The production of physically-foamed panels, made of semi-crystalline polymers in an extrusion process, leads currently to problems. The dissertation introduces an extrusion process for physically-foamed panels of polypropylene and CO2 and presents investigations of the process and the cell nucleation control and of the foamability of different material blends. In this project the effect of the process parameters pressure drop, pressure drop rate and mass temperature on cell nucleation were studied on a laboratory extrusion line at the IKV Leoben using different single hole nozzles. The obtained foam structures were characterized by their density, cell size, cell size distribution and mechanical features. The foam structures were analyzed by scanning electron microscope pictures. The melt strength and extensibility of blends of polypropylenes with different molecular weight were analyzed by using the rheotens test. A correlation of the results with the foamability of the blends was found. An important aspect of the project was to use an environment-friendly foaming agent. CO2 fulfills this requirement. The loss of shear viscosity by adding CO2 to the polypropylene melt was investigated by using an extrusion bypass rheometer (online rheology). The nucleation mechanism in PP/CO2 melts was studied and the arising phenomenon was described. In tests with different nucleating agents the microspheres type Expancel 950MB120 showed the best results. The foaming process could be handled more easily, the gas loss could be reduced and small cell sizes and high expansion rates could be achieved at the same time. Moreover, foamed panels nucleated with microspheres and produced by using a multi-hole nozzle showed the best mechanical features. In sandwich panels with aluminum surface layers a 20 % higher bending strength could be achieved. The shear and the compression modulus were investigated by performing bending tests according to EN ISO 178, compression tests according to EN ISO 604 and by using the calculation model of the sandwich theory.