## 3 Results and Discussion
### Influence of Process Parameters on the Resistivities of Printed Parts  
The influence of three process parameters, i.e., layer thickness, raster width, and air gap (as shown in Figure 2a), was investigated by a three-level, three-factor factorial design. The resistivities of the samples (frequency = 1 kHz) in the vertical and horizontal directions were measured and shown in Figure 2b. The resistivity was higher in the vertical direction than in the horizontal direction for all of the samples. Compared with the resistivity in the horizontal direction, the resistivity in the vertical direction was more readily affected by the process parameters. Specifically, the maximum resistivity in the vertical direction was more than two times greater than that of the minimum value. Data on the resistivity in the vertical direction were further analyzed using statistical software to determine the importance of each factor. Figure 2c shows the effect of each independent variable (\(A\)-\(C\)) on the resistivity in the vertical direction of the samples, namely average values and standard deviations. Among the three parameters, the effect of \(B\) (raster width) was much greater than those of \(A\) (layer thickness) and \(C\) (air gap). With increasing raster width, the resistivity in the vertical direction decreased significantly. With increasing the air gap, the resistivity increased slightly, and the effect of the layer thickness on the resistivity in the vertical direction was not obvious. Moreover, the optimum \(A\), \(B\), and \(C\) showing the lowest electrical resistivity in the vertical direction was \(A1B3C1\) (layer thickness: 0.14 mm, raster width: 0.5 mm, air gap: 0.00 mm). Figure 2: Effect of parameters on the resistivity of printed parts. (**a**) Illustration of the process parameters, including layer thickness, raster width, and air gap; (**b**) resistivities of printed parts in the vertical and horizontal directions for different parameter combinations from factorial design; (**c**) main effects on resistivity in the vertical direction, including average values and standard deviations; and (**d**) main effects on resistivity in the horizontal direction. The effect of the main factors derived from the statistical analysis on the resistivity in the horizontal direction is presented in Figure 2d. \(A\) (layer thickness) exerted the greatest effect on the resistivity of the samples, with the resistivity in the horizontal direction increasing with layer thickness. \(C\) (air gap) also evidently influenced the resistivity, with the resistivity in the horizontal direction increasing with the air gap. In addition, the effect of \(B\) (raster width) on resistivity in the horizontal direction was not significant, and the optimum \(A\), \(B\), and \(C\) showing the lowest electrical resistivity in the horizontal direction was \(A1B2C1\) (layer thickness: 0.14 mm, raster width: 0.4 mm, air gap: 0.00 mm). To further explain these findings, we conducted simulation experiments to clarify the relationship between the resistivity and internal structure for simulated parts similar to the printed ones. The influence of raster width and air gap on the resistivity in the vertical direction and layer thickness and air gap on the resistivity in the horizontal direction was examined as the effect of layer thickness on the resistivity in the vertical direction and raster width on the resistivity in the horizontal direction was not obvious. The result of the simulation examining the influence of the raster width on the resistivity in the vertical direction is shown in Figure 3a. As the resistivity of the fibers remained the same in the models with different raster widths, the differences between these models were the structure. For different raster widths, the void area was the same between parallel fibers, and the difference was the bonding area between fiber layers. It appears that with increasing raster width, the bonding area increased, and this decreased the resistivity in the vertical direction.