## 1 Introduction  
Carbon fiber is approximately 70% lighter, ten times stronger, and three times more elastic than iron, with superior durability, impact resistance, heat resistance, and wear resistance [1, 2, 3, 4]. Carbon fiber reinforced polymer (CFRP) composites can reduce the weight of products and provide high stiffness; thus, their demand has been increasing in various industries, such as transportation vehicle parts, leisure sports products, semiconductor and display manufacturing processes, and more [5, 6, 7, 8, 9, 10, 11]. Among the composite molding methods that involve carbon fibers, pultrusion is the optimal process technology that can continuously produce CFRP of a uniform and simple cross-sectional shape and achieve a high automation rate with low investment costs [12, 13, 14]. In pultrusion, carbon fibers are stretched out while being impregnated with a mixed resin and hardener and are cured at both room temperature and high temperature to produce CFRP products [15].  
Generally, the pultrusion processes that are most widely found in industry involve an open bath impregnation system, in which an open-top bath is completely filled with a mixed resin and hardener, and fibers are fed into the bath tank for impregnation with the resin [16, 17]. The advantage of this open bath method is that a simple impregnation system can be used but the disadvantage is that management of the mixed solution is difficult. Particularly, whereas the variables in the impregnation stage of the pultrusion process include temperature, humidity, discharge rate, and more, the viscosity of the resin varies with temperature, and the resin properties also depend on exposure time to the external environment since penetration of impurities affects the curing time. Because the resin cannot be monitored during the composite production process, there may be defects that cause the final product to vary from the initial product, and it is difficult to analyze the causes of these defects during production. In addition, since the resin is exposed to the atmosphere, problems resulting from its volatility and toxicity should be carefully considered in terms of the safety of the working environment [18].  
To address the disadvantages of the open impregnation device, a pultrusion system using a closed impregnation device was designed and fabricated in-house. In the fabricated system, the resin and hardener are stored separately inside the tank, and the temperature and humidity of the stored resin are managed. In addition, the system is economical because the temperature and amount of the discharged resin can be controlled to meet requirements. The resin is discharged in a sealed space, which eliminates the risk of toxic gas exposure and provides a comfortable work environment for operators. The entire system is fabricated such that it can be controlled with sensors, and the functionality and process of each step can be analyzed, enabling production for various applications with a variety of resins.  
This study aims to improve the fiber impregnation system by using a closed impregnation device for pultrusion developed with in-house technology and ultimately produce high-quality CFRP with an improved speed by producing CFRP bars at various process parameters and analyzing their void fraction, surface roughness, and bending strength.