Optimizing Screw Design for Controlled Filler Dispersion and Distribution in Polyamide66/Silica Composites: An Integrated Simulation–Experimental Study

The influence of screw design on filler dispersion, distribution, and resulting properties of PA66/silica composites was systematically investigated using corotating twin-screw extrusion. Screw configurations were varied by altering the position of the neutral kneading element within a kneading zone, as well as the number and axial placement of kneading zones along the screw. A combined approach of Ludovic process simulation, scanning electron microscopy with energy-dispersive X-ray mapping, differential scanning calorimetry, X-ray diffraction, rheology, and mechanical property evaluation was utilized to establish quantitative process–structure–property relationships. Simulations revealed that changing the neutral kneading element position had minimal impact on global processing parameters such as residence time, specific mechanical energy, and cumulative strain, whereas increasing the number of kneading zones, particularly with an early stage (‘f’) position, substantially elevated these metrics, indicating higher dispersive mixing ability. Based on the morphological analysis, the KN3-fme design (featuring a greater number of kneading zones with optimized placement) produced the smallest filler agglomerates (Dmax = 4.56 μm) and a more spatially uniform distribution, as indicated by the lowest VMR (VMR = 1.09) (Dmax: KN1-f = 19.29 μm → KN3-fme = 4.56 μm; VMR: KN1-f = 1.43 → KN3-fme = 1.09). These microstructural improvements correlated directly with thermal and mechanical performance. Well-dispersed fillers can act as effective nucleating agents, increasing the degree of crystallinity (Xc) up to 46.1% (Xc: KN1-m = 34.6% → KN3-fme = 46.1%). Balanced dispersion and distribution also improved mechanical properties; notably, KN3-fme, which exhibited the most uniform dispersion, achieved the highest tensile strength of 83.2 MPa (tensile strength: KN1-f = 69.9 MPa → KN3-fme = 83.2 MPa). The results demonstrate that optimal screw designs require a balance between dispersion and distribution rather than maximization of dispersion alone. Early stage kneading must be coupled with sufficient downstream conveying length to achieve uniform filler spacing and superior property profiles. This integrated experimental–simulation study provides a practical guideline for designing extrusion screw configurations to tailor the performance of engineering polymer composites.