The results described in the preset work show for the first time for the polymer welding that through the use of laser modulations a reduction of the Heat Affected Zone, and therefore a reduction of the thermal load in both joining partners is possible, without losses in the weld seam strength. The evenly distributed laser energy in the joining area leads to a reduced depth of the weld seam compared to the conventional contour welding of polymers. Based on this aspect the experiments described in Chapter 6.3.2 indicate that the welding of parts with reduced material thickness, for applications where plastic parts with high requirements on the surface finish have to be joining, can be welded without any appearance of the weld seam on the visible surfaces. Therefore, a significant material saving of about 33% can be considered. Furthermore, the reduced thermal load of the joining partners enables the welding of fine structures without thermal deformation of these structures. Especially among the biomedical applications there is a variety of products with microfluidic channels that have to be hermetically sealed, usually by welding a plastic foil or cover on top of them. Such microfluidic channels with walls having a thickness smaller than 0.5 mm require a minimal thermal input in the welding area in order to avoid the deformation or the collapse of these fine structures. The possibility to control the size and shape of the HAZ make the welding of polymers using local beam modulations a strong candidate for such applications.

The welding results discussed in the present work indicate that concerning the weld seam strength and gap bridging ability the new welding approach reaches a similar performance as the other already established laser welding process variants. For all the investigated materials the same weld seam strength as previously reported in the literature can be achieved through the welding using local laser beam modulation; as shown in Figure 76. However, the presented results indicate that in the low laser power range (at the lower limit of the process window) a superior result can be achieved compared to the conventional contour welding. In this region of the characteristic curve, under identical processing conditions (e.g. identical laser power and feed rate) the homogeneous melting of a thin material layer at the welding interface across the weld seam width enables a higher weld seam strength compared to the conventional contour welding (Figure 81, bottom). In this way the new welding approach enables the widening of the process window and contributes to the increase of the process robustness. A similar effect is expected at the upper limit of the process window where the laser beam modulations and the homogeneous energy input in the welding area across the weld seam width could avoid the thermal damage of the material in the middle of the weld seam, typical for this region of the characteristic curve. In this way the process window could be extended in both directions enabling a more robust and reliable process. Intensive investigations within large scale international projects are currently focused to determine the possibility to extend the process limits in this direction by using laser local beam modulation and alternative approached derived from this process (e.g. using optical components to shape the laser beam intensity profile in order to achieve a similar energy distribution in the welding area as for the welding with local beam oscillations).

Concerning the gap bridging ability of the new process proposed within the present work it can be concluded that for spot diameters lower than 2w0= 0.200 mm and reduced oscillation amplitudes the process instability will increase. The reason can be found in the high laser intensities resulting in the welding area for highly focuses laser beams (I= 105 - 106 W/cm2), which will lead to the nearly instantaneous thermal damage of the material in the areas with a residual air gap. However, for larger laser spots and oscillation amplitudes the results discussed in Chapter 6.5 demonstrate the possibility to bridge residual gaps smaller than 0.1 mm, and therefore a similar performance to the conventional contour laser welding. From a wider perspective it can be expected that by using the local beam modulations for the conventional welding with diode laser and typically laser spot 2w0> 0.400 mm a significantly higher volume of molten material can be generated and implicitly a better gap bridging ability.

A further major positive aspect of the welding with local laser beam modulations is the possibility to adjust the laser weld seam with without any changes to the laser welding system. Conventionally, the increase of the weld seam is only possible through the reconfiguration of the welding system (e.g. change of the focusing optics or change of the core diameter for the optical fiber). The possibility to freely select and change the weld seam within one processing step (along a defined welding contour various weld seam width can be realized) enables a higher process flexibility. Depending on the system configuration (selected laser spot diameter) the welding using local beam modulations allows the achievement of a weld seam width starting from around 0.100 mm.

Based on the theoretical considerations discussed in Chapter 5 and experimentally confirmed in Chapter 6 the present work demonstrates for the first time the feasibility of using high brilliance laser sources such as fiber lasers for the laser welding of polymers despite the implicit high intensities resulting due to the high focusability of such laser sources. Beside the process related benefits described above resulting from the possibility to use these new laser sources for the polymers welding, further benefits can be identified in the system technology for the laser welding stations. Fiber lasers through their highly compact construction (e.g. a 20 W system can be built in the size of a typical CD- ROM unit) and lower amperage needed allow the development of laser welding stations with reduced foot print, thus requiring a reduced floor space in the production facilities. Air cooled systems with an optical power of more than 100 W an operation time over 50000 hours, as reported by several manufacturers, make these system virtually maintenance free. Combining these aspects with power requirements for the welding using local beam oscillation under 50 W and modern modular system architecture the ground is laid for the development of advanced laser welding systems with a reduced initial investment cost, and subsequently to the increase of the competitiveness of the laser welding systems.

Schlagwörter: polymer welding; fiber laser; TWIST; microfluidics; laser beam modulations

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