FORMING AND MAKING THE SHEET From the storage chest into which the batches of beaten pulp and chemicals (load, size, and color) are dumped and from which the mixture is continuously pumped to the Jordan, it passes on through to the paper machine, usually a Fourdrinier. In this machine the sheet is formed, dried, and cut into rolls of the desired width. FOURDRINIER PAPER MACHINE The Fourdrinier paper machine is commonly thought of as having a wet and a dry end. The machine consists of three parts-the Fourdrinier part (or the wire), the press part, and the dryer. The Fourdrinier part is made up of a long, flat, endless wire belt or screen, which travels at uniform speed away from the source of pulp-mixture supply, together with suction boxes and a couch roll for removing moisture from the newly formed wet sheet. As the pulp, water, and chemical mixture flows onto the moving wire, it consists of from 99 to 99%1⁄2 percent water. (In board manufacture a thicker mixture is often used.) This mixture flows under pressure with a smooth, flat evenness onto the whole width of the screen. The free water immediately passes through the wire by gravity and by suction, leaving the wet fiber layer. The operation is, of course, continuous, and the thickness of the fiber layer laid down can be controlled. Some paper machines carry a pair of deckle straps which lie flat along the edges of the moving wire and which keep the mixture from flowing off its edge. The formation of a good sheet, however, is not alone a matter of laying down the wet fiber layer. For one thing, it is desirable that the fibers in the newly formed sheet interlace as they are laid down instead of all falling in the same direction. To get this interlacing, the moving wire is often given a continuous cross movement or shake while the fibers are being laid on it. This short, jerking, cross-movement of the screen helps to distribute the fibers in different directions and to build a sheet which is of more uniform strength than if the fibers fell parallel. After the wet sheet has been formed, the wire carrying it passes over the suction boxes, which take out some of the moisture. The sheet then passes over the couch roll; in modern machines a suction couch roll is frequently used. From the couch roll the sheet passes to the press rolls on the press-roll felts. The press part of the paper machine comprises two or more units, each of which consists of a pair of press rolls mounted one above the other. The wet paper sheet is picked up by the first traveling felt belt as it comes from the couch roll and is carried between the rolls of the first press, where the fibers are set in the sheet and some of the water is removed. The sheet is then picked up by a second felt belt and carried in similar manner through a second set of press rolls. If there is a third set, the operation is repeated, additional water being squeezed out as each set of press rolls is passed. The lower roll of each of the presses may also be a suction roll. As the sheet passes through the last press it is usually reversed as to face, so that the impression left by the wire is eliminated through pressure against the smooth, upper (steel or stone) roll. It then goes to the dryers. The dryer part of the paper machine consists of a series of heated, cast-iron cylinders, arranged in two horizontal tiers, one above the Figure 19. Wet end of Fourdrinier machine. Note the "wire" in foreground and belt-like deckle which provides an edge for the sheet as the pulp-and-water mixture flows onto the wire. The driers are in the background. other, with the cylinders of the upper tier placed above the interstices between adjacent rolls in the tier below. There are sometimes as many as 60 of these drying cylinders in the unit. As the sheet passes from the last press of the press part of the paper machine, it leaves the felt and is carried through the drying rolls. The heated rolls dry the sheet, and it comes from the dry end in a wide, continuous sheet of paper. If it is to be given a fine, smooth finish, the sheet is passed through the calender stack, a bank of highly polished cylinders, where, by pressure and friction, it is given much the same appearance as though actually ironed by hand. The sheet is then slit into the desired widths and rewound into rolls. The whole paper-making operation-the laying down of the wet fiber on the Fourdrinier wire, and its successive passing through the press rolls, the dryers, the calenders (if used), the slitters, and the rewinders is a continuous operation. CYLINDER MACHINE No small amount of paper is made each year on the cylinder machine, instead of on the Fourdrinier machine, just described. Tissue, building, wrapping, and absorbent papers, as well as paperboard, are frequently made on this machine. The cylinder machine-invented by a John Dickerson, of Germantown, Pa., in 1809-was, in fact, well developed before the Fourdrinier was even introduced into America. In the cylinder machine a revolving cylinder covered with wire mesh turns, partially submerged, in a vat of the fiber, water, and chemical mixture, and the water drains off as the wire, with its fiber layer, is raised out of the mixture. The wet sheet is taken from the wire by means of a moving felt, is passed through a series of press rolls which squeeze out much of the excess water, and is then conveyed to the drvers. Board is usually made on the cylinder machine, though several cylinders are necessary to build up the desired thickness. The subject is treated in more detail in the paragraphs describing the manufacture of paperboard. WATERMARK Some paper carries a watermark by which it may be identified when held up to the light. This mark may be a trade name or symbol, or it may be any other distinctive mark imposed at suitable intervals. In any case, it is pressed into the sheet by the pressure of a wire roll (the dandy roll) while the sheet is still very wet and before it has left. the Fourdrinier screen. The dandy roll is also used to flatten the upper surface of the sheet so that both sides will have approximately the same finish. The same mesh of wire is used on the roll as on the machine itself. MANUFACTURE OF PAPERBOARD Paperboard of almost any thickness, up to and including wallboard and insulating board, may be made on a Fourdrinier or a modified Fourdrinier machine. On the modified machine the drying rolls are supplanted by a hot-air chamber. By far the greater proportion of paperboard is made, however, on the cylinder machine. The Fourdrinier produces a board of homogeneous cross section, and there is |