EP2393661B1 - Process for producing anilox rollers - Google Patents

Description

The invention relates to a process for the production of anilox rollers, in particular z. For example, those that are used under the name "anilox rolls" in so-called short inking units of offset printing machines to take in conjunction with a doctor blade or a chambered doctor blade color from a paint reservoir and to promote more rollers in the direction of the printing plate. However, anilox rollers are also used in coating plants to dose the amount of paint to be applied and find in addition to the offset printing in other printing processes such. B. the flexographic printing or in the corresponding machines application. It is already known to engrave anilox rolls which in the past have been etched by means of etching processes or which have been mechanically engraved by a stylus penetrating into the soft copper skin on a steel roll body, and have recently also been engraved with laser tools. Such a method is described, for example, in the article " Universal Anilox Rolls for the Printing Press ", published in the journal" Flexo- und Tiefdruck ", issue 2, 2004 ,

Today, the anilox rollers intended for use in anilox inking units are usually provided on the surface with a hard ceramic layer for the purpose of longer service life. In the laser engraving of such rolls, however, problems occur. Often, after a relatively short service life, streaks appear in the print image due to the fact that ceramic particles detach from the anilox roller, jam against the blade of the doctor blade and then generate grooves on the roller surface. The reason for this lies in the manufacturing process of laser engraving. For there were previously used thermal YAG laser or CO ₂ laser whose laser pulses partially evaporate material from the wells produced by them, but partly also drive out in the molten state, in which case the material on both sides of the generated depression as a "mountain" precipitates on the surface.

Since now also to be engraved wells or Haschuren with rotating roller in the circumferential direction (ie in the cross-sectional profile) are driven out with a single relatively long-lasting, high-energy laser pulse at once and, as far as it is elongated wells or Haschuren, this in the longitudinal direction repeatedly juxtaposed recesses were created, namely whenever the roller has rotated further by about one revolution, resulting in this production process along the Haschuren running individual scales of molten ceramic material. These also do not adhere well to each other or to the roll surface and are therefore easily broken by the doctor blade in the printing operation, which then the grooves in the roll surface arise, which can lead to the pre-formed stripes in the printed image. To solve this problem has been proposed to heat the surface of the ceramic roller after laser engraving again with a laser to just below the melting temperature, thereby closing cracks and pores and to compact the surface, as in the EP 1 967 381 A2 is described. Although this can extend the life of the anilox rollers, but it also prolongs the manufacturing process, because by this additional step of over-melting or Zusinterns the surface is once again required considerable additional time. Overall, engraving by this method takes about 10 to 12 hours for an anilox roll having a diameter of 180 millimeters and a length of 560 millimeters.

Furthermore, it has been proposed to solve the problem described to adjust the pulse shape of the laser used so that after an intense first laser pulse immediately followed by a second less intense pulse, then smoothed by the wells generated by the first laser pulse and the remaining therebetween ridges and cured should be. This method, known as "ultramelt" technology, is described, for example, in the journal " German printer "No. 44 from 29.11.2001 , Even with this method, the life of the laser-engraved anilox rollers can be improved, but still not drive in sufficient sizes. For example, a service life of up to 50 million revolutions is required before the anilox roller is replaced and refurbished.

It has also been proposed to subsequently grind the material melted next to the cups engraved by a CO ₂ laser with a finer diamond abrasive to the desired roughness depth in a plurality of grinding passes ( EP 0 396 114 B1 ). After each second grinding pass was The roughness depth and the draw volume of the roller are measured to ensure that the desired optimum is not "overrun" by the grinding process. This is a complex process that is difficult to reproduce and takes a lot of time. DE 10 2007 032903 discloses methods of making anilox rolls for printing presses by laser engraving.

It is therefore the object of the present invention to provide a production process for the laser engraving of anilox rolls with a ceramic surface, with which it is possible to achieve long service lives for the rolls, without thereby drastically increasing the production process itself.

This object is achieved with the measures specified in the characterizing part of claim 1.

According to the invention, the recesses in the laser engraving are produced by a repetitively operated short-pulse laser, wherein in each case a plurality of laser pulses are used to work out the depression in the cross-sectional profile.

It is therefore used here for the production of anilox rolls on known short-pulse laser. These lasers provide light pulses of very short duration in the range of a few hundred nanoseconds or less and generate very high energy densities in a small focus range, due to which the material to be removed is substantially ablated or vaporized and not or only to a very limited extent on the webs precipitates on which the doctor blade slides.

As has been found, the ablated material adheres to the ridges of an anilox roller in a manner such that these remnants can be removed by a short, inexpensive finishing operation, such as honing, lapping or polishing, or by a non-eruptive cleaning operation , Manual overpolishing or wiping off residues from the roller engraved in the manner described with the short pulse laser is already sufficient to produce a very smooth and stable surface on which the doctor blade slides.

Although the use of ultrashort laser pulses in the femtosecond range for the microstructuring of materials is known per se and z. Tie physical sheets 55 (1999, No. 6 on page 41 in the article by Nolte, Momma, Chichkov and Welling ). However, the production of anilox rollers for the inking units of printing presses is not mentioned there as an application field and the problems occurring during the production of such anilox rollers and their removal are also not described. Femtosecond lasers are also currently unable to ablate the amounts of material required for anilox rolls at a processing time of a few hours, which is acceptable for engraving. Because the energy densities and amounts of energy that are needed for it can not be concentrated in the laser pulses at currently possible pulse repetition rates. The lasers used according to the invention, preferably fiber lasers or disk lasers, deliver laser pulses in the range between 20 picoseconds and 200 nanoseconds and average laser powers in the range between 10 watts to several 100 watts. It is important that the used short-pulse laser provides sufficient energy density to optimally use the built-in light energy for ablation of the material to be processed. In addition, the incoming material on the average power must be large enough to ablate, for example, in the case of anilox rolls having a ceramics surface with a pick-up volume of typically 3 cm ^{3 is} at least 0.5 cm ³ ceramic material per hour. Because otherwise the processing time for the laser engraving of a roller would be unacceptably long or the simultaneous use of multiple lasers, the plant technical effort will be too large.

A suitable for the processing of anilox rollers short pulse laser generates z. B. laser beam pulses at a wavelength of 1065nm with a pulse duration, which is expediently below 200 nanoseconds, preferably between 100 and 150 nanoseconds. Below a pulse duration of 200 nanoseconds, it has been found that thermal contributions to material ablation generated in ceramic material are low enough and do not produce any melting of greater extent at the edges of the wells to be engraved. With a pulse peak power of 2 to 6 kilowatts, the well depths customary in anilox inking units for anilox rollers can be easily achieved, with several laser pulses being used to work out the cross-sectional profile be placed one behind the other, typically 5 to 100 laser pulses depending on the width and depth of the wells or Haschuren. With this number of pulses depressions with relatively smooth clean edges can be produced and, moreover, the cross-sectional flank shape of the wells or hafors can be influenced in such a way that, in comparison to the other methods mentioned at the beginning, one for the ink release / acceptance behavior of the wells good geometry yields. Optionally, the anilox roller is then engraved in several passes, so that the cross-sectional shape is worked out in several superimposed layers by the laser pulses. The pulse repetition rate of the laser is several 100,000 kHz. In this way, an anilox roller for a 35/50 cm printing unit can be engraved within a few hours at speeds between 100 and 1,000 rpm. It is useful if the laser engraving is followed by a finishing step and / or a cleaning step, for. B. in the same clamping of the roller as in the previous laser engraving itself, but the finishing step or cleaning step can also be done manually, for example, by polishing or wiping the lasered rollers z. B. with a cleaning agent, either still in the set or after it was taken out of the clamping. Also, cleaning the roll in an ultrasonic bath may be sufficient, and in some cases, such a finishing step may be completely eliminated.

Further advantages of the invention will become apparent from the following description of an embodiment.

Example 1:

1. a) Provided was provided with a bearing pin body made of steel ST52 with a cylinder length L of 560 mm and a diameter D of 180 mm of the cylinder jacket. This turned into shape roll body was then provided in a plasma spraying on the cylinder jacket with a chromium oxide layer with a thickness of 0.275 mm.
2. b) Subsequently, the cylinder surface was cylindrically shaped with the aid of a ceramic-bonded diamond grinding wheel with a tolerance of 0.01 mm ground. Finally, the cylinder surface was subjected to three overflows in a honing machine at 200 rpm. and a feed of 200 mm / min. Smoothed with a contact pressure of 2.5 bar. Here, a honing stone with a grain size of 2000 was used. The thus treated cylinder surface then had a roughness of R _Z = 1.9 microns.
3. c) Thereafter, the thus treated roll body was placed in a laser processing plant. This system contains a repetitively operated Ytterbium fiber laser with a pulse duration of 130 ns at a pulse repetition rate of 500 kHz. The average power of the laser was 150 W, it delivered a peak pulse power of 2.5 kW. He was set to these values in order to engrave on the cylinder surface hafors with a mean depth of 20 microns and a mean width of 90 microns with a web width of 20 microns approximately over a length of 520 millimeters.
   The hafs are said to extend over the cylinder surface at a line of 90 L / cm in a variety of steep spirals. However, since the engraving process takes place with rapid rotation of the workpiece in the circumferential direction, the pulsing of the laser was very synchronized with the speed of the roller. The speed was 150 W to 750 U / min, given the maximum pulse repetition frequency of 500 kHz and the average power of the laser. adjusted to achieve the required width and depth of the Haschuren. In each case, 6 pulses were produced across the cross section of the hash, with which the cross-sectional shape was machined out of the material, before the pulses in the region of the web between the hashes were briefly interrupted, ie 2 laser pulses were suppressed there.
   The feed of the laser unit along the cylinder axis was 5 microns per revolution of the roller, ie at the set speed of 750 rev / min. The engraving process over the cylinder length L of 560 millimeters took about 2.4 hours. During this time, about 1.5 cm ^{3 were} removed in the area of 0.3 m ² to be engraved. Subsequently, the roller was with the same settings engraved again in a second pass, to get to the desired average depth of Haschuren of 20 microns. In total, this resulted in a material removal of 3 cm ³ for a total processing time of 4.8 hours for engraving.
4. d) The thus structured anilox roll was then subjected to a second finishing step. Here, the engraved surface was in two overflows with a 4000 grain honing stone with a lower contact pressure of about 1 bar at a speed of 200 U / min. at a feed of 600 mm / min. smoothed.
   The roller thus treated could then be installed in the printing unit of a sheet-fed offset printing machine of the SM52 type and provided there as anilox anilox roller. The entire machining process from the ceramic coating to the final finishing process took 5.5 hours.

Example 2:

Here, in comparison to the manufacturing process according to Example 1, the production step b) was changed as follows: The honing was carried out with a 4000 grade honing stone in four overflows with a contact pressure of 1.5 bar. The thus treated cylinder surface then had a roughness of R _Z = 1.5 microns.

During subsequent laser engraving, with otherwise identical settings for the laser, the rotational speed of the roller was reduced to 250 rpm. reduced and the axial feed increased to 7.5 microns / revolution. Already with a simple sweeping of the roll surface, sufficiently clean hides with an average depth of about 20 μm were obtained, so that a further engraving could be dispensed with.

After the laser engraving (step c)), the structured anilox roll was not honed again but polished manually for about 10 minutes. The thus obtained anilox roll had comparable good properties to those of Example 1.

Claims (15)

1. Method for manufacturing screen rollers for inking units or varnishing units of offset printing presses or flexographic printing presses by laser engraving, wherein depressions such as cups or tri-helical engravings are formed in rollers that have a ceramic surface in a material removal process with the aid of laser radiation, characterized by the following steps of

   - creating the depressions by means of a repetitively operated short pulse fibre laser that provides a peak pulse power of 2 kW at the minimum and an average power of at least 80 watts,

   - applying between 5 and 100 laser pulses at a pulse repetition rate of between 100 and 1000 kHz to form a depression in the cross-section profile,

   - selecting or adjusting the short pulse laser in such a way that it ablates a minimum of 500 mm³ of material per hour from the roller surface.
2. Method according to claim 1,
   wherein the roller surface is subsequently smoothened and/or cleaned in a mechanical finishing step.
3. Method according to claim 2,
   wherein the finishing step and/or the cleaning step is carried out with the roller in the same chucking as for the previous laser engraving step.
4. Method according to claim 2,
   wherein the finishing step and/or the cleaning step is carried out manually.
5. Method according to claim 2,
   wherein the cleaning step is carried out in an ultrasound bath.
6. Method according to claim 1,
   wherein the laser used in the engraving process generates radiation pulses of a pulse duration in a range of less than 200 ns.
7. Method according to claim 1,
   wherein the number of laser pulses to create the cross-sectional profile is between 10 and 50.
8. Method according to any one of claims 1 to 7
   for the processing of a roller that has a metallic core and a ceramic jacket.
9. Method according to claim 8,
   wherein the ceramic layer is chromium dioxide applied to a metallic roller body, preferably in vapour deposition, sputtering, or plasma spraying process.
10. Method according to claim 9,
    wherein prior to the laser engraving of the roller, a ceramic layer is applied to the jacket of a metallic roller body and the ceramic surface is ground round.
11. Method according to claim 10,
    wherein by the grinding process, the roller receives a cylindrical shape with a tolerance < 0,01 mm.
12. Method according to claim 11,
    wherein prior to the laser engraving, the ground surface is smoothened in a first finishing step in such a way that the average roughness of the surface is R_Z < 2 µm.
13. Method according to any one of claims 1 to 11,
    wherein prior to the laser engraving step, the rollers that have a ceramic surface are ground into shape and are subsequently engraved without a further finishing step.
14. Method according to any one of claims 1 to 13,
    wherein in the engraving process, the laser radiation passes over the roller surface multiple times to create the shape or depth of the cross-sectional profile.
15. Method according to claim 2,
    wherein the finishing step is one or several of the following processes: honing, lapping, polishing.