Application of Vacuum Infusion Technology in the Production of Wind Turbine Nacelles

 

 

Core Tip: “Vacuum Resin Infusion Process” (VRIP), also known as “Vacuum Assisted Resin Diffusion Molding Process” (VARIM), or “Vacuum Assisted Resin Transfer Molding Process” (VARTM). The principle of this process is to inject resin into the prefabricated reinforcement material with the help of vacuum drive, and the mold consists of a flexible membrane and a rigid half mold. Since the reinforcement material is compressed by vacuum, the penetration speed of the resin is generally slow, and it depends on the help of the guide medium (guide cloth or guide tube). This is the patented technology SCRIMP invented by Seemann.

 

Introduction

 

“Vacuum Resin Infusion Process” (VRIP), also known as “Vacuum Assisted Resin Diffusion Molding Process” (VARIM), or “Vacuum Assisted Resin Transfer Molding Process” (VARTM). The principle of this process is to inject resin into the prefabricated reinforcement material with the help of vacuum drive, and the mold consists of a flexible membrane and a rigid half mold.

 

Since the reinforcement material is compressed by vacuum, the penetration speed of the resin is generally slow, and it depends on the help of the guide medium (guide cloth or guide tube). This is the patented technology SCRIMP invented by Seemann. The basic principle of SCRIMP is to use the flow-guiding medium to form a high-flow penetration zone on the surface of the component, so that the resin can quickly reach the entire surface of the product. The impregnation is mainly achieved through the thickness direction, which greatly shortens the penetration path and time of the resin. Relying on high vacuum, the porosity of the product can reach 1%-1.5%, and the fiber volume content is above 50%.

 

Another SCRIMP process is to groove the core material, place the fabric on top of the core material, and the resin flows in the groove at a faster speed than the flow in the flow-guiding medium.

 

The nacelle is an important component of wind power equipment. Since wind turbines always work in a relatively harsh meteorological environment, the wind turbine nacelle must meet the following technical requirements:

 

(1) Aerodynamic load: Bear the aerodynamic load of wind speeds up to 70m/s.

 

(2) Personnel standing load: For installation and maintenance requirements, any point on the nacelle can bear a person standing. The design requires that the material bending deformation does not exceed 0.5cm when subjected to a force of 80Kg for every 5X103mm2 area.

 

(3) Fatigue load: withstand fatigue damage equivalent to 20 years.

 

(4) Material properties do not change significantly after working in the following environment for 20 years.

 

Ultraviolet irradiation: Radiation intensity: 1000W/M2.., Grease resistance: especially grease resistance used in the machine; • Working humidity: up to 95%…, Working temperature: -10℃-+40℃, in extreme cases up to -20℃~+50℃.

 

 

Therefore, the composite materials used for the nacelle cover are required to have the following characteristics:

 

(1) a service life of 20 years;

(2) suitable for the working environment of the nacelle cover, such as oil resistance, moisture resistance, and UV resistance;

(3) the material is easy to buy;

(4) the price is acceptable;

(5) it is repairable, which is necessary for large products;

(6) the mechanical properties of the material meet the design requirements, especially the rigidity;

(7) good fatigue resistance;

(8) certain flame retardant properties.

 

Comparison of Vacuum Resin Introduction Process and Hand Lay-up Process

 

Currently, the materials used in the manufacture of wind turbine nacelles are mainly polyester glass fiber composite materials, and the molding processes are mainly hand lay-up process and vacuum resin introduction process. The advantages and disadvantages of the two are compared as follows:

 

Hand lay-up process (Hand lay-up) is a mold opening process, which currently accounts for 65% of glass fiber reinforced polyester composite materials. Its advantages are that it has a large degree of freedom in changing the shape of the mold, the mold price is low, the adaptability is strong, the product performance is recognized by the market, and the investment is small.

 

Therefore, it is particularly suitable for small companies, and also for the shipbuilding and aerospace industries, which are usually one-time large parts. However, this process also has a series of problems, such as excessive volatile organic compound (VOC) emissions, great impact on the health of operators, easy staff turnover, many restrictions on permissible materials, low product performance, resin waste and large amount of use, etc., especially unstable product quality, the ratio of glass fiber and resin, component thickness, layer material manufacturing rate, and layer material uniformity of the product are all affected by the operator, requiring the operator to have good skills, experience and quality.

 

The resin content of hand lay-up products is generally around 50%-70%. The VOC emissions of the mold opening process exceed 500PPm, and the volatilization of styrene is as high as 35%-45% of the usage. The regulations of various countries are all 50-100PPm. At present, most foreign countries have switched to cyclopentadiene (DCPD) or other low styrene release resins, but there is no good substitute for styrene as a monomer.

 

 

The vacuum resin infusion process is a low-cost manufacturing process developed in the past 20 years, which is particularly suitable for the manufacture of large products. Its advantages are as follows:

 

 

Excellent product performance and high yield rate. Under the same raw materials, the strength, stiffness and other physical properties of vacuum resin infusion molding components can be improved by more than 30%-50% compared with hand-layup components (Table 1). After the process is stabilized, the yield rate can be close to 100%.

 

Table 1 Comparison of typical polyester fiberglass properties

 

 

Stable Product Quality and Good Repeatability.

Product quality is less affected by operators, and there is a high degree of consistency between the same component and between components. The fiber content of the product has been placed in the mold according to the specified amount before injecting the resin, and the component has a relatively constant resin ratio, generally 30%-45%. Therefore, the uniformity and repeatability of the product performance are much better than those of hand-layup products, and there are fewer defects.

 

Improved Fatigue Resistance can reduce the weight of the structure. Due to the high fiber content, low porosity, high product performance, and especially the improvement of interlayer strength, the fatigue resistance of the product is greatly improved. Under the same strength or stiffness requirements, products made by vacuum infusion can reduce the weight of the structure.

 

Environmentally Friendly.

The vacuum resin infusion process is a closed mold process, and volatile organic compounds and toxic air pollutants are confined to the vacuum bag. Only trace amounts of volatiles are emitted when the vacuum pump is exhausted (filterable) and the resin barrel is opened. VOC emissions do not exceed the standard of 5PPm. This also greatly improves the working environment of operators, stabilizes the workforce, and expands the range of available materials.

 

Good Product Integrity. The vacuum resin infusion process can simultaneously form reinforcing ribs, sandwich structures and other inserts, which improves the integrity of the product. Therefore, large products such as fan covers, hulls and superstructures can be manufactured.

 

Reduce the Use of Raw Materials and Labor.

When the same layer is laid, the amount of resin is reduced by 30%. There is less waste, and the resin loss rate is less than 5%. The labor productivity is high, and it can save more than 50% of labor compared with the hand lay-up process. Especially when forming large and complex geometric sandwich and reinforced structural parts, the savings in materials and labor are even more considerable. For example, in the manufacture of vertical rudders in the aviation industry, the number of fasteners is reduced by 365, and the price is reduced by 75% compared with the traditional method. The product weight remains unchanged and the performance is better.

 

Good Product Precision.

The dimensional accuracy (thickness) of vacuum resin infusion process products is better than that of hand lay-up products. Under the same layer, the thickness of general vacuum resin diffusion technology products is 2/3 of that of hand lay-up products. The product thickness deviation is about ±10%, while the hand lay-up process is generally ±20%. The surface flatness of the product is better than that of hand lay-up products. The inner wall of the hood product made by vacuum resin infusion process is smooth, and a resin-rich layer is naturally formed on the surface, so there is no need to apply a top coat. This reduces the labor and materials in the sanding and painting processes.

 

 

Of course, the current vacuum resin infusion process also has certain disadvantages:

 

The Preparation Process Takes a Long Time and is Relatively Complicated. It requires correct layering, laying of flow guide medium, flow guide tube, effective vacuum sealing, etc. Therefore, for small-sized products, its process time exceeds the hand lay-up process.

 

The Production Cost is High and Generates More Waste.

Auxiliary materials such as vacuum bag film, flow guide medium, demoulding cloth and flow guide tube are all one-time use, and currently a considerable number of them rely on imports, so the production cost is higher than the hand lay-up process. However, the larger the product, the smaller this difference. With the localization of auxiliary materials, this cost difference is getting smaller and smaller. At present, research on auxiliary materials that can be used multiple times is a development direction of this process.

 

The Process Manufacturing has Certain Risks.

Especially for large and complex structural products, once the resin infusion fails, the product is easy to be scrapped.

Therefore, good preliminary research, strict process control and effective remedial measures are required to ensure the success of the process.

 

Requirements for Raw Materials in Vacuum Resin Infusion Process

 

Requirements for resin used in cabin covers produced by vacuum infusion process:

 

(I) Low viscosity. Generally around 100-400mPa.s. Preferably not higher than 200mPa.s;

 

(2) Appropriate exothermic peak temperature, generally not higher than 80℃;

 

(3) The FRP layer still has suitable strength before the use temperature reaches 60℃;

 

(4) It still has good bonding strength with the selected glass cloth in a humid environment (relative humidity 95%) for a long time:

 

(5) It can be cured at room temperature;

 

(6) It has a sufficiently long gel time to ensure the completion of the process and can be fully cured in the end;

 

(7) Good weather resistance;

 

(8) Good oil and grease resistance;

 

(9) Good flame retardancy;

 

(10) Low price;

 

(11) Low curing shrinkage, etc.

 

For various components of the resin system, such as resin, curing agent, accelerator, inhibitor, color paste and filler, the corresponding resin fluidity, viscosity and curing reaction kinetics should be studied to ensure the reliability of the process (Figure 3). The research methods include DSC, DTA, dynamic viscometer, etc.

 

Generally speaking, various forms of reinforcing materials, such as chopped strand mat, filament mat, untwisted roving fabric (grid), twisted fabric, woven fabric and sandwich material (foam, balsa wood and honeycomb), can be used, and the maximum surface density of the applied fabric can reach 87kg/M2. However, it should be noted that different fabrics have a great influence on the vacuum introduction process, and fabrics with high permeability and good resin impregnation should be used as much as possible.

 

When using core materials, GPS core materials should be used.

 

Study on Vacuum Resin Introduction Process

 

Study on Resin Fluidity

 

In the vacuum resin introduction process, Darcy’s Law (formula (l)) is mainly used to describe the process of resin flowing through the preform.

 

In Darcy’s Law, the resin is considered to be an incompressible Newtonian fluid whose viscosity is not affected by shear velocity. In the experiment, other liquids can also be used to replace the resin, such as syrup, glycerol and cellulose aqueous solution, which can greatly reduce the experimental cost and increase the test speed. The fabric preform is regarded as a porous medium, and its properties can be characterized by porosity and permeability. They affect the flow direction and speed of the resin in the preform, and thus determine the key parameters such as vacuum pressure, flow (filling) time and flow path required for composite molding, and then affect the design of key structures such as resin inlet, outlet and flow channel to ensure that the resin completes the filling process before gelation.

 

The flow of resin can be divided into two categories:

 

The flow rate of infiltration or macroscopic flow (between yarn bundles) determined by the pressure gradient. (Macroscopic flow).

 

The flow rate of penetration or microscopic flow (in yarn bundles) determined by fiber capillary pressure and surface tension. (Microcosmic flow)

 

The factors affecting flow rate and flow path include: raw materials, flow guide medium, layering and vacuum degree. The two speeds must be equal. If the flow fronts converge, it is difficult to expel the enclosed gas. The removal of gas at the micro level is affected by the viscosity of the resin and the surface tension around the fiber bundle.

 

The study found that the application of high permeability flow guide medium greatly shortens the filling time. The resin flows much faster in the flow guide medium than in the preform, but the difference between the two remains a constant value. The filling time is only a function of the permeability of the flow guide medium and is little affected by the permeability of the preform. The application of flow guide medium reduces the filling time by 50-80%.

 

In the process, it is necessary to prevent the “short circuit effect (cutline)” caused by unreasonable layering, etc. In these low resistance areas, the resin flow rate will increase by 10-100 times, so that the process cannot be carried out as expected.

 

There are currently quite a few software that can simulate the flow process in the vacuum infusion process, including the position and pattern of the resin flow front, which can detect potential problems in the process in advance and optimize the process.