If any pattern or name is required on the paper, it is produced by means of a light skeleton roll, called a “dandy-roll,” covered with raised wires in the form of the desired pattern, {156} placed between the suction-boxes, and pressing lightly on the still moist paper. The paper is thinned where the wire pattern presses, and thus a mark (water-mark) is produced. The other side of the paper has a mark corresponding to the wire-cloth; by using a dandy-roll covered with wire-cloth, the two sides can be obtained alike, such paper going by the name of “wove.”
Paper in which a series of parallel lines are produced is called a “laid” paper.
Some water-marks are produced by means of a dandy-roll in which the pattern is formed by depressions in the surface. The paper is thus thicker where the pattern is formed than in the ground-work. De la Rue (Patent No. 8348, 1884) has patented the use of dandy-rolls so formed as to produce upon the paper the effects both of thickening and thinning.
Imitation water-marks can be produced on the finished paper by subjecting it to pressure in contact with plates on which the design has been produced in relief. In this way very beautiful results can be obtained. (See Patent No. 13,455, 1884.)
It sometimes happens that the wire-cloth slips slightly to one side. This can be obviated by the machine-man shifting, by means of screws, one of the rolls provided for the purpose with a movable journal, until its axis is at a slight angle to that of the other rolls. An automatic apparatus has been invented for this purpose. Two brass plates are fixed, one on each side of the wire-cloth, to a long rod, connected by suitable machinery with the screws working the movable journal, so that, as the wire presses against one or the other of these plates, the roll is shifted so as to correct this.
The paper, which, even after passing the suction-boxes, is still very wet, passes with the wire-cloth between the couch-rolls G G′. These are hollow copper or brass cylinders, covered with a tightly-fitting endless jacket of felt. They may also be made of wood (sycamore or mahogany) or of iron or iron and brass combined. The pressure of the upper couch-roll upon the lower can be regulated by means of {157} screws or levers. They serve to press out water from the paper, and to detach the paper from the wire-cloth. By dexterous manipulation on the part of the machine-men, the paper is transferred to the endless felt, travelling over the rolls k in the direction of the arrows. It is known as the “wet felt,” from the condition of the paper at this stage. In its passage along this felt, the paper passes between two iron rolls K, called the first press-rolls, with the object of having the water squeezed or pressed out of it. These rolls are sometimes covered with a thin brass case, and the top one is provided with an arrangement called the “doctor,” in order to keep it clean, and free from pieces of paper that may have stuck to it. The lower press roll is sometimes covered with an indiarubber jacket. The “doctor” is a kind of knife placed along the whole length of the roll, and pressing against it at every point. The pressure on the rolls can be regulated by means of levers, or, as in the illustration, by the screw k′. It will be readily seen that the under side of the paper that has been next to the felt will, in its still moist condition, have taken to some extent an impression from the felt, while the upper side will have been made comparatively smooth by the pressure against the top roll of the 1st press. In order to make both sides of the paper as nearly as possible alike, it is passed through another set of rolls L, called the 2nd press. This time it is reversed, and enters at the back of the rolls; thus the other side of the paper is next the metal, being taken through by the felt (called the “2nd press felt”) travelling on the small rolls l, the paper, after leaving the wet felt, and before being taken on to the 2nd press felt, travels over the rolls l′. The 2nd press felt is necessary, because the paper is too tender to withstand, unsupported, the pressure of the rolls.
FIG. 53.
The paper, after passing the 2nd press rolls, travels over the drying-cylinders M, the number of which varies somewhat. In the machine shown, there are in all eight cylinders. Sometimes as many as twelve are employed. Between the {158} 2nd press rolls and the cylinders, a passage S allows easy access to the other side of the machine. The paper generally passes alone over the first two, which are only slightly heated; afterwards it is led over the others by means of felts, as shown. The arrangements shown at R are for the purpose of stretching the felts. The cylinders are heated by means of steam, and are generally divided into two sets, between which is a pair of chilled-iron, highly-polished rolls N, called “smoothers,” the function of which is sufficiently explained by their name. They are also heated by means of steam. The cylinders are usually made of slightly decreasing diameter, in order to allow for the shrinking of the paper on drying. Messrs. G. & W. Bertram introduce into some of their machines one or two small drying cylinders, over which the 2nd press felt travels, the object being to drive off some of the moisture absorbed from the paper. This contrivance is said to give excellent results, and to ensure a considerable saving in felts. The arrangement is shown in Fig. 53. It is especially useful in machines running at a high speed, or in those having limited drying power. After leaving the cylinders, the paper should be quite dry; it is then led through the calenders, of which there are in some machines {159} as many as three sets, though only one is shown. These are similar to the smoothing-rolls, just described. Pressure is applied by the screws O′, or by levers and weights. The friction of the hot calenders on the dry paper develops a large quantity of electricity, which occasionally discharges itself in bright sparks.
It is the practice in some mills to cool the paper before passing it through the calenders. This is effected by passing it over a copper cylinder, through which a stream of cold water runs.