RTM Process and 3D Braided Composite Material Manufacturing Technology

 

Three-dimensional braided composite materials are formed by weaving dry preforms using textile technology. The dry preforms are used as reinforcements, and resin transfer molding (RTM) or resin film infiltration (RFI) is used for impregnation and curing to directly form a composite material structure. As an advanced composite material, it has become an important structural material in the fields of aviation and aerospace, and has been widely used in the fields of automobiles, ships, construction, sports goods and medical equipment. The classical laminate theory of traditional composite materials can no longer meet the analysis of its mechanical properties. Domestic and foreign scholars have established new theories and analysis methods.

 

Three-dimensional braided composite materials are one of the imitation woven composite materials. They are composite materials reinforced by fiber braids (also known as three-dimensional preforms) woven using braiding technology. They have high specific strength, specific modulus, high damage tolerance and fracture toughness, impact resistance, cracking resistance and fatigue resistance. As an advanced composite material, three-dimensional braided composite materials have attracted much attention in the engineering community and have become important structural materials in the fields of aviation and aerospace, and have been widely used in the fields of automobiles, ships, construction, sports goods and medical equipment.

 

The development of three-dimensional braided composite materials is because the composite materials made of unidirectional or bidirectional reinforced materials have low interlayer shear strength, poor impact resistance, and cannot be used as main load-bearing parts. L.R.Sanders introduced three-dimensional braiding technology into engineering applications in 1977. The so-called 3D braiding technology is a three-dimensional seamless complete structure obtained by arranging long and short fibers in space according to a certain rule and interweaving them, so that the composite materials no longer have interlayer problems and the damage resistance is greatly improved.

 

Its process characteristics are that it can produce various regular shapes and special-shaped solid bodies, and can make the structural parts multifunctional, that is, weaving multi-layer integral components. At present, there are about 20 ways of three-dimensional braiding, but there are 4 commonly used ones, namely polar braiding, diagonal braiding or packing braiding, orthogonal braiding and warp interlock braiding. There are many types of three-dimensional braiding, such as two-step three-dimensional braiding, four-step three-dimensional braiding, and multi-step three-dimensional braiding.

 

History of Resin Transfer Molding

 

The main molding process of 3D woven composite materials is Resin Transfer Molding (RTM), which is a process method of injecting liquid resin into a closed mold to infiltrate the reinforcing material and solidify it. It is a molding process that has developed rapidly in recent years and is suitable for the production of multi-variety, medium-batch, high-quality advanced composite products. It is a production method for parts close to the final shape and basically does not require subsequent processing.

 

RTM technology originated from the “MARCO” method in the 1940s and was originally developed for molding aircraft radomes. Although RTM has a low cost, it has high technical requirements, especially for raw materials and molds. It is difficult to promote it on a large scale, so it has developed slowly.

 

By the 1980s, due to the increasingly stringent regulations on production environment requirements in industrially developed countries; at the same time, with the development of raw materials, processes and continuous progress in molding technology, coupled with the many advantages of the RTM process itself, such as small tolerances of molded parts, high surface quality, lower molding pressure than SMC (Sheet Molding Compound), various production and processing organization methods, low investment, and high production efficiency, it has attracted the attention of various countries.

 

In the late 1980s, with the changes in the world’s political and economic situation, RTM was considered to be one of the important technologies to solve the high cost problem of advanced composite materials. Japan recommended RTM and pultrusion as the most promising processes. NASA in the United States included RTM technology in its Advanced Composite Materials Program (ACT Program) and organized a lot of research work. At the same time, the civilian composite materials industry has seen a boom in RTM research and application under the pressure of production costs, production cycles and new environmental protection requirements.

 

Around 1985, the second generation of RTM, which aims to shorten the molding cycle, improve the smoothness of the surface quality and improve the quality stability, began to be applied. The third generation of RTM molding process, characterized by higher efficiency, began to be applied in the mid-1990s.

 

The domestic RTM process started in the late 1980s. Influenced by the rapid development of international RTM technology at that time, RTM injection equipment and process methods once became a “hot spot”. However, due to the imperfect raw material supporting system and the lack of basic process theory research at that time, large-scale production failed to be formed, and most of the equipment was idle.

 

After the 1990s, some domestic units (such as the Composite Materials Research Institute of Tianjin Polytechnic University) actively researched and promoted RTM process technology, and carried out systematic research work from raw materials, product design, mold design and manufacturing, surface technology and basic theory, as well as industrial production technology.

 

After entering the 21st century, with the rapid development of three-dimensional weaving technology, RTM process technology has been widely used in aircraft structural components and other military facilities and products. With the application of Light-RTM and SCRIMOP in yachts and wind turbine blades, the application advantages of this type of process have been increasingly recognized by everyone.

 

RTM Process Characteristics

 

An important development direction of RTM process is the integral molding of large parts. Its process methods are represented by VARTM, Light-RTM, and SCRIMP processes. The research and application of RTM process technology involves multiple disciplines and technologies, and is one of the most active research fields in international composite materials.

 

Its main research directions include: preparation of low-viscosity, high-performance resin systems and their chemical kinetics and rheological properties; preparation and permeability characteristics of fiber preforms; computer simulation technology of the molding process; online monitoring technology of the molding process; mold optimization design technology; development of new process equipment; cost analysis technology, etc.

 

RTM is widely used in the fields of ships, military facilities, national defense engineering, transportation, aerospace and civil industry due to its excellent process performance. Its main features are as follows:

 

(1) The mold manufacturing and material selection are highly flexible. According to different production scales, the equipment changes are also flexible, and the product output is between 1,000 and 20,000 pieces/year.

 

(2) It can manufacture complex parts with good surface quality and high dimensional accuracy, and its advantages are more obvious in the manufacture of large parts.

 

(3) It is easy to achieve local reinforcement and sandwich structure; the type and structural design of reinforcement materials can be flexibly adjusted to meet the requirements of different performances from civil to aerospace industries.

 

(4) The fiber content can reach up to 60%.

 

(5) The RTM molding process is a closed mold operation process with a clean working environment and low styrene emissions during the molding process.

 

(6) The RTM molding process has strict requirements on the raw material system, requiring the reinforcement material to have good resistance to resin flow erosion and wettability, low resin viscosity, high reactivity, medium temperature curing, low curing exothermic peak, low viscosity during impregnation, and quick gelation after injection.

 

(7) Low-pressure injection, generally the injection pressure is <30psi (1psi=68.95Pa), and FRP molds (including epoxy molds, FRP surface electroplated nickel molds, etc.) can be used. The mold design has high freedom and low mold cost.

 

(8) The product has low porosity. Compared with the prepreg molding process, the RTM process does not require the preparation, transportation, and storage of frozen prepregs, does not require complicated manual layering and vacuum bag pressing processes, and does not require heat treatment time. It is simple to operate.

 

However, in the RTM process, since the resin and fiber are shaped through the impregnation process during the molding stage, the flow of the fiber in the mold cavity, the fiber impregnation process, and the resin curing process have a great influence on the performance of the final product, which leads to increased complexity and uncontrollability of the process.

 

 RTM Molding Process

 

The RTM molding process is to first lay the reinforcement material preform, core material and embedded parts in the mold cavity, then inject the resin into the closed mold cavity under pressure or vacuum force, infiltrate the fiber, demold after curing, and then perform secondary processing and other post-processing processes. The basic principle is shown in Figure 1.

 

 

Fiber preforming includes manual laying, manual fiber laying plus mold hot pressing preforming, robot jetting chopped fiber hot pressing preforming, three-dimensional weaving and other forms.

 

In the process of mold closing and locking, according to different production forms, some mold locking mechanisms are installed on the mold, some use external mold closing and locking equipment, and vacuum assistance can be used to provide locking force while locking the mold. The mold can be vacuumed to reduce the influence of the internal pressure generated by resin filling on the deformation of the mold.

 

In the resin injection stage, the viscosity of the resin is required to be kept as unchanged as possible to ensure the uniform flow and full impregnation of the resin in the mold cavity. After the filling process is completed, the resin in each part of the mold is required to be solidified synchronously to reduce the influence of thermal stress generated by curing on product deformation.

 

Different types of RTM production layouts

 

With the rapid development of raw material technology, mold technology and equipment technology, RTM production layouts have also appeared in various forms. According to production efficiency, the development of RTM technology can be divided into three generations.

 

The first generation RTM process is usually room temperature curing and external heating, with a production cycle of 80~150min. The production layout often adopts a circular production line. The mold flows in different stations. There are many molds. The production cycle depends on the longest process, usually the curing process.

 

The second generation RTM process is characterized by the mold’s own heating system and a special mold opening and closing locking mechanism. The production efficiency can reach 20~30min. The representative one is the dual-station RTM process layout. When one station is spraying gel coat and laying fiber, the other station can perform injection and curing processes.

 

The third generation RTM process uses a curing temperature of about 120℃. The mold is driven by a special press to realize mold opening, mold closing and locking. The equipment uses high-speed injection equipment and the mold uses a metal mold. The overall layout is similar to the SMC process, and the molding cycle is less than 10min.

 

The process parameters that affect the RTM process include resin viscosity, injection pressure, molding temperature, vacuum degree, etc. At the same time, these parameters are interrelated and affect each other during the molding process.

 

(1) Resin viscosity. Resins suitable for RTM process should have a lower viscosity, usually less than 600mPa·s. When it is less than 300mPa·s, the process performance will be better. Resin viscosity can be reduced by increasing the molding temperature of the resin to facilitate better filling process.

 

(2) Injection pressure. The selection of injection pressure depends on the fiber structure and fiber content as well as the required molding cycle. Research data show that lower injection pressure is conducive to full impregnation of the fiber and to the improvement of mechanical properties. The injection pressure can be reduced by changing the product structure design, fiber layer design, reducing resin concentration, optimizing the location of injection port and exhaust port, using vacuum assistance, etc.

 

(3) Molding temperature. The selection of molding temperature is affected by the heating method that the mold can provide, the resin curing characteristics and the curing system used. Higher molding temperature can reduce the viscosity of the resin, promote the flow and impregnation of the resin inside the fiber bundle, and enhance the interfacial bonding ability of the resin and the fiber.

 

(4) Vacuum degree. Using vacuum assistance during the molding process can effectively reduce the rigidity requirements of the mold, while promoting the removal of air during the injection process and reducing the porosity content of the product. According to experimental data, the average porosity content of the flat plate molded under vacuum conditions is only 0.15%, while the porosity content of the flat plate without vacuum reaches 1%.

 

RTM Equipment and Molds

 

RTM resin injection equipment includes a heating constant temperature system, a mixing agitator, a metering pump and various automatic instruments. Injection machines can be divided into four types according to the mixing method: single-component type, two-component pressurized type, two-component pump type and catalyst pump type. The injection machines currently used for mass production are mainly catalyst pump type.

 

The RI-2 equipment manufactured by Aplicator of Sweden has made a big step forward in the RTM process towards high-quality and high-speed full-system production. The Multiflow RTM equipment manufactured by Liquid Control Systems of the United States can meter, mix and inject reactive resin systems from a few grams to hundreds of kilograms into low-pressure closed molds.

 

The Multiflow CMFH type equipment is used to manufacture large reinforced material parts with an input of 45kg/min. It can be used for a variety of resin systems. The injection machines produced by Plastech TT of the United Kingdom take into account the centralized control of multiple production parameters, among which the Megaject Pro type injection machine is the most automated one.

 

RTM is molded under low pressure, and the mold rigidity requirements are relatively low. A variety of materials can be used to manufacture molds. Commonly used mold types include fiberglass molds, electroformed nickel molds, aluminum molds, cast iron molds, and steel molds.

 

Generally speaking, the RTM process has the following requirements for the mold:

(1) Maintain the shape and dimensional accuracy of the product and the matching accuracy of the upper and lower molds, so that the product can achieve the designed surface accuracy;

(2) Have a device for reliably clamping and pushing open the upper and lower molds and a device for demolding the product;

(3) Sufficient rigidity and strength to ensure that there is no damage and the deformation is as small as possible during mold closing, mold opening and injection;

(4) It can be heated and ensure the service life at a certain resin molding curing temperature, and no cracking and deformation will occur during use;

(5) Have reasonable injection ports, risers, and circulation to ensure that the resin fills the mold cavity and removes the gas in the product;

(6) Have a suitable mold cavity thickness so that the mold has a suitable compression amount on the preform;

(7) The upper and lower molds should have good sealing. For processes without vacuum assistance, the resin leakage rate should be less than 1%. For processes with vacuum assistance, the seal should ensure that there is no leakage to prevent gas from entering the mold cavity;

(8) With appropriate materials and manufacturing costs, meet the requirements of the number of molded products and mold life.

 

Derivative Technologies of RTM

 

RTM technology has developed rapidly. At present, based on the basic molding process mentioned above, some special RTM technologies have been derived, including vacuum-assisted RTM (VARTM), compression RTM (CRTM), Seemann’s composite resin infiltration molding (SCRIMP), resin film infiltration molding (RFI), thermal expansion RTM (TERTM), flexible RTM (FRTM) and co-injection RTM (CIRTM).

 

The yarns inside the three-dimensional braided composite material are interwoven in the plane and three-dimensional space to form a non-layered and complex overall structure. Therefore, at the beginning of the study of braided composite materials, it was mainly observed and studied through test instruments and equipment. In the 1980s, many foreign scholars began various experimental studies on three-dimensional braided composite materials, mainly studying the effects of various parameters of yarns and resins on the mechanical properties of braided composite materials such as tension, compression, bending and interlayer shear. The domestic experimental research started relatively late, and it was not reported until the late 1990s. At present, various experimental studies including low-speed impact and high-energy collision have been carried out.

 

Geometry Model of Microstructure 

 

Since the basis of woven composite materials is textile technology, the study of three-dimensional woven composite materials must first clarify the geometric model of textile structure. The United States is one of the earliest countries to study woven composite materials. The more typical geometric models in the 1980s were the three geometric models first proposed by Frank K. Ko and Tsu-Wei Chou, pioneers of American woven technology, and their collaborators:

 

The first is the “orientation average model” of a unit cell proposed by Ko and Pastore based on yarn fragments in three-dimensional braids.

 

The second is called the “‘M’-shaped branch model”. Ma and Yang et al. regarded the unit cell structure of the four-step woven composite material as composed of three mutually orthogonal yarns and four diagonal yarns, and established a microscopic analysis model for the interaction of these yarns;

 

The third is the “fiber tilt model” of Yang and Ma et al.

 

Yang et al. took the four-way woven composite material woven by the four-step method as the object, and established a fiber deflection model based on the characteristics of the zigzag arrangement of the fiber bundles in its preform. It is believed that the fiber bundles in a single cell are arranged along the four diagonal directions of the cuboid, and form a thin inclined plate after being injected into the matrix. The four inclined one-way plates form a unit, as shown in Figure 2.

 

After entering the 1990s, researchers from various countries conducted more in-depth research on three-dimensional braided composite materials, braiding procedures, and the direction of yarns during the braiding process, and obtained a more complete and reasonable microscopic model of braided composite materials. Among them, Du and Ko introduced four different braiding methods, established a geometric solid model of three-dimensional braiding by the unit cell method, and gave the relationship between key braiding parameters and fiber braiding angle and fiber volume content. Wang and Wang proposed an analytical method to describe the yarn topology structure of three-dimensional braided preforms.

 

First, the method of defining control volumes was used to describe the spatial trajectory of the braided yarns formed during the braiding process. Based on the yarn topology, three different unit cell models were defined, representing the internal, surface, and corner structures of the preform. The geometry of the internal unit cell is a cuboid, containing four groups of interwoven yarns, with a height of one braided knot length. The internal yarn structure is consistent with Li’s results. The geometric shapes of the surface and corner unit cells are both triangular prisms, with a height equal to the length of the braided knot. The surface unit cell contains two sets of interwoven braided yarns, while the corner unit cell contains only one set of parallel straight braided yarns. In the analysis, the cross-sectional shape of the textile yarn is ignored.

 

In China, Wu Delong and Hao Zhaoping first proposed the “three-cell model” based on the four-step method. From the perspective of microscopic analysis, the structure of textile composite materials is composed of face elements (F.C) on the boundaries of repeated internal primitives (B.C) and column elements (R.C) at the corner points, as shown in Figure 3. The characteristic of the three-cell model is that it can well describe the microstructure of the fabric according to the braiding geometry, and can analyze the influence of tensile and compressive dual modulus materials, matrix elastic-plastic materials and interface damage on mechanical properties. Chen Li, Tao Xiaoming and others studied the structure of the four-step three-dimensional braided fabric, revealing the different configurations of yarns in the preformed internal, surface and corner areas, and established the relationship between the braiding structure and the braiding parameters. Pang Baojun and others took four-way braided composite materials as the object, established the geometric structure model of the unit cell, and conducted microscopic experimental verification.

 

 

In recent years, many scholars have paid great attention to the microstructure of rectangular woven materials, and gradually established unit cell geometric models from simple “M” shape to three-dimensional solid, which promoted the development of mechanical models. Zheng Xitao and Ye Tianqi systematically studied the microstructure of preforms and reinforced composites prepared by four-step 1×1 square weaving process. The assumption of elliptical cross-section of yarn was proposed, and the influence of fineness of weaving yarn and filling factor of weaving yarn was considered to create a positive axis model. According to the motion trajectory characteristics of the yarn carrier during weaving, the preform was divided into three different areas, and different control volume units were defined respectively. The relationship between the parameters of the weaving structure was identified, and the design method of three-dimensional woven composite materials was given. The schematic diagram of three-dimensional weaving is shown in Figure 4.

 

 

Feng Wei and Ma Wensuo separated the continuous yarns in the braided fabric and expressed them with special point symbols. They used point groups and space groups to analyze the geometric structure of existing braided materials. On the one hand, they can reasonably describe and classify the geometric structure of existing braided materials; on the other hand, they can also use this theory to derive new and more effective braiding methods for the geometric structure of braided materials. Zhang Meizhong et al. Due to the complexity of the actual structure of braided composite materials, in order to make the research results more realistic, it has become a trend to simulate three-dimensional braided composite materials with existing mature finite element software and study their various mechanical properties.

 

Pandey et al. used CAD modeling technology to describe the representative unit body of three-dimensional braided composite materials, vividly and accurately reproducing the complex internal structure of the composite material. Sun et al. proposed a digital unit method and used this method to simulate the braiding process of three-dimensional rectangular braided structures to accurately know the path of each yarn inside the three-dimensional braided composite material and the microstructure of the preform. With the help of the parametric graphic modeling characteristics of VC++ and SolidWorks software, a software system that can simulate the pore entities of preforms with various braiding parameters and calculate the pore volume and surface area was established.

 

Theoretical Study of Mechanical Behavior

 

The mechanical model of three-dimensional braided composite materials is based on the geometric model of the above-mentioned mesostructure. From the 1980s to the present, representative works include the elastic strain energy method of Ma and Yang, the fiber tilt model of Yang and Ma, the three-cell model of Wu Delong and Hao Zhaoping, the eccentric model of Chen Li and the normal axis model of Zheng Xitao.

 

In the late 1990s, Liang Jun et al. applied Eshelby and Mori-Tanaka theory to conduct mesomechanical analysis of three-dimensional braided composite materials, and then combined with the stiffness averaging method to theoretically predict the elastic constants of three-dimensional braided composite materials containing coin-shaped matrix microcracks. Sun Huiyu developed a fiber tilt model based on foreign models, considered the effect in the thickness direction, used three-dimensional stress-strain analysis, predicted the effective elastic modulus, and introduced this spatial multi-directional laminate mechanical model into the prediction of strength performance.

 

Wang Bo et al. proposed a stiffness synthesis method to predict the shear elastic modulus of woven composite materials, compared the theoretical shear performance of the whole woven specimen and the specimen obtained by cutting, and analyzed the change of shear performance with the number of internal unit cells of the specimen along the width and thickness directions. At present, there are also reports on the elastic properties, damage, strength and performance of woven composite materials under abnormal conditions. Based on the variational principle, Chen Li and Tao Xiaoming et al. proposed to use the finite multiphase unit method to predict the elastic properties of three-dimensional woven composite materials.

 

Liu Zhenguo and Lu Zixing et al. proposed a finite element calculation model of the “M”-shaped body cell to predict the shear performance of woven composite materials. Huang Zhengming established a “bridge model” and analyzed the stiffness and strength properties of woven composite materials. Xu Kun and Xu Xiwu established a progressive damage tensile strength model of three-dimensional four-way woven composite materials based on the octagonal fiber bundle cross-section unit cell model and the microscopic nonlinear finite element method. Zeng Tao et al. proposed a multiphase finite element numerical method using a four-fiber cell model, and predicted the nonlinear response and damage evolution of three-dimensional braided composites based on the Tsui-Wu failure criterion and the Mises criterion. Alzina et al. used a multiscale analysis method to predict the thermoelastic properties of braided composites at low temperatures.

 

Conclusion

 

Three-dimensional braided composite materials have been widely used in many fields such as aerospace, and their application scope will continue to expand in the foreseeable future. Relatively speaking, the theoretical and experimental research on three-dimensional braided composite materials is relatively lagging behind.

 

Due to the complex fiber structure of three-dimensional composite materials, coupled with many factors such as weaving process parameters, structural parameters, extrusion deformation of preforms in the composite process, mechanical properties of braided yarns and matrix, void ratio, and interface damage between textile yarns and matrix, it affects the analysis and estimation of its structure and mechanical properties.

 

The theoretical and process research work of three-dimensional braided composite materials is still in the exploratory development stage. The relevant three-dimensional braiding process theory needs to be further improved, and new process methods need to be developed.

 

The method of analyzing the mechanical properties of three-dimensional braided composite materials needs to be further developed. Establishing a relatively complete strength criterion is the theoretical basis for expanding the use of three-dimensional braided composite materials. At the same time, seeking an accurate solution to the micromechanical solution of three-dimensional braided composite materials is also an urgent problem to be solved.