Resin transfer molding (RTM) process is a typical liquid molding process for fiber-reinforced resin based composite materials, which mainly includes:
(1) Design fiber preforms according to the shape and mechanical performance requirements of the required components;
(2) Lay the pre designed fiber preform in the mold, close the mold and compress it to obtain the corresponding volume fraction of the fiber preform;
(3) Under specialized injection equipment, inject resin into the mold at a certain pressure and temperature to eliminate air and immerse it in the fiber preform;
(4) After the fiber preform is completely immersed in resin, the curing reaction is carried out at a certain temperature until the curing reaction is completed, and the final product is taken out.
The resin transfer pressure is the main parameter that should be controlled in the RTM process. This pressure is used to overcome the resistance encountered during injection into the mold cavity and immersion of the reinforcing material. The time for resin to complete transmission is related to system pressure and temperature, and a short time can improve production efficiency. But if the resin flow rate is too high, the adhesive cannot penetrate the reinforcing material in time, and accidents can occur due to an increase in system pressure. Therefore, it is generally required that the resin liquid level entering the mold during the transfer process should not rise faster than 25mm/min. Monitor the resin transfer process by observing the discharge port. It is usually assumed that the transfer process is completed when all observation ports on the mold have overflow of glue and no longer release bubbles, and the actual amount of resin added is basically the same as the expected amount of resin added. Therefore, the setting of exhaust outlets should be carefully considered.
Resin selection
The selection of resin system is the key to RTM process. The optimal viscosity is 0.025-0.03Pa • s when the resin is released into the mold cavity and rapidly infiltrated into the fibers. Polyester resin has a low viscosity and can be completed by cold injection at room temperature. However, due to the different performance requirements of the product, different types of resins will be selected, and their viscosity will not be the same. Therefore, the size of the pipeline and injection head should be designed to meet the flow requirements of suitable special components. The resins suitable for RTM process include polyester resin, epoxy resin, phenolic resin, polyimide resin, etc.
Selection of reinforcement materials
In the RTM process, reinforcing materials can be selected such as glass fiber, graphite fiber, carbon fiber, silicon carbide, and aramid fiber. Varieties can be selected according to design needs, including short cut fibers, unidirectional fabrics, multi axis fabrics, weaving, knitting, core materials, or preforms.
From the perspective of product performance, the parts produced by this process have a high fiber volume fraction and can be designed with local fiber reinforcement according to the specific shape of the parts, which is beneficial for improving product performance. From the perspective of production costs, 70% of the cost of composite components comes from manufacturing costs. Therefore, how to reduce manufacturing costs is an important issue that urgently needs to be solved in the development of composite materials. Compared with the traditional hot pressing tank technology for manufacturing resin based composite materials, the RTM process does not require expensive tank bodies, greatly reducing manufacturing costs. Moreover, the parts manufactured by the RTM process are not limited by tank size, and the size range of the parts is relatively flexible, which can manufacture large and high-performance composite components. Overall, the RTM process has been widely applied and rapidly developed in the field of composite material manufacturing, and is bound to become the dominant process in composite material manufacturing.
In recent years, composite materials products in the aerospace manufacturing industry have gradually shifted from non load bearing components and small components to main load bearing components and large integrated components. There is an urgent demand for the manufacturing of large and high-performance composite materials. Therefore, processes such as vacuum assisted resin transfer molding (VA-RTM) and light resin transfer molding (L-RTM) have been developed.
Vacuum assisted resin transfer molding process VA-RTM process
The vacuum assisted resin transfer molding process VA-RTM is a process technology derived from the traditional RTM process. The main process of this process is to use vacuum pumps and other equipment to vacuum the inside of the mold where the fiber preform is located, so that the resin is injected into the mold under the action of vacuum negative pressure, achieving the infiltration process of the fiber preform, and finally solidifying and forming inside the mold to obtain the required shape and fiber volume fraction of the composite material parts.
Compared to traditional RTM technology, VA-RTM technology uses vacuum pumping inside the mold, which can reduce the injection pressure inside the mold and greatly reduce the deformation of the mold and fiber preform, thereby reducing the performance requirements of the process for equipment and molds. It also allows RTM technology to use lighter molds, which is beneficial for reducing production costs. Therefore, this technology is more suitable for manufacturing large composite parts, For example, foam sandwich composite plate is one of the commonly used large components in the aerospace field.
Overall, the VA-RTM process is highly suitable for preparing large and high-performance aerospace composite components. However, this process is still semi mechanized in China, resulting in low product manufacturing efficiency. Moreover, the design of process parameters mostly relies on experience, and intelligent design has not yet been achieved, making it difficult to accurately control product quality. At the same time, many studies have pointed out that pressure gradients are easily generated in the direction of resin flow during this process, especially when using vacuum bags, there will be a certain degree of pressure relaxation at the front of resin flow, which will affect resin infiltration, cause bubbles to form inside the workpiece, and reduce the mechanical properties of the product. At the same time, uneven pressure distribution will cause uneven thickness distribution of the workpiece, affecting the appearance quality of the final workpiece, This is also a technical challenge that the technology still needs to solve.
Light resin transfer molding process L-RTM process
The L-RTM process for lightweight resin transfer molding is a new type of technology developed on the basis of traditional VA-RTM process technology. As shown in the figure, the main feature of this process technology is that the lower mold adopts a metal or other rigid mold, and the upper mold adopts a semi rigid lightweight mold. The interior of the mold is designed with a double sealing structure, and the upper mold is fixed externally through vacuum, while the interior uses vacuum to introduce resin. Due to the use of a semi-rigid mold in the upper mold of this process, and the vacuum state inside the mold, the pressure inside the mold and the manufacturing cost of the mold itself are greatly reduced. This technology can manufacture large composite parts. Compared with traditional VA-RTM process, the thickness of the parts obtained by this process is more uniform and the quality of the upper and lower surfaces is superior. At the same time, the use of semi-rigid materials in the upper mold can be reused, This technology avoids the waste of vacuum bags in the VA-RTM process, making it highly suitable for manufacturing aerospace composite parts with high surface quality requirements.
However, in the actual production process, there are still certain technical difficulties in this process:
(1) Due to the use of semi-rigid materials in the upper mold, insufficient rigidity of the material can easily lead to collapse during the vacuum fixed mold process, resulting in uneven thickness of the workpiece and affecting its surface quality. At the same time, the rigidity of the mold also affects the lifespan of the mold itself. How to choose a suitable semi-rigid material as the mold for L-RTM is one of the technical difficulties in the application of this process.
(2) Due to the use of vacuum pumping inside the L-RTM process technology mold, the sealing of the mold plays a crucial role in the smooth progress of the process. Insufficient sealing can cause insufficient resin infiltration inside the workpiece, thereby affecting its performance. Therefore, mold sealing technology is one of the technical difficulties in the application of this process.
(3) The resin used in the L-RTM process should maintain a low viscosity during the filling process to reduce injection pressure and improve the service life of the mold. Developing a suitable resin matrix is one of the technical difficulties in the application of this process.
(4) In the L-RTM process, it is usually necessary to design flow channels on the mold to promote uniform resin flow. If the flow channel design is not reasonable, it can cause defects such as dry spots and rich grease in the parts, seriously affecting the final quality of the parts. Especially for complex three-dimensional parts, how to design the mold flow channel reasonably is also one of the technical difficulties in the application of this process.
Post time: Jan-18-2024
