A detailed description, based on the literature, of the dependence of the foaming process on the type of applied polymeric materials with special reference to polyolefins (PE and PP) was initially presented in the introductory section of this paper. The scope of the work involved comparative studies on the foaming process of waste PE-LD and PP (linear and branched) performed with the aid of chemical foaming medium, and that of particular virgin polymers. Cell composites comprising of PP filled with wood flour (filler-polymer weight ratio = 30:70) were also manufactured. A description of the applied foam extrusion system (Fig. 1) and an evaluation of the thermal decomposition of the blowing agents (Figs. 2 and 3) was made. The dependence of the structure of the polymer and composites obtained in the foaming process on the various reactive extrusion conditions such as the presence of talc as an auxiliary medium, pressure, screw rotational speed, the application of a calibrator, the position of sampling for studies (outer part or middle) or the introduction of foam screen was also determined (Figs. 4-10,13,18-22). The viscosity curves (dependence of viscosity at 190 °C on the strain rate), melt viscosity of the polymers and composites (Figs. 11,14,16) were discussed. Some applicability-related parameters of PP in its bulk and foamed composites such as density, stress, heat distortion temperature (HDT) and thermal conductivity (Figs. 23-25, Table 1) were compared. In the presented studies, porous low-density (0.5-0.6 g/cm³) materials with good heat insulation properties suitable for applications especially in transport and construction were obtained.