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Wind turbine blade forming process
 August 3, 2022
 
As a green and environment-friendly energy, wind energy is one of the most potential renewable energy sources. With the development and maturity of wind power technology, the model has reached more than 5MW, and the blade length has exceeded 60 meters. Blades are one of the key components of wind turbines, with large size, complex shape, high precision requirements, and high requirements for strength, stiffness, and surface smoothness.
 
 
Composite materials offer many advantages in the manufacture of wind turbine blades. Composite manufacturing processes mainly include hand lay-up, compression molding, prepreg molding, pultrusion, filament winding, resin transfer molding, and vacuum infusion molding.
 
Manufacturing processes for composite materials:
1. Hand lay-up process
 
Hand lay-up is a traditional process for producing composite fan rotor blades. In the hand lay-up process, the fiber base material is laid in a single mold, and then the fibreglass cloth and resin are applied with a roller or brush, and the mold is released after curing at room temperature. The hand lay-up method can be used to manufacture large, complex shaped articles at low cost. Because it does not have to be affected by heat and pressure. Simple equipment and molds can be used, and the cost is lower than other feasible solutions.

The main disadvantages of the hand lay-up process for producing fan blades are that the product quality is highly dependent on the operating proficiency of workers and environmental conditions, the production efficiency is low and the uniformity of product quality fluctuates greatly, the dynamic and static balance of the product is poorly guaranteed, and the rejection rate is high. Especially for high-performance complex aerodynamic shape and sandwich structure blades, secondary processing such as bonding is also required. The bonding process requires bonding platforms or frames to ensure the bonding of the bonding surfaces, and the production process is more complicated and difficult. 

Problems in the use of wind turbine blades made by hand lay-up are often caused by uneven glue content, poor fiber/resin infiltration and incomplete curing during the process, such as cracks, fractures and blade deformation.

The hand lay-up process is often accompanied by the release of a large number of harmful substances and solvents, which has certain environmental pollution problems. Hand lay-up is a proven method for producing composite blades, but due to its low yield and discontinuity of components, as well as the difficulty of realizing large-scale products with complex structures and high mechanical properties, it has prompted people to shift their research focus to other production method.
 
2. Molding
 
The compression molding process first places the reinforcement and resin in a two-lobed mold, then closes the mold, applies heat and pressure, and then demolds it for post-curing. The advantages of this process are high fiber content and low porosity, as well as short cycle times, precise dimensional tolerances and a good surface finish.

However, compression molding is suitable for producing simple composite products such as skis, and it is difficult to manufacture complex-shaped parts such as blades including skins, cores and beams. Although it is possible to improve the compression molding process equipment, it requires a large capital investment to improve the heating die that can withstand the pressure of the 20-40 meter span.

Compression molding produces parts with high fiber content and high strength/mass ratios, but it is difficult to manufacture blades with complex geometries in a cost-effective manner.
 
 
3. Prepreg laying process
 
The prepreg method is named after the prepreg reinforcement used. In this process, partially cured resin and reinforcement are laid down in a single mold and cured by heat. In order to avoid the occurrence of resin-rich areas and eliminate air voids, it is necessary to have sufficient overflow of resin in the prepreg. At present, commercial prepregs on the market generally require higher curing temperatures (90-110%). The main advantage of using prepreg is that the fiber reinforcement is well aligned during the production process, so parts with low fiber defects and excellent performance can be produced. Carbon fiber prepregs are widely used in the aerospace industry because they can be used to make parts with complex structures.

The main disadvantage of choosing prepreg for the production of fan blades is the high cost. This material is usually 5 to 10 times more expensive than ordinary resins and reinforcements. In addition, since the prepreg is laid by hand, it is also labor-intensive and low-yield compared to hand lay-up.

Prepreg is an ideal process for producing complex-shaped structural parts. Prepreg is widely used abroad, and its process and equipment have also developed to a mature stage. In actual production, due to the different mechanical properties and process requirements of the skin, main beam, root and other parts of the blade, different prepregs are used for different parts in order to reduce the cost without affecting the performance.
 
4. Pultrusion process
 
The pultrusion process is generally used in the production of continuously formed products with a certain section. In this continuous molding process, the glass fiber reinforcing material passes through the resin dipping tank and is cured and formed. Pultruded products have high fiber content and stable quality, and are suitable for mass production because they are continuously formed and easy to automate. Moreover, the product does not need to be rested in the later stage, the quality is consistent, and there is no need to detect dynamic balance, and the yield is 95%. Compared with other process costs, it can be reduced by 40%. 

Despite the many advantages of the pultrusion process, there are also drawbacks in the manufacture of wind blades. The pultrusion process has been successful in manufacturing vertical axis fan blades and some small horizontal axis fan blades, but it is impossible to manufacture fan blades with variable cross-sections. I-beams and other solid cross-sections are only a small challenge for the pultrusion process, while the hollow parts, including the beam and core material, are difficulty.

The cost of large automated equipment is another consideration for pultrusion applications, since pultrusion cannot currently produce complex-shaped parts with large cross-section variations. Therefore, the pultrusion process has great application potential in the production of small fan blades.
 
5. Filament winding
 
Filament winding is mainly used to make containers and pipes, in which continuous fiber roving is immersed in a dip tank and wound on a machine-controlled mandrel. The winding process controls variables such as fiber tension, production speed and winding angle.

Filament winding enables the manufacture of parts of different sizes and thicknesses. One disadvantage of the winding process applied to blade production is that winding cannot be performed in the longitudinal direction of the blade, and the lack of fibers in the length direction makes the blade prone to problems under high tensile and bending loads. In addition, the rough outer surface produced by filament winding may affect the aerodynamic performance of the blade, so surface treatment is necessary.

Finally, mandrels and computer control are expensive. Clearly, the filament winding feature is suitable for containers and pipes, while additional costs are incurred in blade production.
 
 
6. Resin transfer molding (RTM)
 
The resin transfer molding process is a semi-mechanized composite molding process. The worker only needs to put the designed dry fiber preform into the mold and close the mold. The subsequent process is completely completed and guaranteed by the mold and injection system. The exposure of any resin, and thus the technical and environmental requirements of the workers, are far lower than the hand lay-up process and can effectively control product quality. 

The RTM process adopts a closed-mold molding process, which is especially suitable for forming the whole wind turbine blade at one time (fiber glass chopped strands, cores and joints can be co-molded in one mold cavity) without secondary bonding. Compared with the hand lay-up process, it not only saves various tooling equipment for the bonding process, but also saves working time, improves production efficiency and reduces production costs. At the same time, the quality and production efficiency of composite materials are greatly improved due to the use of low-viscosity resin to infiltrate the fibers and the use of heating and curing processes. 

The RTM process production is less dependent on the technical level of the workers, and the process quality only depends on the determined process parameters. The product quality is easy to ensure, and the product rejection rate is lower than that of the hand lay-up process. The limiting factor for RTM in blade production is first and foremost cost. RTM's mold equipment is very expensive. In addition, since RTM is a closed-mold process, it is difficult to predict the flow of resin, and it is easy to produce substandard products.
 
7. Vacuum infusion molding process
 
The vacuum infusion molding process is to lay the fiber reinforced material directly on the mold, and lay a peeling layer on top of the fiber reinforced material. The peeling layer is usually a thin layer of fiber fabric with low porosity and low permeability. A highly permeable medium is placed on top, then wrapped and sealed with a vacuum film. The vacuum pump is pumped to a negative pressure state, the resin enters the whole system through the rubber inlet pipe, and the main direction of the resin flow is guided by the guide pipe.

The diversion cloth distributes the resin to every corner of the layup, and the release cloth is peeled off after curing, resulting in a dense layup with low glue content. The vacuum infusion molding process is an ideal choice for fan blade manufacturers. Compared with standard RTM, it saves time, has very few machine volatiles, improves labor conditions, reduces operator contact with harmful substances, meets people's requirements for environmental protection, and improves Working environment, process operation is simple. At the same time, vacuum assistance can fully eliminate air bubbles, reduce product porosity, and effectively control product glue content.

The product has high quality stability and good repeatability. The product has good apparent quality, the same layer but thin thickness and high strength. Compared with hand lay-up molding, the tensile strength is increased by more than 20%. This process does not require high molds, and the mold making is simple. Compared with the traditional RTM process, its mold The cost can be reduced by 50-70%. 

With the fiberglass for wind power industry, composite fan blades are developing in the direction of complexity and large-scale. Various processes are used in the manufacture of wind turbine blades. According to the characteristics of different wind turbine blades, a suitable process is reasonably adopted to obtain low-cost high-quality wind turbine blades.
 
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