This article describes the correlation between crystallinity degree and spatial seam homogeneity for laser transmission welding of polymers applying TWIST method, which is based on a superposition of slow regular feed and fast periodic circular or elliptical beam motion. Laser welding experiments and measurements of optical polymer properties (transmittance, reflectance and collimated transmittance) are reported for thermoplastic polypropylene PP and Polyamide PA66 as well as for thermoplastic elastomer PEBAX. The shape of the seam is visualized by measuring the heat affected zone (HAZ). Compared to the circular the elliptical motion leads to a homogeneous HAZ, which is advantageous for the welding process in many cases. Besides TWIST parameters, scattering of the laser radiation influences the welding result considerably. This is shown for high-crystallinity PA66, which needs more laser power to achieve similar weld seams than PP or PEBAX. The correspondence between homogeneous HAZ and good welding results is investigated by measuring the strength of welded T-shape samples with respect to the laser power. The elliptic TWIST motion leads to a broader processing window, if the ratio of the beam diameter and the major TWIST radius is smaller than a certain value (< 1/2). For PEBAX, a local maximum of strength-vs.-power curve is obtained for all TWIST~R radii. For welding PP, the maximum is observed only for big radii.
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