EP1082222B1 - Method for making a flexographic print block - Google Patents

Description

The present invention relates to a manufacturing process a flexographic printing plate.

Such a photograph comprises, in a known manner, a form printer made of a material having a certain compressible character, thus facilitating the transfer of ink, from this printer form to surfaces flexible like cardboard, paper, polyethylene sheets, etc.

We already know cliché manufacturing processes flexographic printing involving a phase of laser engraving, which are for example described in EP-A-0 640 043, EP-A-0 640 044 and US-A-4 994 344 according to which, an ethylene-propylene-alkadiene terpolymer is used as binder of a photopolymerizable layer for the production of a flexographic printing plate. According to the teaching of these documents, a photopolymer layer not yet polymerized, possibly arranged on a flexible support layer, is covered with by means of an upper layer so as to constitute a draft. This top layer, which is likely to be engraved by means of a laser, for example consists of black of carbon. These documents disclose the use of lasers CO2 and solid lasers emitting in the infrared, as by example the Nd-YAG laser, in order to etch the upper layer of this draft.

This etching phase is followed by an exposure step of the photopolymer layer thus accessible through the etched layer, with ultraviolet, then a phase of crosslinking of this photopolymer layer, which tends to polymerize and to harden the zones having undergone the ultra-violet radiations. Uncured areas are then removed.

This known process involving laser engraving, has some drawbacks, however, in that it is necessary to carry out additional exposure steps to ultraviolet and crosslinking. This implies that a such a method has a high cost and a long duration. In addition, these additional steps require doing use of chemical solvents that cause pollution consistent.

DE-A-19507827 describes a process for manufacturing a flexographic printing plate in which a layer of a blank is subjected to a laser etching phase, said layer layer being made of a material having been polymerized before said etching phase with laser, said material comprising a polyester, polyethylene or polyamide and the phase of engraving being carried out by means of a laser operating at a wavelength around 248nm.

The invention proposes to implement a method of manufacture of a flexographic printing plate allowing to overcome all the drawbacks of the prior art mentioned above.

To this end, it relates to a method of manufacturing a flexographic printing plate according to claim 1.

The process according to the invention makes it possible to carry out the previously mentioned objectives. Indeed, it contributes to get rid of the presence of an upper layer likely to be laser engraved and attached above the material photopolymer. According to the invention, the etching face laser is performed directly on the polymerized material.

In addition, this material is subjected to the etching phase at laser after being polymerized, not before being subjected to such polymerization, as was the case in the art prior. In accordance with the invention, we dispense with ultraviolet exposure and crosslinking phases that involved the process of the prior art. Compared to the latter, the invention is therefore notably advantageous in terms of cost, duration and in particular chemical pollution. The process of the invention is essentially mechanical, since it does not use the photochemical processing steps that provides for the prior art method discussed above, but that physical machining of the polymerized material is carried out.

According to a first characteristic of the invention, the etching phase is carried out via a laser excimer.

According to another characteristic of the invention, the phase engraving is carried out via an Nd-YAG laser. In this case, since such a laser usually operates at wavelengths slightly greater than 1064 nm, it should bring the frequency of this laser in the range suitable for carrying out the invention, using known techniques of harmonic generators which allow to double, triple or quadruple the frequency, so to divide the emission wavelength by the same factor.

According to another advantageous characteristic of the invention,
the etching phase is carried out at a fluence of between 2 and 4 J / cm ² , preferably between 2.5 and 3 J / cm ² .

• la figure 1 est une vue schématique illustrant un appareillage permettant la mise en oeuvre du procédé conforme à l'invention ;
• les figures 2 à 4 sont des vues schématiques illustrant trois phases successives d'un mode de réalisation du procédé conforme à l'invention.

The invention will be described below, with reference to the accompanying drawings, given only by way of nonlimiting examples and in which:

• Figure 1 is a schematic view illustrating an apparatus for implementing the method according to the invention;
• Figures 2 to 4 are schematic views illustrating three successive phases of an embodiment of the method according to the invention.

Figure 1 illustrates an apparatus allowing the setting in work of the process of the invention. This equipment includes a laser 2, operating at a wavelength between 200 and 400 nm. At the exit of this laser is placed a mask 4 to rectangular opening that selects the most homogeneous beam delivered by the laser. A lens 6 is placed in the axis of this beam and allows the reproduction of the image of the mask on a blank 8 intended to form a plate flexographic printing.

The distance between lens 6 and laser 2 can be modified so as to vary the surface density of energy, or fluence, present on the surface of the blank 8.

As shown in more detail in Figure 2, this blank 8 comprises a lower layer 10 of mechanical support, performed for example in MILAR (TM). This support layer 10 receives a polymerized layer 12, their subjection mutual insurance, for example, ensured by pressing and / or gluing.

The action of the laser on the polymerized layer 12 is represented in figure 3. The laser forms, by ablation with level of the face of this polymerized layer 12, opposite the support layer 10, cutouts 14 in locations appropriate. The depth of these cuts is, for example, between 500 and 1000 micrometers.

Once the cutouts 14 have been made within the layer polymerized 12, the support layer 10 is separated from the polymerized layer 12, so that the latter constitutes then a flexographic printing plate 16 obtained in accordance to the invention. It should be noted that such a photograph 16, shown in Figure 4, is available immediately after laser engraving phase, compared with the prior art in which such an etching phase was followed by several phases of exposure to ultraviolet light, a phase of polymerization, as well as solvent etching and stabilization.

In the description made with particular reference to Figures 2 to 4, only reference was made to a layer single polymerized on a support layer. It is also possible to make a blank formed of multiple cured layers and subject the layer superior to the action of a laser, so as to form a cliché multilayer flexographic printing.

An example of implementation of the method in accordance with the invention will be described below.

As a polymerized layer 12, a plate is used translucent formed of a mixture EPT, or ethylene-propylene terpolymer, which conforms to the product marketed by the EXXON company under the reference VISTALON 504. This EPT mixture is cross-linked with Percadox BC.

This plate has a density of 1.01, an abrasion of 90 mm ³ under 10N, a Shore A hardness of 65, a breaking strength of 100DaN / cm ² , an elongation at break of 500% and has a thickness that is substantially uniform about 2 mm. It is fixed to a support layer, similar to that designated by the reference 10 in all of the figures, produced in MILAR (TM) and having a thickness of approximately 0.1 mm.

We first submit the blank described above to the action of an excimer laser of the LAMBDA PHYSIK LPX 220 type, whose active medium is based on krypton fluoride and which emits a pulse of 20 ns at 248 nm.

First of all, we study the ablation depth as a function of fluence, namely the surface energy density present at the level of the polymerized layer intended to be etched. To do this, this layer is subjected to 200 shots of the laser described above, operating at a frequency of 10 hertz. Note that the ablation depth has a maximum for a fluence value of around 2.8 J / cm ² .

In a second step, we operate at a fluence value close to the maximum mentioned above, namely equal to 2.6 J / cm ² . Then, we study the variation of the depth as a function of the total amount of energy brought to the level of the surface of the polymerized layer 12, namely as a function of the number of shots. It is noted that a depth of 100 micrometers, which is sufficient for obtaining a flexographic printing plate of satisfactory quality, is obtained for an energy of approximately 600 J / cm ² .

Furthermore, the mass removed after the laser operation was measured. To do this, the laser mentioned above was used, with a fluence of 3.5 J / cm ² . It was found that the mass removed by ablation was 10.21 micro-grams per shot, and that the corresponding volume was 0.0099 mm ³ .

At the same time, the same experiment was carried out using an excimer laser of the LAMBDA PHYSIK LPX 220 type, the active medium of which is based on xenon chloride emitting at 308 nm. The apparatus described with reference to FIG. 1 has been shaped so that the laser can operate at a fluence of 4.2 J / cm ² . It was found that the mass removed by ablation at the level of the polymerized layer 12 was 21.37 micrograms per shot and that the corresponding volume was 0.0207 mm ³ . This therefore means that the loss of material is more sensitive to a wavelength of 308 nm, than to 248 nm.

Finally, a comparative study was carried out in order to estimate the time necessary to remove by ablation a thickness of 0.1 cm on a sector of 10 X 10 cm from the plate described above, by means of the two lasers explained above. above, operating at 248 nm and 308 nm respectively. To do this, these two lasers were placed under identical operating conditions, namely an energy of 2 J, a frequency of 500 hertz, an average power of 1000 watts and a fluence of 3.5 J / cm ² . It was found that 3 minutes and 25 seconds were necessary for the ablation mentioned above, operating at 248 nm, while 2 minutes and 3 seconds were sufficient with a laser operating at 308 nm.

Claims (4)

1. Method of manufacturing a flexographic printing block (16) in which one layer (12) of an outline (8) is subjected to a laser engraving phase, characterised in that said layer (12) is made from a material which has been polymerised before said laser engraving phase, said material being made from EPT (ethylene propylene terpolymer) charged between 20 and 40 % and comprising between 55 and 65 % by weight ethylene and between 35 and 45 % by weight propylene, and in that said engraving phase is effected by means of a laser (2) operating at a wavelength of between 248 and 340 nm, preferably 308 nm and reaching a cutting depth of between 500 and 1000 µm.
2. Manufacturing method according to claim 1, characterised in that said engraving phase is effected through the intermediary of an excimer laser.
3. Manufacturing method according to claim 1, characterised in that said engraving phase is effected through the intermediary of an Nd-YAG laser.
4. Manufacturing method according to any of claims 1 to 7*, characterised in that said engraving phase is effected at a rate of between 2 and 4 J/cm², preferably between 2.5 and 3 J/cm².

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