# Embedded Based Real-Time Monitoring in the High-Pressure Resin Transfer Molding Process for CFRP
## 1 Introduction  
Carbon fiber reinforced plastics (CFRP) are materials that are developed for their high strength and light weight in various industries, including aerospace, automotive, and leisure. The mechanical properties of CFRP such as its strength, allowable stress, and hardness are affected by the type of carbon fiber, resin, and lamination orientation [1; 2]. In particular, numerous studies have investigated various molding methods as the application of CFRP has increased in the automotive industry due to greenhouse gas emission regulation and fuel efficiency improvement requirements. Typical CFRP molding methods include hand lay-up, filament winding, pultrusion, autoclaving, and resin transfer molding (RTM) [3; 4; 5]. Compared to other approaches, RTM molding has the advantages of simplicity and low manufacturing costs. However, it is difficult to apply RTM molding in mass production because it has a long process cycle time and low production efficiency [6; 7; 8]. In the RTM process, a carbon fiber preform is placed into a mold, which into resin is injected at a low pressure (in the range 1-20 bar) to impregnate the fiber, and vacuum is applied to accelerate the diffusion of the resin. Furthermore, a high-pressure RTM process was reported as improving theclassical RTM process for the mass production of CFRP. Unlike classical RTM processes, fast-curing resins are mixed with epoxy resin and hardener at high pressure in the mixing head and are then injected at the maximum pressure of 150 bar. The compression force leads to high cavity pressure in the mold. [9]. The HP-RTM consists of a high-pressure metering pump for precise flow control and a self-cleaning mixing head that has the advantages of high flow rates and short shot times. The molding cycle is completed in approximately 10 min using a thermosetting resin within a short reaction time [10; 11]. However, the fast cycle times and the various process variables mean that the HP-RTM process has many limitations. The process variables that affect the CFRP molding quality include the resin injection rate, the pressure and temperature in the mold, the vacuum, end-gap, pressing force, and binder. In recent years, the process condition monitoring of molding using different types of sensors has been extensively studied to stabilize the HP-RTM process and control the process variables. For instance, [PERSON] et al. used an ultrasonic sensor to monitor the curing state of resin in the RTM process [12]. Controlling the RTM process, shortening the process cycle time, and optimizing the laminate quality were studied using ultrasonic sensor signals. Furthermore, [PERSON] et al. and [PERSON] et al. used a pressure sensor to analyze the HP-RTM process and monitor the effects of process variables. Specifically, the authors analyzed the relationship between the molding quality and the flow of resin from the signal of the cavity pressure sensor and the mechanical properties according to process variables [13; 14; 15]. Likewise, [PERSON] et al. investigated the relationship between the pressing force and reaction temperature through the degree of cure by detecting the reaction temperature of the laminate [16]. Additionally, [PERSON] et al. and [PERSON] et al. measured the change in the light refracted by the resin during the curing process with fiber-optic sensors [17; 18]. [PERSON] et al. used dielectric and optical sensors installed in the mold to detect the curing state of the resin [19]. [PERSON] et al. detected the cure degree of laminates by the developed dielectric coplanar plates sensor in glass-epoxy prepregs [20]. [PERSON] et al. monitored the flow and the cure of CFRP using the electric and dielectric carbon fiber sensors to provide damage and preventive maintenance [21]. However, the results of the analysis of the HP-RTM process monitoring undertaken in previous studies are limited in terms of their applicability in the field.