Laser transmission welding (LTW) is a technique for joining thermoplastics. During contour LTW, any gaps or spaces between the two parts along the weld seam may prevent a weld from forming. This work presents an experimental study on the effects of material property (carbon black level, glass fibers, and crystallinity), process parameters (laser scan power and scan speed), and weld gap thickness on the strength and microstructure of contour welds made of polycarbonate (PC), polyamide 6 (PA6), and PA6 reinforced with 30% glass fiber. Lap specimens, with weld lines parallel to the load direction during mechanical testing, were used to assess the weld shear strength. The results indicated that low concentrations of laser absorbing pigment accompanied by high laser power improve gap bridging. The study also indicated that a novel noncontact test method can be used to search for the optimized process parameters for gap bridging. The maximum gaps bridged were 0.2, 0.4, and 0.25 mm for PC, PA6, and reinforced polyamide 6, respectively.
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