Grenzen der Lochdichte beim Perkussionsbohren mit Ultrakurzpulslasern
Hambach, Nelli; Poprawe, Reinhart (Thesis advisor); Ostendorf, Andreas (Thesis advisor)
Dissertation / PhD Thesis
Many environmental protection tasks, but also in food and cosmetics industry, medical technology and bioanalytics, place high demands on microsieves. Especially in health and environmental protection, fine dust (particles < 60μm in the air) and micro-plastic particles (fibers and particles both in the air and in water < 500μm) constitute a major hazard. Tighter legal requirements have favored the development of fine dust retaining filter systems in recent years, but the need for development is still enormous in the area of temperature resistant, cleanable, fine porous (pore size < 10μm) and highly transparent (open area ratio > 25%) microsieves. In the field of micro-plastics in water, the legal requirements are not yet so strict that industry is forced to take action. However, this will change in the coming years, as some countries have already banned micro-plastics in cosmetic products. Other countries like Germany will follow suit. Nevertheless, the further development of small-pored (d < 10μm) microsieves with large open area ratios is of considerable importance. In the field of fine dust separation, various fields of application are conceivable, e.g. exhaust gases from diesel engines, laser printers, wood firing etc. For the production of micro sieves there is an unprecedented possibility to insert small and high-precision holes into metallic foils and to place them tightly together. These stable, thin metallic surface filters are advantageous for micro-filtration, as the pressure dropping across the individual pores of the filter is minimized and a high flow rate is enabled. Such surface filters cannot be produced today. Ultra-short pulsed laser sources represent a new approach to solving this problem. With short-wavelength laser light, hole diameters of less than 10μm can be achieved. The use of ultra-short pulsed laser radiation reduces thermal influences and thus enables the open area ratio to be maximized. The subject of this thesis is thus a systematic investigation of the physical effects in the production of highly transparent filter foils, so that a hole size of < 10μm with simultaneous increase of the open area ratio to over 20% is to be obtained. To achieve this goal, the work is divided into four parts. In the first step, a theoretical investigation of the temperature evolution in the material is made. A mathematical model should provide a quick estimation of the feasibility of a hole grid in a material with a certain diameter and hole center distance (pitch). Four materials are selected which differ significantly in their thermal properties so that an assessment of the influence of these material properties on the stability of the drilling process can be made. In the second step, the drilling process is analyzed depending on the repetition rate, the number of pulses and the pulse energy using individual holes. Afterwards, hole grids are characterized depending on the repetition rate and the pitch in the different materials. To investigate the thermal effects more closely, grids are drilled with a waiting time between the holes. Results of both studies are compared with the model to identify possible effects and influences.