Nonwoven rCF/PA6 Thermoplastic Composites: Hybrid Commingled Fibers vs. Powder Impregnation—Effects on Microstructure and Mechanical Properties

Fiber-reinforced thermoplastic composites are a promising sustainable alternative to thermoset-based ones, combining recyclability with reduced cycle times. This study compares two impregnation strategies for polyamide 6 (PA6) composites reinforced with recycled carbon fiber (rCF) nonwoven fabrics: powder impregnation (PA6/rCF_p) and hybrid commingled fibers (PA6/rCF_hy). Four-layer laminates were produced by compression molding at two temperatures (270°C and 285°C) and two pressures (5 and 10 bar). For PA6/rCF_p, pressure was the dominant processing variable: increasing it from 5 to 10 bar significantly improved both Young's modulus and tensile strength (p < 0.01), reducing void content from ~11% to ~5%, while temperature had no significant effect. Optimal conditions (285°C, 10 bar) yielded E = 18.3 ± 1.7 GPa and σu = 286 ± 38 MPa. PA6/rCF_hy composites exhibited substantially lower performance (E = 11.7 ± 1.2 GPa; σu = 119 ± 21 MPa) and persistently high void fractions (15%–26%), attributed to the spatial inhomogeneity of the hybrid precursor. A modified rule of mixtures with a quadratic void correction (1 − vv)2 accurately predicted powder composite modulus and tensile strength within ±6%, with orientation efficiency factors for stiffness (η0 = 0.334) and strength (η0,σ = 0.343) closely agreeing, confirming that the same fiber-orientation architecture governs both responses. Applied to the hybrid system, the model captures the overall performance level but with larger scatter, indicating that the combination of high, pressure-independent porosity and precursor inhomogeneity limits the predictive capacity of global mixture models. These findings identify precursor homogeneity as a critical prerequisite for effective consolidation of commingled nonwoven architectures.