The main objective of this study was to investigate the influence of laser beam positioning on the quality of the polymer joint weld. The laser scan path from the centre of the weld was varied, and its effect on the joint performance was evaluated. Three laser transparent-absorbent polymer pairs with different transmissions were subjected to the following tests: burst pressure, thermal shock, and leakage tests. The tests were performed to evaluate the strength and tightness of the weld. The strongest joints, with an average burst pressure value of 6.4 bar, were obtained by positioning the laser beam at a shift of 0.0 mm, and the lowest joint strength of 5.8 bar at a shift of 0.7 mm. The shift of the laser beam from the centre of the weld affected the heterogeneous melting of the polymers, which increased the heating time required to reach the targeted meltdown: 4.2 s was reached at a 0.0 mm shift and up to 10 s at a 0.7 mm shift. This led to joint overheating, forming of pores, and decreased weld strength. The polymer pair with modulated laser radiation transmission and an absorbent part showed inhomogeneous energy deposition across the weld seam and the formation of the heat-affected zone (HAZ) during quasi-simultaneous welding. The quality of this pair was improved using a 0.0 mm shift and laser power from 330 W to 350 W. As a result, no weld leakage was detected after 50, 75, and 100 thermal shock cycles.
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