## 3 Results and Discussion
### Influence of Process Parameters on the Resistivities of Printed Parts
In addition, the air gap also had a significant impact on the resistivity. It is obvious that, with the increasing air gap, the void area was enlarged and this increased the resistivity in the vertical direction. Comparing Figure 3a,b, it is shown that the influence of raster width was more significant than that of the air gap, maybe indicating the effect of the contact condition was greater than voids in the vertical direction. The simulation results for the influence of the layer thickness and the air gap on the resistivity in the horizontal direction are shown in Figure 3c,d. We can see that increasing the layer thickness and air gap enlarged the void area. As the current was passed along the fiber in the horizontal direction, the bonding condition between adjacent fibers did not significantly affect the resistivity. It appears that the voids were the main reason that impacts resistivity in the horizontal direction. Figure 3: Resistivity of the simulated structure similar to the printed parts as a function of the different process parameters. In the figures, the meaning of the color bars has been illustrated in Figure 1. (**a**) Resistivity in the vertical direction for raster widths of 0.3, 0.4, and 0.5 mm; (**b**) resistivity in the vertical direction for air gaps of 0.00, 0.04, 0.08 mm; (**c**) resistivity in the horizontal direction for layer thicknesses of 0.14, 0.20, and 0.26 mm; and (**d**) resistivity in the horizontal direction for air gaps of 0.00, 0.04, and 0.08 mm. For the further investigation of the relationship between the resistivity of 3D-printed parts and their structural properties affected by the process parameters, scanning electron microscopy (SEM) was used to analyze the structural features of the printed parts as shown in Figure 4. We observed from the SEM images that the shape of deposited fibers from different process parameters was similar to that employed in the simulation models, namely, fibers with across-sectional area bound by two straight parallel sides and two rounded sides. Specifically, for images of different layer thicknesses, the void area increased and the number of layers decreased with the increase of layer thickness as indicated in Figure 4a-c. For images of different raster widths, it can be observed that the shape and area of the voids resulting from different raster widths was almost the same and that the bonding area increased with increasing raster width as shown in Figure 4d-f, and both of these observations were the same as the simulation model predictions. For images of different air gaps, the voids significantly increased with the air gap as given in Figure 4a-c, which must induce the increase of resistivities in two directions. A feature of the actual internal structure of printed parts that was different with the simulated structure was that the actual bonding condition was not perfect between adjacent fibers, as shown in Figure 4. The imperfect bonding condition must induce the arising of the contact resistivity between adjacent fibers and increase the resistivity of the printed parts, especially in the vertical direction. In the simulation analysis, when a contact resistivity existed, the decrease of the resistivity in the vertical direction was more substantial with the increasing raster width, as shown in Figure S4c, which can  
Figure 4: SEM images of the internal structure of 3D-printed parts for different parameter values. (**a–c**) Cross-sections in the vertical direction for layer thickness 0.14, 0.20, and 0.26 mm, respectively; (**d–f**) cross-sections in the vertical direction for raster widths 0.3, 0.4, and 0.5 mm, respectively; and (**g–i**) cross-sections in the vertical direction for air gaps of 0.00, 0.04, and 0.08 mm, respectively; in (**e**), it is indicated that the voids existed between adjacent fibers although the air gap was set to zero and the bonding conditions between fiber layers were always imperfect. In addition, the bonding areas between fibers were always less than raster width.