For the injection moulding of thin wall parts with wall thicknesses of less than 1 mm and flow length/wall thickness ratios up to 400:1, an innovative injection moulding technique called expansion-injection moulding (ENGEL X-Melt) was developed by ENGEL Austria GmbH, Austria. The main feature of this process is the separation of the melt compression and injection phases. First, the melt is compressed to a maximum compression pressure in the space in front of the injection screw. When the pressure is relieved, the melt will expand explosively inside the cavity, thus filling the cavity at a high volume flow rate. To reduce development times in the field of expansion injection moulding it is intended to use simulation programs. For this reason, it is necessary to develop suitable calculation software based on a physical model for this technique. Within this research work, a physical model for the simulation of the expansion phase was developed. Based on the physical model, a calculation program called Xmeltsoft V.1.0 was developed for the simulation of the expansion injection mould process for simple thin-wall parts. The calculated compression pressure can be set on the machine. However, in contrast to the compression pressure, the geometry and thickness of the mould cavity and the viscosity of the chosen thermoplastic material influence the end pressure. During the expansion phase of injection moulding, the pressure in the injection unit drops from a pre-set compression pressure to an end pressure. The methodical approach for the calculation of the minimum required filling pressure involves segmentation of the longest flow path into several serially connected small segments. To improve the calculation results, the effect of the frozen skin layer thickness is taken into account. For the purpose of verification, a check card part with a wall thickness of 0.5mm was chosen for the simulation. The check card parts were produced by the expansion injection moulding process using a hybrid Engel injection moulding machine (Type VC 940/130 Electric). The experimental results in comparison with the simulation results of the check card part are presented for a PP, PS and ABS/PC materials. Additionally within this research work, molar mass degradation analyses were done on the moulded parts in order to assess the influence of the processing parameters on material degradation. For the degradation analyses, the samples were collected from different sections of the moulded part. The molar mass analyses were carried out using Size Exclusion Chromatography (SEC) at the Institute of Chemistry of Polymeric Material, Montanuniversitaet Leoben. The material degradation analysis were done on the moulded parts with materials of PP and PS.