Gap Bridging in Laser Transmission Welding of Thermoplastics
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Contour laser transmission welding (LTW) is a technology that has potential for joining large and complicated thermoplastic parts. Thermal expansion is the primary driving force to bridge potential gaps at the weld. A comprehensive investigation into gap bridging was performed using experimental studies, finite element (FE) thermal-mechanical coupled modeling, and analytical analysis of the contour welding process for polycarbonate (PC), polyamide 6 (PA6) , and glass fibres reinforced polyamide 6 (PA6GF). The effects of material properties (carbon black level, glass fibres and crystallinity), process parameters (laser scan power, scan speed) and weld gap thickness on weld shear strength were assessed. The experimental study indicated that low concentration of laser absorbing pigment accompanied with high power laser scan improves gap bridging. Damage on the top surface of the laser-transparent part limited the allowable laser power that could be delivered onto the weld interface. Maximum gaps of 0.2, 0.4 and 0.25 mm were bridged in the experiment for the three types of polymers respectively. The thermal behavior of polymers in contour LTW was analyzed by the 3-D quasi-static thermal FE models. Thermal expansion into the gap was simulated by the simplified 2-D transient, thermal-mechanical coupled FE models. An analytical model describing laser beam transmission and absorption in light-scattering polymers was developed and applied in the FE simulation for PA6 and PA6GF. FE simulated results agree well with the experiment in contour welding with gap of PC and PA6. The optimum material and process parameters have been searched in the model to maximize gap bridging for PC. An analytical model has been developed to predict the temperature rise and the thermal expansion in high speed contour welding of amorphous polymers. The model indicates that the maximum temperature at weld increases linearly with the laser line energy and the laser absorption coefficient. Thermal expansion and hence gap bridging increases with laser line energy. Lower laser absorption coefficient allows higher laser scan energy to be delivered onto the weld interface so helps bridge larger gap. The predicted thermal expansions by the model agree well with the measured maximum gaps bridged for polycarbonate.