Quality assurance IV – Process control by pyrometry
The possibilities for monitoring the joining process in laser plastic welding are wide-ranging. State of the art is primarily indirect methods which do not assess the weld seam itself, but which allow conclusions about the expected seam quality on the basis of various monitoring variables and inputs. In the case of (quasi-) simultaneous welding, for example, this is the so-called collapse control, in which the relative movement of the joining parts to each other is recorded over the process time as a curve, which again can be evaluated inline according to characteristic points and specified limits.
If so-called contour welding without welding collapse is selected as a process variant – and this parameter is therefore not available – pyrometry, i.e. non-contact temperature measurement, can be used for process monitoring and control. In this process, a portion of the thermal radiation generated from the process heat is recorded at the current laser position in order to draw conclusions about the temperature and the energy input by the laser. Factors that can lead to temperature fluctuations are (in addition to the process parameters themselves) the material properties, primarily the laser transmission, irregularities in heat conduction – which can be caused by mold or seam defects – or unwanted burns on the component surface. The recorded temperature signal can be used not only for process evaluation but also for process control. Typically, the signal is used to continuously adjust the laser energy applied via the laser power or feed rate during the joining operation, with the aim of achieving as constant a temperature as possible.
The thermal radiation used in pyrometry is predominantly in the short-wave infrared range and is therefore subject to a certain degree of attenuation when passing through the material. When measuring at the component surface, the scattering and temporal delay of the detected radiation have a disadvantageous effect. These factors are strongly material and geometry dependent and can limit the size of detectable seam defects. Nevertheless, in many cases pyrometry is a suitable means of significantly increasing process reliability.