Drying angle controlled resin dip coating for surface roughness reduction and wettability tuning of FFF-printed TPU: An RSM approach

Fused filament fabrication (FFF) of thermoplastic polyurethane (TPU) offers significant advantages for flexible and wearable applications, but layer-by-layer deposition leaves a rough surface that limits functional performance. While mechanical, thermal, chemical, and irradiation-based post-processing routes for FFF parts are well documented, the influence of drying orientation during resin coating of flexible substrates remains unexplored. This study introduces drying angle as a post-processing parameter, alongside layer thickness, build orientation, raster angle, and infill density, in a 32-run face-centered central composite design (α = 1) for epoxy dip-coated FFF TPU. The quadratic model for roughness reduction (R 2 = 0.9324, adequate precision = 13.43) identified layer thickness, drying angle, and build orientation as the significant factors. RSM identified an optimal parameter combination of (0.2 mm, 0°, 135°, 20%, 0) ° for maximizing the roughness-reduction response (ΔRa). Drying angle controlled the final surface; all specimens dried with the coated face upward (0°) reached Ra of 0.04–0.29 µm (98.8%–99.8% reduction from 4.9–32.6 µm as printed) regardless of the other four factors, whereas specimens dried at 90° and 180° averaged 5.4 and 6.6 µm. The best vertical-drying condition (0.6 mm, 30°, 90° raster, 60% infill, 90°) gave Ra of 2.5 µm (91.1% reduction). FE-SEM, contact-angle, FTIR, and DSC analyses indicated that the resin fills the interlayer valleys and forms a continuous surface layer, shifting wetting from a Cassie–Baxter-like to a Wenzel-like regime, without degrading the chemical or thermal integrity of the TPU. These findings establish drying-angle-controlled resin dip coating as a simple, low-cost route for smoothing FFF-printed flexible polymers, with relevance to prosthetic sockets, wearable interfaces, and other engineering applications.