Investigations on the Interplay Between Focusing and Absorption in Absorber-Free Laser Transmission Welding of Plastics

Purpose: Lab-on-a-chip devices, where small channels and reaction chambers have to be covered with a sealing lid, laser beam welding has already been proven to show considerable potential for standard material combinations. To avoid absorbing additives, the wavelength of the laser has to fit the absorption bands of the polymers. For it to achieve a localized weld seam, the laser beam has to be strongly focused. Methods: To achieve the desired temperature distribution, we establish and discuss the heat rate generated by a focused beam as a function of the absorption coefficient and the Rayleigh length. Thermal simulation of the weld process reveals the dependence of the temperature on Rayleigh length and absorption coefficient. Experiments were carried out with a tunable laser source emitting between 700 and 1000 nm on polycarbonate samples doped with Lumogen 788 dye. Microtome cuts of the welded samples reveal the dimensions of the heat-affected zone and, therefore, of the seam. Results: Experimental and simulation results show that for typical welding conditions a localized seam inside a sample can be achieved with an appropriate focusing of the laser beam and tuning of the wavelength. The dimensions of the heat-affected zone decrease as absorption coefficient decreases and welding velocity increases. The discussion of the functional dependence of the heat rate on absorption coefficient and Rayleigh length shows that Rayleigh length and sample thickness have to be smaller than the optical penetration depth. © 2019, Springer Science+Business Media, LLC, part of Springer Nature.