Theses are particularly useful for the high speed machines, where the fourdrinier wet end would tend to give two-side sheet, since both sides are wire sides and the sheet is formed symetrically on the two sides. Used to make heavy weight board from secondary fiber, which is not de-inked, for folding boxboard, chipboard and gypsum board. A high quality fiber surface may be added for printing upon. A press is a pair of squeeze or wringer rolls designed to remove water mechanically and smooth and compress the sheet. Removing water from the web is accomplished by adding heat to the web and circulating the air.
Heat is applied by the pressurized, steam filled circular steel or cast iron dryer drums.
The initial drums cannot be too hot, otherwise large amounts of liberated steam will cause the paper to cockle. The paper often stretches a few percent in the machine direction while contracting in the cross direction through the course of drying, causing the machine direction to be stiffer and to have a higher tensile strength than the cross machine direction. Sophisticated control of individual dryer motors controls the tension throughout the dryer section and reduces web breaks. Dryer hood - an enclosure around the dryer section and is used to improve drying efficiency by removing the moist air near the surface of the web. If this air is not removed, it quickly becomes saturated with water, preventing further water removal from the web.