Comparison of process monitoring strategies for laser transmission welding of plastics

Laser transmission welding of plastics in industrial applications is expected to be robust against material quality variations and cleanness variations1. The process has to be applicable tolerating all workpiece dimension variations and different assembling positioning within the given tolerances. The inevitable failures of the welding process have to be detected reliably in each part even in large lot sizes.Process monitoring offers an alternative to post-production quality testing which is often destructive, and provides a complete recording of the manufacturing process, as required by official quality standards such as DIN EN ISO 9001.Within the scope of this paper the specific informative capabilities of coaxially spatially-integrated and coaxially spatially-resolved monitoring as well as monitoring of secondary radiation and monitoring by using external illumination are being compared and evaluated. Therefore a welding head is being developed using the already existing modular optical CPC-system of the Fraunhofer ILT. The Coaxial Process Control (CPC) system monitors the machining process in coaxial alignment with the laser beam axis. The welding head allows simultaneous observation using both observation methods.The emitted radiation by the process itself is being detected by a spatially integrating pyrometer and can be spatially resolved by a thermal camera. The visible spectrum is being reflected to a CMOS camera. It allows spatially resolved and spatially integrated monitoring at the same time.The modular system allows also coaxial and off-axis illumination, so coaxial, brightfield and darkfield workpiece illumination is provided and offers different illumination strategies for different materials and welding methods.Laser transmission welding of plastics in industrial applications is expected to be robust against material quality variations and cleanness variations1. The process has to be applicable tolerating all workpiece dimension variations and different assembling positioning within the given tolerances. The inevitable failures of the welding process have to be detected reliably in each part even in large lot sizes.Process monitoring offers an alternative to post-production quality testing which is often destructive, and provides a complete recording of the manufacturing process, as required by official quality standards such as DIN EN ISO 9001.Within the scope of this paper the specific informative capabilities of coaxially spatially-integrated and coaxially spatially-resolved monitoring as well as monitoring of secondary radiation and monitoring by using external illumination are being compared and evaluated. Therefore a welding head is being developed using the already existing modular optical CPC-system of t...