Impact of Ironing on Structural Integrity and Piezoresistive Properties of 3D Printed Sensor

Fused Filament Fabrication (FFF), a Material Extrusion (MEX) technique, offers a practical platform for the fabrication of integrated polymer-based strain sensors; however, sensors based on Carbon Nanotubes (CNTs) typically exhibit limited gauge factors (GF) due to their highly percolated and mechanically robust conductive networks. To increase strain sensitivity, this study investigates a hybrid CNT–Carbon Black (CB) conductive system combined with a surface postprocessing strategy. A hybrid nanocomposite filament containing 3 wt.% CNT and 1 wt.% CB within a polyetherimide (PEI) matrix was developed to form a conductive network in which CB particles may contribute to the strain sensitivity of CNT-based pathways. An ironing process was applied as a surface post-processing step using zigzag and concentric toolpath strategies, while all other processing parameters were kept constant. Electrical measurements show that ironing increases the average baseline resistance from 1.54 × 10⁶ Ω to the order of 10⁷ Ω, suggesting a partial reorganization of the conductive network. Despite this increase in resistance, the piezoresistive sensitivity improves substantially. The average GF rises from 1.30 ± 0.14 for nonironed hybrid samples to 3.67 ± 0.33 for concentric ironed sensors. These results indicate that the combined use of hybrid CNT–CB fillers and concentric ironing provide an effective route for enhancing the strain sensitivity of FFF-fabricated polymer composites for potential aerospace sensing applications.