Higher Strengths When Welding with an Intermediate Layer
Laser welding has now become an accepted welding technique in industrial practice. However, laser welding involves different requirements regarding the optical characteristics of the parts to be joined. While one component must be transparent to the laser beam, the second components must absorb the radiation from the laser.
The intermediate layer process avoids these constraints of laser welding. With his process, the absorbing material in placed between the parts in the form of a film. The radiation from the laser is absorbed solely by this intermediate layer, causing it to melt. As the result of thermal conduction, the parts on either side of film are plasticized and a welded joint is formed. The colors of the actual parts being joined can thus be selected at will, so that even transparent weld seams are possible with this technique.
The thickness of the intermediate layer and its carbon black content have a considerable influence on the strength of the weld. This is discussed by Prof. Dr.-Ing. Edmund Haberstroh and Dipl.-Ing. Wolf-Martin Hoffmann from the Institute for Plastics Processing (IKV), Aachen, in an article appearing the journal Kunststoffe. As part of a research project, welding trials were conducted with polyamide 66 films of different thickness and containing varying amounts of carbon black.
With a carbon black content of 0.5%, almost all of the incident radiation is absorbed within the film. The thicker the film, the greater is the amount of material that must be plasticized via thermal conduction and the smaller is the region melted in the lower of the components being joined. Accordingly, only very low-strength welds can be achieved with thick films. On the other hand, with thin films, weld strength exceed 24N/mm2 . This corresponds to about 70% of the strength achieved in a conventional black/transparent weld seam.
Films with a carbon black content of 0.2% exhibit comparable welding behavior. Due to the lower carbon black content, the radiation penetrates deeper into the material. As a result, a greater portion of the film is plasticized directly by the radiation. The lower component melts more readily and the weld seam is stronger. This positive effect is particularly beneficial in thicker films. Weld seam strengths are better than those obtained with higher carbon black contents in films of the same thickness.
At the lowest carbon black content (0.025%), relatively little radiation is absorbed. As a consequence of the reduced absorption, considerably higher laser outputs are required when welding in order to plasticize the material to a satisfactory degree. At the same time, the speed must be increased to avoid heating the material excessively. The weld seam strengths achieved are in the same range as those obtained with the intermediate carbon black content. Since the radiation from the laser is absorbed within the volume of the film, the amount of plasticized material in the parts being joined is actually about the same.
However, depending on the material used, certain film characteristics are preferable. While the best results were obtained with thin films when using polyamide, comparative trials indicated that when using polycarbonate with a lower carbon black content thicker films yielded better results.
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