US2182559A - Photogravure printing plate and method of and apparatus for producing the same - Google Patents

Description

Dec. 5, 1939. c. L. HENDERSON PHOTOGRAVURE PRINTING PLATE AND METH 2,182,559 on OF AND AME 2- Sheets-Sheet 1 APPARATUS FOR PRODUCING THE 5 Filed Oct. 16, 1936 ol oyravare Prior A'I'Z fi enderson,

fizz/ezzfi U/Zari'esi. Henderson m4 Z a Dec. 5, 1939. c. L. HENDERSON 2,182,559

PHOTOGRAVURE PRINTING PLATE AND METHOD OF AND APPARATUS FOR PRODUCING THE SAME Filed Oct. 16, 1936 2 Sheets-Sheet. 2

v fizz/en? cfarfesiqendrson Patented Dec. 5, 1939,

PHOTOGRAVURE PRINTING PLATE AND METHOD OFAND APPARATUS FOR- PRO- DUCING THE SAME Charles L. Henderson, Appleton, VVis., assignor to Paper Patents Company, Neenah, Wis., a corporation of Wisconsin Application October 16, 1936, Serial No. 105,852

6 Claims.

' My invention relates to the general art of photogravure printing, and has particular reference to intaglio printing plates and to apparatus and methods for making such plates.

The present application is a continuation in part of my co-pending application, Serial N0. 702,549 which was filed on December 15, 1933 and which is assigned to the assignee of the present invention.

As is well known in theart, the printing plates employed in photogravure printing and the cylinders or rolls used in the related process of rotogravure printing are formed with printing surfaces comprising a large number of ink receiving recesses or cells which are separated from each other by relatively narrow, criss-cross areas over which a cleaner or doctor blade rides after the plate or roll has been inked, to remove surplus ink. These cells are of uniform size but of varying depth. The deepest cells, of course, receive the greatest amount of inkand print the darkest tones in the picture. Cells intermediate in depth print shaded or lighter tones in the picture, while the,highlights are printed by very shallow cells holding a small amount of ink. v

In brief, in ordinary photogravure work, the tone value depends on the depth ofthe etched cell, and at first glance, it would seem that the principal problem to be overcome in the provision of satisfactory photogravure printing plates is the attainment of proper gradation in the depth of the ink bearing cells or recesses. There are, however, other considerations which are of equal or greater importance. The cleaning or doctor blade rides directly upon the surface of the plate or roll andconsequently wears that surface to some extent during the printing operation. Hence, the lightest highlight cell must be etched sumciently deeply that its deflningwalls shall not be worn away"during.the several thousand or hundreds of thousands of printing impressions. Thus, to preserve .proper tonal values, the full tone cells must be etched very much deeper. than the highlight cells. The usual result is that the full tone cells carry more ink than is desirable for good printing, and flooding or worminess may result in the deeper tone portions of the finished print, together with loss of detail in the shadows. .A substantial waste of ink is also involved; Moreover, this unequal depth of the ink cells renders the exact duplication of prints, which is essential in work such as wall paper printing, substantially impossible of attainment, because even slight wearing of the supporting surface for the doctor blade during the use of the plate progressively changes the tonal values inthe resultant print sumciently that first impressions from a plate difier materially from later impressions. An important object of the present invention, therefore, is to provide an improved photogravure printing surface for intaglio printing plates and rolls which shall make possible a full range of tonal values with ink recesses or cells which are so proportioned that the normal wearingof the printing surface by the doctor blade has little, if any, effect upon the finished print. To accomplish this object I provide a printing plate wherein the tonal gradations are accomplished by varying the area of the ink containing recesses and,

as shall hereinafter appear, so control the size, shape, and arrangement of these recesses that a full range of tonal values may be accomplished without flooding or excessive use of ink.

is to provide an intaglio printing plate wherein the ink recesses shall be substantially square or rectangular. This object is particularly diflicult of attainment, due to the tendency of gravure dots toetch round and its accomplishment, as will be made apparent in the subsequent description of the preferred method of accomplishing the invention, comprises one of the most important features of the invention.

Heretofore photogravure plates and rotogr vure rolls have been substantially impossible of duplication due to the inability of even a highly skilled operator to accomplish exactly similar etching of an exposed carbon tissue on the surface of the printing plate or roll. A further object of the present invention, therefore, is to provide apparatus and processes whereby exact duplication of photogravure plates and rolls may be accomplished, thereby making possible the use of photogravure rolls for printing wall paper and similar products which of necessity must be very uniform and capable of exact duplication.

A further, and to some extent ancillary, object of the present invention is to provide an improved method for making photographic positives suitable for exposing photogravure printing defining the supporting lands for the doctor or cleaner blade formed therein, and which shall be provided with ink recess defining dots of such shape that the corresponding recesses or cells in the finished plate shall be substantially square after etching is completed. Still further objects and advantages of the invention will be set forth in the concluding portions of this specification.

To assist a clear understanding of the invention and especially the structural characteristics of the intaglio plate itself, the methods of preparing the positive of the picture or design which is to be transferred to the printing plate, and the manner in which the transfer is accomplished, reference may be had to the following description and the accompanying drawings wherein-,-

Figure 1 is a perspective view of a fragment of an intaglio printing plate such as is at present used in photogravure and rotogravure printing;

Figure 2 is a similar view of a portion of my improved intaglio plate for photogravure print- Figure 3 is an illustration copied from an actual photomicrograph showing the dot formation in the full tone darks of the master positive photographic transparency utilized in making intaglio printing plates in accordance with the present invention;

Figure 4 is an illustration similar to Figure 3 showing the arrangement and-the shape of the etched recesses in a photogravure printing plate formed by the processes of the present invention;

Figure 5 is a diagrammatic view of a photographic set up for making the master positive photographic transparencies utilized in the processes of the present invention;

Figure 6 is a fragmentary view of the screen used in the set upshown diagrammatically in Figure 5;

Figure '7 is a view of the stop used in this set up; and

Figure 8 is a diagrammatic view illustrating one manner in which the sensitized surface of the printing plate or roll may be exposed perparatory to the etching operation.

It is to be understood that wherever in this description the term printing plate is used, the

same is intended to include either a flat printing plate or a cylindrical printing roll.

The process of the present invention reduced to its simplest aspects consists in the provision of a master photographic positive of a particular type which can be photographically exposed and thereby imprinted directly upon the printing surface of the printing plate or roll, that surface having previously been sensitized by one of the known sensitizing mediums. The master positive is produced by a line screen photographic process, similar in some respects to that used in half tone work, which produces a finished printing surface on the etched printing plate or roll wherein the ink recesses in the full tone darks constitute substantially square recesses separated by thin lands having the form of intersecting rectangular grids, as is illustrated in Figure 4. Not only is the arrangement of the recess dots in this master positive different from that in any comparable process heretofore known to the art, but the shape of the recess forming dots on the photographic positive is likewise novel and is such that the shape of the ink recess is controlled during the etching process to accomplish the desired square formation.

The first step in the preparation of the master positive consists in the making of a negative of the subject or copy preferably on a photographic dry plate. This negative need not be of the same size as the finished work, but should be large enough to make retouching easy and practical should retouching be desired. The negative is made in the usual way, although in some cases it may require special attention as to density in order to get the proper dot formation on the positive plate. A photographer properly skilled in the photo-engraving art will have no difficulty in obtaining the desired results.

If the finished work is to contain printed matter, the printing can either be added to the copy before the negative is made, or a negative of the lettering can be made and stripped in on the negative in the usual manner.

When the negative has been made, the next step is to make the photographic positive which is subsequently photographically printed upon the sensitized surface of the plate or roll. This operation is as follows:

Referring to Figures 5, 6, and '7, the negative N is mounted in a suitable frame F a foot or so in front of a white background B which is illuminated by two or more lamps L. A conventional half-tone or black line screen S and a ground glass are mounted in the camera. The camera is then adjusted to produce the correct size positive and for focus. The distance between the ground glass and the screen is then so adjusted that when a stop C of the proper size and shape is inserted in the camera lens, the particular required dot formation, which will be described in detail in a later paragraph, will be produced.

As is well understood by photo-engravers, the half-tone screen has the effect of converting the relatively large areas of the picture into small separate dot images corresponding in number and location to the meshes of the screen employed. These images produce developable areas in the resultant photographic positive (or negative) which are in the form of dots, the area of each dot being substantially proportionate to the amount of light which passes through the particular meshes or openings on the screen which are responsible for the formation of that dot. Such effect is well recognized by photo-engravers.

In making a photographic plate or film for use in a half-tone setup, the usual round stop or one of the many conventional stop openings may be used in the camera lens. These stops as now used result in the production of images which, in the intermediate or dark tones, are in the form of black squares, the sides of which are disposed at an angle of 45 degrees to the direction of the lines of the screen employed. The efiect, in the case of a tone of a certain depth, is to produce developable areas corresponding almost exactly to the black squares of a black and white checkerboard. In the lighter portions of the picture the images are in the form of smaller similarly arranged squares with blunt or rounded corners which are slightly separated from each other, and as the tone becomes lighter the separate round corner squares will become smaller and more nearly approach the shape of a circle. In the darker parts of the picture the black check squares of the checkerboard design are larger and consequently overlap each other at their corners so that they lose their separate identity. As the tones get deeper, the areas of the separate white or unexposed parts of the picture degenerate in size and their shape approximates that of a circle, being the reverse of the effect present in .the lightly exposed parts of the picture. a

As mentioned previously, the basic feature of the present invention consists in the provision of a master photographic positive which may be exposed directly upon the sensitized surface of the plate or roll to be etched, this transparency embodying recess forming dots of a particular shape and particular arrangement. In attempting to produce a photographic positive of this type by any of the ordinary photo-engraving processes such as the half-tone process, I found myself confronted by an apparently insuperable difficulty. I" found that the parts of the plate which represented the darks of the picture would consist of large recesses in which small white dots occur as islands separated from each other. Obviously, such a plate could not be employed in photogravure and similar printing processes because of lack of continuous support or bearing for the doctor blade. Therefore, it became necessary to devise some method of enlarging these small islands so that they would be connected together and, in effect, become lines. In other words, I had to have in my dark printing areas, the same. continuous mesh effect as occurs in the ordinary photogravure printing plate or roll made by the carbon tissue method.

To overcome this diiiiculty, I conceived the idea of so controlling the dlspersion through the screen in the usual half-tone photographic setup, that the dot formation in the portions of the photographic positive which correspond to the full tone darks of the picture comprises a series of developable images, each having the general form of a square with indented sides, arranged in rows and separated by undevelopable lines,

which define, in effect, a series of rectangular,.

intersecting grid lines of material thickness and constituting, in effect, a fine, square mesh of screen-like pattern.. The dot arrangement resulting from the process of my invention is not the same as the dot arrangement of the halftone process wherein the dots correspond, in certain tones, to the black square of a black and white checkerboard. In a photographic positive produced by the process of my invention there are dots in the full tone darks corresponding to both the black and the white squares of a black and white checkerboard. Obviously this arrangement accomplishes the mostefiective utilization possible of the available printing surface.

I obtained this result by using a cross-shaped lens stop, the slots of which are parallel with the screen diagonal, i. e. the sides of the slots are arranged at an angle of degrees to the lines of the screen. Practical tests show that this arrangement fully solves the problem and gives the desired result.

The cross-shaped stop C on the camera lens, in cooperation with the screen S. causes the light rays from the negative N being photo graphed to form on thepositive plate or film P a multitude of light-struck dots which in the full tone darks of the picture are disposed in longitudinal and transverse rows and are of varying areas, those from the blackest portion of the subject being relativelylarge and separated by narrow lines, those from the intermediate or shaded portion of the subject being smaller and separated by'wider lines, and those from the high lights of the subject being still smaller and separated by still wider lines or lands. All of the light-struck dots are of substantially equal intensity, tonal variation being accomplished by variation in the area of the dots. This effect is typically illustrated in Figure 2, which shows a fragment of the intaglio printing surface after the transfer from the developed positive transparency to the surface of the printing cylinder, and after the latter has been etched..

The purpose or function of the stop C at the camera lens is to confine the developable light.

sensitized areas between the screen line shadows on the light sensitive surface, and thus maintain the continuity of the nondevelopable parts corresponding to the screen lines, while at the same time producing dot areas in the full tone darks of the transparency which will etch square. The size and shape of the cross-shaped stop C should vary with the screen used to get the best results. This can best be determined by experiment. I have obtained good results from the use of stops in whicheach of the two slots of the cross was one to one and one-half inches long and from one-eighth to three-sixteenths of an inch in width in connection with half-tone black line screens with 100 to 150 lines to the inch with a ratio of lines to spaces of approximately one to one. The fineness of the screen used depends on the character of the work and upon the judgment of the camera operator or. in some cases, it may be specified by the customer. The proper angular relation of the cross stop to the screen line (45 degrees) is shown in Figures 6 and '7. The distance between the plate and the screen necessary to produce the desired dot shape, may be between one-eighth inch and three-elghths inch. These figures apply to a standard engravers lens of twenty-five inch focal length and a 100-150 line screen, for screens of differing fineness other dimensions easily determin'able by the engraver would be used.

When the camera has been adjusted, the operator inserts a photographic plate or film in the camera after removing the ground glass. The plate or film is then exposed, the time of exposure depending on the nature of the subject and preliminary tests made by the operator. The positive plate or film is then removed from the camera and developed and fixed in the usual manner. The positive plate or. film now shows the arrangement but not the shape of the dots as they will appear as etched cells in the finished etching.

A fiat plate suitable for intaglio printing can readily be prepared from the developed master photographic positive made as above described. To do this all that is necessary is to make a contact photographic exposure of the photographic positive upon the resist treated surface of the ultimate metal plate, which is then developed and etched simply by immersion in suitable acid for a predetermined time. The complicated etching processes employed in rotogravure are found to be unnecessary in the processes of this invention, and hence the exact duplication of etching necessary in making duplicate printing surfaces may be readily attained. &

The resultant etched plate is, however, very difierent from the plate which results from a carrying out of the half-tone process or the ordinary rotogravure process. Each of the dots in the master photographic positive which produces the ink recesses or cells in the finished plate has. as mentioned previously, the general form of a square with indented sides. Dots of this general shape tend to etch square, and by proper selection and arrangement of the cross shaped stop, it is a relatively simple matter to provide recess forming dots which in the full tone darks of the finished plate will etch almost perfectly-square, such etched cells being illustrated particularly in Figure 4. This figure has been made from an actual photomicrograph of a printing surface made in accordance with the present invention. The ink recesses in the other portions of the etched plate likewise etch substantially square, those in the full tone darks of the plate being of the maximum possible size and separated by very thin supporting lands for the doctor blade, these lands consisting of two perpendicularly, intersecting sets of equi-spaced, parallel, continuous lines of material, and the recesses in the other portions of the plate being of less area and are separated by wider lands constituting, in effect, extensions of the narrow lands separating the full tone recesses.

All of the ink recesses in the etched plate are of substantially uniform depth over the entire printing surface, hence the normal wearing of that surface by the doctor blade during the printing operation has very little, if any, eifect upon the prints produced by the printing surface. This means that a photogravure printing surface in accordance with the present invention may be utilized for printing wall paper and other types of continuous patterns wherein exact duplication of plates throughout the entire operative life of the printing surface is imperative. Further, since the master positive may be exposed directly upon the sensitized surface of the unetohed printing plate or roll, exact duplication of plates and rolls is possible. In the rotogravure process where the etching is more or less of a blind process, it is, of course, practically impossible to ever exactly duplicate a printing surface.

Because the photogravure method of printing is particularly adapted for use in rotary printing presses, I have developed in connection with my invention a method and apparatus for printing the master positive upon printing cylinders or rolls. Such an apparatus and method is described at length in my co-pending application, Serial No. 702,549, which was filled on December 15, 1933, and which is assigned to the assignee of the present invention.

This process is illustrated diagrammatically in Figure 8, the printing roll being indicated at 5 in that figure and the master positive being indicated at 6. The apparatus includes means for moving the master positive along and in contact with the sensitized surface of the roll while at the same time directing a suitable beam of light through the positive. The reflector system for accomplishing this is indicated at I, and the light source, which may comprise arc lamps or similar equipment, is indicated at 8. The linear speeds of the transparency and the sensitized surface of the roll are, of course, maintained at an equal value by suitable apparatus (not shown), and if the subject is a continuous pattern such as wall paper, suitable precautions are taken to produce a perfect join-up on the printing roll. It is possible by this process to produce any number of rolls which are exact duplicates of each other and which may be used to produce prints which are undistinguishable from one another. The advantages of such an arrangement in the manufacture of wall paper and all-over design patterns is clearly apparent.

From the foregoing it will be seen that I have disclosed an improved method and apparatus for manufacturing a novel type photogravure printing plate or roll which possesses many advantages over the processes heretofore known to the art. I have also disclosed how a photogravure printing plate of this novel type may be readily produced through the use of photo-engraving equipment which is generally available in all well equipped photo-engraving establishments and without the exercise of unusual skill upon the part of the operator. Particularly, I have shown how to produce aphotogravure printing plate having ink recesses of substantially uniform depth which in the full tone darks of the plate comprise substantially square recesses separated by lands having the general form of intersecting, rectangular grids of relatively narrow width, and which in the lighter tones of the plate comprise substantially square recesses of less area separated by wider lands constituting extensions of these rectangular grids. A printing surface of this type wears very evenly and produces prints at the conclusion of its operative life which are practically identical with the prints produced upon the first few impressions of the printing surface.

It might be mentioned that in practice the smallest recesses or cells in my improved printing plate are not of exactly the same depth as the recesses in the full tone darks. This results from the well known fact that an area of relatively large size will etch somewhat more deeply than an area of very much smaller size in the same period of time. However, the lands surrounding the smaller dots are much wider than the lands surrounding the deepest recesses, and the difference in depth is practically, automatically compensated for by the increased resistance to wearing of the lands surrounding the shallower recesses.

In the rotogravure plate, the lands are of the same width throughout the printing surface and the recesses are of widely varying depth. Hence, even slight wear of a rotogravure printing surface will produce considerable change in the finished print, especially in the highlights.

It will be apparent to those skilled in the art that a master photographic positive according to the present invention may be used for producing printing plates or cylinders by methods and apparatus other than these specific methods and apparatus which I have above described. For example, it will be apparent that my improved photographic positive can be used to produce an intaglio plate or rotogravure cylinder by the carbon tissue or other method without the necessity of employing the usual line screen. It is my desire, therefore, that the accompanying claims shall be accorded the broadest reasonable construction consistent with the state of the art.

I claim the following as my invention:

1. An etched intaglio printing surface having in its printing surface ink receiving recesses of substantially uniform depthand of area varying in accordance with the tone depth in the subject to be reproduced, said recesses being substantially rectangular in outline and substantially corresponding in location and arrangement to both the black and the white squares of a black and white checkerboard, said surface having continuous, solid, un-etched portions separating all of said recesses and providing a continuous surface over which a doctor blade may ride, said un-etched portions being in the form of two perpendicularly intersecting sets of spaced, parallel, continuous lines of material of suificient width in the portions of said surface corresponding to th deepest tones of said subject to support a doctor blade and varying in width in the other portions of said surface substantially inversely proportional to the tone depth of corresponding portions of said subject, so that the full range of tones in said subjectmay be reproduced.

2. An intaglio printing plate in which theprinting areas corresponding to the full tone darks of the subject to be reproduced consist of. separate and distinct recesses which are generally rectangular in outline, the walls of said recesses being defined by intersecting sets of substantially parallel lines, said lines in the portions of said plate corresponding to the deepest tones oi said subject being of sufficient thickness to support a doctor blade, and said lines varying in thickness in the other portions of, said plate "substantially inversely to the tone depth in the corresponding portions-of said subject whereby the printing areas for those-portions of said plate corresponding to the lighter tones of said subject consist in separate recesses, the walls of which are defined by lands constituting enlarged extensions of said intersecting lines, all of said recesses being of substantially uniform depth, and the tonal gradation in thesubject to be reproduced by said plate being created through progressive variation in the effective printing area of said recesses.

3. An improved photographic positive for use in making photogravure printing plates, having recessor cell-defining, opaque dot images formed therein which individually have the general shape of a square with inwardly bulging sides and which vary in area in accordance with the tone depth of the subject reproduced in said positive,

said dot images substantially corresponding in location and arrangement to both the black and white squares 01a black and white checkerboard, said photographic positive having continuous, unexposed, transparent areas separating said recess-defining dot images in order to provide lands in the printing plate over which a doctor blade may ride, said unexposed areas consisting 01' intersecting lines, the thickness of which varies substantially inversely with the tonedepth of the subject reproduced in said photographic positive,

said intersecting lines in the portions correspond-. 'ing to the deepest tones of the subject reproduced in said photographic positive being in the form of two perpendicularly intersecting sets of substantially equi-spaced, parallel continuous lines of suflicient width to provide suitable supporting lands for the doctor blade in the etched printing plate. the areas of all of said recess-defining dot images being substantially directly proportional to the tone depth oi the corresponding portions camera and through a conventional half-tone screen, but in which the camera lens opening is so shaped as to form in eiifect a cross having its arms arranged diagonal to the screen lines to maintain the continuity of the non-developable parts corresponding to the screen lines projected while preventing any overlap of the developalble light sensitized areas, even in the full-tone darks oi the picture, then developing the plate, then exposing the resist-treated printing surface through said positive, and then etching the said printing surface.

5. The improved method of making a full-tone positive photograph for use in the production of intaglio printing plates or rolls which consists in making a negative of the subject, and then making therefrom a positive by means of the conventionalcamera and through a conventional halftone, black-line screen, but in which the camera lens'opening is of substantially cruciform shape, the lines of the cross being disposed at an angle of substantially 45 degrees to the lines of the screen, so that the developable light-sensitized areas corresponding to the'dots formed by the screen meshes in the full-tone darks of the picture do not overlap and are confined between the screen lines projected.

. 6. An improved photographic positive for use rating said recess-defining dot images in order to provide lands in the printing plate over which a doctor blade may ride, said unexposed areas consisting of intersecting lines, the thickness of which varies substantially inversely with the tone 'depth of the subject reproduced in said photographic positive, said intersecting lines in the portions corresponding to the deepest tones oi the subject reproduced in said photographic positive being in the form oi two perpendicularly intersecting sets oi spaced, parallel, continuous lines of suflicient width to provide suitable supporting lands for the doctor blade-in the etched printing plate, the areas of all of said recessdeflning dot images being substantially directly proportional to the tone depth of the corresponding portions of the subject reproduced in said photographic positive so that the full range of tones in said subject is preserved in the etched printing plate, and the amount of inward bulgin in the recess-defining dot images being such that the recesses in the full tone darks of the printing L. HENDERSON.

plate shall etch substantially square.

Images