- Laser transmission welding of polymers is a widely used joining technique for automotive components, consumer products, medical applications and others. The process is strongly dependent on the transmission properties of one of the joining partners to ensure a suitable laser intensity distribution in the joining zone between the two components. Scattering at crystalline structures, additives or reinforcements in composites as well as the absorptivity of the matrix can compromise the transmitted beam profile [1, 2, 3]. Successful welding of these materials is therefore often limited to thin parts.
This work describes methods for measuring the spectral absorption, transmission and scattering properties to assess the weldability of scattering materials. Since many of these materials show anisotropic properties due to the manufacturing processes and the fiber orientation, the transmission properties are influenced by the polarization state of the laser beam.
An experimental setup shown in Fig. 1, using a commercially availableLaser transmission welding of polymers is a widely used joining technique for automotive components, consumer products, medical applications and others. The process is strongly dependent on the transmission properties of one of the joining partners to ensure a suitable laser intensity distribution in the joining zone between the two components. Scattering at crystalline structures, additives or reinforcements in composites as well as the absorptivity of the matrix can compromise the transmitted beam profile [1, 2, 3]. Successful welding of these materials is therefore often limited to thin parts.
This work describes methods for measuring the spectral absorption, transmission and scattering properties to assess the weldability of scattering materials. Since many of these materials show anisotropic properties due to the manufacturing processes and the fiber orientation, the transmission properties are influenced by the polarization state of the laser beam.
An experimental setup shown in Fig. 1, using a commercially available Spectrophotometer with integrating sphere, is used to measure direct transmission, forward and backward directed scattering and the reflection of linearly polarized light after interaction with amorphous and semicrystalline polymers as well as particle and fiber reinforced thermoplastic composites. The spectral measurements reveal that compared to commonly used lasers emitting at about 1.0 μm the application of laser wavelengths in the range of 1.3 to 2.0 μm can lead to increased transmission and reduced scattering losses within the materials. Variation of the polarization adds an additional degree of freedom to influence the transmission properties, especially in highly anisotropic materials or composites. For validation of the spectrometric results welding experiments using a conventional diode laser beam source and a unpolarised and a linearly polarized 2.0-μm single-mode Tm-fiber [4] were carried out. Additionally the resulting laser beam profiles after transmission through different amorphous and semicrystalline polymeric materials were measured using a scanning slit beam profiler.
Using a suitable laser wavelength and adjusted polarization successful welding was achieved for highly scattering materials that could not be welded using a conventional diode laser beam source.
[1] X. F. Xu, A. Parkinson, P. J. Bates and G. Zak, Effect of part thickness, glass fiber and crystallinity on light scattering during laser transmission welding of thermoplastics, Optics & Laser Technology 75, pp. 123-131, 2015.
[2] V. Mamuschkin, A. Roesner, M. Aden, Laser Transmission Welding of White Thermoplastics with Adapted Wavelengths, Physics Procedia, 41, p. 172-179, (2013).
[3] T. Frick and A. Schkutow, Laser transmission welding of polymers–Irradiation strategies for strongly scattering materials. Procedia CIRP, 74, p. 538-543, (2018).
[4] K. Scholle, M. Schäfer, S. Lamrini, M. Wysmolek, M. Steinke, J. Neumann, P. Fuhrberg, All-fiber linearly polarized high power 2-μm single mode Tm-fiber laser for plastic processing and Ho-laser pumping applications, Proc. SPIE 10512, Fiber Lasers XV: Technology and Systems, 105120O, (2018).…

