How Paper is Made

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The basic raw material for the papermaking process is wood. To begin the process, pulpwood logs must be reduced to chip form. Prior to chipping, logs are passed through a debarking drum (large, open-ended cylinder). Within the drum, logs collide with one another and rub together removing the bark. The bark falls through slots in the cylinder walls and is collected and burned as fuel in the power boilers. The debarked logs are conveyed to a chipper, which reduces them to small 1.5- to 2-inch squares with a 0.25-inch thickness.

Softwood and hardwood chips are kept separate until the pulp is blended at the paper machine since each has its own physical properties. Wood is made up of small cellulose fibers, bound together by a glue-like substance called lignin. In the pulping process, these fibers are separated by cooking the wood with chemicals to dissolve the lignin.

To accomplish this, the chips are loaded into large vessels called digesters on either a batch or continuous basis. Digesters are designed on the same principle as a kitchen pressure cooker. The chips and chemicals are steamed under pressure for 1.5 to 4 hours until the mixture is reduced to a wet, oatmeal-like mass. The cooking frees the fibers so they can be suspended in water.

The pulp is blown from the digesters under pressure to separate the fibers. It is then washed to remove the cooking chemicals and dissolved lignin and then bleached to the proper shade of whiteness. From there, the pulp is passed through refiners. These refiners roughen the surface of the individual pulp fibers by loosening the threadlike elements from the fiber wall so they cling together when formed into a sheet. Added after refining are dyes and other additives to give the finished paper the desired properties.

Water is then added to the pulp in a ratio of 200 parts water to one part fiber. This furnish, as it is called, is then run onto the forming fabric or wire of the paper machine. The forming fabric is an endless mesh screen that circulates at the wet end of the paper machine. There the fibers become interlaced, forming a mat of paper, and much of the water is extracted.

Traveling at speeds of more than 3,000 feet per minute, the paper is pressed between water-absorbing fabrics and wound through a series of steam-heated cylinders called dryers, where the last of the water in the sheet is removed. At this point, the paper passes through a size press that applies a starch solution to both sides of the sheet. Sizing seals the surface so ink cannot soak into the paper during printing. Since sizing wets the paper, the paper must again be dried by traveling through another series of steam-heated drums.

After drying, the paper goes through a calendering process that provides a smooth finish by ironing the sheet between heavy, polished rollers. At the dry end, the paper is wound onto spools to form a machine reel and then rewound and slit into smaller rolls on a winder. Some of these rolls are sent for sheeting and packing into cartons. Others are rewound to smaller-sized rolls and wrapped for shipment. 

A screen is used to separate larger chips

(From: http://www.internationalpaper.com/Our%20Company/Learning%20Center/How%20Paper%20Is%20Made.html

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Department of Communication, Seton Hall University