Absorber-free laser transmission welding enables the precise and clean joining of plastic films without additives or adhesives. It is therefore particularly relevant for demanding applications in medical technology, optics and the food industry.
Laser beams with spatially adapted intensity distribution can be used to enlarge the process window when welding films and increase the weld seam strength. This allows the energy input to be customised and the temperature field of the welding process to be specifically influenced. When welding a PP or PE film, for example, the process window can be increased by a factor of 3 by using an adapted intensity distribution. The weld seam strength almost corresponds to the strength of the base material.
Absorber-free laser transmission welding is characterised by its non-contact energy input and geometric flexibility and enables the precise and clean joining of polymer films without absorbent additives or adhesives. It is therefore ideal for applications with high process reliability and cleanliness requirements, such as packaging and containers in the medical technology and food industries.
A homogenised weld seam temperature is necessary for a large process window. As part of an ongoing development project in collaboration with OTH Regensburg, Gerresheimer GmbH and AdlOptica Optical Systems GmbH, EVOSYS has been able to optimise the welding process, including beam shaping, in such a way that homogenised energy input is possible. This increases the process window for welding polypropylene or polyethylene films by up to a factor of 3. At the same time, the weld seam strength almost matches the strength of the base material.
Basics
Compared to conventional heat sealing, laser transmission welding of plastic films is characterised by a non-contact and locally flexible energy input with only low mechanical stress. The films to be joined are arranged in an overlap and fixed in place with a clamping device. By using a laser source with a wavelength of around 2 µm, the material can be joined without additional substances such as adhesives or laser-absorbing additives. The plastic, which is transparent to visible light, already sufficiently absorbs the laser radiation in this spectral range.
Absorber-free laser transmission welding is always ideal when additives need to be avoided, especially in applications in the food industry and medical technology. This process is also very suitable for applications in clean rooms. Figure 1 shows the interior of an EvoClear laser welding system with an exemplary film component.
Laser beams usually have a Gaussian distribution of power density, which leads to a temperature increase in the centre of the weld seam during welding. As a result, thermal damage can occur in the centre of the seam, while the edge of the seam is still relatively cold and therefore only has low strength. This challenge is often met by increasing the beam diameter by defocussing the laser beam, but this leads to a widening of the weld seam. In addition, an unnecessarily large amount of power remains unused next to the seam, which worsens the energy efficiency of the process and requires more expensive beam sources with more power than is actually necessary.
A homogeneous weld seam temperature is required for a large process window and therefore a robust process, which is why the typical Gaussian-shaped intensity distribution of the laser beam was converted into a ring-shaped donut and a rectangular flat-top distribution. A process simulation shows that the donut-shaped intensity distribution in particular leads to a significant homogenisation of the weld seam temperature and enlarges the process window. This was proven in welding tests: The process window for welding PP was increased by a factor of three.
Further tests also show that a high seam strength can be achieved regardless of the intensity distribution if a laser power in the centre or at the upper end of the process window is used.
A detailed technical article on this topic has been published in the trade journal Joining Plastics 19 (issue 1-2025), https://www.joining-plastics.info/en
Do you have any questions about the above options? Our experts will be happy to send you more detailed information on request. You can reach us by telephone on +49 9131 – 4088 – 1029 (Mr Holger Aldebert) or by e-mail at sales@evosys-laser.com.
Authors:
EVOSYS Laser GmbH: René Geiger, Lea Kroth
Ostbayerische Technische Hochschule Regensburg: Prof. Dr.-Ing. Stefan Hierl, Frederik Maiwald
Gerresheimer Regensburg GmbH: Dr. Bernhard Schmitt
AdlOptica Optical Systems GmbH: Alexander Laskin
Funding
The results presented in this publication were obtained within the project “CTC-Med”, which is funded by the Bavarian Research Foundation (AZ‑1519‑21).
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