## 3 Results and Discussions
### Influence of the Process Parameters on Tensile Strength
This significant increase in tensile strength can be explained by the orientation of the carbon fibers. In the case of the raster angle of 90\({}^{\circ}\), the load direction is in the direction of deposition of the material and fibers are mostly oriented in same direction, so the tensile strength is mainly dependent of the strength of the carbon fibers. It can be also confirmed by the mechanism of the material fracture that was mainly caused by the tearing mechanism, as shown in Figure 7a. However, in the case of the raster angle of 0\({}^{\circ}\), the load direction was perpendicular to the raster direction, so the tensile strength depended on the cohesion forces between rasters. Thus, the material fracture was caused by the mechanism of stratification between individual rasters. This mechanism results in flat fracture surfaces, as shown in Figure 7b. Additionally, for any other raster angle less than 90 degrees, one component of the load force acts in the direction of the raster, creating shear stresses between the individual rasters, while the other component of the load force acts perpendicular to the raster direction and causes stratification between individual rasters, as well as in the case when the raster angle is 0 degrees. Therefore, the maximum tensile strength will depend on the cohesion force between the individual rasters, so the fracture that occurs in this case will be caused primarily the mechanism of stratification (Figure 7c.). Thus, at the raster angle of 60\({}^{\circ}\), the load force component acting perpendicular to the raster direction and causing the stratification mechanism between individual rasters is smaller than when the raster angle of 30\({}^{\circ}\), resulting in the increase in tensile strength, as shown in Figure 5c. Based on all the above, it can be concluded that the short carbon fibers have a significant impact on the reinforcement of polyamide when the load direction is in the direction of the deposition of the material. Figure 5d presents the stress-strain curves for three different printing speed at the constant values of other process parameters (at zero level for layer thickness, raster angle and wall thickness). It can be seen that by decreasing the printing speed, the tensile strength increases. The influence of the printing speed is related to the heat input, which has an impact on interlayers cohesion and cohesion between individual rasters. At lower printing speeds, the heat input is higher, so the previous layers are heated more intensively, resulting in stronger cohesion between the layers. The maximum tensile strength of 71.97 MPa is achieved at a minimum printing speed of 60 mm/min, which is an increase in tensile strength of 11.46% compared to the tensile strength of 64.57 MPa achieved at a printing speed of 100 mm/min. It can be observed that the printing speed has a secondary impact  
Figure 6: Microscopic images: (a) the layer thickness of 0.1 mm; (b) the layer thickness of 0.3 mm. on the tensile strength while the layer thickness and the raster angle have a primary impact. Figure 8 presents the stress-strain curves for printing speeds of 70 mm/s and 90 mm/s at different layer thicknesses and raster angles. Thus, the tensile strength increases by decreasing printing speed and layer thickness and by increasing the raster angle. Figure 8: The stress–strain curves at printing speed of 70 mm/s and 90 mm/s: (**a**) layer thickness of 0.15 mm and 0.25 mm; (**b**) raster angle of 30\({}^{\circ}\) and 60\({}^{\circ}\). Figure 7: Fracture surface (top view of samples, left, and cross-section of samples, right): (**a**) raster angle of 90 degrees; (**b**) raster angle of 0 degrees; (**c**) raster angle of 45 degrees. To increase the tensile strength of CFPR parts, it is necessary to select smaller values for the layer thickness and printing speed (in this case, a layer thickness of 0.1 mm and printing speed of 60 mm/s), and higher values of raster angle and wall thickness (in this case, a raster angle of 90\({}^{\circ}\) and wall thickness of 2 mm).