Laser welding of plastic is quick, precise, and generates little waste, but it does have its limitations. The process involves shining a laser beam through the edge of an upper sheet of plastic and onto the joining edge of a lower sheet, which has had soot particles mixed into it to absorb the radiation – this means that manufacturers are almost always limited to joining transparent plastic to black plastic. Researchers from Germany’s Fraunhofer Institute for Laser Technology, however, have recently developed a method for welding transparent plastics to one another.

While there are transparent alternatives to black soot for absorbing infrared light in plastic, they are expensive, and have a green tint. The Fraunhofer researchers set about doing away with added absorbers altogether, by studying a variety of transparent polymers to determine at what wavelengths they themselves absorbed laser radiation. Once they had their answers, they then developed laser systems that emitted those wavelengths.

“Before, you didn’t have the right light source,” said project manager Dr.-Ing. Alexander Olowinsky. “It was only during the past few years that laser sources have been developed that emit light in these wavelength ranges.” A wavelength of around 1700 nanometers seemed to be most effective.

The lasers’ lenses are also special, designed to reach maximum density at the beam waist – the narrowest point of the beam. By focusing the waist on the weld point, the highest possible temperature can be applied to a very precise location.

Applications for the technology could include the production of microfluidic lab-on-a-chip devices.