Research Progress in Resin Transfer Molding of Composite Materials – 2

 

Introduction

 

Using composite materials instead of traditional steel and aluminum materials can significantly reduce the overall weight of the car body while improving the mechanical properties of the structure, effectively achieving lightweighting of the car. In recent years, it has been widely used in the automotive field and has achieved corresponding results. However, the preparation process of composite components is affected by many factors. How to prepare composite components with high quality, stable mechanical properties and long service life is a protracted battle that the composite industry will face in the future. At present, the more mature molding processes are molding process, autoclave molding process, hand lay-up process, etc., but they all face the problem of high cost and low efficiency. The rapidly developing RTM process can avoid these problems well.

 

The RTM process is a process that can directly complete the fiber and resin impregnation in the mold. One-step impregnation replaces one or more steps of impregnation in the traditional process. It does not require the preparation of prepregs, vacuum bags, autoclave curing and other processes, which greatly reduces the manufacturing cost and molding time. The fiber volume fraction of the component is high and has good mechanical properties. However, the injection pressure, molding temperature and other process parameters of the RTM process, the process flow, resin viscosity, equipment accuracy and other key factors will affect the quality of the final component. This paper summarizes the research on process optimization, process improvement, resin materials, and equipment at home and abroad, summarizes the problems existing in the current RTM process, and puts forward relevant suggestions.

 

RTM Process and Optimization

 

RTM process is a new process specially used to produce high-performance fiber-reinforced polymer-based composite materials. The entire process does not have a pre-impregnation process between fiber and resin, but directly makes the fiber into a fiber preform, which is then placed in a closed mold. Subsequently, a resin system consisting of a low-viscosity resin, a curing agent, and a catalyst is injected into the membrane cavity at a certain temperature and pressure to completely infiltrate the fiber with the resin, and finally solidifies and demolds. The process flow is shown in Figure 1. Compared with traditional composite materials processes, the RTM process has the characteristics of high production efficiency, low manufacturing cost, high fiber content of components, and smooth surface.

 

 

Figure 1. RTM Molding Process Flow

 

At present, RTM technology has been applied in the automotive field. For example, the body structure of the BMW i3 body life module, the side roof reinforcement of the BMW 7 series sedan, and the middle crossbeam structure of the roof are all manufactured using the RTM process.

However, there are still some problems with the RTM process. On the one hand, the process impregnation time will increase significantly with the increase of the surface area of ​​the component, making it difficult to apply it to large and complex components; on the other hand, in the actual production process, if the viscosity of the resin system used is too high, it is difficult to infiltrate the fiber.

If the resin injection pressure is too high, it is very likely to cause fiber scouring, resulting in dry spots (Figure 2 (a)), pores (Figure 2 (b)) and other defects on the surface of the component.

 

Therefore, the impregnation time of the process, the viscosity of the resin material, the resin injection pressure, the resin flow rate, etc. are all key factors affecting the RTM process. In recent years, many scholars at home and abroad have conducted a lot of research on this.

 

 

Figure 2. Forming Defects of Composite Components

 

Many foreign researchers have used numerical simulation methods to predict and monitor the RTM process online, and optimized its process parameters based on this.

 

Proposed a general software based on finite volume element code (FVM) to establish a virtual model and verify the correctness of the model. Based on this, they combined genetic algorithm (GA) to simulate and optimize the RTM molding process parameters such as injection pressure and injection port position design for a certain Italian body part.

 

Proposed a new high-speed RTM process to reduce the impregnation time of composite materials, and used Fluent 14.0, ANSYS and other simulation software to simulate the RTM process, and obtained the optimal number of resin injection ports and resin permeability ratio that minimized the impregnation time.

 

Used ABAQUS software to predict and simulate the deformation of composite materials in the post-curing stage.

 

Theoretically, the relationship between the pore content of composite components and the resin impregnation speed shows a “V” trend, as shown in Figure 3.

 

 

Figure 3. Theoretical porosity of composite components prepared by RTM method varies with impregnation speed

 

Lower or higher impregnation speed will produce macroscopic or microscopic pores in the components, that is, there is an optimal resin impregnation speed to achieve the lowest porosity. Foreign scholars have used intelligent sensors to control the resin flow rate or microscopic angle to monitor the porosity of composite materials to obtain the optimal impregnation speed.

 

Embedded a pressure sensor in the mold to quickly track and control the resin and used the pressure signal to estimate the front flow position of the resin to achieve online evaluation and control of product quality. At the end of the experiment, a feedback mechanism was established using a computer-controlled pressure regulator to control the resin flow rate and thus reduce the product porosity.

 

Used a microscope to conduct information statistics and online monitoring of the formation of pores in plain fabrics and the location and distribution of pores after formation, and summarized the relationship between porosity and capillary action number, optimizing the optimal impregnation angle and speed.

 

Used fluorescent dyes to improve the contrast between fiber and resin, used digital cameras to collect images, and established an automatic image recognition method to process the collected images, clearly characterizing information such as pore content and size.

 

Combined capillary rise experiments with red thermal imaging technology (IRT), and estimated the optimal impregnation speed of resin from thermal images through inverse algorithms. Finally, the feasibility of using this method in opaque fibers such as carbon fibers was verified through experiments.

 

Domestic scholars have also conducted a lot of research on the RTM molding process. [Cheng Hao et al.] prepared an epoxy vinyl resin system that meets the viscosity requirements of the RTM molding process resin and has good mechanical properties and acid resistance by using styrene to dilute the epoxy resin system.

 

Used the RTM process to prepare basalt fiber/epoxy resin composite materials and optimized the optimal resin system ratio, fiber volume fraction, injection temperature, injection pressure and other process parameters.

 

Studied the effects of these three process parameters on component surface defects by increasing vacuum, changing injection pressure and injection flow rate during RTM molding. The results showed that composite components with vacuum-assisted, low-pressure and low-speed injection had the least surface defects.

 

Derivative Processes of RTM Process

 

Since its development, RTM process has been widely used in various industrial manufacturing fields due to its process advantages. However, it still has problems such as long resin impregnation time and high porosity of molded components. Therefore, RTM process is only suitable for manufacturing small and medium-sized components, and it is difficult to apply it to composite components with large area, complex structure and high precision requirements. In order to solve the drawbacks of RTM process, many scholars have improved it in recent years and designed several derivative processes of RTM process suitable for different application scenarios.

 

① High-Pressure Resin Transfer Molding Process (HP-RTM)

 

When using RTM process to manufacture large components, some components may be completely cured, while other parts of the resin have not been completely impregnated with fibers, resulting in excessive porosity of the components. In order to overcome these problems, the RTM process needs to be improved. Among them, the RTM process of high-pressure equipment injection resin system, namely the high-pressure resin transfer molding (HP-RTM) process, is a common method.

 

The HP-RTM process is divided into high-pressure injection molding process (HP-IRTM) and high-pressure compression molding process (HP-CRTM). When high-pressure RTM equipment is not used, this compression process is usually called compression resin transfer molding (C-RTM). The relevant introduction of HP-RTM is shown in Table 1, and the molding process flow of HP-RTM is shown in Figure 4.

 

Table 1. HP-RTM molding process principle and introduction

 

 

Figure 4. HP-RTM Molding Process Flow

 

With the rapid development and widespread demand in the field of industrial manufacturing, many scholars have conducted relevant research on the differences between different HP-RTM molding methods.

 

Found that when the resin injection time of the HP-IRTM process and the HP-CRTM process was shortened from 30s to 7.5s, it was found that the injection time did not affect the tensile and shear properties of the laminate, that is, both HP-RTM molding processes can efficiently produce large quantities of structural parts.

 

Observed the pore size and distribution of components made by the autoclave molding process, vacuum bag curing process, and HP-IRTM molding process, and found that the porosity of the components made by the HP-IRTM process was the lowest, and was almost equivalent to that of the autoclave molding process.

 

In terms of actual production cost, [Vita et al.] determined the molding cycle of automobile roofs made by C-RTM process, low-pressure resin transfer molding process (LP-RTM), and HP-RTM process respectively through numerical simulation. The results showed that C-RTM molding process significantly shortened the resin injection time and could be cured at a higher temperature, making it the most cost-effective candidate for automobile manufacturing.

 

For the specific HP-RTM molding process, its process parameters are undoubtedly the key factors affecting the quality of composite components. For this reason, [Bodaghi et al.] found that the fiber fabric structure (including woven structure and non-curled structure), the number of fabric layers, and the injection pressure of the laminates manufactured by HP-RTM molding process would affect the generation of dry spots inside the laminates, thereby reducing the mechanical properties of the laminates.

 

Domestic studied the effects of resin flow rate and fiber volume fraction of HP-RTM process on component porosity and mechanical properties through experimental and numerical simulation methods, and found that the potential of resin flow rate on porosity is much greater than that of fiber volume fraction.

 

Optimized the injection pressure, mold opening distance, resin temperature and other process parameters of C-RTM process by orthogonal experiment, and pointed out that lower compression pressure and resin viscosity are important technologies for improving the mechanical properties of parts, while mold preheating temperature has little effect on the mechanical properties of materials.

 

In order to more accurately control the process variables of HP-RTM molding process, some foreign scholars began to use different types of sensors to detect and monitor the state of molding process in real time.

 

Measured the cavity pressure of HP-IRTM process and HP-CRTM process by special high-pressure mold and pressure sensor, and found that the cavity pressure behavior was strongly affected by the selected process. [Kim et al.] proposed an embedded monitoring system based on FPGA, which can monitor the HP-RTM process through high-speed real-time signal processing of multiple sensors such as pressure, temperature and LVDT sensors, realize online control of product quality, and finally verify the feasibility of the system.

 

② Vacuum Assisted Resin Transfer Molding (VARTM)

The VARTM process is a process of curing and molding in a vacuum environment, so its molded components are basically free of pores (pore content is less than or equal to 0.2%) and have high quality. Its process principle is similar to the RTM process, which is also a process of direct molding of fiber preforms and resins. The difference is that it is a single-sided molding process. With the help of a vacuum bag, the vacuum negative pressure is used to push the resin solution through the resin infusion channel into the closed membrane cavity to complete the infiltration of the fiber, and the curing molding process is completed at a certain temperature. The specific molding process is shown in Figure 5.

 

 

 

Figure 5. VARTM Molding Process

 

Due to its single-sided molding characteristics, the VARTM process greatly reduces the mold manufacturing cost and has good flexibility. Compared with the autoclave process, it gets rid of the dependence on autoclave facilities, greatly reducing the manufacturing cost. For large and complex structures, it is a low-cost manufacturing method and is widely used. However, the VARTM process also has some disadvantages, such as:

 

Because the resin flow is completed under vacuum negative pressure, the resin flow rate is too low and the curing cycle is extended;

 

The single-sided molding process causes only one side of the product to be smooth;

 

Compared with the traditional RTM process and autoclave process curing, the fiber volume fraction of the VARTM process is lower, which often leads to lower mechanical properties.

 

The high-quality VARTM process can produce molded components with stable mechanical properties, low porosity and high precision. The research on this process has been valued by more and more scholars and has become a research hotspot in recent years.

 

In order to improve the disadvantages of the VARTM process, many foreign scholars have improved and studied the VARTM process over the years.

 

Improved the VARTM process by setting a pressure chamber above the mold and a flexible silicon heat sheet below the mold to heat the mold. The experiment found that increasing the pressure at the right time can increase the fiber content of the component and reduce the porosity of the component (less than 1%).

 

In order to improve the mechanical properties of the laminate and reduce the cost, the experiment added permanent magnets (NdFeB) above the mold to increase sufficient consolidation pressure for the VARTM process to manufacture laminates. The results showed that the fiber volume fraction of the laminates of the improved VARTM process was greater than 50%, the porosity was much less than 1%, and the mechanical properties were greatly improved.

 

Experimental research used porous molds to improve the VARTM process, so as to control the resin content in the composite material, thereby improving the fiber content and mechanical properties of the composite material components.

 

Studied the effect of adding nano-alumina particles in different weight ratios to epoxy resin on the resin filling time of the VARTM process. They found that the addition of particles would increase the viscosity of the resin and thus prolong the filling time. When the mass ratio of nano-alumina particles was 3%, the filling time could be prevented from increasing significantly.

 

Domestic scholars studied the use of injection port design optimization and numerical simulation methods to further improve the molding quality and efficiency of the VARTM process.

 

Experimentally studied the effect of resin flow inclination on resin filling time and mechanical properties of composite components. The results showed that when the resin flow inclination was negative, the resin filling time was reduced due to the influence of the gravity of the resin itself, and the mechanical properties of the component increased with the increase of the resin flow inclination (from negative to positive).

 

[29] studied the effect of the design of the injection pipe and the exhaust pipe on the filling time and surface defects of high-thick fiber composite materials by numerical simulation. They proposed that when the resin flow pipe was set on the side of the bottom of the model parallel to the length of the model and the exhaust pipe was set on both sides of the bottom of the model parallel to the length of the model, the component molding effect was the best and the time was the shortest.

 

[30] used numerical simulation methods to study the influence of the injection port position and the ejection port position on the injection molding time of composite components of K/T-type joints manufactured by VARTM process, and optimized the best injection molding method.

 

[31] proposed a VARTM process filling simulation algorithm based on a hybrid grid method to absorb the local resin volume change caused by the deformation of the vacuum bag, and applied it to one-dimensional and three-dimensional examples to verify the accuracy of the algorithm.

 

Used the heat conduction equation and curing kinetics theory to simulate the molding process of the VARTM process using Fluent software to study the temperature field change of the laminate, and verified it through experiments.

 

Resin Matrix Development and Modification

 

The fluidity, toughness and applicability of the resin matrix selected in the curing molding process of composite components determine the final molding quality of the components.

 

The RTM process requires the use of a low-viscosity resin system to ensure sufficient impregnation time for the resin to be fully filled in the mold. Excessive viscosity will make it difficult for the resin to flow and unable to fully impregnate the fibers, resulting in defects such as pores and resin-rich on the surface of the component that reduce the mechanical properties of the component. Therefore, it is extremely important to develop resins suitable for RTM technology or modify the original resins to improve the mechanical properties, heat resistance and toughness of components.

 

Table 2 shows the research content and corresponding achievements of domestic and foreign scholars in developing new resins and modifying resins.

 

Table 2. Development and modification of resin matrix at home and abroad

 

Progress of RTM Process Equipment

 

RTM process effectively solves the problems of low component production efficiency and poor quality stability, and has developed into a typical process for manufacturing high-performance composite materials in recent years. However, due to the blockade of foreign technology, the progress of RTM equipment in China is still relatively backward. Equipment research mainly focuses on the integration of injection equipment, feeding devices and production equipment, as shown in Table 3.

 

 

Table 3. Related Research on Domestic RTM Molding Process Equipment

 

Conclusion

 

RTM technology, as a high-efficiency, low-cost molding process, has been widely used in the automotive field in recent years. However, judging from the current development of domestic RTM technology, it is far from meeting the goal of mass production and high-precision production of composite components in the automotive field. In addition, due to the blockade of foreign technology, the equipment research and development of domestic RTM technology is also greatly restricted.

 

At present, the research on derivative processes of RTM process in China is still in its infancy, and the derivative processes of RTM molding can only change certain molding defects. In order to solve these problems and realize the goal of mass production of high-precision composite components as soon as possible, the future research on RTM process parameters should focus more on the optimization design of resin flow channels and the method of intelligent control of resin flow rate and molding pressure.

 

In order to improve the production efficiency of components, the process equipment should be more inclined to the design and development of integrated equipment. Although domestic and foreign scholars have made a lot of efforts in resin modification in recent years, there are still problems such as complex modification process and high cost, which greatly limits the application scope of modified resins. Reducing the modification process and reducing costs are hot issues in resin modification in the future. In addition, new resin matrices suitable for RTM process and its derivative processes should continue to be developed.