The Paper Machine
( Originally Published 1920 )
All paper prior to about 1803 was made by hand on molds which were dipped into the stock to form the sheet. In 1799 Nicolas-Louis Robert, in France, invented a machine in which the stock was delivered to an endless traveling wire cloth by a sort of revolving fan, passed between small squeeze rolls, and wound up in the wet condition in a roll. This was then removed, the paper unwound, passed through press rolls, and hung up to dry. While this machine was capable of continuous operation it was generally considered that a length of 50 feet of paper was the practical limit.
In April, 1801, John Gamble received English patent 2487 for an improved paper machine under the title: "An invention for making paper in single sheets without seam or joining, from one to twelve feet & upwards wide, & from one to forty-five feet & upwards in length." This was an improvement over the Robert machine in respect to the flow of stock onto the wire, and to the pressing of the wet sheet. It should be noted that neither of these machines included means for drying the sheet continuously. Further improvements on a machine of this type were patented by Gamble in 1803, and in the autumn of the same year the first machine ever to be built and operated with success was started at Frogmore, England. Rights to all these machines were later acquired by Henry and Sealy Fourdrinier, and it is because of their financing and promotion that the machines became known as fourdrinier machines.
From the standpoint of the modern papermaker these early machines resemble only that small part of present machines where the stock is formed into a sheet and pressed. This is natural, for the desire of the inventors was to replace the intermittent dipping of the mold by a continuous operation, and they did not visualize the finishing of the paper by drying and calendering in an unbroken web. In accomplishing this one advance the inventors eliminated the skill and pride of workmanship in making superior hand-made paper and opened wide the field for the tremendous changes which have resulted in modern papermaking. In the last hundred and twenty-five years the speed at which paper is turned out on a fourdrinier machine has gone from a maximum of 100 feet per minute in 1867, to 200 feet in 1880, and to well over 2,000 feet at present. In 1897 the maximum width of a machine was 162 inches, making a sheet of 152 inches; now machines are built over 300 inches in width. These changes have involved corresponding changes in engineering design and construction, and it can easily be understood how little a modern high-speed machine resembles the original ones. The cost has also increased from $30,000, which was said to be the value of the first fourdrinier machine installed in the United States at Saugerties, N. Y. in 1827, to well over a million dollars. Such advances as these are in step with the times, but to some they introduce an element of regret. Dr. Dard Hunter, an apostle of the esthetic in papermaking, remarks "The improved paper machine is essential to cope with modern life as it has come to be, but I would be content never to see again one of these long, ponderous, steaming, dragon-like machines, emitting from its great jaws the streams of paper that are thought necessary to our very existence."
The fourdrinier installed at Saugerties was constructed by Bryan Donkin in England and was 60 inches wide. The first machine actually built in the United States was made at South Wind-ham, Conn., sold to Amos D. Hubbard and put into operation in May, 1829, at Norwich Falls, Conn. The first machine-made paper produced in the United States was made in 1817 at the Gilpin Mill, near Wilmington, Del., but this was made on a cylinder machine, developed by Thomas Gilpin, one of the two brothers who operated the mill and who took out a patent on the machine in 1816. The cylinder for this machine was apparently made in England. The cylinder machine has not developed to nearly the extent that the fourdrinier has, and it is far more often used for heavy paper and board stock than for high-grade printing paper. It will be described briefly later in this chapter, but for the present all remarks will apply to the fourdrinier.
To describe a fourdrinier paper machine is not to describe a single machine, but dozens of them, each being designed for some particular purpose and all so co-ordinated and synchronized that they work together with a single objective—the production of a sheet of paper. Each part of the machine has been developed to its present state of perfection gradually, and after many years of study and experience. The main object of all improvements has been the production of better paper, at higher speed and at lower cost, and how well this has been accomplished is shown by the fact that the United States is the wasteful user of more paper per capita than any other country in the world. Whether this is considered a measure of the civilization of a country, as is frequently claimed, seems to be open to question. Nevertheless the race to produce more and more paper goes merrily on and paper ma-chines will undoubtedly be still further improved and speeded up in order to meet this demand.
Paper machines are not built standard, but each is constructed to specifications and though each has the same essential parts these vary in design according to the builder. To describe these in detail would be of little interest to the reader for whom this book is planned, so only brief notes as to the function each serves will be attempted, and before doing this it might be well to summarize the action of the machine. Very briefly it receives prepared paper stock containing about 199 pounds of water for every pound of dry material, passes it through a series of operations to form it into a continuous web and to remove the water, presses the wet sheet to remove still more water and to make the sheet more dense, dries it over heated drier drums, calenders it, and finally reels it up into large rolls. This change from stock containing almost ten tons of water per 100 pounds of dry matter to a sheet of paper which seldom contains over 5 pounds of water in 100 pounds of dry paper takes place in just a few minutes according to the speed of the machine, and is one of the most spectacular operations in the whole field of papermaking. Even to one who is thoroughly familiar with papermaking operations it is always a matter for wonder and admiration.
The preparation of the stock by beating, sizing, loading and coloring has been described; these precede its collection in a large tank, known as a machine or stuff chest. Since the consistency, or solid content, of the stock in the beater chest varies somewhat because of irregularities in the furnishing and dumping of the beaters, it passes a device known as a consistency regulator as it is pumped through the Jordan to the machine chest. This takes stock which is intentionally a little thicker than is wanted at the machine chest and dilutes it with the required water to hold it uniformly at the desired consistency. From the machine chest the stock is pumped to a regulating box which controls the amount of stock flowing to the paper machine and thus maintains the paper at the desired weight. There are various types of regulating boxes, the simplest of which receives an excess of the stock, and by means of adjustable gates sends the desired portion to the machine and the remainder back to the stuff chest. In this simple type the gates are hand-operated, but there are others which are fully automatic and which give much better control of the weight of the sheet.
The stock at the regulator box is of too high consistency to go directly to the wire, so it is largely diluted with some of the water draining through the wire, which is known as "white water." This brings the solid content down to about 0.5 per cent, on an aver-age, and at that consistency it goes through screens or centrifugal devices, or both, to remove lumps and coarse particles. The screens operate according to size of particles by passing the stock through slotted plates, while the centrifugals depend on difference in gravity for the removal of heavy material. Similar purification was formerly accomplished by running the dilute stock over long rifflers to catch sand, and possibly over magnetic devices to remove bits of iron. These older methods were serviceable when most papers were made from rags, which often contained sand, bits of buttons, hooks and eyes, etc., but since the use of chemical pulps became so general they are practically obsolete for the majority of papers.
After passing the screens, the stock goes to a flow-box or head-box whose function is to hold the stock, to keep it sufficiently agitated to prevent flocculation and to build up a sufficient head so that as it is discharged onto the wire it will have nearly the same rate of travel as the wire. The height and design of the head-box vary greatly with the width and speed of the machine; old, slow machines needed very little head, while with modern high-speed machines the stock may flow out under a head of many feet.
When the stock leaves the head-box it passes over an "apron" on its way to the wire. The apron is of some such material as oil-cloth, rubberized cloth, or even a thin sheet of rubber. The apron is fastened to a board at the side next the head-box and extends onto the wire to a distance which depends on the stock used and the paper being made. Just beyond the apron and directly over the wire are one or more slices, which are of sheet brass, placed vertically to the surface of the wire and made adjustable for the width of the sheet and the distance from the wire. The functions of the slices are to make the flow of stock uniform across the wire and to maintain equal stock velocity at every point. This type of apron and slice served well enough on slow machines but was inadequate for modern high-speed operations. In these the functions of the slices are served by giving the high head-box an adjustable lip, or a nozzle type of opening, at the point where it discharges onto the wire, and the flexible apron is not used.
The wire, on which the sheet of paper is formed, is a woven brass or bronze cloth, the mesh of which differs in type and size of opening according to the paper being made. The cloth is woven flat and the ends joined by sewing or brazing to make it endless. The number of drainage openings per inch for standard plain weave wires is:
Mesh Number Openings per Inch
The coarser the wire mesh, the more its diamond-shaped marks will show in the finished paper; hence the finer wires are used for the higher grade papers. For some very thin papers a twilled wire is used, which leaves still less evidence of marks in the paper.
Starting at the end of the machine next to the head-box, the course of the wire is first over a large roll called the "breast roll," and then over a series of relatively small, parallel rolls of steel or brass tubing called "table rolls." It then passes over several suction boxes, under a roll covered with woven or parallel wires, called a "dandy roll," and over one or more additional suction boxes. At its furthest point from the breast roll it passes around another large roll known as a "couch roll" and just before reaching this point it passes over a guide roll which automatically keeps the wire in line and prevents it from running off on either side. After passing around the couch roll the wire returns to the breast roll, and is kept tight in this portion of its travel by a series of rolls which are adjustable in such a way that the wire is kept at the desired degree of tightness; these rolls are known as "stretch rolls."
The power to drive this part of the machine is applied at the couch roll. Old style machines employed a solid bottom couch roll and a felt-covered top couch which was slightly offset from the bottom roll to permit its weight to bear on the wire and thus squeeze water from the paper web through the wire. Modern suction couch rolls have almost entirely replaced the old type because they re-move water from the wet web more efficiently by suction than by pressure, and because no top couch is necessary.
The table rolls serve to support the wire, keep it from sagging under the weight of the stock flowing onto it, and aid in removing water by being in contact with the under side of the wire. Usually the wire is kept as level as possible from side to side and from breast roll to couch roll, but in some machines it can be given a slope with or against the course of the wire as desired. The stretch rolls, which keep the wire under tension, are sprayed with water to prevent the building up of lumps of stock from the slight amount which unavoidably adheres to the under side of the wire on its return trip, thus insuring a clean wire at the breast roll.
Just under the table rolls, and between the flow and return portions of the wire, are trays to catch the white water draining through the wire and deliver it for reuse in diluting the stock from the chest as it goes to the screens. All of this machinery, except the white water trays, is mounted on a heavy frame which is pivoted at the couch roll end, but can be given a slight sidewise shake at the breast roll end. This shake enables the machine tender to obtain a better formation of the sheet when making high grade papers; for modern news machines it is not considered necessary.
As the dilute paper stock crosses the apron it flows onto the wire and immediately begins to lose water through the wire into the trays. The slices of the old type machines tend to back up the flow into a pond and by proper adjustment help regulate the formation of the sheet. To keep the stock from flowing off the edges of the wire, deckle straps are used to form dams. These are of soft rubber, rectangular in section, and made to run in flanged pulleys. They rest on the wire, are carried along by it, and extend from the breast roll to a point beyond the first suction box. Many modern machines have a much shorter, fixed deckle whose rubber edge rests lightly on the wire and retains the stock only until enough water has drained away so the stock will not flow appreciably. From the slices the loss of water through the wire continues less and less rapidly until the suction boxes are reached, when much greater amounts are suddenly removed through the wire and the perforated covers of the boxes by the use of suction pumps. As the web passes under the dandy roll it is still wet enough to receive an imprint of the surface of the dandy, and according to whether the dandy is covered with woven wire or parallel, evenly spaced wires around the roll, the paper is known as "wove" or "laid" paper. Watermarks are applied at this point by attaching to the surface of the dandy a raised design which is then impressed on the wet web, thus making the paper thinner in accordance with the thickness of the design. After the dandy, more water is removed by the remaining suction boxes, the edges are trimmed off by continuously flowing squirts of water, and after passing the suction couch the sheet goes to a series of press rolls.
When the paper leaves the couch roll it contains 80—85 per cent of water, is very easily damaged, and will support its own weight for only a very short distance. It is therefore transferred to a traveling woolen felt which supports it through the first of a series of presses whose function is to remove more water by squeezing and at the same time make the sheet denser and smoother. Two or three presses are used in series, and the paper may go directly through, or it may pass under one press and be reversed through its rolls so that the two sides of the sheet maybe more nearly alike. The lower roll of the presses is of iron covered with rubber, and the top roll is of wood, of cast iron with a brass sleeve, or of polished granite or artificial stone. The top roll of the press stands vertically over the lower roll and it is connected with compound levers and weights which permit regulation of the pressure applied and a maximum pressure much greater than that supplied merely by the weight of the top roll. Each press has a separate felt to carry the web, and just before the web reaches each set of rolls the felt often passes over a suction box to aid in water removal. All felts are kept taut by a series of stretch rolls as they return to the point at which they picked up the paper web. Transferring the web from the couch to the first felt is done when starting by cutting a narrow strip by means of the squirt on the wire and blowing it onto the felt by an air blast; in slow machines it may be done by picking it off the couch by hand and lifting it onto the felt. At each press the web sticks to the top roll and has to be transferred to the next felt by hand or air blast. After leaving the last press to go to the driers the sheet will still contain about 71 to 74 per cent of water, but it has gained enough in strength so that it can be handled to the driers without difficulty.
Many modern machines are equipped with a smoothing press whose function is not to remove water, but to smooth and flatten the sheet after it comes from the true presses and before it goes to the driers. This aids in removing wire and felt marks and produces a superior paper, both for smoothness and strength. The upper roll is of rubber which is somewhat softer than that on ordinary press rolls and the lower roll is of metal; both are ground very smooth. To obtain the desired effect the paper while passing through this press is not supported by a felt.
The felts used on the wet end of the paper machine are not true felts, which are made without weaving, but are actually heavy blankets woven from very high grade woolen yarns. They must be strong to withstand the pull of the machine, but also of a texture loose enough to pass water readily. They are made in great variety to suit the speed of the paper machine and the grade of paper being made, and the surface of the paper is considerably influenced by the length of nap and the fineness of weave of the felt. Felts are easily damaged and must be handled carefully both off and on the machine. To keep them open and porous, and to pre-vent filling up with bits of fiber and clay, they are washed, either on the machine or by removing and giving them a special laundering. They wear out eventually from friction, which removes the nap, and from bacterial action, which causes rotting and tendering, especially in warm weather and while the machine is idle. Felts which are taken off the machine because of injury, or before they are too badly worn, are often washed, dried and sold to local householders for use as blankets.
After leaving the presses the paper goes to the dry section of the machine, the purpose of which is the removal of the water which cannot be taken out by pressing and which amounts to about 70 per cent of the weight of the wet paper at this point. A paper machine drier is a cast iron drum with closed ends, very carefully made so that it may be in good running balance and supplied with a steam inlet and a device to remove condensed water continuously and without loss of pressure. The outer surface is turned and polished as smooth as possible. Driers vary somewhat in size, but those 4 feet in diameter are quite common. They are usually mounted in two rows, one above the other, but staggered, so that an upper drier is over the space between its two neighbors in the bottom row. At the back end of each drier is a gear which meshes with the gears on two driers in the row above or below, so that all turn at the same speed. A row of driers is usually broken into two banks with approximately an equal number of drums, and each bank is driven independently of the other. In some machines the driers are driven by an endless roller chain instead of gears; this is simpler in some respects and is desirable on high-speed news and kraft machines.
When the driers are arranged in two rows, each row usually has a long felt which covers about one half of the surface of each drier and is kept tight by means of stretch rolls. These felts also are not true felts, and most of them are not wool, but a kind of heavy cotton duck; some are made of cotton and asbestos to withstand better the deterioration caused by the heat of the driers, which gradually rots the cotton. The purpose of the felts is to hold the paper firmly against the surface of the driers, except where it passes from one drum to the next, and thus produce a smooth sheet with-out cockles. The wet web is carried from the last press to the first drier and then in turn to the others until it emerges in a dry condition at the end of the two banks. In slowly running machines this is done by hand when first carrying the web over the machine by tucking the end of the web into the nip between the felt and each of the upper drums; in fast machines this cannot be done, so the end of the web is placed between two ropes which run in grooved flanges on the front side of each drier roll and carry the paper through the driers. Proper care and handling of drier felts is a very important part of machine operation, and many devices are employed to keep them running true and at proper tension.
In passing the driers, about two pounds of water must be evaporated for each pound of paper made. In cold climates if this is allowed to escape directly into the room it causes condensation on the ceiling and water drops all over the machine, so it is customary to cover the drier section with a hood from which the vapors are removed by fans. The large volume of vapor-laden air which is removed must be replaced by fresh, warm air, so the ventilation problem is of great importance. The Minton vacuum drier takes care of this problem by inclosing the drier section in a vacuum chamber from which the air is pumped before the paper is put through. The vacuum maintained is about 28 inches and fewer drums are needed than in the exposed systems. This vacuum type of drier is expensive to install and is used more often for chemical pulps than for paper. The lower drying temperature at which it operates is advantageous for pulps, but for papers it does not effect as good rosin sizing as the same stock dried on open driers, except in the case of papers containing calcium carbonate as a filler.
From the driers the paper generally goes through calenders which consist of rolls of cast iron, chilled on the surface to make them hard, and ground and polished to a very smooth surface. A machine calender stack may have as many as eleven rolls, all mounted in a housing at each end of the rolls, and all driven from the bottom roll by friction. The paper is fed in at the top, passes down through the stack and out at the bottom to the reel, where it is wound into large rolls, which are then rewound at full width or through slitter knives which allow the preparation of rolls of other desired sheet widths. The machine calender is designed to compact the sheet and give it a better finish. If this is not desired, the sheet coming from the driers may by-pass the calender and go directly to the reel. Some paper machines use two calender stacks set so that the sheet passes first one and then the other.
It has been necessary to omit so many details of paper machine construction and operation that this description seems very inadequate, but it at least proves the complexity of the machine as a whole. To drive such a machine so that all parts may run at exactly the correct speed is a major problem, especially for the fast ma-chines, yet this has been so well attained that a news machine may run for days or weeks at a time without a break in the paper web. This has been accomplished by driving the individual parts of the machine separately by variable speed devices which permit very accurate adjustment of speed. The old, slow machines used to be driven by belts from a long line shaft and speed adjustments made by cone pulleys, or even by lagging the pulleys to change their diameters slightly. Even by such crude methods good paper was made in those days, but it was not at speeds or in amounts which would supply present-day demands.
The principle of the cylinder machine differs from that of the fourdrinier in several respects. Instead of the sheet being formed on an endless belt of woven wire through which the water drains, leaving the fibrous stock on its surface, the cylinder mold revolves while partially immersed in a vat of the dilute stock and the sheet is formed on the surface of the wire-covered mold. This is accomplished by maintaining the water level within the cylinder lower than that outside, so that the water drains into the cylinder leaving the stock on its surface. The vat in which the cylinder revolves is supplied continuously with the stock being used, and the water from within the mold is removed by a pump and the necessary part delivered to the regulating box for dilution of the new stock going to the vat. The regulating box functions as with the fourdrinier machine by dividing the thick stock pumped from the chest into two parts, one of which goes to the vat, and the other returns to the chest. Before going to the vat the diluted stock may, or may not, be screened according to the grade of stock and the type of paper being made.
As the mold comes up out of the stock its surface is covered with a layer of fiber, and against this a couch roll presses a felt moving at the same speed as the surface of the cylinder. The felt picks the wet sheet off the surface of the mold and carries it along for further treatment. Usually a cylinder machine consists of several cylinder molds and vats supplied with stock entirely independently, and as the felt passes over each cylinder in turn it picks up the sheet from each, thus making a sheet of multiple layers. This makes it possible to form sheets with surface layers differing from those within the sheet, either in color or kind of fiber, or both. The stock from the two end vats forms the two surfaces of the finished sheet; these are called "liners" and the inside portion of the sheet is called the "filler." For many grades of board it is customary to use very cheap material for the filler and much better stock for the liners, so that the final board has a much superior appearance.
After passing the last couch roll and collecting the final sheet of liner, the multi-layer sheet is very thick and contains too large an amount of water to go directly to the driers or even to the heavy presses. It therefore passes between felts through a series of baby-press rolls, the upper roll of each being rubber covered while the lower one is of metal. The distance between the rolls, and therefore the pressure exerted, can be very accurately adjusted by means of hand wheels, and levers and weights. At the first baby-press the sheet is so wet that it is easily injured by too much pressure, so this is set very lightly. In succeeding presses the pressure is increased, so that when it leaves the last baby-press the sheet is dry enough to stand further pressing without being in contact with the felt. The latter returns from this point to the first mold to continue its course all over again, and the sheet goes through still heavier presses to remove more water, and finally to a series of drum driers to complete its drying. This gradual building up of the sheet of wet stock, and the pressing given, welds the various layers from the different cylinders into a coherent whole and the finished board is not easily split into layers. Obviously, cylinder machines are especially useful in the manufacture of heavy boards, but they can be used for light tissues if desired. One characteristic of paper made on a cylinder is the rather wide difference in its tearing strength with and across the grain. This is much greater than in fourdrinier-made papers and is especially noticeable in paper made from very long fibers.
The number of cylinders in a machine varies from one to as many as eight or more depending on the weight of the sheet to be made. Some are built with the first vat next the presses so that the course of the sheet and felt has to be reversed at the end of the vat section and carried back above the vats and molds to the press section. Others are built to give straight line flow right through the machine. Machines with only one vat are used for tissues and for stock which is to be saturated with asphalt for roofing purposes. Such machines may have cylinders over six feet in diameter, but those in most multiple-cylinder machines are smaller than that. No general statement can be made about the speed of cylinder machines because this varies so enormously with the weight of paper or board being made. On weights of paper which are suit-able for preparation on fourdriniers, these will run much more rapidly than cylinder machines. On newsprint, for example, a fourdrinier will run at 1500 feet per minute or over as compared with about 500 feet for a cylinder.
There are various modified paper machines for special purposes and products, but these need only brief mention. The "Yankee machine" dries the paper against a highly polished, single-drum drier, and this may be combined with the wet part of either a fourdrinier or a cylinder. This produces the paper sometimes called "machine glazed." A modified fourdrinier, known as the Harper machine, was built to handle very thin tissues which were too light to be passed from the couch to the wet felt. Other modifications combine portions of the fourdrinier and cylinder machines into various assemblies.