Abstract:
The invention concerns a method for cross-linking photosensitive inks in particular polymersible inks ( 36 ) by ultraviolet radiation consisting in a step ( 35 ) inking dots ( 31 ) on a base ( 38 ) and a subsequent step consisting in applying a concentrated ultraviolet beam ( 32 ) on the inked dots ( 31 ), except for the base non-inked surfaces ( 30 ). The beam is in particular an ultraviolet laser beam. The invention is applicable to jet dot-matrix printing and polychromy.

Description:
This application is based on French Patent Application No. 97/08176, filed on Jun. 23, 1997, which is incorporated by reference herein. 
     BACKGROUND 
     1. Field of the Invention 
     The present invention relates to the field of printing using photosensitive inks, i.e. inks which can be dried or polymerised by light radiation, notably ultraviolet radiation. 
     2. Related Background 
     Printing on supports such as plastics materials which do not absorb traditional inks based on water, alcohol or oil has been made possible by developing solvent-based inks adapted to these materials and concurrently polymeric inks capable of solidifying and adhering to the material. 
     A prohibitive drawback of solvent-based inks is the harmfulness of the solvents used, of the acetone type. Printing with such inks requires complex devices collecting the solvents given off and major precautions in use. 
     Polymeric inks do not have these drawbacks in use and lend themselves particularly well to printing dot by dot, notably by inkjet. 
     In the liquid phase, these inks have a fluidity which makes it possible to mechanically deposit, notably in an offset process, ink drops of very fine size, or to spray drops dot by dot onto a support. 
     The definitive fixing of polymeric inks is effected during a so-called ink cross-linking step which follows the deposition of the ink drops. 
     Cross-linking consists in polymerising or crystallising the ink, the polymers making up the ink being bonded together in order to form longer polymer chains and to be fixed to the support. A cross-linking step therefore enables the ink to be solidified and fixed to the support. 
     The supports consisting of plastics material, such as polyvinyl chloride (PVC), polyethylene (PE), polyethyltetraethylene (PET), polycarbonates (PCs), acrylonitrile-butadiene-styrene (ABS) and other organic polymers are quite naturally suited to printing by polymer ink, the polymers in the ink and the polymers in the support being firmly fixed together during the cross-linking. 
     Cross-linking is obtained by exposure of the ink support to ultraviolet radiation. Ink which can be cross-linked to ultraviolet radiation, abbreviated to UV ink, will therefore be spoken of hereinafter. The energy of the ultraviolet photons allows polymerisation of the polymer chains with each other. However, the support must be exposed to a sufficient ultraviolet radiation power and for a sufficient length of time for the ink to be well fixed to the support and to harden completely. 
     FIGS. 1 and 2 diagrammatically show known techniques of printing using cross-linkable UV ink. FIG. 1 shows diagrammatically a multicolour offset printing of a support. The support  10  advances between a drive cylinder  15  and contact printing rollers  11 ,  12 ,  13  and  14 . Each roller  11  or  12  or  13  or  14  contains a screen of the image to be printed. The hollows in the screens on each roller are inked with a black ink or coloured ink, notably cyan, magenta or yellow. Several screens of colour are thus deposited on the support in order to constitute a final multicolour image. The inking step is followed by a step of cross-linking by continuous exposure  19  of the support  10  under an ultraviolet lamp  18 . Naturally the offset printing can be monochrome by providing a single black or colour inking roller. 
     FIG. 2 shows diagrammatically a method of multicolour printing by inkjet. Several reservoirs  21 ,  22 ,  23  and  24  containing the black polymer ink and those of different colours feed at least one nozzle ejecting drops of ink, each reservoir preferably having its own line of ejection nozzles, the printing line being transverse to the direction of movement of the support. The ink drops are deposited dot by dot on the support, a device for moving the support and for the computer programming of the image to be printed controlling the ejection of the drops through each nozzle in the line with if necessary control of the drop volume ejected. The computer system defines the spatial rotation of the points to be inked and controls the ejection or non-ejection of the drops according to this location. The inking of the support  20  is followed by a cross-linking step, still with continuous exposure, the support moving forward under an ultraviolet lamp. FIG. 2 illustrates an alternative printing in which each inking step is followed by a cross-linking step in order to dry each ink before a subsequent inking of a different colour. The printing device of FIG. 2 therefore has in this example four ultraviolet lamps  25 ,  26 ,  27  and  28  for drying each ink individually. 
     In order to increase the printing rates, it has been proposed to increase the power of the ultraviolet lamps, thus reducing the support exposure time, the support still receiving sufficient energy to dry and fix the ink. 
     However, ultraviolet lamps release a great deal of heat. Printing devices with polymerisable ink must therefore include an expensive and bulky cooling system. The adoption of so-called cold UV lamps, designed to emit less infrared radiation and therefore less heat, do not dispense with the need to have cooling when high printing rates are required. 
     A drawback of the known printing devices with ink which can be cross-linked by ultraviolet radiation is therefore the high release of heat during the cross-linking steps. 
     Another drawback is the premature aging of the supports and their yellowing under the effect of the cross-linking ultraviolet radiation. 
     One aim of the invention is to provide an ink cross-linking method allowing printing at high rate, without the aforementioned drawbacks. 
     A particular aim of the invention is to prevent the yellowing of the support in order to afford durable printing of high quality. 
     SUMMARY 
     Succinctly, these aims are achieved, according to the invention, by providing for the cross-linking to be carried out by an ultraviolet laser beam concentrated on the ink drops deposited on the surface of the support, the white surfaces of the support not being swept by the laser beam. 
     The invention is implemented by providing a method for the cross-linking of photosensitive ink including a step of inking points on a support and a particular step consisting in applying an ultraviolet beam concentrated on the ink dots, to the exclusion of the non-inked surfaces of the support. 
     The inking step preferably consists in depositing, dot by dot on a printing support, drops of polymerisable ink, the ink being polymerisable by ultraviolet radiation. 
     The invention is preferably implemented by the application of an ultraviolet laser beam. 
     A first embodiment of the invention provides for the application of the laser beam to be effected by dot-by-dot sweeping of the support. 
     A second embodiment of the invention provides for the application of the ultraviolet beam to be effected by means of an optical fibre or an array of optical fibres. 
     According to a preferred characteristic of the invention, provision is made for interrupting the ultraviolet beam when it is directed towards the non-inked surfaces of the support, one embodiment of the invention being able to include continuous sweeping of the support. 
     According to an alternative characteristic, provision is made for modulating with respect to power the ultraviolet beam concentrated on the inked dots. 
     The invention applies particularly to printing and cross-linking of ink on a support made of plastics material. 
     Advantageously, the method of cross-linking ink according to the invention applies particularly to a method of printing dot by dot by inkjet and/or a multicolour printing process. 
    
    
     BRIEF DESCRIPTION OF THE FIGURES 
     Other characteristics, aims and advantages of the invention will emerge from a reading of the description which follows, with regard to the accompanying drawings, given by way of non-limitative examples and in which: 
     FIG. 1, described previously, depicts printing and cross-linking of UV ink according to a known method, 
     FIG. 2, previously described, depicts printing and cross-linking of UV ink according to another known method, 
     FIG. 3 depicts a method of cross-linking photosensitive ink according to the invention, 
     FIG. 4 depicts a first embodiment of the method of cross-linking photosensitive ink according to the invention, and 
     FIG. 5 depicts a second embodiment of the method of cross-linking photosensitive ink according to the invention. 
    
    
     DETAILED DESCRIPTION 
     The invention is advantageously intended to be implemented following conventional printing steps. 
     Various known printing methods provide, as illustrated in FIG. 3, an inking of the surface of a support  38 , the inking notably being able to be effected by mechanical contact under a press or by spraying  37  drops  36  of ink, notably during dot-by-dot inkjet printing. 
     The method according to the invention thus includes a preliminary step of inking the support, the inking being effected with a photosensitive ink of the type consisting of ink which can be cross-linked by ultraviolet radiation. Preferably, the inking is effected according to the invention by depositing drops of ink which can be polymerised by point-by-point ultraviolet radiation on a printing support. 
     Upon completion of the printing or more precisely upon completion of this inking step, the support  38  has inked surfaces and non-inked surfaces  30 , the inked surfaces consisting of inked points  31  disposed contiguously, or in isolation. 
     Whatever the relatedness of the inked surfaces, the method according to the invention makes provision for applying an ultraviolet beam concentrated on the ink points, to the exclusion of the non-inked surfaces of the support. 
     FIG. 3 thus shows an optical device  33 ,  34  provided schematically with a source of ultraviolet rays  33  and a beam concentrator  34  for concentrating the ultraviolet rays on an inked dot  31 . 
     One advantage of the method according to the invention is that the light power of the source  3  of ultraviolet rays is concentrated on the single dot  31 , whose cross-linking is then very rapid. Consequently, a very rapid sweeping of the inked dots can be provided, applying the concentrated beam to each dot for a period of time corresponding to the energy which the ink drop must receive in order to be completely cross-linked. 
     The method makes provision, according to the invention, for not applying the ultraviolet beam to the non-inked surfaces. 
     One advantage of such an arrangement is that aging and yellowing of the support are prevented, notably on the non-inked surfaces. 
     Another advantage is that the light energy applied is lesser compared with the methods of exposure to ultraviolet lamps, no radiation power being dispensed unnecessarily on the non-inked surfaces. 
     Such an arrangement is implemented easily by providing for the beam  32  to be concentrated on a surface area substantially equal to the surface area of an ink drop. Means of sweeping the support and of distributing the beam will be detailed below in two preferred embodiments of the device implementing the method according to the invention. 
     The invention is implemented using an ultraviolet laser, although an intense source of ultraviolet of the arc lamp or rotating cathode lamp type can be envisaged. 
     FIG. 4 thus illustrates a laser  43  emitting a coherent ultraviolet radiation beam  42 . The beam  42 ′ is diverted in order to concentrate it on an inked dot  41 ′ to be cross-linked. 
     One advantage of the laser is that the beam  42 ′ of rays emitted can easily have a very much reduced size whilst remaining substantially parallel. The beam  42  can thus be concentrated on a surface as microscopic as the surface of muticolour offset printing dots such as the dots  51   a ,  51   b ,  51   c  and  52   a  to  55   c  depicted in an enlarged view in FIG.  4 . 
     In addition an ultraviolet laser can have a very intense light power, which allows very rapid exposure of each dot to be cross-linked. 
     The cross-linking time for a support having few inked dots is thus advantageously reduced compared with the known methods. It is possible to choose a laser emission device  43  emitting a beam continuously or in pulses. The time of exposure of a drop under the continuous beam or the number of laser pulses applied to the drop is determined so that the drop receives the cross-linking light energy. 
     According to a first embodiment of the method according to the invention, application of the ultraviolet beam is effected by dot-by-dot sweeping of the support. 
     FIG. 4 thus illustrates a sweeping device  46  having a motor orienting a mirror  46  in order to deflect the laser beam  43  to each point on the support. 
     According to the device illustrated in FIG. 4 the device  45 ,  46  for deflecting the beam  42  provides a transverse sweeping of the support  48  by the beam  42 ′,  42 ″,  42 ′″ so as to cross-link all the dots  41 ′,  41 ″,  41 ′″ on a transverse line of the support  48 . The support is then moved in a longitudinal direction in order to cross-link a following line of dots. 
     Preferably, the sweeping device  45 ,  46  is coupled to a dot-by-dot printing computer system, indicating to the sweeping device the exact location of each inked dot of the text or image being printed. The sweeping device can notably receive a command similar to the positioning command for a dot-by-dot print head. 
     The sweeping provided for by the first embodiment can be effected continuously or discretely, according to two variants. 
     In the first variant, the angle of deflection of the ultraviolet beam  42  varies continuously, the beam  42 ′ being deflected progressively all along the transverse line of the support. 
     In order to avoid applying the beam to the “white” surfaces  40 , provision is made for interrupting the beam  42  when it is deflected in the direction of the non-inked surfaces  40 . 
     A component  44  for cutting off the beam  42 , shown diagrammatically in FIG. 4, thus avoids concentrating the beam  42 ′ on non-inked points. This cutoff component is advantageously coupled to the dot-by-dot printing computer system which triggers its obturation when the deflected beam  42 ′ is directed towards the non-inked surfaces  40 . 
     For very rapid cross-linking, the cutoff component  44  must have a very short reaction time. The component  44  is for example a “Q-switch” device as used in optronics. Other means of interrupting the beam  42  are within the capability of a person skilled in the art without departing from the scope of the present invention. 
     It should also be noted that the means of interrupting the beam can form an integral part of the laser  43 . Thus the laser delivers, on demand, ultraviolet radiation pulses when the sweeping device  45 ,  46  is aimed at an inked dot  41 ′ and does not deliver a pulse when the sweeping device  45 ,  46  is aimed at a non-inked point  40 . 
     In the second variant, the sweeping device  45 ,  46  is programmed to deflect the beam  42 ′ to an inked dot  41 ′ and pass directly to another deflection angle, the beam  42 ″ being directed to another inked dot  41 ″. The sweeping command to the device  46  is then discontinued and the position of the mirror  45  passes without transition from one angular value to another discrete angular value. 
     Provision is made for correcting the spread of the beam when the beam  42 ′″ falls on the support at a low angle, i.e. when the deflection of the beam is high. This correction is obtained by providing a so-called flat field correction lens which reduces the spread of the beam under such conditions and focuses it at a point. 
     A second embodiment of the method according to the invention provides for another method of applying the ultraviolet beam to the points on the support, instead of the sweeping step. 
     The second embodiment has, as illustrated in FIG. 5, a linear array  70  of parallel optical fibres  71  to  77 , whose output is disposed opposite the surface of the support to be cross-linked. In an equivalent manner, a two-dimensional array of optical fibres with parallel outputs can be provided. The beam  82  of the laser  83  is injected at the input of the optical fibres  71  to  77 . The fibres  71  to  77  advantageously have their inputs connected together so that the laser radiation entering is distributed substantially equally between all the fibres. 
     Thus the initial laser beam  82  is divided into a multitude of parallel rays, each ray being directed and concentrated towards an inked dot on the support  68 . 
     The optical fibres used are made of quartz or glass transmitting the ultraviolet radiation, an optical fibre made of ordinary glass not transmitting the wavelengths beyond violet. 
     The device  70  for distributing the beam  82  also has means of interrupting the ultraviolet beam, each optical fibre  71  being provided for example with a ray cutoff component in order to avoid exposing a non-inked point  60  on the support  68 . 
     This second embodiment is particularly suitable to printing methods including a screening of points. By adapting the separation pitch of the outlets of fibres in the linear array  20  to the screening pitch of the printing, a series of laser beams is obtained concentrated on the precise coordinates of the points in the printing screen. 
     As illustrated in FIG. 5, the second embodiment applies advantageously to the inkjet printing methods which allow line by line printing, a line of dots being inked instantaneously. 
     A device using an in-line inkjet generally has a linear array  100  of ink drop generators. A series of ink drops  101 ,  102 ,  103  is emitted simultaneously in the direction of the points on the support which it is wished to ink. 
     Such devices are notably used in multicolour offset printing by disposing several generator arrays  100 ,  110 ,  120  fed by reservoirs  109 ,  119 ,  129  of inks of different colours. All shades of colours and tints are obtained by modulating the volume of the ink drops, and using inks corresponding to the primary colours and possibly to black. As detailed in FIG. 4, each coloured dot  51  is formed for example by three or four inked elementary dots  51   a ,  51   b ,  51   c  of primary colours or black. 
     The inked dots in different colours can be cross-linked according to the invention by applying a laser ray to each coloured dot. 
     The elementary dots, generally microscopic, are very close and may possibly overlap. 
     The effect of polychromy is obtained, during offset printing, by modulating the sizes of each microscopic elementary dot in order to reconstitute all possible colours. According to a variant, by modulating the overlap and size of each dot, a multicolour effect is thus obtained. 
     Advantageously, according to the invention provision is made for modulating the concentrated beam applied to such inked dots so that each dot receives sufficient energy for cross-linking the volume of ink in the dot. The laser beam interruption means are then replaced by means of modulating the intensity of the beam. Such a means consists for example of an optical modulator of the orientable diffraction plate type. 
     In general terms, the possibility of modulating the ultraviolet beam with respect to power makes it possible to adapt the cross-linking steps to the inks used and to the printing speed of the support. 
     The cross-linking method can be applied just once after all the colour inking steps as illustrated in FIG.  5 . The beam distribution device  70  then has a tight network of optical fibres, the fibres being distributed spatially according to the maximum screen of inked points which can be formed on printing. 
     Alternatively, a cross-linking can be carried out according to the invention after each inking of a colour during a multicolour printing. 
     The printing installation can then include several cross-linking devices disposed at the output of each monochrome inking device. 
     The method according to the invention advantageously makes it possible to provide a total or partial gelling of the inks during cross-linking between each inking step, the partial gelling being obtained for example by modulating the power of the ultraviolet laser beam. 
     The essential advantage of the cross-linking method according to the invention is, as indicated previously, eliminating the drawback inherent in ultraviolet radiation, namely the discolouring or yellowing action on the polymers making up the support. 
     Provided initially for being applied to a support made of plastics material, the method according to the invention extends to the cross-linking of photosensitive ink on any type of printing support such as paper, cardboard or wood for advantageously replacing printing with ink based on water or solvents whilst preventing any browning of the support. 
     Finally, the rational use of the cross-linking light power according to the invention and the high light intensities which can be obtained with a laser has the advantage of increasing the speed of the cross-linking step compared with traditional insolation UV lamps. 
     Consecutively, the cross-linking method according to the invention advantageously helps to increase the throughput of the printing device into which it is integrated. 
     The method according to the invention thus makes it possible to obtain advantageously a cross-linking speed greater than the speeds of inking by inkjet, so that the printing rate is no longer limited by the cross-linking step. 
     Although the disclosure of the invention is based on ultraviolet radiation, the invention is not limited to a precise light spectrum, but can be applied with any type of light radiation adapted to polymerisation or drying of photosensitive inks. 
     In addition, the cross-linking method can be used with photosensitive paints, the same constituents and the same pigments being used in polymeric inking and polymeric painting. 
     Other advantages, applications and developments of the invention will be clear to a person skilled in the art without departing from the scope of the invention defined in the following claims.