Process for preparing printing plates

Background of the invention

The present invention relates to a process for preparing a printing plate by producing a film bearing a positive image from an original having a positive image of different colours for the printing plate and preparing the printing plate from the film, especially for those useful for printing on corrugated board.

The process of the invention is useful for plates for printing not only on corrugated boards but also for usual paper materials.

Preparation of printing plates usually requires a complex process including many steps as seen in Fig. 7. The conventional process will be described with reference to the case where for a better understanding, the characters "F" and "L" roughly sketched as at 1 are to be printed in different colours.

Step a

The characters are drawn on paper in specified colours to prepare originals 2 for printing plates.

Step b

The originals 2 are photographed by a letterpress camera on an enlarged or reduced scale corresponding to the print 6 to be made, to obtain negative films 3.

The letterpress camera is provided with a front mirror and adapted to photograph reflected images. The image of the original, which is reflected from the mirror and passes through the lens, is projected onto the coating of the film as an inverted normal image having the original right- to-left relationship. Accordingly when the negative film 3 is turned through 180 degrees, the image as seen from the film substrate side is an erect reverse image as seen in Fig. 7, b.

It is to be noted that in the letterpress printing process, the term "normal image" or "reverse image" refers to the image as seen through the film substrate, with the photosensitive coating side of the film positioned outward and does not refer to the image as seen directly on the coated surface of the film. Thus, the reverse image on the negative 3 is a normal image when it is viewed directly on the coated surface.

Step c

Each negative film 3 is placed on the coating 42 of photosensitive paper 41 with the film substrate in intimate contact therewith and with the coated film surface 31 out as shown in Fig. 5, and the paper is exposed to light to obtain a print 4 bearing the image as a positive normal image.

Step d

The print 4 is cut to a piece of paper 5 having a suitable size and bearing the image. The pieces 5 are arranged on a mounting board 7 of the same size as the print 6 to be obtained, the surface of the board 7 is cut away in the same size as the pieces 5, and the pieces 5 are fitted into the mounting board 7, with the surfaces of the pieces 5 flush with the surface of the mounting board 7.

The layout obtained is identical with the printed layout on the print 6 to be finished with the exception of colour.

Step e

The mounting board 7 with the layout of characters is photographed again with the letterpress camera to prepare identical negative films 8 bearing reverse images only and identical in number to the number of colours.

Step f

Only one image on one of the negative films 8 is obliterated with an opaque ink, while only the other image on the other negative film 8 is similarly inked out for colour separation to obtain negative films 9 for the different colours.

Step g

Each of the negatives 9 is placed on a fresh film with the coated surface of the former in contact with the photosensitive surface of the latter, followed by exposure and printing to obtain a colour separation film 10 bearing the image in the form of a positive normal image for the given colour.

Step h

A plate-making colour separation negative 12 is prepared from each film 10 using an enlarger 23 and a fresh film. When the printing plate 11 finally obtained is fitted around a printing roller for printing, an enlarged image will be obtained, so that the negative 12 is such that the image thereon is formed as contracted in the direction of rotation of the printing roller by an amount corresponding to the enlargement.

Fig. 6 shows the enlarger 23, in which the colour separation film 10 is moved horizontally with the coated surface out, while the fresh film is fitted around a drum 24 and rotated at the same time. The colour separation film 10 is exposed to light from above to form an image on the fresh film on the drum 24 through a mirror 25, lens 26 and slit 27 and obtain a print.

Since the peripheral speed of the drum 24 is slightly lower than the horizontal feed speed of the colour separation film 10, the image of the film 10 is formed on the film overthe drum 24 as a reverse image slightly contracted in the direction of rotation of the drum, whereby the plate-making colour separation negative 12 is obtained.

Step i

A photosensitive flexible synthetic resin sheet (for forming a flexible relief printing plate) is exposed to light in intimate contact with each negative 12, whereby the exposed surface area of the resin sheet is chemically reacted and made insoluble. The unexposed area is etched away by a known chemical treatment to obtain a printing plate 11 having a reverse image in relief.

Each flexible printing plate 11 thus prepared is set on a printing roller to print the image on paper 6 in colour by letterpress printing.

Because the positive-negative relationship of the film, as well as of the photosensitive paper, is dependent on the kind of the photosensitive agent forming the coating, the foregoing description is merely exemplary of the conventional process. It is of course possible to print the image of a negative on photosensitive paper or film in intimate contact therewith to obtain a negative image.

The photosensitive agent coating the film and plate-making material may be such that it becomes soluble when exposed. In this case, it is possible to photograph an original of positive image to obtain a film bearing a positive image (positive-positive film).

The normal image-reverse image relationship is also not limitative; it is possible to photograph a normal image with a mirror- or prism-incorporating camera to obtain a normal image by reversing the image twice through the lens and prism.

Various plate-making processes have been practiced which resort to combinations of positive image-negative image relationship and normal image-positive image relationship.

The conventional process described above requires many steps for preparing finished prints and necessitates manual work for the step d of laying out the paper pieces 5 and fitting the pieces 5 into surface cutouts in the mounting board 7 and for the image inking-out and colours separation step f, so that it has been thought impossible to automatically produce printing plates in large quantities.

Furthermore, the negative films 3, 8, photosensitive paper 4, mounting board 7 and colour separation film 10 prepared are used only once and are not reusable. This renders the printing plate expensively to make. While a sample of finished print must be approved of by the customer who placed an order for prints with the rough sketch 1, the plate maker shows the customer a print of the negative films 8 on sensitive paper obtained by contact printing or the mounting board layout 7 for this purpose. Accordingly in the event of a change in the customer's instructions, the same steps must be repeated again to comply with the customer's request. Further because the sample, i.e. the mounting board 7 or the print, bears positive images only, the conventional process involves the inconvenience that no colour image sample is available before the process is executed to the final step (step i).

Summary of the invention

The object of the invention is a process for preparing a printing plate by producing a film bearing a positive image from an original having a positive image of different colours for the printing plate and preparing the printing plate from the film, characterized by producing the image of the original on a transparent film piece for each separated different colour in a positive form, laying out on a separated transparent film the transparent film pieces bearing the positive image for colour separation, and making the printing plate with the use of the transparent film having the transparent film pieces thereon.

The process of the present invention has the advantage that by laying out a positive and separated colour image-bearing transparent film piece on a transparent film of the same size for colour separation as the finished print to be obtained and using the layout directly for preparing a plate-making negative film or a printing plate.

The process wherein an image is laid out on a transparent film not only reduces the waste of film but also reduces the number of steps needed, making it possible to obtain a finished printing plate within a shortened period of time.

Further for multicolour printing, an image-bearing transparent film piece is laid out on a transparent film for each colour, such that when all the transparent films are superimposed, the assembly shows how the print to be obtained appears, whereas the transparent films, when separated from one another, individually serve as plate-making films for different colours. This eliminates the image obliterating and colour separation procedure conventionally required to greatly reduce the number of steps and shortens the time taken.

Brief description of the drawings

• Fig. 1 is a diagram showing the steps of a process of the invention;
• Fig. 2 is a diagram showing another process embodying the invention;
• Fig. 3 is a diagram showing the steps of preparing a print sample;
• Fig. 4 is a sectional view showing an exposure method for preparing an intaglio plate;
• Fig. 5 is a sectional view showing how the assembly of a negative film and a photosensitive paper is exposed to light;
• Fig. 6 is a sectional view showing an enlarger; and
• Fig. 7 is a diagram showing a conventional plate-making process.

Detailed description of the invention

Fig. 1 shows an exemplary process embodying the present invention for preparing printing plates 11 from a rough sketch 1. The plates 11 are adapted for making finished two-colour prints 6. The process comprises the following steps.

First step

The rough sketch 1 is separated into colours by drawing originals 2 on paper for the printing plates 11 for different colours.

Second step

Each of the originals 2 is photographed with a letterpress camera on the specified enlarged or reduced scale to prepare a negative film 13 bearing a reverse image for the given colour.

The negative films 13 for the different colours can of course be prepared by making a multicolour original 18 first without making the originals 2 for the different colours, and photographing the original 18 in the usual manner with use of colour filters.

Third step

The negative film 13 is placed on the same kind of film with the coated surface of the film 13 out and with the film substrate thereof in contact with the latter, and the assembly is exposed to light to obtain a transparent film piece 14 bearing a normal image thereon.

Fourth step

Transparent films 15 of the same size as the print 6 to be obtained are prepared in the same number as the number of the colours. Each film piece 14 is laid out on each transparent film 15 for colour separation and is affixed thereto with an adhesive tape 16 or an adhesive.

Thus images of the same colour are collected on the same transparent film 15, so that this step corresponds to the conventional image obliterating colour separation step.

The transparent film 15 having the film piece 14 affixed thereto and prepared for the particular colour corresponds to the colour separation film 10 prepared from the image-obliterated negative film 9 for colour separation by the conventional process of Fig. 7. Accordingly the step of preparing the transparent film 15 is followed by the same steps as in various known plate-making processes for preparing relief plates, lithographic plates and like printing plates. The transparent film 15 is used for producing the printing plate 11 by further steps which will be described with reference to Fig. 1.

Fifth step

Each transparent film 15 thus obtained for the given colour is set on the enlarger 23 with the film 15 up and the film piece 14 down and then illuminated with light to obtain a plate-making negative 12 having a reverse image contracted in the direction of rotation of the printing roller.

Sixth step

The printing plate 11 is prepared from the negative 12 in the same manner as heretofore known.

Unlike the conventional process shown in Fig. 7, the present process includes the step of affixing transparent film pieces to transparent films 15 individually for different colours to thereby totally eliminate the most cumbersome, inefficient manual steps of layout, image oblieration and colour separation. The present invention further reduces the number of films to be consumed.

The film piece 14 used can be separated from the transparent film 15 for storage and repeated use by removing the adhesive tape 16 or adhesive. This assures efficient work and a reduced cost.

In the first step, the originals may be prepared from the rough sketch 1 by drawing the images on transparent films to obtain fair copies which are identical with the positive film pieces 14 resulting from the third step. The second step can then be omitted.

Fig. 2 shows another embodiment of the invention which comprises three steps of preparing from a multicolour original 18 transparent films 15 having film pieces 14 laid out thereon.

First step

The images to be printed are drawn on a sheet of paper to obtain the multicolour original 18.

Second step

The multicolour original 18 is photographed on positive-positive films directly through the lens without using a mirror for reflection but using colour filters for colour separation to obtain positive film pieces 14.

Third step

The positive film pieces 14 are laid out on transparent films individually for the different colours and then affixed thereto with an adhesive tape 16 or adhesive to prepare colour separation transparent films 15.

The same steps as in the embodiment of Fig. 1 or in the conventional plate-making process are thereafter performed to prepare plate-making negatives 12 for the different colours with use of an enlarger, and printing plates 11 with use of the negatives 12. Thus the desired print 6 can be obtained.

Fig. 3 shows a process wherein the colour separation transparent films 15 obtained by the process of Fig. 1 or 2 are used for making lithographic plates 17, which are used for offset- printing a proof sheet during the plate-making process.

Step A

The transparent films 15 each bearing a layout for the given colour as stated above are used as original films for the different colours.

Step B

As shown in Fig. 4, the transparent film 15 having the film piece 14 affixed thereto is placed on an aluminum plate (PS plate) 17a coated with a photosensitive agent, and the assembly is exposed to light. The exposed area only of the photosensitive agent, which is insoluble, is made soluble by photoreaction and then chemically etched away to obtain the lithographic plate 17.

As already known, an ink is applied to each lithographic plate 17 and then transferred to a blanket cylinder 28 and further to paper to obtain a finished multicolour print sample 48.

When the transparent film 15 having the film piece 14 thereon is fitted over the photosensitive surface of the aluminum plate 17a and exposed to light for preparing the print sample as shown in Fig. 4, the thickness of the film piece 14, as well as of the adhesive tape 16, is likely to produce a shade line, with the result that a thin line which is absent from the original will appear on the print sample 48. Since the thin line appearing on the sample 48 is attributable to the fact that the exposure lamp is a point light source, the shade line can be eliminated by using an increased number of exposure lamps or by placing a film 22 rough-surfaced by chemical etching or a rough-surfaced mat glass plate over the film piece 14 to hold the film piece and exposing the assembly to light to scatter the light as shown in Fig. 4.

While several embodiments of the invention have been described above with reference to the drawings for illustrative purposes, the invention can be practiced in various modified modes by changing the positive-negative film relationship and the normal-reverse image relationship as already known. Such modifications are to be construed as being embodiments of the invention unless they depart from the inventive scope defined in the appended claims.