Introduction to Autoclave Forming Process

 

Do You Know How The Autoclave Molding Process is Carried Out?

Autoclave Process

The autoclave process refers to laying the prepreg on the mold according to the layering requirements, sealing it in a vacuum bag and putting it into the autoclave, heating and pressurizing the autoclave equipment to complete the material curing reaction, so that the prepreg blank becomes the required shape and meets the quality requirements of the component.

 

Prepreg: It is a combination of resin matrix and reinforcement made by impregnating continuous fibers or fabrics with resin matrix under strictly controlled conditions. It is an intermediate material for manufacturing composite materials;

 

Autoclave: Autoclave is a process equipment designed for the characteristics of polymer-based composite material molding process.

 

Autoclave Equipment

 

HRC Autoclave Equipment

 

Autoclave Process

1. Select pre-impregnated carbon fiber cloth and cut it with automatic cutting equipment;
2. Attach 3 to 7 layers of carbon fiber prepreg to the mold surface;
3. Lay vacuum auxiliary materials;
4. Seal the vacuum bag, evacuate, and perform pressure test;
5. Place the mold in the autoclave and heat it to about 130 degrees/bake it for 2 to 4 hours in an environment with a vacuum and pressure of 0.6 to 1.0 MPa to solidify it.

 

Schematic Diagram Of Autoclave Packaging Process Simple Flow Chart Of Autoclave Process:

Schematic Diagram Of Autoclave Packaging Process:

 

Advantages Of Autoclave Process:

 

Uniform pressure in the tank: Use compressed air or inert gas (N2, CO2) or mixed gas to inflate and pressurize the autoclave, and the pressure acting on the normal line of each point on the surface of the vacuum bag is the same, so that the component is formed and cured under uniform pressure;

 

Uniform air temperature in the tank: The heating (or cooling) gas circulates at a high speed in the tank, and the gas temperature at each point in the tank is basically the same. Under the premise of a reasonable mold structure, it can be ensured that the temperature difference at each point during the heating and cooling process of the component sealed on the mold is not large;

 

Stable and reliable molding process: The pressure and temperature in the autoclave are uniform, which can ensure the stability of the quality of the molded parts. The components manufactured by the autoclave process have a low porosity and uniform resin content. Compared with other molding processes, the mechanical properties of the parts prepared by the autoclave process are stable and reliable. The carbon fiber autoclave process products have high strength and hardness (3 times the hardness of steel) and are very light.

 

Can maintain the perfect appearance of carbon fiber fabrics: All carbon fiber surface textures in the autoclave process are free of any disorder or damage, and the carbon fiber surface is very complete and beautiful.

 

Wide range of applications: The mold is relatively simple and efficient, suitable for the molding of large-area complex-shaped skins, wall panels and shells, and can be used to mold various complex structures and parts of different sizes. The temperature and pressure conditions of the autoclave can almost meet the molding process requirements of all polymer-based composite materials;

So far, most composite parts requiring high load in the aerospace field use the autoclave process.

 

Disadvantages Of The Autoclave Process:

 

Large investment and high cost: Compared with other processes, the autoclave system is large and complex in structure. It is a pressure vessel. It is very expensive to invest in a large autoclave; each curing process requires a large amount of expensive vacuum bags, sealing strips, isolation films, breathable felts, demolding cloths and other auxiliary materials, and a large amount of water, electricity, gas and other energy is consumed during molding.

 

 

The Main Applications Of Autoclave Technology:

 

Aerospace: skin parts, ribs, frames, fairings, etc.;

Automobile: body panels and body structural parts, such as hood inner and outer panels, door inner and outer panels, roof, fenders, door sill beams, B-pillars, etc.;

 

Rail Transportation: pillow beams, side beams, etc.;

Others: boat industry, high-end consumer goods, etc.

 

The autoclave process is the main method for manufacturing continuous fiber reinforced composite parts. It is widely used in high-tech fields such as aerospace, rail transportation, sports and leisure, and new energy. Composite products produced by autoclave technology 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%.

 

Vacuum Auxiliary Materials For Autoclaves

Vacuum Bag Film

Vacuum Bag Film is a process auxiliary material for the curing and molding of composite components. It can form and maintain a vacuum state under certain environmental conditions. Its function is to form a vacuum system.

Vacuum bag film – temperature resistance 150℃

Specifications: 2*100m, 4.06*115m, 6*100m, 8*100m

Material: PA+PE+PA

Thickness: 50-65-75u

 

Vacuum bag film-temperature resistant to 200℃

Specifications: 2*280m, 3*153m, 4*229m

Material: PA

Thickness: 50-75u

 

Vacuum bag film-temperature resistant 230℃

Specifications: 2.9*150m, 2.9*100m

Material: PA

Thickness: 50-75u

 

Release Cloth

Release Cloth is usually used as the first isolation product and other vacuum auxiliary materials. The release cloth will not stick to the product and is easy to tear off after curing and molding. It can reach about 170-230 degrees Celsius.

Specifications: 1.8*200m, can be cut to the narrowest 10cm

Material: Polyester fiber-temperature resistant 170 degrees Celsius, nylon 66-temperature resistant 230 degrees Celsius

Weight: 85g/square meter

 

Polytetrafluoroethylene High Temperature Cloth (temperature resistant 260℃) uses high-quality glass fiber as the weaving material, and is woven into a high-grade glass fiber cloth base material in plain, twill, satin or other weaving methods. Then, through unique process technology, it is repeatedly fully impregnated with suspended polytetrafluoroethylene emulsion to produce various thicknesses and ultra-wide high temperature resistant cloths.

Specifications: 1*100m, 1.22*100m

Thickness: 0.75mm

 

Release Film

In most cases, the release film is in direct contact with the laminate and separates the laminate from the non-release breathable felt. Existing release films are selected based on the curing temperature, pressure, complexity of the part and the resin system. Release films are usually provided with holes to ensure the removal of air and volatiles embedded in the laminate.

 

Name: Perforated Release Film

Color: Default Blue

Material: PE

 

Product Index

Thickness Index: 30μm

Gram Weight: 30g/m2

Softness Index: Very Soft

Width (mm): 1.56m

Length per Roll: 400m

Heat Resistance (℃): 120

 

Perforated Isolation Film – Temperature 200℃

Name: Perforated Isolation Film/Non-porous Isolation Film

Color: Purple

Material: PMP

 

Product Index

Thickness Index: 30μm

Weight: 30g/m2

Softness Index: Very Soft

Width (mm): 1.5m

Length Per Roll: 200m

Temperature Resistance (℃): 200

 

230℃ Perforated Isolation Film – Temperature 230℃

 

Name: Perforated Isolation Film/Non-porous Isolation Film

Color: Blue

Material: ETFE

Product Index

Thickness Index: 20μm

Weight: 20g/m2

Softness Index: Very Soft

Width (mm): 1.5m

Length per Roll: 127m

Temperature Resistance (℃): 230

 

Breathable Felt

The needle punching process is strictly controlled, the surface density is uniform, and the thickness is appropriate. The whole process is 100% metal detected, and there will be no metal broken needles mixed in. Good strength, pressure resistance, temperature resistance, and good conformability.

Specifications: 1.5*20, 1.5*50m, 1.5*100m

Weight: 150g, 340g

Heat Resistance: 200℃

 

Aramid Breathable Felt

 

 

Applied to the autoclave process, 170 degrees is its softening point, the melting point is greater than 240 degrees, and it can withstand short-term high temperatures below 200 degrees.

 

Sealing Tape

Sealing tape is used to seal vacuum bags and mold parts. The sealing tape must be sticky enough to stick well to the mold surface, but not too sticky so that the sealing tape on the vacuum bag film cannot be torn off for repositioning. At the same time, after curing, the sealing tape must be torn off cleanly from the mold surface. We provide a series of sealing tapes with different curing conditions.

Thickness: 3mm

Width: 12mm

Length: 15m or 150~300m/box

Temperature Resistance and Color: 120℃ (black), 204℃ (yellow)

 

Thickness: 3mm

Width: 12mm

Length: 7.5 or 7.6m or 150~300m/box or 40 rolls/box

Temperature Resistance and Color: 230℃ (gray), 400℃ (green or brown-red)

 

Vacuum Pipes

Vacuum Pipe fittings include vacuum bases, anti-backflow male and female joints, and various pipes used in vacuum conditions. Using these pipe fittings can ensure that there is no leakage during vacuum operation.

Specifications: 6*8, 8*10, 10*12, 12*14, 16*19, etc.

Length: 100~200 meters/roll

 

Vacuum Hose Temperature Resistance 80℃

Vacuum Valves, Quick Connectors, etc., Temperature Resistance 150~260℃

Temperature Resistance 232℃ (Blue/Green) or Temperature Resistance 250℃ (Red)

Length: 3m~5m~8m

Size: inner diameter 9.5*outer diameter 18mm

Specification: 1/4NPT

 

Vacuum Leak Detector

Vacuum Leak Detector is to effectively detect the leaking parts of the vacuum system

 

Vacuum Pump System

 

Application of Autoclave Molding Process

Autoclave is a molding method that is widely used in carbon fiber products. The specific steps are as follows.

 

1. Preparation Process

This process mainly includes the preparation of tools and materials, mainly including the preparation of molds, carbon fiber prepregs, and auxiliary materials.

 

2. Material Laying

This step includes the cutting, laying and compacting of prepregs. It is mainly to cut the carbon fiber prepreg according to different sizes and requirements. The cutting can be done by a sample or an automatic cutting machine. Finally, the prepreg is cut into the desired size, laid according to the designed laying method, and finally pre-compacted. At present, carbon fiber products such as carbon fiber tubes, carbon fiber plates or other special-shaped parts are all laid by hand, which is more flexible.

 

3. Curing Preparation Process

This step mainly includes the bagging of carbon fiber products, the cutting and laying of auxiliary materials, etc. Usually, the curing preparation work includes the preparation of parts, molds, vacuum bags and auxiliary materials. Among them, the auxiliary materials that must be used include demolding materials, breathable materials (used to evenly disperse the vacuum in the vacuum bag system), etc.

 

4. Curing and Molding

Curing is a necessary step for every carbon fiber product. It is the process of heating and pressurizing the thermosetting resin in the prepreg under fixed process conditions to form a relatively stable three-dimensional mesh structure. For thermosetting carbon fiber composite materials, once cured, all defects caused by the layering or curing process are immutable. Therefore, it is very important to strictly control the process parameters when curing in an autoclave.

 

5. Testing

Detection is mainly divided into three methods: visual inspection, ultrasonic or X-ray non-destructive testing. When the carbon fiber product is cured and formed, it is first necessary to visually inspect the appearance to see if there are white spots, lack of glue, glue collection and other problems. The internal inspection usually uses ultrasonic or X-ray non-destructive testing, which can show different forms of existence according to the different densities of the material, and can also detect the dense gaps and stratification inside.

 

6. Post-Processing

It is mainly because the carbon fiber products after processing and forming will have burrs or flash. They can be trimmed or machined by polishing machines or milling machines, five-axis engraving machines or five-axis machining centers to meet the design requirements.

 

In addition to the above-mentioned fixing steps, some composite parts need to be subjected to secondary molding in an autoclave, co-bonded or co-cured, and then composite parts with complex shapes are prepared.

 

Carbon Fiber Composite Molding Technology: Autoclave Molding Process

 

The wide application of carbon fiber composite materials has attracted great attention in the field of modern engineering. This lightweight and high-strength material has unique advantages in aerospace, military, rail transportation and other fields. This article will study the molding process of carbon fiber composite materials in depth, focusing on the traditional autoclave molding process, and introduce the latest low-temperature molding technology, in-situ molding method and other key processes.

 

 

1. Overview of Autoclave Forming Process

 

1.1. Principle of Autoclave Forming Process

The autoclave forming process is a process in which the prepreg is laid on the mold according to the design requirements, sealed in a vacuum bag and placed in an autoclave, and the material curing reaction is completed through the process of heating and pressurizing. This process is widely used in aerospace, military and other fields, and can achieve high-quality manufacturing of composite components.

1.2. Application Fields of Autoclave Molding Process

The autoclave molding process has unique advantages in aerospace, military and other fields. Taking carbon fiber composite materials as an example, this process can achieve lightweight internal and external quality of UAV composite components, uniform resin content and excellent mechanical properties. Therefore, it is favored in the manufacturing of composite components of main load-bearing components and high-speed requirements of UAV.

 

1.3. Challenges and Limitations

However, the autoclave molding process also has some shortcomings, mainly reflected in poor economic efficiency and high requirements for manufacturing equipment, initial investment and processing costs. This has restricted the popularization of this technology to a certain extent. Considering economic benefits, in some actual manufacturing processes, low-temperature and low-pressure molding technology often becomes an alternative to autoclave molding technology.

 

2. Domestic and Foreign Development

 

2.1. Foreign autoclave manufacturers and technical characteristics

Well-known foreign autoclave manufacturers such as Italy’s Terruzz, the United States’ ASC Process Systems, and Germany’s SCHOLZ have provided advanced equipment for the molding of carbon fiber composite materials through different technical characteristics.

 

Taking Italy’s Terruzz as an example, its autoclave has innovative designs such as independently supported tank doors, toothless three-ring structure doors, and self-sealing rings, so that the tank door does not need to rotate when opening and closing, greatly improving the sealing effect. Terruzz’s heating system uses a unique multi-layer heating tube, and the finned design improves the utilization efficiency of thermal energy during the heat exchange process. These innovative designs provide more stable and efficient process support for composite material molding.

 

2.2 Progress in Domestic Autoclave Molding Technology

In China, AVIC Composites Co., Ltd. already has a large autoclave with a diameter of φ7m×30 m, and has the ability to integrally mold and manufacture 30 m long components. In terms of technical reserves, it has successfully developed multiple models of wing/fuselage reinforced wall panels, multi-wall integrated wall panels/box sections, and full-height honeycomb integral rudders, and is in a leading position in the domestic industry.

 

Up to now, the number of domestic autoclave molding equipment has increased year by year, and the investment and use of large-scale equipment has played a key role in improving the production efficiency of carbon fiber composite materials. Taking AVIC Composites Co., Ltd. as an example, its large autoclave not only provides the production capacity of overall molding, but also has achieved a series of innovative results in technology research and development.

 

III. Emerging Technologies Emerge

 

3.1. Emerging Low-Temperature Molding Technology

With the continuous development of the field of carbon fiber composite materials, low-temperature molding technology has gradually emerged. According to statistics, the application of low-temperature molding technology in composite material manufacturing has increased year by year. Its process cost is relatively low, energy consumption is controllable, and curing reaction can be completed under low temperature conditions. This brings a more economical choice for composite material manufacturing. While ensuring quality, low-temperature molding technology reduces the manufacturing cost of drones and other products, becoming one of the current development hotspots.

 

3.2. Potential of in-situ Molding Method

In-situ molding method is an emerging process, which is characterized by online molding without post-processing process, greatly shortening the product circulation cycle and improving production efficiency. According to data, although the performance of thermoplastic composite materials prepared by in-situ molding method is currently only 80% of that of traditional autoclave molding technology, it has great potential. Breakthroughs in the preparation technology of laying-level prepregs and the design and manufacturing technology of heated laying heads will be key challenges for future development.

 

4. Opportunities and Challenges

The opportunities in the field of carbon fiber composite materials are mainly reflected in the emergence of emerging technologies, such as the gradual maturity of low-temperature molding technology and in-situ molding methods. The application of these technologies will bring more choices to the industry and improve the breadth and depth of the application of carbon fiber composite materials in aerospace and other fields.

 

In the face of challenges in the field of carbon fiber composite materials, it is an important task at present to break through the preparation technology of laying-level prepreg and the design and manufacturing technology of heated laying heads. This involves innovation in both materials and processes, and requires the joint efforts of R&D personnel in the industry. At the same time, domestic companies need to introduce advanced technologies through international cooperation to improve their own R&D level.

 

5. Future Outlook

The future of the field of carbon fiber composite materials is full of expectations. The development of emerging technologies will provide more possibilities for the manufacture of carbon fiber composite materials. Further research and application of low-temperature molding technology and in-situ molding methods will promote the manufacturing process of carbon fiber composite materials to a higher level. Through international cooperation, my country’s carbon fiber composite material manufacturing level is expected to be comprehensively improved, providing strong support for my country’s development in aerospace and other fields.

 

The molding process of carbon fiber composite materials is constantly evolving, from traditional autoclave molding to emerging low-temperature molding technology and in-situ molding methods, which brings more choices to the industry. In the future, we need to overcome technical difficulties, improve the performance of emerging technologies, realize the application of carbon fiber composites in a wider range of fields, and make greater contributions to the innovation and development of aerospace and other fields. With the continuous innovation in the field of carbon fiber composites, we are full of confidence in the future.

 

References:

[1] Chen Bo. Review of the development of composite material process equipment at home and abroad VIII – Autoclave molding [J/OL]. Composite Materials Science and Engineering: 1-14 [2022-03-09].

[2] Ding Lili, Jia Rui, Yin Jiagan, et al. Application of resin-based carbon fiber composites in rail vehicle body and overview of manufacturing process [J]. New Materials for High-speed Railway, 2023, 2(03): 29-34.

[3] Xu Zhu. Research on autoclave molding process of UAV motor housing [J]. Contemporary Chemical Industry, 2023, 52(02): 370-373.

 

Detailed Explanation of Carbon Fiber Composite Autoclave Process Technology

 

There are dozens of molding processes for carbon fiber composites, among which autoclave process is the most widely used and common molding method. This article gives a general explanation 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

Put the carbon fiber prepreg on the mold according to the layering requirements, seal the blank in a vacuum bag and place it in a carbon fiber autoclave. Under vacuum, the autoclave equipment heats up, pressurizes, keeps warm, cools down and releases pressure, and solidifies by using the uniform temperature and uniform pressure provided 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 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 is one of the molding methods for carbon fiber composite components and 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 processes. 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. It generally 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. It can achieve modularization and serialization, and can 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, and an air duct plate insulation layer 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 to form a pressure transmission and control system. 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 tube, high temperature fan, air duct plate, insulation layer and temperature control system. The heating power meets the maximum temperature requirement and heating rate requirement of the cavity.

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

(8) Vacuum system: It is composed of vacuum pump, pipeline, vacuum gauge and vacuum valve to provide vacuum conditions for the encapsulated composite preform. It requires multiple vacuum pipe joints to meet the molding process requirements.

(9) Automatic control system: It adopts PLC control system to realize 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 places.

(2) Uniform and adjustable temperature: The circulating hot air flow in the tank heats the workpiece, and the temperature difference at each place 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 easy to discharge.

 

1.3.4 Disadvantages Of The Autoclave Process

Large investment and high cost, the autoclave connection is complex, the cost is high, and the investment is large. Each use not only consumes water, electricity, gas and other energy, but also requires auxiliary materials such as vacuum bag film, sealing strips, suction adhesives, 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 to 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 process, and it is corrosion-resistant and has a long service life.

2.2. Mold Material

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 must have 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 will be 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 stacking 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 different process parameters set according to the required performance of the products.

 

The following is a detailed description of the carbon fiber battery box designed and customized by Wuxi Weisheng for a small new energy vehicle in China.

 

3.1. Stacking Blanks

The procedure includes three links: prepreg cutting (cutting), stacking, and pre-compacting. The whole structure and layer design should consider the stackability. Take the battery box of Wuxi Weisheng 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 to 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 vacuum bags. During the packaging process, the prepreg should be trimmed again through the surface of the vacuum bag with tools such as rollers, and the air should be squeezed out 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, single-piece curing is suitable. For small-sized components 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. When the temperature in the tank is higher than 150°C or the pressure is greater than 1.0MPa, inert gas should be used for pressurization.

 

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 atmospheric ventilation measures are taken 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, wait until the temperature and pressure in the tank gradually drop below the safety index before shutting down, open the tank door and remove the mold and components. When the mold temperature drops below 50°C, 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 form of defect 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. 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 processes. 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, during which the only chemical or thermal reaction 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.

 

Detailed Explanation of the Autoclave Molding Process of Composite Materials

Autoclave molding refers to a process method in which a composite material blank formed by stacking a single layer of prepreg in a predetermined direction is placed in an autoclave and the curing process is completed at a certain temperature and pressure. The autoclave is a special pressure vessel that can withstand and regulate a certain temperature and pressure range. The blank is laid on the surface of the mold with a release agent, and then covered with a porous anti-sticking cloth (film), adhesive felt, and breathable felt in turn, and sealed in a vacuum bag, and then placed in an autoclave. Before heating and curing, the bag is first evacuated to remove air and volatiles, and then heated, pressurized and cured according to the curing system of different resins.

 

 

Autoclave 3D Model Diagram

 

The formulation and implementation of the curing system are the key to ensuring the quality of autoclave molded parts. After heating to a certain temperature, the blank is pressurized. The pressure is appropriate to ensure the compaction of the parts, that is, to ensure the maximum discharge of air and volatiles without squeezing out too much resin. The heating and pressurization procedures need to be determined by measuring the changes in the viscosity, dielectric constant or reaction heat of the resin during the curing process. The mold used has a simple structure, the pressed parts are dense, and the porosity is low; the fiber direction and volume fraction, shape and geometric dimensions of the parts can be guaranteed. It is suitable for composite molding of laminates and sandwich structures. Many large load-bearing structural parts in the aviation and aerospace fields are widely used in this molding process.

 

The autoclave process is the most common molding method for fiber composite materials. 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%. The formed components are mostly used in the main and secondary load-bearing structures in the aerospace field, as well as in many fields such as national defense, rail transportation, electronic communications, automobile manufacturing, sports equipment, etc.

 

-1- Introduction to the Autoclave Process and Basic Principles

Autoclave molding is to seal the composite blank, honeycomb sandwich structure or adhesive structure on the mold with a vacuum bag, place it in an autoclave, and make it into one of the molding methods of advanced composite materials and their components under vacuum (or non-vacuum) conditions.

The principle is to use the high-temperature compressed gas inside the autoclave to generate pressure to heat and pressurize the composite blank to complete the curing molding.

 

-2- Composition of the Autoclave System

 

Schematic Diagram of Autoclave System

 

Schematic Diagram of Autoclave System

 

Composition and Description of Each System of Autoclave

 

3-Application Fields and Process Characteristics of Autoclave Molding Process

Autoclave molding process is widely used in aerospace, military industry, rail transportation, vehicles and sports goods.

Application of autoclave molding process

 

Characteristics of Autoclave Molding Process

 

 

4-Materials for Autoclave Molding Process

The materials used in autoclave molding process include prepreg, process auxiliary materials and mold materials.

Materials and descriptions for autoclave molding process

5-Autoclave Molding Process and Defect Control

 

Autoclave Molding Process and Defect Control

Schematic Diagram Of the Autoclave Heating Process

 

Schematic Diagram of the Autoclave Insulation Process

 

Schematic Diagram of Autoclave Cooling Process

 

From the perspective of improving product quality and efficiency, the autoclave process has the following four key links.

Key links in the autoclave molding process

Defect Types and Control Methods in Autoclave Molding Process

 

6-Autoclave Integrated Molding Technology

Integrated molding is a common technology in autoclave process.

Integrated molding technology features

 

7-Well-Known Composite Autoclave Manufacturers at Home and Abroad

Well-known composite autoclave manufacturers at home and abroad

Introduction to the Advantages and Disadvantages of Autoclave Forming Process

 

The autoclave forming process is to seal the composite blank, honeycomb sandwich structure or adhesive structure on the mold with a vacuum bag, and then heat it, pressurize it, keep it warm (medium or high temperature), cool it down and release it in the autoclave under vacuum (or non-vacuum) to make it into the required shape and quality.

 

The autoclave forming process is one of the main forming methods for advanced composite materials, honeycomb sandwich structures and metal or composite adhesive structures that are widely used. The manufactured components can be used in the main and secondary load-bearing structures in the aerospace field. The autoclave forming process is mainly suitable for the forming of most thermosetting composite materials, honeycomb sandwich structures and adhesive structures.

 

The autoclave system is to create an ideal temperature and pressure environment for the polymerization reaction of composite materials. It is composed of the autoclave body and a series of subsystems, mainly including the following parts:

1. Autoclave Body;
2. Pressurization System;
3. Vacuum System;
4. Heating System;
5. Cooling System;
6. Feeding System;
7. Control System;
8. Blowing System;
9. Instrument Valve

 

 

As the main production equipment for composite parts, the autoclave is a large pressure vessel with an integral heating system. The structure of a typical autoclave is shown in the figure. Its common structure is a cylinder with one end closed and the other end open, which provides the necessary heat and pressure for the compaction and curing of composite parts. The curing temperature of commonly used aviation thermosetting materials (such as high-temperature curing epoxy resin) is 175°C and the curing pressure is 600kPa, so the autoclave must be strong enough at high temperatures. This makes the manufacture of autoclaves quite expensive.

 

At present, most aviation applications are medium temperature and pressure. At the same time, there are some composite materials that require higher pressure and temperature for forming, such as advanced thermoplastic matrix composites (such as PEEK, PEI, etc.) and high-temperature thermosetting composites (such as polyimide PMR-15, etc.), which require temperatures between 300~400°C and pressures above 1.0MPa.

 

For example, the curing pressure and temperature of the LP-15 polyimide composite material (material product brand of Beijing Institute of Aeronautical Materials) in the figure are relatively high. For this situation, a special autoclave needs to be built, which will be quite expensive.

 

 

According to the curing temperature and pressure, the autoclave can be divided into ordinary type and high temperature and high pressure type. The manufacturing cost of high temperature and high pressure autoclave is much higher than that of ordinary autoclave, and it is mainly used to manufacture polyimide resin and thermoplastic composite material structure. The autoclave consists of tank system, vacuum system, blast system, heating system, pressurization system, cooling system, control system and others.

 

Designed according to the size, temperature and pressure of the cured product, the strength meets the requirements of high temperature and high pressure, and the outer wall temperature of the tank is ≤60℃.

 

Autoclave Subsystem

▶ Heating System: The temperature difference of the gas in the tank is ≤5℃, the heating rate is 1～8℃, and the requirements are adjustable.

▶ Cooling System: circulating water cooling, the cooling rate is 0.5～6℃/min, which is adjustable.

▶ Pressurization system: Inflation and pressurization, the cutoff is air or inert gas, the maximum pressure is determined by the process, and the pressure is adjustable; there are safety explosion-proof and deflation settings.

▶ Blowing System: The wind speed in the tank is 1～3m/s, and the noise is ≤60dB.

▶Vacuum System: The vacuum pipeline and joints in the tank meet the requirements of the vacuum process, usually with multiple connections and adjustable vacuum degree.

▶Control System: Automatic control with computer display and control, which can be manually controlled; temperature, pressure, and vacuum have indicating instruments and recorders, the workpiece temperature and the gas temperature in the tank can be independently controlled, the temperature and pressure are equipped with safety control, and the door opening and closing are equipped with safety inspection and self-locking mechanism.

▶Feeding System Instrument: trolley, connecting bridge, ground track and instrument valve…

 

 

Main Features of Autoclave Molding

 

 

One of the main advantages of autoclave forming is that it is suitable for the production of a variety of materials. As long as the curing cycle, pressure and temperature are within the limits of the autoclave, composite materials can be produced. The applicable scope of the autoclave forming process is shown in the following table.

 

Autoclave Molding Materials

 

Autoclave Molding Process

 

 

Autoclave Molding Mold—Mold Material Characteristics and Requirements

Should 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, simple use and maintenance, easy transportation, etc.

 

Material:

Aluminum: light, good thermal conductivity, but high thermal expansion coefficient, low surface hardness, suitable for products with low precision and coordination relationship.

Steel: high processing precision, high rigidity, high hardness, long service life, suitable for most products. The disadvantage is large mass and high heat capacity.

Cast Steel or Cast Iron: 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.

Rubber: good shape-following, easy to match, suitable for manufacturing co-curing molds. The disadvantage is poor dimensional stability.

Glass Fiber Composite Material: light weight, low cost, suitable for simple surface products. The disadvantage is low modulus of the material.

Carbon Fiber Composite Material: light, high modulus, good rigidity, consistent with the thermal expansion coefficient of carbon fiber composite components, can give products high precision. The disadvantage is high cost.

Wood: Lightweight and cheap, suitable for making disposable molds.

 

Autoclave Molding Molds—design and use requirements

Precautions for Hot Press Molding

 

Composite Material Autoclave Molding Process

The autoclave molding process is a commonly used composite material preparation process, which is mainly used to prepare high-performance, complex-shaped composite materials. It has high efficiency, stable production efficiency and good repeatability. Therefore, it is widely used in aerospace, automobile, construction, sports equipment and other fields.

 

The Autoclave Molding Process Mainly Includes the Following Steps:

1. Preparation of Prepreg

 

The production process of prepreg preparation is relatively simple, but the requirements are strict. Since the quality and performance of prepreg directly affect the performance of the final product, the quality of each link needs to be strictly controlled during the production process to ensure the stability of the quality and performance of prepreg. The production process of prepreg usually includes the following steps:

 

1. A) Raw material preparation: Select suitable carbon fiber cloth and resin for preparation.

 

1. B) Carbon fiber pretreatment: In order to improve the adhesion and surface energy of carbon fiber, it needs to be surface treated. This usually involves soaking the carbon fiber in an acidic or alkaline solution and drying it at high temperature.

 

1. C) Mixing: Mix the resin according to the formula to achieve the best performance in one go.

 

1. D) Pre-impregnation: Impregnate the carbon fiber cloth in the resin solution to ensure that the resin fully wets the carbon fiber to achieve the best performance.

 

1. E) Remove bubbles: Put the impregnated carbon fiber cloth into a vacuum bag and remove bubbles in the resin to ensure uniform quality between the resin and the fiber.

 

1. F) Drying: Dry the impregnated carbon fiber cloth to remove moisture and achieve the ideal resin content.
2. G) Cutting: Cut the prepreg cloth into the required shape and size for subsequent molding and processing.
3. H) Packaging: Pack and store the cut prepreg for subsequent use.

 

2. Prepreg Cutting

The cutting of prepreg is mainly divided into manual cutting and automatic cutting, and varies according to production requirements and specific shapes.

 

1. A) Manual cutting: Manual cutting is a simple and common method. During the manual cutting process, the operator uses tools such as cutting knives or scissors to manually cut according to the required shape and size. This method is suitable for small-scale production and prepreg cloth with simple shapes.
2. B) Die cutting: Using die cutting can ensure the accuracy and consistency of prepreg cloth. During the die cutting process, the prepreg cloth is placed in the mold, and then the knife in the mold cuts the prepreg cloth by pressure or vibration. This method is suitable for mass production and prepreg cloth that requires high-precision shape.
3. C) CNC cutting: CNC cutting is an automated cutting method. By placing the prepreg cloth on the CNC cutting machine and controlling the movement of the cutting knife by a computer, high-precision and high-efficiency cutting can be achieved. This method is suitable for mass production and prepreg cloth with complex shapes.
4. D) Laser cutting: Laser cutting is a non-contact cutting method. Cutting the prepreg cloth by a laser beam can achieve high-precision and high-efficiency cutting. This method is suitable for cutting that requires high-precision shape and thinner prepreg cloth.
5. Laying Of Prepreg

The laying of prepreg is mainly divided into manual laying and automatic laying.

 

Manual laying is suitable for small and complex structures. Laser projection positioning is required in the project, and pre-compacting is required during the process.

 

Automatic laying is mainly divided into automatic tape laying technology and automatic wire laying technology.

 

Automatic tape laying technology refers to the process of laying pre-impregnated tape on the mold surface according to the design requirements using automated equipment. After pre-treatment, the pre-impregnated tape is accurately laid on the mold surface by the automatic tape laying equipment according to the path on the design drawing. The advantages of automatic tape laying technology are high production efficiency and stable product quality, and it can realize the manufacturing of products with complex shapes and large sizes. At the same time, due to the use of automated production methods, manual intervention and errors can be reduced, thereby improving the reliability and consistency of production.

 

Automatic Fiber Placement technology refers to the process of using automated equipment to accurately place carbon fiber or other fibers on the mold surface according to design requirements. Through the automatic fiber placement equipment, the pre-treated carbon fiber is accurately placed on the mold surface according to the predetermined path, and then pre-impregnated, cured and other process treatments are performed. The advantages of automatic fiber placement technology are high efficiency and good repeatability in the preparation of large composite materials. At the same time, multiple fibers can be placed alternately during the preparation process to improve the performance and strength of the material.

 

4. Packaging

The placement of each component during packaging is shown in the figure.

Vacuum Bag: Provides a Vacuum Environment.

Breathable felt: Maintains uniform vacuum pressure (partial pressure) in the vacuum bag.

Perforated isolation membrane 1: Prevents resin from flowing to the breathable felt, but requires small molecule gases to pass through the breathable felt.

Glue-absorbing felt: Absorbs excess resin that is squeezed out.

Perforated isolation membrane 2: Allows resin and small molecule gases to pass through.

Release cloth: Allows the surface of the composite product to have a cloth pattern for subsequent bonding or painting processes, and the release cloth should be able to be peeled off from the surface of the part.

Release agent: Prevents the resin from sticking to the mold surface.

 

5. Curing and Molding in Autoclave

When using the autoclave process to prepare composite materials, it is necessary to reasonably set parameters such as temperature, pressure and time according to the selected resin system to ensure the preparation of high-quality composite materials. At the same time, the characteristics and processing requirements of different resin systems need to be considered when setting parameters. The following are some common resin systems and their parameter requirements in the autoclave process:

 

1. Thermosetting Epoxy Resin System: In the autoclave process, the temperature needs to be set between 120~180℃ for about 1~2 hours. At the same time, the pressure needs to be controlled between 1~10MPa to ensure that the resin is fully cured. Thermosetting epoxy resin systems usually require post-curing, that is, re-heat curing at high temperature to improve the performance of the material.

 

2. Thermosetting Polyimide Resin System: In the autoclave process, the temperature needs to be set between 300~350℃ for about 1~2 hours. Due to the good thermal stability of polyimide resin, a higher temperature is required during the preparation process. At the same time, the pressure needs to be controlled between 10~30MPa to ensure that the resin is fully cured. Materials prepared by thermosetting polyimide resin systems usually have excellent high temperature resistance and mechanical properties.

 

3. Thermosetting Phenolic Resin System: In the autoclave process, the temperature needs to be set between 130 and 180 ° C, and the time is about 1 to 2 hours. At the same time, the pressure needs to be controlled between 5 and 15 MPa to ensure that the resin is fully cured. Materials prepared by thermosetting phenolic resin systems usually have good wear resistance, corrosion resistance and mechanical properties.

 

Part.1. Advantages of Autoclave Molding Process

 

1. Composite components with high strength, high stiffness, high heat resistance and corrosion resistance can be prepared, and the autoclave can prepare very complex component shapes to meet design requirements.

 

2. The autoclave molding process has high production efficiency and can continuously produce a large number of composite components.

 

3. The process parameters of the autoclave molding process can be precisely controlled, and the quality of the prepared composite components is stable and has good repeatability.

 

4. The autoclave molding process can fully cure the resin in the prepreg material, thereby improving the performance of the composite material.

 

Part.2. Disadvantages of Autoclave Molding Process

 

1. A large amount of capital needs to be invested in purchase, and the investment requirements for production enterprises are high.
2. It requires more process steps, such as material preparation, placement, heating, curing, etc., and the process flow is relatively cumbersome.
3. Since molds are required in the preparation process, if the mold surface is not smooth or suitable, it may affect the appearance quality of the final composite material component.
4. Since the autoclave and materials need to be heated, the energy consumption of the autoclave process is relatively high.