured, due...
or steel
Steel
Steel is an alloy that consists mostly of iron and has a carbon content between 0.2% and 2.1% by weight, depending on the grade. Carbon is the most common alloying material for iron, but various other alloying elements are used, such as manganese, chromium, vanadium, and tungsten...
, and "soft" rollers coated with polymeric composites. This widens the working nip and distributes the specific pressure on the paper more evenly.
Supercalender
A supercalender is a stack of calenders consisting of alternating steelSteel
Steel is an alloy that consists mostly of iron and has a carbon content between 0.2% and 2.1% by weight, depending on the grade. Carbon is the most common alloying material for iron, but various other alloying elements are used, such as manganese, chromium, vanadium, and tungsten...
and fiber
Fiber
Fiber is a class of materials that are continuous filaments or are in discrete elongated pieces, similar to lengths of thread.They are very important in the biology of both plants and animals, for holding tissues together....
-covered rolls through which paper
Paper
Paper is a thin material mainly used for writing upon, printing upon, drawing or for packaging. It is produced by pressing together moist fibers, typically cellulose pulp derived from wood, rags or grasses, and drying them into flexible sheets....
is passed to increase its density
Density
The mass density or density of a material is defined as its mass per unit volume. The symbol most often used for density is ρ . In some cases , density is also defined as its weight per unit volume; although, this quantity is more properly called specific weight...
, 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
Cast iron is derived from pig iron, and while it usually refers to gray iron, it also identifies a large group of ferrous alloys which solidify with a eutectic. The color of a fractured surface can be used to identify an alloy. White cast iron is named after its white surface when fractured, due...
and highly compressed paper, while the rolls used for coated paper
Coated paper
Coated paper is paper which has been coated by a compound to impart certain qualities to the paper, including weight, surface gloss, smoothness or reduced ink absorbency. Kaolinite or calcium carbonate are used to coat paper for high quality printing used in packaging industry and in magazines...
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
Halftone is the reprographic technique that simulates continuous tone imagery through the use of dots, varying either in size, in shape or in spacing...
s because they print well from type and halftones, although for the latter they are not as good as coated paper.
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 vinylVinyl
A vinyl compound is any organic compound that contains a vinyl group ,which are derivatives of ethene, CH2=CH2, with one hydrogen atom replaced with some other group...
and ABS
Acrylonitrile butadiene styrene
Acrylonitrile butadiene styrene is a common thermoplastic. Its melting point is approximately 105 °C ....
polymer sheets, and to a lesser extent HDPE, polypropylene
Polypropylene
Polypropylene , also known as polypropene, is a thermoplastic polymer used in a wide variety of applications including packaging, textiles , stationery, plastic parts and reusable containers of various types, laboratory equipment, loudspeakers, automotive components, and polymer banknotes...
and polystyrene
Polystyrene
Polystyrene ) also known as Thermocole, abbreviated following ISO Standard PS, is an aromatic polymer made from the monomer styrene, a liquid hydrocarbon that is manufactured from petroleum by the chemical industry...
.
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 is also used in the production of certain types of secondary battery cells (such as spirally-wound or prismatic Lithium-ion cells) where it is used to achieve uniform thickness of electrode material coatings on current collector foils.