Enabling pyrometry in absorber-free laser transmission welding through pulsed irradiation

Laser transmission welding has become an established joining technique in series production of plastic components. Besides its unique process related properties, it also offers several monitoring methods to ensure a constant welding result. In most cases, pyrometry is the method of choice which contactless detects the heat emitted from the interface of the joining partners. Considering the transmission properties of polymers and the optical components between the workpiece and pyrometer, only a small fraction of the thermal process emission is able to penetrate to the detector of the pyrometer. Typically, in classic laser transmission welding, the thermal emission is measured in the spectral range 1.1–2.5 μm limited by the laser wavelength on the one side and the absorption capability of polymers/optical elements in the optical path on the other side. In absorber-free laser transmission welding, laser sources in the range of 1.6–2 μm are used in order to exploit the intrinsic absorption of thermoplastics. With the laser emitting in the sensitivity range of the pyrometer, optical filters have to be used to isolate the thermal radiation from the laser radiation which, however, attenuates the already weak thermal signature even further. An approach that does not require any optical filters is presented in this paper. The concept is operating the laser in a pulsed mode which enables detection of thermal emissions between two consecutive pulses without being overlaid by the laser radiation. Though laser radiation is not delivered continuously, the result demonstrates that it is still possible to obtain a nearly homogeneous seam when choosing an appropriate pulse regime. It is also shown how the detected signal can be utilized to adjust the focal position, which is an important but time-consuming aspect in absorber-free laser welding.