An article to understand the composite autoclave molding process

There are dozens of molding processes for carbon fiber composite materials, among which the autoclave process is the most widely used and most common molding method. In this paper, the process is summarized from the aspects of process principle, process characteristics, common problems and countermeasures, and typical technical applications, and the process flow is described in detail.

 

1 Basic Concepts

1.1 Process principle

 

The carbon fiber prepreg is laid on the mold according to the layup requirements, and the blank is sealed in a vacuum bag and placed in the carbon fiber autoclave. In the vacuum state, through the autoclave equipment heating, pressurization, heat preservation, cooling and pressure relief procedures, the use of the autoclave at the same time to provide a uniform temperature and uniform pressure to achieve solidification, so that the formation of high surface and internal quality, complex shape, huge area of carbon fiber composite parts.

 

 

1.2 Process development

 

Autoclave process is a process developed for the production of the second generation of composite materials, formed in the 40s of the 20th century, until the 60s was gradually popularized, and later widely used in aerospace, composite materials, electronics, weapons, transportation, sports equipment and new energy and other high-tech fields, especially in the production of various skin parts has played a huge role, has become a mature process. The composite products produced by the autoclave process account for more than 50% of the entire composite product output, 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

 

Autoclave is a kind of process equipment for the characteristics of polymer matrix composite molding process, and the use of autoclave molding is the main method for manufacturing continuous fiber reinforced thermosetting composite parts, which is suitable for the molding of advanced composite structures, honeycomb sandwich structures and metal or composite glued structures. Generally, it has the advantages of high temperature and pressure control accuracy, safe and reliable structure, good system stability, low energy consumption, simple operation and maintenance, etc., and can realize modularization and serialization, which can meet the needs of different fields, different processes and different specifications.

 

 

1.3.2 Equipment Composition

 

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

(1) Overall structure: It is composed of the tank, the tank door mechanism, the high-temperature motor, the air duct plate insulation layer, etc., to form a high-pressure, high-temperature tank.
(2) Safety interlocking device: It is composed of automatic pressure interlocking, manual interlocking and ultra-high pressure alarm device.
(3) Quick door opening device: manual electric dual-purpose fast door design, can open and close the tank door normally when the power fails;
(4) Sealing device: The tank door is sealed with silicone inflatable type, which is resistant to high pressure.
(5) Pressure system: The pressure transmission and control system is composed of compressor, gas tank, pressure control valve, pipeline, pressure transmitter and pressure gauge, etc., the pressure in the tank can reach 1.5-2.5MPa, the error is not more than 0.05MPa, and it is equipped with a safety explosion-proof device.
(6) Heating system: It is composed of stainless steel electric heating pipe, high-temperature fan, air duct plate, heat insulation layer and temperature control system, and the heating power meets the requirements of the maximum temperature of the cavity and the heating rate.
(7) Temperature circulation system: It is composed of a circulating fan, an air deflector and a shroud, which accelerates the heat flow conduction and circulation and forms a uniform temperature field.
(8) Vacuum system: It is composed of a vacuum pump, a pipeline, a vacuum gauge and a vacuum valve, which provides vacuum conditions for the encapsulated composite prefabricated parts, and requires a plurality of vacuum pipe joints to meet the requirements of the molding process.
(9) Automatic control system: PLC control system is used to realize high-precision control and real-time recording of process parameters such as pressure, temperature, and cooling.

 

1.3.3 Advantages of the autoclave process

 

(1) Uniform pressure: The pressure is pressurized by gas, and the pressure acts on the surface of the product through the vacuum bag, and the normal pressure at each point is equal, so that the parts are solidified and formed under the same pressure.
(2) The temperature is uniform and adjustable: the tank is heated by circulating hot air flow to the workpiece, and the temperature difference is small everywhere. At the same time, the cooling system is configured, so that the temperature can be strictly controlled within the range of process settings.
(3) Wide range of application: the mold is relatively simple and has high efficiency. It is not only suitable for plates and shells with large areas and complex surfaces, but also suitable for plates, rods, tubes and blocks of simple shapes, and can also be used for gluing assembly, and small parts can be cured at the same time for multiple pieces at one time.
(4) The molding process is stable and reliable: the pressure and temperature are uniform, adjustable and controllable, so that the quality of the molding or handover products is consistent and reliable; Low porosity, controllable and uniform resin content; Vacuum can be evacuated when pressurized, so that low molecular matter can be easily discharged.

 

 

1.3.4 Disadvantages of the autoclave process

 

The investment is large and the cost is high, and the autoclave is complex, the cost is high and the investment is large. Each time it is used, it not only consumes water, electricity, gas and other energy, but also needs auxiliary materials such as vacuum bag film, sealing strips, adhesive absorption, 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, which is a composite material processed by carbon fiber yarn, epoxy resin, release paper and other materials through coating, hot pressing, cooling, lamination, coiling and other processes, also known as carbon fiber prepreg. Its advantages are high strength, low density, strength can reach 6-12 times of steel, density is only a quarter of steel, can be made into any shape according to different molds, easy to form, easy to process, and corrosion resistance, long life.

 

 

2.2 Mold Materials

 

Carbon fiber autoclave requires the mold to have the characteristics of 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, especially good thermal conductivity, hot rigidity and air tightness, and the design requirements for the mold are also relatively high. The following are the materials used by carbon fiber composite product manufacturers in actual production:

1) Aluminum: good thermal conductivity and processing technology, light weight, but the coefficient of thermal expansion is relatively large, because the hardness is low and it is easy to be damaged, so it will be limited in use.
(2) Steel: The processing accuracy, stiffness and hardness are relatively high, the service life is long, suitable for most products, the disadvantage is that the quality is large, and the heat capacity is also high.
(3) Cast steel or cast iron: It can replace steel to reduce costs, but the temperature difference between each point is large, and the surface is easy to produce sand holes.

 

 

 

2.3 Ancillary Materials

 

There are many kinds of auxiliary materials required for the carbon fiber autoclave process, and they are all consumable, and they can only be used once, because the amount is large, so the cost is relatively high, but the use of auxiliary materials has a great impact 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, the components in the molding are sealed on the mold with putty in order to form a vacuum bag.
(3) Sealing tape: It is sticky at room temperature, has good sealing at high temperature, and is easy to clean after curing.
(4) Adhesive absorbing materials: It can quantitatively absorb the excess resin in the composite blank, and has certain air permeability, such as adhesive felt, glass cloth, adhesive paper, etc.
(5) Release material: refers to a layer of material placed between the composite material blank and the mold or cover plate to isolate it, so that the blank does not adhere to it after curing, and is divided into porous isolation film, non-porous isolation film, breathable film, breathable film, breathable film, etc.
(6) Pressure-sensitive tape: It can play a positioning and fixing role in the autoclave molding process.
(7) Block: Limit the flow of resin from the edge of the component, which helps to control the fiber content of the component.
(8) Peelable cloth: allow volatiles to pass through, absorb a certain amount of excess resin, and easily remove it after curing, providing a surface that can be glued or sprayed.
(9) Release agent: The purpose is to make it easy for the component to be separated from the mold after curing.
(10) Solvent: used to clean the surface of the mold.
(11) Auxiliary tools: vacuum nozzles, vacuum tubes, thermocouples, pressure rollers, etc.

 

3 Autoclave process

 

When different carbon fiber products use the autoclave process, the methods and steps are similar, basically in accordance with the order of laying blanks, making vacuum bags, vacuum leak detection, workpiece into the tank, opening the heating procedure, pressurization, heat preservation and pressure holding, cooling, pressure reduction, shutdown, and workpiece 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 description of the carbon fiber battery box designed and customized by Wuxi VIA New Material Technology Co., Ltd. for a small new energy vehicle in China.

 

3.1 Laying blanks

 

The process consists of three steps: prepreg cutting (blanking), laying, and pre-compaction. The whole structure and layup design should consider the stackability, taking the battery box of Wuxi VIA New Material Technology Co., Ltd. as an example, the whole box is 600mm× 400mm×200m, The cuboid with a wall thickness of 1.5mm, because the mold itself adopts a negative mold, so it is necessary to consider the integrity and operability of the prepreg layup, the prepreg needs to be cut into the plane expansion type of the cube, before the layup, the mold should be placed in the autoclave for preheating, and then the release agent is brushed on the inner wall of the mold, and the cut prepreg is expanded into a flat surface as much as possible, carefully put into the abrasive, and gradually compacted, so that the prepreg and the mold are tightly fitted together, and all sides must be kept flat and wrinkle-free. Then, along the outer edge of the mold, cut off the excess prepreg. For irregular products, if they cannot be expanded into a flat surface, they should be made into strips of appropriate width, and some parts can be opened or spliced.

 

 

According to the required bearing capacity of the battery box, the layup of the prepreg is determined to be two layers of 3K plus three layers of unidirectional prepreg for layup, and the interlayer inclusions should be removed in time during the layup process, so as to ensure the interlayer performance, so the pre-compaction stage is to remove volatiles in the layup operation, and the air between the prepreg layers is compacted at the same time, and its ultimate purpose is to improve the dimensional accuracy of the components. The layup is then vacuumed at room temperature or under heating conditions for 5-15 minutes. In the case of thick parts, pre-compaction is generally carried out 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 laying, the product should be positioned and assembled on the mold to seal the vacuum bag. In the encapsulation process, the prepreg should be trimmed again with pressure rollers and other tools across the surface of the vacuum bag, and the air should be pressed out as much as possible, so that the prepreg can go through the processes of resin flow, resin curing, and fiber compaction, and finally achieve curing.

 

The mold and components encapsulated in the vacuum bag are connected to the vacuum pump (buffer), pushed to the autoclave, and the process parameters of the autoclave are formulated according to the material process characteristics, the shape and size of the components, and the equipment situation. For large components that do not obstruct the air circulation in the autoclave cavity, it is suitable for single piece curing, and for small parts with uncomplicated shapes, it is possible to have multiple pieces in one can. The use pressure of the autoclave used in the composite molding process is generally less than 1.6MPa, and the curing temperature of the composite matrix resin for the domestic important aviation structure is up to 180±5 °C, and the upper limit temperature of general use is 250 °C. In the process of working of the autoclave system, temperature measurement points can be set up in each heating zone and the relevant parts of the product, and the temperature distribution can be collected and displayed by the central control system, and the heating and cooling rate can be adjusted according to the process requirements to ensure the curing quality of the product.
 

 

For critical products, inert gas protection is required in the tank during the curing process to prevent combustion or explosion caused by flammable volatiles escaping from the product during the curing process. When the temperature in the tank is higher than 150°C or the pressure is greater than 1MPa, inert gas should be pressurized. After the autoclave starts to be pressurized, there may be air leakage in the vacuum bag, and in this case, it should be judged and dealt with according to the actual situation. If it is a slight air leak, it is allowed to continue curing, and air ventilation measures are used to slow down the rupture of the vacuum bag, and in severe cases, it should be stopped for repair.　

 

3.3 Demoulding

 

After the autoclave curing process is completed, wait for the temperature and pressure in the tank to gradually drop below the safety index before shutting down, open the tank door and take out the mold and components. When the mold temperature drops below 50°C and then the component is removed from the mold, if the temperature is too high, the mold will be released, which can easily cause product deformation or resin cracking.

 

4 Common problems and countermeasures

 

After curing and demoulding, the appearance quality of carbon fiber products can be detected by visual inspection and measuring tools, non-destructive testing with the help of ultrasonic, X-ray, infrared, etc., and destructive testing by microscopic observation and sampling testing. When using the autoclave process to make carbon fiber composite products, there are often problems such as delamination, porosity, and deformation.

 

4.1 Stratification

 

One of the main drawbacks of carbon fiber composite components made by the autoclave process is delamination. Delamination is the separation between layers caused by interlayer stresses or manufacturing defects, i.e., degumming or cracking between layers, which can be reduced by design and process improvements, or by improving the toughness of the resin

 

4.2 Porosity and porosity

 

Porosity refers to the voids formed during the molding process of carbon fiber composites, which are generally expressed by porosity, which refers to the content of microscopic pores in the resin between the fiber layers. Porosity grows to a certain extent to form pores, which is a form of defect that appears in a macroscopic state, and its formation mechanism is the same as that of pores, but the size of the defect is different. Sometimes, due to late pressurization, uniform porosity can occur between the fibers and resins and between the resins in the composite layer. The first countermeasure is to control the resin pressure, which is greater than the product of volatile content and saturated vapor pressure, the second is to form an effective gas path between the prepreg layers, and the third is to adopt zero glue absorption process.

 

4.3 Transformation

 

Deformation is a form of defect in which carbon fiber composite products do not conform to the design standards, and the shape curvature and other parameters change. It can be controlled by the following methods: adjust the layup design from the angles, proportions, sequences, etc.; Process optimization in terms of curing temperature and cooling rate; Change the type or structural form of the mold material; Correction is forced by stiffeners, stress, etc.

 

In addition, if there is a large-scale disconnection or poor gluing between the glued interface of the two layers of composite materials, it is often caused by mold assembly and human operation errors, and it is necessary to strengthen standardized operation management.

 

5 Integral molding technology

 

Monolithic molding is one of the advantages and characteristics of composite materials, and it is a common technology in the autoclave process. The use of co-curing or co-gluing can greatly reduce the number of parts and fasteners, so as to realize the integrated molding of composite materials from structural design to manufacturing, and further reduce the weight of the structure and reduce the cost, especially the manufacturing cost, on the premise of meeting the overall performance requirements of the structure. This technology is suitable for the manufacture of large composite structures such as wing integration and integral fuselage segments

 

5.1 Technical Approach

 

However, the defects of this technology are also obvious, on the one hand, local defects can easily affect the entire component, once the large parts are scrapped, the manufacturing risk will be greatly increased; On the other hand, as molds become larger and more complex, and design requirements are high, tooling costs are rising, and the demand for various adhesives and special materials is increasing, and it is necessary to carefully balance molding and assembly costs.

 

In short, the autoclave process, as one of the commonly used processes in the molding process of carbon fiber composite materials, is suitable for the production of sandwich structural parts and laminate components, for example, the carriage board of the light rail car body such as high-speed rail, and can also be used to make composite components and glued components, such as large automotive parts. From the manufacture of large-size and complex aviation and aerospace CFRP components to the in-depth application of automobiles, high-speed rail and other fields under the influence of lightweight trends, the autoclave process will continue to play an important role.