Fig. 40.
Mr. Wyatt on American Super-calendering.—Mr. Wyatt, on a recent visit to America, had many opportunities of witnessing the systems of manufacture adopted there, and subsequently delivered an interesting address to the members of the Paper-Makers' Club,[27] in which he acknowledged the superiority of the high-class printing papers for book-work, which has so often been the subject of recognition in this country. Indeed, if we compare the surface of the paper used even for ordinary technical journals in America and that generally adopted for our own periodicals of a similar class, we are constrained to admit that the difference is in favour of our transatlantic competitors. "In the manufacture of high-class super-calendered printing papers," Mr. Wyatt observes, "for fine book-work, or as they call them book papers, the Americans certainly excel. Whether this be due to the kind of raw material used, to the almost universal use of the refining-engine, which renders the pulp very soft and mellow, or to the state of perfection to which they have brought the art of super-calendering, or perhaps due to all three, I could not exactly determine. The material generally used for this class of paper is poplar chemical fibre and waste paper to the extent of 50 per cent., and even up to 75 and 80 per cent. of the total fibre, the balance being rags, or, in cheaper qualities, sulphite wood pulp; the stuff is all mixed together in large beaters, holding from 800 lbs. up to 1,500 lbs. of pulp, where it is about half beaten, and then finished in one or other form of refining-engine.
"The Americans have, I think, more thoroughly studied the question of super-calendering paper than we, and in this respect get better results and better work. The paper is mostly slit and trimmed on the paper-machine, and reeled up in from two to four widths by an ingenious contrivance called the Manning-winder, which automatically keeps the tension constant on each of the reels, whatever the diameter, and is super-calendered in narrow widths on small calenders. These calenders are from 36 inches to 42 inches wide, and consist of a stack of 9 to 11 rolls, alternately chilled iron, and cotton or paper; the paper is passed through the rolls two or three times, never less than twice, under great pressure applied by hand-screws. The power required is very high, being from 40 to 50 h.p. for each calender, and the speed from 450 feet up to 600 feet per minute. The paper is not usually damped before calendering, but is left rather under-dried from the machine; neither is steam heat used in the rolls, which get very warm, owing to the high speed at which they run. The rolls are driven entirely by straps, the arrangements for the fast and slow speed and for reeling on and off the paper being well designed and worked out; the main strap, running at high speed, runs on a loose pulley on the shaft of the bottom roll, by means of a powerful friction clutch; this pulley can be made a tight one. On this same bottom shaft is keyed a multiple V-shaped grooved friction pulley. Another, and independent shaft, driven from the main shaft by a crossed belt, has a small grooved pulley keyed on it, which can be thrown in and out of gear with the large grooved pulley. Strap-driving is thus secured throughout, and the speed can be increased gradually without jerks, from the starting up to the fastest speed by working the levers, gearing the friction clutch and pulleys slowly."
In reference to the high finish of American papers, we are disposed to attribute this mainly to the nature of the chief raw material used—wood fibre. In the year 1854, when specimens of Mr. Charles Watt's wood-fibre paper were first printed upon, the remarkable gloss of the wood paper attracted much attention, and it was noticed that the impression of the ink appeared to be well on the surface of the paper, and not, as was often the case with ordinary printing papers of the time, partially absorbed by the paper itself. Mr. Wyatt states that poplar chemical fibre and waste paper to the extent of 50 per cent., and even up to 75 and 80 per cent., are used, the balance being rags; now since the waste paper in all probability would be composed largely of wood fibre, and as, in the cheaper qualities, sulphite wood pulp is used in lieu of rags, it will be fair to assume that the chief basis of the highly-finished papers for which the Americans are justly famous is wood fibre, and we believe that there is no other variety of cellulose which is so susceptible of producing a naturally glossy paper as that which is obtained from wood by the soda process.
Mr. Arnot on Finishing.—Mr. Arnot makes the following observations respecting the finishing of paper:—"The paper may be slit into widths, suitable for wet calenders, or may be cut up into sheets, and glazed by the plate or board calenders. The former method of surfacing or finishing has come extensively into use in recent times, the labour involved being much less than in the older method of finishing in sheets. Still, however, the plate calenders are kept at work upon the higher classes of goods, it being possible to give almost any degree of surface to good paper by that means. There is little doubt, too, that the paper glazed by the plate rolls retains its original softness to a greater degree than that passed through web calenders. In the latter it is exposed in one thickness to great pressure, and is thinned in consequence; whereas, when the sheets are made up into piles, along with copper or zinc plates, there is a certain amount of spring or elasticity in the treatment which largely counteracts the crushing action of the rolls. The web calenders consist of a series of rollers erected in a vertical frame, and between these the paper winds, beginning at the top and coming downwards, so that the pressure gradually increases as the paper moves on its journey. It will be observed that the under rolls have to bear the weight of the upper ones, and that consequently the pressure on the paper will be greater the lower down it descends. Many of the rollers themselves are now made of paper, and as these possess a slight degree of elasticity, and take a high polish, they are alternated with iron rollers with good effect. The paper-rolls are made by sliding an immense number of circular sheets, perforated in the centre, on to an iron core or shaft, pressing these close together by hydraulic action, and trimming them off on the lathe. The plate or broad calenders consist only of two rollers, the upper one heavily weighted, preferably by compound levers. Between these rollers the sheets of paper, alternated with plates of copper or zinc, and made up into bundles about an inch in thickness, are passed backwards and forwards, the reciprocating action being produced by the movement of a lever in the hand of an attendant. The metal and paper sheets of different bundles may be interchanged, and the process repeated with the effect of increasing the beauty and equality of the finish."
Fig. 41.
Cutting.—Revolving Knife.—When paper is to be used in a continuous printing-machine, or, as is often the case, has to be exported in the web, it is supplied in rolls; otherwise it is cut into sheets before leaving the mill. The form of cutter generally used is what is termed the revolving knife-cutter, an illustration of which, as manufactured by Bertrams, Limited, is shown in Fig. 41. At A is shown a series of webs, the paper from which is drawn forward by the rolls, B, and is then slit into suitable widths, and the margin at the same time pared by circular knives, one of which is shown at C. It then passes through a pair of leading-rolls, after which it comes in contact with a knife, D, attached to a revolving drum, E, pressing against a dead knife not shown in the engraving. The sheets, as they are thus cut, drop upon a travelling felt or apron, F, from which they are lifted and placed in piles, by boys or girls standing on each side of the felt. These machines will cut eight webs at one time.
Fig. 42.