In this paper the results of micromachining of polycarbonate polymer foils and SUS304 stainless steel thin sheets are reported, performed by an industrial femtosecond laser operated at 1030 nm and 515 nm (SHG) wavelength. For a typical Galvoscanner setup the ablation scribing was investigated at a spotsize of 20 μm which is typical for contemporary feature size used for medical stents. At this value a maximum peak intensity of 6.4*1013 W/cm² can be reached, which enables significant nonlinear absorption in the polymer.
Laser pulse overlap was varied to optimize overlap values for best edge quality and for best ablation rate.
From the results some guidelines for complete cutting of thin sheets were derived. For an acceptable edge quality a maximum ablation rate of 2.6 mm³/min was demonstrated for stainless steel thin sheets, whereas up to 9,4 mm³/min have been reached for polycarbonate.
For SUS304 the use of the SHG does not increase ablation rate or edge quality, whereas for polycarbonate the cutting quality is better, but at a smaller ablation rate.
In medical device manufacturing there is an increasing interest to enhance machining of biocompatible materials on a
micrometer scale. Obviously there is a trend to generate smaller device structures like cavities, slits or total size of the
device to address new applications. Another trend points to surface modification, which allows controlling selective
growth of defined biological cell types on medical implants.
In both cases it is interesting to establish machining methods with minimized thermal impact, because biocompatible
materials often show degradation of mechanical properties under thermal treatment. Typical examples for this effect is
embrittlement of stainless steel at the edge of a cutting slit, which is caused by oxidation and phase change. Also for
Nitinol (NiTi alloy) which is used as another stent material reduction of shape-memory behavior is known if cutting
temperature is too high. For newest biodegradable materials like Polylactic acid (PLA) based polymers, lowest thermal
impact is required due to PLA softening point (65°C) and melting temperature (~170 °C ).
Laser machining with ultra-short pulse lasers is a solution for this problem. In our work we demonstrate a clean laser cut
of NiTi and PLA based polymers with a high repetition-rate 1030 nm, 400-800 fs laser source at a pulse energy of up to
50 μJ and laser repetition rate of up to 500 kHz.
Continuous carbon fibre reinforced plastics (CFRP) are recognized as having a significant lightweight construction potential for a wide variety of industrial applications. However, a today‘s barrier for a comprehensive dissemination of CFRP structures is the lack of economic, quick and reliable manufacture processes, e.g. the cutting and drilling steps. In this paper, the capability of using pulsed disk lasers in CFRP machining is discussed. In CFRP processing with NIR lasers, carbon fibers show excellent optical absorption and heat dissipation, contrary to the plastics matrix. Therefore heat dissipation away from the laser focus into the material is driven by heat conduction of the fibres. The matrix is heated indirectly by heat transfer from the fibres. To cut CFRP, it is required to reach the melting temperature for thermoplastic matrix materials or the disintegration temperature for thermoset systems as well as the sublimation temperature of the reinforcing fibers simultaneously. One solution for this problem is to use short pulse nanosecond lasers. We have investigated CFRP cutting and drilling with such a laser (max. 7 mJ @ 10 kHz, 30 ns). This laser offers the opportunity of wide range parameter tuning for systematic process optimization. By applying drilling and cutting operations based on galvanometer scanning techniques in multi-cycle mode, excellent surface and edge characteristics in terms of delamination-free and intact fiber-matrix interface were achieved. The results indicate that nanosecond disk laser machining could consequently be a suitable tool for the automotive and aircraft industry for cutting and drilling steps.
Diode laser systems have been established for material processing and pumping solid state lasers in the recent
years, due to flexibility, efficiency and lifetime. In the meantime, diode laser bars with an output power of more
than 120 W and a beam parameter product less than 70 mm mrad are available (see fig. 1). Depending on the
optical system an energy density in focus of more then 106 Wcm-2 can be achieved. But for several applications
like hardening metal surfaces or welding thin blanks/plates the output power is insufficient. To increase optical
output power several diode laser bars are arranged vertically and/or horizontally. With these so called stacks
an optical output power of more than 4 kW can be achieved. Due to the incoherent beam coupling the beam
parameter product is increased at the same rate. But the energy density or intensity in focus is rather less than
constant. Other applications, e. g. welding or marking, require higher intensities, which can not be achieved
with diode lasers. For these applications diode pumped solid state laser are mostly applied.
The main challenge to address single emitters in a high-power diode-laser-bar is the thermal and electrical management to avoid crosstalking. Especially p-side up assembly leads to increasing thermal influence of neighbouring emitters due to the low thermal conductivity of GaAs. Electro-magnetic fields inside and outside the laser-bar, for example caused by high frequency modulation (10 MHz) at a high current (up to 1 A), induce voltages into neighbouring electric circuits, hence the output power of neighbouring emitters can be affected.
High beam quality can be achieved by accurate adjustment of the mechanical and micro-optical components in the manufacturing process of high power diode laser stacks. A charaterization device which can determine these parameters by automatically measuring the radiation properties of high-power diode-laser stacks has been developed. The result is a mechanically robust, easy to use characterization device of high reliability suited for applications in quality control and product optimization.
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