The Art of Paper-Making / A Practical Handbook of the Manufacture of Paper from Rags, Esparto, Straw, and Other Fibrous Materials, Including the Manufacture of Pulp from Wood Fibre

by Alexander Watt

By another patent, dated May, 1884, No. 8348, the inventor forms the surface of the dandy-roll of wire-gauze embossed in such a manner that parts of the surface of the gauze, corresponding to the configuration of the design of the water-mark, are raised, and project out from the general surface, and other parts corresponding to the line shading of the design are depressed below the level of the general surface. The accompanying drawing, Fig. 34, shows diagrammatically, and greatly enlarged, a section of a portion of the surface of a dandy-roll made in accordance with this invention. a represents the section of a ridge or projection raised on the surface of the gauze; b represents the section of a groove or depression in the wire-gauze, which, with other similar grooves, serves to produce an opaque shading to the design. c is an auxiliary ridge or projection, serving to define the shading line, and to intensify it by driving the pulp into the groove or depression b. Further effects may be obtained by attaching wires to the dandy-roll, either in the usual way, where the surface is unembossed, or upon the raised parts a, which give the configuration to the water-mark. In place of forming the ridges or projections a, which produce the configuration of the water-mark, by raising portions of the wire-gauze above the general surface, they may be formed by sewing on suitably shaped slips of wire-gauze, or of sheet metal perforated all over with fine holes, on to the surface of the gauze which is embossed with the grooves b, but it is much to be preferred that both the ridges a and the grooves b should be produced by embossing the gauze. Water-marks may also be produced by placing sheets of finished paper in contact with plates of copper or zinc, bearing a design in relief, and submitting them to heavy pressure.

Fig. 35.

Suction-Boxes.—These boxes, which are fitted under the wire, are made of wood, and are open at the top, the edges being lined with vulcanite. The ends of the boxes are movable, so that they may be adjusted to suit the width of the paper required; they are also provided with air-cocks for regulating the vacuum, which is obtained by means of two sets of vacuum pumps, having three 6-inch barrels to each set: a vacuum pump of this form is shown in Fig. 35. As the wire travels over these boxes, the action of the pumps draws the wire upon them with sufficient pressure to render them air-tight; by this means a large portion of the water which the pulp still retains at this point becomes extracted, thereby giving to it such a degree of consistency that it can stand the pressure of the couch-rolls without injury. The backwater extracted by the suction-boxes, as also that collected in the save-all, is added to a fresh supply of pulp before it flows on to the sand-tables.

Couch-Rolls.—At the extreme end of the wire-cloth from the breast-roll, and inside the wire, is the under couch-roll, from which the wire receives its motion. This roll, which is of brass, is usually about 14 inches in diameter, is carried upon a cast-iron framing with brass bearings, and is ground to a working joint with the top roll, which is also of brass, and 20 inches in diameter. Both these rolls are covered with a seamless coating of woollen felt. The upper roll rests upon the lower one, and the wire-cloth, and the web of paper upon it, pass between the rolls, receiving gentle pressure, by which the paper becomes deprived of more water, rendering it still more compact. It is at this stage that the web of paper leaves the wire-cloth, and passes on to a continuously revolving and endless web of woollen felt, termed the "wet felt," from the moist condition of the paper. This felt, which is carried on wooden rollers, is about 20 feet long, and is manufactured with considerable care.

The Press-Rolls.—The paper now passes on to the first press-rolls, which deprive it of a still further quantity of water, and put it in a condition to bear gentle handling without injury. The upper roll is fitted with a contrivance termed the "doctor," which keeps the roll clean by removing fragments of paper that may have become attached to it. The doctor is furnished with a knife which passes along the entire length of the roll, pressing against it from end to end. These rolls are generally of iron, jacketed with brass, the under one being 14 inches in diameter, and the top roll 16 inches. Sometimes this roll is made of fine-grained cast-iron. When the roll is of iron the doctor blade is steel; but when this roll is brass the knife is of the same material. The under surface of the paper, which has been in contact with the felt, and necessarily being in a moist condition, receives more or less an impression from the felt over which it travelled, while the upper surface, on the other hand, will have been rendered smooth by the pressure of the top roll of the first press. To modify this, and to render both surfaces of the paper as nearly uniform as possible, the paper passes through another set of rolls, termed the second press-rolls, in which the paper becomes reversed, which is effected by causing it to enter at the back of the rolls, which rotate in a reverse direction to those of the first press, by which the under or wire side of the paper comes in contact with the top roll of the press. By this arrangement the underside of the paper is rendered equally smooth with the upper surface. The second set of press-rolls is provided with an endless felt of its own, which is usually both stronger and thicker than that used in connection with the first press-rolls. In some mills each set of press-rolls is provided with a doctor, to prevent the web of paper from adhering to the metal. Sometimes the doctor knives are made from vulcanite, a material which would seem specially suited for a purpose of this kind. From this point the paper passes to the first set of drying cylinders.

The Drying Cylinders.—The invention of the steam drying cylinder is due to Mr. T. B. Crompton, who, in the year 1821, obtained a patent for this useful addition to the paper-machine. Since that period, however, the system of drying the paper by steam-heat has been brought to a high state of perfection; not only this, but the number of cylinders has gradually increased, while the heat to which they are raised has proportionately decreased, and as a consequence the size, which is injuriously affected by rapid drying, is gradually deprived of its moisture, and thus renders the paper closer and stronger, while at the same time a very rapid speed can be maintained. The drying cylinders in the machine shown in the engraving are 4 feet in diameter and 12 in number, being arranged in two groups of 8 and 4 cylinders respectively, and in the aggregate present a very large drying surface, it being very important that the operation should be effected gradually, more especially at its earlier stages. There is a passage between the second press-roll and the cylinders, through which the machine-men can pass from one side of the machine to the other. The first two or three of the first section of cylinders are only moderately heated, and having no felt on them, allow the moisture from the paper to escape freely. The next five cylinders, however, are provided with felts, which press the paper against the heated surfaces, by which it becomes smooth and flattened, thus putting it into a proper condition for passing between the smoothing-rolls. The cylinders are heated by steam, and are generally of decreasing diameter, to allow for the shrinking of the paper during the drying.

Smoothing-Rolls.—These consist of highly polished cast-iron rolls, heated by steam. The paper being in a somewhat moist condition when it passes through these rolls, they have the effect of producing a fine smooth surface.

The paper next passes over the last four drying cylinders, all being provided with felts, to keep the paper closely pressed against their heating surfaces, by which the remaining moisture becomes expelled and the paper rendered perfectly dry. The paper now passes through the calender rolls, and is then wound on to reels at the extreme end of the machinery. The operation of calendering will be treated in the next chapter.