Processing by injection is the technological process by that the thermoplastics material is injected, under pressure, in the cavity of a mould, where it cools down and solidifies. This process is the most common method for obtaining plastic materials. Injection moulding of thermoplastics has emerged as the premier vehicle for delivering high quality, value added commercial products. Continued global competitiveness has increased standards for product capability and quality while requiring reduced product development time and unit cost. Despite advanced design methods and new process technologies, it is becoming apparent that the injection moulding process is neither flexible nor robust. This paper presents a set of experiments that focused on particular processing conditions of injection through narrow section, thin-wall injection and microinjection. In these cases, the ultrasonic activation does not play an important role as single influence factor but could amplify or strengthen the influence of classical setting parameters of the process: mould temperature, injection pressure and temperature