## 4 Discussion  
A significant level of variability in the measured mechanical properties was observed. Factors that influence the variability in the measured mechanical properties are to be found along the whole manufacturing chain, as well as in the test procedure. All manufacturing processes induce uncertainties. For the 3D printing process, the prepreg filament, ambient conditions, manufacturing process parameters and machine inaccuracies all influence the structure of the printed material and therefore, the resulting material properties. Some factors certainly influence the test specimens of a given set equally. This is the case, among others, for the distance between the printing nozzle and the print bed (adjusted before printing), for the wear of the different components of the 3D printer, for the variations in filament quality, and for the variations in environmental conditions like temperature, ambient pressure and humidity, during manufacturing and material storage. The equipment used for mechanical testing also influences, equally, the test results. These factors can be used to explain the observed dispersion between the individual sets. Other factors, in contrast, have a direct influence on each particular specimen, within each set. For instance, variations in the print precision (print head movement speed, filament feeding speed) can lead to local differences in the microstructure (e.g., location and size of voids), with a direct impact on the resulting mechanical properties. These factors influence the standard deviation observed for each set, and are at the origin of the differences in standard deviation observed between the different sets. The international standards for determining the flexural properties of polymer matrix composite materials (ASTM D 7264/D 7264M [63] and ISO 14125:1998 [52]) suggest to characterize at least five specimens per test to achieve valid nominal values and standard deviation results. In the present study, three times this number of specimens were tested, whereby these were manufactured with three different CF/PA6 filament rolls (differentiated as sets). The test results show that this minimum number of specimens, as recommended by the test standards, is not enough to characterize the mechanical properties of 3D printed CFC with sufficient accuracy, especially in terms of variability. The comparison between the different sets in Figures 11 and 12 show indeed, that a significant amount of uncertainty would remain, if one set only was considered (i.e., five specimens). In that case, the mechanical properties would either be overestimated or underestimated. Therefore, there is a need to define new test standards for 3D printed CFC materials. \begin{table}
\begin{tabular}{l c c c} \hline \hline
**Mechanical Property** & **Mean Value** & **Standard Deviation** & **Coefficient of Variation** \\ \hline Flexural modulus \(E_{\parallel}\) [GPa] & 38.3 & 3.39 & 8.85 \\ Flexural strength \(\sigma_{\lambda_{\parallel}}\) [MPa] & 547 & 54.2 & 9.91 \\ Maximum strain \(\epsilon_{\parallel}\) [\%] & 1.35 & 0.0542 & 4.01 \\ \hline Flexural modulus \(E_{\perp}\) [GPa] & 2 & 0.369 & 18.4 \\ Flexural strength \(\sigma_{\lambda_{\perp}}\) [MPa] & 40.1 & 8.77 & 21.9 \\ Maximum strain \(\epsilon_{\perp}\) [\%] & 3.1 & 0.416 & 13.4 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Mechanical properties of 3D printed CF/PA6 from bending tests (statistical values). (\(\parallel\)) corresponds to the longitudinal direction (fiber direction is 0\({}^{\circ}\)). (\(\perp\)) corresponds to the transverse direction (fiber direction is 90\({}^{\circ}\)). In addition to significantly larger sample sizes, improved test methods are also required. The three-point bending test is probably sensitive to the location of voids in the specimen. The standards ASTM D7264/D7264M [63] and ISO 14125:1998 [52] both suggest three-point and four-point loading configurations. Both tests are performed with identical specimen geometries. However, the standards do not specify which test configuration should be preferred.