Welding of large thermoplastic parts with a MultiSpot-welding head
Verena Wippo and Julian Kuklik highlight the need for advanced beam shaping for welding large thermoplastic parts
Thermoplastics are increasingly being used as structural components in a wide variety of areas, such as to implement new lightweight construction concepts. For this reason, there’s a growing need for reliable, fast and economical joining processes for thermoplastic structures.
Laser transmission welding is an industrially widespread and established process for joining thermoplastics. So far, this process is used primarily to produce narrow weld seams for electronic components, containers or micro fluid applications. Nowadays the parts which have to be joined are becoming larger, and so are the forces which have to be transferred over the weld seam. Since the weld seam strength is lower in plastic welding than in metal welding, it is necessary to produce wider connection cross-sections for plastics.
In principle, homogenised laser focuses can be used to produce wide weld seams. However, as research has shown, the maximum process temperatures when joining with a large laser focus with small radii lead to partial overheating of the material in the inner area of the radius. This affects the strength of the weld. To solve this problem, the Laser Zentrum Hannover (LZH) together with six partners as part of the KMU-innovativ project MultiSpot has developed a new type of welding head based on modularly controllable laser spots (see Figure 1).
Figure 1: The MultiSpot welding head can be used to join large parts and can be conducted using a robot
This welding head makes it possible to locally and temporally vary the intensity distribution within a laser focus, which means that for the first time, the laser power can be adapted to the weld seam geometry and the material properties.
The basis for this is a diode laser system with laser diodes that can be addressed independently of one another, i.e., the output power of each diode laser module can be set individually. The diode laser system and the control software were developed by neoLASE. It consists of nine independently emitting fibre-coupled laser diodes, each with a maximum output power of about 100W and emission at 970nm. These laser diodes were provided by the associated project partner Coherent Dilas Diodenlaser. The laser radiation is guided to the processing head through individual optical fibres and formed there into a modular laser focus via optics.
The development and production of the optics was carried out by the project partner Sill Optics. The optical assembly consists of nine individually adjustable lenses, which are arranged in a 3 x 3 pattern. The lenses are made of quartz glass and are coated according to the laser wavelength. These optics were integrated into a processing head by LMB Automation. The company later intends to use the processing head together with the diode unit in new systems. It was also designed to be robust against vibrations, which will enable the processing head to be mounted on a robot.
In order to be able to calibrate the system, the project partner Primes developed a new measuring device for measuring the beam properties. This device has a maximum measuring field of 40 x 40mm² and a resolution of 2,048 x 2,048 pixels. Furthermore, a multi-caustic measurement as well as a subsequent separation into individual caustics is possible with this device. This enables the nine individual beams to be measured individually. The measuring device thus lays the basis for quality monitoring, since deviations such as performance losses can be detected and corrected.
Functional test on demonstrator components from the automotive industry
The project partners successfully tested the MultiSpot welding head on two different demonstrators with different weld seam profiles from the automotive sector. One was a servo oil container and the other was a door demonstrator.
The course of the weld seam of the servo oil tank had straight sections as well as differently curved radii. The weld seam width was 10mm. The LZH has developed different welding patterns for the welding process, which depict different energy distributions within the laser focus. These welding patterns can be changed automatically to achieve a constant temperature distribution in the weld seam, regardless of the component geometry.
The door demonstrator was a hybrid component consisting of a plastic outer panel and a metallic door structure (see figure 2).
Figure 2: An automotive demonstrator part consisting of mixed materials
To prepare for the joining process, the joining areas of the metal component were first roughened using another laser system. Both components were then pressed together and the metal was heated with the new MultiSpot welding head. The metal melted the plastic through heat conduction and this flowed into the structured areas. After the plastic had cooled down, both components were joined together. Different energy distributions were also developed for this component to account for the heat conduction in the metal component.
So that automation concepts can be implemented as easily as possible in the future, the partner neoLASE has equipped the diode laser system with an OPC-UA interface both for communication with the robot and the measuring device of the project partner Primes. The firm wants to ensure secure communication between the individual components.
A new welding technology for joining thermoplastics and thermoplastics with metal has been developed that can be used in a wide variety of areas from automotive engineering to medical technology.
The MultiSpot project (modular controllable laser spot for temperature field-adapted welding of complex plastic components) was funded by the Federal Ministry of Education and Research as part of KMU-innovativ (FKZ.: 13N14693). In addition to the Laser Zentrum Hannover, LMB Automation, neoLASE, Primes and Sill Optics were involved. Associated partners were Coherent and Volkswagen.
Verena Wippo and Julian Kuklik are research associates within the Composites Group at the Laser Zentrum Hannover e.V. in Hannover, Germany.