Research On The Technology Of Laser Transmission Welding Of Polypropylene

With the rapid development of plastic product and its increasing trend on lightening,there is a growing demand in many industries for complex plastic parts. At the same time,the joining of complex plastic structure has become a big difficulty. Nowadays, laserwelding of thermoplastics has drawn a lot of attention because of its advantages, such asexcellent welding quality, high efficiency and easy to control, which can overcome theshortages of traditional plastic joining methods.Firstly, this paper introduces the mechanism of laser transmission welding on plasticon the basis of its optical, thermal and physical properties. Then experiments onopaque-transparent (black-white) and transparent-transparent (white-white) polypropylene(PP) are conducted respectively.For the opaque-transparent PP, we gained perfect welding quality. The surface ofwelds is good and with defects, all welds fractured at the base metal. Study found that aclamping force is necessary to obtain high quality in plastic laser welding. Weldingparameters have great influences on the quality. If the energy is too small, little materialcan melt which means poor welding and low shear strength. If the line energy is too large,the material will decompose which means low shear strength too. In addition, orthogonaltests show that the order of processing parameters affecting welding quality is laser power＞welding speed＞spot size, and the optimal parameter is laser power80W, welding speed2mm/s, focus spot diameter0.4mm for our experiment.Transparent-transparent plastic laser transmission welding is a big difficulty in thefield of plastic laser welding and the key to solving this problem is absorbent. This studytried different absorbents to conduct transparent-transparent plastic laser welding, whichturns out that black paint can be used to gain good welding quality. We studied theinfluence of welding parameters, including the thickness of the material and the blackpaint layer. Results show that shear strength increses with increasing of the laser power orreducing of the welding speed and then reduces again when the line energy is too large. Ifthe the material is too thick, its transmittance to laser will decrease and leads to poor welding quality. In this experiment, double-layer paint has better welding quality thansingle-layer paint. Meanwhile, defects such as porosity, thermal decomposition, surfaceroughness appeared in the welding process and measures such as using argon to shield,choosing appropriate process parameters and using higher purity raw material ofpolypropylene can be taken to controll the defects.