Quality assurance III – Weld seam defects made visible
A wide range of options is available for ensuring quality and monitoring the joining process during laser plastic welding. Even before welding, necessary part properties, such as laser transmission, can be easily and reliably checked (see also article Quality assurance II). During the joining operation, the process parameters are continuously tracked and, if necessary, readjusted. These are, in particular, the laser power, the joining force or also the feed rate of the laser beam. In (quasi-) simultaneous welding, another established method is the so-called collapse control: here, the principle-related relative movement of the joining partners to each other is recorded as a curve over process time and can be evaluated with regard to various criteria. Taken together, these measures permit a reliable detection of fluctuations in process-relevant variables and thus allow for indirect conclusions about the weld seam quality.
Direct evaluation of the weld seam is also possible using imaging methods, especially where, for example, collapse control cannot be applied (such as in the contour welding process). Intego GmbH in Erlangen has developed the TMS Planar and Radial Vision devices for this purpose: these are camera systems that operate in the infrared spectrum and thus enable a non-destructive view into the welding plane, even if the laser-transparent joining partner is opaque in the visible wavelength range. The result is a display similar to an X-ray image – the weld seam becomes “visible” and can be checked by intelligent software algorithms for defects such as irregularities, interruptions or thermal damage. The devices can be integrated inline for process evaluation or are available as easy-to-use stand-alone devices for accompanying quality assurance. Time-consuming and cost-intensive post-processes, such as leak tests, can thus often be dispensed with.