Laser welding for the little Bob
Daan Technologies SAS develops, manufactures and markets innovative electronic devices. The manufacturer's best-seller is the small mini dishwasher "Bob", which can be used in a variety of ways thanks to its small dimensions and flexible handling. In contrast to conventional dishwashers, the detergent in Bob is not added with every wash cycle, but is contained in a cartridge that is pre-filled for a good 30 wash cycles.
This cartridge, called "cassette", consists of a black plastic housing with three chambers, a small circuit board and a white lid. The lid must be joined to the housing in such a way that the chambers are both tight and reliably separated from each other. Laser plastic welding was chosen for this step, as only this method could guarantee tightness and also high strength. The durable welded joint also ensures reusability: empty cassettes can be sent back to the factory and refilled there.
Other arguments in favour of laser welding the assembly were the short cycle time, the small space requirement of the joining process as well as the high process reliability and the possibility to monitor the process online. This process also posed the least risk to the internal electronic board. Since the energy input is contactless, such sensitive components are not affected in the welding process.
The cartridge is made of the material PPC (polypropylene copolymer), and a white lid is welded onto the black housing.
Today, laser welding of plastics is mainly used in the transmission process. In this process, the laser radiation is focused through the plastic, which is transparent to the laser wavelength, onto the absorbing moulded part, causing it to melt on the surface. The transmissive partner is also plasticised via heat conduction, resulting in a bond. During the welding process, the two joining partners must be clamped.
In addition, this process distinguishes between contour, simultaneous and quasi-simultaneous welding. Quasi-simultaneous welding is used for the Bob Cassette. This means that the laser is guided along the welding contour at very high speeds. The aim is to scan the contour several times so fast that the weld seam is heated and melted at the same time. This offers the possibility of bridging a joining path to compensate for mould part tolerances. Monitoring this joining path during the welding process also allows online process control.
The compact EVO 2800 was selected as a suitable machine for the manufacturing of the cassette. Due to the manual assembly, this complete system is predestined for production. The system is also suitable due to its intuitive operation, easy process changeover using EvoLaP process software and integrated process monitoring. The cartridge parts have to be assembled in the insertion station and then welded in a second station. The sorting out of good and bad parts also takes place in the insertion station.
An indispensable module for production is the positioning and clamping unit for the two components to be joined. Interchangeable, component-specific holders and clamping glasses ensure exact positioning of the components before the welding process. As soon as both parts to be joined are in the correct position, clamping is carried out with a pneumatic cylinder that presses the entire assembly against the clamping glasses from below and doses the necessary forces for the welding process. The pure welding process only takes a little more than three seconds despite the demanding geometry.
User-friendly operation of the system in everyday production is ensured by the human-machine interface based on an industrial PLC. The HMI also handles the visualisation and evaluation of the process data, the storage of data records and other system functions. Supplemented by the EvoLaP contour software, the flexibility of the system is increased. The user can quickly program in the data for the welding contour, feed speeds and laser power. Due to the intuitive user interface, which is specially designed for use on touch screens, the user can start directly.
High output, high machine availability
The production of the cassette for the Bob shows that laser welding is an extremely reliable process. The experiences in series production - both with the laser welding of plastics process and with the EVO 2800 welding system used - are consistently positive. The clean and robust process guarantees a high output and the system itself shows a high availability with extremely low follow-up costs. The low-maintenance laser and the reliable mechanical and electrical components contribute to this.