Slice, headbox

Subsequent to stock preparation and proper dilution, the paper furnish usually is fed to the paper machine through one or more screens or other devices to remove dirt and fiber bundles. It then enters a flow spreader which provides a uniform flowing stream and which is the width of the paper machine. The flow spreader, or manifold, discharges the slurry into a headbox, where fiber flocculation is minimised by microturbulence and where the proper pressure head is provided to cause the slurry to flow at the proper velocity through the slice and onto the moving Fourdrinier wire. 

The stock is pumped through a manifold into the headbox of the paper machine, where the stock flow is decelerated and distributed over the width of the machine. Various baffles and step diffusors are used to avoid vortex flow and stagnation zones. The furnish leaves the headbox through the slice, a narrow gap with controlled profile, and impacts on one or two endless screens, the so-called papermakers wire. Water is removed from the fiber mat by the action of foils and vacuum. 

The furnish is fed to the headbox of a paper machine (Fig. 15.12), about the length of a city block. Sufficient additional fresh water (white water) is added to bring the pulp concentration in the headbox down to about 0.5%. White water, the water removed from the fibers as the paper sheet is formed on the machine, is continuously recycled. This low-concentration of pulp in water is necessary to obtain an even distribution of fiber (a smooth formation ) as the sheet is formed. The diluted furnish is fed through an adjustable horizontal slot (the slice) in the headbox onto an endless wire screen (the Fourdrinier wire), which moves rapidly away from the headbox. Direct drain-... 

The diluted stock may then be pumped by the fan pump through a series of cleaners and into the machine headbox. An inlet distribution header spreads the stock flow evenly over the width of the headbox. The flow is stabilised by the headbox and passes from the headbox through an adjustable orifice plate called the slice. It is discharged onto the moving wire screen (which is usually of plastic construction), dewatered by a series of devices called table rolls, foils and flat boxes leaves the wire as a formed web of paper. It is further dewatered and dried in the succeeding operation before being wound on the reel at the dry end of the machine. 

The suspension is dehvered from the approach flow system through piping that has a circular cross section. The flow must be turned and distributed uniformly and with great accuracy across the full width of the headbox. It must then flow out of a narrow slit (normally called a slice, 6-25 mm in height, and more in certain applications) that can be more than 10 m wide. In modem headboxes, this uni-... 

SHce lip actuators Linear displacement actuators with a spacing of between 75 mm and 150 mm deflect the slice hp of the headbox nozzle. 

The headbox distributes the stock across the full width of the wire through a slit called the slice. The slice opening can be adjusted to give a consistent cross machine profile. The "wire" is a continuous belt made of woven polyester or similar synthetic fiber, or of phosphor-bronze for certain paper grades. Many different weave patterns are used, from the basic square weave to complex multilayer weaves designed to optimize water drainage, paper formation, and retention of fines and fillers. 

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