Part 13

Notwithstanding the excellent qualities of the stop-cylinder press, commercial necessities often demand a sacrifice of quality to speed, and this has brought the two-revolution press into very general use. As the name implies, the cylinder makes two revolutions, one to print the sheet, and the other, an idle one, to allow the bed to return. While the bed is returning, the impression cylinder is lifted to clear the type-form. As the cylinder rotates continually at a uniform speed, the type-bed must also travel at a constant speed. The reversal of the movements of the bed must, therefore, take place in a short space of time.

The study of inventors has been concentrated upon this subject more than upon any other connected with flat-bed presses, and hundreds of patents for "bed motions" have been taken out. Considering the fact that in the larger presses the weight of the bed and form is about one and a half tons and that this weight moving at a speed of about six feet in a second must be brought to a full stop and put into motion again in the opposite direction at full speed in about one-quarter of a second, it is obvious that the problem was not an easy one, especially when the reversal of the bed must be accomplished without a jar or vibration. The mechanism employed has always been a driving gear and one or two toothed racks. In Koenig's original movement, the driving gear on the end of a rising and falling shaft ran on top of a rack attached to the bottom of the bed in order to drive the bed in one direction, and then descending around the end of the rack ran in the bottom to the same rack to drive the bed in the other direction and ascending at the other end to repeat the movement. This, as already stated, has proven a very efficient mechanism and is employed, with improvements, by some of the press manufacturers of the present time.

In a pamphlet entitled "A Short History of the Printing Press" (New York, 1902), by Robert Hoe, the writer describes a method of reversing the bed. Although somewhat technical, it seems desirable to quote him as follows: "As early as 1847, Hoe & Co. patented an entirely new bed-driving mechanism. To a hanger fixed on the lower side of the bed were attached two racks facing each other, but not in the same vertical plane, and separated by a distance equal to the diameter of the driving wheel, which was on a horizontal shaft and movable sideways so as to engage in either one or other of the racks. By this means, a uniform movement was obtained in each direction. The reversal of the bed was accomplished by a roller at either end of the bed entering a recess in a disc on the driving shaft, which in a half-revolution brought the bed to a stop and started it in the opposite direction. This involved a new principle; a crank action operating directly upon the bed from a shaft having a fixed centre, and within recent years modifications of this patent have been successfully employed to drive the type-bed at a high velocity and reverse it without a shock or vibration."

This invention appears to have been the forerunner of the more recent improvements in bed motions. A notable one is that employed in the Miehle presses, which have gained much celebrity, run at a high rate of speed, and are used in many printing-offices in this and other countries. The reversal of the bed movement is accomplished by a so-called "true crank" movement and with an absence of jar and vibration never before obtained in any other than the stop-cylinder presses.

At the present time, the latest development in printing presses is Hoe & Co.'s new two-revolution press, in which, also, the reversal of the bed is accomplished by the true crank movement, but with an improvement which brings it to an easy stop and returns it without the least vibration.

On all two-revolution presses there are employed, to assist in the reversal of the bed, air-chambers or cylinders, without which the reversing mechanisms could not withstand the enormous strain to which they are subjected. These are iron cylinders, closed at one end, approximately six inches in diameter and eighteen inches long, and varying in size according to the size of the press. Some presses have two and others four of these cylinders, one or two at each end. The open ends of the cylinders are toward the bed, and attached to the bed are two or four pistons which enter the air-chambers as the bed nears the end of its stroke. The compression of the air in the cylinders makes a cushion and checks the momentum of the moving bed. The pistons can be adjusted to regulate the air compression to suit the velocity of the bed and the weight of the form, which vary in different kinds of work.

The delivery of the printed sheets is performed either by a delivery cylinder or by a front delivery with the printed side of the paper uppermost as already described for the stop-cylinder presses. Grippers are not used in the front delivery carriage, as the sheet is discharged from the cylinder by its continuous rotation.

The average running speed of a two-revolution press is about one-third greater than that of a stop cylinder, or about eighteen hundred impressions an hour, as against from one thousand to thirteen hundred and fifty impressions from the stop cylinder, this being the comparison in presses of the average size, printing sheets about 33 x 46 inches. The driving power required is in the proportion of about five for the two-revolution press to three for the stop cylinder, and the wear and tear is in about the same proportion.

Another press, which is still employed to a small extent for book-work, is the flat-bed perfecting press. This press is virtually two two-revolution presses combined into one, with the advantage that they require only one man as "feeder," but with the disadvantage that they produce only about two-thirds as much work as two separate single-cylinder, two-revolution presses. Their greatest disadvantage lies in the difficulty of preventing the fresh ink on the side of the sheet first printed from "setting off" on the packing of the cylinder which prints the reverse or second side. Mechanisms are employed to move the "tympan sheet" or outside covering of the second cylinder along at fixed intervals, but they are complicated and troublesome. These presses are expensive and cumbersome, and can generally be used only for inferior grades of work in large editions. Under the care of a skilful and painstaking pressman, good work can be produced from them, but fine book-work is always done on stop-cylinder and two-revolution, single-cylinder presses, which have now been brought to a high state of perfection.

Nearly a hundred years ago Hansard wrote, "The printing machine in its present state appears susceptible of little improvement." He was, in truth, right so far as the main principles of the flat-bed cylinder press are concerned, but there have been immense improvements in many of the details. With the introduction of automatic sheet-feeding devices, and improvements in the driving, inking, and delivery arrangements, mechanical ingenuity seems to have been exhausted. The temptation is strong to apply Hansard's prediction to the flat-bed cylinder press of the present day, but with the many surprises that meet us in other fields this would border on temerity.

Already there have been great advances in adapting the entirely rotary principle to the printing of high-grade work, although its use is still restricted to the production of large editions.

As early as 1852 Hoe & Co. made a rotary press for D. Appleton & Co., especially for printing the famous Webster spelling-book. The types were locked up on the cylinders in curved beds, called "turtles," and the sheets were delivered by a sheet-flier. Probably thirty million copies were printed on this press, which was dismantled nearly twenty-six years ago.

In 1886 this same concern made a press which is still used for printing some of the forms of the Century Magazine. This press had two pairs of cylinders, and curved electrotype plates were used on it. The paper was in a roll at one end, and at the other end there were delivered, to each revolution of the cylinders, eight eight-page signatures already folded to the size of the Century page. This was the first rotary press made for a good grade of book-work. Two similar presses were afterward made for Harper's Weekly and for the Strand Magazine of London.

What is known as the rotary art press was made in 1890 for printing the fine half-tone illustrations in the Century Magazine.

This has one plate cylinder and one impression cylinder, and curved electrotype plates are used. The sheets are "fed" by hand in the usual manner, and are printed on one side at a time and delivered by a sheet-flier. It produces as much work as four flat-bed cylinder presses and of better quality. The plates are inked by sixteen rollers. The performance of this press is another demonstration of the superiority of the rotary over the flat-bed principle of printing.

Since then hundreds of rotary presses have been made for magazine and book printing, most of them equipped with attachments for folding the sheets as they are printed, and all having a high rate of speed. C. B. Cottrell & Co. have made many rotary presses for magazine printing, most of which deliver the sheets flat, without folding, and most of them made to suit some predetermined size or sizes of sheets or pages.

In the evolution of the printing press there are three sharply defined stages: first, the flat impression surface and the flat printing surface, requiring the exertion of all of the impressing power upon the entire surfaces; second, the cylindrical impression surface and the flat printing surface, requiring the exertion of all of the impressing power upon only a narrow line or a small portion of the printing surface; third, a cylindrical impression surface and a cylindrical printing surface, still further reducing the area upon which all the impressing power is exerted.

Just as the second stage has, particularly for book-work, virtually superseded the first, so the third is destined to supersede the second. It is only an adaptation of the means to the ends. The mechanical principles of the rotary press are, in fact, simpler than those of the flat-bed cylinder press, and it may be said that so far as the purely mechanical part of the press is concerned, they have been fully developed, but much still remains to be done in other directions. The variety in the sizes of the pages of different books, the smallness of the editions, and the fact that the finer grades of paper, especially coated paper, cannot be obtained in roll form, are obstacles to be removed. As most book forms are electrotyped for flat-bed presses, and as it requires but little additional expense to curve the plates, this one item is not much of an obstacle to overcome. It is, however, still difficult to curve the plates perfectly, and the pressmen, even if they can produce excellent work from flat-bed presses, require considerable training if they have had no experience on rotary presses. All these difficulties are sure to be overcome in time.

PRINTING INK

By James A. Ullman.

The process of making printing ink consists of grinding a pigment, black, white, or colored, into a suitable varnish. The pigment is that constituent which makes the impression visible, while the varnish is the vehicle which carries the pigment during the operation of grinding and during its distribution on the press to the type, from the type to the paper, and ultimately binds it to the paper.

A complete factory for the production of printing ink consequently consists of three distinct plants,--one for the production of the varnishes, one for the manufacture of the pigments, and one for the grinding of the pigments into the varnishes.

Roughly speaking, the varnishes are divided into three classes, the first and second of which are the varnishes proper, i.e. the resin and the linseed varnishes, while the third class consists of dryers, etc., whose purpose is to influence the drying and consistency of the inks.

Taking up first the proper varnishes, we find that these are produced by the destructive distillation of resin in huge cast-iron stills. By this process, the solid resin of colophony is split up into water, various resinic acids or naphthas, and resin oils of various specific gravities and consistencies, all of which are separated from each other into separate containers which are ready to receive them. As one distillation is not sufficient to purify the resin oils from the water and acid, which would not only give the resulting ink an obnoxious odor but be detrimental to type, plates, etc., the distillation is repeated a number of times until the oils become perfectly pure. The grades of varnishes made from these resin oils are used for the cheaper classes of printing inks, not only on account of their lower cost, but because they are more suitable for the class of work for which such inks are used.

The linseed varnishes are made by boiling refined linseed oils at a very high temperature. The linseed oil loses its acrid elements by volatilization, and gradually becomes thick and viscous, the various "numbers" or consistencies of these varnishes being dependent upon the length of time during which the oil is subjected to the process, and to the temperature applied.

The dryers are made by adding to the linseed oil during the boiling, suitable oxidizing agents, such as compounds of lead or manganese, by means of which the oil is chemically affected, i.e. it is oxidized. Such dryers, when added to printing ink, attracts the oxygen of the air and transfer it by catalytic action to the varnish of the ink, thus causing it to oxidize more rapidly, or to become, as it is commonly called, dry.

Having disposed of the manufacture of the varnishes and dryers, we now come to the manufacture of pigments. This is such a large field that it can be only cursorily covered within the limits of a short article. The pigments are of many kinds and classes. The blacks alone would form a large chapter by themselves; yet all of them consist of carbon, produced by the combustion of hydrocarbons of various kinds, and according to their origin they are the so-called carbon blacks, lamp blacks, spirit blacks, oil blacks, Frankfort blacks, etc., each of which has its distinct and peculiar properties and value for its specific purpose.

The other pigments fall naturally into two divisions,--chemical colors and the so-called "lakes." The chemical colors are in general of mineral origin, produced by the action of one chemical upon the other, or in some cases by physical or chemical action upon earths and ores. In the first group, we have such colors as vermilions, white lead, chrome yellows, the ferrocyanide blues (Milori blues, bronze blues, Prussian blues, Chinese blues, Antwerp blues, Paris blues, Berlin blues), ultramarines, etc.; in the second group, such colors as cyanides, umbers, Indian red, and many others.