Research On Themoplastic PP Laser Transmission Welding Technology

With the continuous development of plastic products industry and development requirements of product lightweight, many industries represented by automobile manufacturing industry are on the growing demand for plastic products, especially complex plastic parts. Because of its economic, simple, fast, reliable, both achieving the tight junction of whole or complex structures in process and very suitable for mass production of plastic parts, plastic welding has become the preferred processing methods of connecting thermoplastic parts. Laser transmission welding, as a new kind of welding technologies, make up the shortage of conventional plastic connection means with its unique technical advantages and features, and is becoming a hot research.Firstly this paper introduced reflection, absorption, scattering and transmission effects starting from the optical properties of plastic. The thermodynamic properties of plastics were analysed in depth, including three different mechanical states, flow characteristics of the melt. Based on the principle and process of laser transmission welding, the lap joint form was taken and the key technology and difficulty of welding were also considered. Secondly, experimental scheme was determined, including choosing laser equipment and welding materials, designing welding method and experiment process. Shear strength test and metallographic microscopy after welding were made to study the effects of welding process factors on the welding quality.Compared with the results of high-power fiber laser welding plastic, diode laser with relatively uniform energy distribution was more suitable for plastic welding. Using 808nm diode laser, the effects of welding process factors such as laser power, welding speed, clamping force and absorbent on the PP laser transmission welding quality were discussed. Under a certain laser spot size, an optimal scope of the welding energy (laser power and welding speed ratio)existed to obtain good plastic weld formation. Metallographic microstructure observation showed that the two kinds of PP material crosslinked at the interface and new chemical bond was formed. Good weld joints generally showed a higher shear tensile strength, while the strength of weld joints would be greatly reduced as a result of stress concentrations when decomposition, erosion and cavity appeared in the weld. Maintaining a certain clamping force was a prerequisite to achieve plastic laser welding. No clamping force or relatively small clamping force was liable to occur unbinding, lack of weld or uneven weld. Clearweld absorbent could be used to weld two pieces of transparent PP, but the strength of all joints generally was not very high.