In laser transmission welding of thermoplastics the optical properties of the joining parts determine the quality of the welding result. Especially, the scattering of laser radiation in the transparent welding part has an impact on weld seam properties. This scattering is caused by additives. For polycarbonate (PC) with different additives the transmittance, the reflectance and the collimated transmittance are measured with a UV-VIS-NIR spectrometer. From this data, the optical properties, such as scattering coefficient, absorption coefficient, and anisotropy factor are calculated. The calculations are made with the aid of the four-flux model of radiation transport in the diffusive approximation. The results show that the additives have a significant influence on the scattering coefficient. For most additives under consideration the scattering is forward directed, which means that most of the radiation is transmitted into the absorbing welding part. However, the power density distribution of the transmitted radiation may differ significantly from PC without additives. So, the weld seam may also differ due to different additives
The increasing use of plastics as a construction material has given rise to the problem of how to join dissimilar materials. For the production of hybrid plastic-metal components, several processes are possible. In a new joining method LIFTECA (R), recently developed at the Fraunhofer Institute for Laser Technology, a part or a defined area of a part is heated by laser radiation through the plastic component with which it is to be joined. The part is held in contact with the plastic component under mechanical pressure and heated; then, further mechanical pressure is applied to press it into the plastic material. A stable positive bond is achieved during cooling, provided that a suitable component geometry has been selected. An important aspect of the process is that the part to be heated should have a higher melting point than the plastic component with which it is to be joined. The more heat resistant of the two parts/components can be metal, ceramic, or a heat-resistant plastic. The high energy density of the laser beam permits the heating stage to be accomplished very rapidly. Another approach is pursued within the Cluster of Excellence "Integrative Production Technology for High-Wage Countries" of the RWTH Aachen University. Multiple irradiation strategies in line with the choice of the material, the beam source, and the pretreatment of the samples are investigated. In a first experimental series, the influence of the surface structure on the joining process is examined. A structure in dot, line, and cross pattern is implemented on the surface of stainless-steel samples with Nd:YAG laser radiation. Afterwards, these samples are joined with transparent plastic samples. For the laser beam transmission joining process, contour or quasi-simultaneous irradiation strategies are considered while using diode laser power. The results show promising achievements, with good strength being obtained
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