Welding plastic with light

Stuttgart, Germany – Thermoplastic plastics have been welded using laser penetration welding since the end of the 1990s, but the process is still considered extremely innovative. Generally, one of the two components for the wavelength used by the laser is transparent; the other absorbs the laser energy, for example, by mixing carbon black.

Heat is generated in the component absorbing the energy, which is transferred to the upper component by gently compressing the two components. The parts fuse in the edge zone and form a welded seam, which reliably connects the two components. Diode lasers are used for the most part because their relatively low power and beam quality are completely sufficient for fusing the plastics.

"The process of laser plastic welding differs significantly to the process of welding metallic materials," explains Frank Brunnecker, VP of laser welding at LPKF Laser & Electronics AG in Erlangen, as "the laser beam first penetrates the upper laser-transparent material and releases its energy to the lower, laser-absorbing component." The result is a clean, safe, and sophisticated welded seam using a particularly efficient process, he added.

Peter Bruns, application and technical service manager, LIMO Lissotschenko Mikrooptik GmbH, Dortmund, adds that "laser penetration welding permits applications that are not possible or are only possible with great difficulty using traditional bonding technology".

The decisive advantage over most of the other bonding processes is the contactless, direct interaction of the laser light with the plastic, Bruns continued. For example, the rival, ultrasonic welding, is dependent on good mechanical contact of the sonotrodes with the plastic; laser penetration welding makes possible visual integration of online monitoring of the bonding process, for example, by monitoring the power of the diode laser, and monitoring or control of the process temperature, which is not possible with 'mechanical' welding processes."

"Laser, as a valuable tool, has become an integral part of material processing, primarily in the automotive industry," states Thilo von Olnhausen, marketing manager, ACSYS Lasertechnik GmbH, Kornwestheim. company's laser processing system has been modified especially for welding tachometer panels, where the housing is welded to the transparent pane using laser, he added.

"The advantage of this method is that no screws or any type of fixtures such as adhesives or clamps are required, and parallel loading is made possible through the reversing rotary table," von Olnhausen says.

The laser penetrates the transparent tachometer pane and is absorbed by the opaque tachometer housing underneath. The plastic heats up to fusion point and becomes liquid in a fraction of second, the concentrated heat of the dark plastic fuses the transparent plastic of the tachometer pane so that the two different plastics are welded together after cooling."

New developments based on longer diode laser wavelengths have in the meantime paved the way for the bonding of two transparent plastics. Frank Brunnecker from LPKF says, "New products with long wavelengths between 1470 and 1550 nm with sufficient laser output are available, which make possible sufficiently high intrinsic absorption in transparent plastics for shorter wave engths so that these can be bonded."

In this spectral range, most plastics can absorb the laser beam also without the addition of a laser absorber. "The challenge for every laser weld lies in the coordination of the component design and material, a suitable laser welding process, as well as the proper system technology." Such transparent bonding with the highest visual requirements is becoming increasingly important, particularly in the medical industry, where particularly safe and hygienic processing is demanded, he added.

Brunnecker went on to say the laser process is abrasion-free and does not need any additional materials such as adhesives. Furthermore, laser welding offers several possibilities for quality assurance in the process.

"Examples include monitoring of the weld path, pyrometer control of heating the welded seam or also the patented reflection diagnostics which reliably identify the successful welding process," says Bruns of LIMO Lissotschenko Mikrooptik.

Approximately 200 exhibitors will showcase their innovations and further developments in laser technology at LASYS 2012 in Hall 4 of Messe Stuttgart from June 12 to 14.