The Role of Backpressure on Mixing Performance in Injection Molding: Polypropylene/Polystyrene Blends

In conventional thermoplastic processing, polymer blending is typically performed using a two‐step method involving co‐rotating twin‐screw extrusion followed by injection molding. This study explores a novel single‐step approach by utilizing backpressure within the injection molding process itself, thereby eliminating the need for a separate extrusion step. The objective of the research is to determine whether sufficient mixing and homogeneity can be achieved solely through the plasticization phase of injection molding by optimizing backpressure levels. A constant composition of polypropylene (80 wt%) and polystyrene (20 wt%) was used to investigate the influence of backpressure on the morphological, thermal, and mechanical properties of the blends. Various backpressure values (0, 1, 3, 5, and 10 bar) were applied, and the results were compared with a control blend pre‐compounded using a co‐rotating twin‐screw extruder. The results indicate that a backpressure of 5 bar results in optimal phase dispersion and mechanical performance, comparable to that of the extruded control sample. However, higher backpressures (e.g., 10 bar) resulted in particle agglomeration, which negatively affected the properties of the compounds. The study demonstrates that by controlling the backpressure, polymer blends can be effectively produced in a single injection molding step—with potential benefits in terms of energy savings, reduced material degradation, and simplified processing.