Editing Paper machine (section)

== Operation ==
There are four main sections on this paper machine. The forming section makes the pulp into the basis for sheets along the wire. The press section, which removes much of the remaining water via a system of nips formed by rolls pressing against each other aided by press [[felt]]s that support the sheet and absorb the pressed water. The dryer section of the paper machine, as its name suggests, dries the paper by way of a series of internally [[steam]]-heated cylinders that evaporate the moisture. Calenders are used to make the paper surface extra smooth and glossy. In practice calender rolls are normally placed vertically in a ''stack''.

{{clear}}
[[File:Fourdrinier.svg|thumb|centre|upright=4.0|Diagram showing the sections of the Fourdrinier machine]]

=== Forming section or wet end ===
[[File:Florida Pulp and Paper Company mill, Cantonment, Florida.jpg|thumb|right|300px| A worker inspecting wet, bleached wood pulp on an old-fashioned [[Hollander beater|Hollander pulper]] or "beater"]]

From the machine chest stock is pumped to a head tank, commonly called a "head tank" or ''stuff box'', whose purpose is to maintain a constant head (pressure) on the fiber slurry or ''stock'' as it feeds the basis weight valve.  The stuff box also provides a means allowing air bubbles to escape. The consistency of the pulp slurry at the stuff box is in the 3% range.  Flow from the stuff box is by gravity and is controlled by the basis weight valve on its way to the fan pump suction where it is injected into the main flow of water to the fan pump.  The main flow of water pumped by the fan pump is from a whitewater chest or tank that collects all the water drained from the forming section of the paper machine. Before the fiber stream from the stuff box is introduced, the whitewater is very low in fiber content.  The whitewater is constantly recirculated by the fan pump through the headbox and recollected from the wire pit and various other tanks and chests that receive drainage from the forming wire and vacuum assisted drainage from suction boxes and wet fiber web handling rolls.  On the way to the head box the pulp slurry may pass through centrifugal cleaners, which remove heavy contaminants like sand, and screens, which break up fibre clumps and remove oversized debris. The fan pump ultimately feeds the headbox, whether or not any centrifugal cleaners or screens are present.<ref name="TAPPI_Machine_ops">{{cite book
|title=Paper Machine Operations Short Course Notes
|last=Technical Association for the Pulp and Paper Industry
|author2=Various
|year=2004 |publisher= TAPPI Press
}}</ref><ref name="TAPPI_Wet_End">{{cite book
|title= Paper Machine Wet End, The
|last=Technical Association for the Pulp and Paper Industry
|author2=Various
|publisher= TAPPI Press}}</ref><ref name="TAPPI Wet End Ops">{{cite book
|title= Wet End Operations Short Course Notes
|last=Technical Association for the Pulp and Paper Industry
|author2=Various
|year= 2005|publisher= TAPPI Press
 }}</ref>

The purpose of the headbox is to create turbulence in order to keep the fibers from clumping together and to uniformly distribute the slurry across the width of the wire. Wood fibers have a tendency to attract one another, forming clumps, the effect being called flocculation.  Flocculation is lessened by lowering consistency and or by agitating the slurry; however, de-flocculation becomes very difficult at much above 0.5% consistency.  Minimizing the degree of flocculation when forming is important to physical [[properties of paper]].<ref name="TAPPI_Wet_End"/><ref name="TAPPI Wet End Ops"/>

The consistency in the headbox is typically under 0.4% for most paper grades, with longer fibres requiring lower consistency than short fibres.  Higher consistency causes more fibres to be oriented in the '''z''' direction, while lower consistency promotes fibre orientation in the '''x-y''' direction. Higher consistency promotes higher calliper (thickness) and stiffness, lower consistency promotes higher tensile and some other strength properties and also improves formation (uniformity).<ref name="TAPPI_Wet_End"/><ref name="TAPPI Wet End Ops"/>  Many sheet properties continue to improve down to below 0.1% consistency; however, this is an impractical amount of water to handle. (Most paper machine run a higher headbox consistency than optimum because they have been sped up over time without replacing the fan pump and headbox. There is also an economic trade off with high pumping costs for lower consistency).

The stock slurry, often called ''white water'' at this point, exits the head box through a rectangular opening of adjustable height called the ''slice'', the white water stream being called the ''jet'' and it is pressurized on high speed machines so as to land gently on the moving fabric loop or ''wire'' at a speed typically between plus or minus 3% of the wire speed, called ''rush'' and ''drag'' respectively.  Excessive ''rush'' or ''drag'' causes more orientation of fibres in the machine direction and gives differing physical properties in machine and cross directions; however, this phenomenon is not completely avoidable on Fourdrinier machines.<ref name="TAPPI_Wet_End"/><ref name="TAPPI Wet End Ops"/>

On lower speed machines at 700 feet per minute, gravity and the height of the stock in the headbox creates sufficient pressure to form the jet through the opening of the slice.  The height of the stock is the head, which gives the headbox its name.  The speed of the jet compared to the speed of the wire is known as the ''jet-to-wire ratio''.  When the jet-to-wire ratio is less than unity, the fibres in the stock become drawn out in the machine direction.  On slower machines where sufficient liquid remains in the stock before draining out, the wire can be driven back and forth with a process known as ''shake''.  This provides some measure of randomizing the direction of the fibres and gives the sheet more uniform strength in both the machine and cross-machine directions.  On fast machines, the stock does not remain on the wire in liquid form long enough and the long fibres line up with the machine.  When the jet-to-wire ratio exceeds unity, the fibers tend to pile up in lumps.<ref name="TAPPI_Wet_End"/><ref name="TAPPI Wet End Ops"/>  The resulting variation in paper density provides the antique or parchment paper look.

Two large rolls typically form the ends of the drainage section, which is called the ''drainage table''.  The ''breast roll'' is located under the flow box, the jet being aimed to land on it at about the top centre.  At the other end of the drainage table is the suction (''couch'') roll.  The couch roll is a hollow shell, drilled with many thousands of precisely spaced holes of about 4 to 5&nbsp;mm diameter.  The hollow shell roll rotates over a stationary suction box, normally placed at the top centre or rotated just down machine.  Vacuum is pulled on the suction box, which draws water from the web into the suction box.  From the suction roll the sheet feeds into the press section.<ref name="TAPPI_Wet_End"/><ref name="TAPPI Wet End Ops"/>

Down machine from the suction roll, and at a lower elevation, is the ''wire turning roll''. This roll is driven and pulls the wire around the loop. The wire turning roll has a considerable angle of wrap in order to grip the wire.<ref name="TAPPI_Wet_End"/>

[[File:Ultrasonic foil.JPG|thumb|Ultrasonic foil installed under the wire on a paper machine]]

Supporting the wire in the drainage table area are a number of drainage elements.  In addition to supporting the wire and promoting drainage, the elements de-flocculate the sheet.  On low speed machines these table elements are primarily ''table rolls''.  As speed increases the suction developed in the nip of a table roll increases and at high enough speed the wire snaps back after leaving the vacuum area and causes stock to jump off the wire, disrupting the formation.  To prevent this drainage foils are used.  The foils are typically sloped between zero and two or three degrees and give a more gentle action.  Where rolls and foils are used, rolls are used near the headbox and foils further down machine.<ref name="TAPPI_Wet_End"/><ref name="TAPPI Wet End Ops"/> Ultrasonic foils can also be used, creating millions of pressure pulses from imploding [[cavitation]] bubbles which keep the fibres apart, giving them a more uniform distribution.

Approaching the dry line on the table are located low vacuum boxes that are drained by a barometric leg under gravity pressure. After the dry line are the suction boxes with applied vacuum.  Suction boxes extend up to the couch roll.  At the couch the sheet consistency should be about 25%.<ref name="TAPPI_Wet_End"/><ref name="TAPPI Wet End Ops"/>

=== Variations of the Fourdrinier forming section ===
The forming section type is usually based on the grade of paper or paperboard being produced; however, many older machines use a less than optimum design.  Older machines can be upgraded to include more appropriate forming sections.

A '''second headbox''' may be added to a conventional fourdrinier to put a different fibre blend on top of a base layer. A ''secondary headbox'' is normally located at a point where the base sheet is completely drained. This is not considered a separate ply because the water action does a good job of intermixing the fibers of the top and bottom layer. Secondary headboxes are common on [[linerboard]].

A modification to the basic Fourdrinier table by adding a second wire on top of the drainage table is known as a '''top wire former'''. The bottom and top wires converge and some drainage is up through the top wire. A top wire improves formation and also gives more drainage, which is useful for machines that have been sped up.

The '''Twin Wire Machine''' or '''Gap former''' uses two vertical wires in the forming section, thereby increasing the de-watering rate of the fibre slurry while also giving uniform two sidedness.<ref>Technology choice in a global industry : the case of the twin-wire in Canada, Ofori-Amoah, Benjamin, 1989
Thesis (Ph.D.) – Simon Fraser University, 1990, http://ir.lib.sfu.ca/handle/1892/6373</ref>

There are also machines with entire Fourdrinier sections mounted above a traditional Fourdrinier. This allows making multi-layer paper with special characteristics. These are called '''top Fourdriniers''' and they make multi-ply paper or [[paperboard]]. Commonly this is used for making a top layer of bleached fibre to go over an unbleached layer.

Another type forming section is the '''cylinder mould machine''' invented by [[John Dickinson (inventor)|John Dickinson]] in 1809, originally as a competitor to the Fourdrinier machine.<ref>''Paper Machine Clothing: Key to the Paper Making Process'' Sabit Adanur, Asten, CRC Press, 1997, p. 120–136, {{ISBN|978-1-56676-544-2}}</ref><ref>{{Cite web |url=https://www.britannica.com/technology/cylinder-machine|title=Cylinder machine {{!}} device |website=Encyclopedia Britannica|language=en|access-date=2020-04-05}}</ref> This machine uses a mesh-covered rotating cylinder partially immersed in a tank of fibre slurry in the wet end to form a paper web, giving a more random distribution of the [[cellulose]] fibres. Cylinder machines can form a sheet at higher consistency, which gives a more three dimensional fibre orientation than lower consistencies, resulting in higher caliper (thickness) and more stiffness in the machine direction (MD). High MD stiffness is useful in food packaging like cereal boxes and other boxes like dry laundry detergent.

'''Tissue machines''' typically form the paper web between a wire and a special fabric (felt) as they wrap around a forming roll. The web is pressed from the felt directly onto a large diameter dryer called a ''yankee''. The paper sticks to the yankee dryer and is peeled off with a scraping blade called a ''doctor''. [[Tissue paper|Tissue]] machines operate at speeds of up to 2000&nbsp;m/min.

=== Press section ===
[[File:Paper machine.ogv|thumb|Paper machine]]
[[File:Granite Press Roll.JPG|thumb|Granite press roll at a granite quarry site]]

The second section of the paper machine is the press section, which removes much of the remaining water via a system of nips formed by rolls pressing against each other aided by press [[felt]]s that support the sheet and absorb the pressed water. The paper web consistency leaving the press section can be above 40%.<ref name="Technical Association for the Pulp and Paper Industry">{{cite book|last1=Atkins |first1=Jim |year=2004 |title=The Paper Machine Dry End |publisher=[[TAPPI|TAPPI Press]] |series=13. Making pulp and paper |isbn=978-1-59510-031-3 |oclc=803290898 |location=Norcross |language=en |postscript=[[Compact disc|CD]] format}}</ref>  Pressing is the second most efficient method of de-watering the sheet (behind free drainage in the forming section) as only mechanical action is required.

The number of press rolls, their arrangement and the arrangement and type of felts used are influenced by the grades of paper being produced and the desired operational characteristics of the machine.<ref>{{cite book
|title= Pressing and Drying course notes
|last=Technical Association for the Pulp and Paper Industry
|author2=Various
|year= 2005|publisher= TAPPI Press}}</ref>

Press felts historically were made from wool. However, today they are nearly 100% synthetic. They are made up of a [[polyamide]] woven fabric with thick batt applied in a specific design to maximise water absorption.

Presses can be single or double felted. A single felted press has a felt on one side and a smooth roll on the other. A double felted press has both sides of the sheet in contact with a press felt. Single felted nips are useful when mated against a smooth roll (usually in the top position), which adds a two-sidedness—making the top side appear smoother than the bottom. Double felted nips impart roughness on both sides of the sheet. Double felted presses are desirable for the first press section of heavy paperboard.

Simple press rolls can be rolls with grooved or [[Blind hole|blind drilled]] surface. More advanced press rolls are suction rolls. These are rolls with perforated shell and cover. The shell made of metal material such as bronze or stainless steel is covered with rubber or a synthetic material. Both shell and cover are drilled throughout the surface. A stationary suction box is fitted in the core of the suction roll to support the shell being pressed. [[End face mechanical seal]]s are used for the interface between the inside surface of the shell and the suction box. For the smooth rolls, they are typically made of granite rolls.<ref>{{cite journal|title=Papermaking: Papermachine – Pressing|pages=2, 3, 12, 13|url=http://www.fibrelab.ubc.ca/files/2013/01/Topic-14-Papermaking-Pressing-text.pdf|access-date=25 August 2014|publisher=UBC Fibre Lab}}</ref> The granite rolls can be up to {{Convert|30|ft|adj=on}} long and {{Convert|6|ft}} in diameter.<ref>{{cite journal| last1=Richter| first1=Dorothy A.|title=Barre granite quarries, Barre, Vermont|journal=Geological Society of America Centennial Field Guide—Northeastern Section|date=1987| volume=5| pages=239–242| doi=10.1130/0-8137-5405-4.239| isbn=0-8137-5405-4}}</ref>

Conventional roll presses are configured with one of the press rolls is in a fixed position, with a mating roll being loaded against this fixed roll. The felts run through the nips of the press rolls and continues around a felt run, normally consisting of several felt rolls. During the dwell time in the nip, the moisture from the sheet is transferred to the press felt. When the press felt exits the nip and continues around, a vacuum box known as an Uhle Box applies vacuum (normally -60 kPa) to the press felt to remove the moisture so that when the felt returns to the nip on the next cycle, it does not add moisture to the sheet.

Some grades of paper use suction pick up rolls that use vacuum to transfer the sheet from the couch to a lead in felt on the first press or between press sections.  Pickup roll presses normally have a vacuum box that has two vacuum zones (low vacuum and high vacuum). These rolls have a large number of drilled holes in the cover to allow the vacuum to pass from the stationary vacuum box through the rotating roll covering. The low vacuum zone picks up the sheet and transfers, while the high vacuum zone attempts to remove moisture. Unfortunately, at high enough speed [[centrifugal force]] flings out vacuumed water, making this less effective for dewatering. Pickup presses also have standard felt runs with Uhle boxes. However, pickup press design is quite different, as air movement is important for the pickup and dewatering facets of its role.

Crown Controlled Rolls (also known as CC Rolls) are usually the mating roll in a press arrangement. They have [[hydraulics|hydraulic]] cylinders in the press rolls that ensure that the roll does not bow. The cylinders connect to a shoe or multiple shoes to keep the crown on the roll flat, to counteract the natural "bend" in the roll shape due to applying load to the edges.

Extended Nip Presses (or ENP) are a relatively modern alternative to conventional roll presses. The top roll is usually a standard roll, while the bottom roll is actually a large CC roll with an extended shoe curved to the shape of the top roll, surrounded by a rotating rubber belt rather than a standard roll cover. The goal of the ENP is to extend the dwell time of the sheet between the two rolls thereby maximising the de-watering. Compared to a standard roll press that achieves up to 35% solids after pressing, an ENP brings this up to 45% and higher—delivering significant steam savings or speed increases.  ENPs densify the sheet, thus increasing tensile strength and some other physical properties.

=== Dryer section ===

[[File:Papermaking machine at a paper mill near Pensacola.jpg|thumb|right|300px| Dryer section of an older Fourdrinier-style paper-making machine. These narrow, small diameter dryers are not enclosed by a hood, dating the photo to before the 1970s.]]
The dryer section of the paper machine, as its name suggests, dries the paper by way of a series of internally [[steam]]-heated cylinders that evaporate the moisture. Steam pressures may range up to 160 psig. Steam enters the end of the dryer head (cylinder cap) through a steam joint and condensate exits through a siphon that goes from the internal shell to a centre pipe.  From the centre pipe the condensate exits through a joint on the dryer head.  Wide machines require multiple siphons.  In faster machines, centrifugal force holds the condensate layer still against the shell and turbulence generating bars are typically used to agitate the condensate layer and improve heat transfer.<ref name="Technical Association for the Pulp and Paper Industry" />

The sheet is usually held against the dryers by long felt loops on the top and bottom of each dryer section.  The felts greatly improve heat transfer.  Dryer felts are made of coarse thread and have a very open weave that is almost see through,  It is common to have the first bottom dryer section unfelted to dump broke on the basement floor during sheet breaks or when threading the sheet.

Paper dryers are typically arranged in groups called ''sections''  so that they can be run at a progressively slightly slower speed to compensate for sheet shrinkage as the paper dries. Some grades of paper may also stretch as they run through the machine, requiring increasing speed between sections.  The gaps between sections are called ''draws''.

The drying sections are usually enclosed to conserve heat.  Heated air is usually supplied to the pockets where the sheet breaks contact with the driers.  This increases the rate of drying. The pocket ventilating tubes have slots along their entire length that face into the pocket.  The dryer hoods are usually exhausted with a series of roof mounted hood exhausts fans down the dryer section.

=== Size Press ===
Additional [[sizing]] agents, including [[resin]]s, [[glue]], or [[starch]], can be added to the web to alter its characteristics. [[Sizing]] improves the paper's water resistance, decreases its ability to fuzz, reduces abrasiveness, and improves its printing properties and surface bond strength.  These may be applied at the wet (internal sizing) or on the dry end (surface sizing), or both. At the dry end sizing is usually applied with a ''size press''. The size press may be a roll applicator (flooded nip) or Nozzle applicator . It is usually placed before the last dryer section.  Some paper machines also make use of a 'coater' to apply a [[coated paper|coating]] of [[fillers]] such as [[calcium carbonate]] or [[china clay]] usually suspended in a binder of cooked starch and [[styrene-butadiene]] latex.  Coating produces a very smooth, bright surface with the highest printing qualities.

[[File:Bundesarchiv Bild 183-W1015-015, Technitz, Schreibpapier AROS aus Altrohstoffen.jpg|thumb|right|upright=1.2|Paper leaving the machine is rolled onto a ''reel'' for further processing.]]

=== Calender section ===
{{Main|Calender}}
<!-- Note: This is the correct spelling of 'calender' as used in this sense. -->
A [[calender]] consists of two or more rolls, where pressure is applied to the passing paper. Calenders are used to make the paper surface extra smooth and glossy. It also gives it a more uniform thickness. The pressure applied to the web by the rollers determines the finish of the paper.

=== Reel section ===

After calendering, the web has a moisture content of about 6% (depending on the furnish). The paper is wound onto metal spools using a large cylinder called a ''reel drum''. Constant nip pressure is maintained between the reel drum and the spool, allowing the resulting friction to spin the spool. Paper runs over the top of the reel drum and is wound onto the spool to create a ''master roll''.

To be able to keep the paper machine running continuously, the reel must be able to quickly switch from winding a finished roll to an empty spool without stopping the flow of paper. To accomplish this, each reel section will have two or more spools rotating through the process. Using an overhead crane, empty spools will be loaded onto two ''primary arms'' above the reel drum. When the master roll reaches its maximum diameter, the arms will lower the new spool into contact with the reel drum and a machine behind the drum will run a tape along the moving sheet of paper, swiftly tearing it and attaching incoming paper onto the new spool. The spool is then lowered onto the ''secondary arms'', which steadily guide the spool away from the reel drum as the diameter of paper on the spool increases.
 
The [[Roll hardness tester|roll hardness]] should be checked, obtained and adjusted accordingly to insure that the roll hardness is within the acceptable range for the product.

===Winder section===
Reels of paper wound up at the end of the drying process are the full trimmed width, minus shrinkage from drying, of the web leaving the wire. In the winder section reels of paper are slit into smaller rolls of a width and roll diameter range specified by a customer order. To accomplish this the reel is placed on an unwind stand and the distances between the slitters (sharp cutting wheels), are adjusted to the specified widths for the orders. The winder is run until the desired roll diameter is reached and the rolls are labeled according to size and order before being sent to shipping or the warehouse. A reel usually has sufficient diameter to make two or more sets of rolls.