Design, Modeling, and Validation of a Compact, Energy-Efficient Mixing Screw for Sustainable Polymer Processing

This study presents the design, modeling, and validation of a mixing screw for energy-efficient single-screw extrusion. The screw features a short length-to-diameter (L/D) ratio of 8:1 and incorporates double flights with variable pitch and counter-rotating mixing slots. These features promote enhanced plastication by breaking up the solid bed and improving thermal homogeneity through backflow mechanisms relieving a 3.75 compression ratio. Non-isothermal, non-Newtonian simulations modeled the thermal and flow behavior for high-impact polystyrene (HIPS) and recycled polypropylene (rPP) under various operating conditions. Experimental validation was conducted using a 20 mm pilot-scale extruder with screw speeds ranging from 10 to 40 RPM and barrel temperatures of 220 °C and 240 °C. Results showed a strong linear dependence of mass output on screw speed, with maximum mass throughputs of 0.58 kg/h for HIPS and 0.74 kg/h for rPP at 40 RPM. Specific energy consumption (SEC) was calculated as 0.264 kWh/kg for HIPS and 0.344 kWh/kg for rPP, corresponding to efficiencies of 31.5% and 56.5% relative to theoretical minimum energy requirements. Compared to traditional general-purpose and barrier screws with L/D ratios of 27:1, the mixing screw demonstrated improved energy efficiency and reduced residence time distributions. These findings suggest the potential of the mixing screw for compact extrusion systems, including 3D printing and other sustainable polymer and bioplastics processing applications.