Absorber-free laser beam welding of transparent thermoplastics

The laser welding of thermoplastics has been established in several industrial applications, but it has not yet been able to weld transparent thermoplastics without an IR-absorber. However, by the use of special optics in combination with a proper wavelength, absorber-free laser beam welding can be utilized in a large number of applications primarily in medical device technology or in high-tech products. The availability of high brilliance fiber lasers with new wavelengths makes these applications possible. The different wavelengths are achieved by doping the fibers with the laser-active elements ytterbium (1060 – 1080 nm), erbium (1530 – 1600 nm) or thulium (1800 – 2100 nm). Starting from 1200 nm the intrinsic absorption of several visually transparent polymers reaches a sufficient value to allow laser welding without any IR-absorber. To generate a heat source in the joining area, the intensity distribution and the wavelength of the laser must be aligned to the absorption characteristics of the polymer. The usage of special optics with high numerical aperture keeps the laser intensity on top of the material and below the melting threshold. Only in the welding area does the intensity reach the necessary value to determine the welding of the transparent components.