Influence of die geometry on the cellular structure and expansion behavior of extruded HDPE foams using CO 2 blowing

Abstract

It is well known that polymeric foams, as a significant class of engineering materials are of growing importance due to their unique properties. In this study the role of die geometry in shaping the microstructure of extruded HDPE/talc foams was investigated by establishing a correlation between flow dynamics and morphological evolution. The extrusion process was conducted using three dies with nozzle diameters of 1, 2, and 3 mm at die temperatures of 130, 133, and 136°C, utilizing CO 2 as the physical blowing agent. Results revealed that die geometry significantly modulates the melt’s rheological behavior, the 2 mm die (L/D = 5) provided an optimal balance between a sufficient pressure drop rate (≈25 bar/s) and an intermediate apparent viscosity (between 18,072 to 22,200 Pa.s). This configuration at 130°C yielded the best structural characteristics, including a foam density of 0.420 g/cm 3 an expansion ratio of 2.25 and a cell density of 1.9 × 10 5 cells/cm 3 . Compared to the 1 mm die, the foam density decreased by 19.23%. While the 1 mm die offered the highest pressure drop rate (≈87 bar/s) its performance was limited by excessive back-pressure exceeding the gas injection threshold, which hindered gas entry and reduced solubility. Conversely the 3 mm die caused structural deterioration due to a low pressure drop rate and excessive melt viscosity. Achieving desirable foam structures requires a precise synergy between die-induced thermodynamic instability and rheological resistance. Ultimately, optimizing die geometry and temperature is crucial to ensure sufficient gas entry and maintain the melt strength necessary for superior microcellular morphology.