Joining thermoplastics over large areas with multifocal laser optics
With a new type of welding head that has nine individual laser spots, thermoplastics can be joined to thermoplastics and thermoplastics to metal over large areas. Equipment and process were developed in the MULTISPOT project, in which the Laser Zentrum Hannover e.V. (LZH) was involved together with four companies and two associated partners.
Laser welding processes for polymer components have so far been used primarily to create narrow weld seams, such as for microfluidic applications or for welding electronic components and containers. A large laser spot is needed to join thermoplastic over larger connecting surfaces. However, it can usually only introduce energy into the work piece with a constant intensity distribution. This is particularly relevant for curves: in the outer area of the curve, too little energy is applied in a classical approach, while too much energy is applied in the inner area.
In the project MULTISPOT, the LZH, together with four small and medium-sized enterprises (SMEs) and two associated partners, has developed a new process to join thermoplastic to thermoplastic but also thermoplastic to metal over large areas.
Setting laser power individually in a field
For this purpose, neoLASE GmbH and COHERENT Inc. have developed a diode unit with nine individually controllable diode stacks. The special feature: The laser power of the spots can be adjusted independently of each other. With a specially developed optics from Sill Optics GmbH & Co. KG, it is thereby possible to adjust the intensity distribution. In this way, the temperature in the weld can be adjusted according to the local thickness and nature of the material, as well as the weld geometry. LMB Automation GmbH has then combined the components in one welding head. The development was made possible by a measuring device from PRIMES GmbH, with which multifocal optics can be measured for the first time.
Process and software development from the LZH
The process for the new welding head was developed by the scientists of the LZH. For optimal connection of thermoplastic to metal, they structure the metal in advance. Then, they heat the metal of the work piece to such an extent that the plastic melts via heat conduction and joins firmly to the metal. Using the process, they were able to successfully join thermoplastic door elements to a metal frame.
The automation of the process is important for its use in series production in automobile manufacturing. For this purpose, the scientists, together with LMB Automation GmbH, have developed concepts for the use of the welding head on a robotic arm and have also written the necessary software programs. As a result, the specified laser power profile can be precisely maintained when moving the robotic arm. Volkswagen AG provided support in the practical implementation and made demonstrator parts available.
The project MULTISPOT was funded by the German Federal Ministry of Education and Research as part of KMU-innovativ