This study aims to propose a novel dual-nozzle fused deposition modeling (FDM) approach in which a secondary nozzle is used to fill interfilament air gaps using either the same material (thermoplastic polyurethane [TPU–TPU]) for the single-material case or a different, compatible material (TPU–PLA, TPU–ASA and TPU–ABS) for multimaterial cases. The objective is to mitigate the adverse effects of the inherent layered structure of FDM-printed parts, which introduces voids between filaments and layers, increases porosity and reduces mechanical strength.
The effectiveness of both single- and multimaterial gap-filling strategies is evaluated in terms of stiffness and strength. Classical Laminate Theory (CLT) is used to assess the elastic behavior, while finite element analysis (FEA) of a representative volume element (RVE), using Dirichlet boundary conditions, is conducted to evaluate the elastic–plastic response. For the single-material case, air gaps between TPU filaments are filled with the same material (TPU). For the multimaterial case, the gaps between TPU filaments are filled with compatible secondary materials (PLA, ASA and ABS). In the composite formulations, TPU is treated as the matrix, while PLA, ASA and ABS are considered reinforcing fibers.
The predicted results indicate a significant reduction in porosity, decreasing from approximately 11% to 4% for a 0.4 mm layer height case. In addition, we found that, As the layer height increases, the porosity percentage remains substantially lower than that of the no-filling baseline case regardless of the printing materials. For example, at a 0.8 mm layer height, porosity is reduced from approximately 21.5% to 8%. This behavior counteracts the typically adverse effects of increasing layer height, where larger voids are expected; in contrast, the proposed gap-filling technique utilizes the increased gap size to accommodate larger gap-filling filaments. The proposed approach also demonstrates strong potential for improving mechanical properties due to porosity reduction. For single-material gap-filling, stiffness is predicted to increase by approximately 18% in the transverse direction (normal to printing direction), 155% in the longitudinal direction (aligned with printing direction) and 119% in shear modulus. For multimaterial gap-filling, where stiffer materials are used as fillers and deposited simultaneously with TPU, stiffness along the fiber direction increases from 12.73 MPa to 134.6 MPa, 95.8 MPa and 103.59 MPa when using PLA, ASA and ABS, respectively. This results in intermediate part stiffness, which can be useful in many industrial and medical applications such as snap-fit components, seals and joints. In terms of strength, gap-filling with PLA filaments improves tensile strength along the fiber direction by approximately 90%, compressive strength by around 70% and shear strength by up to 57% along the 90° plane compared to the no-filling case. When comparing gap-filling with the same material (TPU) versus different materials (PLA, ASA and ABS), the improvement in tensile strength along the fiber direction reaches 43.6%, 40.3% and 40.3%, respectively. At the ply scale, strength enhancements reach approximately 190% for bidirectional configurations and 230% for unidirectional layouts at a 0.4 mm layer height case. The reported results show how implementing higher material strength to fill air gaps of FDM-printed elastic material such as TPU can increase part strength and leads to outstanding part mechanical properties. The effect of printing direction is also investigated, revealing a tradeoff between stiffness and anisotropy, with a 0 / 90/90 / 0 ply orientation identified as optimal for in-plane stiffness.
This work demonstrates, through theoretical and numerical modeling, the potential of a dual-nozzle gap-filling strategy to significantly improve the mechanical performance of FDM-printed parts by reducing porosity and enhancing both stiffness and strength. This work intentionally focuses on mechanical feasibility and design potential.
