CH618527A5 - Method for pulling proofs in photogravure printing - Google Patents

Abstract

The object of this method is to allow the pulling of proofs permitting possible checking and correcting of photogravure prints without having to use, for pulling the proofs, the definitive photogravure printing press. A frame (2) is applied to the surface of the photosensitive layer of an unexposed offset plate (1); a continuous-tone photogravure printing film (3) is applied to the surface of the frame; the plate (1) is exposed to light, on the side containing the film (3), so as to reproduce the image of the film on the plate, the plate is developed so as to produce a printing forme and this forme is used to pull a proof intended to permit the checking and correcting of the photogravure print. <IMAGE>

Description

The present invention relates to a process for printing rotogravure prints and, more generally, the correction of rotogravure prints. The process according to the invention constitutes a process for printing proofs, the implementation of which is simple and which is intended to allow verification and correction of rotogravure prints.

According to current practice, in the printing process, proofs are taken, before proceeding with the final printing, so as to allow the client or the person responsible for printing to verify that the printed matter will have the desired characteristics. This operation is generally designated by the term proof printing or proof printing and retouching. The number of printed proofs is very low since it is of the order of a few sheets and hardly exceeds a dozen sheets. The purpose of the proof print is to allow correction of errors presented by the surface of the printer form as well as defects in appearance, uniformity and color. In all cases, this operation presupposes that the printer form used for printing proofs is intended to undergo retouching.

It is also clear that, if the proof printing was intended to enable the production of printed matter having characteristics different from those of the printed matter to be supplied to customers, i.e. the final printed matter or the printed matter editing, this operation would be of no interest with regard to the verification of the final prints. However, for the verification of the final prints, it is not necessary that the printing of proofs is carried out by implementing a process corresponding in all points to the process used for the final printing. This is why it is commonly used to draw proofs from special proofing presses.

However, in rotogravure printing, which differs from letterpress and offset printing, proof printing is not easy to perform and it is difficult to make corrections during or after the implementation of operations to prepare the printer form. This is why, up to now, we replaced the printing of rotogravure prints by the printing of prints of the type called "blue" or "cyanotype" these prints being prepared from the positive with continuous tones d rotogravure before manufacturing the printer form. The “blue” print allows the examination of the dimensions and the arrangement of the images or the characters as well as the photographic gradation and the other characteristics of the print which will be obtained after the preparation of the printer form. However, this method does not allow examination of the tonal effect of the final print in the case of color gravure printing. This is why, in this case, it is necessary to carry out the printing of proof prints by means of the final printing press or by using a proof printing press having an arrangement similar to that of the final printing press. This is a drawback.

The object of the present invention is therefore to eliminate the drawbacks which have just been indicated by providing a process for printing proofs which has a simple implementation.

To this end, the method according to the invention is characterized in that a screen is applied to the surface of the photosensitive layer of a blank offset plate; a continuous-tone gravure film is applied to the surface of the screen; the plate is exposed to light, on the side of the film, so as to reproduce the image of the film on the plate, the plate is developed so as to produce a printing form and this form is used to draw a proof intended to allow the verification and correction of the gravure printing.

The rotogravure printing process and the offset printing process differ from each other by the way in which the printing is carried out as well as by the nature of the stains that make up the prints obtained. However, the existence of these differences simply means that the procedure used to obtain an image identical to the original is different from one process to another and does not imply that it is impossible to reproduce at by one of these two methods the kind of prints obtained by means of the other method. The difference in the composition of the spots of the print simply indicates that there is a difference that can be observed on microscopic examination, it being understood that the print is generally observed not under the microscope but at the naked eye. In view of this last fact, it is not necessary to carry out the printing of proofs by means of the same process and under the same working conditions as when printing the final prints. In fact, the present invention stems from the discovery of the fact that one can carry out the printing of proofs in s

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rotogravure using the offset printing process without any drawback from the point of view of the effectiveness of the printing verification.

The invention will be better understood in the light of the detailed description of examples of implementation of the method which goes? follow, referring to the drawing attached in this:

Figure 1 is a schematic section illustrating the procedure for superimposing a screen and a continuous tone film for gravure printing on the surface of a blank offset plate, as well as the manner in which the 'photographic printing in accordance with a first example of implementation of the method according to the invention;

FIG. 2 is a comparative diagram showing three reproduction curves by printing;

Figure 3A is a section similar to that shown in Figure 1 and illustrates how to make a main exposure using a continuous tone positive, in accordance with a second example of implementation the method according to the invention;

Figure 3B is a diagram showing how halftone spots form on a blank offset plate as a result of the main exposure;

FIG. 4A is a schematic section illustrating the manner in which an additional exposure is carried out; and Figure 4B is a diagram illustrating graphically how the halftone spots form on the offset plate as a result of the additional exposure.

FIG. 1 illustrates the principle of a first form of implementation of the method according to the invention. For the implementation of this method, a blank plate 1, a screen 2 and a continuous-tone film 3 3 are used for gravure printing, which may be a continuous-tone positive or a continuous-tone negative.

As a blank offset plate, any conventional type plate can be used. For example, one can use a “polymetal” type plate, a deep etching plate, a dry offset plate and a presensitized plate, each of these plates being provided with a photosensitive layer on one or other of its faces. In the case where a continuous tone positive is used as the continuous tone film 3, a blank offset plate can be used comprising a positive photosensitive layer decomposed by photolysis. In the case where a continuous-tone negative is used, on the other hand, a blank plate can be used provided with a photopoly-merisable or photocrosslinkable negative photosensitive layer. In the case of a negative, a presensitized plate is used more particularly, but the use of this latter type of plate has great advantages both in the case where a positive film is used and in that in which one uses a negative film, because the use of this type of plate facilitates the printing of proofs. 50

As screen 2, a screen such as that used in the gravure printing process and in the offset printing process can be used. Preferably, a screen such as those used in the offset printing process is used. 55

As indicated above, a continuous tone positive 3 or a continuous tone negative can be used as continuous tone gravure film 3. A continuous tone positive is almost always used in the usual offset printing process in which the photosensitive element used is charcoal paper. However, in the rotogravure printing process corresponding to the mode of implementation according to which an electronic engraving machine is used, it is possible to use not only a positive with continuous tones but also a negative with continuous tones. The continuous tone positive can be prepared in the form of a film in which the colors are separated, for example by photographing an original using a camera or by subjecting the original to the action of a device for exploring colors and then preparing the film of the desired dimensions. The continuous tone negative can be prepared, for example, by photographing with a camera the continuous tone positive in which the colors have been separated as described above and then giving the negativ thus obtained the dimensions desired.

The screen 2 and the continuous-tone film 3 are superimposed, in this order, on the photosensitive layer of the blank offset printing form 1, this screen and this film being in intimate contact with this layer, as shown in FIG. 1 and then the exposure to light is carried out, on the side of the film 3, as indicated by the arrows. This exposure to light can be performed in the same way as when printing a halftone positive in the offset printing process.

After exposure to light and photographic printing, the obtained printer form 1 is subjected, as in the case of an ordinary offset printer form, to the subsequent operations of the offset printing process such as, development, washing with water, the sensitization treatment, another washing with water, washing with elimination water, exfoliation and drying so as to obtain a printing form. The development operation indicated above can be carried out by a usual method such as development by brushing or by spraying.

Next, a proof is printed by the offset printing process, using the printer form obtained in the manner just described and following the usual procedure for printing offset prints so as to produce a printed. During this operation, it is of course made so that the color of the ink used for the printing of the offset proof print approximates that of the ink which will be used for the printing of the rotogravure prints and preferably an ink with a high degree of transparency is used. It has been observed that the effect of continuous tones suitable for reproduction can be obtained by the gravure printing process by using a screen having fine lines such as a screen comprising, for example, more than 78 lines / cm.

It appears, from the description above, that the process described constitutes a process for printing rotogravure prints, the implementation of which is very simple in comparison with the way in which the preparation of printed matter is currently carried out. in the rotogravure printing process. In addition, this process allows significant rationalization and advantages such as a considerable reduction in the processing time and the cost price of the preparation of the printed matter. It should be noted that the process according to the invention has great advantages not only in the case of the traditional gravure printing process in which etching is carried out by chemical attack, but also in the case of the gravure printing process by electronic engraving.

The nature and the advantages of the process which has just been described will become more clearly apparent from the description of a detailed example of implementation which will follow.

Example 1

A positive type screen (59 lines per centimeter) is placed on the photosensitive layer of a presensitized plate, the face of the screen applied on the photosensitive layer being turned downwards, and a positive with continuous tones is placed on the screen. for photogravure this positive being brought into close contact, by its face turned downwards, with the screen. Exposure to light is carried out to impress the photosensitive layer, by irradiating the positive with continuous tones,

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for ten minutes, using a single metal-halide lamp having a power of 2 kW.

The presensitized plate thus obtained is then developed by spraying for one minute (with a plate displacement speed of 30 sec./m. Using a solution obtained by diluting a solution of 5 to 6 times reserve of a development agent (caustic soda, sodium silicate) for presensitized plate. The plate is then washed with water, it is subjected to an sensitization treatment with a development ink (manufactured by the Fuji Company. Shasl 'n Film Kabushiki Kaisha (Fuji Photographic Film Company) Japan), and then washed with water. Remove unwanted parts with a suitable solution and wash the plate again with water. .

The plate is then subjected to coating with gum and to drying, and a printing form is thus obtained.

Printing plates are thus prepared, according to the operating mode which has just been described, for four colors, namely cyan, black, magenta and yellow.

Printing is carried out in several colors, using the printing forms prepared in the manner just described, on a proofing press (manufactured by JG Mailänder Durckmaschinenfabrik, Federal Republic of Germany). the order of printing of the colors being: cyan, black, magenta and yellow with proof printing inks (manufactured by the Company Morohoshi Ink Kabushiki Kaisha, Japan). This produces prints allowing the correction of the final gravure prints.

Curve A of FIG. 2 is a curve representative of the reproduction of printed matter obtained during the printing of proofs using a printer shape obtained, in accordance with the first embodiment of the method according to the invention, using a continuous tone positive film. The mouse B in FIG. 2 is a curve representative of the reproduction of a print obtained during final printing, by the gravure process, using a printer form obtained by the usual process, using the same positive film with continuous tones as in the case of curve A. The comparison of curves A and B shows that, in the shadow domain of the positive with continuous tones, the photographic density of the print obtained by printing 40 d proofs according to the process according to the invention is higher than that of the print obtained by final printing. On the other hand, in the clear areas of the positive, the photographic density of the print obtained by printing according to the process according to the invention is lower than that of the print obtained by final printing. This means that the print obtained by proof printing has a harder tone than that which is obtained during the final printing and which has no gradation of hues in the shaded areas.

If the amount of light is increased during exposure, 50 in order to obtain a good gradation of shades in the shaded areas, the tone of the print obtained by proof printing becomes even harder and the trend of insufficient density in light to medium ranges is further accentuated. On the contrary, if the amount of light is reduced, the insufficient density in the light to medium portions is remedied, but the gradation of the tint in the shaded areas becomes even worse.

In addition, the preparation of a contact screen allowing good coincidence of the reproduction curve of the print obtained by proof printing with the reproduction curve B is very difficult.

Studies carried out with the aim of improving the result obtained during the implementation of the first embodiment of the method described above, when a positive film with continuous tones is used, have made it possible to discover that, by subsequently removing only the continuous tone positive for rotogravure printing, during the implementation of this process, and by carrying out an additional exposure, one can obtain a print reproduction curve approaching the curve representative of the hues '' a print obtained by the gravure printing process. This discovery is the basis of a second embodiment of the method according to the invention which will be described below.

As indicated in FIG. 3 A, a contact screen 2 and a continuous-tone positive for gravure printing 3a are superimposed successively on the photosensitive layer of a blank offset plate 1, so as to form an assembly with several layers which are expose to light to obtain the photo-impression. This exhibition will be referred to below as the “main exhibition”. During this operation, an exposure rate corresponding to approximately 60 to 90% of the exposure rate used in the first embodiment of the method according to the invention is used.

Figure 3B is an explanatory diagram showing graphically how the dots are formed on the blank offset plate during the main exhibition. It is assumed, in this case, that the continuous tone positive 3a for gravure printing includes a range in which the distribution of optical densities corresponds to that represented by the part of the diagram designated by the number 4 and that a frame 2 having an optical density distribution indicated by the part of the diagram designated by the number 5. In this case, the quantity of transmitted light corresponds to the distribution indicated, in a simplified manner, by the part of the diagram designated by the number 6 and , as a result, halftone dots 7 are formed on the offset plate 1.

If you set the amount of light exposure, during main exposure, to a value lower than that which you choose when you do not perform the additional exposure, the area of the dots in halftone formed on the offset plate 1 increases and the print density becomes higher throughout the range. As a result, the photographic density in the light to medium ranges also increases, and the aforementioned density deficiency in the light to medium ranges, which occurs during the implementation of the first mode of implementation of the method according to the invention is corrected.

Thus, during the main exposure, the insufficient density in the light to medium parts of the image is corrected. . On the other hand, due to the decrease in exposure intensity, the gradient of the shades in the dark parts of the image is darker than in the case of the first embodiment of the method according to the invention. Consequently, to correct this effect, the continuous tone positive for gravure printing 3 is removed so as to leave only the screen 2 in contact with the plate 1, and then an additional exposure is carried out on the side of screen 2, as shown in Figure 4A. During this operation, the intensity of the exposure is preferably of the order of 0.3 to 7%, more particularly 0.5 to 4% of that of the main exposure which is described above.

FIG. 4B is an explanatory diagram which graphically shows the way in which the semitone dots are formed on the offset plate in the case where an additional exposure is carried out which is superimposed on the main exposure described above . The distribution of the light transmitted through the frame 2, the distribution of optical density of which is indicated by the part of the diagram designated by the number 5, is indicated by the part of the diagram designated by the reference number 6a. As a result, the shape of the semitone dots 7, produced by performing only the main exposure, changes to that of the semitone dots 7a.

This additional exposure makes it possible to obtain a gradient of shades, in the dark parts of the print, in

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practically having no influence on the light to medium ranges, in fact, the degraded semitone points of the screen 2 are used effectively. More precisely, when the additional exposure is carried out, an increase of the exposure in the parts of low optical density of the frame s 2 and, as a result, a degree of nuances of the dark parts of the print is obtained which cannot be reproduced completely only by the main exposure. In addition, the light to medium ranges include small semitone dots, obtained during the main exposure, which are masked by the high optical density portions of the frame, and therefore the additional exposure n has virtually no effect on these parts of the printout. Thus, the additional exposure makes it possible to obtain a suitable shade gradient in the dark parts of the print. i s

Curve C in FIG. 2 constitutes the print reproduction curve of a proof print obtained using a printer form prepared in the manner indicated above. As can be seen, the curve C is very close to the print reproduction curve B of a gravure printing 2u obtained during the final printing using a printer form, prepared in the usual manner using the same gravure positive continuous tones. This print is not inferior in quality to that of ordinary gravure printing.

The nature and the advantages of the second mode of implementation of the method according to the invention, described above, will appear even more particularly in the light of the detailed example of implementation and of the following comparative examples:

Example 2 3 „

A positive type screen (59 lines per centimeter) is placed, the surface of its film being turned downwards, on the photosensitive layer of a presensitized plate and a continuous continuous-tone positive overlay is superimposed on the screen. the film surface is also turned downward and placed in close contact with the frame. The main exposure is then carried out, the light coming from the positive side mentioned above, with an exposure time of 8 minutes and 30 seconds using a single lamp of the metal-halide type having a power of 2 kW. 4I)

After which, only the continuous tone positive for gravure printing is removed and, after placing the screen again in close contact with the surface of the plate, an additional exposure to light is carried out, for 10 seconds over the entire surface of the plate, on the side of the frame. 45

After which, the presensitized plate, thus exposed, is developed by spraying it for one minute (with a plate running speed of 30 sec / m.) With a solution obtained by diluting a stock solution 5 to 6 times. a development agent (caustic soda, sodium silicate) 50 for presensitized plate. The plate is then washed with water,

it is subjected to an sensitization treatment using a development ink (manufactured by the Fuji Shashim Film Kabushiki Kaisha Company (Fuji Photographic Film Company), Japan), and is washed with water. The unwanted parts 55 are then removed using an appropriate removal solution and the plate is washed again with water.

After which, the plate is subjected to a coating of gum, in the usual manner and it is dried, which makes it possible to obtain a printing form. 6 ()

The procedure is as described above to produce printing forms corresponding respectively to the four basic colors, that is to say to cyan, black, magenta and yellow.

A color proof print is prepared on a proof press (manufactured by JG Mailänder Druckmaschinenfabrik, Federal Republic of Germany), using the printer forms prepared as described above and special inks proofs (Made by the Morohoshi Ink Kabushiki Kaisha Company, Japan) in the following order of colors: cyan, black, magenta and yellow. This produces prints that are very suitable for correcting rotogravure printing.

The prints obtained by proceeding in the manner described in Example 1 are also suitable for the correction of rotogravure prints, but, in comparison with the proof prints obtained in the manner described in Example 2 above, they have harsh shades and poor gradation in the dark parts of the image.

The tests obtained by proceeding in accordance with the first and second mode of implementation of the method according to the invention which have just been described do not give complete satisfaction from the point of view of their qualities of gloss and transparency in comparison with the printed matter. final obtained by printing on the final printing press. More precisely, the difference in the printing process results in the fact that the ink film transferred to the print obtained by the offset process is more than that which is transferred to the surface of the print obtained in gravure printing. Consequently, in order to obtain the same photographic density as in the case of gravure printing, it is necessary to correspondingly increase the quantity of pigments contained in the ink.

However, in the case of offset printing, because the ink film applied to the print is thin, the pigment particles protrude outside the film and the latter has an irregular surface. In addition, because the pigment content, expressed as a percentage, is higher than in gravure printing, the basic color of the pigment appears. For this reason, which is not the only one, it is difficult to improve the quality of the print by increasing the pigment content of the ink.

This drawback can be remedied by a third embodiment of the invention which will be described below. In the same way as this mode of implementation, the print intended for use in the printing of rotogravure prints is prepared by proceeding in the same way as in the first and second modes of implementation of the invention. However, a transparent layer of resin is then applied to the surface of the test thus obtained. The proof thus coated is used to make alterations and corrections to the rotogravure print.

In order to apply a transparent layer of resin over the entire surface of the test, the transparent resin is dissolved in a solvent, to which filler substances are added, if necessary, and the solution thus obtained is applied in the form of a coating layer, by a method such as the gravure inking method of the rotogravure type, the direct inking method or the reverse roller inking method. The amount of transparent resin thus applied can advantageously be of the order of 5 to 45 grams / m2 (this weight corresponding to the dry weight of resin).

As transparent resin, it is possible to use a resin which does not affect the qualities of the test such as the reproduction of the colors and the shape of the dots in semitones and which is capable of giving the test an aspect of transparency and of gloss recalling that of the prints obtained by the gravure process. The amount of resin obtained in the solution is advantageously from 5 to 60% by weight, preferably 10 to 40% by weight. As an example of a resin, the use of which is particularly advantageous, in the context of the invention, mention may be made of the resins which are suitable for the gravure printing process, in particular cured rosin, nitrocellulose, polyamides and chloride chloride copolymers. vinyl - vinyl acetate. These resins are very suitable for the preparation

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of a coating layer and they give the test a transparency and a gloss similar to those of the prints obtained by the gravure process. As solvent, liquids such as toluene, xylene and normal hexane can be used, the amount of each of these solvents being preferably approximately equal to the amount by weight of the transparent resin. As optional addition material, fillers such as calcium carbonate and magnesium carbonate or other additives can be used, the amount of these substances being from 10 to 50% by weight, based on the weight of solution.

It is also possible to apply the transparent resin in the molten state.

The third embodiment of the method according to the invention which has just been described can be illustrated by the following detailed example:

Example 3

The procedure is as indicated in Examples 1 and 2 and two kinds of tests are thus obtained.

A rotogravure cylinder is prepared, having an entirely solid surface (depth 30 microns), according to the usual process for preparing printing forms. Then, using a gravure printing press, two layers of a gravure printing agent, consisting of 50 parts by weight of cured rosin and 50 5 parts by weight of toluene, are applied to the two kinds of prints indicated above, so as to form transparent layers of resin having a dry matter content of the order of 15 g./m2.

m • The two kinds of proofs thus obtained have a brightness and transparency which are very similar to those of the prints obtained by the gravure printing process. The quality of these proofs is as good as that of rotogravure prints and they effectively allow the correction of i5 prints obtained by rotogravure. In addition, the two kinds of proofs thus coated with the transparent resin layer are superior, from the point of view of their surface brightness and their transparency, to the tests devoid of the transparent resin coating, the preparation of which is described in the examples. 1 and 2 and their characteristics are more similar to those of the prints obtained by the gravure printing process.

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Claims (11)

618,527 1. A process for printing rotogravure prints, characterized in that a screen (2) is applied to the surface of the photosensitive layer of a blank offset plate (1); a continuous tone gravure film (3) is applied to the surface of the screen; the plate (1) is exposed to light, on the side of the film (3), so as to reproduce the image of the film on the plate, the plate is developed so as to produce a printing form and this form is used to draw a test intended to allow the verification and correction of the rotogravure print. 2. Method according to claim 1, characterized in that said continuous tone film (3) is a positive. 2 3. Method according to claim 1, characterized in that said continuous tone film (3) is a negative. 4. Method according to claim 1, characterized in that the blank offset plate (1) is a presensitized plate. 5. Method according to claim 2, characterized in that, after the exposure of the plate (1) to light on the side of the positive continuous tone (3a), the latter is removed so as to leave only the frame (2) on the plate (1) and that the plate (1) is then subjected to a second exposure to light on the side of the frame (2). 6. Method according to claim 5, characterized by the fact that the intensity of said second exposure is between 0.3 and 7% of that of the first exposure. 7. Method according to claim 1 or claim 5, characterized in that, after having prepared a test print by means of the finally obtained printer form, a transparent resin coating is applied to the surface of this print. 8. Method according to claim 7, characterized in that the transparent resin coating is prepared by dissolving a transparent resin in a solvent, then applying the solution thus obtained to the surface of the test by a method chosen from the gravure inking process, the direct roller inking process and the reverse roller inking process. 9. Method according to claim 8, characterized by the fact that said transparent resin is chosen from hardened collophanes, nitrocellulose, polyamides and vinyl chloride-vinyl acetate copolymers, and by the fact that said solvent is chosen from toluene, xylene and n-hexane. 10. Method according to one of claims 7,8 or 9, characterized in that one adds to the resin such as calcium carbonate or magnesium carbon. 11. Method according to one of claims 7,8,9 or 10, characterized in that the quantity of the transparent resin coating corresponds to a dry weight of resin from 5 to 45 g / m2.