Alexander Watt

CALENDERING, CUTTING, AND FINISHING.

Web-glazing.—Glazing Calender.—Damping-Rolls.—Finishing.—Plate Glazing.—Donkin's Glazing Press.—Mr. Wyatt on American Super-calendering.—Mr. Arnot on Finishing.—Cutting.—Revolving Knife Cutter.—Bertrams' Single-sheet Cutter.—Packing the Finished Paper.—Sizes of Paper.

To impart a higher gloss, or, as it is technically termed "glaze," to paper after it leaves the machine, it has to be subjected to further calendering, which is accomplished either in the web, or in sheets, according to the quality of the paper.

Web-Glazing.—Glazing Calender.—When paper has to be glazed in the web, it is passed between a series of rolls, which are constructed upon several different systems. In one form of this machine the rolls are alternately of finely polished iron, and compressed paper, or cotton, the iron rolls being bored hollow to admit of their being connected to steam pipes, for heating them when necessary. In this machine there are eight rolls, the centre pair being both paper rolls, which have an effect equivalent to reversing the paper, by which both sides are made alike. Another form of glazing calender, of American origin, but which has been improved upon by our own engineers, consists of a stack of rolls made from chilled iron, the surfaces of which are ground and finished with exquisite precision upon a system adopted in America. A representation of this calender as manufactured by Messrs. Bentley and Jackson is given in Fig. 37. Such rolls as require heating are bored through, and their ends fitted with brass junctions and cocks, to regulate the admission of steam. The standards are of cast iron, planed and fitted with phosphor bronze bearings; the bearings to carry the top roll of the stack are furnished with wrought-iron screws and hand wheels, and wrought-iron lifting links can be attached to raise one or more of the rolls, according to the finish required on the paper. Compound levers are also supplied, to regulate and adjust the pressure on the ends of the rolls.

Damping Rolls.—An important improvement in connection with the calendering of paper was introduced by Messrs. G. and W. Bertram a few years since, by which a higher finish is given to the paper than had previously been attainable. This consists of a damping apparatus A (Fig. 38) which is placed between the last drying cylinders B of the machine and the glazing calenders C. The damping-rolls consist of two brass or copper rolls, about 14 inches in diameter, through which a constant stream of cold water is passed, while a line of steam jets, issued from finely-perforated pipes, plays over the face of the rolls. The cold water within the rolls condenses the steam, thereby imparting a uniform moisture to the under surface of the paper, which enables it to take a better surface when passing through the glazing rolls. The steam-pipes can be regulated so as to give any amount of dampness required by adjusting the steam cocks accordingly. By reference to the engraving, it will be observed from the disposition of the rolls that the web of paper is reversed, thus equalising the moisture on both sides, by which the paper-maker is enabled to produce an evenly-finished paper.

The chilled-iron glazing-rolls, as originally introduced, were fitted up in stacks of seven, and sometimes as many as nine rolls, but it was found in practice that so large a number of rolls gave unsatisfactory results; the heavy pressure, acting on the paper immediately after leaving the drying cylinders, had the effect of "crushing" the paper, giving it a thin feel. It is now considered preferable to use calenders having not more than four, or at most five rolls. An arrangement of this description, manufactured by Bertrams, is represented in Fig. 39. The system recommended by Mr. Dunbar is to employ three sets of rolls, disposed as follows:—"First, a set of three rolls; second, a set to consist of four rolls, and a stack of five to give the finishing or dry surface. With this arrangement of calenders, and the assistance of the damping apparatus, any desired surface can be got by varying and regulating the drying of the paper, which any careful machine-man can do with ordinary attention."

Finishing.—To give a still higher finish to the paper, it is subjected to what is termed "friction-glazing," which consists in passing it through a stack of rolls, formed alternately of small iron rolls and larger paper ones, the iron rolls revolving at a much higher speed than the paper-rolls. The effect of this final glazing operation gives the paper a very fine surface.

Plate-Glazing.—Donkin's Glazing Press.—This term, which is also called "super-calendering," is applied to a method of glazing hand-made paper, and is also adopted for the better qualities of machine-made paper. It consists in placing sheets of paper between highly polished plates of either copper or zinc, the latter being more generally used. The metal plates, with the sheets of paper placed alternately between them, are made up into packs or "handfuls" (the operation being usually performed by women), and these are passed between two powerful rolls, giving a pressure of from twenty to thirty tons, and each pack, consisting of about forty plates and as many sheets, is passed through the rolls several times, the pressure being regulated by means of screws or levers and weights acting on the ends of the top roll. A machine for glazing paper in packs, manufactured by Messrs. Bryan Donkin and Co., is shown in Fig. 40. Some descriptions of paper, as "antique" and "old style," for example, are surfaced with good cardboard instead of copper or zinc plates. As soon as the handful has passed through the rollers, the motion of the machine is reversed, by which means the pack is made to pass forwards and backwards repeatedly, according to the extent of gloss or smoothness required.

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."

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.

Bertrams' Single-sheet Cutter.—In cases where it is necessary that the sheets should be cut with great uniformity, as in the case of paper bearing a water-mark, in which it is requisite that the design should appear exactly in the centre of the sheet, the ordinary cutter is not found to be sufficiently reliable; a machine termed a "single-sheet cutter" is therefore used for this purpose, of which an illustration is shown in Fig. 42. The paper is led direct from the paper-machine, or from a reel frame, to the drawing-in rolls, A; after which it passes through the circular slitting-knives, B; from here it is led by the roller C to a large wood-covered drum, D, and at the front of this drum the sheets are cut by the cross-cutting knives, E. There are two cast-iron tapered cones, with belt guide for adjusting the speed; a fly-wheel to promote steadiness in working; a series of wrought-iron levers, cranks, eccentrics, shafts, etc., for accurately regulating the travel of paper and the cut of the horizontal knives; a small pasting table is also fitted across the machine for mending broken sheets.

Packing the Finished Paper.—The paper, after it leaves the cutting-machine, is conveyed to the finishing-house, where it is carefully examined by women, who cast aside all defective or damaged sheets, which, under the trade names of "imperfections" or "retree," are sometimes disposed of, at a lower rate, to the customer for whom the order is executed. In the warehouse these imperfections are marked with a capital R on the wrapper, or two crosses, thus X X. If the paper is broken, it is sometimes marked B X X; it is not generally the custom, however, to sell imperfections, but to return them to the beater-man, to be re-converted into pulp. The perfect sheets are then counted, and packed up in reams consisting of 480 to 516 sheets.

Sizes of Paper.—The various sizes of paper are known in the stationery trade under different designations, as demy, crown, double crown, royal, imperial, etc. As paper is generally purchased according to weight, the various weights per ream are also distinguished with the size of the paper, as 16 lb. demy, 22 lb. double crown, and so on. The following table shows the sizes of some of the writing and printing papers in common use:—