## 3 Results and Discussion
### Characteristics of Composites  
After the pultrusion and direct processes, the PA6/GF composite specimens were injection-molded by adjusting the glass fiber weight content (target--40 wt%, 50 wt%) and resin types (1011 BRT, GP 1100A (W)). Table 5 shows the physical characteristics of eight types of composites (Processes--P and D, resin types--B and G, target fiber contents--40 wt% and 50 wt%). Although the target fiber contents could not be exactly matched in some conditions, the error did not significantly impair the overall process of determining the optimized conditions. The glass fiber and resin weight contents, density, fiber volume, and void content of the PA6/GF composites are summarized in Table 5 with regard to the processes, resin types, and target weight contents. The fiber volume content (V\({}_{t}\)) of the composites increased with increasing fiber weight content. The density of the composites was closely related to the void content in the materials. For composites with similar void contents, the composite density increases with increasing fiber weight content (W\({}_{t}\)). In addition, composites with a low void content generally have a higher density than those with a high void content. However, several composite voids are defects that significantly degrade the mechanical properties. Regarding the physical characteristics, all the samples were expected to have excellent mechanical properties because they had a relatively low void content. The mechanical properties of composites, such as their strengths and modulus, are generally proportional to their fiber volume content. Table 6 shows the mechanical properties of one reference composite and eight types of composites. This phenomenon can be observed in Figure 7, which shows the relationship between the mechanical properties of the composite, such as tensile and flexural characteristics, and the fiber volume content. These graphs indicate that composites fabricated via the pultrusion process have better mechanical properties than those fabricated via the direct process. This was more apparent for the impact strength than for the tensile and flexural strengths (Figure 8). The increment in the modulus is dramatically higher than that of the strength with increasing fiber volume content. Figure 9 also shows the increment rate of the mechanical properties when the target fiber volume fraction increases from 40 wt% to 50 wt%. These results indicate that the fiber volume fraction has a greater influence on the modulus of the composites than the mean fiber length does [14]. Furthermore, the fiber lengths obtained from the pultrusion and direct processes were measured and compared. Figure 10 shows the OM images of the glass fibers after the burning process. The fiber lengths of the composites were observed to be hundreds of micrometers in the OM images. Generally, the decre \begin{table}
\begin{tabular}{c c c} \hline \hline
**Sample** & **IFSS (MPa)** & **Standard Deviation** \\ \hline
1011 BRT & 20.27 & 1.45 \\ GP 1100A (W) & 16.25 & 0.59 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: IFSS calculation result and standard deviation of the GF and PA6 interface. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Specimens** & **Fiber Content** & **Resin Content** & **Density** & **Fiber Volume Content** & **Void Content** \\
**(Process-Resin-Fiber** & **(wt\%)** & **(wt\%)** & **(g/cc)** & **(vol\%)** & **(\%)** \\ \hline P-B-40 & 38.9 & 61.1 & 1.419 & 21.55 & 2.37 \\ P-B-50 & 47.4 & 52.6 & 1.515 & 28.06 & 2.10 \\ P-G-40 & 40.0 & 60.0 & 1.409 & 21.89 & 3.03 \\ P-G-50 & 51.7 & 48.3 & 1.581 & 31.96 & 1.66 \\ D-B-40 & 33.8 & 66.2 & 1.346 & 17.77 & 4.04 \\ D-Bof the composites is influenced by the increase in the interactions among the fiber-fiber, fiber-polymer, and fiber-processing-equipment surface wall, which is more significant for high-volume content composites [13; 14; 15]. In these images, the mean fiber length of the composites prepared via the pultrusion process was longer than that of the composites prepared via the direct process.