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Paper Making

( Originally Published 1920 )



WE have now reviewed the various steps preparatory to the process of beating, and this process is perhaps the most important of all. The output of a mill depends, first, upon the quality of stock which is furnished to the beaters, and secondly, on the way the stock is handled in the beaters. A formula, better known as a "furnish," is prepared by the superintendent and given to the beater engineer. This tells him exactly how to blend his raw materials. Very few papers are made from one kind of material alone, most papers being a mixture of different fibers, with the addition of mineral filler, sizing and coloring. All the ingredients are put together into the beating engine with a large volume of water similar to a washer, minus the washing drums.

BEATING

The process is called beating because it has displaced the original method of maceration by mallets and later by the machine described in Chapter I as a "stamper."

The ultimate characteristics of the paper are dependent upon the handling of the beater roll and the character of the knives. For example, a blotting-paper is made by a quick beating with sharp knives. This cuts the fibers clean and short and leaves them in a most absorptive condition. The very same fibers, treated with dull knives and slowly beaten, would have an entirely different character. Their ends would be teased out and ragged, and in the process of manufacture they would part very slowly from the water absorb-ed. The paper produced would have the characteristics of a writing-paper, hard and strong. This instance will afford some idea of the wide variation in results which may be brought about by varying the treatment in the beaters. So important is this step in manufacturing that it has been said with a good deal of truth that "the paper is made in the beaters."

After the process has been continued a sufficient length of time, the stuff is emptied into a chest called the "Jordan chest," because it acts as a reservoir for another type of refining engine known as the "Jordan." This engine is conical in shape and the inside is lined with knives. A cone-shaped plug, also shod with knives, fits into this shell, and by the turn of a screw may either be moved in or out, thus varying the space between the two sets of knives. By this adjustment the refining of the pulp which flows through the engine is regulated.

The stock passes through one or more of these "Jordans" into the machine chest. Thence it is pumped to a level higher than the machine, and flows through "sand settlers" to a screen. The "sand settler" is a long, open trough containing a series of baffle boards which collect any sediment, preventing it from getting into the paper.

Screens are of various types, the main feature consisting of bronze plates pierced with fine slots through which the fibers are forced. The object is to give uniformity to the stock which reaches the machine, and to exclude any knots of stock, strings or foreign substances.

The width of the slots is varied to suit different stocks—some slots being as fine as 10/1000 of an inch.

We have now described the process of paper-making up to the point where the stuff is formed into paper, and must pause for a description of the paper-machine itself.

PAPER-MACHINE

The paper-machine may be considered in three parts: The wet end where the paper is formed and pressed, the middle, where it is dried, and the dry end, where it is calendered, slit and wound.

There are two distinct types of wet ends—the Fourdrinier and the cylinder. Both are mechanical reproductions in continuous process of the steps taken in the ancient hand methods, a brief consideration of which impresses clearly on one's mind the rationale of the machine.

HAND PROCESS

The tools of the primitive paper-maker consisted of a pulp vat for the fiber-laden water, a frame, or mold, across which was stretched a mesh of closely woven wire, and a removable frame, known as the deckle, which fitted around the edge of the mold to keep the moist pulp from overflowing and to help regulate the thickness of the paper.

Grasping the. mold by two opposite sides, the vatman submerged the mold in the water; then raised it out, holding it level. By this means a film of pulp was caught up, being deposited on the bottom of the mold by the passage of the water in which the fibers had been suspended. A lateral shaking motion served to knit the fibers together, and to deposit them as evenly formed as possible all over the mold. As the water drained through, the film of pulp solidified. Then the deckle frame was removed, and there, on the top of the mold, was a sheet of moist pulp. The edges of this sheet would be thin and feather-like as a result of the pulp leaking under the deckle. Hence the term deckle edge.

It required a great deal of skill to remove this film, while preserving it intact. This was accomplished by inverting the mold and pressing the sheet upon a moist felt cloth. If the act was skilfully performed, the mold could be lifted away from the sheet, leaving it unbroken upon the felt. Then it was covered by a second piece of felt and the process was repeated until a small pile had accumulated.

The pile was removed to a screw press, wherein as much water as possible was squeezed out of the paper. Cellulose fibers have a strong affinity for water, however, and it is said that under any pressure which such a pile could withstand, without becoming crushed and gruelly, the paper would retain water equal to one-half its weight. Hence, the last vestiges of moisture, excepting of course that amount normally retained by air-dried paper, had to be removed by evaporation. In the old days, this was accomplished by hanging the sheets over poles to dry.

After that, if the paper required sizing, the sheets were dipped one by one into a pot of animal size, then dried once more. Lastly they were finished to the desired surface by being placed between smooth plates and pressed.

FIBER CHARACTERISTICS

A few moments' consideration of the changes which the fibers undergo from their condition of isolation as they exist mixed in the vat, to their status as components of a sheet of paper, will help to make clear much that seems obscure about the behavior of a sheet of finished paper, as well as to explain the reason for the different processes executed on the paper-machine.

The fiber is a hollow, collapsed tube, the ends bruised and frayed by the treatment in the beating and refining engines. Absorptive in nature to a marked degree, it swells with the water it takes up and is limp and flaccid. As the mold is raised horizontally out of the vat in the process of forming sheets, all the fibers which had been suspended in the water which passed through the meshes of the mold are caught like so many fish in a net, and lie spread in a limp, impression-able mass over the surface of the mold until they are transferred by the "coucher man" to the felt. Little alteration can take place in the general position of the fibers after they have been "couched," consequently the formation of the sheet is the most important stage of the process. As the water is pressed out, each fiber contracts to some extent, and, from a consistency like gruel, the formed sheet passes to a more stable state, wherein it can be gently handled without disintegrating.

As the drying proceeds there is a marked shrinkage in the dimensions of the sheet, caused by the shrinking of each individual fiber, until the fibers are thoroughly set, enmeshed one with the other.

The addition of size glazes over each fiber and makes it less susceptible to moisture. The addition of clay permeates the structure, filling up the interstices. Up to a certain point the clay does not materially weaken the structure, as a certain percentage of empty air space would exist without it. Beyond that point the clay will fill places that conceivably would be filled by fibers, and having no adhesive strength, the structure of a sheet overloaded with clay is weakened in proportion to its overload.

While the fibers are more or less moist, they are susceptible to alteration in structure, and may in this state be flattened by calendering to a smooth surface, and the presence of clay helps to fill in the microscopic valleys between the fibers so that the surface becomes level to human vision.

THE FOURDRINIER

Now to return to a sketch of the wet ends of paper-machines. The Fourdrinier part consists of a head box, which resembles the case of an upright piano. Where the keyboard might, be, is a broad portal for the passage of a stream of pulp, the width of the machine, onto a horizontal, endless wire belt. This wire belt is suspended in a frame some thirty feet long and held taut by being stretched over a number of rolls. The large roll near the head box is known as the breast roll. The still larger roll at the other extreme of the frame is called the lower couch roll, on top of which is a felt-jacketed couch roll. The wire is kept level by a trans-verse series of "table rolls" closely set, and the under part of the wire is held down by stretch rolls. Directly under the top part, and continuing from the breast roll for about two-thirds the length of the frame, is a shallow tray called a "save-all, as it catches all the drippings which contain filler, and some fine fibers which are returned to the screens by stuff-pumps, maintaining a continuous circulation so that nothing goes to waste. Into this save-all water may be admitted to regulate the consistency of the stuff.

Near the couch roll the wire passes over two or three suction boxes, and on top of the wire, between the suction boxes, turns a wire-covered roll called a "dandy."

On either side of the machine is a frame which may be contracted or expanded. It carries a series of pulleys over which run rubber deckle straps, the under parts of which rest on the wire and keep the wet pulp within bounds. By this means the width of the web of paper is regulated. As a little pulp leaks under these straps machine-made paper has deckle edges on both sides of the web. Artificial deckle edges may also be produced by squirting a fine stream of water upon the web near the couch roll, but it is not possible to produce this effect across the web. Except on special papers the deckle edges are trimmed off by slitters at the end of the machine.

Near the flow box, running at right angles across the machine, are two so-called "slices" about eight inches apart. These may be adjusted at various heights from the wire, in order to regulate the thickness of the paper. Their most important function is to make the thickness uniform from one side to the other of the sheet, and to create a pond which assists in forming the paper.

The frame of the Fourdrinier has a joint near the first suction box, and a mechanical arrangement called a "shake" is located near the head box to impart a lateral shaking motion to the frame while the wire runs straight ahead, thus imitating the shaking of the hand mold.

Beyond the couch roll is a series of press rolls, between which run endless felts to carry the soft, moist paper.

Then follows a large series of steam-heated cylinders. Next a stack of iron calender rolls, and a set of reels. As soon as one reel is full a new reel is started and the paper from the first reel is slit by rotary slitters and made up into rolls of the desired widths on the winder.

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.*-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.



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