Studies on polypropylene blended with inorganic fillers show a more diverse picture. Delva showed negligible differences for the MFR between pure PP and blends from montmorillonite (MMT) and PP until eleven extrusion cycles. In opposite the introduction of MMT and a compatibilizer caused a severe increase of MFR compared to pure PP [33]. On the other hand Wang et al. [52] states that talc as inorganic filler lowers the MFR of polypropylene significantly for different extrusion cycles. They concluded that inorganic talc particles increase the viscosity by hindering plastic flow. Multiple processing steps break talc aggregates and increases particle dispersion which results in amplification of the viscosity increase [52]. Microscopic pictures of the samples of this study also show that aggregates break and iron particles disperse better in the matrix after multiple extrusions (Fig. 3). Nevertheless there seems to be no difference in MFR in T1 and even a higher MFR in T2 for PP_20%SP. Therefore it seems that, if the mechanism of Wang et al. [52] is correct, degradation is more dominant than the viscosity increasing effect of inorganic particles, even after only one extru- sion. It has also to be taken into consideration that the mass fraction of iron powder is clearly lower than the talc-fraction stated by Wang et al. [52].