Introduction and Application of Vacuum Assisted Resin Transfer Molding Process

 

 

Vacuum Assisted Resin Transfer Molding (VARTM/SCRIMP) is suitable for products with high quality requirements, small batches and large sizes. Compared with the traditional autoclave molding process, it has outstanding features such as low mold cost, room temperature resin curing and almost unlimited product size. VARTM has been successfully used in foreign countries in the fields of ships, military facilities, defense engineering, aviation and civil industries.

 

At present, one of the most widely used processes in vacuum assisted resin transfer injection molding, SCRIMPTM molding process, was developed in the late 1980s on the basis of RTM process. It is a low-cost molding process suitable for making large composite products. The finished product of this process has good quality, such as low porosity, high fiber content, and good mechanical properties, and the emission of volatile toxic gases can be controlled to a minimum.

 

SCRIMP vacuum assisted resin transfer injection molding is a process that uses a film to seal the reinforcement material on a single-sided mold, completely uses vacuum to absorb low-viscosity resin, uses a high-permeability medium to quickly impregnate along the surface of the reinforcement material, and simultaneously infiltrates in the thickness direction of the reinforcement material. The composite materials processed by this method have high fiber content, excellent mechanical properties of products, and unlimited product size, which is especially suitable for making large products.

 

Recently, due to the continuous development of resin systems and textile reinforcement molding technology, aviation manufacturers have also shown great interest in VARTM, mainly in the use of composite materials such as carbon fiber-epoxy resin and carbon fiber-bismaleimide resin.

 

Compared with the traditional mold forming process, the SCRIMP molding process has many advantages. The SCRIMP process saves about 50% of the cost compared with manual laying, and the resin waste rate is less than 5%. In particular, the environmental protection of the processing process is the most prominent advantage of the SCRIMP process. Under the same raw material conditions, compared with hand lay-up components, the strength, stiffness or hardness and other physical properties of composite materials can be increased by more than 30%-50%.

 

Product quality is not affected by the operator, and the uniformity and repeatability of product performance are much better than open mold products, and there are much fewer defects. Since SCRIMP adopts a closed mold molding process, volatile organic compounds and toxic air pollutants are controlled to a great extent, and VOC emissions do not exceed the standard of 5PPm, while the volatility of styrene in open mold molding exceeds 500PPm.

 

The SCRIMP process is particularly suitable for manufacturing larger products, and can be used for one-time molding of core materials and reinforced structural parts, as well as the manufacture of thick, large and complex geometric shapes, which improves the integrity of the product, and the savings in materials and labor are considerable. The components made by SCRIMP have good repeatability, whether they are the same component or between components. The consumption of resin during SCRIMP molding can be strictly controlled, the fiber volume ratio can be as high as 60%, and the product porosity is less than 1%.

 

Many domestic and foreign composite material manufacturers, especially some shipyards, yacht factories and wind turbine blade factories, have already adopted the SCRIMP method, but most manufacturing technologies are used in a “trial and error” way to accumulate processing experience, which greatly affects the quality of the products.

 

In order to give full play to the characteristics of products using the SCRIMP process and achieve a high level of quality, manufacturers should pay attention to factors such as: the choice of vacuum during impregnation, the characteristics of the reinforcing material, the viscosity of the resin, the type of resin, the degree of impregnation, the gelation and curing of the resin, etc.

 

Textile reinforcement materials for LCM process At present, the commonly used reinforcement materials are mainly glass fiber, carbon fiber, aramid and high-density polyethylene fiber, etc. There are many types of reinforcement materials to choose from. Figure 1 shows the structural forms of commonly used reinforcement materials, such as woven gingham, three-dimensional orthogonal woven fabrics, warp knitted multi-axial fabrics, 2D and 3D woven fabrics and stitched fabrics. In the processing of composite materials, the structure, material and laying form of the reinforcement materials directly affect the resin impregnation and processing technology.

 

Application of LCM process technology The high fiber content, excellent product performance, good production repeatability and especially low-cost rapid prototyping of the SCRIMP process make its product performance comparable to the hot autoclave process widely used in aviation, aerospace and other fields. With the expansion of SCRIMP technology from the early naval military and defense military industry to the civilian industry,

 

SCRIMP technology has been widely used in the manufacture of hulls, ship decks, patrol boats, wind turbine blades, bridges, transport shells and other civilian and marine infrastructure projects. At present, the SCRIMP process can form products with an area of ​​185m2, a thickness of 700mm, a fiber weight content of 70-80%, and a porosity of less than 1%. The resin waste rate is less than 5%, which saves more than 50% of labor costs compared with manual laying. The main application area of ​​SCRIMP products is the hull structure. The Swedish Navy’s stealth frigate Visby, with a length of 72m, is the largest FRP sandwich structure built so far. The ship’s components such as the hull, deck and superstructure are also manufactured using the SCRIMP method.

 

Another major application area of ​​the SCRIMP process is the manufacture of wind turbine blades. Due to the influence of market, technology, materials and funds, most of the current domestic wind turbine blade manufacturers use wet hand lay-up process and room temperature curing. It can achieve mass production of 600KW and 750KW unit blades. The wet hand lay-up process is relatively simple, does not require heating and pressurizing devices, and does not require expensive tooling equipment.

 

The molding process has the disadvantages of low production efficiency, high labor intensity, poor labor hygiene conditions, difficult to control product quality, low performance stability, and low product mechanical properties. This process is usually only used to produce blades with short lengths and small batches. However, for large megawatt-class wind turbine blades, due to the huge size of the blades, the widest part is about 300cm and the highest part is greater than 200cm, the hand lay-up molding process is not competent.

 

At present, foreign countries use closed-mold vacuum-assisted molding technology to produce large blades (blade length is more than 40m) and large-scale production. This process is suitable for one-time molding of integral wind turbine blades (fibers, sandwich cores and joints can be co-molded in one mold cavity) without secondary bonding. The world-renowned blade manufacturer LM has developed a 56M all-glass fiber blade produced using this process.