Brief Analysis of Carbon Fiber Wrapping Composite Material Forming Process

 

 

A pressure vessel is a pressure-bearing device used to store gas or liquid. The emergence of metal-lined carbon fiber wound composite pressure vessels is an important milestone in the development of the entire pressure vessel technology.

 

With the improvement of molding process equipment and the improvement of the performance of raw materials (high-strength carbon fiber materials), high-pressure, lightweight all-composite pressure vessels have become an important direction for the future development of pressure vessels, especially the development of carbon fiber wound composite pressure vessels.

 

In the preparation process of carbon fiber wound composite pressure vessels, the selection of winding molding process parameters has a great influence on the performance of composite products.

 

In order to improve the performance of wound composite materials, it is particularly important to optimize its main process parameters. The existing literature on carbon fiber wound composite materials mainly focuses on process design, and less on process control during the actual winding operation. This article conducts experimental research on the main process control parameters of winding molding, and compares the two processes of wet winding and dry winding, providing necessary reference for carbon fiber wound composite molding process experiments.

 

Winding Process

 

The fiber winding process is a process in which high-strength fibers (such as carbon fibers) impregnated with a certain mass fraction of resin are continuously wound onto the outer surface of a core mold or an inner liner according to the linear shape and layering sequence pre-set by the program under the action of winding tension, and then heated to obtain a product.

 

The working principle is that through the regular movement relationship between the winding equipment guide wire head and the core mold, the fibers impregnated with resin are repeatedly wound onto the rotating core mold along the designed path until the winding is completed, and finally heated and cured to obtain a composite material product.

 

The winding process is usually divided into wet winding process, dry winding process and semi-dry winding process according to the different states of fiber impregnation with resin during the winding process. Among them, wet winding process and dry winding process are more commonly used.

 

Wet Winding Process

The process of using continuous fibers to directly impregnate resin glue under a certain winding tension, and continuously and repeatedly winding onto the surface of a core mold or an inner liner according to the set path, and then heating and curing to obtain a product is called wet winding. Because of its relatively simple process, low requirements for raw materials, and wide selection, it is the most widely used.

 

Dry Winding Molding Process

Before winding, the fiber has been pre-impregnated with resin and rolled up. The pre-impregnated fiber needs to be unwound before winding. No resin dipping is required during the winding process, but it needs to be heated. After the pre-impregnated resin is in a viscous flow state, it can be wound onto the core mold or lining surface according to the set linear shape. Finally, it is heated and cured to obtain the product. This process is called dry winding.

 

Winding Molding Process Parameters

In the winding molding process, the factors that affect the performance of composite materials include resin mass fraction, winding tension, ply sequence, winding speed and curing process.

 

Resin Mass Fraction

 

Composites are composed of two or more substances with different properties. Only when the materials work together can the composite materials with the best performance be obtained. The resin mass fraction directly affects the thickness, mass distribution and effective performance of carbon fiber strength of composite products, thereby affecting the performance of composite materials. Selecting the most suitable resin mass fraction is the first problem to be solved in process research.

 

When the resin mass fraction is too low, the carbon fiber is not fully impregnated, the product porosity increases, the interface bonding performance is poor, and the interlaminar shear strength, air tightness and aging resistance of the composite product are affected; when the resin mass fraction is too high, the corresponding carbon fiber content is reduced, the effective strength of the carbon fiber is reduced, and the comprehensive performance of the composite product is reduced. Studied the effect of different resin mass fractions on carbon fiber winding composite materials and pointed out that when the resin mass fraction is 30% to 35%, the tensile strength of the carbon fiber is relatively high (see Table 1).

 

In the wet winding process, the resin mass fraction is closely related to the resin viscosity, winding tension and winding speed. However, there are problems such as high resin consumption, serious glue dripping and leakage, and uneven control of the resin mass fraction. Resin viscosity is the basis for uniform and stable control of the resin mass fraction.

 

Generally, the resin viscosity is required to be controlled between 200 and 800 MPa·s. If the resin viscosity is too low, although it has good impregnation on the carbon fiber, if the winding tension is large, the winding speed is fast, and the impregnation time is short, the resin mass fraction brought out with the fiber cannot meet the requirements; if the resin viscosity is too high, the impregnation performance is poor, the bubble entrainment is large, and the resin mass fraction is difficult to control. Although the resin mass fraction is difficult to control in the wet winding process, the carbon fiber yarn spreading effect is relatively good, and the air tightness of the wet wound product is better.

 

In the dry winding process, due to the pre-impregnation of the carbon fiber in advance, the fluctuation of the resin mass fraction can be accurately controlled within 2%, and the resin mass fraction does not need to be adjusted during the winding process.

 

However, the fluidity of dry winding resin is poor, and the heating process needs to be strictly controlled by adjusting the heating temperature and winding speed to make the resin of the pre-impregnated carbon fiber in a viscous flow state, and then wind it onto the surface of the mandrel according to the set line shape. Otherwise, the carbon fiber yarn spreading effect will be affected. Even if the resin mass fraction meets the requirements, the porosity of the dry winding product will be high.

 

Winding Tension

The winding tension is an important process parameter in the winding process. The purpose is to make the fiber neatly and evenly wound onto the mandrel, thereby ensuring the uniformity of the tension between each fiber and each winding layer, and giving full play to the strength of the carbon fiber.

 

Winding Tension

If the winding tension is too high, it will cause certain damage to the carbon fiber during the carbon fiber transfer process, affecting the strength of the carbon fiber and causing the performance of the composite material product to decrease; but if the winding tension is too low, the interlayer shear strength will also decrease, which also affects the performance of the composite material product.

 

Studied the NOL ring tensile properties and interlaminar shear strength properties of Toray T700 carbon fiber under different winding tensions (see Figures 1-2). The performance of the composite material was better within the winding tension range of 60-70 N.

 

 

There is a resin impregnation system in wet winding. The winding tension has a greater impact on the resin mass fraction and less damage to the carbon fiber. The tension is mainly applied by the tension roller of the fiber impregnation. The flow of resin plays a lubricating and protective role, which can effectively reduce the damage of the carbon fiber during transmission. However, it is easy to cause slip when the fiber is wound on the edge of the core mold. In addition, under the action of winding tension, it is conducive to the migration and discharge of bubbles attached to the resin, making the composite product more compact.

 

There is No Resin Impregnation System In Dry Winding. The winding tension has little effect on the resin mass fraction and is beneficial to the yarn spreading effect of pre-impregnated carbon fiber. The tension is mainly applied by the friction resistance of the pre-impregnated fiber rotation, which makes the carbon fiber relatively wear during transmission. However, it is not easy to slip when the fiber is wound on the edge of the core mold, and the positioning is relatively accurate.

 

Winding Tension Gradient

If the winding tension is constant, as the number of winding layers increases, the outer fibers will exert radial compressive stress on the inner fibers, forcing the inner fiber winding layer to undergo radial compression deformation and produce relaxation and displacement, presenting a state of loose inside and tight outside. When the composite material is subjected to stress, the fiber winding layers cannot be loaded evenly, which greatly affects the strength and fatigue performance of the composite product. Therefore, choosing a suitable tension gradient is an important condition for improving the performance of composite materials.

 

The use of a regular layer-by-layer tension reduction system can effectively solve the problem of loose inside and tight outside in the composite winding layer, ensuring that the initial stress state of the inner and outer layers of the fibers is the same and the tension they bear is equal, thereby ensuring that the inner and outer fiber winding layers can be loaded evenly when the composite material is subjected to stress, greatly improving the strength and fatigue resistance of the composite product.

 

Studied the effect of different tension reduction laws on carbon fiber winding composite materials under the initial tension of 70 N. When the winding tension reduction law was reduced from 0.5 N per layer to 0.25 N, the fatigue cycle number of the 56 L gas cylinder increased from 979 times to 1112 times, and the burst strength increased from 93 MPa to 95 MPa; the fatigue cycle number of the 130 L gas cylinder increased from 1152 times to 1358 times, and the burst strength increased from 91 MPa to 94 MPa.

 

Studied the effect of different tension systems on the tensile strength of NOL rings. The tensile strength of NOL rings made by winding tension decreasing system (20 N to 10 N) was the highest, which was 6.7% higher than that of NOL rings made by winding tension constant system (20 N) and 19.2% higher than that of NOL rings made by winding tension constant system (10 N).

 

The believed based on experience that for the sake of simplicity, the tensile strength can be decreased every 2 to 3 layers.

 

Laying Order

 

In the winding molding process, there are usually two laying methods: one is to concentrate all the hoop winding fiber layers in the inner or outer layer of the composite material, and all the spiral winding fiber layers in the outer or inner layer of the composite material; the other is to alternate the hoop winding and spiral winding regularly.

 

From the literature that in the composite material with “concentrated” winding, there is only one intersection surface when the circumferential winding and spiral winding are alternated. When the composite material is damaged, stress concentration is easily generated at the intersection surface, resulting in delamination failure, resulting in poor overall structural performance; while with “alternating” winding, an intersection surface is generated every time the circumferential winding and spiral winding are alternated, which can effectively improve the overall structural performance of the composite material.

 

Studied the effects of different winding ply methods on the burst strength and limited life of carbon fiber fully wound composite gas cylinders. Different winding methods were used, such as 10C/4H (i.e., 10 circumferential windings followed by 4 spiral windings), 4H/10C (i.e., 4 spiral windings followed by 10 circumferential windings), and 5C/2H/5C/2H (i.e., 5 circumferential windings and 2 spiral windings alternately). The burst pressure test results are shown in Table 2, and the limited life test results of different winding methods are shown in Table 3.

 

 

It can be seen from Tables 2 to 3 that when the circumferential winding is close to the inner shell, the burst pressure is higher and the limited life cycle number is lower. On the contrary, when the spiral winding is close to the inner shell, the limited life cycle number is higher and the burst pressure is lower. Alternating circumferential winding and spiral winding can effectively improve the comprehensive performance of the composite material.

 

The circumferential winding layer and the longitudinal winding layer should be “alternated” and the distribution should be as uniform as possible to make the force distribution of the fiber more reasonable, so that the performance of the composite product can be met with the least material.

 

Winding Speed

 

The winding speed generally refers to the linear speed of the fiber winding onto the mandrel, which directly reflects the production efficiency and capacity during the winding process. If the winding speed is too small, the production efficiency is low.

 

The winding speed in wet winding is limited by the fiber impregnation time and the capacity of the winding equipment. When the winding speed is too large, the carbon fiber impregnation time is short, the resin mass fraction is low, and the speed of the impregnation roller and the mandrel is too high. The resin glue is easy to migrate and splash from the inside of the container to the outside under the action of the rotating centrifugal force, and the on-site environment is poor.

 

Therefore, the wet winding speed cannot exceed 54 m/min. The winding speed in dry winding is mainly limited by the heating time. When the winding speed is too fast, the resin heating time of the pre-impregnated fiber is short, and the ideal viscosity flow state cannot be achieved. The pre-impregnated fiber yarn spreading effect is poor, but the dry winding speed can reach 200 m/min, and the production efficiency is relatively high.

 

Curing Process

 

The curing process system of winding molding mainly includes curing temperature, curing time, heating rate and core mold rotation speed. The curing process of composite materials is mainly determined by the curing characteristics of the resin matrix, and a three-platform scheme is usually adopted.

 

Pre-Curing Platform, to ensure that the temperature of the resin system reaches the predetermined value uniformly, to prevent the local reaction of the resin system from being too fast, so as to ensure the uniformity of the curing reaction of the second platform. Generally, the curing degree of the resin does not exceed 30%. This platform is very critical for the discharge of resin bubbles in wet winding molding;

 

Curing Platform, the core stage of the curing reaction. In the formulation of the curing process, the heating rate of the curing platform is particularly important. The preferred heating rate is usually 0.5 K/min;

 

Post-Curing Platform. The temperature of this section is the highest. The purpose is to ensure that the resin curing reaction is complete, but it must be started after the second platform is completely finished to prevent overheating.

 

During the Curing Process, the winding composite product needs to rotate evenly and slowly to ensure that the resin matrix is ​​heated evenly and prevent glue flow [19]. The mandrel rotation speed is generally 3 to 5 m/min, which is the linear speed of the mandrel winding surface. For wet winding, mandrel rotation is necessary, mainly to ensure the uniformity of the resin mass fraction during the curing of the composite material and to reduce the loss of resin; for dry winding, mandrel rotation is also beneficial, which can compensate for the uneven temperature field in the curing furnace.

 

Conclusion

 

Compared with wet winding, although dry winding has large equipment investment and greater damage to the carbon fiber strength, it has a good on-site environment, fast winding speed, can accurately control product quality, and better realize mechanical automation.

 

Winding is one of the main molding processes for composite materials. Although the wet winding process is relatively common at present, the dry winding process will surely develop better in the future as the automation level of winding equipment continues to increase, environmental protection requirements become increasingly stringent, and the winding process continues to improve.