Guido Hennig

Processing of dielectric materials and metals with PS laserpulses

Ever since industrially applicable ps laser systems have been available, cold ablation with ultra short laser pulses is of huge interest when high requirements concerning accuracy, defined surface roughness and small heat affected zone are demanded. Interesting applications are in the fields of surface and 3-d structuring with direct and induced processes. For a profitable industrial use of this technology high process efficiency is required, which is enabled by the development of systems with high average power of more than 100 W. The process efficiency directly scales with the average power when the repetition rate of the system is properly chosen. But aside from process efficiency often a high surface quality (low surface roughness, minimized surface melting and no oxidation processes) is desired. These measures are not only strongly affected by the laser parameters but also by the strategy of structuring. Especially for surface structuring the corresponding requirements for the equipment of the beam guiding system are often not accomplishable and therefore there is a strong demand for new technologies which have to be developed.Ever since industrially applicable ps laser systems have been available, cold ablation with ultra short laser pulses is of huge interest when high requirements concerning accuracy, defined surface roughness and small heat affected zone are demanded. Interesting applications are in the fields of surface and 3-d structuring with direct and induced processes. For a profitable industrial use of this technology high process efficiency is required, which is enabled by the development of systems with high average power of more than 100 W. The process efficiency directly scales with the average power when the repetition rate of the system is properly chosen. But aside from process efficiency often a high surface quality (low surface roughness, minimized surface melting and no oxidation processes) is desired. These measures are not only strongly affected by the laser parameters but also by the strategy of structuring. Especially for surface structuring the corresponding requirements for the equipment of the beam g...

Ultra-high-precision surface structuring by synchronizing a galvo scanner with an ultra-short-pulsed laser system in MOPA arrangement

For surface and 3D structuring the ultra short pulsed laser systems are mostly used in combination with galvo scanners. This work reports on the synchronization of the scanner mirror motion with the clock of the laser pulses, which is usually in the range of 100 kHz and higher, by a modification of the electronic scanner control. This synchronization facilitates the placement of the small ablation craters from single pulses with the precision of about 1 μm relative to each other. The precise control of the crater positions offers the possibility to test and optimize new structuring strategies. Results of this optimization process with respect to minimum surface roughness, steepness of wall, accuracy to shape and efficiency will be presented.

Factors controlling the incubation in the application of ps laser pulses on copper and iron surfaces

For laser micro processing with short and ultra-short pulses the threshold fluence is affected by the incubation and changes with the number of pulses applied. In general the incubation effect is described by a power function including the incubation coefficient S. Beside the threshold fluence also the energy penetration depth is subject to the incubation effect; moreover it is a main cause for the change of the threshold fluence with increasing pulse number. The behavior of the threshold fluence can be explained by varying absorption (due to changes in the surface reflectivity), chemical changes of the surface (e.g. due to oxidation) or changes in the microstructure of the material whereas the behavior of the energy penetration depth could be explained by the latter two effects but should not be affected by a change in the absorption. To try to distinguish between these three effects a systematic ablation study with 10 ps pulses at 1064nm wavelength on copper and iron under different gases atmospheres and pressures was done. The results show on the one hand the change of the energy penetration depth is the main cause of the incubation and that on the other hand an adapted model better fits the trend of the threshold fluence and the penetration depth as a function of the number of pulses applied. The influence of the gas (air, oxygen, nitrogen and argon) is only marginal whereas a reduction of the pressure from normal atmosphere down to 50 mbar results in a 25% increase of the maximum removal rate. Induced changes in the microstructure were detected by a high resolution X-ray diffraction analysis on single crystal (111-orientation) copper and iron samples.

Laser engraving in gravure industry

Digital imaging methods like Computer to Plate (CTP) solutions are progressive technologies to improve the quality and efficiency of prepress and printing workflows. In this environment the application of lasers for print form fabrication plays an increasingly important role because of the high machining rate, the high spatial resolution, the achievable high energy densities by fine focusing and the ability of digital modulation. This paper gives an overview of our laser based processes in flexography and gravure with a focus on the precise micro structuring of gravure print forms by direct laser ablation. Direct laser engraving into metallic cylinders is performed with a high power Q-switched Nd:YAG laser system at 70 kHz repetition rate tuned for high reproducibility and stability of the mean pulse energy (σ2 < 0.8%). An exact modulation technique allows fast variation and precise calibration of the energy of each single laser pulse as well as active modulation of the intensity profile of the beam. This method permits to engrave with each single laser pulse a complete cell and to define diameter and depth of each cell (the aspect ratio and the cell shape) independently and freely from pulse to pulse at a rate of 70 kHz, controlled by digital image data. Thus the cell shape can be optimized for the best ink transfer characteristic on different print substrates. Future developments in gravure industry and the requirements on fiber lasers and ultra short pulse lasers for an efficient industrial engraving process of print cylinders are discussed.

High throughput laser micro machining on a rotating cylinder with ultra short pulses at highest precision

For surface and 3D structuring ultra-short pulsed laser systems are used in combination with mechanical axes, whereas the mechanical axes can include electrical motor as well as beam deflecting systems like a galvo scanner. The motion of the axes is synchronized with the clock of the laser pulses, which is usually in the range of 100 kHz and above, by a modification of the electronic axes control. This work shows the scalability of the ablation process up to MHz-regime in relation to surface quality and ablation efficiency. Furthermore the transfer of the machining strategy from a synchronized galvo scanner to a rotating cylinder setup is shown.

High precision and high throughput surface structuring by synchronizing mechanical axes with an ultra short pulsed laser system in MOPA arrangement

For surface and 3D structuring ultra short pulsed laser systems are mostly used in combination with galvo scanners. This work reports on the synchronization of the scanner mirror motion with the clock of the laser pulses, which is usually in the range of 100 kHz up to several MH, by a modification of the electronic scanner control. This synchronization facilitates the placement of the small ablation craters from single pulses during the mirror motion with the precision of about 1 µm relative to each other. The precise control of the crater positions offers the possibility to test and optimize new structuring strategies. Results of this optimization process with respect to minimum surface roughness, steepness of wall, accuracy to shape and efficiency will be presented.For surface and 3D structuring ultra short pulsed laser systems are mostly used in combination with galvo scanners. This work reports on the synchronization of the scanner mirror motion with the clock of the laser pulses, which is usually in the range of 100 kHz up to several MH, by a modification of the electronic scanner control. This synchronization facilitates the placement of the small ablation craters from single pulses during the mirror motion with the precision of about 1 µm relative to each other. The precise control of the crater positions offers the possibility to test and optimize new structuring strategies. Results of this optimization process with respect to minimum surface roughness, steepness of wall, accuracy to shape and efficiency will be presented.

Large scale laser microstructuring of gravure print rollers

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 cm3/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 (σ2 < 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.

Surface structuring of metals and non-metals for printing tools and embossing dies with an ultrafast ps-laser machining system

With roll-to-roll processes, millions of reproductions (e.g. RFID-antennas or Fresnel-lenses) can be produced in a fast and economical way. The processing of replica tools for such printing and embossing applications requires in many cases sub-μm and μm-structures. Ultra-short pulse lasers with ps- and fs-pulse durations and in single pulse or burst mode operation are appropriate tools to generate this micro- or nanostructures. In recent years the ongoing development of these laser sources and of fast beam delivery optics allows higher ablation rates combined with a superior quality for several materials like copper and brass as well as glass and dielectrics. Different ps-laser systems at 10 ps with up to 80 W at 1064 nm with a pulse repetition rate up to 8 MHz and energies up to 50 μJ (at lower repetition rate) have been used in a micro-engraving system for large cylindrical workpieces. This setup allows the micro structuring of cylinder surfaces as well as the processing of thin film substrate sheets up to a thickness of approximately 300 μm. Dimensions up to 7 m face length and circumferences up to 1,3 m can be processed with an accuracy of about 1μm. A variety of metals have been investigated by structuring 2D and 3D elements. The process is nearly melt-free, but the resulting surface structure of the ablated zone depends on the sort of metal. The high fluencies also enable the engraving of transparent materials which allow a much faster micro processing speed compared to metals. This work shows examples of micro-structuring melamine-resin coated cylinder surfaces and hybrid materials.

Influence of particle shielding and heat accumulation effects onto the removal rate for laser micromachining with ultra-short pulses at high repetition rates

High throughput is a key aspect in laser micro machining. In order to achieve excellent processing quality with high power, high repetition rate ultrafast lasers new and fast beam deflecting systems as e.g. polygon line scanners with marking speeds up to 100 m/s and more or – alternatively – fast rotating cylindrical work pieces are useful. These devices facilitate to work at high repetition rates in the MHz-regime and it has to be clarified if particle shielding and heat accumulation effects may become evident in this regime of repetition rates.Experiments up to 43 W of average power at 6.8 MHz repetition rate and a pulse to pulse distance of one half of a spot radius have been performed on five different steel grades and on copper. Hints for a particle shielding effect were only found for steel 1.4301, but the effect is weak and, if it really exists, only in the range of 10%. For all other steel grades and copper no shielding or heat accumulation effects were observed up the maximum power of 43 W of the laser system.Experiments with a galvo scanner and reduced pitches showed an oxidation effect which is not caused by the heat accumulation during the marking of a single line but eventually by the short time interval between marked lines which may lead to a long time heat accumulation.High throughput is a key aspect in laser micro machining. In order to achieve excellent processing quality with high power, high repetition rate ultrafast lasers new and fast beam deflecting systems as e.g. polygon line scanners with marking speeds up to 100 m/s and more or – alternatively – fast rotating cylindrical work pieces are useful. These devices facilitate to work at high repetition rates in the MHz-regime and it has to be clarified if particle shielding and heat accumulation effects may become evident in this regime of repetition rates.Experiments up to 43 W of average power at 6.8 MHz repetition rate and a pulse to pulse distance of one half of a spot radius have been performed on five different steel grades and on copper. Hints for a particle shielding effect were only found for steel 1.4301, but the effect is weak and, if it really exists, only in the range of 10%. For all other steel grades and copper no shielding or heat accumulation effects were observed up the maximum power of 43 W of the...

Laser processing in printform fabrication

Laser engraving is the fastest and most versatile process for gravure cylinder fabrication. New experiments with high power fiber lasers (cw lasers and pulsed MOPA systems) resulted in improved cell precision, screen resolution and efficiency.