## 3 Experiment Procedure
### Sample Preparation and Experiment
#### 3.3.2. Supplementary Test and Introduction of Fiber Overlap  
In the previous section, the effects of fiber print layer thickness, fiber print speed, and fiber print temperature parameters on the mechanical properties of specimens were discussed. The thickness of fiber print layer and fiber print speed are significant factors. The resulting curve is not curved and is linear. In order to improve the accuracy of the results, several groups of experiments were added for further analysis and verification.  
Firstly, the data were interpolated in the significant factors, fiber print layer thickness and fiber print speed, for analysis. Secondly, considering that the most significant factor fiber print layer thickness, the lower the fiber print layer thickness, the better the mechanical properties of the tensile specimen. It is speculated that the difficulty in improving the mechanical properties of FFF-printed parts may be due to the bubbles distributed inside the specimen. These bubbles will lead to a poor bond between the matrix and the fiber, which will lead to uneven force and easier fracture. At present, many scholars are trying to solve this problem. Some people use manual intervention during specimen forming, such as printing under vacuum or low pressure or vibration to reduce the formation of bubbles. Others choose to add a roller to the printer to bind the reinforced fibers closer to the matrix and reduce bubble formation.  
For the independent extrusion 3D printer selected in this paper, the fiber printing nozzle is in contact with the hotbed by a plane with rounded corners, as shown in Figure 1b. This allows adjusting the layer height to exert a pressing effect on the fibers and parts that have been printed on the hotbed. Considering this reason, the smaller the thickness of the fiber printing layer, the better the mechanical properties of the print specimen.  
When the thickness of the fiber layer is low, the carbon fiber may be flattened and dispersed, that is, the width of continuous carbon fiber-reinforced PLA will become larger. At some time, there will be a certain overlap with the adjacent reinforced fibers. When the thickness of the PLA filament is 0.05, overlap may occur. The effect of overlap degree on printing needs to be excluded or verified. In this section, the concept of \"fiber overlap\" is introduced in order to explore the reasons why small thickness of fiber printing layer leads to better mechanical properties.  
To explore whether air bubbles can be reduced by the pressure of the print nozzle on the printed specimen or by the overlapping extrusion of two adjacent fibers. Using the print parameters in the previous section, the print temperature of continuous carbon fiber-reinforced PLA was set at a median value of 220 \({}^{\circ}\)C due to its non-significant factors. In this experiment, the overlap degree was adjusted by controlling the fiber print line width. When the fibers were required to be non-overlapping, the fiber print line width was 1.3 mm, and when the fibers were required to be overlapping, the fiber print line width was 0.9 mm. The test scheme and results are shown in Table 6. The side view of the printed specimen of the 0.1 mm prepreg filament layer and the printed specimen of the 0.15 mm prepreg filament layer is shown in Figure 4.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**No.**} & **Fiber Print Layer** & **Fiber Print** & \multirow{2}{*}{**Fiber Overlap**} & **Maximum** \\  & **Thickness** & **Speed** & & **Tensile Stress** \\ \hline
12 & 0.05 & 300 & Yes & 179.03 \\
13 & 0.05 & 300 & No & 170.77 \\
14 & 0.05 & 350 & Yes & 177.19 \\
15 & 0.1 & 250 & No & 165.19 \\
16 & 0.15 & 250 & No & 152.56 \\
17 & 0.125 & 325 & Yes & 163.84 \\
18 & 0.2 & 250 & Yes & 141.08 \\ \hline \hline \end{tabular}
\end{table}
Table 6: Test table of fiber print layer thickness, fiber print speed, fiber overlap.