Laser Sintering (LS) is widely used to produce plastic parts with complex designs, for prototyping or custom low volume production. But achieving good and reproducible mechanical properties, suitable for end-use parts, is still a challenge. The aim of this work is to better understand how the physical mechanisms of the material transformation influence the final part properties. LS parts are known to exhibit less ductility than their injection-molded counterparts. The crystalline microstructure is not the only feature affecting the mechanical properties. The degree of “welding” between powder particles and the residual porosity certainly have a major role, too. A variety of parameters influence the parts final microstructure, especially the LS machine settings and the physical features of the material. It is well known that heat control throughout the entire processing is crucial in determining the parts final properties. Previous work has shown some relations between the variables of the laser exposition, part microstructure and mechanical properties. The present study addresses specifically the phenomena occurring in the molten phase, which contribute to the consolidation and densification of the layer prior to its solidification. In a semi-crystalline polymer powder under laser exposure, particles first melt, then viscous coalescence occurs, followed by a densification step through gas bubbles removal. The anisotropy of mechanical properties in LS parts suggests that macromolecular diffusion is a key phenomenon to the bulk formation. In this work, a PA12 powder was chosen to investigate the polymer interdiffusion dynamics through interfaces and the bubble resorption through gas diffusion in the melt. These results allow to better understand how material characteristics and process conditions can affect the bulk formation and the resulting microstructure in order to correlate with mechanical properties.