Microstructure and Mechanical Properties of Welded Thermoplastics

Thermoplastics have been used increasingly for automobile components for both interior and under-the-hood applications. The plastic parts are made through various molding process such as compression molding, injection molding and blow molding. For parts with large or complicated geometry, small portions of the part may have to be molded first, then joined together using a welding process. The welded regions usually exhibit inhomogeneous and inferior mechanical performance compared to the bulk regions due to the differences in thermal history.

The microstructures and mechanical properties of welded thermoplastics have been examined using hot-plate welded polyethylene. The specimens are prepared at various thermal conditions to simulate the real welding process. The thermal properties in welds are monitored using DSC (Differential Scanning Calorimetry) and the crystallinities are calculated. The microstructures in welds are examined using optical microscope and TEM (transmission electron micrograph) and smaller lamellar size is observed. The mechanical properties of the weld are examined using tensile test and special-constructed microindentation test. Both tests show that the mechanical properties in the welds are inferior to that in the bulks but they can be improved by proper heat treatments such as annealing.