## 3 Results and Discussions
The decrease of MFI values could be attributed to an increase of composite viscosity, induced by the formation of a nanofiller network. In practice, the low MFI value filament materials lead to many difficulties during the FDM feeding and extrusion processes; therefore, high MWCNT content composites are not favorable for commercial applications. FESEM measurement was conducted to analyze the surface fractures of the broken printed parts and the morphology of MWCNTs distributed within the samples. Figure 6 presents an FESEM image of the destruction surface of an FDM composite sample after the tensile test. A layered structure is revealed, along with the presence of voids between layers and segments. Figure 5: Melt flow index of composites as a function of MWCNT fraction at 230 \({}^{\circ}\)C with applied load of 2.16 kg. Figure 6: FESEM image of the fracture surface of the printed part with 2 wt% MWCNT content. The fracture morphologies under 1.0 wt%, 2.0 wt%, and 3.0 wt% content of ABS-MWCNT composites, as depicted by FESEM measurements, are presented in Figures 7a, 7b and 7c, respectively. The 1.0 wt% MWCNT sample exhibited a typical fracture surface with shear lips and few dimples, which means the sample retained high ductility during the tensile test, compared to the ABS polymer. In Figure 7b,c, with higher MWCNT content, a more considerable degree of shear lips can be observed. Although the addition of the MWCNT filler could enhance the tensile strength of the ABS polymer by over 40%, as mentioned in the previous part, as the increase was not significant for concentrations above 2.0 wt%, adding an appropriate amount of nanofillers to the matrix is essential. Importantly, voids and crack tracks were generated more significantly in the surface of higher MWCNT content samples, revealing the optimum loading content of MWCNTs in the ABS matrix as 2.0 wt%. The pull-out of the MWCNTs from the fracture surface, shown in Figure 6 at the selected regions, is presented more clearly in Figure 8a,b, having a higher magnification. This observation confirmed that the dispersion and distribution of MWCNTs in the matrix were better with lower MWCNT content, resulting in an improvement of the tensile strength. However, it was observed that the dispersion of MWCNTs was not perfect with higher MWCNT content loading into the ABS matrix: they were aggregated and pulled out by the tensile stress, as shown in Figure 8c,d. The agglomeration in 2.0 wt% and 3.0 wt% samples was more severe than in the case of 1.0 wt% MWCNTs, and the tensile strength no longer increased for the 3.0 wt% sample. The presence of MWCNT agglomerates in the matrix demonstrated the poor separation effect during the production and printing of the  
Figure 7: FESEM micrographs (magnification: x20,000) of the fracture surfaces of printed samples with MWCNT content of: (**a**) 1 wt%; (**b**) 2 wt%; and (**c**) 3 wt%. ABS-MWCNT composite filaments. In addition, the agglomeration of MWCNTs reduced the adhesion level between MWCNTs and the ABS matrix, due to the lower degree of interfacial contact between them. Furthermore, the aggregation of MWCNTs could result in cracking when tensile strength is applied to the sample. Therefore, in this study, we suggest maintaining the content of MWCNTs at 2 wt% or less in the composite filaments.