WO2002009941A1 - Method for engraving a printing form using a laser beam - Google Patents

Abstract

The invention relates to a method for engraving a printing form using a laser beam. According to the invention, an absorbent layer is applied to the surface of the printing form and the layer's absorption for laser light is greater than the absorption of the printing form material. This results in reducing the laser power required for melting or vaporizing the printing form material. The absorbent layer can be produced by chemically modifying the surface of the printing form by means of electrodeposition or a mechanical method such as vapor depositing, melting-on or varnishing. In addition, the absorption of the surface of the printing form can be increased by roughening the printing form material or the absorbent layer. The method is preferably used for engraving rotogravure forms having a copper surface.

Description

Process for engraving a printing form with a laser beam

The invention relates to a method for engraving a printing form with a laser beam, for example for gravure printing, offset printing, letterpress printing, screen printing or flexographic printing. When producing printing forms, it is necessary to produce very fine structures on the surface of the printing form, since high-resolution image information such as text, raster images, graphics (linework) and line patterns are to be reproduced.

Engraving methods known from the prior art for the direct engraving of gravure printing forms either work with a diamond stylus, an electron beam or a laser beam in order to work out depressions - so-called cups - which each form a halftone dot, from the printing form material. In gravure printing, printing cylinders are normally used as printing forms, the surface of which is made of copper, into which the structures required for printing are engraved into the printing cylinder in the form of cups by means of a diamond stylus. After their completion, the printing cylinders are placed in a printing press, in which the cells are filled with printing ink and the printing ink is transferred to the printing material during the printing process.

Copper cylinders are used because of their long service life in the printing process. A long service life is required for long runs, especially in magazine printing or packaging printing, since the surface of the printing plate wears out during the printing process. In order to extend the service life even further, the printing cylinders are also galvanically chrome-plated after engraving. It follows that copper is the most suitable material for the surface of gravure cylinders. Materials other than copper have so far not proven themselves for long runs.

When creating the cells, the diamond stylus cuts indentations in the rotating printing cylinder along a circumferential line, while the engraving system moves in the axial direction along the printing cylinder. The lifting movement of the diamond stylus takes place via an electromechanically driven magnet system with an oscillating armature on which the diamond stylus is attached. Such an electrical The tromechanical vibration system cannot be made as quickly as desired due to the forces that are required to engrave the cells. In order to increase the engraving speed even more, in today's engraving machines several such engraving systems are arranged side by side in the axial direction of the printing cylinder. When using several engraving systems, several so-called strands are engraved into the surface of the printing cylinder at the same time. Such a strand contains, for example, one or more entire magazine pages. A problem that arises is that because of the different engraving systems which are controlled independently of one another, wells of different volumes are produced in the individual strands with the same tonal value to be engraved, which leads to differences in the individual strands which the eye recognizes when looking at them later. For this reason, e.g. B. in packaging printing only worked with an engraving system so that these errors, which are tolerated in the magazine print, do not occur.

When engraving the cells, the cell volume is varied depending on the image content of the template to be printed. Here, the respective tonal value of the original is to be reproduced as precisely as possible when printed. In order to generate the corresponding volumes, the depth of penetration of the diamond stylus into the copper surface is varied by actuating the magnet system, the geometry of the cups being approximately between 120 μm in diameter at a depth of 40 μm and approximately 30 μm in diameter at a depth of 3 μm changes. Because there is only a very small range of variation in the depth of the wells between 40 μm and 3 μm, the penetration depth of the stylus with which the wells are engraved must be controlled with very high precision in order to reproducibly achieve the desired tonal range , Since the geometry of the engraved cells depends directly on the shape of the stylus, there are also very high demands on the geometry of the diamond stylus. In addition, the diamond stylus is subject to wear, because when engraving a large impression cylinder, approximately 20 million cells have to be engraved by a stylus. If one of the diamond styluses breaks off during the engraving of a printing cylinder, the entire printing cylinder is unusable, which on the one hand causes considerable financial damage and on the other hand considerable damage Loses time because a new cylinder has to be engraved and the start of printing is postponed for hours.

The engraved cells, which are later to receive the printing ink, are arranged on the surface of the printing form in accordance with the printing screen, with a separate printing cylinder being produced for each printing ink, each of which is given a different screen with a different angle and screen size. With these grids, narrow webs remain between the individual cells, which, when printed in the printing press, carry the doctor blade, which scrapes off the superfluous ink after inking. Due to the restriction that the cells can only be engraved along a circumferential line, there is only a limited selection of screen angles available for the screens. Another disadvantage of the mode of operation of electromechanical engraving is that texts and lines must also be reproduced in a raster, which leads to the contours of the characters and the lines which the eye recognizes as disturbing. This is a disadvantage of intaglio printing compared to offset printing, in which this offset can be kept so small that the contours of characters and lines appear completely smooth to the eye.

Likewise, in gravure engraving, due to the mechanical boundary conditions in the engraving machine that the cells can only be engraved along a circumferential line, it is not possible to generate frequency-modulated screens in which the size and position of the cells are randomly distributed according to the tonal value. Such frequency-modulated screens would have the advantage of improved image quality because details can be reproduced more sharply and because no moiré can occur between the basic colors of the print.

All in all, the electromechanical engraving is well suited to producing high quality gravure cylinders. But it has a number of weak points and is very complex. These disadvantages would like to be eliminated by another engraving method that does not work with the limitations of electromechanical engraving. Furthermore, it is known to use the electron beam engraving method used in material processing to produce the wells, which has shown very good results because of the high energy of the electron beam and the precision with regard to beam deflection and beam geometry. This process is in the printing step "Fast electron beam engraving process for the engraving of metal cylinders", optics 77, no. 2 (1987) pages 83-92, Wissenschaftliche Verlagsgesellschaft mbH Stuttgart. Because of the very high expenditure required for the hardware and electronics, electron beam engraving has so far not been used in practice for engraving copper cylinders for gravure printing, but only in the steel industry for surface engraving of so-called texture rollers for sheet metal production with which textures are rolled into the sheets.

In the specialist literature and also in the patent literature, it has repeatedly been proposed to engrave the copper cylinders using a laser. However, since copper is a very good reflector for laser radiation, very high powers and in particular very high power densities of the lasers are required in order to penetrate the copper and melt or vaporize it. So far, there is no laser engraving unit with laser radiation sources corresponding to high power density and energy, with which it is possible to provide the copper cylinders for gravure printing with the required cell structure in the copper conquered pool.

Nevertheless, attempts have been made to use lasers for gravure engraving by switching to materials other than copper. For example, it has been proposed in DE-A-19 27323 to prepare copper cylinders by chemical etching in such a way that the surface of the copper cylinder already has cells with a volume that corresponds to the maximum printing density. These cells are filled with a solid filling material such as plastic. The filling material is then removed by means of a laser until the desired well volume is reached. Although this process requires less laser power than would be necessary to melt and vaporize the copper as in electron beam engraving. With this method, however, the remaining plastic is attacked and decomposed by the solvent of the printing ink in the printing process, so that only a small printing edition is possible. This method has not proven itself in practice and has not been used.

It is also published in the VDD seminar series "Direct laser engraving for metal-coated gravure cylinders", as part of a "Colloquium by the Association of German Printing Engineers e.V. and the Department of Printing Machines and Printing Processes, Department of Mechanical Engineering, Technical University Darmstadt", by Dr. phil. nat. Jakob Frauchiger, MDC Max Dätwyler, AG, Darmstadt, December 12, 1996, proposed to engrave zinc-coated gravure cylinders using a Q-switched Nd: YAG high-performance solid-state laser pumped with arc lamps. In this method, the volume of the cells is determined by the optical power of the laser.

A disadvantage of this method is that the arc lamps required for pumping the laser have a relatively short lifespan and must be replaced after about 500 hours of operation. If the pump light source fails during engraving, the engraving cylinder becomes unusable. This corresponds to a failure of the diamond stylus in the electromechanical engraving and has the same disadvantages.

A decisive disadvantage of this method, however, is that zinc is considerably softer than copper and is not suitable as a surface material for printing cylinders. Since the doctor blade with which the excess ink is scraped off before printing in the printing press is a steel knife, the zinc surface is damaged after a certain time and the printing cylinder becomes unusable. A printing cylinder with a surface made of zinc therefore does not have nearly as long a service life in printing as a printing cylinder with a surface made of copper. Printing forms with a zinc surface are therefore not suitable for long runs.

Even if, as has also been suggested, the zinc surface is chrome-plated after the engraving in order to increase the service life, the durability does not come close to that of normal copper cylinders. Chromium does not adhere as well to zinc as it does to copper and the so-called "hot chrome plating", which is successfully used in copper cylinders, In order to achieve an optimal adhesion of the chromium to the copper, zinc is not possible because the zinc would melt. Since the chrome layer does not adhere very well to the zinc, it is also attacked by the doctor blade, which leads to a relatively early failure of the printing cylinder. If, however, copper cylinders are chrome-plated using this method, extremely long runs are possible, since the chrome adheres firmly to the copper surface, so that these copper cylinders far surpass the chrome-plated zinc cylinders.

From the document EP-B-0473 973, which likewise relates to the method described above, it is apparent that in this method for lifting a well with a diameter of 120 μm and a depth of 30 μm, zinc has an energy of 6 mWsec is required. For copper, an energy of 165 mWsec is specified in this document, which is a factor of 27.5 for the required laser power. This means that lasers with a continuous wave output of a few kilowatts and good beam quality are required to produce cells in copper at a speed that is acceptable for the printing industry. However, such a power cannot be generated with the laser arrangement described above. For this reason it is only possible to engrave on a zinc surface.

Furthermore, a complex modulator would be required for such an arrangement for the engraving of copper cylinders. Modulators for very high laser powers are slow, which leads to a reduction in the modulation frequency and thus the engraving frequency. However, if the engraving frequency is too low, the energy diffuses into the area around the processing spot on the surface without lifting a well. Therefore, in addition to the high energy required for excavation, it is also necessary to have a high output.

If you want to take advantage of the copper surface and engrave it with a laser, it is essential to apply the high power density required to penetrate the surface of the copper and the high energy required to melt or evaporate the copper. So far, however, this has not been possible with a solid-state laser. It is therefore an object of the present invention to avoid the disadvantages of the prior art methods described and to specify a method for engraving printing forms - preferably copper cylinders for gravure printing - with a laser beam which manages with a lower power density and energy and therefore is inexpensive. This object is solved by the features of claims 1 to 8.

The method according to the invention is based on the observation that the absorption of the copper surface for light energy increases sharply when part of the copper in the processing spot has already evaporated. In the publication "The Laser in the Printing Industry" by Werner Hülsbuch, page 540, publisher W. Hülsbusch, Konstanz, it is described that in material processing with power densities from typically 10 ⁷ to 10 ⁸ W / cm ² for all materials to one spontaneous evaporation of the material occurs, which is associated with a sudden increase in absorption, which is particularly advantageous since the laser power is then no longer reflected by the metal surface.

The aim of the invention is to achieve the onset of melting or vaporization of the copper and the associated increase in absorption even at significantly lower power densities. If the absorption is correspondingly increased due to the onset of evaporation, the lower power density is also sufficient to continue the removal process, so that further material is removed from the copper surface. In this way it becomes possible to maintain the engraving process and to engrave wells of variable depth in the copper surface.

The lowering of the power density threshold for the onset of evaporation is achieved by the method according to the invention by increasing the absorption of the copper surface by pretreating the gravure cylinder. The surface is preferably converted in a thin layer into a chemical copper compound which has a high absorption for laser light, for example into a copper oxide layer. The chemical conversion of the copper surface can be achieved by the action of acids, alkalis or salt solutions, whereby the process can be accelerated and intensified if necessary by heating or by electrolysis.

According to an alternative solution of the method according to the invention, a thin layer of another metal with higher absorption for the laser light is applied to the copper surface, preferably by galvanic means. This metal layer should not be so thick, as in the known methods, that the cells are engraved exclusively in this other metal. It should only lower the performance threshold for copper evaporation by evaporating this thin layer and increasing the absorption associated with it. The actual engraving of the cells is then carried out in the copper layer.

Another solution for increasing the absorption of the copper surface is the mechanical application of a thin, highly absorbent layer, for example a plastic layer. Techniques such as vapor deposition, spraying, melting or varnishing can be used for this, possibly supported by a subsequent baking process. Here, too, the applied layer should only lower the threshold for copper evaporation, so that the actual engraving of the cells is then carried out in the copper layer.

An increased absorption of the copper surface can finally also be achieved by increasing the roughness of the surface, for example by etching the surface. The increase in roughness can also be combined with the aforementioned solutions, i.e. after the generation or application of the thin absorbent layer, this layer is additionally roughened.

Claims

claims 1. A method for engraving a printing form with a laser beam, characterized in that an absorbing layer is applied to the surface of the printing form, whose absorption for the laser light is higher than that Absorption of the printing form material. 2. The method according to claim 1, characterized in that the absorbent layer reduces the laser power required for the melting or evaporation of the printing plate material. 3. The method according to claim 1 or 2, characterized in that the absorbent layer is produced by chemical change in the surface of the printing form. 4. The method according to claim 1 or 2, characterized in that the absorbent layer is produced by electrodeposition. 5. The method according to claim 1 or 2, characterized in that the absorbent layer by a mechanical method such as evaporation, spraying, Melting or painting is applied. 6. The method according to any one of claims 1 to 5, characterized in that the absorption of the surface of the printing form is increased by roughening the Druckfomnmater- material or the absorbent layer. 7. The method according to any one of claims 1 to 6, characterized in that the printing form is an intaglio printing form. 8. The method according to any one of claims 1 to 7, characterized in that the printing form material is copper.