US2692198A - Methods of producing half-tone printing plates - Google Patents

Description

Oct. 19, 1954 M. D. WHITNEY 2,692,198

METHODS OF PRODUCING HALF-TONE PRINTING PLATES 2 Shee t s-Sheet 1 Filed Aug. 11 1950 wmmmlu ilfllfllflii F me i X; 7 INVENTOR.

Wave/e50 Mwr/vfr BY ATTORNEY Oct. 19, 1954 H TNE 2,692,198

METHODS OF PRODUCING HALF-TONE PRINTING PLATES Filed Aug. 11 1950 2 Sheets-Sheet 2 J 'INVENTOR. L Mal/Mae Q h H/T/YEY Patented Oct. 19, 1954 UNITED STATES PATENT OFFICE METHODS OF PRODUCING HALF-TONE PRINTING PLATES 4 Claims.

This, invention relates, to a method and means for producing printing plates for use in printing pictorial, illustrations upon conventional printing presses.

The principal object of the invention is to provide a method and means whereby the usual acid-etched metallic plates and the steps involved in their manufacture will be eliminated so that the process may be used by small town newspapers and other printing establishments not having the present engraving facilities available.

With the usual acid-etched plates, the depth of etch decreases as the fineness of the screen increases. This decreases the height of the printing protuberances so greatly that fine screen engravings cannot be satisfactorily used on soft papers. Another object of this invention is to produce a printing plate which will have a uniform depth of etch at all screen meshes.

Other objects and advantages reside in the detail construction of the invention, which is designed for simplicity, economy, and efficiency. These will become more apparent from the following description.

In the following detailed description of the invention, reference is had to the accompanying drawing which forms a part hereof. Like numerals refer to like parts in all views of the drawing and throughout the description.

In the drawing:

Fig. 1 is an illustration of a photographic negative from which the improved printing plate is to be produced;

Fig. 2 is a magnified view of a screen employed in the process;

Fig. 3 illustrates a sheet of transparent plastic material employed in the improved process;

Fig. 4 illustrates a plastic sheet carrying a photographic light-sensitive emulsion which forms the final printing plate in the process;

Fig. 5 is a greatly magnified enlargement of a portion of the screen of Fig. 2;

Fig. 6 is an enlarged cross-section illustrating the plate of Fig. 4, with the transparent plastic sheet of Fig. 3, the screen of Fig.2, and the negative of Fig. l superimposed thereon;

Fig. '7 is a greatly magnified view of a portion of printed matter which is finally produced from the negative of Fig. 1;

Fig. 8 is a still further magnified view of a portion of the matter of Fig. 7;

Fig. 9 illustrates the completed emulsion plate mounted on a backing block for use in the printing press; and

Fig. 10 is a diagrammatic view illustrating the use of a photographic enlarger in the improved process.

In Fig; 5 lines have. been used tov designate variations in opaqueness. In the actual screen the variations would be in density only.

The general method of reproducing photographic pictures in newspapers and. the like consists of the following steps: (1) exposing a photographic film in a press camera; (2.) making a photographic print of the film; (3,) re-photographing the photographic print through a line screen to produce a negative film of the picture in dots through the interstices of the screen; (4) photographically printing this screened negative on a metal plate having a light-sensitive emulsion to form a screened positive on the plate; and (5) immersing the screened positive plate, after development, in acid to etch away the unexposed portions of the plate face to produce the final printing plate.

The improved process eliminates steps 2, 3 and 5, that is, the original print, the screened negative and the acid etching are no longer required,

and a printing plate is produced directly from the image formed on the original negative in the press camera. In the improved process only the original press camera negative, diagrammatically indicated at I0, and a flexible final printing plate, indicated at H, are produced. There is some loss of pictorial quality each time an image is reproduced from one photographic;

material to another. By eliminating the image transfer from camera negative to photographic print and from this print to a line screen negative much more of the original quality of the image is retained by the use of the process, herein outlined.

The final printing plate consists of a sheet of synthetic resin or other flexible plastic material.

One surface of the plate I I is coated with a rela-.

tively heavy, photographic, light-sensitive emulsion l2 to which a reversed positive image of the negative H1 is transferred through a screen 14 and through a transparent plastic sheet l3.

The positive image on the emulsion I2 is then developed, treated, and hardened. The treatment of the emulsion causes the portions thereof which have not been exposed to light to be lowered and the portions which have. been exposed to be raised above thenormal emulsion surface to produce the final printing plate ll. Only the raised light-exposed portions of the plate will contact the inking rollers of the'press and the paper upon which the picture is to be printed.

The usual line screen consists of a series of opaque crossed lines, the lines being of uniform density throughout their widths. The improved screen l4, however, consists of a series of crossed lines of varying density throughout their widths, as shown in the magnified view of Fig. 5. Each lineconsists of a completely opaque medial line 3 I6 bordered at both sides with portions II of varying density. The variable density borders II on the lines It shade out gradually from the completely opaque line I6 to substantially transparent edges which border transparent interstices or open spaces I8. The complete width of the variable density lines, including portion I6 and both portions I1, is substantially equal to the width of the interstices I8. The center to center spacing of the lines is uniform in both directions and can conform to the various meshes of the standard photo-engravers screens. For most uses the lines wi l be spaced 125 per inch.

The plate I I is formed from a relatively heavy, cellulose acetate sheet having a thickness of at least 0.0073 inch. The emulsion I2 on the acetate sheet has a thickness of from 0.0009 inch to 0.004 inch, and the light-sensitive portion of the emulsion is of any suitable variety having high contrast, high resolving power, and fine grain. The plate I I has a thicker emulsion coating and the coating has a greater silver salt content than the usual photographic emulsion in order to give a greater thickness or depth to the final printing surface, and greater height to the exposed dot formation which is finally formed thereon.

In the simplest form of the improved process, the plate II is placed upon a flat supporting surface, preferably a vacuum plate, as shown at I9 in Fig. 10, with its emulsion face upward. The screen I4 is placed over and tightly against the emulsion I2 and, for direct contact work, the camera negative I is placed over the screen I4, as shown in Fig. 6 and exposed to a suitable printing light. Preferably and usually, however, the negative III is placed in a photographic enlarger, as shown at in Fig. 10, and the image is projected onto the emulsion I2 through the screen I4. In either case, the negative is inverted so that its emulsion side faces upwardly in order to effect lateral reversal of the image on the printing plate and the exposed emulsion I2 is then treated as will be later described.

In the preferred form of the process, the sheet I3 of clear plastic is placed between the screen I4 and the emulsion face I2 of the plate I I before the exposure is made either by direct contact printing, as shown in Fig. 6, or by means of the enlarger projection, as shown in Fig. 10. The sheet I3 is allowed to remain in place for approximately one-sixth of the total exposure time. The transparent plastic sheet is then removed and the exposure is completed with the screen I4 in tight contact with the emulsion I2. Any desired means may be employed for holding the screen in register with the exposed emulsion I2. The vacuum plate I9 has been found satisfactory for this purpose.

After exposure, the plate II with its exposed emulsion is developed in an extremely high contrast developing solution, preferably having the following formula:

Water to make 32 oz.

The developed emulsion is then fixed in the usual hypo-fixing bath to set the exposed silver and dissolve out the unexposed silver to cause a resulting shrinking of the unexposed portions of the emulsion. After washing, the emulsion is treated in a solution, preferably having the following formula:

Potassium bichromate 3 Hydrochloric acid 2 Water to make 1 gallon.

This causes further shrinkage of the unex posed portions of the emulsion from which the silver has been dissolved and an expansion of the silver containing exposed portions thereof.

After a second washing the rough, indented surface of the emulsion is hardened by any suitable hardening solution, such as a solution consisting of:

Iodine grams 50 Potassium iodide -do Water cc 1000 The completed, hardened printing plate is then cemented or tacked to a wooden or metal backing block I5 which is used in the printing press in the same manner as the usual zinc or copper etching.

The final printing surface consists of a multiplicity of projections or dots. With the screen illustrated, these dots will be quadrangular in shape and of varying surface area corresponding to the variations in light and shade in the original negative III. For instance, the letters illus trated on the negative of Fig. 1 are transparent at their tops and become increasingly opaque as their bottoms are approached. It will be seen from the magnified view in Fig. '7 that in the final printing from the plate II, produced from the negative III, the quadrangular dots are relatively large and close together at the tops of the letters and diminish in size as the bottoms are approached.

The above results from the fact that the more intense light passing through the more transparent portions of the negative can penetrate further into the variable density borders I! of the screen I4. Thus, the width of the dots is in direct proportion to the intensity of the light striking the screen which, of course, will produce the final half-tone shading on the printed piece.

It is desired to call attention to the factthat the size of the final dots may also be regulated by the use of the plastic sheet I3. The thickness of this sheet controls its effect on the dot sizes. If the sheet I3 were exceedingly thin or eliminated entirely, the dots in the shadow areas would approximate the open spaces in the screen and this has been found to produce a flat image with very little contrast. A transparent sheet having a thickness of 0.02 inch causes the dots to run together to produce a muddy, unsatisfactory plate. It has been found that if the sheet I3 has a thickness of from .003 to'.013 inch the light from the printing lamp will spread after it passesthrough the interstices in the screen to produce larger dots on the emulsion separated by clear narrow lines which greatly increase the contrast of the final printing by creating deeper black in the dark areas. Therefore, the sheet I3 and its thickness and the control of its time of use dur-. ing the exposure are the important control features of the invention. The short exposure with the plastic sheet I3 in place produces a larger and more solid dot structure for the shadow portion of the image on the emlsion I2 with the loss of detail in the lighter portions of the camera negative I0. The longer exposure 'of the image with the screen 14 in direct contact with the emulsion I2, however, produces the high light detail. Variations of the total exposure and the ratio of exposure, with or without the plastic sheet [3 is used to control both contrast and total density in the emulsion 12.

While a specific form of the improvement has been described and illustrated herein, it is desired to be understood that the same may be varied, within the scope of the appended claims, without departing from the spirit of the invention.

Having thus described the invention, What is claimed and desired secured by Letters Patent is:

1. A method of producing a printing plate from a photographic negative of a picture, for use in a printing press, comprising: placing a transparent spacing sheet over a light-sensitive gelatin silver salt emulsion carried by a backing plate, placing a varying density halftone screen over said transparent spacing sheet, placing said negative over said screen, passing light through the superimposed negative, screen, and spacing sheet to expose said light-sensitive emulsion, then removing said sheet and completing the exposure with the screen in contact with said emulsion, developing the exposed positive emulsion to produce a positive image of said negative thereon, thence dissolving away the unexposed light-sensitive salts from said emulsion to cause a shrinkage of the latter in correspondence with the varying densities of said image, treating with acid bichromate solution for shrinking those portions of the gelatin from which the unexposed salts have been dissolved to produce an indented printing surface on said backing sheet, then hardening the emulsion by treating the same with a solution of potassium iodide and iodine, and

thence applying printing ink to the protuberances of said surface, so that said ink will offset against a sheet brought into contact with said surface.

2. The method according to claim 1 wherein said varying density halftone screen consists of a series of intersecting lines of varying density throughout their widths.

3. The method according to claim 1, wherein said acid bichromate solution comprises a proportion of 3 oz. potassium bichromate, 2 oz. hydrochloric acid, and water of a volume to make 1 gallon in all.

4. The method according to claim 1, wherein said hardening solution comprises a proportion of grams iodine, grams potassium iodide, and 1000 0.0. of water.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 381,091 Sutton Apr. 10, 1888 878,505 Cochran Feb. 11, 1908 1,354,001 Fifer Sept. 28, 1920 1,464,139 Ruhland Aug. 7, 1923 1,793,070 Eldridge Feb. 17, 1931 2,150,805 Meulendyke Mar. 14, 1939 2,244,187 Cochran June 3, 1941 2,384,857 Terry Sept. 18, 1945 2,405,291 Corbett Aug. 6, 1946 2,533,650 Wattier Dec. 12, 1950 FOREIGN PATENTS Number Country Date 5,008 Great Britain of 1892 168,578 Great Britain June 1, 1922

Claims (1)

1. A METHOD OF PRODUCING A PRINTING PLATE FROM A PHOTOGRAPHIC NEGATIVE OF A PICTURE, FOR USE IN A PRINTING PRESS, COMPRISING: PLACING A TRANSPARENT SPACING SHEET OVER A LIGHT-SENSITIVE GELATIN SILVER SLAT EMULSION CARRIED BY A BACKING PLATE, PLACING A VARYING DENSITY HALFTONE SCREEN OVER SAID TRANSPARENT SPACING SHEET, PLACING SAID NEGATIVE OVER SAID SCREEN, PASSING LIGHT THROUGH THE SUPERIMPOSED NEGATIVE, SCREEN, AND SPACING SHEET TO EXPOSE SAID LIGHT-SENSITIVE EMULSION, THEN REMOVING SAID SHEET AND COMPLETING THE EXPOSURE WITH THE SCREEN IN CONTACT WITH SAID EMULSION, DEVELOPING THE EXPOSED POSITIVE EMULSION TO PRODUCE A POSITIVE IMAGE OF SAID NEGATIVE THEREON, THENCE DISSOLVING AWAY THE UNEXPOSED LIGHT-SENSITIVE SALTS FROM SAID EMULSION TO CAUSE A SHRINKAGE OF THE LATTER IN CORRESPONDENCE WITH THE VARYING DENSITIES OF SAID IMAGE, TREATING WITH ACID DICHROMATE SOLUTION FOR SHRINKING THOSE PORTIONS OF THE GELATIN FROM WHICH THE UNEXPOSED SALTS HAVE BEEN DISSOLVED TO PRODUCE AN INDENTED PRINTING SURFACE ON SAID BACKING SHEET, THEN HARDENING THE EMULSION BY TREATING THE SAME WITH A SOLUTION OF POTASSIUM IODIDE AND IODINE, AND THENCE APPLYING PRINTING INK TO THE PROTUBERANCES OF SAID SURFACE, SO THAT SAID INK WILL OFFSET AGAINST A SHEET BROUGHT INTO CONTACT WITH SAID SURFACE.

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