Processing-Route Dependence and Adhesion of Polymer–Adhesive–Steel Interfaces with Recycled PA6/GF30 Composites Modified with Functional Additives: Injection Molding Versus Large-Format Fused Granulate Fabrication

Large-scale additive manufacturing (LSAM) using fused granulate fabrication (FGF) offers a potential route for the high-value utilization of recycled thermoplastic composites. This study investigated recycled PA6 containing nominally 30 wt.% glass fibers as a candidate feedstock for large-format FGF. An unmodified reference and formulations containing SF315/PTFE, SF315/U15, and MIX-OH were examined. The granulates were characterized using melt mass flow rate measurements, thermogravimetric analysis, and differential scanning calorimetry, while mechanical properties were compared after injection molding and FGF processing. The best FGF performance was obtained for SF315/PTFE (polytetrafluoroethylene) and SF315/U15. Their tensile strengths reached 117.3 and 118.7 MPa, respectively, compared with 56.8 MPa for the FGF-processed reference. Flexural strengths reached 158.8 and 152.4 MPa, respectively, versus 88.0 MPa for the reference. The tensile and flexural improvements produced by MIX-OH after injection molding were not retained after FGF processing, confirming strong processing-route dependence. Preliminary pull-off screening demonstrated that TPU-bonded polymer–steel joints could be produced. Under the investigated conditions, SF315/PTFE and SF315/U15 were identified as promising recycled PA6/GF30 formulations for large-format FGF and further tooling-oriented development.