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Once the pulp has been produced, paper is manufactured in two stages: stock preparation and on the paper machine.
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In stock preparation, the fibres in the pulp are beaten to give the paper special properties, such as optimum strength and fibre binding. In addition to pulp, the stock ingredients usually comprise stock size, calcium carbonate, shading dye, and 99 per cent water. Paper production begins with the stock mixture being fed into the head box, which is the starting point of the paper machine, and sprayed onto what is called the wire. The wire section is basically the paper machine’s first dewatering process. Using one or two wires or straining cloths, the water in the stock is removed with dewatering elements and a vacuum, and the moisture content of the mixture decreases considerably. It is also in the wire section that the paper begins forming, and the fibres orientate themselves mainly in the direction of the paper web. From the wire, the still wet pulp moves on to the press section, where it is dewatered further using cylinders and felts. The pressure in the press section influences the finished paper’s bulk, stiffness, opacity, strength and roughness.
The next stage is the drying section, where the paper passes a number of steam-heated steel cylinders, and the majority of the remaining water is removed. Only now, after the drying section in which almost all the moisture has been removed, can the word paper be used in its true sense. As the paper comes out of the pre-dryer it is surface sized, whereby a thin film of starch is added to the surface for extra strength and printability. The paper then goes on to the after-dryer still at the same high speed, where it is dried after starch has been added. This stage can be omitted if the paper is an unsized quality.
Before the paper is completely ready and wound onto a large reel called a tambour reel, it is pressed between cylinders and calandered to ensure a smooth, fine surface. In the final stage of the process, the finished paper is wound onto large tambour reels, and then cut down into smaller reels or sheets.
The entire process in the paper machine takes very little time, between 10 and 30 seconds from stock to finished paper. During this brief sequence of events, the stock is dewatered and becomes paper. The paper machine is therefore quite simply a large dewatering machine. |
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