Large scale laser microstructuring of gravure print rollers

Transcription

1 Invited Paper Large scale laser microstructuring of gravure print rollers Guido Hennig a, Karl-Heinz Selbmann a, Silke Pfinninger a, Johannes Brendel a, Stephan Brüning b a MDC Max Daetwyler AG, Flugplatz, CH Bleienbach, Switzerland b Schepers GmbH, Karl-Benzstr. 7, D Vreden, Germany ABSTRACT Application of lasers for print form fabrication plays an increasingly important role in the printing industry due to the high machining rate, the high spatial resolution and the ability of digital modulation. This paper gives an overview of our laser based processes in gravure and embossing with a focus on micro-structuring of gravure print forms by direct laser ablation. The precise large scale micro-fabrication by laser engraving is the fastest and most versatile process for gravure cylinder fabrication (ablation rate up to 1 cm 3 /min). Direct laser engraving into metallic cylinders is performed with high power Q-switched Nd:YAG laser systems and fiber lasers at up to 100 khz repetition rate, tuned for high reproducibility and stability of the mean pulse energy (σ 2 < 0.6%). Flexible aspect ratios and designs of the cell profile are achieved by fast modulation of the laser beam profile for each single pulse. This allows for optimization of the cell shape to get the best ink transfer interaction on a specific print substrate. New experiments with high power fiber lasers (cw lasers and pulsed MOPA systems > 500W@ 100kHz) resulted in improved cell precision, screen resolution and production efficiency. Future large scale cylinder engraving with ultra short pulse lasers (ps) is discussed. Keywords: large scale micro-fabrication, ablation, high power Q-switched Nd:YAG laser, fiber laser, beam profile modulation, gravure printing, ink transfer characteristics, process efficiency 1. INTRODUCTION TO GRAVURE PRINTFORM MANUFACTURING The traditional ways of print form fabrication using analog copying methods based on films have been replaced effectively by modern digital imaging processes. Computer to Plate (CTP) or even direct Computer to Press methods allow for improvements in quality and efficiency of prepress and printing workflows. The application of lasers for print form fabrication gives excellent tools, to push these developments because of the high spatial resolution, the ability of fine focusing, the achievable high energy densities and the digital modulation. The traditional imaging processes of gravure cylinders and the micro-fabrication of embossing tools and Anilox rollers are currently being replaced by laser micro structuring. 1.1 Imaging of gravure cylinders A gravure print form consists of a steel cylinder electroplated on the surface with a homogeneous and even layer of copper or zinc. The image information is represented pixel by pixel by small cells which are engraved into the copper or zinc surface and are to transfer the ink in the printing process. Most gravure print forms are imaged by one of the following procedures: - chemical etching after film based analog exposure and developing of a protection mask layer - chemical etching after digital laser structuring (imaging) of a mask resist - digital electromechanical engraving with diamonds - digital imaging by laser induced direct ablation of metal After imaging the surface is additionally plated with chromium to ensure a long lifetime under strongly abrasive conditions in the printing press 1. Fig.1 shows the principle of the gravure printing process: patterns of microscopically small cell volumes, engraved into the surface of a printing cylinder and representing grey values of the pixels of an image, take off the ink and transfer it to the substrate. By using a doctor blade only the amount of ink defined exactly by the cell size will be transferred. Photon Processing in Microelectronics and Photonics VII, edited by Andrew S. Holmes, Michel Meunier, Craig B. Arnold, Hiroyuki Niino, David B. Geohegan, Frank Träger, Jan J. Dubowski, Proc. of SPIE Vol. 6879, 68790O, (2008) X/08/$18 doi: / SPIE Digital Library -- Subscriber Archive Copy Proc. of SPIE Vol O-1

2 Substrate Doctor blade Gravure Cylinder Ink Fig.1. a) Gravure printing b) Gravure cylinder after direct laser engraving c ) Cell structure on the surface Chemical etching Traditionally the gravure print forms were imaged with an analog process based on film masters. The image is copied by direct contact exposing from the film to a photosensitive mask layer on the copper surface of the gravure cylinder. After exposure, developing steps and chemical etching are performed to produce the cells in the copper. Two kinds of cell types must be distinguished: conventional (depth variable) etching in which the cells vary only in depth to represent different steps of the grey scale, whereas all diameters are constant. The processing is very complex and includes multiple chemical steps to produce the printing form. In autotypical (area variable) etching only the diameter of the cells varies from the highlights to the shadows of the greyscale and all cells are equal in depth. This processing is easy to control by the exposed areas of the mask and by the time of etching. The definitions mentioned before are also valid to distinguish different types of laser engraved cells. The chemical etching method for gravure print form fabrication can also be applied after digital structuring (imaging) of a mask resist by laser ablation Electromechanical gravure The electromechanical (EM) engraving method allows the realisation of digital imaging in gravure. An oscillating diamond stylus cuts the cells directly into the Copper or zinc surface of the slowly revolving gravure cylinder. The diamond draws a helical or circular line of cells as the engraving head moves continuously or by stepping along the axis of the cylinder. At common screens each oscillation generates one cell. The volume of the cell depends on the amplitude of the diamonds oscillation and is controlled by digital image data. The maximum frequency of the oscillation is about 8-10 khz restricted by mechanical resonance of the diamonds holder and thermal heat dissipation capacities of the electromagnetic drive. According to the tonal values, the diamond stylus cuts into the Cu surface producing cells with variable diameter and variable depth, but always with the fixed aspect ratio defined by the diamonds geometry (Fig. 2). This is described as halfautotypical cell type..4 _ % 50 % 5 % Fig. 2. Electromechanical gravure head, diamond holder and cells for shadows, mid-tones and highlights Proc. of SPIE Vol O-2

3 1.1.3 Direct ablation of gravure cylinders In gravure the print form can also be engraved by direct laser ablation of the metallic surface material 2,3,4 (Fig. 4). Due to the large dimensions of the print form and the fine image resolution, especially for packaging applications, high pulse repetition rates and high average power of the laser are required to manage the imaging process in an economically acceptable time (i.e. 1 m 2 / 11 min). The dimensions of gravure cylinders cover a range of 0.3 m to 4.4 m in length with a circumference of 0.3 up to 2.2 m and a surface area of up to 10 m 2. With screen resolutions (image resolutions) of 60 lines/cm to 100 l/cm for publication products and 100 l/cm up to 400 l/cm for packaging applications, the typical number of cells on a cylinder is within the range of 10 8 up to On the Direct Laser Engraving System (DLS) of MDC Max Daetwyler Corp. the optical and mechanical arrangements are adapted to these requirements. 2. INTRODUCTION TO THE MDC DIRECT LASER SYSTEM (DLS) FOR GRAVURE The direct laser gravure is free of the instabilities of chemical etching processes or the restrictions imposed by the mechanical engraving process. The keywords for acceptance of this new print form fabrication process on the market are precision, repeatability, productivity and efficiency as well as flexibility and economy. The MDC-DLS approach satisfies these requirements. In gravure printform fabrication the large dimensions of heavy workpieces meet the micro precision of laser treatment representing an industrial large scale laser microfabrication application. 2.1 Optical device and the process of direct engraving Fig. 3. shows the basic scheme of the optical arrangement. Two Q-switched Nd:YAG multimode laser beams, each with 400 W at 1064 nm and at a repetition rate of 35 khz, are externally modulated by an acousto-optic modulator device (AOM) that controls the energy of each laser pulse directly by the digital image data. The maximum first order efficiency is 75 % and the beams are combined to a 70 khz, 600 W laser beam. After transmission through a fiber optic system with 92 % transmission efficiency more than 500 W mean power can be focused on the cylinder surface. The lasermaterial interaction at the cylinder surface is a thermo-optical ablation by the laser pulse (Fig. 4). To achieve precisely defined cells and to reduce the heat interaction (heat conduction and convection of melted material) to a minimum, the main part of the material is ablated directly by evaporation. Thereby the melting area and burrs can be reduced to a thin deposit of no more than 2-3 µm around the cell. For this purpose, a special electro-plated Zn material with organic additives is used. This has been developed by Daetwyler in order to get a lower heat conductivity than in normal Zinc structures. The combination of melt with vapour is advantageous for a high ablation efficiency, because the molten parts need less energy for heating, but are removed by the high pressure of the vaporised particles also. Every laser pulse creates an entire cell by local evaporation of the Zn layer. The shape of the cell is defined by the intensity profile of thelaser beam. Halfautotypical cells can be generated using a gaussian beam profile and conventional cells with constant diameter for each gray value using a top hat profile. The volume depends on the pulse energy. The dust of evaporated zinc and zinc oxide is removed from the processing area by a cross jet airflow assuring that consecutive cells of the Image data Laser table AOM Laser A (35 khz) Laser B (35 khz) Exhaust dust suction Q-switch Nd-YAG 70kHz 500 W Fiber 70 khz 500 W Head Cylinder LLI I Ap: 2 Machine bed Fig. 3. MDC Direct Laser System (DLS) for Gravure. Fig. 4. Direct melting and vaporisation of Zn Proc. of SPIE Vol O-3

4 100µm a) b) 100µm Fig. 5. a) Conventional cells with flat bottom ( SEM-photo) b) Halfautotypical cells with round bottom Cr Zn Cu I. S - Fig. 6. Single shot screen, 100 l/cm, 30 and masterscreen screen can be formed precisely without serious shadowing of the laser beam by the evaporated dust. A vacuum system guides the airflow into filters to retain the dust. The laser beam operates as a non-contact tool. This is a crucial advantage compared to the electromechanical gravure with the diamond stylus. Since there is no wear, uniformity of the gravure is assured over the entire width of the cylinder, providing predictability and repeatability for the printing process. Typical cell diameters for maximum color gravure cells range from 28 µm (for 400 l/cm, 30 angle) up to 150 µm for 70 l/cm, 30 angle), and are adjusted with a zoom optic. Examples of cells and screens are shown in Fig. 5. A so called Masterscreen can be synthesized by overlapping several small cells to a honeycomb-shaped compound cell (Fig. 6). This allows to combine the high resolution of the small cell with the ink holding volume of the larger synthesized cell. This combination is especially advantageous in package printing. To cover the whole surface of the cylinder the laser beam engraves one single continuous helical track at a rate of 70,000 cells/s. While the cylinder rotates with up to 20 revolutions/s, (depending on the circumference of the cylinder and the screen resolution), the processing head moves parallel to the cylinder axis with a cross feed of 15 to 150 µm per revolution (depending on the resolution and the angle of the screen). The thickness of the walls is about 4 to 6 µm between the cells at maximum tone value. This requires a beam pointing accuracy on the cylinder of less than 1 µm (Fig. 8). With a cylinder weight of up to 2.5 tons rotating at 20 Hz very high precision as well as stability of the mechanical components of the engraving machine are required. The synchronisation and the jitter of the two laser sources have also to be optimised to avoid wall thickness variations. In addition, the focus distance to the cylinder is stabilized actively with an adaptive autofocus unit that is able to learn, during engraving, the topology of the cylinder surface (the deviations from the concentricity) with an accuracy of less than 5 µm. This is necessary because the strongly focussed high power multimode beam has a short Rayleigh range of only µm, depending on the focus size. 2.2 Efficiency Due to the high repetition rate and the concept of one pulse, one cell the laser is about 9 times faster than the electromechanical gravure (i.e. for a 70 Lines/cm screen 11.6min/m 2 with laser vs min/m 2 with EM, Fig. 7). min low resolution EM 8 khz Engraving Time / m 2 Laser 70 khz high resolution screens [L/cm] 'ASS. Fig. 7. Engraving speed and screen resolutions Fig. 8. Homogeneous screens require beam pointing stability Proc. of SPIE Vol O-4

5 In EM extremely long engraving times would be needed for high resolution screens with small cell diameters. In addition, depth and ink volume of the cells are limited for small cell sizes due to the fixed shape of the diamond stylus. However, the variable design of laser engraved cells allows high screen resolutions with big ink volumes which can be engraved within an acceptable time of a few hours. 3. ACTIVE MODULATION OF THE BEAM PROFILE Lots of different substrate materials are on the printing market, each with individual surface properties. The way to optimise the ink transfer depends on the surface structure of the substrate (roughness, ink absorptive capacity), the ink parameters like viscosity and size of the pigments and the printform. The optimisation can be matched for each individual case only using different shapes of the gravure cells. An important parameter for this is the aspect ratio of the cells. 3.1 Aspect ratio and ink transfer The aspect ratio (AR) defined as depth per diameter of the cell is a crucial parameter for the interaction processes between ink, cell and substrate. It determinates the filling and the release of the ink: In case of a too shallow cell (aspect ratio AR < 0.1 for low quality substrates or AR < 0.05 for high grade paper or foils) problems like ink drying, missing dots, doughnut shaped printout and unstable ink transfer occur (Fig. 9). If the cell is too deep (aspect ratio > 0.5), no ink at all is filled into the cell, or capillary forces inhibit the ink release. Fine surfaces like foils are more tolerant, raw paper grades more critical. AR < 0.05 AR > 0,5 2 µm - ink dries before substrate contact - unstable ink transfer - missing dots - doughnut print - no ink transfer because of capillary forces Fig. 9. Aspect ratio and ink transfer To control the aspect ratio freely, an independent control of depth and diameter of the cell is necessary, that means, in terms of the laser beam an independent control of the power and the intensity profile is required. 3.2 Intensity profile modulation C- Apart from heat conduction and convection processes the cell represents an exact copy of the intensity profile of the laser pulse at the focal point. (Looking at this, our machine can be used as an analyser of the profile with a shot to shot time resolution of 70 khz.) To get a specifically defined shape of each cell, the intensity profile of the beam is formed 2x 35 khz Laser Power modulation Image Data Fiber Optics Intensity profile modulation Fig. 10. Independent modulation of power and intensity profile Cylinder Proc. of SPIE Vol O-5

6 and controlled actively and in realtime. The general scheme of this control is shown in Fig. 10, for details see reference 5. Cell shape, diameter and depth can be assigned independently for each single cell by separate modulation of the focus diameter and the energy of each laser pulse. This new cell type in printform fabrication is named superhalfautotypical cell (SHC) as it is an extension of the halfautotypical cell properties (their depth and diameter are variable, but not independently controllable). Thanks to the SHC cell shape modulation, it is possible to generate completely new cell configurations to optimise the ink transfer characteristic and the printability for each individual %-tone value and for each print substrate. Examples for different designed shapes of engraved cells at different tone values are shown in Fig. 11 and Fig. 12. The profiles are measured with a white light interference microscope. These examples show how an intensity profile can be designed ranging from a gaussian to a top hat profile, also allowing for various bottom structures. The cell shapes are well defined and repeatable as it is shown by the measurement for neighboured, consecutive cells. The SHC modulation permits to engrave all kinds of cell types (conventional, autotypical, halfautotypical) with one laser system for which formerly different processes (electromechanical engraving, chemical etching,) were needed. depth a) :. a) b) b) I a1 c) c) aw d) d) x-profile [mm] Fig. 11. Examples of measured SHC cross section profiles a) Gaussian intensity profile for a light tone b) Top hat approach for a mid-tone c) Intensity profile with bottom ring structure(test) d) Same Ring structure with changed parameters Fig D view and top view of SHC cells, measured with white light interferometer a) - d) are the same as in Fig. 11.) Proc. of SPIE Vol O-6

7 4. NEW APPROACHES Apart from the direct one shot one cell method with SHC beam profile modulation the desired shape of gravure cells can also be designed by other techniques, e. g. by combination of multiple consecutive laser pulses which have a smaller focal spot (i.e. a diameter of 10 µm) than the required cell structure (i.e. 100 µm) and are step by step superposed. The resulting combined cell has a shape and internal structure dependent on the modulation, overlapping and feed forward scheme of the laser pulses (like an advanced masterscreen cell). New laser sources like high brightness fiber lasers (cw and pulsed) or ultra short pulse lasers allow for this advanced approach of engraving due to their excellent beam quality (TEM00) and high peak power at the focus. Apart from Zn the high brightness allows also engraving of other materials like Cu or ceramics which are more difficult to engrave with the Nd-Yag laser system described above due to their specific material properties. 4.1 Material properties - zinc versus copper For etching and for EM gravure the traditionally used surface material of the cylinder is copper, and users are supplied with adequate plating and finishing machines for this process. However, zinc is much more efficient for laser ablation as shown in the comparison in Table 1. Table 1. Zinc versus copper Zn Cu 1064nm Solid 50 % Solid < 4 % Melting Heat [kj/mol] Melting Point [ K] Vaporisation Heat [kj/mol] Vaporisation Point [K] Heat conduction [W/m*K] Zinc has better material properties for the ablation process: higher absorption allows for pulse energy, lower heat conduction inhibits the fast transversal dissipation of the energy out of the cell area and easier entry of the the smaller required melting heat per mol and the low evaporation point enforce the process speed compared to copper. Thus, the required energy input for heating, melting and vaporizing of the material is much lower for Zn. The material gives a precise response to the laser treatment, and due to relatively low interaction with the surrounding material only small molten debris of 2 3 µm remains at the borders of the cell. This allows for an exact fine control of tone values even at high ablation rates and enables highly efficient engraving. For copper the energy loss by heat diffusion is about 3-4 times more than for Zn resulting in large and strong melt residuals. The unpredictability of the shape of those deposits leads to inhomogeneous surface structures, which requires more efforts at the following cleaning or polishing process. A problem of copper is the strong change of reflectivity related to the transition from solid to fluid phase. Due to this the exact fine control of grey steps especially at the regime of light tones is very difficult, because as soon as the melting process starts above the ablation threshold, the absorption of energy increases by more than a factor of 10. This can be compared to breaking through ice on the surface of a lake. 4.2 Comparison of different engraving approaches using different laser sources To engrave other metals than zinc the power density at the focus spot has to be enhanced. This can be done by finer geometrical focussing or by focussing of the laser pulses along the time axis, apart from increasing the overall power itself, which would require more expensive pump diode modules. Finer geometrical focussing requires high power laser beams with TEM 00 mode beam quality as fiberlaser or disklaser can provide. This can be cw- or pulsed systems. Focussing the energy on the time scale means that ultrashort pulses (ps, fs) are applied. Fig.13 visualizes the options for new laser sources for the gravure printform fabrication in comparison to the current MDC laser engraving process and Fig 14 explains the different engraving strategies. Proc. of SPIE Vol O-7

8 Geometry TEM 00 Cw -fiberlaser disklaser MDC Multimode Laser pulsed, one shot - one cell Nd:YAG rod laser fiberlaser cell shape = beam profile power time Time power power time Ultrashort pulse laser (ps) multiple shots time Fig. 13. Alternative laser sources and engraving algorithms Screen 70 l/cm Screen 140 l/cm Screen 70 Screen Track width 140µm 70µm 10µm feed forward steps Fig. 14. a) One shot one cell concept b) Image Setter One shot one cell Engraving strategy As explained in chapter 3, the most efficient method for large scale micro engraving is the one shot - one cell method, where a pulsed laser beam generates a complete cell with each single pulse. In gravure printing the minimum usable and required cell size is about 25 µm, because smaller cells don t take up or don t release ink as well. Thus, if the intensity is sufficient for the ablation process, the focus of the laser doesn`t have to be smaller. Using zinc and following the concept of the active beam profile modulation, the most efficient way of engraving is not to minimize M 2, but to be able to adapt the beam profile to a spot size which matches precisely the desired cell diameter and cell shape for the best ink transfer and to create a complete cell with only one laser pulse. This is the high speed alternative compared to the slower concepts of creating one cell with multiple shots using ultrashort pulses or TEM 00 beams. The method is especially advantageous and efficient for large screens with big full tone cell diameters (up to 150 µm) and with low pixel resolution. Using the MDC multimode Nd:YAG system described above the drilling of cells is Proc. of SPIE Vol O-8

9 performed in Zn with micrometer accuracy at volume ablation rates up to 1 cm 3 /min and with an area rate up to 0.1 m 2 /min for a 70 lines/cm screen. For this method a pulsed, high power multimode fiber laser system (MOPA) with 500 Watt at 1062 nm has been used as well. The pulse repetition rate is tunable from 25 khz to 100 khz. Therefore, this system allows the one shot drilling of large cells (i.e. 140 µm diameter for a 70 l/cm screen), at 30 khz repetition rate at which more energy per pulse is available, whereas smaller cells (i.e. with 25 µm diameter for 400 l/cm screen, which need less pulse energy), can be engraved at higher repetition rates (100 khz). The higher repetition rate compensates at a certain rate for the higher number of cells at high resolution screens. Due to the flexibility of the repetition rate this fiber laser has - apart from the easier handling and maintenance - advantages for the production efficiency at high resolution screens compared to the Nd-YAG system whose 70 khz repetition rate cannot be varied on demand. For a given maximum power of the laser the best production efficiency is defined for each screen by the highest achievable pulse rate at which still sufficient pulse energy is provided for an ablation of the complete cell. In Fig. 15 examples of engraving results with the pulsed fiber laser are shown which are made for applications in packaging and publication printing. The round cells are advantageous, because the high symmetry allows for free (and different) choice of screen angles for each color. This enables printouts which are free of color drift and free of Moirée - patterns when the different colors (typically Cyan, Magenta, Yellow and Black) are printed on top of each other. The improved beam quality of the fiber laser allows also for the engraving of other materials than zinc like for example copper. However, still more molten residual parts remain for copper than for zinc, which has much less residuals around the cell and sharp cell walls. a) Cu b) Zn c) Zn Fig. 15. Full tone cells in Cu (a) and Zn (b) for different screens direct after engraving with pulsed fiber laser and in Zn after additional cleaning (c). Cu shows a molten area around the cell, which is not to be seen for Zn Engraving strategies and applications using high power cw - TEM 00 sources Cw fiberlasers and disklasers with fundamental mode are already commercially available in the high power regime of some kw. The ability of fine focussing of the laser beam below the diameter of the gravure cell (down to 5-15 µm) allows for (and requires) new concepts of engraving algorithms, because one single cell must be composed of multiple overlapping parts. For example, scan processes as used for image setters are applicable for this purpose instead of the one shot - one cell concept (Fig.14.a). A cw operating laser is on/off - or greyscale modulated and engraves small Proc. of SPIE Vol O-9

10 overlapping stripes which form together e.g. a rhombic cell. An advantage compared to the one shot one cell method is the higher resolution of the image (i.e l/cm for 10 µm feed forward steps and a laser spot size of µm). A disadvantage is the loss of productivity and efficiency. At the same pulse or modulation rate and the same duty cycle the image setter algorithm would be slower by a factor of about 50 times, compared to a 140 l/cm single shot screen, which has feed forward steps of 70 µm. This factor increases to 200 regarding a screen of 70 l/cm. To compensate for this, frequencies in the MHz range are required for extracavity modulation of the cw laser beam, and the duty cycle must be optimised according to the minimum required interaction time for the ablation. Moreover, multibeam engraving heads are used. The image setter scan process algorithm is applicable to numerous high resolution 2D (printing) and 3D (embossing) applications as shown below. Especially for 3D applications, multipasses are required to achieve engraving depths in the mm range. Due to the high brightness of the TEM 00 laser sources the processing of other metals than Zn is possible as well as of other materials like ceramics or EPDM. Below, some examples are discussed for which this engraving method is used for special applications. Application 1: Anilox gravure rollers for glue transfer with large hexagonal or rhombic cell sizes represent an application of high volume material transfer (Fig.16, Fig. 17). For Glue transfer large and very deep cells are required due to the required volume and the high viscosity of glue. Cells with small diameter are not applicable, because they wouldn`t take on the glue. The rhombic and hexagonal cellstructure allows for high area coverage of the glue (higher quantity than for round cells). Fig. 16. Glue transfer in carpet production Depth 400 µm Cell diameter 1 mm Fig. 17. Glue transfer to car glass foils Depth 90 µm Cell diameter 150 µm Application 2: Printed electronics is an upcoming new technology in which the required high precision of electronic components and circuits define new benchmarks for the accuracy and homogenity of the printouts. The conducting and semiconducting organic or inorganic inks are mostly paste-like and difficult to print. The precise control of the cell S/Il!! I. Fig. 18. Engraving tests of RFID using cw fiber laser and image setter technique; resolution is 1000 l/cm Proc. of SPIE Vol O-10

11 geometry and surface texture of the gravure print form is essential for homogeneous, pinhole free layers of these functional inks. Fig. 18 shows test engravings for RFID circuits and antennas based on the image setter method. The outline has a broadness of only 10 µm. A cw fiber laser with 600 W and M 2 = 1.2 was used. Application 3: The high resolution and high power of the TEM 00 laser beams allow for the engraving of large scale embossing rollers. For the microfabrication of 3D - embossing forms a 3-dimensional data rip controls and synchronizes the motions of the axes and the modulation of the laser. The material is ablated subsequently layer for layer by applying an image setter process for each layer with e.g µm feed forward resolution. The final depth of the structures is in the range of about 1000 µm. The touchless laser tool allows for free and precise micromanufacturing of each layer. The production time per layer of about m 2 /h at this high resolution is appropriate to efficient production. Fig 19 shows an embossing roller for roll to roll structuring of ingrain wallpaper. The engraved material is Zn, coated with chromium. The structure was generated by stepwise engraving of five subsequent layers, each 200 µm deep with a 200 W cw fiber laser. Between the five engraving steps the residual zinc oxid was removed by brushes. In Fig. 20 embossing tools from rigid EPDM dedicated to apply optical design functions like surface finery and shadowing into smooth materials used for automotive components are shown. 300 µm 300 µm Fig. 19. Embossing cylinder for ingrain wallpaper Fig. 20. Cubic and prismatic tools from EPDM for surface structuring of optical design functions Ultrashort pulses To be even more independent from the material properties and to reduce the melting influences ultrashort pulses (ps and sub-ps) can be applied. Because of the high peak power in this case the ablation process is dominated by vaporisation and because of the short interaction time the heat dissipation into the material is negligible and in the ideal case no molten zone or burrs occur. This allows for exact ablation results and precise cell shaping at almost all cylinder materials. The reduced heat dissipation helps to use nearly 100% of the absorbed energy for the ablation process. However, on the other hand, the complete vaporisation of the material requires more energy input than the combination of melting and vaporisation in which the molten material is blown out of the cell by the pressure of the vapor as used for our one shot - one cell process. Therefore, to get the same ablation rates with ultrashort pulse systems as with our DLS Q switched system, it will not be sufficient to provide the same sum of integrals of pulse energies, but also the gap between vaporisation and melting process has to be filled. The ablation rate for Zn with the MDC Direct laser System is of 1 cm 3 /min for a 70 l/cm screen and mm 3 /min for a 350 l/cm screen. The efficiency of alternative laser sources has to be improved up to these benchmarks. At the current state, the achievable average power of ultrashort pulse lasers (between some ten and 100 W 6, 7, 8, 9 ) is still too low compared to the Nd:YAG and fiberlaser sources described above to ensure an efficient and fast cylinder production as it is required for the industrial gravure applications. Also up to now, no fiber transportation of high power ps pulses to the engraving head is possible. This prohibits mechanical separation and acoustical isolation of the laser source from vibrations of the engraving machine. Due to a very thin ablation depth per ps - pulse (typically some nm) some hundred shots have to be placed exactly at the same position on the engraving cylinder to achieve a sufficient cell volume. However, experiments using ps lasers in the region of 20 W showed that melting can occur, if the repetition rate is enhanced and the sum of the energy impact per time and area is too high at one point. To avoid this effect, the total local energy impact must be controlled to be below the melting threshold. Therefore, the future research on ps ablation has to elaborate the best process algorithms for high throughput conditions which allow to control the total local energy input rate at one point, but to maintain the overall ablation efficiency at the same time. Proc. of SPIE Vol O-11

12 5. SUMMARY In gravure, the laser technology - combined with digital imaging methods - has improved the traditional processes of print form fabrication regarding efficiency, range of screens, precision and quality of the printout. In case of applying appropriate production algorithms many of the current laser types can be utilized. The one shot one cell - SHC process with modulated laser beam profiles is today the fastest process for gravure and can be matched to different printing conditions, e.g. to various ink or substrate materials. New engraving algorithms with high power TEM 00 laser sources enable the extension of laser ablation methods to a higher number of industrial applications like rollers for large area material transfer, high precision gravure patterns for printed electronics or for 3 dimensional embossing tools. The high brightness of new laser sources allows for the efficient treatment of other materials than Zn. Ultrashort - pulse lasers will push and improve these methods if the required laser power will be achieved and new appropriate engraving algorithms and strategies for multishot processing will be developed to avoid melting at high efficient process speed. It is a challenge for the future to optimise the ablation process with ps - ultrashort pulse lasers. REFERENCES A. Brockelt, G. A. Lausmann, M. Stichler, Ein neues Tiefdruckformherstellungsverfahren Die MDC Lasergravur in Verbindung mit einer Spezialverchromung. A new process for rotogravure form preparation MDC laser engraving in connection with special chrome plating, yearbook surface technique, 55, ISBN X, p , 1999, Direkte Lasergravur metallbeschichteter Tiefdruckzylinder ( Direct laser engraving of metal coated rotogravure cylinder ), Deutscher Drucker, 25, 1997 G. Hennig, J. Frauchiger: Die Bebilderung von Tiefdruckzylindern mit Lasertechnik, Deutscher Drucker, 12, p , 2001 G. Hennig, J. Frauchiger: Neues Gravurverfahren eröffnet neue Perspektiven im Tiefdruck, Flexo + Tiefdruck, Vol.2, p.14-17, 2001 G. Hennig, J. Frauchiger, Direct Processing of Rotogravure Cylinders using a High Power Laser with highly dynamical Beam Profile, Proceedings Lane 2001, p , 2001 J. Limpert, T. Clausnitzer, T. Schreiber, A. Liem, H. Zellmer, H. J. Fuchs, E. B. Kley, A. Tünnermann. 76 W average power femtosecond fiber CPA system Trends in Optics and Photonics, TOP 83, p. 414, 2003 H. Zellmer, M. Reich, A.Liem, T. Schreiber, J. Limpert, A. Tünnermann, Faserlaser Innovative Strahlquellen für Forschung und Industrie, Fraunhofer IOF Annual Report, 2004 U. Keller, Sättigbare Halbleiter-Absorberspiegel für ultraschnelle Festkörperlaserspiegel, Photonik 4, p , 2004 R. Paschotta et al., Picosecond pulse sources with multi-ghz repetition rates and high output power, New J. Phys. 6, 174, 2004 Proc. of SPIE Vol O-12