PAPER IN PROCESS
It is an almost dramatic moment when the machine is ready to start. The machine tender opens the valves which admit the stuff from the flow box and a stream spreads out onto the wire. At a given signal the back tender starts the wire, and the endless white stream moves smartly forward. Then ensues the mechanical imitation of making paper by hand, only instead of forming sheet by sheet, the formation is a continuous process in the web. The shake of the machine mixes the position of the fibers in the “pond” behind the slices; the water runs like a downpour of rain through the moving wire into the save-all, leaving behind its burden of fiber, or “stuff,” as the mixture is at that stage called, in a white film.
The suction boxes accelerate the expulsion of water, and the dandy roll closes the fibers together as the film passes beneath it. Then the web is carried between the couch rolls, when the water fairly pours out in the squeeze. As the top roll is felt-jacketed, the film sometimes sticks to it, as a slight suction is created in the pores of the felt. The back tender stands by with a hose to wash down the paper if it starts to adhere to the jacket. The paper is prevented from completely going around this top roll by a guard board which is fixed across the top. Many machines are now equipped with a “suction couch roll,” which does away with the need for a top roll, as the water is sucked, instead of pressed, out of the paper.
At a given signal the back tender starts one edge of the film forward, by a skilful slap of the hand, which picks up the edge of the film and transfers it to the felt carrier between the press rolls. The remainder of the web is made to follow the lead of the first section, till finally the full width is transferred to the first felt, which carries it through the first series of press rolls.
An arrangement similar to the guard board, called a “doctor,” runs across the top press roll, so that the paper may be allowed to roll up if desired, while the machine tender regulates the flow of water until the consistency of the stuff is right. The doctor also keeps the press roll clean. Quite often the long end of paper first started at the couch roll is passed right along from the first felt to the second, carried through the second set of press rolls, and the third, if three there be, to the steam driers, and thence over the entire battery of driers, through the calenders onto the reel.
From the press rolls it is led by the back tender, assisted by a third hand, and if all goes well the paper may be winding up on the reel inside of ten minutes.
But there is many a chance for mishaps before the wet end of the machine is adjusted and the heat in the driers is regulated to a nicety.
The weight of the paper depends upon the quantity of stuff let onto the machine, the dilution of the stock, and the speed at which the machine is run. Given a certain volume, the faster the wire runs the thinner the stuff is spread, and vice versa. Before things are settled down, considerable worthless paper may be turned off.
The width of the web is controlled by the distance between the deckle straps. These are adjustable, but an allowance of ten inches or so must be made for the shrinkage of the web in drying.
The preliminaries to a run of paper may be likened to the make-ready on a printing-press, though they do not, as a rule, last nearly so long. Yet this is the reason why small odd sizes and odd shades of paper are not popular with the manufacturer, unless he can get a sufficient extra price to compensate for the “make-ready” costs.
Water-marks.[C]—The water-mark in paper is effected by raised lines on the dandy roll. The design, being impressed in the moist web, displaces the fibers and leaves thin areas in the paper, which consequently show when the sheet is held against the light, as they are more translucent than the adjoining areas.
[C] The study of ancient water-marks is quite fascinating in connection with early Printers’ marks. See “A New Light on the Renaissance,” by Harold Bailey.
FOURDRINIER MACHINE, S. D. WARREN & CO.
View showing the “Fourdrinier” part of a modern book paper-machine.
CHAPTER SEVEN
PAPER-MAKING—Continued
Technique.—The importance of the formation of the sheet on the machine wire is the same as on the hand mold, as subsequent pressing and calendering can only modify faulty formation. The stuff should be uniform and even in texture. The press rolls must be ground with absolute accuracy, and slightly crowned to allow for their sagging. Otherwise water would be unevenly expelled from the web, possibly causing a damp streak throughout the entire run of paper, which would show in the finished product.
If a portion were pressed too hard it would contain less moisture as it reached the driers and become dry before adjacent sections. If the paper were calendered, the moister parts would take on a smoother surface than the drier parts.
Another feature to be closely watched on particular papers is to eliminate, as far as possible, the impress of the weave of the wire cloth, which is left in the under side of the web. This can be accomplished to so fine a degree, by a skilful man, that the difference between the two sides of the paper is scarcely discernible. The fineness of weave of the one cloth also is an important bearing in securing an even sided sheet.
Thus we see that it is well-nigh impossible to reduce the making of paper to an exact science, and a reasonable variation must be expected, both in weight and finish. The successful management of a paper-machine depends, from start to finish, on careful, experienced judgment and alert attention. If the beater-man dilutes one batch of stuff more than another, the variation will show the minute the altered stuff appears on the machine, and only an immediate readjustment at the wet end can avoid considerable variation in the product. Then, from end to end, the long machine must be watched carefully, so that the pressing, drying and calendering may all be kept uniform. A bungler should find no place in the machine-room, but it is desirable that consumers have sufficient appreciation of human limitations, as applied to paper-making, to admit proper allowances for normal variations.
Cylinder Machine.—The cylinder machine, invented by John Dickinson about ten years after the Fourdrinier, is much the same as the wet machine described in [Chapter II], with the addition of press rolls, driers and calenders. The single-cylinder machine is used for making light-weight tissues and other thin papers. Cylinder vats can also be arranged in series, as on board machines, so that the webs formed on each cylinder can be combined. This is accomplished by an arrangement of felts which run tangent to the cylinders, picking off the formed paper automatically from each successive mold.
The felt runs between squeeze rolls, so that the various plies of paper are pressed together, forming a single thickness. Machines of this type can make very thick sheets, and are used for making bristol boards, blanks, boxboard, strawboard, etc.
The number and arrangement of driers on any machine depends on the product to be derived. Fast-running machines, such as the large news mills are equipped with, have necessarily a large number of driers, as they turn off fifty tons or so a day and require a great drying capacity. Slow running machines, such as are used in fine writing-paper mills, need a much smaller number, as the average fine writing-paper machine produces little over three or four tons a day.
CYLINDER VATS, MADE BY THE PUSEY & JONES CO.
The felts which convey the paper are omitted so as to get a clearer view of the molds.
Harper Machine.—There is a type of Fourdrinier called the Harper which differs from it in that it is turned end for end. A long felt carries the paper from the couch rolls back over the Fourdrinier part, delivering it to the first press. This is considered advantageous in making very light papers which otherwise are with difficulty led from the couch to the press rolls and are apt to break down in the passage.
Yankee Machine.—There is even one type of machine known as the “Yankee” which has but one drier of very large diameter. This is used in making machine-glazed wrapping papers, which are very smooth on the side of the sheet which comes in contact with the drier and rough on the other side. The “wet” end of this machine is a Fourdrinier type.
The arrangement and number of smoothing and calender rolls is also dependent on the class of paper to be made. Most writing-paper machines have no calenders at all as the surface is obtained on special machinery such as platers and sheet calenders after the paper comes from the drying loft. One can easily appreciate that, while the general principles of all paper-making are identical, there is a call for a wide variety of arrangements, such as those cited, to meet the varying requirements of different classes of paper.
Surface Sizing.—Surface sizing, or animal sizing, necessitates a vat with squeeze rolls. The paper is first run over enough driers to dry it; then introduced into the vat of hot size. On the cheaper grades the size is dried on the machine by a special skeleton drying apparatus, but the better grades are cut off and piled up by the “lay-boy” at the end of the machine, then transferred to drying lofts and hung up over poles to dry. Hence the term “loft-dried.” Any special finish has then to be applied sheet by sheet.
Finishing Paper.—Finishing paper is accomplished either on the paper-machine itself, or after the paper is turned off on the machine it may be treated by special apparatus.
Wove and Laid Papers.—A so-called wove paper is made with a plain dandy, covered with fine wire cloth the same texture all over. Laid paper is really a water-marked paper, in which the whole surface is marked by a specially constructed dandy which imprints a mark in imitation of the early hand molds. There are heavy lines running with the grain of the paper and lighter lines running across.
Antique.—An “antique” surface is obtained by skipping the calender rolls and leaving the paper rough as it comes off the felt to the driers. A medium finish is obtained by a slight calendering, while the highest machine finish, and the so-called English finish, is obtained by a heavy calendering.
Water Finish.—A common method of obtaining a high finish on heavy papers is by the use of “water doctors,” which keeps two or more of the calender rolls moist, dampening the paper while it is being calendered. The surface thus imparted is called a “water finish.”