The two most common machines in the current use are the Fourdrinier and the cylinder machines. Both produce paper sheet from pulp in a continuous process. In the Fourdrinier the pulp and water mixture flows at a controlled rate through a headbox and onto a moving wire mesh screen. As the screen moves away from the pulp, leaving a sheet of matted pulp that still contain a high proportion of water. A wire covered roll holding a sheet surface to impress a watermark. The sheet then moves on to a woolen felt screen, which takes it through a series of presses, where more water is removed. Finally, the sheet passes over a number of heated drums that evaporate the remaining water.
The cylinder machines differs from the Fourdrinier principally in the "wet end," or forming operation. Instead of the moving wire screen, a screen covered rotary cylinder is half-submerged in the pulp vat. As the cylinder rotates, a sheet of matted pulp is formed on its exterior surface and is then picked up by a moving belt, where it is treated to remove the remaining water, as in the Fourdrinier process. A series of cylinders may be used, each one depositing an additional layer of pulp on the belt, so that thicker, multilayer sheets are built up.
As it leaves the paper-forming machine, the dried paper is wound onto large reels. The rolled paper may be sit to the widths required, cut into sheets, trimmed, and packaged. Other finishing operations include calendaring, coating or operations that convert the paper roll into special products.
In addition to the paper used for writing and printing, paper is made into a wide variety of end products, from the absorbent papers used for toweling, toilet, and blotting papers to paperboards that are made up into containers; papers used in building construction (such as roofing paper); and special papers that are designed for other particular uses.
Because of their need for water and lumber for pulp, paper mills are often located on the banks of rivers, in remote, forested areas. Papermaking processes require the heavy use of chemicals, and the by-products have included dioxins and other toxins, which have been components of the wastewater that is flushed into the river.
Working with the industry and with paper-manufacturing-states, the environmental Protection Agency (EPA) has been developing new standards for paper mill effluents in order to reduce pollution.