## 2 Materials and Experimental Methods
### Manufacturing of Test Specimens  
A CFF desktop 3D printer Mark Two (Markforged) [50] was used to manufacture the bending test specimens. The Mark Two 3D printer uses the prepreg extrusion method, which prints with a pre-impregnated composite filament. A feeding system transports the filament to the heated print head during the 3D printing operation, where the thermoplastic matrix material is melted before filament deposition (see Figure 1). The round filament is compressed to a smaller layer height during deposition, which results in a larger path width. The bending test specimens were manufactured using a Markforged prepreg filament made of a 1K roving of continuous carbon fiber, which was pre-impregnated with a nylon matrix (CF/PA6) [51]. The test specimens were designed according to the standard ISO 14125:1998 [52]. Their dimensions are 60 \(\times\) 15 \(\times\) 2 mm (length \(\times\) width \(\times\) thickness). The slicer software Eiger (Markforged) [53] was used to define the printing sequence and create the G-code. Figure 2 shows the printing path for one layer of the bending test specimen with 0 deg fiber direction. Based on [PERSON] et al. [14], who reported higher strength and stiffness for specimens sliced in the thickness direction rather than in the width direction, the specimens were sliced in the thickness direction. All test specimens were manufactured in the same way. The slicer software automatically adds at least one bottom and one top nylon layer. In previous bending tests [54], a flexural modulus of 20.4 GPa \(\pm\) 18.2% for specimens with the extra top and bottom nylon layers was reported. However, a flexural modulus of 38.3 GPa \(\pm\) 8.2% was observed for specimens without nylon layers. One possible explanation for this difference is the significantly lower flexural modulus of nylon compared to CF/PA6. To get rid of this effect, the extra nylon bottom and top layers were removed, which also resulted in the reduction of the specimen thickness. The flexural modulus \(E_{f}\) is calculated according to the test standard [52] from geometrical parameters, the change of applied force \(\Delta F\) and the change of deflection \(\Delta s\) as:  
\[E_{f}=\frac{L^{3}}{4 bh^{3}}\frac{\Delta F}{\Delta s}, \tag{1}\]  
where \(h\) is the specimen thickness, \(L\) is the length between the supports, and \(b\) is the width of the test specimen. If the specimen height decreases by 10%, the flexural modulus increases by 37% arithmetically, assuming that the change of force \(\Delta F\) over the change of deflection \(\Delta s\) does not change. In addition, the nylon layer may cause a delay in bending deformation of the CF/PA6 layers. Instead of enforcing strain on the compression side of the specimen (upper side), the loading pin first deforms the nylon layer (low surface hardness). As a result, the measured stress-strain curve shifts towards higher strains and the determined flexural modulus decreases. Figure 3 shows both specimens before and after nylon removal. Figure 1: Illustrations of the CFF 3D printing. Schematic representation of the printing process in the print head in **(a)** front view and **(b)** lateral view. Complementary process parameters used for the 3D printing of the bending test specimens are shown in Table 1. The temperature and layer height are the default settings of the 3D printer. In total, 30 test specimens were manufactured using three CF/PA6 filament spools. The purchased spools were identical, but the material properties may still have differed from one spool to the other. A set of ten specimens (five with 0\({}^{\circ}\) and five with 90\({}^{\circ}\) fiber direction) was manufactured per spool. The nomenclature of the specimens is as follows: name of the 3D printer (here: Mark Two, or M2), fiber direction in degrees (0 or 90), filament spool index (I to III), and specimen number (1 to 5). The manufacturing of one set (i.e., 10 specimens) of specimens required about 12.6 cm\({}^{3}\) of CF/PA6 filament.