Laser welding

Welding is one of the most  used techniques in joining hollow parts. Besides this technique exist more than fifteen separately techniques for welding thermoplastics, some of which have been commercially available. These include manual processes such as hot gas welding and extrusion welding, processes using vibration and frictional heating between the materials such as ultrasonic, linear vibration welding and processes using an electromagnetic heat source such as resistive implant welding, dielectric welding and welding laser. 

Welding laser is a technology suitable for joining sheet, film, molded thermoplastics and textiles, using for that a laser beam to melt the plastic on the  region where the laser beam passes. This process can be done by several lasers, like  Diode, fiber, Nd:YAG and CO2. the most common processes are welding by transmission or direct. CO2 laser is used mainly for doing direct welding, because the beam is absorbed rapidly by the polymer surface. for that reason this laser is be used more for cutting or film welding. the other laser are use to weld by transmission, because the polymers are transparent to the wavelength of beam emitted by the lasers. 

The main advantages of laser welding is the non-contact, non contaminant process, flexible and easy to control, automate and with no thermal stresses, small stresses and low particle release. This reduced stress into the joining materials due to the highly localized energy deposition. This process produces also minimum heat affected zones that are extremely important for the biological probes handled within the microfluidic biochips, where typically, the thermal gradients should not exceed 45ºC –70ºC. Process is mainly based on through-transmission laser welding of polymeric materials. 

The welding is made by a focused laser beam directed to two overlapping thermoplastic parts. The first part is designed as transparent to the laser wavelength and the second part is designed as absorbent in the infra-red spectrum. Laser–polymer interaction, happens in the wavelength range of  0.830–1.064 μm, this welding is induced only by thermal effects, this mean that heat is transfer essentially by heat conduction between the absorbing and the semi-transparent plastic part. During the laser propagation process through the part three possible interactions can happen, the light can be reflected, absorbed, or scattered. Normally less then 5% of the energy of the laser beam is reflected by the transparent part.  

figure: laser welding