Thermal Degradation Of POM During Laser Transmission Welding

Laser transmission welding(LTW) is widely used in polymer connection. The thermal degradation of materials strongly influences the weld strength during LTW. Weld strength decreases at high temperatures because of material thermal degradation. Hence, study on the thermal degradation of materials has a theoretical and practical application in industry. It lays a theoretical foundation on selecting and optimizing weld parameters during LTW.The most widely used engineering plastic polyformaldehyde(POM) was choosed as experimental material. Weld experiments were conducted for natual POM and black POM. The 0.2wt% carbon black(CB) was contained in black POM. The thermal analysis model based on the volumetric heat source and pyrolysis kinetic model of POM were developed during LTW. And the thermal degradation of POM is predicted by combining the above two models.Firstly, laser transmission welding experiments were conducted for natural POM and black POM with semiconductor laser. The influence of laser power on the weld width and weld strength was discussed under the condition of fixed scanning speed and clamping force. The fracture appearance was analyzed through digital microscope. The weld defects and shear stiffened structures were analyzed. It was found that weld width increased with the increase of laser power, while weld strength shows a trendency of first increase and then decrease with the increase of laser power. It lays a foundation on the comparison between the thermal degradation of POM and weld strength at the different laser power.Secondly, the transmittance of natural POM and the reflectance of black POM were measured by a power meter and a UV-vis-near infrared spectrophotometer, respectively. And the volumetric heat source model was developed using the measured optical data. The thermal analysis model of POM during LTW was developed through loading the volumetric heat source with APDL language in ANSYS. The temperature field of POM during LTW can be simulated by this thermal analysis model. It is found that the simulated weld width has a reasonable agreement with the experimental ones. It indicates the reliability and accuracy of this thermal anslysis model. The temperature-time data of the point located at the maximum temperature was predicted through this thermal analysis model, laying a foundation on the study of thermal degradation below.Finally, DSC experiments and TGA experiments of natural POM and black POM were performed on differential scanning calorimetry and thermogravimetric analyzer. The influences of carbon black on the thermal properties and thermal stability of POM were studied. It was found that the carbon black had little effect on the thermal properties and thermal stability of POM. For black POM, kinetic parameters of POM were calculated through solving the TGA data at the heating rate of 10K/min using an nth order model with Freeman-Carroll method in MATLAB. The pyrolysis kinetic model of POM was developed with those calculated kinetic parameters. It was found that the conversion of POM could be predicted through this pyrolysis kinetic model. The thermal degradation of POM was studied by combining the temperature-time function in thermal analysis model and the pyrolysis kinetic model. It was demonstraed that the predicted power at which POM started to degrade was generally consistent with the power at which weld strength began to decrease. The developed models(thermal analysis model and kinetic model) can predict the thermal degradation of POM, providing a reference on selecting and optimizing weld parameters during LTW.