LPKF’s Laser Plastic Welding Application Centre in Fürth was challenged by an automotive subcontractor, specialising in the development, design and high-end production of door-sill strips, to securely weld the surface of an 800mm long and 200mm wide film onto a white carrier housing. Following comprehensive initial material testing, a sample production phase, and prototype production, a customised, robot-controlled laser welding machine, with special clamping technology, was delivered to the customer.

Laser welding

Laser welding was considered the most suitable method for this application. Only a narrow strip approximately 3mm wide was available on the component for the welding process, and no additional material could be added during welding. The precise control of the energy input protects the electronics and stabilises the narrow welding surfaces. Laser welding demonstrated its advantages here because only a very narrow welding seam was required. This ensures that the black paint on the film directly adjacent to the weld seam remained undamaged.

A special feature is the white LED film welded onto the housing. This required a special absorber in the carrier housing. The laser beam penetrates the laser-transparent LED film and releases its energy in the laser-absorbent housing. Applying a constant clamping pressure transfers the heat to the overlying LED film. Both of the joining components then melt locally along the weld seam to create a permanent joint once cooled.

A constant clamping pressure everywhere is required for a reliable weld seam, and this in turn requires a suitable clamping mechanism. The solution was found in a bottom-up glass clamping method. This involves the component being transported on a tool carrier and pressed upwards against a quartz glass plate. This lifting process needed to be extremely precise to ensure very accurate welding, and therefore boasts a repeatability of 50μm. The clamping pressure reaches up to 10,000 Newton over the whole area. This in turn demands very robust tool carriers, guides and glass plates. The clamping technology can be changed very quickly when a different model variant is required, as part of a robot-controlled special machine designed for mass production with short cycle times.

The laser welding system delivered to the end user uses a conveyor belt to perfectly synchronise the work-pieces. The system has both input and output areas whose compartments close when the component has been presented, so that the system achieves laser safety class 1 during production operations. The laser source is housed in a switching cabinet outside of the machine and a robot controls the welding on large work-pieces. Sensors are used to identify the coded work-piece and automatically select the values for clamping pressure and laser parameters. Leaks or optical defects along the weld seam are immediately identified by an integrated process control system.

In this application, laser welding has demonstrated its economic efficiency when joining films with large dimensions. The system can produce one-off pieces, small batches, as well as production level volumes with a high degree of automation. The laser welding method ensures that door-sills for top of the range cars display the highest levels of optical quality.