Resin Transfer Molding Technology Has Developed Rapidly In Recent Years, Relying Not Only On Low Cost

 

Resin Transfer Molding (RTM) technology belongs to the category of liquid composite molding technology. The RTM process originated in the late 1940s and has developed rapidly due to its outstanding characteristics such as good product quality, high production efficiency, low manufacturing cost, and easy production of integral composite components.

The process of RTM technology: First, lay out the reinforcement material preform consistent with the structural form of the workpiece in the membrane cavity; then, under a certain temperature and pressure, use injection equipment to inject low-viscosity liquid resin into the closed membrane cavity. While impregnating the preform, the resin displaces all the gas in the membrane cavity; after the mold is filled with the mold cavity, the resin is cured by heating, and then the product is demolded.

At present, RTM technology has been widely used in aerospace, construction, communications, transportation, health, sports equipment and other fields. Especially in the aviation field, RTM technology has been widely used in the load-bearing and non-load-bearing structures of advanced aircraft with its advantages of high weight reduction and low cost.

 

 1. Characteristics of RTM technology

 

Compared with other molding processes, RTM technology has the following advantages.

 

1. High Production Efficiency

The manufacturing process can be mechanized or automated; it can form a variety of composite parts from small precision to large overall complex; for large and complex components with sandwich, reinforcement and embedded parts, it can be formed by one-step impregnation and curing, replacing the traditional molding process of 2-step or multi-step molding.

 

2. High Product Quality

The surface quality of the parts is high; the dimensional accuracy of the parts is high, and the tolerance is precisely controllable, with good stability and repeatability; the fiber volume content of the parts is high, about 55% ~ 60%; the void ratio is low, about 0~0.5%; the design freedom of the preform is large, and it can adopt all or part (stitching, Z-Pin, etc.) reinforcement and (thermoplastic resin, etc.) toughening technology, and can also adopt three-dimensional fabrics with strong integrity, and the parts have high tolerance for delamination damage.

 

3. Good Economic Effect

RTM process is a “non-autoclave” low-cost manufacturing process. The preforms and some resin systems used can be stored at room temperature, which can realize the manufacturing of net size and near net size of composite parts, reducing waste and machining costs; large, integral and complex components with sandwich, reinforcement and embedded parts can be impregnated in one step, replacing part or all of the assembly, saving assembly time and cost. The larger the batch, the lower the cost.

 

4. Environmentally Friendly

The closed mold process is beneficial to human health and environmental protection.

At the same time, the RTM process has the following limitations:

(1) It is difficult to design and manufacture molds with complex cavities.

(2) It is still difficult to dynamically monitor the resin filling of large-sized and complex-structured parts.

(3) The mold cost accounts for a large proportion of the process cost, and the cost advantage of small-scale production is limited.

 

2. Development of RTM Technology

After years of development, RTM technology has further improved production efficiency and reduced production costs. On the basis of the above basic molding process, some new RTM technologies have been derived, including Liquid Pressure Molding (LPM), Multiple Insert Molds (MIT-RTM), High Pressure RTM (HP-RTM), Compression RTM (Compression RTM), Thermal Expansion RTM (TERTM), Flexible RTM (FRTM), Co-injection RTM (CIRTM), Reaction Injection Molding (RIM), Solvent-Assisted RTM Process (SARTM), Resin Injection Cycle RTM (RIRTM) and Zero Injection Pressure RTM (ZIPRTM).

 

LPM Technology

LPM technology is a new liquid molding technology developed by AVIC Composites Co., Ltd. in 2012. The principle is to lay a pre-formed body designed according to the performance and structural requirements of the LPM process in the rigid mold cavity, and then use a pressure device to inject a special resin into the resin rapid flow channel around the pre-formed body in the closed mold cavity, and by adjusting the pressure of the pressure device on the glue outlet channel, the composite material part is subjected to the pressure required for curing, and finally the temperature is raised and cured according to the process specifications of the resin.

As a new and efficient composite liquid molding process technology, LPM technology has the advantages of high molding accuracy, the volume content of the molded composite fiber is equivalent to the prepreg/autoclave molding level, strong process applicability, high molding efficiency, low resin requirements, simple molding equipment and low manufacturing cost.

 

MIT-RTM Technology

 

MIT-RTM technology is an efficient RTM process technology developed by PlastechTT, a British company, in the late 1990s to address the shortcomings of low molding efficiency of traditional RTM technology.

The MIT-RTM device consists of multiple insert tooling (MIT) and a mold cavity support, positioning, sealing and heating system. Multiple insert liner molds, i.e. part molding surfaces, are multiple lightweight thin shells (about 4~7mm) separated from a reusable master mold with high dimensional accuracy, good dimensional stability, and these liner molds can be independently sprayed with gel coat and laid with reinforcing materials outside the mold cavity, saving the process waiting time occupied by the molding mold during injection and curing, and effectively improving production efficiency.

 

The Advantages of MIT-RTM technology include:

1. Most of the processes in the traditional RTM process occur inside the mold cavity. The MIT-RTM technology can decompose the processes occurring inside the mold cavity and move some of them outside the mold cavity to complete them, which greatly reduces the process waiting time and has high molding efficiency;
2. The cost of the liner mold accounts for a small proportion of the total equipment cost, and the cost advantage of mass production is obvious;
3. After the liner mold is damaged, it is easy to repair and replace, and the production process is basically unaffected.

 

HP-RTM Technology

HP-RTM is a new RTM process technology for mass production of high-performance thermosetting composite parts launched in recent years for low-cost molds. It adopts a high-temperature curing resin system. After the mold is filled, the resin is quickly cured to achieve a high degree of curing. It avoids the problem of low dimensional accuracy of products produced by low-cost molds.

The principle of HP-RTM technology is to lay a preform designed according to performance and structural requirements in the rigid mold cavity, and then use the high pressure provided by the injection equipment to inject special resin into the closed mold cavity under vacuum-assisted exhaust, and finally use the high pressure provided by the hydraulic press to complete the resin impregnation and curing.

 

The advantages of HP-RTM process technology mainly include:

1. Short resin filling time;
2. Rapid curing resin system can be used with short curing cycle;
3. High-pressure injection and curing are conducive to air removal and low product porosity;
4. High product surface quality and high dimensional accuracy;
5. High process stability and repeatability.

 

Compression RTM Technology

 

Compression RTM technology is a new liquid molding technology developed to solve the shortcomings of traditional RTM molding technology in molding composite products with high fiber volume content, such as low permeability of preforms, high pressure required for resin injection, and low molding efficiency.

The principle of compression RTM technology is to lay a preform designed according to performance and structural requirements in a rigid mold cavity, partially close the mold, inject the resin system into the mold cavity under low injection pressure, pressurize the mold to make it completely closed to the final position, and at the same time, the resin completes the full infiltration of the preform, and finally heats and cures according to the process specifications of the resin.

 

The advantages of compression RTM technology mainly include:

1. The resin flows and infiltrates along the thickness direction of the preform, and the filling time is short;
2. Composite products with high fiber volume content (63%) can be molded;
3. The porosity of the product is smaller than that of traditional RTM products;
4. It has high process stability and repeatability.

 

SARTM Technology

In view of the process characteristics of the polyimide resin system used in high-temperature RTM technology, such as polycondensation reaction, a large amount of volatile matter in the early stage, high melting point, and high viscosity, when using high-temperature resin system injection, solvent-based polyimide resin is mainly used, that is, during the process, a low-viscosity (<200mPa·s) solvent-containing resin solution is used for infiltration at room temperature. The operation period can be infinitely long, avoiding the high equipment requirements and short operation period caused by high-temperature injection. However, the main problem is that the solvent and polycondensation by-products need to be discharged before curing. The process is relatively complicated, which will make it difficult to control the glue content and thickness of the parts, and local pores are prone to appear. At present, foreign countries use polyimide resin systems such as PMR-15, PMR-Ⅱ, AFR700B for autoclave process, combined with three-dimensional weaving technology and SARTM process to successfully manufacture some complex composite components, such as engine air intake, fuel and lubricating oil external pipelines, adjustable stator blade sleeves and other components, as well as corrugated plates, diffuser blades, etc.

 

RTM（SRIM）

 

Structural Reactive RTM (SRIM)

This process first pre-places the long fiber reinforced mat in the mold cavity, and then uses the high-pressure impact force provided by the high-pressure metering pump to mix the two monomer materials evenly in the mixing head. Under certain temperature conditions, the mixed resin system is injected into the mold to cure and form a composite product.

 

The advantages of the structural reactive RTM process mainly include: 1. Short resin filling time; 2. Short molding cycle.

 

3. RTM process Equipment

 

RTM process equipment mainly includes three parts: mold, injection equipment and curing equipment, and each part will have a decisive influence on the entire development and production process.

RTM molding process is a closed mold operation process. The resin flow process, pressure and heat transfer process and curing process in molding all occur inside the mold. The quality of the molding mold will be directly “copied” on the component. Therefore, mold material selection and design are the prerequisites for obtaining high-quality, high-precision and good surface quality parts.

Generally speaking, the mold material selected for RTM molding process should have the advantages of fast thermal conductivity, low specific heat capacity, good thermal stability, small thermal expansion coefficient, good processing technology, light weight, long service life, low cost, easy use and maintenance. For RTM parts with large quantities or high surface quality requirements, steel, zinc-aluminum alloy or nickel-plated steel shell molds are generally selected.

Different molding methods have different requirements for mold structure. Compared with the traditional autoclave molding process, the mold used in the RTM molding process not only has to meet the rigidity and heat transfer requirements, but also has higher requirements on the matching accuracy of the upper and lower molds, the mold surface quality, the design of the injection port and the overflow port, the mold sealing and the demolding device of the product.

A successful RTM molding mold should have the following characteristics: exquisite structure, uniform temperature field, good system sealing, simple and easy operation such as mold closing and demolding, and the internal quality and external dimensional accuracy of the molded parts meet the technical requirements.

 

For the traditional RTM process, injection equipment is usually used for resin injection. The equipment generally includes a resin storage tank, a resin supply system (constant pressure injection and constant flow injection), a resin delivery pipeline, a metering pump and various automatic instruments. Injection equipment can be divided into three types according to the mixing method: single-component type, two-component type (pressurized type and pump type) and catalyst pump type.

The production efficiency of the product depends on the degree of automation of the molding equipment. In order to improve the injection efficiency, save the amount of resin as much as possible, and achieve product quality control, manufacturers around the world continue to develop new injection equipment. Aplicator of Sweden has developed a series of injection equipment, such as RI-2, RI-15 and IPR2-8000 series. These equipment are simple to use and easy to operate, and can efficiently and safely inject the mixture of resin and catalyst into the mold.

The Spartan injection equipment developed by GRACO of the United States has been well received by domestic and foreign manufacturers for its stable low-pressure characteristics, complete and reliable circulation system, perfect mixing effect and reliable cleaning device. ISOJET of France has developed professional two-component, single-component and special RTM injection equipment suitable for polyimide resins. The company’s equipment has also played a role in the research and development of RTM technology at home and abroad.

Compared with injection, the curing of resin can be carried out in different equipment, such as ovens, presses, and integral heating molds. When choosing a heating method, it is necessary to comprehensively consider the heating efficiency of each heating method, the temperature uniformity of the heating system, the controllable range of the heating and cooling rates, as well as the temperature control accuracy, operating costs and environmental protection. For oven heating, it is also necessary to consider whether the size of the oven and the bearing capacity of the inner and outer tracks can meet the needs of the parts.

 

With the development of RTM technology, more and more foreign aviation companies are using RTM molding platforms with heating systems to replace traditional RTM process equipment with high-power ovens + injection equipment.

The RTM molding platform system is mainly composed of pre-molding molds/mold frames, RTM molds/mold frames, heating units, hydraulic systems, control systems and RTM injection systems. The platform system can simultaneously realize RTM molding processes such as mold pressurization/heating, resin injection and curing. It can not only simplify the operation process, improve production efficiency, reduce energy consumption, and shorten the manufacturing cycle, but also reduce the design rigidity and manufacturing cost of RTM molding molds, and significantly improve the molding quality of parts.

In order to break the monopoly of imported equipment in the domestic market, Shanghai Bairuo Testing Instrument Co., Ltd. in China has developed the first RTM integrated molding equipment in China. The system organically integrates multiple equipment such as reactors, vacuum equipment, injection exhaust systems, heating systems, pressure holding systems, and molds according to the RTM process.

The upper and lower molds are moved separately, turned over, molded, and molded and turned over automatically, and the whole process is continuous and positioned. Mold heating, pressure holding and injection are all controlled by human-machine control, which is easy to operate. This set of equipment meets the requirements of RTM molding process and meets the requirements of RTM production automation of composite material manufacturers.

Recently, Pinette Emidecau of France has developed a complete automated high-speed RTM work unit for aviation manufacturing. The work unit has four work stations: robot preforming mold loading; mold closing and transfer; resin transfer molding (RTM) injection; and hot furnace curing after injection.

Among them, the processes of preform transfer, mold loading, mold closing, and transfer between workstations are handled by robots or other automated facilities, and the hot furnace can hold multiple molds. The work unit improves process consistency and production efficiency, and has been verified by manufacturing carbon fiber/epoxy resin aviation parts.

 

IV. Application Status of RTM Technology in the Aviation Field

 

The development direction of aviation composite materials is high performance, low cost and integration. RTM molding technology, as a low-cost composite material manufacturing method suitable for integral molding, has been widely used in the aviation field, and some applications have achieved continuous mass production.

With the improvement and perfection of the process level, RTM technology in the aviation field has been used only for non-load-bearing components, such as flap guide fairings and engine rear hanger fairings, and has been used for secondary load-bearing parts, such as doors, etc., and has now been successfully applied to main load-bearing components, such as aircraft propeller blades.

 

Aircraft Fuselage Structure

 

In the field of military aviation, more efficient and more integrated advanced composite materials are used. While improving strength, both material and structure weight reduction is achieved, meeting the high performance requirements of the new generation of combat aircraft. The F-22 is a significant example of the application of RTM technology in the main load-bearing structure. In the F-22 prototype project produced by Lockheed Martin in the United States, more than 400 parts are manufactured using this technology, typically wing sinusoidal corrugated beams, weapon bay caps, reinforcements, horizontal and vertical tails, front fuselage fuel tank frames, front fuselage bulkheads, and upper, lower and rear beams of the wing. Among them is the RTM-molded wing sinusoidal corrugated beam produced by subcontractor Dow-UT, which is about 4.5m long, has high dimensional accuracy, low manufacturing cost, and a 50% reduction in the number of parts.

In the development of the F-35 aircraft, RTM technology is used as a major integrated high-precision molding technology. The number of parts of the multi-cavity structure integral composite vertical tail developed using Co-RTM technology has been reduced from 13 to 1, and the number of fasteners has been reduced by thousands. While improving the degree of integration, the cost has been greatly reduced.

In the field of civil aviation, RTM technology has also been widely used, such as the vertical tail box connecting the ear piece and the cantilever beam of the horizontal tail of the Airbus A330/340 wide-body aircraft, and the propeller blades of the SAAB 340 and Fokker50 turboprop aircraft formed by braided RTM. In addition, there is also an integrated rudder (2.8m long, 1.1m wide, and 25.4mm thick) formed by RTM technology on the G250 aircraft, which is known as the “real Gulfstream”.

This component is manufactured by North Coast Composites in Ohio, USA. It is an integrated structure integrating ribs, wing beams, rudder shells and lightning protection devices. 95% of the parts have net size molding characteristics, and only 3 shell edges need post-processing. Boeing also used RTM molding technology in the development of the latest 787 aircraft, the most representative of which is its all-composite landing gear arm. This is the application of RTM technology in the main load-bearing structure.

RTM molding technology is also widely used in the helicopter field. For example, the European NH90 helicopter, its landing gear torque arm and longitudinal thrust rod are both molded using RTM technology. In addition, the joints of the American Comanche helicopter are molded using RTM, and the American V.SYSTEM company uses RTM technology to mold the special-shaped fairing shell of the helicopter.

 

In addition to aircraft structures, RTM technology has also been widely used in the engine industry due to its low cost, high efficiency and lightweight design. Although the core components of the engine are usually too hot for composite materials, the cold end components are the most important potential application areas for composite materials, such as the front cone of the cap, rotor blades, fan case and containment ring, fan outlet guide vanes, fan stator blades, engine nacelle and reverse thrust device, muffler structure and other components on high bypass ratio engines.

 

The most representative ones are the F-135 engine intake case, Genx engine fan case, LEAP-X engine fan blades and fan case, and the reverse thrust devices of engines such as PW 4000, V2500, RB21-Trent/A330, etc., among which the RTM composite parts of the reverse thrust devices of engines such as RB21-Trent/A330 are more than 36% lighter than aluminum alloy parts.

 

 Conclusion

 

After decades of development, the RTM process has become increasingly mature and has been successfully applied to the manufacture of aircraft bodies, engines and other structures, and has become one of the key technologies for high-performance aircraft manufacturing. In the foreseeable future, with the continuous innovation of technology, the RTM process will develop towards a trend of higher speed, higher efficiency, automation, and focus on low energy consumption and environmental protection, and the scope of application will be further expanded.