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Laser transmission welding is a non-contact and efficient process technology for joining thermoplastic polymers. In the conventional process, laser sources in the wavelength range of 1 μm are usually used. Therefore, most of the laser radiation is transmitted through the upper joining partner and absorbed only in the lower joining partner. As a result, the possibilities to influence the temperature field especially in the upper joining partner are limited. To overcome these limitations, an additional thulium fiber-laser with a wavelength of 1.94 μm is used in this study and coaxially aligned with a diode laser. The use of an additional thulium fiber-laser leads to a significant absorption in the upper joining partner. Through this approach, it is shown that the temperature field and the weld seam geometry can be influenced by using these two different discrete laser wavelengths. Depending on the intensity distribution of both lasers, an increase of the size of the heat affected zone in the upper joining partner can be observed. In order to develop a better process understanding, a thermal finite element model is built up and verified by comparing the calculated size of the heat affected zone for different process parameters with the experimental data. The model is able to represent the influence of both laser sources on the temperature field and is used to calculate characteristics of the temperature field, such as maximum temperatures or cooling rates. The characteristics are then used to explain the weld seam morphology, such as occurrence and size of spherulitic structures in the weld seam.
Laser transmission welding is a well-known joining technology for welding thermoplastics. Although the process is already used industrially, fundamental process-structure-property relationships are not fully understood and are therefore the subject of current research. One aspect of these mentioned process-structure-property relationships is the interaction between the temperature field during the welding process, the weld seam morphology of semi-crystalline thermoplastics, and the weld seam strength. In this study, the influence of the line energy on the weld seam morphology of polypropylenes is analyzed. For this purpose, the size of spherulites in the weld seam is investigated, as well as different occurring phases of polypropylene (a- and b-phase). It is shown that both the spherulite size of the a-phase and the amount of b-phase increase with increasing line energy. For the explanation and discussion of the results, a temperature-dependent thermal simulation model is used to derive characteristic attributes of the temperature field (maximum temperatures, cooling rates, temperature gradients).
In this paper, the use of electrophotographic polymer powder transfer for the preparation of multi-material layers is discussed with respect to the application in powder bed-based additive manufacturing technologies as selective laser sintering (SLS). Therefore, the challenges of this task are considered verifying the critical process steps in order to develop a concept for an electrophotograhic laser sintering machine. On that basis, an experimental setup with a two-chamber design is realized which enables the investigation of the electrophotographic powder transfer at typical process conditions of SLS. Using this setup, transfer tests of polypropylene powder patterns were performed and qualitatively analyzed by photographic imaging. The results confirm the high potential of the application of electrophotography for multi-material powder deposition and show how a residual electrophotographic powder deposition can be achieved in general, which is independent from the already produced part height, in order to build up three-dimensional multi-material components.