Analysis of the interaction process during laser transmission welding of multilayer polymer films with adapted laser wavelength by numerical simulation and thermography

The field of application of multilayer polymer films ranges from the packaging of food and other goods to medical technology and the encapsulation of organic electronics. The layer structure of the polymer films is adapted to the respective areas of application. Usually, polymer film systems contain several individual layers. A trend toward more flexible production and batch sizes down to one can be identified in all the above-mentioned areas of application. This requires new production processes. Laser transmission welding with adapted laser beam sources represents a promising approach for this. It is characterized by a contactless and a locally limited energy position. A very high degree of format flexibility is also possible, thanks to a purely numerical adjustment of the welding contour. In many applications, it is desired to keep the polymer film as it is available on the market. If two multilayer polymer films should be welded with laser radiation in an overlapping arrangement, sufficient absorption of the laser radiation in the material is required. Each individual polymer layer has wavelength-dependent material-specific optical properties. Due to the vibrations of polymer chains, polymers have absorption peaks in the near infrared range. If a suitable laser beam source is now used for the welding process, the energy can be deposited directly in the material without modifying it beforehand. The energy input should be the highest at the point of contact or in the sealing layers of the two polymer films so that it is locally heated and melted. The surrounding film layers are also thermally influenced by heat conduction processes. In addition, the optical properties of the other layers influence the propagation of the laser radiation through the multilayer polymer film. For a comprehensive understanding of the process, it is imperative to examine these processes in detail. Therefore, the interaction process is considered in depth and, especially, the propagation of the temperature field within the material, which is not directly accessible for real measurements, is analyzed exactly. For this purpose, on the one hand, a numerical temperature model is set up, which depicts the real experimental setup and takes into account the optical properties and heat conduction variables of the individual layers. The numerical model allows the temporal resolution of interaction processes within the multilayer polymer film to be viewed from different angles. On the other hand, thermographic images of the real samples are taken during the welding process. For this purpose, the emitted heat radiation of the polymer films is measured off-axis. The data obtained are evaluated and compared with each other. For the final discussion of the results, additional analyses of the weld specimens microscopy and thin sections are used.