Fraunhofer ILT Project Demonstrates Thulium Laser Welding

Using a thulium laser, a team from the Fraunhofer Institute of Laser Technology (ILT) has developed an innovative process for producing the smallest weld seams in transparent plastic components. The development builds on the successful completion of the research project called SeQuLas (segmental quasi-simultaneous laser irradiation) and demonstrates high-precision welding without the presence or use of additional absorbers. The research team consists of experts from three additional Fraunhofer industrial partners: Amtron GmbH, Ortmann Digitaltechnik GmbH, and Bartels Mikrotechnik GmbH.

Thulium lasers deliver a specific advantage to the production process, given how well plastics absorb their corresponding wavelengths. Because it does not require the use of soot or any other additional absorbers, this new joining process could increase efficiency in industrial production. It also bodes potential application to a range of medical technologies.

Microfluidic chips are a primary component of focus in the SeQuLas project, which Fraunhofer ILT, its subsidiaries, and partners launched in 2017. Microfluidic chips work well with liquids, effectively mixing and filtering them for use. The encapsulation of the microchannels integrated in these chips is media-light, however, and conventional joining technology reaches its limits in the micrometer range. Even when the researchers turned to the process of volume absorption, in which they used a thulium fiber laser with an emission wavelength of 1940 nm (a wavelength range in which plastics have natural absorption, eliminating the need for added absorbers), the procedure created a common deterrent: a heat-affected zone (HAZ), over the full cross section of the component.

In the SeQuLas research project, the partners developed an electronically monitored process for gentle, high-precision laser transmission welding of small plastic components for medical technology (in the picture: microfluidic chip from Bartels Mikrotechnik). Courtesy of Fraunhofer ILT, Aachen, Germany.

To prevent the HAZ from expanding vertically and eventually reaching a point that it envelops the cross section, the scientists guided a laser beam along the weld contour several times at high speed with the aid of a scanner system. The method, known as quasi-simultaneous irradiation, heated the entire seam contour at once.

With laser beam sources in the near-infrared (NIR) range enabling high precision and flexibility, absorber-free laser transmission welding (LDS) presents as an “ideal solution” to conventional joining technology, a Fraunhofer ILT press release said.

Quasi-simultaneous irradiation is also amenable in/with materials that tend to warp, such as flat components in volume absorption.

With the developed joining process, in which a thulium fiber laser is used, high-precision welding of microfluidic components can be achieved. Courtesy of Fraunhofer ILT, Aachen, Germany.

Fraunhofer ILT tested with polycarbonate components to show that heat dissipates at the outer surfaces while it accumulates inside the material during the welding process. In contour welding, bits of the plastic melt in a sequence. Moreover, the increasing number of passes and the high scanning speed reduces the HAZ’s vertical growth by up to 30% compared to contour welding.

The project also developed a control for the laser welding process; a pyrometer integrated into the beam path measures the temperature in the component during welding. By synchronizing the signal from the pyrometer with the position of the scanner mirrors, the team, via information from the component itself, can gauge and record heat distribution in a spatially resolved manner. The system also yields information on the precise location of potential thermal damage. The new welding process can react to deviations in temperature almost immediately, and tweak control of the laser power as needed to ensure homogeneous seam properties along the contour of the seam.

“SeQuLas - Laser Welding of Absorber-Free Thermoplastics by Segmental Quasi-Simultaneous Irradiation” was funded by the European Regional Development Fund (ERDF) and the state of North Rhine-Westphalia (NRW) under the Produktion.NRW program of the LeitmarktAgentur NRW. Researchers completed the project in February this year.