Editing Paper machine (section)

=== 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"/>