This paper presents a new monitoring principle for laser transmission welding of plastics. The principle uses two
independent detectors: a CCD or CMOS camera to supply an image of the weld seam and a pyrometer to detect heat
radiation from the welding process. In laser transmission welding, the weld seam is usually hidden by the laser
transparent joining partner, which may still be colored in the visible spectrum. Therefore, a special lighting laser will be
used to acquire an image of the weld for CCD or CMOS cameras. If the upper joining partner is not only partly
transparent for the laser wavelength of the welding laser, but also for the laser wavelength of the lighting laser, a direct
visualization of the weld is possible. Factors influencing the weld quality or leading to defects within the weld are kerfs
on the surfaces of the joining partners, moisture uptake of the plastic material and the surface finish of the materials.
Which weld defects result from these factors and how they can be detected by the monitoring system are shown. Not all
of the resulting defects can be detected with a single detector. Therefore, the information from the two detectors,
camera and pyrometer, is correlated.
In dental prosthodontics, miniaturized unit assembly systems are increasingly used in order to enable the optimized adaptation
of the prosthesis to the individual anatomic situation. To manufacture these miniaturized systems, there is the
need to cover the complete device, in the reported case made of polycarbonate, comprising among others springs that
apply pressure to fix and adjust the prosthesis. The laser is a suitable joining tool for applications in micro technology.
Therefore, laser transmission welding was investigated for joining the specific miniaturized components, so-called retention
modules. The housing was made of PC with carbon black. The cover consisted of transparent PC with a thickness of
400 μm along the joining contour. The wall thickness of the joining partners amounted to 400 μm.
The investigations presented in this paper include detailed examinations of the welding process with and without laser
mask. For both process variants, the influence of the main process parameters laser output power, welding speed and
focal position was studied. The process was qualified especially with regard to joining strength, swelling, process time,
reproducibility, accuracy and functionality of the complete assembly. It was examined how the positioning of the mask
determines the formation of the weld seam. The geometry of the retention module and the clamping were optimized. It
turned out that the clamping of the components is crucial for a reliable process. Optimized process conditions enable the
micro welding of plastic components for dental products considering the high requirements regarding functionality,
biocompatibility, lifetime, and esthetics. Laser transmission micro welding proved to be a suitable method to package the
final assembly without any refinishing operation.
Due to their precise focusing properties and the possibility of an excellent non-tactile energy coupling, lasers represent a suitable machining process for miniaturised components of shape memory alloys (SMAs) in micro-system and biomedical technology. SMAs find increasing application in these fields because of their special properties such as super-elasticity and shape memory effect.
Current research is concentrated on the development of suitable and economic process strategies for machining micro-components and -actuators using ultra-short-pulse lasers. The application of this laser type enables the realisation of miniaturised components made of SMAs without damaging the grain structure and therefore maintaining the shape memory effect. The processed miniaturised components are characterised with respect to the achieved geometrical resolution, the surface quality of the processed areas and their mechanical functionality. For real production, a reduction of the processing time is necessary. Therefore, different process strategies consisting in different numbers of process cycles (simultaneous material removal and smoothing, material removal and subsequent smoothing) are investigated machining NiTi-components. In this paper, results of the laser based method regarding processing time and surface quality will be presented. In addition, the application of these strategies for the realisation of exemplary actuator or component geometries (e.g. for minimal invasive surgery) will be demonstrated.
Due to special material properties, among others their two-way memory effect (TWME), shape memory alloys (SMA) are finding increasing attention in micro system technology. However, only slow and quite complicated training methods are available to induce the TWME. At the Laser Zentrum Hannover e.V., investigations are being carried out to realize the induction of the TWME into SMA components using laser radiation. By precisely heating SMA components with laser radiation, local tensions remain near the component surface. Concerning the shape memory effect (SME), these tensions can be used to make the components execute complicated movements. Compared to conventional training methods to induce the TWME, this procedure is faster and easier. Further, higher numbers of thermal cycling are expected because of the low dislocation density in the main part of the component. Results regarding the dependence of the laser induced TWME on material and machining parameters will be presented.
In the course of increasing miniaturization of components in minimal access surgery the superelastic properties of nickel titanium shape memory alloys (NiTi-SMA) find more and more attention. However, only a few processes are available for machining of miniaturized NiTi-components. Changes of the mechanical properties due to heat input or mechanical tensions have to be avoided. Especially for complex geometries with dimensions in the submillimeter-range, these requirements are hardly to meet. Finding new methods for manufacturing micro-instruments of NiTi wires with high geometrical resolution and superelastic mechanical properties for applications in endoscopic surgery are the main goals of the investigations presented here. Because of the precise focussing properties and the possibility of an excellent non-tactile energy coupling, material processing by lasers is a suitable alternative to conventional machining-processes. Comparing investigations with laser systems with different wavelengths and pulse duration show the suitability of ultra-short pulse Ti:Sapphire lasers for this machining process. Only by the use of ultra-short laser pulses it is possible to structure micro-components of NiTi almost without thermal influence. Results of mechanical and metallographic examinations show that the special properties of miniaturized SMA-components can be maintained. Experimental results as well as example geometries produced with ultra-short pulse lasers are presented in the paper.