In plastics industry it is common practice to mechanically recycle waste material arising from productions. However, mechanical recycling affects material inherent structure and consequently its rheological properties and processability; therefore it needs to be quantified. Mechanical recycling of high density polyethylene (HDPE) was simulated by one-hundred consecutive extrusions. During the extrusion also processing parameters, namely melt pressure and torque were recorded. After every cycle, portion of material was removed for the purpose of characterization. After the processing, rheological properties of the material were characterized in terms of measuring melt flow index and viscosity. In addition, gel permeation chromatography and solubility tests were performed in order to study structural changes. The results of rheological measurements, i.e. MFI and complex viscosity, showed unfavourable effect of mechanical recycling on HDPE processability. With increasing number of the reprocessing cycles MFI drops practically to zero after 30th extrusion cycle. Further, viscosity significantly increases between 5th and 20th cycle. Moreover, changes in processability were directly monitored through the melt pressure and extrusion torque. Both values rise in first 30 extrusion cycles, pressure reaches more than 500% in respect to first extrusion. GPC measurements and solubility tests suggest that chain-scission and chain branching occur simultaneously from the start of the reprocessing. Through the first 30 extrusion cycles the molecular structure is being dominated by chain branching, while after 30th extrusion cycle chain scission becomes predominant over the chain branching. After 60th extrusion cycle, the branched molecule chains begin to cross-link. More can be read in |1|. [1] P. Oblak, J. Gonzalez-Gutierrez, B. Zupančič, A. Aulova and I. Emri, "Processability and mechanical properties of extensively recycled high density polyethylene," Polymer degradation and stability, vol. 114, pp. 133-145, 2015.