Abstract:
A method for producing a flexographic plate that includes a base layer and a layer of a photosensitive material fixed on the base layer. An image is produced on the photosensitive layer by selective reticulation. Areas to be reticulated are exposed to amplitude-modulated laser light with a wavelength ranging from 390 to 410 nm, which scans the surface of the photosensitive layer. The non-reticulated areas are then removed.

Description:
FIELD OF INVENTION 
     The invention relates to a method for the production of a flexographic printing plate, particularly by digital means, which has a base layer and a layer of light sensitive material, according to which an image is produced on the light sensitive layer by bringing about a selective crosslinking by insolation of the zones which are to be crosslinked using light with a predetermined wavelength and by removal of the non-crosslinked zones. The invention also relates to a flexographic printing plate obtained according to this method. 
     BACKGROUND 
     Methods and flexographic printing plates are already known. A known method for production of a flexographic printing plate consists of insolating with ultraviolet light a photo-polymer of elastomeric nature through a mask which is opaque with respect to ultraviolet (UV) light. This method has the disadvantage that the mask is produced digitally by selective ablation in situ of a surface layer which is opaque with respect to UV with a laser operating with infrared light. 
     According to another method, the image is produced by direct writing of the photo-polymer plates using amplitude modulated ultraviolet sources. These sources can be lasers typically operating at wavelengths of 350 to 370 nm. These sources have the major disadvantages of having an exorbitant cost, of having a low energy efficiency and also low available power, of using optics with power losses and of being expensive to maintain. 
     Yet another known method involves the use of photo-polymers which are sensitive to visible light with, for example, silver base technologies in the manner of photography films. This method is very limiting because it requires strict measures of protection against daylight. 
     SUMMARY OF THE INVENTION 
     The invention aims to propose a method which palliates the disadvantages just disclosed. 
     In order to attain this aim, in the method according to the invention for the production of the image, an amplitude modulated laser light is used, having a wavelength is on the order of 390 to 410 nm, and which is made to sweep the surface of the plate. 
     According to a of the invention, laser sources consisting of a bundle of diodes functioning at wavelengths around 405 nm are used. 
     According to of the invention, the removal of the non-crosslinked zones is done by liquefying of these zones by thermal means, without the use of solvents. 
     The arrangement of the flexographic printing plate according to the invention includes a tubular sleeve on a rigid support, which has a composite base and, attached on this base, a layer made of light sensitive material which is free of solvents. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       The invention will be better understood and other aims, characteristics, details and advantages of it will appear more clearly in the following description in reference to the appended diagrammatic drawings given only by way of example illustrating an embodiment of the invention and in which: 
         FIG. 1  is a diagrammatic radial view in section of the flexographic printing plate arrangement in the form of a sleeve according to the invention; 
         FIG. 2  is a radial view in section with tearing away and on a larger scale of another embodiment of the flexographic printing plate arrangement according to the invention, and 
         FIG. 3  is a perspective view of yet another embodiment of the flexographic printing plate arrangement according to the invention. 
     
    
    
     DETAILED DESCRIPTION 
     A flexographic printing plate according to the invention is advantageously present in the form of tubular sleeve  1  mounted on rigid support  2  which is known. Plate  1  has composite base  4  of suitable thickness, between approximately 0.2 and 40 mm, preferably 0.3 mm, and layer  5  of light sensitive material with a thickness between approximately 0.5 and 2 mm, preferably 1.5 mm, which is attached to the exterior surface of base  4 . This sleeve can be manufactured using an extrusion method or any other known method. According to another variant, it could be produced by thermally projecting pre-formulated powder onto a support cylinder or sleeve, for example, made of a composite material or any other appropriate material. 
     It should be noted that the exterior surface of the light sensitive layer can be machined and polished in order to ensure strict compliance of the dimensions. 
     According to the invention, the image on the light sensitive layer is produced by direct writing using light with a wavelength in a range on the order of 390 to 410 nm approximately, which is emitted by a laser modulated in terms of amplitude by software and which sweeps the surface of the plate. In this way, the light used is situated between the boundary of the visible and the ultraviolet. Preferably, the laser source consists of a bundle of diodes functioning at wavelengths around 405 nm. 
     As light sensitive material which is sensitive to such a laser, materials are used which contain one or more high molecular weight polymers, functionalized monomers or oligomers, photo-initiators, reactive or non-reactive diluents, inhibitors and protective agents and pigments. The diluents and the oligomers generally allow adjustment of the viscosity. 
     The photo-initiators used must of course be sensitive to the light which is used. It would be possible to consider, for example, photo-initiators commercially available under the trade names Irgacure 819 and 1850 of Ciba, Genocure CQ of Rahn, Darocure TPO of Ciba, TPO lucirin of BASF, Genocure TPO of Rahn and Quantacure CPTX of Rahn. 
     These photoinitiators are capable of undergoing a photoreaction that destroys light absorption, which produces a bleaching effect that renders already crosslinked zones of the light sensitive layer transparent to the laser light. This enables a progressively deeper penetration of the light into the light sensitive layer in order to allow uniform crosslinking throughout the thickness of the layer, even in the case of thick layers. 
     The photo-polymer used can have two or more complementary crosslinking systems, namely a main system used for creating the image and a complementary system for completing the crosslinking and increasing the chemical and mechanical resistance. Another system could generate different compressibilities. Such a system is described in the document FR 2 803 245. 
     The photo-polymers used can be partially crosslinked in order to adjust the viscosity and to prevent cold creep during prolonged periods of storage or transport. It should also be noted that the photo-polymer could be sensitized by a flash of light before the laser treatment in order to increase the effectiveness of the laser. 
     Preferably, a photo-polymer which is sensitive to the laser light used in the context of the invention is a material based on SBS or containing SBS, SIS or else made of SEBS and with a hardness between approximately 60 and 70 ShA. 
     Another particularity of the invention lies in the fact that the washing of the zones not crosslinked by exposure to the laser light is preferably done by a known thermal method described in the document U.S. Pat. No. 3,264,103. For this purpose, the sleeve is heated to a temperature ensuring the liquefying of the non-crosslinked zones, allowing these zones to be eliminated without solvents. To this end, the material not crosslinked by the laser light could be specially formulated by known means in order to have a great variation of viscosity at a temperature between 60 and 140° C. Ranges of viscosity necessary for thermal development under good conditions are between 10,000 and 1,000,000 centipoise in solid phase and below 1000 centipoise in fluid phase of development. 
     It should also be noted that the energy necessary for the insolation is advantageously between 20 and 1000 mJ/cm 2 . 
     Diluents considered as examples, which can also be reactive, are the following, with use of the abridged names of the molecules and classification of them into two distinct families: 
     Aliphatic and functionalized mono-acrylates and mono-methacrylates with different molecular weights: HEA, HPA, EMA, IBMA, HMA, I-DMA, EMMA, C13MA, C17.4MA, IBOA, HPMA; 
     Aliphatic and functionalized urethane acrylates, diacrylates and dimethacrylates with different molecular weights: HDDA, TEGDA, TTEGDA, TPGDA, NPGDA, BDDMA, DEGDMA, HDDMA, PG200DMA, N-IBMMAA, GDMA. 
     Multifunctional urethanes acrylates and methacrylates of the type of: TMPTA, TMPTMA, DTMPTA, DPEMPA. 
     The invention as described has numerous advantages. Thus, the production of the sleeves, on one hand, and the creation of the image, on the other hand, can be done very rapidly, with perfect registering and without the use of an intermediate film. The wavelength of the laser light used, which is lower than that of the light used up to now, ensures greater spatial resolution. The possibility of developing the sleeve without solvent provides the very important advantage of protecting the environment. Finally, the use of light in the wavelength band between 390 and 410 nm makes it possible to use very high performance laser diodes  4  with relatively low cost of purchasing and maintenance. A large number of photo-initiators sensitive to this wavelength are available. The constraints of protection against daylight are limited. At these wavelengths, the laser beam optics which are used are simple. The direct crosslinking according to the invention has the advantage of requiring less energy than ablation which is a competing digital production technology. 
     The sleeve according to the invention as described in reference to  FIG. 1  can be supplemented by addition of other layers, as already mentioned above, in order to obtain more complex flexographic printing plate arrangements. 
     Thus,  FIG. 2  shows an arrangement in which compressible layer  6  as described in French patent No. 2 803 245 is interposed between light sensitive layer  5  containing the image in relief represented by  7  and composite base  4 . 
       FIG. 3  illustrates the possibility of using, in the manner and for the reasons described in the document EP 0 711 665, inserted sleeve  8  made of a polymer material between support  2  and sleeve  1  formed by base  4  and light sensitive layer  5 . 
     Described in the preceding, as example of implementation of the invention, is a flexographic printing plate arrangement in which this plate is produced in the form of a tubular sleeve. Of course, the arrangement can also be obtained by rolling and attachment of plates on support cylinders or sleeves. 
     It should be noted that numerous modifications can be made to the invention as just described as an example. In effect, it is possible to use several lasers which act in parallel. The flexographic printing plate can have a base made of polyester film in place of the rigid support. This plate can have two or more layers of light sensitive materials, and it can be capable of being etched with water or with an aqueous solution under pressure, at high temperature or by simple brushing.