After the paper stock (consisting of the fibrous material, and, usually, sizing materials, loadings and coloring matter) is compounded, it is collected in the machine (or mixing) chest where it is further diluted from about 3 to 6% down to about 2 to 3%. The stock is then pumped into a regulating box which controls the flow of the stock to the papermaking machine.
Subsequent to further dilution of the stock with white water (which is water containing fibers and other materials retrieved from the papermaking machine), the stock is screened through sand traps or other purifying devices to remove lumps or clumps of fibers, impurities, etc. The screened stock then enters the flow (head) box of the papermaking machine, where it is agitated and spread onto the wire, which by means of forward movement and lateral agitation, forms the fibers into a matted web of paper. The wire, which is endless, is supported by the breast roll, followed by a series of foils or table rolls. It also moves over several suction boxes which remove water from the suspension, and then under the DANDY ROLL . before returning over a suction couch roll. The breast roll and the suction couch roll represent the two extremes of the wire On its return to the breast roll the tautness of the wire is controlled by a number of stretch rolls. The water draining through the wire, i.e., the white water, is collected and returned to the head box. The fibrous slurry is prevented from running off the edges of the wire in various manners, e.g., by means of a DECKLE on either side of the wire. When the paper web passes under the dandy roll it is still sufficiently wet to receive an impression. It is at this point that the web is given a WATERMARK , or, if desired, a laid or wove finish. The former is produced by covering the roll with evenly spaced parallel wires, while the wove finish requires that the roll be covered with woven wire. The paper web proceeds from the wire to a series of wet presses. When the web reaches the couch roll it still contains approximately 80 to 85% water and therefore cannot support itself. The required support is provided by woolen felts which carry the web through the presses, each press being supplied with its own felt. Suction boxes are also usually provided to remove water from the felt. The wet presses reduce the water content of the web to about 70%, and at this point the web is self supporting. The web than passes from the "wet end" of the papermaking machine to the "dry end." This section consists of a series of cast iron driers arranged in two or more rows. The web is held tightly against the drying cylinders by means of felts. By the time the web emerges from the last drying press, its water has been reduced to less than 10. Depending on the finish desired, the paper may pass from the drying section to the calender stack or it may be reeled initially without calendering. Thus, modern papermaking involves essentially seven basic operations: 1) fiber pretreatment; 2) fiber blending; 3) furnish cleaning and screening; 4) slurry distribution and metering; 5) web formation and water removal by mechanical means; 6) web compaction and water removal by means of heat; and 7) sheet finishing, by means of calendering, sizing, coating, or glazing.
The method used today to prepare cellulosic material for papermaking is almost entirely mechanized. Wood chips are treated with chemicals, under heat and pressure (see: SULFATE PROCESS ;SULFITE PROCESS ), various impurities (notably LIGNIN ) being removed in the process. The chips are partially processed into pulp and are then bleached to the desired brightness and washed. Following washing, the partially processed pulp is ready to be processed into paper pulp. The pulp, at this stage called "half stuff," is ground in a beater, by means of refiners (see: JORDAN ;REFINER ), and, in the usual process, loading agents, sizing and coloring materials are also added.
The properties of the finished paper depend largely on the loading agents, as well as the fibrous materials used and the degree of their treatment. These agents consist of mineral substances. principally one of the clays (see: KAOLIN ), which affect the opacity of the paper, its suitability for printing, etc. SIZING (2) materials are usually added to the pulp to impart water (ink) resistance. See also: ADDITIVES (2) ;AIR-DRIED ;ALKALINE FILLER ;ALKALINE PROCESS ;ALKALINE RESERVE ;ANIMAL SIZED ;ASH ;ATTAPULGITE CLAY ;BEATER SIZED ;CALENDER ;CELLULOSE ;CELLULOSE CHAIN ;CELLULOSE FIBERS ;CHAIN LINES ;CHEMICAL WOOD PULP ;CLAY ;COATED ;COATING ;COATING CLAY ;COUCH (3 , 4 );COUCH ROLL ;DIGESTER ;FILLER CLAY ;HARD SIZED ;MACHINE DIRECTION ;MECHANICAL WOOD PULP ;NEUTRAL SULFITE PROCESS ;PULP ;ROSIN SIZE ;SEMI-CHEMICAL PULP ;SIZING ;SLACK SIZED ;SUPERCALENDER ;SURFACE SIZED ;TUB SIZED ;UNBLEACHED SULFATE PULP ;UNBLEACHED SULFITE PULP ;WELL-CLOSED FORMATION .
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Timestamp: Saturday, 19-Nov-2011 13:18:43 PST
Retrieved: Tuesday, 24-Apr-2018 03:01:12 GMT