Resin Transfer Molding (Resin transfer molding, RTM) technology has developed rapidly in recent years, relying on more than low cost. It belongs to composite liquid forming (Liquid Composite molding).

RTM technology originated in the late 1940s, because of its good product quality, high production efficiency, low manufacturing cost, easy to produce the whole composite components and other outstanding features have been rapidly developed.

RTM Technology: First of all, in the membrane cavity zhong with the workpiece structure in the form of enhanced material pre-forming body.

Then, at a certain temperature, pressure, using injection equipment to inject low viscosity liquid resin into the closed membrane cavity, resin in the impregnation of the preformed body, the replacement of all the gas in the membrane cavity; After the mold is filled with mold cavity, the resin is cured by heating and then demoulding to obtain the product.

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 bearing force and the non bearing structure of advanced aircraft by virtue of the advantages of high weight reduction and low cost.

Characteristics of a RTM technology

The RTM process has the following advantages compared with other molding processes.

1. High production efficiency

The manufacturing process can be mechanized or automated; can be formed from small precision to large-scale complex composite parts; for large and complex components with sandwich, stiffened and embedded components, the process of 2 steps or multi-step forming can be replaced by one-step impregnation and solidification molding process.

2. High product quality

The surface quality of workpiece is high, the dimension precision of workpiece is high, the tolerance is accurate and controllable, the stability and repeatability are good, the fiber volume content of the workpiece is high, about 55% ~60%, the void rate is low, about 0~0.5%.

The design of the preform is large in degree of freedom, not only can use all or local (suture, z-pin, etc.) enhancement as well as (thermoplastic resin, etc.) toughening technology, but also use the whole more strong three-dimensional fabric, the workpiece resistance to delamination damage tolerance is high.

3. Good economic effect

RTM process is a “non-hot pot” low-cost manufacturing process, the use of preformed and part of the resin system can be stored at room temperature, the composite material can be achieved net size and near-net size manufacturing, reducing waste and mechanical processing costs.

Can be one-step impregnation with sandwich, stiffened and embedded parts of large, complex components, instead of part or all of the Assembly, can save assembly hours and costs, the larger the volume, the lower the cost.

4. Environmentally friendly

Closed-mode technology is beneficial to human health and environmental protection.

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

(1) Complex cavity mold design and manufacturing difficult.

(2) It is difficult to monitor the dynamic resin filling mode of large size and complex parts.

(3) Die cost accounted for the cost of the process, small-scale production cost advantage limited.

The development of two RTM technology

After years of development, RTM technology to further improve production efficiency, reduce production costs, based on the basic process of the above-mentioned forming a number of new RTM technology, including liquid pressure molding LPM (Liquid pressuremolding) technology, Multiple insert lining RTM (MIT-RTM), high-pressure RTM (HP-RTM), compression RTM (Compression RTM), thermal expansion RTM (TERTM), flexible RTM (FRTM), injection Molding (CIRTM), Reaction Injection Moulding (RIM), The solvent-assisted RTM process (SARTM), resin-injected cyclic RTM (RIRTM) and 0 injection pressure RTM (ZIPRTM).

LPM Technology

LPM technology is a new liquid forming technology developed in 2012 by Avic Composite Co., Ltd.

Principle is in the rigid mold cavity laying in the performance and structure requirements of the design of the LPM process-specific predetermined body, and then the use of pressure equipment to inject the special resin in the closed mold cavity around the predetermined type of resin fast flow gum channel.

The pressure of pressure device on the glue channel is adjusted to apply the curing requirement to the composite material, and finally, a new composite material forming technology is developed according to the resin process specification.

As a new and efficient composite liquid forming technology, LPM technology has the advantages of high forming precision, forming the volume content of composite fiber and the molding level of prepreg/hot pressing tank, high process applicability, low molding efficiency, lower resin requirement, simple forming equipment and low manufacturing cost.

MIT-RTM Technology

MIT-RTM technology is an efficient RTM technology developed by the British Plastechtt Company in the late 1990s for its low molding efficiency in traditional RTM technology.(Multiple Insert Tooling).

The MIT-RTM device consists of a plurality of insert lining modes mit (ListBox Insert tooling) and a support, positioning, sealing and heating system for the lining die cavity.

A plurality of insert-type linings, that is, parts forming surfaces, are made of a plurality of lightweight thin shells (about 4~7mm) separated by a high precision, stable size, and reusable master dies.

These lining molds can be independently sprayed in the mold cavity, paving and strengthening the operation of the material, saving the injection, curing the process of taking up molding mold waiting time, effectively improve the production efficiency.

MIT-RTM Technical advantages include:

1. Most of the traditional RTM process occurs in the mold cavity, MIT-RTM technology can be occurred in the mold cavity inside the process decomposition, the partial removal of the mold cavity outside the completion, greatly reducing the process waiting time, molding efficiency is high.

2. The cost of the liner is small, and the cost of mass production is obvious.

3. After the lining mold is damaged, the patching replacement is easy, the production process is basically unaffected.

HP-RTM Technology

Hp-rtm is a new RTM technology for high performance thermosetting composite parts, which is introduced in recent years for low cost tooling.

High temperature curing resin system, after the completion of filling, resin rapid curing, to achieve a high degree of curing. Avoid the low cost of the mold produced by the product size accuracy is not high problem.

The principle of HP-RTM technology is to design the preformed moulding body in the rigid mould cavity, according to the performance and the structure requirement, then in the case of vacuum auxiliary exhaust, the high pressure provided by the injection equipment is used to inject the special resin into the closed mold cavity, and finally the high pressure supplied by the hydraulic press is used to finish the impregnation and solidification of the resin.

The main advantages of HP-RTM technology include:

1. Resin Mold filling time is short.

2. Can use fast curing resin system, curing period is short.

3. High-pressure injection and solidification, conducive to the elimination of air, product porosity is low.

4. The product surface quality is high, the dimensional precision is high.

5. High process stability and repeatability.

Compression RTM Technology

The compression RTM technology is a new liquid forming technology which is developed for the high fiber volume content composite products of the traditional RTM forming technology, the permeability of the preformed form is small, and the injection resin requires a great deal of pressure and low molding efficiency.

The principle of the compression RTM technology is to design the preformed moulding body in the rigid mould cavity laying according to the performance and the structure requirement, the mould is partially closed, the resin system is injected into the mold cavity under low injection pressure, and the mould is pressurized so that it is completely closed to the final position, and the resin completes the full infiltration of the preformed body. Finally, a new type of composite liquid forming technology is developed according to the process specification of resin.

The advantages of the compression RTM technology mainly include:

1. The resin is flowing in the thickness direction of the preformed body, and the filling time is short;

2. High fiber volume content (63%) of composite products;

3. The porosity of products is smaller than that of traditional RTM products;

4.High process stability and repeatability.

SARTM Technology

The process characteristics of the polyimide resin system used in high temperature RTM technology-polycondensation reaction, a large number of volatile points in prophase, high melting point and large viscosity.

In the use of high-temperature resin system injection, the main use of solvent-based polyimide resin, that is, in the process of the process at room temperature through low viscosity (<> containing solvent resin solution infiltration.

The operation period can be infinitely long, avoid the high temperature injection equipment requirements, short operation period and other problems, but the main problem is the need to be cured before the solvent and polycondensation by-products discharged.

The process is relatively complex, will lead to the workpiece content, thickness and so difficult to control, easy to appear local pores.

At present, the PMR-15, Pmr-ⅱ, afr700b and other polyimide resins used in hot-pressing tank technology have been successfully manufactured in combination with the three-dimensional braiding technology and the SARTM process.

For example, the engine inlet, fuel and lubricating oil external pipeline, adjustable stator blade bushings and other components, such as corrugated plate, diffuser blades.

Structural reaction type RTM (SRIM)

The process first put the long fiber reinforced pad preset in the mold cavity, and then use high-pressure metering pump to provide the high-pressure impact, the two monomer materials mixed in the mixed head, in a certain temperature conditions, the mixture of good resin system injected into the mold, curing molded composite products.

The main advantages of the structural reactive RTM process include: 1. Resin filling time is short, 2. The molding cycle is short.

Three RTM process equipment

RTM process equipment mainly includes 3 parts, including mould, injection equipment and curing equipment, and all parts will have a decisive influence on the whole development and production process.

RTM Molding process belongs to a closed-mode operation process, the resin flow process, pressure and heat transfer process and solidification process are all occurred inside the mold, the quality of the molding mold will be directly “copied” on the component.

Therefore, the selection and design of molds is the prerequisite for obtaining high quality, high precision and good surface quality workpiece.

In general, the RTM molding process selection of mold materials should have a fast thermal conductivity, low heat, good thermal stability, small thermal expansion coefficient, good processing technology, light quality, long service life, low cost, easy to use and maintenance and other advantages.

For large batches or surface quality requirements of the RTM workpiece, general selection of steel, zinc alloy or nickel-plated steel shell mold.

Different molding methods have different requirements for mold structure. Compared with the traditional hot-pressing tank molding process, the RTM molding process used in addition to meet the stiffness and heat transfer requirements, but also on the upper and lower mold, the mold surface quality, injection and overflow mouth design, mold sealing and workpiece demoulding device, such as higher requirements.

The successful RTM molding mold should have the following characteristics: The structure is exquisite, the temperature field is homogeneous, the system seals is good, the clamping, demoulding and so on operation is simple and easy, the internal quality and the shape dimension precision of the molded workpiece meet the technical requirements.

For the traditional RTM process, injection equipment is usually used for resin injection, the equipment generally includes resin storage tanks, resin supply system (constant pressure injection and constant flow injection), resin conveying pipeline, metering pump, and a variety of automation instruments. The injection equipment can be divided into one-component type, two-component type (pressure type and pump type), and 3 kinds of catalyst pump by mixing method.

Production efficiency depends on the degree of automation of the molding equipment, in order to improve injection efficiency, as far as possible to save the amount of resin, to achieve quality control of products, manufacturers around the world continue to study the new injection equipment.

The Swedish Aplicator Company has developed a series of injection equipment, such as RI-2, RI-15 and ipr2-8000, which are easy to use, easy to operate, and efficiently and safely injected with a mixture of resins and catalysts into the mold.

The Spartan injection equipment developed by American Graco Company has been praised by manufacturers at home and abroad for its stable low pressure characteristics, complete and reliable circulatory system, perfect mixing effect and reliable cleaning device.

French Isojet Company has developed a specialized dual-component, single component and special RTM injection equipment for polyimide resins, and the company’s equipment has also played a role in the development of RTM technology at home and abroad.

Compared with the injection, the curing of the resin can be in different equipment, such as general in the oven, press, the overall heating die.

When choosing heating mode, it is necessary to consider the heating efficiency of each heating mode, the temperature uniformity of heating system, the controllable range of ascending and cooling rate, and the control precision of temperature, operating cost and environmental protection.

For the oven heating, but also consider the size of the oven and the inner and outer rail bearing capacity to meet the needs of the workpiece.

With the development of RTM technology, more and more foreign aviation enterprises adopt RTM molding platform with heating system instead of traditional high power oven + injection equipment RTM process equipment.

RTM molding platform system is mainly composed of predetermined mould/mould frame, RTM mould/mould frame, heating unit, hydraulic system, control system and RTM injection system.

The platform system can realize the RTM molding process, such as die pressurization/heating, resin injection and solidification.

It can not only simplify the operation process, improve the production efficiency, reduce energy consumption, shorten the manufacturing cycle, but also can reduce the design rigidity and manufacturing cost of RTM mold, and can significantly improve the molding quality of the parts.

In order to break the domestic market situation of imported equipment monopoly, Shanghai Hundred if test Instrument Co., Ltd. developed the 1th domestic RTM integrated molding equipment

According to RTM process, the system integrates the reaction kettle, vacuum equipment, injection and exhaust system, heating system, pressure-holding system, mould and other equipments.

The upper and lower modes are individually moved, flipped, closed, and die-turned to be automatically processed, and the whole process is continuous and positioned to be prepared.

Mold heating, holding pressure, injection are controlled by the human machine, simple operation.

The equipment meets the requirements of RTM molding process and meets the requirements of RTM production automation in composite material production enterprises.

Recently, the French Pinette Emidecau company has developed a complete automation high-speed RTM working unit for Aviation Manufacturing.

The working unit has a robotic die-forming mode, closed mode and transmission, resin transfer Molding (RTM) injection, and 4 workstations after injection.

In which the pre-molding body transport, mold, closed mode, transfer between workstations, such as the use of robots or other automated facilities to deal with, while the heat furnace can be loaded with multiple molds.

The unit enhances process coherence, improves productivity, and has been validated by the manufacture of carbon fiber/epoxy aero parts.

Application status of four RTM technology in aviation field

The development direction of aero-composites is high performance, low cost and integration, and RTM molding technology, as a low-cost composite manufacturing method, is widely used in aviation field, and some applications realize continuous mass production.

With the improvement of the technological level, RTM technology in the field of aviation from the beginning only for the non-load components, such as flap guide fairing and engine rear hanger fairing, to the secondary bearing parts, such as the hatch, has been successfully applied to the main bearing components, such as aircraft propeller blades.

Airframe structure

In the field of military aviation, a more efficient and integrated advanced composite structure has been adopted, and the double weight reduction of materials and structures has been realized at the same time to improve the strength, which satisfies the high performance requirements of the new generation of fighter aircraft.

F-22 is a notable example of the application of RTM technology in the main bearing structure, in the F-22 prototype project produced by Lockheed Martin in the United States, more than 400 parts are manufactured using the technology, typically with wing sine corrugated beams, weapon cabin caps, stiffened, horizontal and vertical tail fins, front fuselage tank boxes, Front fuselage bulkhead and wing upper, lower beam and rear spar.

This includes the RTM-shaped corrugated beam of the wing produced by the subcontractor, Dow-ut, about 4.5m, high dimensional precision, low manufacturing cost and 50% reduction in the number of parts.

In the development of F-35 aircraft, RTM technology is used as one of the main integrated high-precision molding technology, and the number of parts of the overall composite material with CO-RTM technology is reduced from 13 to 1, and the fastener is reduced to 1000 number.

While increasing the degree of integration, the cost is greatly reduced.

In the field of civil aviation, RTM technology has also been widely used, such as the vertical fin-box of the Airbus a330/340 wide-bodied airliner and the cantilever beam of the horizontal fin, as well as the propeller blades of the Saab (SAAB) 340 and FOKKER50 turboprop aircraft using braided RTM.

In addition, there is the G250 aircraft, known as the “Real Bay Stream”, using RTM technology to form an integrated rudder (length 2.8m, Width 1.1m, thick and thin. 4mm).

The component is manufactured by the North Coast Composite company of Ohio, Coast Composites, and is a monolithic structure with fins, wing beams, rudder housings and lightning-proof devices.

95% of the components have a net size molding characteristics, only 3 kinds of shell edges need to be processed after.

Boeing has also adopted RTM technology in the development of the new 787 aircraft, the most representative of which is its fully composite undercarriage arm.

This is the application of RTM technology in the main bearing structure.

RTM technology is also widely used in the helicopter field, such as the European NH90 helicopter, the landing gear torque arm and the longitudinal thrust rod of the 2 components are used RTM process molding.

In addition, the American Comanche Helicopter connector using RTM molding, the United States v. System using RTM technology to shape the helicopter shaped rectifier shell.

Engine structure

In addition to the aircraft structure, RTM technology has been used in the engine industry with the advantages of low cost, high efficiency and lightweight design.

Although the core components of the engine are usually too hot for composites, the cold end components are the most important potential applications of composite materials.

such as Big Culvert road than the hood of the engine on the front cone, rotor blades, fan casing and containment ring, fan outlet diversion vane, fan stator blades, engine short cabin and backstepping device, muffler structure and other components.

The most representative of which is the F-135 engine inlet casing, Genx engine fan casing,-engine fan blades and fan casing, as well as the PW 4000, V2500, rb21-trent/a330 and other engines of the backstepping device.

Among them, the RTM composite workpiece of rb21-trent/a330 and other engine is more than 36% less than that of aluminum alloy workpiece.

Conclusion

After several decades ‘ development, RTM technology has been successfully applied to the manufacture of aircraft body and engine, and has become one of the key technologies of high performance aircraft manufacturing.

In the foreseeable future, the RTM process with the continuous innovation of technology, will be more high-speed, efficient, automation and focus on low energy consumption and environmental protection trends, the scope of application will be further expanded.