DE69619764T2 - Device for producing a lithographic printing plate with development taking place on the printing press - Google Patents

Description

1. Technical field of the invention.

The present invention relates to a printing device for producing a lithographic printing plate. The present invention particularly relates to a printing device that can be started immediately after the imagewise exposure and development of the lithographic printing plate.

2. General state of the art.

Lithographic printing is the process of printing from specially manufactured surfaces, certain areas of which will attract lithographic ink and other areas will repel the ink when wetted with water. The ink-attracting areas form the printing image areas, and the ink-repelling areas form the background areas.

In the field of photolithography, a photographic material is made to attract oily or greasy colors in the photo-exposed areas (negative working) or in the non-exposed areas (positive working) on a hydrophilic background.

In the production of conventional lithographic printing plates, also referred to as surface lithoplates or planographic printing plates, a support that has an affinity for water or has acquired such an affinity through chemical processing is coated with a thin layer of a radiation-sensitive composition. Suitable layers with a radiation-sensitive composition are light-sensitive polymer layers that contain diazo compounds, dichromate-sensitized hydrophilic colloids and a variety of synthetic photopolymers. Diazo-sensitized layer combinations in particular are widely used.

During image-wise exposure of such a light-sensitive layer, the exposed image areas become insoluble and the non-exposed areas remain soluble. The printing plate is then developed with a suitable liquid to remove the diazonium salt or diazo resin contained in the non-exposed areas.

Commercial diazo plates typically use an anodized and roughened aluminum support with a hydrophilic surface. However, there are also commercially available plates with a flexible support such as a paper support coated with a hydrophilic layer. For example, the Lithocraft 10008 PHOTOPLATETM is a diazo plate that contains a hydrophilic layer on a paper support and a photosensitive diazo layer coated over it. Depending on the plate supplier's instructions, a plate can be made by imagewise exposing the lithographic plate precursor or imaging element, mounting the exposed imaging element on the press, and wiping the plate surface with Lithocraft® 10008 Developer/Desensitizer. The plate supplier's instructions also include a process that does not use a developer/desensitizer. However, such a process usually results in poor lithographic performance, which means that in practice a developer/desensitizer is almost always required.

The radiation-sensitive imaging elements described above, used to make a printing plate, have the particular disadvantage that they must be protected from light. This disadvantage is particularly evident in on-press development (development on the printing press), since the clamping of the image-wise exposed imaging element is usually carried out in normal daylight and the handling time for clamping the imaging element is therefore limited. In addition, diazo printing plates of the aluminum Type not suitable for on-press development. EP 580393 describes a lithographic printing plate that can be produced on-press from a heat-sensitive, ablatable imaging element.

On the other hand, processes are known for producing printing plates using imaging elements that are more sensitive to heat than to light. For example, Research Disclosure No. 33303, January 1992, describes a heat-sensitive imaging element that contains a cross-linked hydrophilic layer with thermoplastic polymer particles and an infrared-absorbing pigment such as carbon black on a support. When exposed imagewise to an infrared laser, the thermoplastic polymer particles coagulate imagewise, making the surface of the imaging element ink-receptive in these areas without further development. A disadvantage of this process is the high susceptibility to damage of the resulting printing plate, because the non-printing areas can become ink-receptive when light pressure is applied to these areas. In addition, the lithographic performance of such a printing plate may be poor under critical conditions and such a printing plate will therefore have a limited lithographic printing latitude.

3. Summary of the present invention.

The object of the present invention is to provide a method in which a printing plate with excellent printing properties is produced in a practical and ecological manner.

Further objects of the present invention will become apparent from the following description.

The objects of the invention are achieved by a process characterized by the following steps for producing a lithographic printing plate.

(1) imagewise exposure to light from an imaging element containing (i) on a hydrophilic surface of a lithographic support an imaging layer comprising hydrophobic thermoplastic polymer particles dispersed in a hydrophilic binder and (ii) a compound capable of converting light into heat contained in the imaging layer or a layer adjacent thereto,

(2) and developing an image-wise exposed imaging element thus obtained by mounting it on a printing drum of a printing press and supplying an aqueous fountain solution and/or printing ink to the image-forming layer while the printing drum is rotating.

The present invention also provides a method for producing multiple copies from an original, characterized by the following steps.

(1) imagewise exposure to light of an imaging element comprising (i) on a hydrophilic surface of a lithographic support an imaging layer comprising hydrophobic thermoplastic polymer particles dispersed in a hydrophilic binder and (ii) a compound capable of converting light into heat contained in the imaging layer or a layer adjacent thereto,

(2) mounting, without prior development, an image-wise exposed imaging element thus obtained on a printing drum of a printing press,

(3) rotating the printing drum, whereby an aqueous fountain solution and/or ink are supplied to the imaging layer of the imaging element, and (4) transferring ink from the imaging element to a receiving element, typically a sheet of paper.

The present invention further provides a method for producing a lithographic printing plate characterized by the following steps

(1) mounting an imaging element which is (i) supported on a hydrophilic surface of a lithographic support an image-forming layer comprising hydrophobic thermoplastic polymer particles dispersed in a hydrophilic binder and (ii) a compound capable of converting light into heat and contained in the image-forming layer or a layer adjacent thereto, onto a printing drum of a printing press,

(2) imagewise laser or LED exposure of the imaging element, and

(3) developing an image-wise exposed imaging element thus obtained by supplying an aqueous dampening solution and/or printing ink to the imaging layer while the printing drum is rotating.

The present invention also provides a method for producing multiple copies from an original, characterized by the following steps.

(1) mounting an imaging element comprising (i) on a hydrophilic surface of a lithographic support an imaging layer comprising hydrophobic thermoplastic polymer particles dispersed in a hydrophilic binder and (ii) a compound capable of converting light into heat contained in the imaging layer or in a layer adjacent thereto on a printing drum of a printing press,

(2) imagewise laser or LED exposure of the imaging element,

(3) developing an image-wise exposed imaging element thus obtained by supplying an aqueous dampening solution and/or printing ink to the imaging layer while the printing drum is rotating, and

(4) the transfer of ink from the imaging element to a receiving element.

The present invention also describes a printing device comprising the following parts.

(i) at least one printing station comprising the following parts

- a printing drum

- a means for mounting an imaging element on the printing drum,

- at least one laser or LED device,

- a means for directing and focusing the laser or LED radiation onto the surface of the imaging element,

- a means which controls the mutual movement of the conducting means and the carrier element so that the surface of the image-forming element is scanned by the laser or LED radiation,

(ii) a means for conveying dampening water to the printing drum,

(iii) means for conveying ink to the printing drum, (iv) means for conveying a receiving member to the printing station, and

(v) a means for transferring ink from the printing drum to the receiving member.

In yet another aspect, the present invention relates to a method for producing a lithographic printing plate characterized by the following steps:

(1) imagewise exposure to light from an imaging element containing (i) on a hydrophilic surface of a lithographic support an imaging layer comprising hydrophobic thermoplastic polymer particles dispersed in a hydrophilic binder and (ii) a compound capable of converting light into heat contained in the imaging layer or a layer adjacent thereto,

(2) and developing an image-wise exposed imaging element thus obtained by rinsing with tap water.

4. Short description of the characters

The present invention is illustrated by the following figures, but is not limited thereto. Fig. 1 is an isometric view of the cylindrical embodiment of an image generating device usable according to the invention and cooperating with a diagonal writing matrix,

Fig. 2 is a schematic representation of the embodiment shown in Fig. 1, illustrating its working mechanism in more detail,

Fig. 3 is a front view of a writing matrix used for image formation according to the invention, in which the image-forming elements are arranged in a diagonal matrix,

Fig. 4 is an isometric view of the cylindrical embodiment of an image generating device usable according to the invention and cooperating with a linear writing matrix,

Fig. 5 is a cutaway perspective of a separate laser and a beam guidance system,

Fig. 6 is an enlarged partial sectional perspective view of a lens element that focuses a laser beam from an optical fiber onto the surface of an imaging element,

Fig. 7 is an enlarged partial sectional perspective view of a lens element with a built-in laser, and

Fig. 8 is a circuit diagram of a laser driver circuit usable according to the invention.

5. Detailed description of the present invention

An imaging element useful in accordance with the invention comprises, on a hydrophilic surface of a lithographic support, an imaging layer comprising hydrophobic thermoplastic polymer particles dispersed in a hydrophilic binder. The hydrophilic binder used in the present invention is preferably not or only slightly cross-linked. The imaging element further contains a compound capable of converting light into heat. Although this compound is preferably incorporated in the imaging layer, it can also be contained in a layer adjacent to the imaging layer.

According to one embodiment of the invention, the lithographic support may be an anodized aluminum support. A particularly preferred lithographic support is an electrochemically grained and anodized aluminum support. The anodized aluminum support in the present invention may be subjected to processing to improve the hydrophilic properties of the support surface. For example, the aluminum support may be siliconized by processing the support surface with a sodium silicate solution at an elevated temperature, e.g. 95°C. Alternatively, phosphate processing may be carried out, wherein the alumina surface is processed with a phosphate solution optionally further containing an inorganic fluoride. Furthermore, the alumina surface may be rinsed with a citric acid or citrate solution. This treatment may be carried out at room temperature or at a slightly elevated temperature between about 30°C and 50°C. Another interesting method is to rinse the aluminum oxide surface with a bicarbonate solution. It is also obvious that one or more of these post-treatments can be carried out separately or in combination.

According to a further embodiment of the invention, the lithographic support comprises a flexible support, such as a paper support or a plastic film, coated with a cross-linked hydrophilic layer. A particularly suitable cross-linked hydrophilic layer can be obtained from a hydrophilic binder cross-linked with a cross-linking agent such as formaldehyde, glyoxal, polyisocyanate or a hydrolyzed tetraalkyl orthosilicate. The latter binder is particularly preferred.

Hydrophilic binders that can be used are hydrophilic (co)polymers such as homopolymers and copolymers of vinyl alcohol, acrylamide, methylolacrylamide, methylolmethacrylamide, acrylic acid, methacrylic acid, hydroxyethyl acrylate, hydroxyethyl methacrylate or maleic anhydride-vinyl methyl ether copolymers. The hydrophilicity of the (co)polymer or (co)polymer mixture used is preferably higher than or equal to the hydrophilicity of at least 60% by weight, preferably at least 80% by weight, hydrolyzed polyvinyl acetate.

The amount of crosslinking agent, in particular tetraalkyl orthosilicate, is preferably at least 0.2 parts by weight per part by weight of hydrophilic binder, is preferably between 0.5 and 5 parts by weight, particularly preferably between 1.0 part by weight and 3 parts by weight per part by weight of hydrophilic binder.

A cross-linked hydrophilic layer in a lithographic support used according to this embodiment preferably also contains substances which improve the mechanical strength and porosity of the layer. For this purpose, colloidal silica can be used. The colloidal silica can be used in the form of any commercially available water dispersion of colloidal silica with, for example, an average particle size of up to 40 nm, e.g. 20 nm. In addition, inert particles with a larger grain size than the colloidal silica can be added, e.g. silica prepared as described in J. Colloid and Interface Sci., Volume 26, 1968, pages 62 to 69, by Stöber, or alumina particles or particles with an average diameter of at least 100 nm, which are particles of titanium dioxide or other heavy metal oxides. By embedding these particles, the surface of the cross-linked hydrophilic layer acquires a uniform rough texture with microscopic peaks and valleys that serve as storage sites for water in background areas.

The thickness of a cross-linked hydrophilic layer in a lithographic support used according to this embodiment can vary between 0.2 µm and 25 µm and is preferably between 1 µm and 10 µm.

Particular examples of suitable crosslinked hydrophilic layers usable according to the invention are described in EP-A 601 240, GB-P 1 419 512, FR-P 2 300 354, US-P 3 971 660, US-P 4 284 705 and EP-A 514 490.

As a flexible support of a lithographic support according to this embodiment, a plastic film is particularly preferred, for example a subbed polyethylene terephthalate film, cellulose acetate film, polystyrene film, polycarbonate film, etc. The plastic film support can be opaque or translucent.

A polyester film support coated with an adhesion-improving layer is particularly preferred. Adhesion-improving layers that are particularly suitable for use according to the invention contain a hydrophilic binder and colloidal silica, as described in EP-A 619 524, EP-A 620 502 and EP-A 619 525. The amount of silica in the adhesion-improving layer is preferably between 200 mg/m² and 750 mg/m². Furthermore, the ratio of silica to hydrophilic binder is preferably above 1 and the specific surface area of the colloidal silica is preferably at least 300 m²/g, particularly preferably 500 m²/g. In the present invention, an image-forming layer is cast onto a hydrophilic surface. One or more intermediate layers are optionally inserted between the lithographic support and the image-forming layer. An image-forming layer according to the invention contains thermoplastic polymer particles dispersed in a hydrophilic binder.

Suitable hydrophilic binders for use in an image-forming layer according to the invention are, for example, synthetic homo- or copolymers such as a polyvinyl alcohol, a poly(meth)acrylic acid, a poly(meth)acrylamide, a Polyhydroxyethyl (meth)acrylate, a polyvinyl methyl ether or natural binders such as gelatin, a polysaccharide such as dextran, pullulan, cellulose, gum arabic and alginic acid.

The hydrophobic thermoplastic polymer particles used in the invention preferably have a coagulation temperature above 35°C and more preferably above 50°C. Coagulation can occur as a result of softening or melting of the thermoplastic polymer particles under the action of heat. Although the coagulation temperature of the thermoplastic hydrophobic polymer particles is not subject to a specific upper limit, it should be sufficiently below the decomposition temperature of the polymer particles. The coagulation temperature is preferably at least 10°C below the temperature at which decomposition of the polymer particles occurs. If the polymer particles are exposed to a temperature above the coagulation temperature, they coagulate and form a hydrophobic agglomerate in the hydrophilic layer, whereby the hydrophilic layer becomes insoluble in tap water or an aqueous liquid at these locations.

Specific examples of hydrophobic polymer particles usable according to the invention are, for example, polyethylene, polyvinyl chloride, polymethyl (meth)acrylate, polyethyl (meth)acrylate, polyvinylidene chloride, polyacrylonitrile, polyvinylcarbazole, etc. or copolymers thereof. Polyethylene is particularly preferred.

The weight average molecular weight of the polymers can vary between 5,000 and 1,000,000 g/mol.

The particle size of the hydrophobic particles can vary between 0.01 µm and 50 µm, more preferably between 0.05 µm and 10 µm and most preferably between 0.05 µm and 2 µm.

The polymer particles are contained as a dispersion in the aqueous coating liquid of the image-forming layer and can be prepared by the processes described in US-P 3 476 937. In another, for the Process particularly suitable for preparing an aqueous dispersion of the thermoplastic polymer particles.

- the hydrophobic thermoplastic polymer is dissolved in an organic water-immiscible solvent,

- the solution thus obtained is dispersed in water or an aqueous medium, and

- the organic solvent is evaporated.

The amount of hydrophobic thermoplastic polymer particles contained in the image-forming layer is preferably between 20 wt.% and 65 wt.%, more preferably between 25 wt.% and 55 wt.%, and most preferably between 30 wt.% and 45 wt.%.

As suitable light-to-heat-converting compounds, infrared-absorbing components are preferably used, although the absorption wavelength is not of great importance as long as the absorption of the compound used falls within the wavelength range of the light source used for image-wise exposure. Particularly useful compounds are, for example, dyes and in particular infrared dyes, carbon black, metal carbides, metal borides, metal nitrides, metal carbonitrides, oxides with a bronze structure and oxides

with a structure related to the bronze family, but without the A component, e.g. WO2,9. Conductive polymer dispersions can also be used, such as conductive polymer dispersions based on polypyrrole or polyaniline. The lithographic performance achieved and in particular the print run durability achieved depends on the heat sensitivity of the imaging element. In this respect, it has been found that very good and favorable results can be achieved with carbon black.

Although a light-to-heat converting compound according to the invention is most preferably embedded in the image-forming layer, at least part of the light-to-heat converting compound can also be incorporated into an adjacent layer. Such a layer can, for example, be the crosslinked hydrophilic layer a lithographic support according to the second embodiment of lithographic supports explained above.

According to a method according to the invention for obtaining a printing plate, the image-forming element is exposed image-wise and then mounted on a printing drum of a printing press. In a preferred embodiment, the printing press is then started and, while the printing drum with the image-forming element mounted thereon is rotated, first the dampening rollers supplying dampening water and then the ink form rollers are lowered onto the image-forming element. In general, the first clear and usable prints are obtained after about 10 revolutions of the printing drum.

According to an alternative method, the inking rollers and dampening rollers can be lowered simultaneously or the inking rollers can be lowered first.

The fountain solutions that can be used according to the invention are aqueous liquids that generally have an acidic pH value and contain an alcohol such as isopropanol. The fountain solutions that can be used according to the invention are not subject to any particular restrictions and commercially available fountain solutions can be used.

It may be advantageous to wipe the image-forming layer of an image-wise exposed imaging element with, for example, a water-soaked cotton ball or sponge before mounting the imaging element on the printing press or at least before starting the printing press. This will remove certain non-image areas but will not initiate the actual development of the imaging element. However, this procedure has the advantage of avoiding the risk of significant contamination from the dampening system of the printing press and the printing ink used.

In a further process, the image-forming element is first mounted on the printing drum of the printing press and then exposed image-wise directly on the press. After exposure, the image-forming element can be developed as described above.

According to yet another method according to the invention, the imaging element can be exposed imagewise and subsequently developed by rinsing with tap water.

The imagewise exposure according to the invention is preferably an imagewise scanning exposure using a laser or an LED diode laser. According to the invention, the use of a laser emitting in the infrared range (IR) and/or near infrared range, i.e. in the wavelength range between 700 and 1500 nm, is very particularly preferred. Laser diodes emitting in the near infrared range are particularly preferred for use in the present invention.

A preferred imaging device for image-wise scanning exposure according to the invention preferably comprises a laser whose power is applied directly to the surface of the imaging element through lenses or other beam directing elements or whose power is transferred from a remote laser to the surface of an unexposed imaging element through a fiber optic cable. A controller and associated positioning hardware ensure precise orientation of the radiation to the surface of the imaging element, ensure radiation scanning across the surface, and turn the laser on at locations adjacent to selected points or areas of the imaging element. The controller is responsive to input image signals corresponding to the original document and/or image to be copied onto the imaging element to produce a precise negative or positive image of that original. The image signals are stored in a computer in the form of a bit match table file. Such files can be generated by a raster image processor (RIP) or other suitable means. For example, a RIP can accept input data in page description language, which defines all elements to be transferred to the image-producing element, or as a combination of page description language and one or more several image files. The bit correspondence tables are used to define the color tone as well as the screen frequencies and screen angles in amplitude-modulated screening. However, the present invention is particularly suitable for use in combination with frequency modulation screening, as described for example in EP-A 571010, EP-A 620677 and

EP-A 620674.

The imaging device can be autonomous, acting only as a plate automaton, or can be built directly into a lithographic printing press equipped with a dampening system. In the latter case, printing can begin immediately after imagewise exposure and development, thereby achieving a significant reduction in press preparation time. The imaging device is a flatbed printer or a flat drum printer, in which the unexposed lithographic printing plate is attached to the cylindrical inner or outer surface of the drum. Of course, the structure of the outer drum is more suitable for in-situ use on a lithographic printing press, with the printing drum itself embodying the drum component of the recorder or plotter.

In a preferred drum configuration, the required relative movement between the laser beam and the imaging element is obtained by rotating the drum (and thus also the imaging element mounted thereon) about the drum axis and by moving the laser beam parallel to the axis of rotation, thereby scanning the circumference of the imaging element and gradually obtaining a "growing" image in the axial direction. The movement of the laser beam can also be set parallel to the drum axis, whereby the scanning angle increases after each scanning pass over the imaging element, thereby obtaining a circumferentially "growing" image on the imaging element. In both cases, after complete laser scanning and development, a faithful image on the surface of the imaging element. In the flatbed configuration, the beam scans along one of the imaging element's axes and is indexed after each pass along the other axis. Of course, the required relative motion between the laser beam and the imaging element can be achieved by motion of the imaging element rather than by (or in addition to) laser beam motion.

Regardless of the scanning method of the laser beam, it is usually preferable to use a large number of lasers (for speed reasons) whose power is used to drive a single writing matrix. The writing matrix is then indexed after each pass over or along the image-forming element to be spaced apart by a distance determined by the number of laser beams originating from the matrix and the desired image resolution (i.e. the number of pixels per unit length).

A more detailed description of a preferred embodiment of an image forming device usable in accordance with the invention follows below.

a. External drum recording

Referring to Figure 1 of the various figures, there is shown an external drum embodiment of an imaging system useful in accordance with the present invention. The assembly includes a drum 50 around which an imaging member 55 is wound. Drum 50 includes a hollow segment 60 in which the outer edges of imaging member 55 are secured by conventional clamping elements (not shown). The size of the hollow segment can vary widely depending on the environment in which drum 50 is used.

Drum 50 is preferably installed straight forward in the structure of a conventional lithographic printing press equipped with an element supplying dampening water to the image-forming element and serves as Printing drum of the press. In a typical printing press design, the supply of ink and fountain solution to the imaging element 55 is accomplished by means of an ink supply line or a series of fountain rollers, the end rollers of which are in rolling contact with drum 50. The latter drum is also in rolling contact with a blanket cylinder which transfers ink to the receiving element, typically a sheet of paper. The printing press may be equipped with more than one such printing assembly in a linear matrix. Alternatively, a plurality of printing assemblies may be arranged around a large central printing cylinder in rolling contact with all of the blanket cylinders.

Drum 50 is mounted within a support frame and is driven by a standard electric motor or other conventional means (see schematic diagram in Fig. 2). The angular position of drum 50 is monitored by a hollow shaft encoder (see Fig. 4). A writing matrix 65 mounted on a spindle 67 and a guide rod 69 writes to the rotating imaging element 55. The axial movement of writing matrix 65 is controlled by the rotation of a step motor 72 which drives spindle 67 and thereby shifts the axial position of writing matrix 65. After each complete passage of writing matrix 65 over the entire surface of imaging element 55, step motor 72 is energized during the time that writing matrix 65 is above cavity 60. By rotation of step motor 72, writing matrix 65 shifts to the appropriate axial location to begin the next image formation pass.

The axial index spacing between successive image forming passages depends on the number of image forming parts in the writing matrix 6S and their configuration in the matrix, as well as on the desired resolution. Fig. 2 shows a series of laser sources L1, L2, L3 driven by suitable laser driver circuits designated as a whole by reference number 75. ... Ln, the power of which is fed into a fiber optic cable. Gallium arsenide lasers are preferred as lasers, although other laser models are also possible.

The cables carrying the laser power are collected in a bundle 77 and exit as separate cables connected to the writing matrix 65. To avoid loss of laser power, it may be desirable to arrange the bundle in such a configuration that it does not have to be bent above the critical refraction angle of the fiber (thus maintaining total internal reflection).

Fig. 2 also shows a control unit 80, which then controls the laser drivers 75 when the associated lasers come to lie opposite the points to be described on the image-forming element 55. Control unit 80 also ensures the drive of the step motor 72 and the drum drive motor 82.

Controller 80 is supplied with data from two sources. A sensor 85 continuously monitors the angular position of drum 50 relative to writing matrix 65 and supplies controller 80 with the corresponding position signals. In addition, data signals from an image data source (e.g. a computer) are also supplied to controller 80. The image data correspond to points on image-forming element 55 which are to be described with pixels. To do this, controller 80 establishes a correlation of the actual relative positions (as indicated by sensor 85) of writing matrix 65 and image-forming element 55 with the image data, thereby switching on the relevant laser drivers at the appropriate times during scanning of image-forming element 55. The control circuit required to implement this scheme is well known to those skilled in the art of scanners and plotters.

At their end, the laser power cables run into lens arrangements 96 mounted within the writing matrix 65, which preferably direct the laser beam precisely onto the surface of the imaging element 55. The following is a description of the construction of a suitable lens array, such assemblies being generally designated by reference numeral 96 in the present invention. An illustration of a suitable arrangement is shown in Fig. 3, in which the lens assemblies 96 are arranged in a staggered manner over the surface of the body 65.

The staggered arrangement of the lens systems makes it easier to arrange a larger number of lens assemblies in a single head than would be possible with a linear arrangement. Since the image formation time is directly proportional to the number of lens elements, a staggered arrangement makes it possible to speed up the overall image formation. Controller 80 is either supplied with image data already arranged in a vertical column, each belonging to a different lens assembly, or can gradually extract the contents of a memory buffer, in which a complete bit correspondence table of the image to be transmitted is stored, from the buffer, column by column. In both cases, controller 80 recognizes the various relative positions of the lens assemblies with respect to image-forming element 55, and only controls the appropriate laser when the associated lens assembly is located over a point to be imaged.

An illustration of an alternative matrix structure is shown in Fig. 4, which also shows the sensor 85 attached to drum 50. In this case, the writing matrix, designated by reference number 150, comprises a long linear body which is loaded with light guide cables from bundle 77. The inside of writing matrix 150, or a certain portion thereof, contains threads engaging spindle 67, with rotation of spindle advancing writing matrix 150 along image forming element 55, as discussed above. The individual lens assemblies 96 are each equidistantly spaced apart at a distance B from each other. Distance B corresponds to the difference between the axial length of plate 55 and the distance between the first and last lens array. Distance B corresponds to the total axial distance traveled by writing matrix 150 during a complete scanning pass. During each passage of writing matrix 150 across cavity 60, step motor 72 is energized and rotates to advance matrix 150 an axial distance corresponding to the desired spacing between imaging passes (ie, print density). This distance is smaller than the distance indicated in the embodiment described above (writing matrix 65) by a factor of n, where n is the number of lens arrays incorporated in writing matrix 65.

b. Performance (guide and lens arrangement)

An illustration of suitable means for delivering laser power to the surface of an imaging element is shown in Figures 5 through 7. With respect to Figure 5, this shows a separate laser assembly that uses a fiber optic cable to transmit the laser pulses to the imaging element. In this assembly, a laser source 250 is powered by an electrical cable 252. Laser 250 is located in the rear segment of a housing 255. Two or more focusing lenses 260a and 260b are located in the front segment of the housing. The lenses focus the radiation from laser 250 onto the end face of a fiber optic cable 265, which is preferably (although not necessary) secured within the housing 255 by means of a removable retaining capsule 267. Cable 265 conducts the power from laser 250 to an output assembly 270 illustrated in detail in Fig. 6.

Fig. 6 shows how fiber optic cable 265 is passed through a (preferably removable) retaining cap 274 in assembly 270. Retaining cap 274 fits onto a generally tubular body 276 which contains a series of threads 278.

In the front part of body 276 there are two or more focusing lenses 280a and 280b. Cable 265 is connected by a Barrel 280 passes through a portion of body 276. Body 276 forms a hollow channel between inner lens 280b and the end of barrel 280. The end face of cable 265 is thereby located a predetermined distance A from inner lens 280b. Distance A and the focal lengths of lenses 280a and 280b are sized so that the radiation supplied to cable 265 is precisely focused on the surface of the imaging element at a normal beam distance to the imaging element 55. This distance can be adjusted to vary the size of an image element.

Body 276 may be secured to writing matrix 65 by any suitable technique. In the illustrated embodiments, a nut 282 engages threads 278 and compresses an outer flange 284 of body 276 against the outer surface of writing matrix 65. The flange may optionally be provided with a translucent window 290 to protect the lenses from possible damage.

Alternatively, the lens assembly can be mounted on a pivot point within the writing matrix which ensures rotation of the lens assembly in the axial direction (i.e., in Fig. 6, through the plane of the paper). This pivot point allows the axial position to be precisely adjusted. At a rotation angle of 4º or less, the circumferential error caused by the rotation can be compensated for electronically by shifting the image data before it is fed to the controller 80.

Fig. 7 shows an alternative embodiment in which the laser source irradiates the surface of the imaging element directly without transmission through a fiber optic cable. From this FIGURE it can be seen that laser source 250 is arranged within the rear segment of an open housing 300. In the front part of the housing 300 there are two or more focusing lenses 302a and 302b which precisely focus the radiation from laser 250 onto the surface of the imaging element 55. The housing can optionally be covered with a light-transmitting lens arranged in the same plane as the open end arranged window 305 and a heat sink 307.

It should be noted that although optical fibers are used in the above discussion of the imaging embodiments and the associated FIGURES, in any event the use of fibers can be eliminated by using the embodiment shown in Figure 7.

c. Driver circuit

A suitable driver circuit for driving a diode-type laser (e.g., a gallium arsenide laser) is schematically illustrated in Fig. 8. The operation of the driver circuit is controlled by controller 80, which applies a fixed pulse width signal (preferably between 5 and 20 µsec in duration) to a high speed, high voltage MOSFET driver 325. The output terminal of driver 325 is connected to the gate of a MOSFET 327. Since driver 325 is capable of providing a high output voltage to quickly charge the capacitance of the MOSFET gate, the turn-on and turn-off times of MOSFET 327 are very short (preferably less than 0.5 µsec) despite the capacitive loading. The source terminal of MOSFET 327 is connected to a ground potential.

Once the MOSFET 327 is set in the conductive state, a laser diode 330 is controlled with current. Between the MOSFET 327 and the laser diode 330 there is an adjustable current limiting resistor 332 that controls the diode power. This control can be used, for example, to correct different diode powers and to generate identical output strengths in all lasers belonging to the system, or to vary the laser power in order to control the image size.

Opposite the terminals of laser diode 330 there is a capacitor 334 which prevents damage from current overshoots, which may occur, for example, due to conductor inductance in combination with low Interelectrode capacitance of the laser diodes is prevented.

The present invention will now be illustrated by the following examples, but is not limited thereto. All parts are by weight unless otherwise stated.

EXAMPLE 1 Preparation of a lithographic support

A 0.2 mm thick aluminium foil is degreased by immersing the foil in an aqueous solution containing 5 g/l sodium hydroxide at 50ºC and rinsing it with demineralised water. The foil is then electrochemically grained with alternating current at a temperature of 35ºC and a current density of 1,200 A/m² in an aqueous solution containing 4 g/l hydrochloric acid, 4 g/l hydroboric acid and 5 g/l aluminium ions to obtain a surface topography with an arithmetic mean roughness Ra of 0.5 µm.

After rinsing with demineralized water, the aluminum foil is etched with an aqueous solution containing 300 g/l sulfuric acid for 180 s at 60ºC and then rinsed with demineralized water for 30 s at 25ºC.

The foil is then anodized at a temperature of 450°C, a voltage of about 10 V and a current density of 150 A/m² for about 300 s in an aqueous solution containing 200 g/l sulfuric acid to obtain an anodic oxidation foil containing 3 g/m² Al₂O₃, washed with demineralized water, then post-processed with a solution containing 20 g/l sodium bicarbonate for 30 s at 40°C, then rinsed with demineralized water for 120 s at 20°C and dried.

The lithographic support thus obtained is then incubated for 60 s in an aqueous solution containing 5 wt.% citric acid immersed, rinsed with demineralized water and dried at 40ºC.

Manufacturing of the imaging element (material)

To prepare an imaging element according to the invention, the following coating composition is prepared, coated in an amount of 30 g/m² (wet layer amount) on the lithographic support described above and dried at 35°C.

Preparation of the casting composition

7 g of a 15% carbon black dispersion, 1 ml of an aqueous wetting agent solution and 73 g of water are added successively to 6.75 g of a 20% dispersion of polymethyl methacrylate (average particle diameter of 90 nm) stabilized with Hostapal B (1%, based on the polymer) in demineralized water, and finally 12 g of a 5% aqueous solution of a 98% hydrolyzed polyvinyl acetate with a weight average molecular weight of 200,000 g/mol are stirred in.

Making a printing plate and printing prints

The imaging element described above is mounted on the printing drum of a printing press equipped with a dampening system.

The imaging element described above is scanned with a Nd:YLF infrared laser emitting at 1050 nm (scanning speed 4 m/s, beam width 15 µm and laser power on the surface of the imaging element 670 mW).

After the imaging element has been exposed, the press is started and the printing drum with the imaging element mounted on it is allowed to rotate. The dampening rollers of the printing press are then applied to the surface of the imaging element and supply dampening solution to the imaging element. After 10 revolutions of the printing drum, the ink supply rollers are also lowered onto the imaging element. After a further 10 revolutions, clear prints are obtained without ink absorption in the non-image areas.

EXAMPLE 2

An imaging element is prepared as described in Example 1, except that the lithographic support is not immersed in a citric acid solution and is immediately coated with the coating composition.

A printing plate is prepared as described in Example 1 and after a total of 20 revolutions of the printing drum, clear prints are obtained with no ink uptake in the non-image areas.

EXAMPLE 3 Preparation of a lithographic support

195 g of an aqueous hydrolyzed 22% tetramethyl orthosilicate emulsion and 12 g of a 10% solution of a wetting agent are stirred successively into 398 g of a dispersion containing 21.5% TiO2 (average particle size between 0.3 µm and 0.5 µm) and 2.5% polyvinyl alcohol in demineralized water.

395 g of demineralized water are added to this mixture and the pH is brought to 4.

The resulting dispersion is cast in a wet layer thickness of 50 g/m² onto a polyethylene terephthelate film support (coated with a hydrophilic adhesive layer), dried at 30°C and then subjected to a 1-week curing period at a temperature of 57°C.

An imaging element and a printing plate are then prepared using the lithographic support described above as described in Example 1. After 20 revolutions of the printing drum, clear prints are obtained with no ink uptake in the non-image areas.

Claims (3)

1. A process for making lithographic printing plates characterized by the following steps. (i) Mounting on the printing drum of a printing device, which includes at least one printing station comprising the following parts. - a printing drum - a means for mounting an imaging element on the printing drum, - at least one laser or LED device, - a means for directing the laser or LED radiation and focusing it on the surface of the imaging element, - a means which controls the mutual movement of the conducting means and the carrier element in such a way that the surface of the image-forming element is scanned by the laser or LED radiation, from an imaging element which contains (i) on a hydrophilic surface of a lithographic support an imaging layer comprising hydrophobic thermoplastic polymer particles dispersed in a hydrophilic binder and (ii) a compound capable of converting light into heat and which is contained in the imaging layer or in a layer adjacent thereto, (ii) information-wise exposure of the imaging element by means of a laser or LED device, (iii) supplying dampening water to the printing drum by means of a means designed for supplying dampening water to the printing drum, (iv) after, before or simultaneously with step (iii), supplying ink to the printing drum by means designed for supplying ink to the printing drum, (v) feeding a receiving element to the printing station by means designed for feeding a receiving element to the printing station, and (vi) supplying ink from the printing drum to the receiving member by means designed to supply ink from the printing drum to the receiving member. 2. A method for producing lithographic printing plates according to claim 1, characterized in that the laser is an infrared laser or a plurality of infrared lasers. 3. A method for producing lithographic printing plates according to claim 1, characterized in that the printing device contains a plurality of printing stations.