EP0888577A1

EP0888577A1 - Industrial method and device for preparing lightened or pre-lightened positive plates for offset printing

Info

Publication number: EP0888577A1
Application number: EP97915511A
Authority: EP
Inventors: Jean-Marie Nouel
Original assignee: Individual
Current assignee: Individual
Priority date: 1996-03-19
Filing date: 1997-03-18
Publication date: 1999-01-07
Also published as: JP2000506997A; WO1997035233A1; FR2746519A1; FR2746519B1; BR9708124A

Abstract

An industrial method and device for preparing lightened or pre-lightened positive plates (for use with a positive film) for (wet or waterless) offset printing, wherein said plates include or may include small ink-impervious areas in the ink-pervious portions, are disclosed. The method typically includes continuously burning a pre-sensitised positive band (1) fed around the wall of a burning cylinder (C2). Developed or undeveloped pre-sensitised positive bands with a burnt light-sensitive layer such that, once developed, said bands include small ink-impervious areas in the ink-pervious portions, are also disclosed.

Description

Industrial process and device for the preparation of positive plates, intended for offset printing, lightened or pre-lightened.

The main objects of the present invention are an industrial process and device for preparing positive plates (for use with a positive film), intended for offset printing (dry offset or wet offset), pre-lightened or lightened, ie called present or having small non-encrophilic surfaces in the encrophilic parts. In other words, it relates to the industrial production, on positive pre-sensitized plates, intended for offset printing, of small non-ink-absorbing surfaces in the ink-absorbing parts (surfaces). The present invention also relates to positive pre-sensitized bands, the photosensitive layer of which has been treated to present, after development, small non-ink-absorbing surfaces. It relates to said bands, developed or not. These strips are intended to be cut into plates; pre-lightened or lightened positive plates, to be implemented, in a conventional manner, in offset printing technique.

It is already specified here that, in the present text, the expression "positive plate" or "positive pre-sensitized plate" is to be taken in the sense which is moreover recognized to this day by man of the trade namely "plate usable by the printer with a positive film". Incidentally, it is noted here that such positive plates generally comprise a photosensitive layer of the positive type, but there are however some with a photosensitive layer of the negative type, which is known to the skilled person.

Before describing in detail each of the aspects of the present invention, the Applicant intends to place the invention in context. To this end, he gives an update below on the one hand on the positive pre-sensitized plates used to date in offset printing and on the other hand on the principle of lightening and its implementation.

The offset process uses plates consisting of a support which has, for printing, encrophobic parts and encrophilic parts. Said support is generally based on a metal plate (aluminum, steel, brass ...).

In wet offset, the encrophobic (hydrophilic) parts consist of said metal plate, advantageously treated (matt aluminum, steel covered with matt chrome, etc.); the encrophilic (hydrophobic) parts generally consist of an encrophilic photosensitive varnish disposed on said metal plate. In dry offset, the ink-absorbing parts consist of an ink-absorbing surface (varnish, metal, etc.) and the encrophobic parts in a coating based on silicones.

A photographic operation with exposure and development makes it possible, in wet offset and in dry offset, to obtain on said support said encrophilic parts and said encrophobic parts.

The offset plates are used by the printer either with positive films, on which the motif to be printed is printed in opaque black, or with negative films, on which the motif to be printed appears in the transparent parts of the negative opaque film. The photosensitive layers can either be of the positive type and then become soluble in a developer for their parts which have received light during exposure, or of the negative type and then become insoluble in a developer for their parts which have received light during l 'sunstroke. The positive layers (of the positive type) are mainly used on the positive pre-sensitized plates using a positive film at the printer while the negative layers (of the negative type) are generally used on the negative pre-sensitized plates using a negative film at the printer.

The offset plates usable by the printer with positive films are, as already indicated above, called "positive plates" or "positive pre-sensitized plates".

They are widely used in Europe, the Far East and South America.

The Applicant proposed an improvement in printing, and in particular in this offset printing technique which uses positive films. In particular to reduce the "fattening" and facilitate said printing, he recommended to lighten it, more precisely to create small non-encrophilic surfaces in the encrophilic surfaces. The basic principle of this lightening technique has been described in patent application FR-A-2 660 245. A preferred implementation thereof - according to a stochastic screening - has been described in the patent application WO -A-9602868.

In order to create said small non-ink-absorbing surfaces in the ink-absorbing surfaces of positive offset plates, the use of a negative (opaque) film has been described having small transparent surfaces through which light passes through on exposure. In wet offset, the small, insoluble ink-absorbing surfaces go into the developer and create the small non-ink-absorbing (and hydrophilic) surfaces in the ink-absorbing parts. In dry offset, small areas insolophobic encrophobes do not leave in the developer and create said small non-encrophilic surfaces in the encrophilic parts. Those skilled in the art can easily understand that the last plates (dry offset) can only be used, pre-lightened, with only one final development. It was recommended to remove from 2 to 26% of the printer surface. The desired reduction ratio is obtained by varying the number and / or the dimensions of said small areas.

We have more precisely described, in Example 1 of said application WO-A-96 02868 the use of a negative film, having small transparent surfaces of approximately 362 μm ~ and the number of which has made it possible to obtain a reduction rate of 5% of the encrophilic surface of a positive pre-sensitized plate. The results, when printed, are very interesting.

The preparation of these lightened positive plates, a preparation which therefore involves a negative film and additional insolation, is however a fairly cumbersome technique. It is necessary in particular, to obtain the film, of adequate dimensions (corresponding to those of the plate, sometimes important), of a rather high price and to handle it delicately (it is advisable not to scratch the fragile surface of the gelatin-silver layer ). The cumbersome nature of the said technique therefore slows down the industrial development of the reduction, a reduction which has already been shown to be of interest.

The Applicant therefore wished to develop a process for the preparation of pre-lightened positive plates (called to present, after development, small non-encrophilic surfaces in the encrophilic parts (surfaces)) or lightened (having small non-encrophilic surfaces in the encrophilic parts (surfaces): industrial process, not involving the handling of a negative film. After having tried various techniques, he retained one which constitutes the first object of the present invention. He has in fact found, surprisingly, that it is possible to produce a quality insolation, continuously, on coils or strips, at relatively high speeds of production lines. It has also developed the device to ensure said quality sunshine, continuously.

According to its first object, the invention therefore relates to an original process for the preparation of positive plates, intended for offset printing (dry offset or wet offset), pre-lightened or lightened. Said method typically comprises: - continuous exposure of a positive pre-sensitized strip, comprising a support coated with a photosensitive layer, put to run, said photosensitive layer in contact with the side wall of a cylinder at least partially hollow said cylinder d 'sunstroke; the running speed of said strip being kept equal to the speed of rotation of said cylinder; said side wall of said insulating cylinder being essentially opaque over at least part of its thickness with the exception of small transparent areas and said insulating cylinder containing means for ensuring said insolation through said small transparent areas; - the cutting of said exposed strip into plates.

For the industrial preparation of positive pre-lightened or lightened plates, therefore, according to the invention, a positive pre-sensitized strip is used. The qualifier "positive" used here with reference to "the strip" has the same meaning as when used with reference to "the plate". Said strip comprises, in a manner known per se, a support (generally metallic) coated with a photosensitive layer; its preparation does not raise any particular problem. Said photosensitive layer of said positive strip - of the positive or negative type - would have to be exposed through positive films by the printer.

Said photosensitive layer of strips useful in the context of the preparation, according to the invention, of plates intended for printing in wet offset, mainly consists of a layer (of a varnish), photosensitive, positive photosensitive. It is not excluded that such a layer is covered with a protective varnish and / or an anti-blur.

In dry offset, the strip useful in the context of the present invention consists of a support coated with a photosensitive ink-absorbing layer, itself coated with an silicone-based ink-absorbing layer. The photosensitive nature of such a photosensitive layer (layer resulting from the combination of at least two layers: encrophilic varnish + encrophobic silicones) is generally due to the encrophilic layer entering into its composition (photosensitive encrophilic varnish). However, other types of "Watcrless" plates have been described, including in their structure:

a support coated with an silicone-based encrophobic layer, itself covered with an ink-absorbing photosensitive varnish,

- a support coated with a photosensitive encrophobic layer based on silicones. It is proposed, in the context of the method of the invention, to continuously insulate said photosensitive layer of said strip, by making it pass in contact with the wall of an insulating cylinder. Said cylinder is a cylinder, at least partially hollow, which has an essentially opaque side wall over at least part of its thickness with the exception of small transparent zones through which the sunshine can be implemented. Said small transparent zones obviously have the characteristics - dimensions, distribution - adapted to the desired lightening.

Said small transparent zones are generated on the surface of said insolation cylinder by any conventional technique familiar to the printing technician, and may be distributed:

- according to a conventional screening or screening with amplitude modulation;

- According to a stochastic framing or framing with frequency modulation: one will then implement a reduction, according to a stochastic framing, as described, in principle, in application WO-A-9602868;

- according to a mixed screening: with frequency modulation whose points have variable areas;

- completely randomly: it is thus possible to generate said small transparent areas without using a frame, for example, by simple projection.

Said small transparent areas are generally distributed according to a certain screening and in the context of a particularly advantageous variant of the invention, they consist of points of the same area, distributed randomly, according to a stochastic screening. Inside the insulating cylinder - at least partially hollow - there are means (light source (s)) for insulating said photosensitive layer through said small transparent areas. The internal geometry of said cylinder is obviously such that said means at least partly illuminate the contact surface between said cylinder and said photosensitive layer. The latter is scrolled at a speed equal to the speed of rotation of said cylinder. It is obviously advisable to avoid any relative displacement, any sliding of said photosensitive layer relative to said cylinder and to ensure the best possible contact. The aim is thus to ensure optimum insolation (without diffusion of light), without damaging the photosensitive layer. When the positive pre-sensitized strip passes, said exposure cylinder can be free to rotate. In this hypothesis, it is driven, in rotation, by said strip. Advantageously, however, in particular to support industrial rates, said cylinder is motorized; its motorization device generally including improved means for controlling its speed of rotation. Obviously, care will be taken to ensure that the exposure implemented through the side wall of the exposure cylinder is carried out under conditions where the expected effect is ensured. To this end, we can play on many parameters:

- the sensitivity of the photosensitive layer,

- the power of the light source, - the exposure time.

This last parameter depends on the speed of travel of the strip in contact with the side wall of the insolation cylinder and on the size of the contact surface between said strip and said cylinder.

This contact surface can be reduced to its simplest expression, i.e. to a straight line if said strip is set to run tangentially to said insolation cylinder. This variant implementation of the process of the invention can only be envisaged if there are very sensitive photosensitive layers: sensitive to a "flash" of sunshine. This is by no means excluded, in the relatively near future. Photosensitive layers, on the market to date, however, with commonly used light sources (ultraviolet lamps), require relatively long exposure times. Thus, the process of the invention has been developed with real contact surfaces between the insolation cylinder and the strip put to run in contact with it; said contact surfaces obviously consisting of a portion of the side wall of said cylinder. Advantageously, such a portion corresponds to at least about 1/4 of a circle. In a particularly preferred manner, such a portion corresponds to approximately 1/2 circle. In the context of this particularly preferred variant, the pre-sensitized strip is made to run in a direction D to come into contact with the insolation cylinder and it leaves, exposed, in the direction opposite to said direction D.

The exposure cylinder is generally arranged so that most, if not all, of the contact surface between said cylinder and the plate is exposed at time t.

The pre-sensitized strip, exposed on contact with the exposure cylinder - thus pre-lightened - can then be cut into plates. According to a first variant of the method of the invention, said pre-lightened strip is, before or after its cutting into plates, developed. We then obtain, at the end of the two successive stages: development / cutting or cutting / development, lightened plates. Said plates are then only subjected to a second exposure through a positive film which contains the document to be printed, then to a second development. The use of such light plates has certain advantages. There is better contact between said lightened plate and said film comprising the document to be printed, intrinsically an anti-blur is implemented ...

According to a second variant of the method of the invention, said pre-lightened strip is cut into plates. The plates, thus obtained, pre-lightened, are not developed. They are used directly for a second exposure through the positive film which contains the document to be printed; second sunstroke followed by a single development.

Whether one or the other of the above variants is implemented (it is recalled here that only the second of said variants can be implemented in the context of the preparation of dry offset plates), the Applicant has found, from surprisingly, that there was no urgency to develop the exposed strips or plates (pre-lightened). Indeed, it has been observed that most of the photosensitive layers can on the one hand be exposed, stored in the state, and developed several weeks later, and on the other hand be exposed, developed, stored in the state and re -solated several weeks later ... This gives the implementation of the process of the invention great flexibility. According to the invention, the supply of lightened plates is therefore proposed, and more precisely, depending on whether or not the process for preparing such plates includes a development stage, the supply of plates that are actually lightened or pre-lightened.

We now propose to describe the second object of the present invention, namely an industrial device for preparing positive or pre-lightened or lightened plates, a device which is particularly useful for implementing the method described above. Said device essentially comprises in an entirely original manner, an insolation cylinder.

It more precisely comprises: - a cylinder at least partially hollow, said insolation cylinder having a substantially opaque side wall over at least part of its thickness with the exception of small transparent areas and containing means for ensuring through said small transparent areas the desired exposure; - Means for the rotation of said cylinder. Several embodiments can be conceived, within the reach of those skilled in the art, of said cylinder. It may in particular have a side wall, of a translucent material such as glass or of a plastic material, made opaque by treatment (metallic deposit or layer of photosensitive varnish or not, on said translucent material) and on which small surfaces have been generated. transparent. Said small transparent surfaces could thus be generated by treatment of the surface made opaque and in particular by exposure, chemical bite, laser engraving, diamond engraving thereof. According to another alternative embodiment, the insulating cylinder may have a side wall made of an opaque material (opaque side wall over its entire thickness) perforated with windows.

Said side wall can be rotatably mounted around a fixed central axis. It can also, according to another embodiment, be secured and rotated by a central axis.

The small transparent zones, for the implementation of the desired insolation, allow the light coming from adequate means - light source (s) - arranged inside the cylinder. Said suitable means may in particular consist of a light source - tube or ramp, in particular - of ultraviolet light arranged along the axis of the cylinder or in at least one such light source of ultraviolet light disposed off-center relative to the axis of said cylinder . Such a light source, offset, is advantageously associated with a reflector. This allows the light emitted to be concentrated on all or part of the surface to be exposed. Advantageously, at least two such light sources are used, each equipped with a reflector.

In general, the means involved in ensuring sunshine do not rotate, during the implementation of said sunshine, so that they continuously expose the entire strip put to run in contact with the side wall of the insolation cylinder (which turns).

Advantageously, the insolation cylinder also contains deflector means for limiting, or even avoiding any parasitic insolation by the part of its side wall not brought into contact with the strip to be exposed. The aim is thus to protect the environment of the device of the invention. Furthermore, insofar as the insolation means, arranged inside the cylinder, do not fail to release calories, provision is made advantageously, inside the insolation cylinder, cooling means. Such means can be arranged according to different variants. In the event that said exposure cylinder is rotatably mounted around a central axis, said central axis will advantageously be arranged so that it constitutes a cooling means, a cooling channel. It is thus possible to provide, forced or not forced circulation (by natural convection) inside such a hollow axis, of a cooling fluid (gas or liquid).

The device of the invention also generally associates with the insolation cylinder - the essential constituent means of said device - at least one other cylinder. It should in particular be able to perfectly control the running speed of the strip to be exposed and its plating on the exposure cylinder.

According to its last object, the invention relates to positive pre-sensitized strips, in particular useful for the manufacture of pre-lightened or lightened positive plates intended for offset printing. Said plates are obtained by cutting said strips. Said strips comprise a support, generally metallic, coated with a photosensitive layer. As specified above, said photosensitive layer consists mainly of an ink-absorbing layer (context of the wet offset) or may comprise an ink-absorbing layer coated with a coating of ink-repellent silicones (context of the dry offset). Typically, said bands are pre-lightened or lightened. In other words, they have been exposed to present, after development, small non-encrophilic surfaces in their encrophilic parts (pre-lightened strip, undeveloped) or have been exposed and developed, so that they contain small surfaces not encrophilic in their encrophilic parts (lightened strip). A person skilled in the art understands that with a (bi-metallic) strip intended to generate bi-metallic plates, it is advisable, after development, to use a chemical bite to reveal the non-encrophilic surfaces, including the small non-encrophilic (lightening) surfaces in the encrophilic surfaces. Said small non-ink-absorbing surfaces are advantageously distributed according to a stochastic screening. The Applicant has developed, in application WO-A-96 02868, the advantages of implementing this type of reduction. To implement it in the context of the present invention, provision is made, on the surface of the insulating cylinder, for the adequate distribution of the small transparent zones (of the same area). It is recalled here that, as specified above, the present invention is not however limited to the context of lightening according to a screening stochastic and notably includes lightening according to a mixed screening and totally random lightening (not involving a frame for its implementation).

A person skilled in the art will have understood, on reading the remarks above, illustrated by the examples below and the appended figures, that the method of the invention can be implemented, with the appropriate supports, each time the small transparent zones of the insolation cylinder, by letting the light pass, create on the surface of said support non-ink-absorbing zones in ink-absorbing parts. The invention is now described with reference to the accompanying figures. It is then illustrated with a few examples.

In said figures, there is shown schematically an embodiment of the device of the invention.

Figure 1 is a side view of such a device; Figure 2, a half front view, in section, of the insolation cylinder of said device; Figure 3 shows a detail (enlargement of area A of Figure 1) of said insolation cylinder.

The device of Figure 1 has three cylinders C1, C2 and C3. The cylinders C1 and C3 are motorized and cause the positive pre-sensitized strip to travel 1. The cylinder C2 is the insulating cylinder of said strip 1. Said insulating cylinder C2 is rotatably mounted around a central axis 2 , fixed.

It is mounted, as shown in FIG. 2, by means of a ball bearing 3 and a support flange 4.

Said central axis 2 of said insolation cylinder C2 is hollow and opens freely at the two ends of said cylinder. It constitutes a cooling channel, the ambient air ensuring the role of cooling fluid.

Said insolation cylinder C2 contains two UV tubes 5, each mounted on a reflector 6. It is also equipped with a deflector 7. Said tubes 5, associated with their reflector 6 and with the deflector 7 are mounted on the central axis 2, fixed. They don't spin. The exposure is carried out, by means of the tubes 5, through small transparent surfaces 10 formed on the external surface made opaque of the side wall of the cylinder C2 (said side wall being of a translucent material treated on the surface to be rendered opaque). Said small transparent surfaces 10 are not represented in FIGS. 1 and 2. They can only be seen in FIG. 3. In said FIG. 3, it has also been clearly shown that the strip 1 comprises a metallic support 1 ′ coated with a layer encrophile photosensitive 1 ". Said strip 1 is obviously made to pass, within the framework of the method of the invention, so that its photosensitive ink-sensitive layer 1" is brought into contact with the side wall of the insulating cylinder C2. Said strip is useful for the preparation of positive plates intended for wet offset printing, pre-lightened or lightened. Example 1

An aluminum coil 30/100 mm thick and 80 cm wide is used. This metal was matted, anodized, sealed and coated with an ink-absorbing positive photosensitive varnish to obtain a positive pre-sensitized strip. Such a strip is thus prepared, as are most of the positive pre-sensitized offset plates on the market today, plates intended for printing in wet offset.

Before its final cutting in the format of the plates, the positive pre-sensitized strip is treated according to the method of the invention. It is scrolled around three cylinders of 1 meter in diameter, the middle cylinder being the sunshine cylinder, the other two having the purpose of making said strip press against said sunshine cylinder and ensuring its drive ( the device used is similar to that of FIG. 1). The three cylinders are motorized and all rotate at the running speed of the strip to avoid any slipping of the latter. The insolation cylinder is supported by two flanges each located on one side of the cylinder. Said cylinder rotates on a fixed axis by means of ball bearings.

The exposure cylinder has a length of 1.10 meters.

Two 1.07 meter long UV tubes with a power of 40 Watts per centimeter, each fitted with a reflector, are fixed inside the insolation cylinder so that the wall of the cylinder which passes through rotating in front of the tubes is constantly lit during its rotation on the fixed shaft. The insolation cylinder also contains two deflector plates (one on each side to prevent annoying dispersion of the light rays). Aspiration is implemented so as to avoid annoying heating.

The cylinder is made of a transparent plastic which, after a very light matting by sandblasting, has been covered by electrolysis with a copper film 50 microns thick.

After fine grinding to avoid any roughness, the copper film is pierced in a stochastic pattern so as to present "holes" (small areas) of approximately 620 square microns and representing a surface total transparent equal to approximately 8% of the total surface of the cylinder. The drilling of said holes is carried out by a technique used for the engraving of copper surface cylinders used in printing with the gravure process.

The positive pre-sensitized strip, moving at a speed of 10 meters per minute, rolls up and is pressed against a portion of cylinder representing approximately

50% of the circumference of the insolation cylinder; the side, covered with the photosensitive ink-absorbing layer being in close contact with the wall of said cylinder.

Said layer is thus exposed to light passing through the transparent parts produced on the opaque surface of the wall of said exposure cylinder. Due to the side deflector plates, only 40% of said surface, facing the tubes, receives the light rays. The metal strip is exposed over its entire length and over its entire width. It is then coated, without being developed, using electrostatic guns with a usual anti-blur coating then with a protective paper and finally cut and packaged in packs of 10 to 20 plates. After delivery, the printer takes a plate in 30/100 thickness, format 785 x 1,030, proceeds to the exposure of positive films representing the texts and the subjects to be printed, to the development, to the erasures and to the erasing of the plate and, that, without changing his habits. With a small magnifying microscope

50 times, he can see that all the ink-absorbing parts are filled with small surfaces without an ink-absorbing layer, distributed in a random manner. The driver notes, with such sheets being put on the machine, that the paper weighing 70 grams per m2 easily leaves the blanket despite a large charge of ink, and that the printing is of superior quality due to '' better stripping in values between 15 and 85% Example 2

The procedure is as in Example 1, but after exposure, the strip passes through a development tank, provided with brushes, and in which the positive layer exposed in small areas is dissolved. After rinsing, drying and cutting, it is possible to see with the same microscope as that of Example 1 that the entire photosensitive ink-absorbing surface of said strip and then of the plates obtained by cutting it is filled with very small surfaces without a layer. encrophile. The thickness of the positive ink layer being approximately 2 microns, these millions of small holes of approximately 620 square microns of surface then facilitate the contact, during exposure, between the positive films and said lightened pre-sensitized ink layer . Example 3

The procedure is as in Examples 1 and 2 above but with a different exposure cylinder. Said insolation cylinder, made of transparent plastic, was covered with a layer composed of gelatin and silver salts, identical to the layers used in printing on films. After exposure, development-fixing then coating of a thin transparent varnish, the cylinder has an opaque wall with transparent surfaces.

Example 4

The procedure is as in examples 1, 2, 3 above, but only one drive cylinder (roller) is used, downstream of the insolation cylinder.

Example 5

The procedure is as in Example 1, but with a plate which includes a support made of stainless steel (encrophobic and hydrophilic metal) covered with two micrometers of copper (encrophilic metal), themselves covered with a positive ink-absorbing photosensitive layer. After exposure by the printer with a positive film (including the motif to be printed) and development, the copper not protected by the photosensitive layer is destroyed by a bite based on iron perchloride, revealing stainless steel. Using a small microscope magnifying 50 times, it is possible to see that the encrophilic parts are filled with tiny holes at the bottom of which the stainless steel is visible. When using such a plate on the machine, the ink is taken up by the ink-absorbing layer and then, as the latter wears, by the underlying copper.

Some of the plates produced in examples 1 to 5 above were, after exposure to sunlight, stored for three months and their use by the printer, at the end of these three months, did not raise any problem. This result is relatively unexpected since printers have the habit of always developing their plates within hours of their insolation. Such a habit has created a real prejudice to the development of the presently claimed invention.

Claims

- Claims -

1. Process for the preparation of positive plates intended for offset printing, pre-lightened or lightened, ie called to present or having small non-ink-absorbing surfaces in the ink-absorbing parts; said process being characterized in that it comprises:

- continuous exposure of a positive pre-sensitized strip (1), comprising a support (1 ′) coated with a photosensitive layer (1 "), put to run, said photosensitive layer (1") in contact with the side wall of an at least partially hollow cylinder, said insolation cylinder (C2); the running speed of said strip (1) being kept equal to the speed of rotation of said cylinder (C2); said side wall of said insulating cylinder (C2) being essentially opaque over at least part of its thickness with the exception of small transparent zones (10) and said insulating cylinder (C2) containing means (5) for ensuring said exposure through said small transparent areas (10);

- the cutting of said strip (1) exposed in plates.

2. Method according to claim 1, characterized in that said insolation cylinder (C2) is motorized.

3. Method according to one of claims 1 or 2, characterized in that said strip (1) is set to run tangentially to said insolation cylinder (C2).

4. Method according to one of claims 1 or 2, characterized in that said strip (1) is scrolled while being held in contact with said insolation cylinder (C2) on a portion of the cylinder corresponding to at least about 1 / 4 of a circle and advantageously about 1/2 a circle and in that it is exposed on at least part of this portion of cylinder.

5. Method for preparing light positive plates according to any one of claims 1 to 4, characterized in that it further comprises the development of said strip (1) exposed before cutting or the development of the plates obtained by cutting said strip (1) exposed, before their new exposure with a positive film which includes the document to be printed.

6. Device in particular useful for implementing the method according to any one of the preceding claims, characterized in that it comprises:

- a cylinder at least partially hollow, said insolation cylinder (C2) having a substantially opaque side wall over at least part of its thickness with the exception of small transparent zones (10) and containing means (5) for ensuring through said small transparent areas (10) the desired exposure;

- Means for the rotation of said cylinder (C2).

7. Device according to claim 6, characterized in that said insulating cylinder (C2) has a side wall made of a translucent material treated on the surface to be made essentially opaque with the exception of small transparent surfaces or has a side wall made of an opaque material with perforated windows.

8. Device according to one of claims 6 or 7, characterized in that said means (5) for insulating consist of a UV light source arranged along the axis (2) of said insulating cylinder (C2) or in at least one UV light source, advantageously equipped with a reflector (6), disposed off-center with respect to the axis (2) of said insulating cylinder (C2).

9. Device according to any one of claims 6 to 8, characterized in that said insulating cylinder (C2) also contains deflector means (7) to limit, or even eliminate any parasitic insolation by the part of its side wall not brought into contact with the strip (1) to be exposed.

10. Device according to any one of claims 6 to 9, characterized in that said insolation cylinder (C2) also contains cooling means.

11. Device according to any one of claims 6 to 10, characterized in that said insolation cylinder (C2) is rotatably mounted around a central axis (2), advantageously arranged to constitute a cooling means.

12. Tape (1) positive pre-sensitization, in particular useful for the preparation of pre-lightened or lightened positive plates, intended for offset printing, comprising a support (T) coated with a photosensitive layer (1 "), characterized in that it has been exposed to present, after development, small non-encrophilic surfaces, advantageously distributed according to a stochastic screening, in its encrophilic parts.

13. Positive pre-sensitized strip (1), in particular useful for the preparation of lightened positive plates intended for offset printing, comprising a support (1 ′) coated with a photosensitive layer (1 ″), characterized in that its encrophilic parts contain small non-encrophilic surfaces, advantageously distributed according to a stochastic screening.