US2978324A - Half-tone printing blocks - Google Patents

Description

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April 4, 1961 G. HEussNl-:R 2,978,324

HALF-TONE PRINTING BLOCKS Filed Feb. 25, 1954 PAPE@ mee/Ez SHEET 13 apague f`:" 74

Jef/my F 2 ELS P/@Mavma 1g' 1 TnJ 16 TeA/@PARENT v 26 2, (Cfa'fw CAQB/EZ SHEET META/ j 10 11 l 13a F1554. C j H \76 15 F 3 TRANSPARENT L/sHr/ v 0g NMEA/T50 BY /M*/; TTORNEY HALF-TONE PRINTBWG BLOCKS Gerda Henssner, Cologne-Nippes, Germany, assignor to Koch Processes Limited, London, England, a British company Filed Feb. 23, 1954, Ser. No. 411,802

Claims priority, application Great Britain Feb. 27, 1953 9 Claims. (Cl. 96-35) The present invention relates to half-tone printing blocks. Y

The diameters of the upstanding dots of a half-tone printing block vary considerably between high-light and shadow areas.

It, therefore, the printing surfaces of all the dots of a half-tone block lie in a ,common plane the pressure between the dots and the paper on which a print is being made varies in dependence upon the diameters of the dots.

This variation in the printing pressure produces undesirable effects in the print and to avoid these effects it is common practice to use interlays in a technique known as make ready. The interlays consist of patches of paper inserted between the plates and the mounts of the blocks. These patches lie under the shadow areas and serve to raise these areas relatively to the high-lights. The difference in height may need to be no more than say a' thousandth of an inch to avoid the aforesaid undesirable I effects. Examples of the technique of make-ready by interlays is `described on pages 132 and 133 of thebook Practical Printing and Binding, published by Odhams Press Ltd., London.

Make-ready is a time-consuming operation, however,

Y and may for example require as much as 30 minutes for a `single half-tone block.

One object of `the present invention is to provide an improved method of producing a half-tone printing block in which the need for make-ready by interlays can be eliminated.

It has already been proposed to produce a half-tone printing block by a method in which a screened photographic negative is printed down on to an even layer of transparent colloidal material, such as gelatin, which has been made sensitive to light and washing out the unexposed .parts of the layer.

In this proposed method exposure takes place right 4through the layer and hence when washing out"has been effected the tops of the upstanding dots ofthe block lie in acommon plane.

Accordingto the present invention in an improvement of the aforesaid proposed method of producing a half-tone printing block, the layer of transparent colloidal material is superimposed upon a' second, relatively thin, light-sensitive, infraposed-layer yof colloidalmaterial containing pigment of a nature ,and concentration such that light reaching the second layer from areas of relatively large dot diameter penetrates further than light reaching the second layer from areas of relatively small dot diameter, whereby when washing out is effected the heights of the upstanding dots vary in dependence upon the diameters of the dots. Thus the need for interlays can .be avoided. Furthermore .the change in height with change in dot diameter is progressive, whereby unlike make-ready techniques, there is no abrupt change in height as may occur at the edge of an underlying patch. t

.The invention also provides a foil for carrying out the method according to the invention, the foil comprising two layers -ofcolloidal material the layers being one on the Y. Fatented Apr. 4, 1961 other, a first of the layers being relatively thick and transparent, and the second layer being relatively thin and containing pigment ot a nature and concentration such that light directed on to the second layer penetrates the second layer to a depth dependent upon the intensity of the light, the said layers preferably being supported on a carrier.

The invention will now be described, by way of example, with reference to the accompanying drawing in which Figs. 1 to 3 illustrate three steps respectively in the production of a half-tone printing block by a method in accordance with the invention, Fig. l illustrating, inter alia, and in diagrammatic cross-section, a iirst species of foil according to the invention; Fig. 2 the application of such foil to a base plate after its exposure; and Fig. 3 the printing block produced therefrom, and in which Fig. 4 is a diagrammatic cross-section of a second species of foil according to the invention.

Fig. l shows schematically and to an enlarged scale a screened half-tone negative 11 carried on a glass plate 11i. The dots of the negative are shown to be in the form of transparent areas 12 in the negative 11 and the size of the dots varies in dependence upon the variations in light and shade over the picture represented by the halftone negative.

The half-tone negative is placed on a foil comprising four layers 13 to 16 respectively. The lowermost layer 13 is a paper carrier and the first layer 14 adjacent the paper 13 is a layer of gelatin which is heavily pigmented with dense black pigment to render this rst layer opaque for a purpose to be described later.

The next `layer 15 is of gelatin which-is relatively lightly pigmented with burnt sienna andthe uppermost layer 16 is a transparent layer consisting of a mixture of gelatin and -fish glue. The layer 14 may be 0.1001 inch thick, the layer 15 may likewise be 0.001 inch thick and .the thickness of the uppermost layer 16 is dependent .upon the screen ruling employed in producing `the screenedl halftone negative 11 and may vary between 0.001 land 0.010 inch. In one example the layer 14 contains 1 ounce .of vegetable black per 4 ounces of gelatin weighed dry; the layer 15 0.5 ounce of burnt sienna pigment per 4 ounces of gelatin weighed dry; and the layer "16 .Contains 10 ounces of gelatin per 2 ounces of fish glue per one ,ounce of glycerine. The foil is made light-sensitive when required b'y treatment with, for example, ammonium bichromate in known manner.V

1n printing down, light is directed through the `screened half-tone negative on to the foil as indicated ,bythe arrows 17 the exposure ,time being'dependent lupon the intensity of the light and the screen ruling :of the half-tone negative. The exposure time may be v'from 5to 12 minutes. The only light reaching the foil is that :which passes through the transparent dots A12 and this passes through the clear layer 16 to the layer 15. The pencil Inf light 18l passing through a transparent dot of'small diameter penetrates the layer 15 to only a small extent, as shown. The pencil of light 19 of ,slightly larger diameter penetrates'further into the layer A15 and so on, thepencil of light 20 passing through the dot of the largest diameter penetrating the layer 15 right down to the heavily pigmented layer 14.

' Those parts of the layer 16 which are traversed by the pencils of light are hardened and hardening of the layer 15 is eiiected to varying depths Vin dependence upon the diameters of the pencils of light, that is to say lin dependence upon the variations in dot size.

After this exposure of the foil the half-tone negative is removed and the foil fiooded with light for a short period, say 10 to 15` seconds. This hardens the upper skin of the layer 16 as depicted at 26 in Fig. 1. The light is then switched oit and the foil linverted and stuck to a .metal base 21 as shown in Fig. 2. This assembly is then immersed in warm water (38 to 40 centigrade) first of all to remove the paper carrier 13. One function of the heavily pigmented layer 14 is to facilitate removal of the paper by preventing light from reaching the paper during the exposure illustrated in Fig. l. It will be appreciated that without the layer 14 exposure would take place through to the paper in the dot areas of maximum diameter. Such exposure woud cause the paper to stick to the layer and would result in damage to the layer15 when the paper carrier is removed.

When the paper carrier is fioated off by the action of the warm water, the water is then able to dissolve the unexposed parts of the layers 15 and 16 with the result shown in Fig. 3.

It will be seen from Fig. 3 that the heights of the undissolved parts 22 to 25 increase with increase in the dot diameters, the increase in height being progressive with increase in dot diameter. As is evident from Fig. 2, all of the dots when prepared by the present process, comprise a base portion of transparent material derived from the layer 16 of uniform depth, and a tip portion derived from the layer 15 of pigmented material, the tip portions of the dots of larger diameter being of proportionately greater depth than those of smaller diameter. The half-tone block shown in Fig. 3 is therefore ready for use in printing without the need for make-ready.

lf a block as shown in Fig. 3 is required for use in producing a printing cylinder for newspaper printing it may be incorporated in a forme with type and the forme used in the well known dry fiong method to make an impression on dry fiong without heat. The mould so produced is employed to make a printing cylinder by casting in known manner.

If the wet ong method should be employed there is a substantial quantity of steam produced when the moist ong is heated in contact with the block. There is therefore a tendency for the block to swell and disintegrate and it is necessary in order to prevent this to coat the block with a substance resistant to steam at least for the short time in which it is present during the pressing operation. A suitable substance for the purpose is nitro-cellulose varnish.

The foil may take forms other than that shown in Fig. 1. The principal feature of the forme consists of the provision of the transparent layer 16 on the specially pigmented layer 15.

In a further foil provided by the invention and illustrated in Fig. 4, the carrier 13a is in the form of a transparent sheet, for example Celluloid, the layer 14 is dispensed with, the layer 16 is next the Celluloid and the layer 15 on top of the layer 16. The half-tone negative is placed against the transparent carrier and the light is directed through the negative, the carrier, and the layer 16 to the layer 15. Y

After the exposure the transparent carrier is aixed to a stiff support and washing out is then effected.

I claim:

1. A method of producing a half-tone printing block that comprises (a) preparing on a carrier a foil comprising at least two layers of colloidal material superposed one on the other, one of the layers being relatively thick and transparent and the other being relatively thin and slightly pigmented; (b) photo-sensitizing said layers; (c) printing down on said foil with light directed through a screened half-tone negative and said relatively thick layer in succession to said relatively thin layer to a degree sufficient to harden the light exposed areas of the colloidal material in said relatively thick and thin layers to a depth in said relatively thin layer proportional to the dot size in said screened half-tone negative; (d) hardening the surface of said relatively thick layer remote from said relatively thin layer;V (e) afiixing the exposed foil to a support with said hardened surface contiguous thereto; and (f) washing out the unexposed parts of said layers to produce a printing block in which the heights of the dots is proportional to the diameters thereof.

2. A method according to claim 1 in which the foil prepared on the carrier in step (a) is maintained on said carrier at least during the performance of said steps (b), (C), (d),a11d (e)- 3. A method according to claim 1, in which said carrier is in the form of a sheet, and further comprising the steps of preparing the foil by supporting the two layers of colloidal material on said carrier sheet with the transparent layer remote therefrom and, after affixing the exposed foil to the support in step (e), washing away said carrier sheet.

4. A method according to claim l, in which said carrier is in the form of a transparent sheet, and further comprising preparing the foil by supporting the two layers of colloidal material on said transparent carrier sheet with the lightly pigmented layer remote therefrom, and afiixing the exposed foil to the support in step (e) with said carrier sheet contiguous to said support.

5. A method of producing a half-tone printing block that comprises: (a) preparing a foil comprising a carrier sheet with a first layer of heavily pigmented colloidal material, a second layer vof lightly pigmented colloidal material, and a third layer of transparent colloidal material thereon; (b) photosensitizing said layers, (c) printing down on said foil with light supplied through a screened half tone negative to a degree sufficient to harden the light exposed areas of the colloidal material of said third and second layers to a depth in said second layer proportional to the diameters of the so exposed areas, (d) hardening the upper surface of said third.layer, (e) mounting the foil on a base plate by affixing said hardened upper surface to the plate, (f) and washing away said carrier sheet and first layer and the unexposed parts of said second and third layers to produce a printing block in which the heights of the dots are proportional to the diameters thereof.

6. A method of producing a half-tone printing block that comprises: (a) preparing a foil comprising a carrier with a layer of lightly pigmented colloidal material and a transparent layer of colloidal material thereon, (b) photosensitizing said layers, (c) printing down on said transparent layer with light supplied through a screened half-tone negative to a degree sufficient to harden the light exposed areas of the colloidal material of said transparent and lightly pigmented layers to a depth in said lightly pigmented layer proportional to the diameters of the so exposed areas, (d) hardening the surface of the transparent layer, remote from the pigmented layer, (e) mounting the foil on a base plate with the transparent layer between the base plate and the lightly pigmented area, and (f) washing away the unexposed areas of the layers to produce a printing block in which the heights of the dots are proportional to the diameters thereof.

7. The invention of claim 6, in which, in step (a) one prepares the foil using a transparent sheet as the carrier and applies the lightly pigmented layer as a relatively thin layer and the transparent layer as a relatively thick layer with the transparent layer interposed between said sheet and said lightly pigmented layer; in which, in step (c) one prints down on said transparent layer through said transparent carrier; and in which, in step (d) one afixes the exposed foil to the base plate with said carrier interposed between said transparent layer and said base-plate.

8. A foil for use in producing a half-tone printing block, said foil comprising a paper carrier sheet, a first layer of colloidal material adjacent said paper carrier sheet, said first layer being sufficiently heavily pigmented to render it opaque, a second layer of colloidal material overlying said first layer, said second layer being lightly pigmented so as to be penetrable by light to a depth proportioned to the intensity of the light, and a third layer of colloidal material overlying said second layer, said third layer being transparent, said layers being bichromate sensitized, whereby an exposure of said foil by passing light through respective areas of said transparent sensitized layer into the underlying area of said lightly pigmented layer hardens the colloidal material of said respective areas of said transparent layer and further hardens the colloidal material of said underlying areas to a depth in said lightly pigmented area proportioned to the intensity of the light impinging on said underlying areas, the colloidal material of said transparent and pigmented sensitized layers, where not hardened as aforesaid, being soluble in a solvent, but where hardened as aforesaid being nonsoluble in said solvent, whereby said non-hardened portions may be washed from the light hardened portions thereof, which light hardened portions will have depths proportional to the intensity of light impinged thereon as aforesaid for forming printing dots proportional in height to the diameter thereof.

9. A foil for use in producing a half-tone printing block,

said foil comprising a transparent Celluloid carrier sheet,

a proximate layer of colloidal material directly underlying said transparent carrier sheet, said proximate layer being transparent, and a further layer of colloidal material directly underlying said proximate layer, said further layer being lightly pigmented so as to be penetrable by light passing thereto from said transparent layer to a depth proportional to the intensity of said light, said layers being bichromate sensitized, whereby exposure of said foil by passing light through respective areas of said transparent carrier and said transparent sensitized layer into the underlying areas of said lightly pigmented layer hardens the colloidal material of said respective areas of said transparent layer and further hardens the colloidal material of said underlying areas to a depth in said lightly pigmented area proportioned to the intensity of the light impinging on said underlying areas, the colloidal material of said transparent and pigmented sensitized layers, where not hardened as aforesaid, being soluble in a solvent, butrwhere hardened as aforesaid being non-soluble in said solvent, whereby said non-hardened portions may be washed from the light hardened portions thereof, which light hardened portions will have depths proportional to the intensity of light impinged thereon as aforesaid for forming printing dots proportional in height to the diameter thereof.

' ReferencesV Cited in the file of this patent UNITED STATES PATENTS 428,472 Dubois May 20, 1890 1,098,610 Aylsworth June 2, 1914 1,408,312 Thornton Feb. 28, 1922 1,532,696 Hassard et al. Apr. 7, 1925 1,846,230 Stuber Feb. 23, 1932 2,058,396 Baker Oct. 27, 1937 2,088,399 Gibson July 27, 1937 2,251,965 Verkinderen Aug. 12, 194,1 2,373,732 Wolfson Apr. 17, 1945 2,405,513 Mullen Aug. 6, 1946 2,692,198 Whitney Oct. 19, 1954 2,900,255 Charlton Aug. 18, 1959

Claims (1)

1. A METHOD OF PRODUCING A HALF-TONE PRINTING BLOCK THAT COMPRISES (A) PREPARING ON A CARRIER A FOIL COMPRISING AT LEAST TWO LAYERS OF COLLOIDAL MATERIAL SUPERPOSED ONE ON THE OTHER, ONE OF THE LAYERS BEING RELATIVELY THICK AND TRANSPARENT AND THE OTHER BEING RELATIVELY THIN AND SLIGHTLY PIGMENTED; (B) PHOTO-SENSITIZING SAID LAYERS; (C) PRINTING DOWN ON SAID FOIL WITH LIGHT DIRECTED THROUGH A SCREENED HALF-TONE NEGATIVE AND SAID RELATIVELY THICK LAYER IN SUCCESSION TO SAID RELATIVELY THIN LAYER TO A DEGREE SUFFICIENT TO HARDEN THE LIGHT EXPOSED AREAS OF THE COLLOIDAL MATERIAL IN SAID RELATIVELY THICK AND THIN LAYERS TO A DEPTH IN SAID RELATIVELY THIN LAYER PROPORTIONAL TO THE DOT SIZE IN SAID SCREENED HALF-TONE NEGATIVE; (D) HARDENING THE SURFACE OF SAID RELATIVELY THICK LAYER REMOTE FROM SAID RELATIVELY THIN LAYER; (E) AFFIXING THE EXPOSED FOIL TO A SUPPORT WITH SAID HARDENED SURFACE CONTIGUOUS THERETO; AND (F) WASHING OUT THE UNEXPOSED PARTS OF SAID LAYERS TO PRODUCE A PRINTING BLOCK IN WHICH THE HEIGHTS OF THE DOTS IS PROPORTIONAL TO THE DIAMETERS THEREOF.

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