EP0993378A1

EP0993378A1 - Ink cross-linking by uv radiation

Info

Publication number: EP0993378A1
Application number: EP98932230A
Authority: EP
Inventors: Paul Morgavi
Original assignee: Gemplus SCA; Gemplus Card International SA
Current assignee: Gemplus SA
Priority date: 1997-06-23
Filing date: 1998-06-18
Publication date: 2000-04-19
Anticipated expiration: 2018-06-18
Also published as: ES2165688T3; EP1162079A2; JP4125796B2; DE69833974D1; FR2764844B1; CA2293551C; WO1998058806A1; ATE206090T1; JP2002504873A; EP0993378B1; CN1260753A; US6562413B1; EP1162079B1; CA2293551A1; EP1162079A3; DE69833974T2; DE69801823D1; DE69801823T2; FR2764844A1; ES2262582T3

Abstract

The ink drop fixing method has a preliminary stage of applying an ultraviolet beam (32) concentrated on the ink drops (31) on the surface, avoiding exposure of the plastic support (30) for the printing. The ink drops are deposited point-by-point on the surface. The ink is made of a material that is polymerisable under ultra-violet radiation. The ultra-violet radiation is provided by an ultra-violet laser (43), which is swept point-by-point across the printed surface. The ultra-violet beam is directed by an optic fibre (71) of a network (70) of optic fibres. The beam is interrupted when it is directed toward the un-inked surface.

Description

U.V. INK CROSSLINKING

The present invention relates to the field of printing based on photosensitive inks, that is to say inks that can be dried or polymerized by light radiation, in particular by ultraviolet radiation.

Printing on substrates such as plastic materials which do not absorb traditional water, alcohol or oil-based inks has been made possible by the development of solvent-based inks adapted to the material and concurrently with polymer inks capable of solidifying and adhering to the material.

A crippling 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 rejected solvents and important processing precautions.

Polymer inks do not have these disadvantages of use and are particularly suitable for point-to-point printing, in particular by inkjet.

In the liquid phase, these inks have a fluidity which makes it possible to mechanically deposit, in particular according to an offset process, drops of ink of very fine dimension or to spray drops point by point on a support.

The final fixing of the polymer inks is carried out during a so-called ink crosslinking step which follows the deposition of the ink drops.

Crosslinking consists of polymerizing or crystallizing the ink, the polymers constituting the ink bonding together to form longer polymer chains and attaching to the support. A crosslinking step therefore makes it possible to solidify the ink and to fix it to the support. The supports made of plastic, such as polyvinyl chloride (PVC), polyethylene (PE), poly-ethyl-tetraethylene (PET), polycarbonates (PC), 1 acrylonitrile-butadiene-styrene (ABS) and other organic polymers are quite naturally suitable for printing by polymer ink, the polymers of the ink and the polymers of the support fixing themselves firmly together during crosslinking.

Crosslinking is obtained by exposure of the inked support under ultraviolet radiation. We will therefore speak thereafter of ink crosslinkable by ultraviolet radiation, abbreviated to UV ink. The energy of ultraviolet photons allows polymerization of the polymer chains between them. However, the support must be exposed under a power of ultraviolet radiation and for a sufficient time, so that the ink fixes well to the support and hardens completely.

Figures 1 and 2 schematically show known printing techniques based on crosslinkable UV ink. Figure 1 shows a polychrome offset printing of a support. The support 10 advances between a drive cylinder 15 and rollers 11, 12, 13 and 14 for contact printing. Each roll 11 or 12 or 13 or 14 contains a frame of the image to be printed. The screen weights of each roll are inked with black or colored ink, especially cyan, magenta or yellow. Several color screens are thus deposited on the support to constitute a final polychromy image. The inking step is followed by a crosslinking step by continuous exposure 19 of the support 10 under an ultraviolet lamp 18. Of course the offset printing can be monochrome by providing a single black or colored inking roller .

Figure 2 schematically shows a polychrome inkjet printing process. Several tanks 21, 22, 23 and 24 containing the black polymer inks and of different colors supply at least one nozzle for ejecting ink drops, each tank preferably having its own line of ejection nozzles, the line of impression being transverse to the direction of movement of the support. The ink drops are deposited point by point on the support, a device for moving the support and computer programming of the image to be printed controlling the ejection of the drops by each nozzle of the line with a possible drop volume control ejected. The computer system defines the spatial locations 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 crosslinking step, always by continuous exposure, the support advancing under an ultraviolet lamp. FIG. 2 illustrates an alternative printing in which each inking step is followed by a crosslinking step in order to dry each ink before a subsequent inking of a different color. The printing device of FIG. 2 therefore comprises in this example four ultraviolet lamps 25, 26, 27 and 23 for drying each ink individually. To increase the printing rates, it has been proposed to increase the power of ultraviolet lamps thereby reducing the exposure time of the support, the support always receiving sufficient energy to dry and fix the ink.

However, ultraviolet lamps give off a lot of heat. Printing devices with polymerizable 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, does not exempt from providing cooling when high printing rates are desired.

A disadvantage of known printing devices with ink crosslinkable by ultraviolet radiation is therefore the high heat generation at the crosslinking stages.

Another drawback is the premature aging of the supports and their yellowing under the effect of crosslinking ultraviolet radiation.

An object of the invention is to provide an ink crosslinking process allowing printing at high speed, without the above-mentioned drawbacks.

A particular object of the invention is to avoid yellowing of the support in order to allow lasting quality printing.

Briefly, these objects are achieved, according to the invention, by providing that the crosslinking is effected by an ultraviolet laser beam concentrated on the drops of ink deposited on the surface of the support, the white surfaces of the support not being scanned by the laser beam. The invention is achieved by providing a method of crosslinking photosensitive ink comprising a step of inking points of a support and a particular step consisting in applying an ultraviolet beam concentrated on the inked points, excluding non-metallic surfaces. inked from the support.

The inking step preferably consists in depositing point by point on a printing medium drops of polymerizable ink, the ink being polymerizable by ultraviolet radiation.

The invention is preferably carried out by applying an ultraviolet laser beam.

A first embodiment of the invention provides that the application of the ultraviolet beam is carried out by point-by-point scanning of the support.

A second embodiment of the invention provides that the application of the ultraviolet beam is carried out via an optical fiber or a network of optical fibers. According to a preferred characteristic of the invention, provision is made to interrupt the ultraviolet beam when it is directed towards the non-inked surfaces of the support, an embodiment of the invention possibly comprising continuous scanning of the support. According to an alternative characteristic, it is planned to modulate in power the ultraviolet beam concentrated on the inked points.

The invention is particularly applicable to printing and crosslinking ink on a plastic support.

Advantageously, the ink crosslinking method according to the invention is particularly applicable to a point-to-point inkjet printing process and / or a polychromatic printing process.

Other characteristics, objects and advantages of the invention will appear on reading the description which follows, with reference to the appended drawings, given by way of non-limiting examples and in which: FIG. 1, previously described, represents a printing and a crosslinking of UV ink according to a known method, - Figure 2, previously described, represents a printing and a crosslinking of UV ink according to another known method, Figure 3 shows a method of crosslinking photosensitive ink according to the invention, FIG. 4 represents a first embodiment of the photosensitive ink crosslinking method according to the invention, and

- Figure 5 shows a second embodiment of the photosensitive ink crosslinking method according to the invention.

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 being able in particular to be carried out by mechanical contact in press or by spraying 37 with drops 36 of ink, in particular during point-to-point inkjet printing. The method according to the invention thus comprises a preliminary step of inking the support, the inking being carried out with a photosensitive ink of the type ink crosslinkable by ultraviolet radiation. Preferably, the inking is carried out according to the invention by depositing drops of ink polymerizable by ultraviolet radiation point by point on a printing medium.

At the end of the printing or more precisely of this inking step, the support 33 includes inked surfaces and non-inked surfaces 30, the inked surfaces being made up of inked points 31 arranged contiguously or in isolation.

Whatever the connectedness of the inked surfaces, the method according to the invention provides for applying an ultraviolet beam concentrated on the inked points, to the exclusion of the non-inked surfaces of the support. FIG. 3 thus shows an optical device 33 34 schematically provided with a source of ultraviolet rays 33 and a beam concentrator 34, for concentrating the ultraviolet rays on an inked point 31. An advantage of the method according to the invention is that the light power of the source 33 of ultraviolet rays is concentrated on the only point 31 whose crosslinking is then very rapid. Consequently, it is possible to provide for a very rapid scanning of the inked points, by applying the beam concentrated on each point for a period of time corresponding to the energy which the drop of ink must receive in order to be completely crosslinked.

The method provides according to the invention not to apply an ultraviolet beam on the non-inked surfaces. An advantage of such an arrangement is that aging and yellowing of the support is avoided, especially on non-inked surfaces.

Another advantage is that the light energy applied is less compared to the methods of exposure to ultraviolet lamps, no radiation power being unnecessarily dispensed on non-inked surfaces.

Such an arrangement is easily achieved by providing that the beam 32 is concentrated on an area substantially equal to the area of a drop of ink. Means for scanning the support and distributing the beam will be detailed below in two preferred embodiments of the device implementing the method according to the invention.

The invention is achieved by using an ultraviolet laser, although an intense source of ultraviolet type of arc lamp or rotating cathode lamp can be envisaged. FIG. 4 thus illustrates a laser 43 emitting a coherent beam 42 of ultraviolet radiation. The beam 42 'is deflected to focus on an inked point 41' to be crosslinked.

An advantage of the laser is that the beam 42 'of emitted rays can easily have very small dimensions while remaining substantially parallel. The beam 42 can thus be concentrated on a surface as microscopic as the surface of offset printing dots in polychromy such as the dots 51a, 51b, 51c and 52a to 55c shown in enlarged view in FIG. 4. In addition, an ultraviolet laser can have a very intense light power, which allows very rapid exposure of each point to be crosslinked.

The crosslinking time of a support comprising few inked points is thus advantageously reduced compared to known methods.

One can choose a laser emission device 43 emitting a beam continuously or by pulse. The exposure time 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 crosslinking light energy.

According to a first embodiment of the method according to the invention, the application of the ultraviolet beam is carried out by point-by-point scanning of the support.

FIG. 4 thus illustrates a scanning device 46 comprising a motor orienting a mirror 46 for deflecting the laser beam 43 towards each point of the support.

According to the arrangement illustrated in FIG. 4, the device 45, 46 for deflecting the beam 42 ensures a transverse scanning of the support 43 by the beam 42 ', 42 ", 42"' so as to crosslink all the points 41 ', 41 ", 41 "'of a transverse line of the support 48. The support is then moved in a longitudinal direction to crosslink a next line of points.

Preferably, the scanning device 45, 46 is coupled to a point-to-point printing computer system, indicating to the scanning device the exact location of each inked point of the text or image being printed. The scanning device can in particular receive a command similar to the command for positioning a print head point by point.

The scanning provided by the first embodiment can be carried out continuously or discretely, according to two variants.

In the first variant, the deflection angle of the ultraviolet beam 42 varies continuously, the beam 42 'being deflected progressively along the transverse line of the support.

To avoid applying the beam on "white" surfaces 40, it is planned to interrupt the beam 42 when it is deflected towards the non-inked surfaces 40. A component 44 for cutting the beam 42, shown diagrammatically on the Figure 4, thus prevents the beam 42 'from being concentrated on non-inked points. This cut-off component is advantageously coupled to the computerized point-to-point printing system which triggers its closure when the deflected beam 42 ′ is directed towards non-inked surfaces 40.

For very rapid crosslinking, the cut-off component 44 must have a very short reaction time. Component 44 is for example a "Q-switch" device as used in optronics. Other means of interrupting the beam 42 are within the reach of those skilled in the art without departing from the scope of the present invention.

It will also be noted that the beam interruption means can be an integral part of the laser 43. Thus, the laser delivers on command pulses of ultraviolet radiation when the device scanning 45, 46 targets an inked point 41 'and does not deliver a pulse when the scanning device 45, 46 targets an non-inked point 40.

In the second variant, the scanning device 45, 46 is programmed to deflect the beam 42 'towards an inked point 41' and pass directly to another angle of deflection, the beam 42 '' moving towards another inked point 41 ' '. The scanning command of the device 46 is then discontinuous and the position of the mirror 45 passes without transition from an angular value to another discrete angular value.

It is planned to correct the spread of the beam when the beam 42 "'falls at a slight angle on the support, that is to say when the deflection of the beam is high. This correction is obtained by providing a correction optic so-called flat field which reduces the spreading of the beam under such conditions and focuses it in a punctual manner. A second embodiment of the method according to the invention provides another mode of application of the ultraviolet beam on the points of the support, location of the scanning step.

The second embodiment comprises, as illustrated in FIG. 5, a linear strip 70 of optical fibers 71 to 77 parallel, the outlet of which is arranged facing the surface of the support to be crosslinked. Equivalently, a two-dimensional network of optical fibers with parallel outputs can be provided. The laser beam 82 83 is injected at the input of the optical fibers 71 to 77. The fibers 71 to 77 advantageously have their inputs assembled so that that the incoming laser radiation is distributed almost equally between all the fibers.

Thus the initial laser beam 82 is divided into a multitude of parallel rays, each ray being directed and concentrated towards an inked point of the support 63.

The optical fibers used are quartz or glass transmitting ultraviolet radiation, an ordinary glass optical fiber does not transmit wavelengths beyond purple. The beam distribution device 70 also comprises means for interrupting the ultraviolet beam, each optical fiber 71 being provided for example with a ray cutting component to avoid exposing an un-inked point 60 of the support 68 This second embodiment is particularly suitable for printing methods comprising a dithering of dots. By adapting the spacing pitch ent of the fiber outputs of the linear bar 70 to the dithering pitch of the printing, a series of laser rays is obtained concentrated on the precise coordinates of the points of the printing frame.

As illustrated in FIG. 5, the second embodiment advantageously applies to ink jet printing methods which allow printing line by line, a line of dots being inked instantly.

An in-line inkjet device generally includes a linear array 100 of ink drop generators. A series of drops of ink 101, 102, 103 is emitted simultaneously in the direction of the points of the support which it is desired to ink. Such devices are used in particular in polychrome offset printing by having several generator strips 100, 110, 120 supplied by reservoirs 109, 119, 129 of inks of different colors. All the nuances of colors and tint are obtained by modulating the volume of the drops of inks, and by using inks corresponding to the fundamental colors and possibly to black. As detailed in FIG. 4, each colored point 51 is formed for example of three or four elementary points 51a, 51b, 51c inked in fundamental colors or in black.

The inked dots of different colors can be crosslinked according to the invention by applying a laser beam to each colored point.

The elementary points, generally microscopic, are very close and may possibly overlap.

The polychromy effect is obtained, during offset printing, by modulating the dimensions of each microscopic elementary point to reconstitute all the possible colors. According to a variant, by modulating the overlap and the dimension of each point, one also obtains a polychromy effect.

Advantageously, provision is made according to the invention to modulate the concentrated beam applied to such inked points so that each point receives sufficient energy to crosslink the ink volume of the point. The laser beam interrupting means are then replaced by means for modulating the intensity of the beam. Such a means is made for example an optical modulator of the orientable diffraction plate type.

In general, the possibility of modulating the ultraviolet beam in power makes it possible to adapt the crosslinking step to the inks used and to the printing speed of the support.

The crosslinking method can be applied only once after all the steps of color inking as illustrated in FIG. 5. The beam distributor device 70 then comprises a tight network of optical fibers, the fibers being distributed spatially according to the maximum frame of dots. ink that can be formed in printing.

Alternatively, crosslinking according to the invention can be carried out after each inking of a color during a full color printing.

The printing installation can then include several crosslinking devices arranged at the output of each monochromatic inking device. The method according to the invention advantageously makes it possible to provide a total or partial gelation of the inks during the crosslinking between each inking step, the partial gelation being obtained for example by modulating the power of the ultraviolet laser beam.

The essential advantage of the crosslinking method according to the invention is, as indicated above, to eliminate the drawback inherent in ultraviolet radiation, namely the bleaching or yellowing action on the polymers constituting the support.

Initially intended to apply to plastic supports, the method according to the invention extends to the crosslinking of photosensitive ink on any type of printing medium such as paper, cardboard, wood to advantageously replace printing with ink based on water or solvents while avoiding any browning of the medium .

Finally, the rational use of the crosslinking light power according to the invention and the high light intensities which can be obtained with a laser have the advantage of increasing the speed of the crosslinking step compared to UV lamps. traditional sunstroke.

Subsequently, the crosslinking method according to the invention advantageously contributes to increasing the rates of the printing device in which it is integrated. The method according to the invention thus makes it possible advantageously to obtain a crosslinking speed greater than the ink jet inking speeds, so that the printing rate is no longer limited by the crosslinking step. Although the description of the invention is based on ultraviolet radiation, the invention is not limited to a precise spectrum of light, but can be applied with any type of light radiation suitable for the polymerization and drying of photosensitive inks.

In addition, the crosslinking process can be used with photosensitive paints, the same constituents and the same pigments being used in polymeric inking and in polymeric paint. Other advantages, application and development of the invention will appear to a person skilled in the art without depart from the scope of the invention defined in the claims below.

Claims

1. A method of crosslinking photosensitive ink comprising a step (35) of inking dots (31) of a support (38) characterized in that the method comprises a subsequent step consisting in applying a concentrated ultraviolet beam (32) on the inked dots (31), excluding non-inked surfaces (30) of the support.

2. Method according to claim 1, characterized in that the inking step consists of depositing point (37) by point (31) on a support (38) for printing drops (36) of polymerizable ink, l ink being polymerizable by ultraviolet radiation.

3. Method according to claim 1 or 2, characterized by the application of a beam (42) of laser (43) ultraviolet.

4. Method according to one of claims 1 to 3, characterized in that the application of the ultraviolet beam (42) is carried out by scanning (42 ', 42 ", 42"') point (41 ') by point (41 ", 41" ') of the support (48).

5. Method according to one of claims 1 to 4, characterized in that the application of the ultraviolet beam is carried out via an optical fiber (71) or a network (70) of fibers (71- 77) optical.

6. Method according to one of claims 1 to 5, characterized in that it comprises a step consisting in interrupting (44) the ultraviolet beam (42) when it is directed towards non-inked surfaces (40) of the support. (48).

7. Method according to one of claims 1 to 6, characterized in that it comprises a step consisting in modulating in power the ultraviolet beam concentrated on the inked points.

8. Method according to one of claims 1 to 7, characterized in that the support (33,48,68) is made of plastic.

9. Method according to one of claims 1 to 8, characterized in that it (70) fits into a method (100) of point-by-point printing by ink jet.

10. A polychromatic printing method (100,110,120) with photosensitive inks of different colors, characterized in that it comprises at least one application (70) of the crosslinking method according to one of claims 1 to 9.