Foam extrusion offers the possibility to realize products with reduced weight and material costs. Besides this, it is possible to realize unique product properties such as low thermal conductivity, low dielectric constant and improved impact properties by foam extrusion. However, it is necessary to understand the nucleating mechanism at the best to achieve these properties. Recently, a new nucleation theory was published in the literature. This theory describes the important influence of the flow conditions on the nucleation rate of the bubbles. It was shown that the nucleation rate increases with increasing shear and elongation rate. Both, shear and elongation rate causes an orientation of the polymer chains, which leads to a reduced solubility of the blowing agent. However, it is important that these orientations or in other words deformations relax more or less quickly. The time, which is needed for the relaxation of the deformations, is determined by the viscoelastic properties of the polymer melt. This thesis was carried out to analyse the influence of the relaxation behaviour, respectively the relaxation time of the polymer melt, on the foam morphology (mean cell diameter, mean cell density and density). Therefore material formulations with different contents of two polymers were used (branched and linear polyethylene) to vary the rheological properties over a wide range. By the use of a literature research, various methods have been found, which enable the determination of the viscoelastic behaviour of the melt. As the rheological studies have shown, the best method to determine the relaxation time, is the creep and recovery test. In the next step the material formulations were foamed with three different round nozzles. It was shown, that there is a significant impact of the viscoelastic properties of the melt on the foam morphology. The experiments have shown, that the nucleation rate increases and the mean cell diameter decreases with increasing relaxation times. However, this is limited by the occurrence of flow instabilities due to the insufficient relaxed deformation. Therefore, these experiences must be taken into account in the design of nozzles for physical foaming. The comparison of the measured rheological parameters and the foam morphology has shown, that the creep and recovery test and the parameter b from the cross model could be used for the development or improvement of material formulation for physical foaming.