## 3 Analytical and Experimental Results  
### Microstructure and Fiber Content  
The manufacturer (Markforged) provides information about the material properties in a data sheet [51] and an official retailer (Mark3D) specifies the CFF nozzle diameter [62]. However, none of them provides information on the CF/PA6 filament diameter and the fiber amount in the filament. Figure 5 shows the result of the micrograph analysis of the unprinted CF/PA6 filament, where the filament boundary is clearly visible. The filament diameter was determined to be 388 \(\upmu\)m at one position, using the image processing program. However, the filament is not perfectly circular. The distribution of carbon fibers within the filament cross-section is irregular. The fibers are distributed within three bands with regions of pure matrix material in between. This structure was also reported by [PERSON] et al. [41]. Furthermore, the micrograph shows voids in the filament, which are recognizable as dark round spots. In contrast, the elongated spots likely result from the grinding process within the specimen preparation (see Section 2, Microstructural analysis) and are not actual defects, originally present in the filament. Table 2 shows the TGA results of two unprinted CF/PA6 filament samples. The samples were weighted before heat treatment, as well as the fiber residues afterward. The ratio of residue weight to filament weight is the fiber's weight fraction in the composite material. Using these TGA measurement results, the fiber volume fraction f of the carbon prepreg filament was then calculated from Equation (2). With the measured fiber weight fraction (\(f_{w}\) = 47.3 \(\pm\) 0.1%) and the densities of carbon fiber (\(\rho_{f}\) = 1.8 g/cm\({}^{3}\), [58]) and PA6 matrix (\(\rho_{m}\) = 1.08 g/cm\({}^{3}\)[51]), an average fiber volume fraction of \(f\) = 35.0 \(\pm\) 0.1% in the carbon prepreg filament was obtained. This value is in accordance with results reported by other authors, such as [PERSON] et al. [45], who reported an average filament diameter of 359 \(\pm\) 4 \(\upmu\)m and an average carbon fiber volume fraction of 34.9 \(\pm\) 0.8%. Figure 6 shows a micrograph of the cross-section from a specimen with 0\({}^{\circ}\) fiber direction. The layer-wise structure resulting from the 3D printing process is well visible in the micrograph due to interlayer voids and irregular fiber and matrix distribution. The micrograph width is approximately equal to the path width of two filaments printed next \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Sample** & **Sample Weight [mg]** & **Residues [mg]** & **Residues [w\%]** \\ \hline CF/PA6 filament a & 8.008 & 3.778 & 47.17 \\ CF/PA6 filament b & 8.159 & 3.868 & 47.41 \\ \hline Average carbon weight fraction & & & 47.3 \(\pm\) 0.1 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Results of the CF residues after the TGA measurement (up to 600 \({}^{\circ}\)C), which are used to calculate the fiber ratio in the CF/PA6 filament. Figure 5: Microscopic image of an unprinted CF/PA6 filament cross-section showing an irregular distribution of continuous fibres in three bundles, and areas of pure matrix material in between. to each other (the average width of a printed filament is 910 \(\upmu\)m [54]). The height of the micrograph is approximately equal to eleven printed layers. There is a locally varying fiber distribution, likely a consequence of the already irregular fiber distribution in the CF/PA6 filament. Also, a columnar pattern of the void concentration appears. There are areas with more voids (at the left and the right edges of the micrograph) and areas with fewer voids (at the center of the micrograph). For a better view of the columnar pattern, the reader is referred to [PERSON] et al. [43], where an X-ray computed tomography image shows an entire specimen cross-section using a larger scale. The printing path can explain the occurrence of the columnar pattern.