Paper Making - Part 2
( Originally Published 1920 )
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, straw-board, 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.
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 Fourdriner 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."
Fine writing-papers may be finished in a variety of ways. A plain, smoothed surface is obtained by passing the sheets, which are automatically fed, by a system of tapes, through calender stacks, called sheet calenders.
PLATING.—Plating was first resorted to as a means of smoothing paper in the sheet, but when a linen, or pebbled, or any other special finish is desired, it is also accomplished in a plating machine. This consists of two heavy rolls. The sheets of paper, with a metal plate top and bottom, are passed through the rolls under heavy pressure. If a linen finish is desired, pieces of linen are placed between the plates on both sides of the sheets so that the linen texture is embossed into the paper. Similarly any other substance may be used for other effects.
EMBOSSING.—Embossed papers are usually finished from the roll by running between iron rolls with embossing pat-terns engraved upon them. An extra strength is required of paper for this purpose, otherwise the pattern will cut through the sheet.
SUPERCALENDERING.—Supercalenders are machine s, apart from the paper-machine itself, for making high-finished paper: The rolls, vary in number. Each alternate roll is made of hard paper. In treating uncoated stock there are also one or two steam boxes to moisten the paper before it is calendered. This softens the surface fibers, and they can then be rolled flatter and hence take a shinier surface. The alternate rolls in a stack for calendering coated papers are made of cotton, and no steam boxes can be used, because the moisture would injure the coating.
The paper is run through the calenders in the web. All smooth, or special, finishes are gained only at added cost. Where the process takes place on the machine, more breakage is occasioned and more paper has to be sorted out, as the hard-finishing accentuates spots in the paper, and little lumps of fibers, which would pass unnoticed in an uncalendered or antique paper, are squashed down and blackened by calendering. Hence the higher cost of such papers.
Supercalendering and plating bring into play different 'workman, so that the labor cost is increased, and any finishing, sheet by sheet, is necessarily slow and more costly than that accomplished in a continuous process from the roll.
COMBINING.—Many kinds of papers, as photo-mounts, double-thick covers and cardboards, are made by pasting two or more thicknesses together. This was formerly done in the sheet, but most of the pasting is now effected in the web. The papers are run over a paste roll, combined, and passed either through a drying chamber or over a battery of driers like those of the paper machine. The pasted paper is lastly made into rolls and taken to the finishing room to be sheeted.
COATED PAPERS.-Coated papers are made by covering the surface of ordinary paper with a veneer of clay, mixed with some adhesive, as casein or glue, and suitably colored.
The process is done from the roll; the paper first goes through the machine where the liquid coating is brushed onto the surface, passing directly in automatically formed festoons through a long, heated room to dry, and finally is rewound.
The rolls are then taken to the supercalender room and the paper is given the desired finish.
Dull-finish coated papers require a special kind of coating and receive very light calendering after being coated.
High-finished coated papers of the best grades are double-coated and run several times through the calenders.
Another method of producing a high finish is known as "flinting." In this process the paper is mechanically polished by smooth flint stones and gains a very high luster. Such papers are most widely used as box covering. A similar effect is obtained by friction calenders, which consist of two chilled iron rolls with an intermediate roll of hard paper. The top roll rotates at a higher speed than the others.
The coating may be dyed to any color desired, so that coated and glazed papers are obtainable in a wide variety of shades.
GUMMED PAPERS.—Gummed papers are made by passing the web through a machine, which coats it with glue, after which it passes over drying apparatus and is wound into rolls ready for finishing.
Gummed paper for labels is usually finished in sheets, while for sealing tape and box stays it is ordinarily made up into rolls.
WAXED PAPERS.—WAXED papers are made by applying a coating of parrafin. This renders the stock water proof, and it is used largely as a wrapper for food products.
GLASSINE PAPER.—By a special treatment in the beaters and jordans cellulose fiber is so treated as to become hydrated. This hydration makes the paper produced grease proof, and by heavy supercalendering the character of the sheet is again greatly altered, it becoming almost perfectly transparent. In this state it makes a most attractive and hygiene wrapper.