EP0976573A1

EP0976573A1 - Flexibly supported lithographic printing plate having an improved dimensional stability

Info

Publication number: EP0976573A1
Application number: EP98202539A
Authority: EP
Inventors: Bartholomeus c/o Agfa Gevaert N.V. Verlinden; Johan C/O Agfa-Gevaert N.V. Van Hunsel
Original assignee: Agfa Gevaert NV
Current assignee: Agfa Gevaert NV
Priority date: 1998-07-29
Filing date: 1998-07-29
Publication date: 2000-02-02
Also published as: JP2000043438A

Abstract

The present invention provides an element comprising on a flexible support a lithographic surface having ink accepting and ink repellent areas or a latent lithographic surface resulting in a lithographic surface upon development, said imaging element being laminated with its side opposite to the lithographic surface to a dimensionally stable assembly, the imaging element having a high friction coefficient with respect to the dimensionally stable assembly.

Description

1. Field of the invention

The present invention relates to a method for making a lithographic printing plate comprising a flexible support and laminated thereto a dimensionally stable base.

2. Background of the invention

Lithographic printing is the process of printing from specially prepared surfaces, some areas of which are capable of accepting ink (oleophilic areas) whereas other areas will not accept ink (oleophobic areas). The oleophilic areas form the printing areas while the oleophobic areas form the background areas.

Two basic types of lithographic printing plates are known. According to a first type, so called wet printing plates, both water or an aqueous dampening liquid and ink are applied to the plate surface that contains hydrophilic and hydrophobic areas. The hydrophilic areas will be soaked with water or the dampening liquid and are thereby rendered oleophobic while the hydrophobic areas will accept the ink. A second type of lithographic printing plates operate without the use of a dampening liquid and are called driographic printing plates. This type of printing plates comprise highly ink repellent areas and oleophilic areas. Generally the highly ink repellent areas are formed by a silicone layer.

Lithographic printing plates can be prepared using a photosensitive lithographic printing plate precursor, also called imaging element. Such imaging element is exposed to electromagnetic radiation or heat in accordance with the image data and is generally developed thereafter so that a differentiation results in ink accepting properties between the exposed and unexposed areas.

Examples of photosensitive lithographic printing plate precursors are for example: the silver salt diffusion transfer (hereinafter DTR) materials disclosed in EP-A-410500, EP-A-483415 and EP-A-423399; imaging elements having a photosensitive layer containing diazonium salts or a diazo resin as described in e.g. EP-A-450199; imaging elements having a photosensitive layer containing a photopolymerizable composition as described in e.g. EP-A-502562, EP-A-491457, EP-A-503602, EP-A-471483 or DE-A-4102173.

Alternatively a lithographic printing plate may be prepared from a heat mode recording material as a lithographic printing plate precursor. Upon application of a heat pattern in accordance with image data and optional development the surface of such heat mode recording material may be differentiated in ink accepting and ink repellent areas. The heat pattern may be caused by a direct heating source such as a thermal head but may also be caused by a light source as e.g. a laser. In the latter case the heat mode recording material will include a substance capable of converting the light into heat. Heat mode recording materials that can be used for making a lithographic printing plate precursor are described in e.g. EP-A-573091, DE-A-2512038, FR-A-1.473.751, Research Disclosure 19201 of April 1980 or Research Disclosure 33303 of January 1992.

As supports for the above mentioned lithographic printing plates there are known metal supports such as e.g. aluminium and flexible supports such as e.g. paper or polyester film supports. Generally the flexible supports are used for short run jobs where they have a cost advantage over metal supports. Furthermore, if a transparent flexible support is used, exposure of the lithographic printing plate precursor may proceed through the support which allows the use of cameras without special optics. Moreover, imaging elements with a flexible support can be used on all kinds of laser imagesetters for film, whereas it is expensive and complex to use imaging elements having a metal support on laser imagesetters using internal drum technology, since automatic plate loading and unloading is complex due to the low flexibility of the metal support. Thus, when computer-to-plate exposure on an imaging element having a metal support is to be made, heavy investment costs are required.

A disadvantage of the use of flexibly supported lithographic printing plates is the inherently low dimensional stability which is one of the causes hampering or making totally impossible the use of the printing plate especially when two or more images need to be printed over each other in register such as e.g. in colour printing. Furthermore the handling and mounting on a printing press of a lithographic printing plate having a flexible support is cumbersome, surely when the printing plate has considerable dimensions. Attempts have therefore been made to increase the dimensional stability of a flexibly supported printing plate by e.g. using a thicker flexible support. Although this brings some improvement, there is still a need for further improvement. Furthermore, the use of a thicker support may cause problems in some processing equipment used for processing the printing plate precursor. The use of a thicker support can also cause problems on the printing press because on most printing presses only printing plates with a thickness of at most 0.30 mm or at most 0.40 mm can be used.

Patent application EP-A-0 194 111 discloses a polyester printing plate that is mounted on a press after double-creasing the head side of plate. The plate is preferably mounted over an underpacking with which it has a high effective coefficient of friction. The trailing side of the plate is attached with a low-tack repositionable adhesive. A disadvantage of this mounting method is the repositioning of the plate when the plate is being inked up on the press, at slow press rotation; this repositioning is not reproducible and hence problematic for colour printing. Another disadvantage is that the method does not cope with plate stretch during printing, which is probably due to water penetration in the polyester base.

European patent application No. 97 20 1336 filed on 3 May 1997 discloses a printing plate that is laminated to a dimensionally stable base via an adhesive intermediate layer. A disadvantage is that a lamination under heat or under pressure is required for every new printing plate. Moreover, although re-use of the dimensionally stable base is fairly simple, there is still room for improvement.

3. Objects of the invention

Accordingly it is an object of the present invention to provide an improved method for making a lithographic printing plate, combining the ease of exposure in an imagesetter of a printing plate precursor with a flexible support, with the dimensional stability and ease of handling of a lithographic printing plate with a metal support.

It is a further object of the present invention to provide an improved method for making a lithographic printing plate combining the cost advantage of a printing plate with a flexible support, with the dimensional stability and ease of handling of a lithographic printing plate with a metal support, especially when used in printing images on top of each other in register, such as e.g. in colour printing. In obtaining this cost advantage, it is important that the dimensionally stable base can be reused.

It is another object of the present invention to provide a convenient method of producing a laminate comprising a printing plate and a dimensionally stable base, and a convenient method of reusing the dimensionally stable base.

It is yet another object of the present invention to cope with plate stretch during printing.

Further objects of the present invention will become clear from the description hereinafter.

4. Summary of the invention

The above mentioned objects are realised by a laminate, having the specific features defined in claim 1.

The above mentioned objects are realised by a method, comprising the steps defined in claim 10.

Specific features for preferred embodiments of the invention are set out in the dependent claims.

The term "laminate" stands for a sandwich of two or more layers adhering to each other; each layer can be a separate sheet or plate, or a continuous web.

A lithographic printing plate obtained according to the claimed method combines the advantages of a printing plate having a flexible support without showing its disadvantages.

An embodiment according to the present invention comprises an imaging element on a flexible support, laminated, with its side opposite to the side carrying the lithographic surface, to a dimensionally stable assembly. The flexible support has a low modulus of elasticity (e.g. 4 GPa, the modulus of polyethylene terephthalate), while the dimensionally stable assembly has a high modulus of elasticity (e.g. 210 GPa, the modulus of steel). According to the invention, the flexible support has a high friction coefficient with respect to the dimensionally stable assembly. Such a laminate behaves like a printing plate on a non-flexible support, such as an aluminium printing plate, without having the cost of such a printing plate.

In a first embodiment according to the present invention, the dimensionally stable assembly comprises a dimensionally stable base. In another embodiment, the dimensionally stable assembly comprises a dimensionally stable base and an intermediate layer, having a high friction coefficient with respect to the imaging element. Both embodiments will be discussed more in detail hereinafter.

In one embodiment according to the invention, both the dimensionally stable assembly and the imaging element are separate sheets or plates. In another embodiment, the dimensionally stable assembly is a separate plate and the imaging element is a continuous web. In still another, third embodiment, the dimensionally stable base is a separate plate, while the intermediate layer and the imaging element both are continuous webs. In a fourth embodiment, the dimensionally stable base is a separate plate, the intermediate layer is a continuous web and the imaging element is a separate sheet or plate. In a fifth embodiment, the dimensionally stable base is a continuous web.

Further advantages and embodiments of the present invention will become apparent from the following detailed description.

5. Detailed description of the invention

An imaging element according to the invention comprises a flexible support. Flexible supports suitable for use in accordance with the present invention may be opaque or transparent, e.g. a paper support, a plastic film, an aluminium foil. When a paper support is used, preference is given to one coated at one or both sides with an alpha-olefin polymer. Preferably, a plastic film is used, e.g. a poly(ethylene terephthalate) film or a poly-alpha-olefin film. The thickness of such plastic film is preferably comprised between 0.07 and 0.35 mm. When an aluminium foil is used, its thickness is preferably between 0.05 and 0.1 mm.

The present invention may be used with any type of lithographic printing plate (precursor), also called an imaging element, having a flexible support and capable of laser image recording. Examples are the printing plate precursors mentioned in the introduction. The present invention is however especially suitable for use with a lithographic printing plate precursor that is processed according to the DTR-process to obtain a lithographic printing plate. The present invention is also especially suitable for use with a driographic printing plate precursor. The present invention is also especially suitable for a heat mode recording material, particularly for a heat mode recording material comprising on a support, having an oleophilic surface, (i) a recording layer containing a light-to-heat converting substance capable of converting radiation into heat and (ii) a cured oleophobic surface layer and wherein said oleophobic surface layer and recording layer may be the same layer.

Said printing plates (precursors) are exposed, preferably by using a laser or a light emitting diode. The light source used is depending on the spectral sensitivity of the imaging element. Argon lasers, helium-neon lasers, semiconductor lasers, e.g. Nd-YAG or laser diodes can be used. Said imaging elements are preferably exposed in an apparatus including means for a scanning exposure such as an imagesetter, in particular one of the internal drum type.

A preferred imaging apparatus suitable for image-wise scanning exposure in accordance with the present invention preferably includes a laser output that can be provided directly to the imaging elements surface via lenses or other beam-guiding components, or transmitted to the surface of an imaging element from a remotely sited laser using a fiber-optic cable. A controller and associated positioning hardware maintains the beam output at a precise orientation with respect to the imaging elements surface, scans the output over the surface, and activates the laser at positions adjacent to selected points or areas of the imaging element. The controller responds to incoming image signals corresponding to the original document and/or picture being copied onto the imaging element to produce a precise negative or positive image of that original. The image signals are stored as a bitmap data file on a computer. Such files may 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 the features required to be transferred onto the imaging element, or as a combination of page-description language and one or more image data files. From the commands formulated according to the page description language and from the image datafiles, at least one bitmap is constructed. Since image data are defined in terms of continuous tone (contone) levels, and a printing plate is a binary device, i.e. either ink or no ink can be accepted and transferred to the reproduction, a conversion from a contone image to a binary image is required. This conversion is referred to as halftoning. According to a first class of halftoning techniques, referred to as periodic amplitude modulation halftoning, halftone dots are laid out on a periodic grid, defined by a screen angle and a screen ruling (see e.g. EP-A-0 748 109). The size or area of the halftone dots is in accordance with the corresponding optical density of the contone image. According to a second class of halftoning techniques, referred to as frequency modulation halftoning, the number of halftone dots per unit area rather than their size is modified according to the corresponding optical density of the contone image (see e.g. EP-A-0 639 023). In either halftoning technique, the bitmap is thus a binary representation of the image, i.e. each pixel in the bitmap has a value of either 0 or 1. In case of colour contone images, usually three or four bitmaps are created, i.e. for a cyan, a magenta, a yellow and optionally a black colour component. Each bitmap will generate one printing plate. Each printing plate is inked with ink of the corresponding colour during printing, when the various colour layers are printed in register on top of each other. The present invention is particularly suitable for use in combination with frequency modulation screening as disclosed in e.g. EP-A-620674.

The imaging apparatus can be configured as a flatbed recorder or preferably as a drum recorder, with the imaging element mounted to the interior or external cylindrical surface of the drum.

In a preferred drum configuration, the laser beam is directed along the axis of the drum and is deflected at right angles onto the imaging element by means of an optical system, preferably an optical prism, that rotates around the drum axis. The beam source is moved along the drum axis, thereby scanning the imaging element circumferentially, so that the image "grows" in the axial direction. In another preferred drum configuration, the required relative motion between the laser beam and the imaging element is achieved by rotating the drum (and the imaging element mounted thereon) about its axis, and moving the beam source parallel to the drum axis. Alternatively, the beam source can move parallel to the drum axis and, after each pass across the imaging element, the laser beam is rotated at a small angle, either by rotating an optical system or by rotating the drum, so that the image on the imaging element "grows" line by line circumferentially. In both cases, after a complete scan of the imaging element by the beam and optional development, an image corresponding to the original will have been applied to the surface of the imaging element. In the flatbed configuration, the beam is drawn across either axis of the imaging element, and is indexed along the other axis after each pass. Of course, the requisite relative motion between the beam and the imaging element may be produced by movement of the imaging element rather than (or in addition to) movement of the beam.

The scan-wise exposed imaging material requires in most cases a development step in order to yield a lithographic printing plate. Depending on the imaging element, said development step proceeds by rubbing the exposed imaging element with e.g. a cotton pad and is then completely dry. So, there is a method for making a lithographic printing plate requiring no dampening liquid comprising the steps of:

by a laser beam, image-wise exposing using a heat mode recording material comprising on a support having an oleophilic surface or an oleophilic layer thereon (i) a recording layer containing a light-to-heat converting substance capable of converting the laser beam radiation into heat and (ii) a cured oleophobic surface layer and wherein said recording layer and surface layer may be the same layer
rubbing the exposed heat mode recording material thereby removing said oleophobic surface layer in the exposed areas so that the underlying oleophilic surface is exposed and
avoiding the swelling of said oleophobic surface layer by carrying out said rubbing without the use of a liquid or with the use of a non-solvent for said oleophobic surface layer.

In most cases the development step requires the treatment of the exposed imaging element with an aqueous solution, particularly an aqueous alkaline solution. The development is preferably done in a developing means.

As mentioned, the present invention is especially suitable for use with a lithographic printing plate precursor that is processed according to the DTR-process to obtain a lithographic printing plate.

Such lithographic printing plate precursor comprises on a flexible support in the order given an optional base coat, a silver halide emulsion layer and an image-receiving surface layer.

To obtain a lithographic plate from such a precursor the precursor is scan-wise exposed e.g. by means of a laser or a LED, and is subsequently developed in an alkaline processing liquid in the presence of a developing agent and a silver halide solvent. The plate surface may then be neutralised with a neutralising agent. After processing the image-receiving layer will bear a silver image that is capable of accepting greasy ink in a printing process using a dampening liquid.

As an alternative said lithographic printing plate precursor comprises in the order given on the hydrophilic surface of a flexible support an image-receiving layer and a silver halide emulsion layer.

To obtain a lithographic plate from such a precursor the precursor is scan-wise exposed e.g. by means of a laser or a LED, and is subsequently developed in an alkaline processing liquid in the presence of a developing agent and a silver halide solvent. The printing plate precursor is then treated to remove the layer(s) on top of the image-receiving layer. After processing the image-receiving layer will bear a silver image that is, optionally after a treatment with a finisher, capable of accepting greasy ink in a printing process using a dampening liquid.

A dimensionally stable base according to the invention comprises a material having an E-modulus of at least 15 GPa, preferably in the range between 30 and 300 GPa.

Suitable bases are for instance metal bases, preferably aluminium or stainless steel bases.

Preferred bases are also bases made of a composite material comprising fibres and a resin matrix. The fibres are preferably selected from the group of carbon fibres, boron fibres, silicon carbide fibres, and mixtures thereof, although other fibres such as glass fibres, aramid fibres, polyamide fibres and natural fibrous materials such as jute may also be used. Preferably the fibres have an average diameter of from 3 µm to 20 µm. The flexural E-modulus of the fibres is preferably greater than 200 GPa.

The majority of the fibres suitably have lengths which extend from one end of the base to the other in the direction of the load, although mixtures of long and short fibres can be used. In fact, the maximum length of the fibres can be somewhat greater than the length of the base where the fibres are applied diagonally.

The fibres may constitute from 30% to 70% by volume of the composite material, although a higher packing fraction is possible with fibres of mixed diameter.

The resin is preferably selected from thermo-setting and thermoplastic polymers, and mixtures thereof, such as epoxy resins, furane resins, silicone resins, polyester resins, phenolic resins, vinylester resins, polyamide (PA) resins, polypropylene (PP) resins, polyethylene resins (PE), poly(ethylene terephthalate) resins (PETP), poly(butylene terephthalate) resins (PBT), and polyphenyloxide resins (PPO).

The thickness of the dimensionally stable base is preferably comprised between 20 µm and 400 µm, more preferably between 25 µm and 350 µm, most preferably between 50 µm and 300 µm.

As mentioned hereinbefore, a laminate according to the present invention - comprising an imaging element on a flexible support and a dimensionally stable assembly - behaves like a printing plate on a non-flexible support, such as an aluminium printing plate. This is now explained in detail.

When an imaging element on a flexible support, having a low modulus of elasticity E₁ and a thickness d₁, is laminated to a dimensionally stable base, having a high modulus of elasticity E₂ and a thickness d₂, the laminate will have an equivalent modulus of elasticity E_eq with respect to a tensile load: Eeq = (d1*E1 + d2*E2)/(d1 + d2) as is easily calculated using the basic formula σ = ε*E, well known as "Hooke's law" in the theory of strength of materials (in Hooke's law, σ is the stress, ε is the strain and E is the modulus of elasticity).

To obtain a laminate that has the same elongation under a tensile load as an aluminium plate, in the above formula E_eq is set to 70 GPa, i.e. the modulus of elasticity of aluminium, and E₁ and E₂ are set to the moduli of the materials constituting the laminate, e.g. poly(ethylene terephthalate) (PET) and steel. The required ratio d₁/d₂ can then be calculated.

A laminate preferably meets the following requirements, in order to behave like a printing plate on a non-flexible support, such as an aluminium printing plate. Such a printing plate on a non-flexible support is referred to below as "an aluminium plate".

In the first place, the laminate preferably behaves as an entity, so that no delamination occurs, during mounting on the press. In the second place, when tensioning the laminate on the press, the laminate preferably behaves like an aluminium plate (these first two requirements mainly apply to embodiments wherein the laminate is a separate sheet or plate). In the third place, shear forces on the laminate during printing, e.g. due to the forces between the printing cylinder and the blanket cylinder, preferably do not result in delamination of the laminate, since this may result in register errors.

We have found that a laminate according to the present invention meets these requirements.

The friction coefficient µ between two surfaces is the ratio of the force W required to start moving one surface over the other to the total force N pressing the two surfaces together: µ = W/N. The friction coefficient as defined above is the so-called static friction coefficient: the force W is increased up to the value where one surface would start moving. Once the surface is moving over the other one, the force required to keep the surface moving changes (decreases) to the value Wdynamic = µdynamic * N, wherein µ_dynamic is the so-called "dynamic friction coefficient" or "sliding friction coefficient". It is customary to call the static friction coefficient simply "the friction coefficient", especially if no sliding movements are involved.

In a preferred embodiment, the friction coefficient µ between the imaging element and the dimensionally stable assembly fulfils the following relation (1): µ > 0.1, preferably µ > 0.2, still more preferably µ > 0.4 and most preferably µ > 0.6. If the friction coefficient µ is smaller, shear forces during printing may cause delamination.

Preferably, lamination of the imaging element to the dimensionally stable base is reversible, so that the imaging element and the dimensionally stable assembly can still be peeled apart. In a preferred embodiment, the imaging element has a peel adhesion value P to the dimensionally stable assembly (expressed in N/m width) that fulfils the following relation (2) : P < 200 N/m, preferably P < 100 N/m, still more preferably P < 10 N/m. The peel adhesion value is measured in a tensile testing machine, such as an "Instron" machine: a 5 cm wide and 20 cm long sample of the laminate is mounted in the machine, the law speed is set at 20 cm/min, and the imaging element and the dimensionally stable assembly are pulled in opposite directions.

An advantage of reversible lamination is that the dimensionally stable base can be reused in many makings of a lithographic printing plate according to the invention, which can lead to an important cost saving. Preferably, the dimensionally stable base can be used in at least 100 makings of a lithographic printing plate.

In a preferred embodiment, the friction coefficient µ and the peel adhesion value P fulfil the relations (1) and (2) stated above, such that the imaging element is fixedly adhered to the dimensionally stable assembly during printing. "Fixedly adhered" means that the imaging element is not repositionable with respect to the dimensionally stable assembly. If the imaging element is repositionable with respect to the dimensionally stable assembly, instead of being fixedly adhered, temporary or permanent delamination of the laminate may occur. Preferably, delamination is avoided, especially in colour printing, since it may result in register errors.

In a first embodiment according to the present invention, the dimensionally stable assembly comprises a dimensionally stable base. The imaging element is laminated with its side opposite to the lithographic surface to a side of the dimensionally stable base. Said side opposite to the lithographic surface, or said side of the dimensionally stable base, or both said sides are preferably roughened so that a high friction coefficient is obtained.

In another preferred embodiment, the dimensionally stable assembly comprises a dimensionally stable base and an intermediate layer, that has a high friction coefficient with respect to the flexible support. Preferably, the intermediate layer has a thickness between 0.5 µm and 100 µm. We have found that an intermediate layer, comprising a partly cured silicone layer, has a high friction coefficient with respect to a flexible support of poly(ethylene terephthalate) (PET).

A partly cured silicone layer as intermediate layer provides additional advantages. After printing, the imaging element is easily peeled off from the dimensionally stable assembly. The intermediate layer is then cleaned from dirt, printing ink, etc. The intermediate layer may have printing ink on its surface, if the size of the imaging element is smaller than the size of the dimensionally stable assembly (one - large - dimensionally stable assembly may be used for imaging elements having different sizes, i.e. smaller and larger sizes). After peeling off and cleaning, a new imaging element can be laminated to the dimensionally stable assembly, which is reused. A partly cured silicone layer is easily cleaned; cleaning it is easier than cleaning a cured (i.e. completely cured) silicone layer. A partly cured silicone layer is easily reused. A partly cured silicone layer moreover has a lower peel adhesion value P than a cured silicone layer, with respect to the imaging element.

In another embodiment according to the present invention, the intermediate layer comprises EPDM-rubber; EPDM is an abbreviation for a rubber containing the three monomers ethylene, propylene and an unconjugated diene. Like a partly cured silicone layer, a layer of EPDM-rubber has a high friction coefficient with respect to poly(ethylene terephthalate) (PET) and to many other materials. The EPDM-layer can be a sheet that is adhered to the dimensionally stable base by means of a primer (i.e. a kind of adhesive). In another embodiment, small grains of EPDM-rubber are strewn over a dimensionally stable base that is covered with a primer. In yet another embodiment, the EPDM-layer comprises poly-tetra-fluorethylene (PTFE), so that cleaning of the intermediate layer is improved. Preferably, the EPDM-layer comprises at most 30 % of poly-tetra-fluor-ethylene.

Preferably, the thicknesses of the flexible support and of the dimensionally stable assembly are such that their equivalent modulus of elasticity E_eq is at least equal to the modulus of aluminium. On the printing press, the laminate is bent. Preferably, the bending strength of the laminate is at least equal to the bending strength of an aluminium plate with approximately the same thickness.

A laminate according to the invention is a sandwich of two or more layers, comprising a dimensionally stable assembly and an imaging element. Embodiments according to the invention can be grouped according to the geometry of the laminated layers: each layer can either be a separate sheet or plate, or a continuous web. Moreover, the laminate can be created in several ways: it can be created by manual lamination, or in a laminating means called a laminator, or by on-press lamination, or by tensioning one layer over another one, or by other means known in the art. Both the combinations of separate plates and continuous webs in the laminate, and the ways to create the laminate, are now discussed in detail.

In a first embodiment according to the invention, both the dimensionally stable assembly and the imaging element are a separate sheet or plate. As an example, the sheets or plates may have a width of 51 cm and a length of 40 cm (these are the dimensions of a plate for a Heidelberg GTO52 press). Lamination of the separate sheets or plates can be effected manually, by pressing the sheets or plates together, but preferably a laminating means called a laminator is used. A laminator preferably comprises a pair of rollers, having an adjustable pressure to each other and moving at a fixed or an adjustable speed. Lamination with a laminator is effected by bringing the two elements which have to be laminated in close contact with each other. The thus formed sandwich is then put through between the two rollers of the laminator, so that the air between the two elements is removed, at least partly, and a laminate is formed. Lamination may occur before or after the optional development of the scan-wise exposed imaging element. Both manual lamination and lamination in a laminating means called a laminator are "off-line" or "off-press" methods, as opposed to "on-press" lamination. Lamination of the separate sheets or plates may also be effected by on-press lamination; this may be realised as follows. First, the dimensionally stable base is mounted on the printing cylinder of the printing press (mounting on the printing press is described in detail hereinafter). Then, the head side of the imaging element is mounted on the printing cylinder. Subsequently, the printing cylinder is slowly being rotated, while the tail side of the plate is being held manually. The dimensionally stable assembly and the imaging element are thus slowly put through between the printing cylinder and the blanket cylinder of the printing press, whereby the laminate is formed. In fact, in on-press lamination the printing cylinder and the blanket cylinder of the printing press act as laminating rollers. In the above description of the first embodiment, the dimensionally stable base is a separate sheet or plate that has to be mounted on the printing cylinder of the press. Alternatively, the dimensionally stable base may be comprised in the printing cylinder of the press. In this case, lamination on-press is preferred.

In a second embodiment according to the invention, the geometry of the laminated layers is the following: the dimensionally stable assembly is a separate plate and the imaging element is a continuous web. Preferably, the printing press comprises means for providing a portion of the imaging element at the position of the dimensionally stable assembly, and means for forming a laminate of said portion and the dimensionally stable assembly. Said portion of the imaging element may cover the entire length of the dimensionally stable assembly, and it may cover the complete width or only part of the width of the dimensionally stable assembly. Preferably, the imaging element is exposed on the press. Exposure of the imaging element may occur before or after the laminate is formed. US-A-5 355 795 discloses a printing press wherein the imaging element is a continuous web. In US-A-5 355 795, the means for providing a portion of the imaging element at the position of the dimensionally stable assembly are a supply spool containing a rolled supply of lithographic plate material (i.e. the imaging element) and an uptake spool for "used" plate material. The portion of lithographic plate material between the supply spool and the uptake spool is wrapped around the plate cylinder of the press. Mechanical means provide for tensioning the wrapped plate material, so that the plate material is in intimate contact with the plate cylinder. After printing, tension of the plate material is decreased, the supply roller provides a new portion of plate material, and tension is increased again. In the present invention, to provide a laminate according to the invention, the plate cylinder comprises at its outer surface a dimensionally stable assembly, that preferably has a high friction coefficient and a low peel adhesion value with respect to the imaging element. For colour printing purposes, each plate cylinder of the printing press comprises a dimensionally stable assembly (each plate cylinder being used to print a different colour). A laminate according to this second embodiment may be created by tensioning the web-like imaging element over the plate-like dimensionally stable assembly, thus pressing the imaging element and the dimensionally stable assembly together. Preferably, the web-like imaging element has a high friction coefficient with respect to the plate-like dimensionally stable assembly. The thus formed laminate behaves like a printing plate on a non-flexible support, such as an aluminium printing plate. In the second embodiment described above, the dimensionally stable base may be comprised in the printing cylinder of the press, or the dimensionally stable base may be a separate sheet or plate that has to be mounted on the printing cylinder.

In a third embodiment according to the invention, the geometry of the laminated layers is the following: the dimensionally stable base is a separate plate, while the intermediate layer and the imaging element both are continuous webs. This third embodiment is thus similar to the second embodiment described above, but now not only the imaging element but also the intermediate layer is a continuous web. Just like in the second embodiment, the imaging element is preferably exposed on the press, and exposure of the imaging element may occur before or after the laminate is formed. Just like in the second embodiment, the printing press may comprise means for providing a portion of the imaging element at the position of the dimensionally stable base, e.g. an imaging element supply spool and an imaging element uptake spool. Additionally, in this third embodiment the printing press may comprise means for providing a portion of the intermediate layer at the position of the dimensionally stable base, e.g. an intermediate layer supply spool and an intermediate layer uptake spool. Preferably, the intermediate layer has a high friction coefficient with respect to the imaging element. Operation may now be as follows. After printing, tension of the web-like imaging element is decreased, and tension of the web-like intermediate layer is decreased. Then, the imaging element supply roller provides a new portion of imaging element, and the intermediate layer supply roller provides a new portion of intermediate layer. Subsequently, tension of the intermediate layer and tension of the imaging element are both increased, so that a laminate is formed comprising the dimensionally stable base at the outer surface of the plate cylinder, the intermediate layer and the imaging element. In the third embodiment described above, the dimensionally stable base may be comprised in the printing cylinder of the press, or the dimensionally stable base may be a separate sheet or plate that has to be mounted on the printing cylinder.

In a fourth embodiment according to the invention, the geometry of the laminated layers is the following: the dimensionally stable base is a separate plate, the intermediate layer is a continuous web and the imaging element is a separate sheet or plate. This embodiment is similar to the third embodiment described above, but now only the intermediate layer is supplied as a web, e.g. by a supply spool and an uptake spool. The imaging element is laminated to the intermediate layer and the dimensionally stable base, preferably by on-press lamination (on-press lamination is discussed above). The dimensionally stable base may be comprised in the printing cylinder of the press, or the dimensionally stable base may be a separate sheet or plate that has to be mounted on the printing cylinder.

In a fifth embodiment according to the invention, the geometry of the laminated layers is the following: the dimensionally stable base is a continuous web, e.g. supplied by a supply roller, wound around the plate cylinder and then onto an uptake roller. The dimensionally stable base preferably has a small thickness, e.g. a thickness of 180 µm.

In embodiments wherein the dimensionally stable base is a separate sheet or plate, the dimensionally stable base may be mounted on the printing cylinder of the printing press in several ways, as known in the art. The dimensionally stable base may comprise register punch holes, required for mounting a lithographic printing plate on a printing press with register pins. When required by the printing press used, the dimensionally stable base comprises several holes for attaching the dimensionally stable base onto a press pin bar. When required by the printing press used, the dimensionally stable base comprises one or more bends for mounting it onto the printing cylinder.

In embodiments wherein the imaging element is a separate sheet or plate, the imaging element may comprise register punch holes, required for mounting a lithographic printing plate on a printing press with register pins. When required by the printing press used, the imaging element comprises several holes for attaching the imaging element onto a press pin bar. When required by the printing press used, the imaging element comprises one or more bends for mounting it onto the printing cylinder.

In embodiments wherein the imaging element is a separate sheet or plate, the imaging element is preferably clamped at both sides, head side and tail side, on the press. This is especially preferred if a polyester flexible support is used, because of the stretching of this support during printing, as will be explained now. If both the dimensionally stable assembly and the imaging element are a separate sheet or plate, then, when tensioning the laminate on the press, both the dimensionally stable assembly and the flexible support are elongated. Preferably, tension and strain in the flexible support are so high that, even after the stretching of the (polyester) flexible support during printing, the flexible support is still tensioned. In this way, plate stretch during printing - which is probably due to water penetration in the polyester base - does not cause delamination of the flexible support and the dimensionally stable assembly. Plate stretch due to water penetration is especially important after a few thousand impressions.

In a preferred embodiment, tension and strain in the imaging element are so high that, during printing, the imaging element remains in contact with the dimensionally stable assembly, so that delamination is avoided, even if the imaging element is e.g. stretched during printing. In this preferred embodiment, the imaging element may be a continuous web or it may be a separate sheet or plate.

As mentioned hereinbefore, in a preferred embodiment, the laminate comprises an intermediate layer. Preferably, the intermediate layer is adhered to the dimensionally stable base. In another embodiment, the intermediate layer is adhered to the flexible support. The intermediate layer can be applied in several ways, some of which are explained already hereinbefore. Preferably, the intermediate layer is adhered to the dimensionally stable base via a primer. The intermediate layer can also be applied by sputtering.

In a preferred method of producing and using a laminate according to the invention, an intermediate layer is applied to a dimensionally stable base, thus forming a dimensionally stable assembly. An imaging element is laminated to the dimensionally stable assembly. After printing, the imaging element is removed, the dimensionally stable assembly is cleaned, and reused, i.e. another imaging element is laminated to the dimensionally stable assembly. This method presents several advantages. The dimensionally stable base as well as the intermediate layer can be reused. A laminate according to the present invention only requires a simple lamination, without the application of heat or high pressure, whereas the presence of an adhesive layer, such as a thermal adhesive layer (TAL) or a pressure-adhesive layer (PAL), requires a lamination under heat or pressure. Because of the above features, a method for making a lithographic plate according to the invention is easy and convenient. As explained hereinbefore, lamination can be done manually, or in a laminating means called a laminator, or by on-press lamination, or by tensioning one layer over another one, or by other means known in the art.

Having described in detail preferred embodiments of the current invention, it will now be apparent to those skilled in the art that numerous modifications can be made therein without departing from the scope of the invention as defined in the appending claims.

Claims

A laminate comprising:

a lithographic element having a first surface and a second surface, opposite to the first surface, the first surface being:

a) a lithographic surface having ink accepting and ink repellent areas; or

b) a latent lithographic surface resulting in a lithographic surface upon development;

and

a dimensionally stable assembly, laminated to the second surface of the lithographic element;
characterised in that:

the lithographic element has a peel adhesion value P to the dimensionally stable assembly, with P < 200 N/m; and

the lithographic element has a friction coefficient µ with respect to the dimensionally stable assembly, with µ > 0.1.
The laminate according to claim 1, wherein the lithographic element comprises a flexible support.
The laminate according to any one of the preceding claims, wherein the dimensionally stable assembly comprises:

a dimensionally stable base; and

an intermediate layer, laminated to the second surface of the element and to the dimensionally stable base.
The laminate according to claim 3, wherein said dimensionally stable base is comprised in a printing cylinder of a printing press.
The laminate according to claim 3, wherein said dimensionally stable base is mounted on a printing cylinder of a printing press.
The laminate according to claim 3, wherein said dimensionally stable base is selected from the group consisting of metal and a composite material comprising fibres and a resin matrix.
The laminate according to any one of claims 3 to 6, wherein said intermediate layer is a partly cured silicone layer.
The laminate according to any one of the preceding claims, wherein said lithographic element is a continuous web.
The laminate according to any one of claims 1 to 7, wherein said lithographic element is a separate plate.
A method for making a lithographic printing plate comprising the steps of:

(i) image-wise exposing an imaging element in accordance with an image pattern;

(ii) laminating a thus obtained image-wise exposed imaging element, with its side opposite to the side carrying a lithographic surface, to a dimensionally stable base, such that said imaging element has a peel adhesion value P to the dimensionally stable base, with P < 200 N/m, and said imaging element has a friction coefficient µ with respect to the dimensionally stable base, with µ > 0.1;

(iii) optionally developing said image-wise exposed imaging element before or after laminating it to said dimensionally stable base.
The method according to claim 10, further comprising the step of applying an intermediate layer to said dimensionally stable base or to said image-wise exposed imaging element.
A method for making a copy, comprising the steps of:

making a printing plate by a method according to any one of claims 10 to 11;

placing said printing plate on a printing cylinder;

bringing said printing plate in contact with a printing ink and optionally with a dampening liquid;

printing said copy by said printing plate;

peeling the imaging element off from the dimensionally stable base; and

reusing said dimensionally stable base for making another lithographic printing plate.
A method for making a copy having at least two colours, comprising the steps of:

i) making at least two printing plates by a method according to any one of claims 10 to 11;

ii) placing said at least two printing plates each on a printing cylinder;

iii) bringing said at least two printing plates in contact with a printing ink and optionally with a dampening liquid; and

iv) printing said copy by said printing plates.