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.

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“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.