## 3 Results and Discussions
### Influence of the Process Parameters on Tensile Strength  
The interaction influences of each input parameter on the tensile strength are shown on the 3D response surface plots (Figure 4a-f). It can be concluded that the most significant influence on the tensile strength is contributed by the layer thickness and raster angle. For parts with a 100% infill structure, the wall thickness has no significant effect on the tensile strength. Slight increases in tensile strength values were observed with an increase in the wall thickness (Figure 4c,e,f). The results of the tensile strength tests as well as the generated surface plots clearly show that the analyzed process parameters have a significant influence on the mechanical behavior of polyamide reinforced with short carbon fibers. Based on the experimental data in Figure 5a, it was presented the stress-strain curve for the test specimens with the best and worst conditions of analyzed process parameters. It can be observed that the properly selection of optimal process parameters leads to a significant increase in the tensile strength. The range of maximum stress varies from 45.77 MPa (sample 19 from Table 4) to 91.53 MPa (sample 18 from Table 4) for the highest tensile strength value, which is improved by 99.98%. It can also be seen that polyamide reinforced with short carbon fibers shows extremely brittle behavior with very small deformations before fracture, ranging from 2.6% (sample 21 from Table 4) to 5% (sample 19 from Table 4), in contrast to neat polyamide which shows plastic behavior under tensile stresses [14,17]. Hence, it can be concluded that the presence of carbon fibers reduced the ductility of polyamide and improved the tensile strength. Figure 3: Tensile strength: (**a**) normal plot of residuals; (**b**) predicted vs. actual plot. Figure 4: The 3D response surface plot: (**a**) printing speed vs. layer thickness; (**b**) raster angle vs. layer thickness; (**c**) wall thickness vs. layer thickness; (**d**) raster angle vs. printing speed; (**e**) wall thickness vs. printing speed; (**f**) wall thickness vs. raster angle. Figure 5(b) presents the stress-strain curves for three different layer thicknesses at the constant values of other process parameters. A trend of a significant increase in the value of the tensile strength from 45.77 MPa at the layer thickness of 0.3 mm (sample 19 from Table 4) to 73.93 MPa at the layer thickness of 0.1 mm (sample 2 from Table 4) can be observed. Thus, by increasing the layer thickness, the tensile strength significantly decreases. It also confirmed by the generated surface plots in Figure 4(a)-c. The decrease in the value of tensile strength by increasing the layer thickness can be explained by the increase in porosity between individual layers. As the raster width increases with increasing of the layer thickness, the air gap increases due to the technique of deposition the material. The air gaps are clearly visible at the layers thickness of 0.3 mm, as shown in Figure 6(b). When the raster width is smaller, it allows better raster stacking, resulting in better interlayer cohesion and a larger effective cross-surface area, as shown in Figure 6(a). Figure 5: The stress–strain curves: (**a**) material behavior for different process parameters; (**b**) Tensile stress–strain curves at different layer thickness; (**c**) tensile stress–strain curves at different raster angle; (**d**) tensile stress–strain curves at different printing speed. Figure 5c presents the stress-strain curves for five different raster angles. It can be seen that the trend of a significant increase in the value of the tensile strength from 58.9 MPa for raster angle 0\({}^{\circ}\) (sample 7 from Table 4) to 91.53 MPa for raster angle 90\({}^{\circ}\) (sample 18 from Table 4). Thus, by increasing the raster angle, the tensile strength increases. It is also confirmed by the generated surface plots in Figure 4b,d,f. The significant increase in the tensile strength was observed at the raster angle of 90\({}^{\circ}\).