| This study describes a systematic approach for characterizing the microstructure formation during the hot plate welding process. The properties, such as viscosity, stress relaxation modulus, relative crystallinity, of PP were characterized by experiments and also correlated by proper models. The quiescent and flow induced crystallization model which covered completely melted and partially melted regions was developed. This three-zone crystallization model is suitable for the processes consisting of completely melted and partially melted polymer.;The microstructures of the PP welded samples were revealed through optical microscopy. The micrographs of the welded samples in Heat Affected Zone showed strong orientation at HAZ boundary. These spherulites at the HAZ boundary were deformed and might crystallize under the influence of the stress. However, the spherulites at the joining interface keep the same round shape without deformation. It suggests that the spherulites at joining interface crystallize under no stress. The proposed model could predict this phenomenon and also provide a reasonable explanation.;A computer simulation based on control volume finite element method was adopted to model the heating, squeezing, and cooling during the hot plate welding process. The measured viscosity model, stress relaxation model and crystallization model were implemented into the simulation to determine the microstructure formation. The computer simulation can determine the material distribution, temperature profile, crystallization rate, and stress profile after joining. Consequently, the simulation could predict the stress affected microstructure formation and the size of HAZ. However, this simulation would not be able to provide the information of spherulite size and shape. The model for determining the shape, density and size of spherulite still need further investigation.;A diffusion time based model to correlate joining strength with welding parameters was proposed. It is only a preliminary result. Further investigation and verification on this aspect is necessary. |
