The paper presents a set of experiments focused on the study of rheological behavior of a polymer flowing through a narrow section at the forming by injection of thin-walled plastic parts. The paper addresses the use of ultrasonically activated injection for fabrication of polymeric parts with thin wall features. In the experiment, a part with six different geometric features has been created. The design-of-experiments approach is applied to correlate the quality of the parts with the processing parameters. Four processing parameters are investigated using a screening factorial experimentation plan to determine their possible effect on the filling quality of the moulded parts. The experiments have been conducted on a hot runner mould with two nests in which the final (nest) nozzle has been modified to host, as the central element, the ultrasonic horn of a sonic system. It has been found that the ultrasonic activation applied on the active part of the mould does not play an important role as a stand-alone factor but could amplify or strengthen the effect of classical setting parameters (and influence factors) of the process: the melt temperature and injection pressure. Because it is easier to stimulate and to control rheological properties of the melt by setting the intensity of ultrasonic energy and, more important, the effect is forthwith, the paper recommends the runner systems with ultrasonic activation as an alternative for the hot runner with heating elements.