Resin Transfer Moulding (RTM) is a process method in which resin is injected into a closed mold to impregnate the reinforcing material and cure it. This technology can eliminate the need for prepregs and hot press tanks, effectively reducing equipment costs and molding costs. The technology has been developing rapidly in recent years, and has been widely used in the aircraft industry, automobile industry, naval industry and other fields, and has developed various branches such as RF1, VARTM, SCRIMP, SPRINT, etc., to meet the needs of different fields of application.
RTM started in the 1950's and is a closed molding technology that improves on the hand lay-up molding process SMC, BMC molding, injection molding, RTM, VEC technology are all closed molding processes. Due to the development of environmental laws and improved product requirements, open molding of composite materials is increasingly restricted, prompting the application of closed molding technology, in recent years, especially to promote the innovation and development of RTM technology Basic Principle.
The basic principle of RTM is to put in a well-designed mold in advance by the reasonable design, cutting or by the mechanization of the pre-formed reinforced material. The mold needs to have peripheral sealing and fastening, and to ensure the smooth flow of resin; after the mold is closed and injected with a fixed amount of resin, the resin can be removed from the mold to obtain the desired product after curing.
To sum up, through RTM process, we can
① manufacture two-side glossy products;
② high molding efficiency;
③ closed-mold operation, no pollution of the environment, no harm to the health of workers,
④ reinforcing materials can be placed in any direction, it is easy to realize the reinforcing materials according to the stress condition of the product example;
⑤ raw materials and energy consumption is low;
⑥ less investment in the construction of the plant.
Origin of RTM process
The molding process of composite materials is the key to improve and enhance the performance of advanced resin matrix composites. The traditional molding processes are hand lay-up molding, molding, winding molding, pultrusion molding, spray molding, injection molding, etc. Hand lay-up molding, which requires manual work to lay the fiber reinforcing material and resin alternately in the mold, the pre-production process is cumbersome, the production efficiency is low, the safety of the production is poor, and the cost of labor is higher, which is unsuitable for large-scale production.
Hand Layup Molding Flow Chart
Nowadays, the rapidly developing resin transfer molding (RTM) molding process can effectively avoid the many shortcomings of hand-layup molding, has the advantages of efficient production of large-area complex components, low labor and consumption costs, high-efficiency rapid prototyping and environmentally friendly, etc., and the mold is easy to make, strong designability, good stability of the laminate size and structure, excellent mechanical properties, and the surface finish of the manufactured laminates are good. Beautiful and other excellent features, for the mass industrialization of advanced resin matrix composites production provides the conditions, so as to meet the growing demand for composite material applications.
Conventional RTM Process
The conventional RTM process process is shown below. The presence of air in the mold can cause significant defects in the finished product. In order to be able to output air from inside the mold, the RTM mold must have at least one inlet for resin injection and one outlet, and sometimes multiple inputs and outputs. Depending on whether the hardener and resin are mixed before or after injection, this molding process is divided into one-component injection and two-component injection, respectively, where the two-component injection is mixed in a mixer at a specific ratio before injection into the mold cavity.
Compared to traditional processes, such as hand molding and compression molding, RTM has the advantages of simplifying the steps of the production process, improving the production efficiency, greatly reducing the labor cost, avoiding the safety and performance problems caused by the workers' contact with the resin and other raw materials, and providing a way to mass produce advanced composite materials.
However, RTM molding process still has some disadvantages, such as.
1) The impregnation rate of resin to the reinforcing fiber is not high, and there are defects such as air holes, dry spots and rich resin, which seriously affect the performance and quality of the products.
2) Due to the reinforcing material in the mold cavity to go through the flow of resin under pressure and mold filling process, will drive or even wash away the fiber, resulting in composite molding products in the fiber flexure, uneven distribution of fiber or even less fiber content, so that the mechanical properties of the product is greatly reduced.
3) When making large products, the cavity area is large, the phenomenon of uneven flow of resin may occur during the molding process, and it is difficult to predict and control the actual flow of resin and the degree of fiber infiltration to a certain extent.
Based on the above advantages and disadvantages of RTM, the late improvement and development of high-pressure resin transfer molding (HP-RTM), vacuum-assisted resin infusion process (VARTM), SCRIMP resin impregnation technology (SCRIMP) molding process, lightweight RTM (LRTM) and other processes.
Advanced development of the RTM process
HP-RTM molding process is a derivation of the development of the RTM process, mainly divided into High-pressure injection molding (HP-IRTM) and high-pressure compression molding (HP-CRTM). The overall purpose of these processes is to produce products with low porosity and high fiber volume fraction by means of high pressure. This type of process requires high mold hardness and other requirements, otherwise it is prone to deformation, which can cause problems such as dry fibers and fiber precipitation.
HP-IRTM Process
The HP-IRTM molding process begins with the placement of the fiber precursor into the mold cavity to form a completely closed cavity, followed by vacuuming, these steps are the same as those of the traditional RTM process. The difference is that the resin and hardener are mixed and then injected into the mold cavity under high pressure, which can greatly shorten the filling time and provide production efficiency, while maintaining the surface quality and good morphology of the product, and can produce products with larger fiber volume fraction.
HP-CRTM process
HP-CRTM molding process is to put the fiber precast body into the mold cavity, there is a gap between the surface of the upper mold and the fiber precast body, after the same vacuum, so the injection pressure of this process can be much smaller than HP-IRTM, the mold hardness requirements are also lower than the HP-IRTM, but after the injection need to have compression process mold pressure control to close the mold gap, as the gap is reduced the resin is completely extruded into the fiber to get the thickness of the final product. Decrease the resin completely extruded into the fiber, the gap is completely closed to obtain the thickness of the final product this step will significantly affect the product's fiber volume fraction, while the fiber volume fraction is a key factor in characterizing the balance between mechanical properties and processing performance.
Working Principle of VARTM
VARTM is a new cost-effective process for single-sided molding, which is suitable for the production of large-scale parts. After the prefabricated body is put into the mold, the top is sealed with a vacuum bag, and the resin is injected under vacuum (the injection pressure is usually <0.6985 MPa) or directly sucked into the resin by using the negative vacuum pressure, which effectively avoids defects such as air bubbles arising from resin impregnation of fiber prefabricated body, and the resin has better mobility at this time, and it is able to impregnate the fiber prefabricated body sufficiently, and then it is finally cured and de-filmed at ambient temperature, and then the composites products are obtained. In aerospace, VARTM is utilized to produce composite products.
In aerospace, VARTM molding process can be used to obtain better satellite antenna reflector. The manufacturing of the reflector requires that the reflective surface should avoid deformation as much as possible during the fabrication process, which improves the precision and stability of the dimensions and ensures the good quality of the reflector. Some researchers have done experiments to verify that the rebound deformation of the reflector reflector surface manufactured using the autoclave manufacturing process is 0.37 mm; while the rebound deformation of the reflector reflector surface manufactured using the VARTM process is 0.35 mm, which is a difference of nearly 5%, and this fully reflects the performance advantages of the VARTM molding process in the manufacture of satellite reflectors.
VARTM molding process not only has low process cost and high economic benefits, but also has high mechanical properties and stability of the products, etc., and also reduces the VOC pollution to a certain extent, which is one of the effective ways to make large-scale composite products in the future. RTM Process Application Prospect
In the wide application of composite materials today, the derived process of RTM has been improved and supplemented by the researchers, and it has a unique status among the molding processes, and has shown a good performance among the molding processes. Today, the RTM process is widely used in composites, through the continuous improvement and supplementation of researchers, it is unique among various molding processes and shows enduring vitality.
Improvements in tooling and control technology continue to reduce waste, VOC emissions and costs, which is not only beneficial to the large-scale commercial production of composites to meet the growing demand for materials, but also conducive to green and sustainable development, and to achieve the environmental protection requirements of the "green process".
Whether it is the automobile industry, which is closely related to people's livelihood, or the military and aerospace industry, which is related to the strength of the country, RTM and its derivatives can meet the requirements.
In the future, the development of RTM and its derivatives will be in line with the Internet era, and the support of computer simulation technology will bring unlimited possibilities to the RTM process.
Source: cnfrp.com