## 2 Experiment
### Test Methods  
Differential scanning calorimetry (DSC) (TA Instruments, Q100) and thermogravimetric analysis (TGA) (TA Instruments, Q500) were performed to determine the thermal properties of the PA6 resins in a nitrogen atmosphere. DSC analysis was performed in a scan range from room temperature (RT) to 300 \({}^{\circ}\)C at a scan rate of 10 \({}^{\circ}\)C/min. TGA was performed in the scan range from RT to 800 \({}^{\circ}\)C at a scan rate of 20 \({}^{\circ}\)C/min.  
A rotational rheometer (TA Instruments, DHR-1) was used to measure the viscoelasticity of the resin. The PA6 film was prepared using a hot-press process at 250 \({}^{\circ}\)C for 15 min. Rheometer tests were performed in the scan range of 230-290 \({}^{\circ}\)C at a scan rate of 2 \({}^{\circ}\)C/min. The maximum torque was 150 mN\(\cdot\)m, and the strain was 5% at 1 Hz.  
The interfacial shear strength (IFSS) between the PA6 resin and the glass fiber was measured using a single-fiber pullout test, which was performed using a self-made pullout test system (Figure 2). The self-made pullout test system comprised a micro-tensile tester (DAYEONG M.T.C., EMC-001) connected to an optical microscope (OM) (Olympus, CKX41). The maximum force (\(F_{\text{max}}\)) was determined from the force-displacement curve. This maximum force originates from the debonding and friction between the resin and the fiber [11; 12; 13]. The IFSS is calculated as follows:  
\[\text{IFSS}=\frac{F_{\text{max}}}{\pi d_{f}L_{e}} \tag{1}\]  
where is the fiber diameter with a value of 17 \(\upmu\)m. The range of embedded fiber length (\(L_{e}\)) was 200 to 300 \(\upmu\)m. The crosshead speed of the micro-tensile tester was 0.5 mm/min at RT.  
The physical properties of the PA6/GF composites, such as the fiber weight content, matrix weight content, void content, and fiber volume content, were measured via a burning process using a muffle furnace (DAIHAN, WiseTherm(r)) according to the ASTM D3171-06 standard. The residues of the PA6/GF composite were then diffused in distilled water to measure the fiber length of the composite. The residual solution was dried in a convection oven at 100 \({}^{\circ}\)C for 1 h. The residues were observed using the OM (Olympus, CKX41). Furthermore, the fiber length of the composites in the residues was analyzed using ImageJ(r), an imaging analysis program.  
The tensile strength was measured in accordance with the ASTM D638 standard using a universal testing machine (Tinius Olsen, H5 KT). The flexural strength was measured  
\begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline \multirow{2}{*}{**Direct Extrusion**} & \multicolumn{4}{c}{**1 Extruder**} & \multicolumn{4}{c}{**2 Extruder**} \\ \cline{2-9}  & **B1** & **B2** & **B3** & **AD** & **B1** & **B2** & **B3** & **B4** & **Die** \\ \hline Temperature (\({}^{\circ}\)C) & 240 & 245 & 250 & 255 & 240 & 245 & 250 & 255 & 260 \\ \hline \hline \end{tabular}
\end{table}
Table 3: Extruder temperature profile for the LFT direct extrusion manufacturing processes.  
Figure 2: Single-fiber pullout test setting.  
using ASTM D790, which is a three-point bending test. The impact strength was measured using ASTM D256, which is an Izod impact test.  
After the tensile test, the fracture surfaces of the PA6/GF composites were inspected using field-emission scanning electron microscopy (FE-SEM) (Hitachi, SU8010). The surfaces of the fractured composites were coated with platinum via sputtering to prevent charging.