Lea Sauerwein, Christian Ebenhöh and René Geiger, of Evosys Laser, describe the development of a new optimised polymer welding process
The joining of polymers via laser welding is an established process in the plastics industry, and also in others such as the automotive, medical and consumer industries.
The most commonly used technique, through-transmission welding, joins two components in an overlapping arrangement with a single laser beam source.
The upper joining part is laser-transparent, so the laser beam transmits through this part and is focused onto the lower laser-absorbing part. The laser energy is absorbed in this part and warms it up directly. Through heat transfer, the upper part also heats up and plasticises, creating a material bond with the lower part.
A diode laser with a wavelength of approximately 800nm to 1,000nm is commonly used for this process. In this wavelength range most engineering plastics show a relatively high transmission – most natural, uncoloured materials can be used as the transparent joining part. Through the use of additives, for example carbon black, the laser absorbing properties can be adjusted for the lower part.1,2
This process has some disadvantages, however, mainly because of its dependency on heat transfer. A high geometrical accuracy is crucial for the process to maintain proper thermal contact between the parts – gaps in the joining interface can hardly be bridged. Because the upper joining part is warmed up mainly through heat transfer, a temperature gradient develops between both parts, potentially leading to residual stresses. These effects carry the risk of limiting the process window and reducing the weld seam quality.3
A state-of-the-art approach to reducing these drawbacks is the so-called hybrid welding technique. Secondary radiation (commonly from a polychromatic light source) supplements the primary laser beam source to warm up the upper joining part directly and reduce the dependency on heat transfer, thus decreasing the residual stresses and geometric challenges.4
A new approach
Evosys has developed an alternative approach to polymer joining, called advanced quasi-simultaneous welding (AQW). In the newly developed process variant, the primary laser source, a diode laser with a wavelength of 980nm, is supplemented with a secondary laser source, a fibre laser with a deviating wavelength of 1,940nm. At the wavelength of the secondary radiation, most engineering plastics show a relatively low transmission, so with this wavelength it is possible to heat the upper joining part more directly. Both laser sources are switched on in an alternating time pattern, with the length and variation of the pulses being adjusted to fit the welded material. In contrast to simultaneously irradiating the plastics with both beam sources, this method, combined with a purposeful selection of the applied wavelengths, enables the deposition of a selective amount of energy into each joining part and thereby leads to a more controllable and efficient process.
It is possible to integrate the new technique into systems with a galvanometric scanner.
The spot diameters of both laser beams can be set individually to fit the welded product by focusing both beams separately using a specially designed beam expander, and then combining them. By using two laser beam sources it is possible to integrate the new technique into systems with a galvanometric scanner. This is necessary for a quasi-simultaneous welding process, where the weld seam is not only irradiated once, as is the case in contour welding, but multiple times with a high feed rate.
Putting it to the test
Different tests were carried out with polycarbonate to investigate the benefit of AQW. Welding time and tensile strength were investigated, the latter being an indicator for the weld seam quality. Cross sections through the weld seam were also produced to evaluate the size of the weld nugget in the laser-transparent part.
The welding time can be analysed when employing a so-called collapse-controlled welding process. The weld seam plasticises (quasi-) simultaneously, resulting in a movement of the lasertransparent part towards the absorbing part, which is called the welding collapse. It can be measured by a tactile distance measuring sensor. A welding collapse of 0.2mm and a laser feed rate of 800mm/s were set to reflect typical industrial mass-production parameters. The welding time until reaching the welding collapse, and the total collapse after a certain cooling time, were measured.
A weld nugget can be seen in this joined laser-transparent part.
To investigate the influence of the new process on the welding time, the upper process limit for the two conventional processes (each with only one laser source) were first determined. The laser power was set to the highest possible level that does not lead to negative effects, such as burn marks or thermal decomposition of the base material in the weld seam. In a second step, both lasers were alternated in a sequential time pattern, but with the same laser powers as the first step. The welding times until reaching the set welding collapse were compared for each different process.
To test the tensile strength of the samples, the parts were arranged in a flat-to-flat overlapping geometry and welded with a fixed number of overruns. The parameters feed rate, clamping force and AQW frequency were set as they were in the previous tests, which were conducted using a T-joint geometry. Additionally to the parameters investigated before, the impact of the energy delivered by the two lasers – that is, more laser power from the primary laser source in combination with less power from the secondary – was also determined. The laser powers of the standard processes were set as 100 per cent laser power. Hence 120 per cent primary radiation, for example, meant the laser power was increased by 20 per cent, in comparison to the standard process. To keep the total amount of energy constant in all variations, when increasing the primary laser power by 20 per cent, the secondary laser power was decreased by 20 per cent.
Results
With a welding collapse of 0.2mm, the upper process limit was reached for the primary laser source at a laser power of 50W, with the secondary laser source at 51W. It was not possible to improve the welding time further by increasing the laser powers, as this leads to negative effects, as mentioned previously. When using AQW, the lasers were alternated with a switching frequency of 100Hz. The laser powers were the same as those in conventional processes. As can be seen in figure 1, a significant reduction in the welding time – by approximately 10 per cent – was achievable with AQW, in comparison to the standard process with only one laser source. This reduction of the welding time is not limited to certain feed rates, as it can be reproduced at different feed rates.
Figure 1: Reduced welding times achieved using advanced quasi-simultaneous laser welding.
When investigating the tensile strengths, the new process was compared to the standard processes, as it was in the previous test. As can be seen in figure 2, the samples welded with only the primary laser source were the relevant comparison in this process, as the ones welded with only the secondary laser source showed a significantly lower tensile strength. Parts welded with the same amount of energy as the primary and secondary laser source showed a similar tensile strength as those welded with only the primary laser source. Whereas it could be seen that parts welded with a greater amount of primary laser power show higher tensile strengths.
Figure 2: Increased tensile strength and weld nugget height achieved using advanced quasi-simultaneous laser welding.
The best results were achieved with 140 per cent primary radiation and 60 per cent secondary radiation. Welding with the AQW process has an influence on the height of the weld nugget in the laser-transparent part, confirming that this process leads to an increase of the plasticised material in the laser-transparent part through direct heating.
Conclusion and outlook
In multiple trials, the benefit of the newly developed AQW process was confirmed. Purposeful selection of the applied wavelengths and enhanced system technologies enable a more efficient process with improved weld seam quality. A reduction of about 10 per cent in welding time is possible through the direct and controlled heating of each joining partner by laser radiation. By increasing the amount of plasticised and fused material in the laser-transmissive part, the weld seam strength and quality is increased.
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Lea Sauerwein is a development engineer at Evosys Laser.
Christian Ebenhöh is a key account manager at Evosys Laser.
René Geiger is a development manager at Evosys Laser.
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References
[1] J. Eichler, H. Eichler (2010). Laser – Bauformen, Strahlführung, Anwendungen. Springer Berlin Heidelberg. 978-3-642-10461-9
[2] T. Frick (2007). Untersuchung der prozessbestimmenden Strahl-StoffWechselwirkungen beim Laserstrahlschweißen von Kunststoffen. Bericht aus dem Lehrstuhl für Fertigungstechnologie. Bd 189
[3] R. Klein (2011). Laser Welding of Plastics. WileyVCH. Weinheim, Germany. 978-3-527-40972-3
[4] A. Hofmann (2006). Hybrides Laserdurchstrahlschweißen von Kunststoffen. Bericht aus dem Lehrstuhl für Fertigungstechnologie. Bd 174
