## 3 Results  
### Measurement of Resin Viscosity and Impregnation Behavior  
To understand the effect of the resin injection temperature on viscosity, the viscosities of the resin, hardener, and mixed resin were measured using the LV2 spindle of the DV-II + Pro viscometer (Brookfield Co., Ltd., Sydney, Australia) depending on injection temperature, and the optimal resin injection temperature was determined to achieve the desired viscosity range (Table 1). In addition, [PERSON]'s law can be used to predict the rate at which the resin is impregnated between fibers. The relationship between the unsaturated permeability of fibers and the variables, such as the viscosity of the resin used, impregnation distance, time, and fiber volume fraction, was investigated by [PERSON] and [PERSON] with the following equation [22]:  
\[K_{unsat}=\frac{l_{f}^{2}(t)}{2t}\frac{\mu\cdot\left(1-V_{f}\right)}{P_{in}} \tag{2}\]  
where \(K_{unsat}\) is the unsaturated permeability of fibers, \(\mu\) is the resin viscosity, \(l_{f}(t)\) is the impregnation distance, \(t\) is the impregnation time, \(V_{f}\) is the volume fraction, and \(P_{in}\) is the injection pressure. The above equation can be used to predict the rate at which the resin is impregnated into dry-state fibers inside the closed impregnation system. Since the fibers are pultruded at a constant speed and the inlet length of the impregnation nozzle is 35 mm, as shown in Figure 5, the resin should be sufficiently impregnated in the fibers at the given speed, or within the given impregnation time, even if the pultrusion speed is increased. In this impregnation system, the thickness of the fibers that should be impregnated is approximately 2.2 mm, which is 10% thicker than the final bars produced with the fiber volume fraction of 60%. Therefore, the process time was predicted based on the permeability of the fiber with a volume fraction of 55%. Through this analysis, it is possible to specify the required resin viscosity or the required process temperature according to the given pultrusion speed. Additionally, for experimental accuracy, the temperature of the resin storage tank as well as that of the pipe were controlled to maintain a constant temperature during the resin discharge and impregnation. Figure 4: Cantilever test of a CFRP bar. To predict the impregnation speed, the out-of-plane fiber permeability depending on the fiber volume content was based on the experimental result reported by [PERSON] et al., who used a similar fiber type, resin injection pressure, and fiber volume fraction. In this study, when the fiber volume fraction was 50%, the out-of-plane fiber permeability was measured to be 1.13 \(\times\) 10\({}^{-12}\)[23]. Since the impregnation time is determined by the above-mentioned unsaturated permeability in the out-of-plane direction, the length of resin inlet, and the pultrusion speed, the impregnation time at a given pultrusion speed was substituted into Equation (2) to calculate the resin viscosity required for the impregnation thickness of 2.2 mm, as shown in Table 2 below. The required viscosity of the mixed resin was found to decrease as the pultrusion speed increased, and a mixed resin viscosity of 368 mPa-s or lower was necessary to impregnate the fibers with a thickness of 2.2 mm at the improved speed of 600 mm/min. As shown in Table 1, the viscosity of the mixed resin was 220.4 mPa-s at 40 \({}^{\circ}\)C, and therefore, it appears that the resin can be sufficiently impregnated in the fibers at a pultrusion speed of 600 mm/min if the temperature of the closed impregnation system is maintained at 40 \({}^{\circ}\)C. Figure 6 shows the changes in the viscosity depending on the temperature and the changes in the required viscosity depending on the pultrusion speed. The resin viscosity required for sufficient impregnation at the pultrusion speeds of 400 and 600 mm/min was identified and used to set the process temperature.