Detailed Explanation Of Carbon Fiber Composite Autoclave Process Technology

 

There are dozens of molding processes for carbon fiber composite materials, among which the autoclave process is the most widely used and common molding method. This article gives a general description of the process from the aspects of process principle, process characteristics, common problems and countermeasures, typical technical applications, etc., and describes the process flow in detail.

 

1 Basic concepts

 

1.1 Process Principle

The carbon fiber prepreg is laid on the mold according to the layering requirements, and the blank is sealed in a vacuum bag and placed in a carbon fiber autoclave. Under vacuum, the autoclave equipment undergoes heating, pressurization, insulation, cooling, and pressure relief procedures, and solidification is achieved using the uniform temperature and uniform pressure provided simultaneously in the autoclave, so that carbon fiber composite parts with high surface and internal quality, complex shapes, and large areas can be formed.

 

 

1.2 Process Development

The autoclave process is a process developed for the production of second-generation composite materials. It was formed in the 1940s and was gradually promoted and used in the 1960s. Later, it was widely used in high-tech fields such as aerospace, composite materials, electronics, weapons, transportation, sports equipment and new energy. In particular, it has played a huge role in the production of various skin parts and has now become a mature process.

 

Composite products produced by the autoclave process account for more than 50% of the total output of composite products, and the proportion in the aerospace field is as high as more than 80%. At present, the autoclave process, as one of the molding methods of carbon fiber composite components, is also widely used by many carbon fiber composite parts manufacturers.

 

1.3 Process Equipment

 

1.3.1 Autoclave

The autoclave is a process equipment designed for the characteristics of polymer-based composite molding process. The use of autoclave molding is the main method for manufacturing continuous fiber reinforced thermosetting composite parts. It is suitable for the molding of advanced composite structures, honeycomb sandwich structures and metal or composite bonding structures.

 

Generally, it has the advantages of high temperature and pressure control precision, safe and reliable structure, good system stability, low energy consumption, simple operation and maintenance, etc. It can be modularized and serialized to meet the needs of different fields, different processes and different specifications.

 

 

1.3.2 Equipment Composition

As a system equipment for curing carbon fiber composite materials, the carbon fiber autoclave can realize the timing and real-time online control of process parameters such as temperature, pressure, vacuum, cooling, and circulation. The equipment includes the following components:

 

(1) Overall structure: It is composed of a tank body, a tank door mechanism, a high-temperature motor, an air duct plate insulation layer, etc. to form a high-pressure and high-temperature tank body.

 

(2) Safety interlock device: It is composed of pressure automatic interlock, manual interlock, and ultra-high pressure alarm device.

 

(3) Quick-opening device: It is a manual and electric dual-purpose quick-opening door design, which can open and close the tank door normally during power outages;

 

(4) Sealing device: The tank door adopts a silicone inflatable seal, which is resistant to high pressure.

 

(5) Pressure system: It is composed of a compressor, a gas storage tank, a pressure control valve, a pipeline, a pressure transmitter, and a pressure gauge. The pressure in the tank can reach 1.5-2.5MPa, and the error is not more than 0.05MPa. It is equipped with a safety explosion-proof device.

(6) Heating system: It is composed of stainless steel electric heating tubes, high-temperature fans, air duct plates, insulation layers, and temperature control systems. The heating power meets the maximum temperature requirements of the cavity and the requirements of the heating rate.

(7) Temperature circulation system: It is composed of a circulating fan, air guide plate, and air guide cover to accelerate the conduction and circulation of heat flow and form a uniform temperature field.

(8) Vacuum system: It is composed of a vacuum pump, pipelines, vacuum gauges, and vacuum valves to provide vacuum conditions for the encapsulated composite preforms. Multiple vacuum pipe joints are required to meet the molding process requirements.

(9) Automatic control system: It adopts a PLC control system to achieve high-precision control and real-time recording of process parameters such as pressure, temperature, and cooling.

 

1.3.3 Advantages Of Autoclave Process

(1) Uniform pressure: Gas is used for pressurization. The pressure acts on the surface of the product through the vacuum bag. The normal pressure at each point is equal, so that the product is cured and formed at the same pressure at all locations.

 

(2) Uniform and adjustable temperature: The workpiece is heated by a circulating hot air flow in the tank, and the temperature difference at each location is small. At the same time, a cooling system is configured so that the temperature can be strictly controlled within the process setting range.

(3) Wide range of applications: The mold is relatively simple and the efficiency is high. It is suitable for large-area complex-shaped plates and shells, as well as simple-shaped plates, rods, tubes, and blocks. It can also be used for gluing and assembly, and small parts can be cured at the same time.

(4) The molding process is stable and reliable: The pressure and temperature are uniform, adjustable and controllable, so that the quality of the molded or transferred products is consistent and reliable; the porosity is low, and the resin content is controllable and uniform; vacuum can be drawn when pressurized, so that low molecular weight substances are easily discharged.

 

 

1.3.4 Disadvantages of the Autoclave Process

Large investment, high cost, complex autoclave connection, high cost, and large investment. Each use not only consumes water, electricity, gas and other energy, but also requires auxiliary materials such as vacuum bag film, sealing strips, suction adhesive, isolation cloth, etc., which greatly increases the production cost.

 

2 Production Materials

 

2.1 Prepreg

The raw material used in the carbon fiber autoclave process is carbon fiber prepreg. Carbon fiber prepreg is a composite material made of carbon fiber yarn, epoxy resin, release paper and other materials through coating, hot pressing, cooling, laminating, winding and other processes. It is also called carbon fiber prepreg.

 

Its advantages are high strength and low density. The strength can reach 6-12 times that of steel, and the density is only one-fourth of steel. It can be made into any shape according to different molds. It is easy to form and easy to process. It is also corrosion-resistant and has a long service life.

 

 

2.2 Mold Materials

Carbon fiber autoclaves require molds with fast thermal conductivity, low specific heat capacity, high rigidity, light weight, small thermal expansion coefficient, heat resistance, good thermal stability, long service life, low manufacturing cost, easy use and maintenance, and easy transportation. In particular, they require good thermal conductivity, hot rigidity, and air tightness. At the same time, the design requirements for the mold are relatively high. The following are several materials that carbon fiber composite material manufacturers use more in actual production:

 

(1) Aluminum: Good thermal conductivity and processing technology, light weight, but relatively large thermal expansion coefficient. Because of its low hardness, it is easily damaged, so it is subject to certain restrictions in use.

 

(2) Steel: High processing accuracy, rigidity, and hardness, long service life, suitable for most products, but the disadvantage is that it has a large mass and high thermal capacity.

 

(3) Cast steel or cast iron: It can replace steel to reduce costs, but the temperature difference between each point is large, and sand holes are easy to form on the surface.

 

 

2.3 Auxiliary Materials

The auxiliary materials required for the carbon fiber autoclave process are of various types and have different uses. They are all consumable and can generally only be used once. Because of the large amount used, the cost is relatively high, but the use of auxiliary materials has a great influence on the control of molding quality.

 

(1) Breathable felt: It is a channel for the gas in the blank to flow outward, which can ensure the quality of composite material molding.

 

(2) Vacuum film: It has good strength, ductility, temperature resistance, wear resistance and toughness. When used, putty is used to seal the molding component on the mold to form a vacuum bag.

 

(3) Sealing tape: It is sticky at room temperature, has good sealing performance at high temperature, and is easy to clean after curing.

 

(4) Glue-absorbing material: It can quantitatively absorb excess resin in the composite blank and has certain air permeability, such as glue-absorbing felt, glass cloth, glue-absorbing paper, etc.

(5) Release material: refers to a layer of material placed between the composite blank and the mold or cover plate to prevent the blank from adhering to it after curing. It is divided into porous isolation film, non-porous isolation film, breathable film, breathable and adhesive film, etc.

(6) Pressure-sensitive tape: can play a positioning and fixing role in the autoclave molding process.

(7) Block: restricts the flow of resin from the edge of the component, which helps to control the fiber content of the component.

(8) Peelable cloth: allows volatiles to pass through, absorbs a certain amount of excess resin, is easy to remove after curing, and provides a surface that can be glued or sprayed.

(9) Release agent: The purpose is to facilitate the release of the component from the mold after curing.

(10) Solvent: used to clean the mold surface.

(11) Auxiliary tools: vacuum nozzle, vacuum tube, thermocouple, pressure roller, etc.

 

3 Autoclave Process

 

When using the autoclave process, different carbon fiber products have similar methods and steps. Basically, they are carried out in the order of laying up blanks, making vacuum bags, vacuum leak detection, putting workpieces into the tank, starting the heating program, pressurizing, keeping warm and keeping pressure, cooling, stopping the machine for reducing pressure, and taking workpieces out of the tank. The main difference lies in the different molds and the process parameters set according to the required performance of the product. The following is a detailed explanation of the carbon fiber battery box designed and customized by Wuxi Weisheng New Materials Technology Co., Ltd. for a small domestic new energy vehicle.

 

3.1 Laying Up Blanks

The procedure includes three links: prepreg cutting (cutting), laying up, and pre-compacting. The whole structure and layer design should consider the stackability. Take the battery box of Wuxi Weisheng New Material Technology Co., Ltd. as an example. The whole box is a cuboid with a size of 600mm×400mm×200m and a wall thickness of 1.5mm. Because the mold itself uses a negative mold, the integrity and operability of the prepreg layer should be considered. The prepreg needs to be cut into a cuboid flat unfolding style. Before laying, the mold should be placed in an autoclave for preheating, and then the mold release agent should be brushed on the inner wall of the mold. The cut prepreg should be unfolded into a plane as much as possible, carefully placed in the mold, and gradually compacted so that the prepreg fits tightly with the mold. Each surface must be kept flat and wrinkle-free, and then the excess prepreg is cut along the outer edge of the mold. For irregular products, if they cannot be unfolded into a plane, they should be made into strips of appropriate width, and opening or splicing can be used locally.

 

 

According to the required bearing capacity of the battery box, the prepreg layer is determined to be two layers of 3k plus three layers of unidirectional prepreg. During the layering process, the air between the layers should be removed in time to ensure the interlayer performance. Therefore, the pre-compacting stage is to remove the volatile matter and the air between the prepreg layers during the layering operation while compacting the layers. The ultimate goal is to improve the dimensional accuracy of the component.

 

Then the layers are vacuumed at room temperature or under heating conditions for 5-15 minutes. If it is a thick part, it is generally pre-compacted once every 3 layers, which has a great impact on the molding quality. In general, this link is labor-intensive, time-consuming and costly.

 

3.2 Autoclave Curing

 

After the layers are laid, the products should be positioned and assembled on the mold and sealed in a vacuum bag. During the packaging process, the prepreg should be trimmed again through the surface of the vacuum bag with tools such as a roller to squeeze out the air as much as possible, so that the prepreg undergoes resin flow, resin curing, fiber compaction and other processes, and finally achieves curing.

 

Connect the vacuum bagged mold and components to the vacuum pump (buffer) and push them into the autoclave. According to the material process characteristics, component shape and size, and equipment conditions, the process parameters of the autoclave are formulated.

 

For large components that do not hinder the air circulation in the autoclave cavity, it is suitable to cure one piece. For small-sized parts with simple shapes, multiple pieces can be cured in one tank.

 

The use pressure of the autoclave used in the composite material molding process is generally less than 1.6MPa. The curing temperature of the composite material matrix resin used in important domestic aviation structures is up to 180±5℃, and the upper limit temperature commonly used is 250℃.

 

During the operation of the autoclave system, temperature measurement points can be set up in each heating zone and relevant parts of the product. The temperature distribution can be collected and displayed by the central control system, and the heating and cooling rates can be adjusted according to the process requirements to ensure the curing quality of the product.

 

 

For important products, inert gas protection is required in the tank during the curing process to prevent the flammable volatiles released during the curing process from causing combustion or explosion. Inert gas should be used for pressurization when the temperature in the tank is higher than 150℃ or the pressure is greater than 1MPa.

 

After the autoclave starts pressurizing, the vacuum bag may leak. In this case, it should be handled according to the actual situation. If it is a slight leak, it is allowed to continue curing, and the air is vented to slow down the rupture of the vacuum bag. In severe cases, the machine should be stopped for repair.

 

3.3 Demolding

After the autoclave curing process is completed, the temperature and pressure in the tank should be gradually reduced to below the safety index before shutting down, opening the tank door to remove the mold and components. When the mold temperature drops below 50℃, remove the component from the mold. If the temperature is too high, demolding is performed, which can easily cause product deformation or resin cracking.

 

4 Common Problems and Countermeasures

 

After curing and demolding, the appearance quality of carbon fiber products can be inspected by visual inspection and measuring tools, non-destructive testing can be carried out with the help of ultrasound, X-ray, infrared, etc., and destructive testing can be carried out through microscopic observation and sampling testing. When using the autoclave process to make carbon fiber composite products, delamination, porosity, and deformation are common problems.

 

4.1 Delamination

One of the main defects of carbon fiber composite components made by the autoclave process is delamination. Delamination is the separation between layers caused by interlayer stress or manufacturing defects, that is, debonding or cracking between layers. The residual stress can be reduced by improving the design and process, and the probability of occurrence can be reduced by improving the toughness of the resin.

 

4.2 Porosity and Looseness

Porosity refers to the voids formed during the molding process of carbon fiber composite materials. It is generally expressed by porosity, which refers to the content of microscopic pores in the resin between fiber layers. When the pores grow to a certain extent, pores will form. It is a defect form that appears in a macroscopic state. Its formation mechanism is the same as that of pores, but the size of the defects is different.

 

Sometimes, factors such as too late pressurization can also lead to uniform pores between the fibers and resins and between the resins in the composite material layer. The first countermeasure is to control the resin pressure, which should be greater than the product of the volatile content and the saturated vapor pressure; the second is to form an effective air path between the prepreg layers; and the third is to adopt a zero-adhesion glue process.

 

4.3 Deformation

Deformation is a defect form in which carbon fiber composite products do not meet the design standards and the parameters such as the curvature of the appearance change.

 

It can be controlled by the following methods: adjust the layer design from the angle, proportion, sequence, etc.; optimize the process in terms of curing temperature and cooling rate; change the type or structure of the mold material; and force correction by strengthening ribs, applying stress, etc.

 

In addition, if a large area of ​​detachment or poor bonding occurs between the bonding interfaces of two layers of composite materials, it is often caused by mold assembly and human operation errors, and standardized operation management needs to be strengthened.

 

5 Integrated Molding Technology

 

Integrated molding is one of the advantages and characteristics of composite materials and is a common technology in autoclave process. The use of co-curing or co-bonding can greatly reduce the number of parts and fasteners, thereby realizing integrated molding of composite materials from structural design to manufacturing, and further reducing the weight of the structure and reducing costs, especially manufacturing costs, while meeting the overall performance requirements of the structure. This technology is suitable for the manufacture of large composite structures such as wing integration and integrated fuselage sections.

 

5.1 Technical Method

Co-curing refers to a process method in which two or more parts are cured and molded once to form an integral part.

 

Secondary bonding refers to two or more pre-cured composite parts connected together by bonding, and the only chemical or thermal reaction during the bonding is the curing of the adhesive film.

 

Co-bonding is a process method in which one or more parts that have been cured and molded and one or more parts that have not yet been cured are cured and bonded into an integral part through an adhesive (generally an adhesive film) in one curing, which is a combination of co-curing and secondary bonding.

 

5.2 Technical Advantages

This technology can effectively reduce the number of parts and connectors, and is easy to establish a large product fusion layout, avoid drilling, and reduce component processing damage, thereby increasing the smoothness and integrity of the body surface.

 

5.3 Technical Risks

However, the defects of this technology are also obvious. On the one hand, local defects can easily affect the entire component. Once a large part is scrapped, the manufacturing risk will be greatly increased. On the other hand, because the mold becomes larger and more complex, the design requirements are high, so the tooling cost increases, and the demand for various adhesives and special materials increases accordingly. It is also necessary to carefully balance the molding and assembly costs.

 

 

In short, the autoclave process is one of the commonly used processes in the molding process of carbon fiber composite materials. It is suitable for making sandwich structures and laminate components, such as the carriage panels of light rail vehicles such as high-speed rail. It can also be used to make composite components and adhesive components, such as large automotive parts. From the manufacture of large-sized and complex-shaped aviation and aerospace CFRP components to the in-depth application in the fields of automobiles and high-speed rail under the influence of the lightweight trend, the autoclave process will continue to play an important role.