Composite molding ----- autoclave molding

The autoclave molding process is the main molding process of composite materials, and the equipment used in the autoclave is a large pressure vessel with an integral heating system, as shown in the figure below. An autoclave is a pressure vessel with a body that is a cylinder closed at one end and open at the other, providing the necessary heat and pressure for compaction and curing of advanced composite parts. Commonly used thermoset materials such as high-temperature epoxy resins have a curing temperature of 180±5°C and a curing pressure of 6 bar, so it is important to ensure that the autoclave is strong enough at high temperatures, which makes the manufacture of autoclaves quite expensive.

 

Temperature and pressure are usually determined by the material system employed. In the molding of thermoset composites, high temperatures are required to reduce the resin viscosity and initiate curing chemistry, while pressure is used to press the layers together and inhibit the formation of pores.

 

1、 Autoclave

 

1.1 The main system and composition of the autoclave

 

The autoclave is composed of a tank system, a vacuum system, a blast system, a heating system, a pressurization system, a cooling system, a control system and a rack car. Its main systems and components are shown in the table below.

 

Table Autoclave system composition

1.2 Autoclave molding process characteristics

1.2.1  Advantages

 

▷ Uniform pressure in the tank: Because compressed air or nitrogen is used to inflate and pressurize the autoclave, the pressure acting on the normal of each point on the surface of the vacuum bag is the same, so that the components in the vacuum bag are formed and solidified under uniform pressure (the air has the risk of spontaneous combustion, so it is only used for medium and low temperature curing systems).

▷ Stable and reliable molding process: the pressure and temperature are uniform, adjustable and controllable, so that the quality of 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. Based on the above points, it can more effectively ensure the mechanical properties and product quality of products than other curing molding methods.

▷Uniform temperature, adjustable: The tank is heated by circulating hot air flow to the workpiece, and the temperature difference is small. At the same time, the cooling system is configured, so that the temperature can be strictly controlled within the range of process settings.

▷Wide range of application: the mold is simple and efficient; 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 handover assembly; Small pieces can be cured at the same time in multiple pieces at a time.

 

1.2.2  Cons

 

▷Large investment and high cost: 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 strip, breathable felt, isolation cloth, etc., which greatly increases the production cost.

▷ The safety operation steps are strict and cumbersome: it is no exaggeration to say that the heated autoclave is equivalent to a bomb, so the industrial autoclave is generally equipped with several pressure safety valves, and the operators must be trained in special equipment pressure vessels and hold certificates.

▷Long curing cycle: the thermal inertia of the autoclave and the mold is generally much greater than the thermal inertia of the workpiece, so it limits the heating and cooling rate of the processing process, so the autoclave is not suitable for rapid processing with a short potential cycle, and the mass production of thermoplastic composite parts is not suitable for autoclave molding.

 

2、Autoclave quality control points 

2.1 Temperature control

 

▷ The autoclave should be equipped with three sets of instruments for temperature control, temperature recording and alarm, and a stabilized power supply device.

▷The temperature uniformity in the autoclave should be verified once a year, and the temperature control system should be calibrated once a year.

▷ Thermoclaves of φ1.2m×4m and above specifications should have no less than 8 thermocouples for temperature control, detection and recording of the temperature of the workpiece.

▷ Within the operating temperature range, the measurement accuracy of the thermocouple should reach ±1°C.

▷The automatic temperature recorder should be able to automatically plot the temperature-time curve of the entire process.

▷ Under the condition of full load, the autoclave should meet the following requirements: the temperature uniformity should be good, and the temperature difference between any two points in the tank should not be greater than 10 °C; The air heating and cooling rate should be adjustable in the range of 0.3~5°C/min; The autoclave temperature should be controlled to an accuracy of ±3°C.

 

2.2 Pressure control

 

▷ The accuracy of the pressure instrument should be ±1.5% of the full scale reading at 0-2.0Mpa.

▷ Pressure instruments and automatic pressure recording instruments should be verified once every six months.

▷ The accuracy of the vacuum pressure instrument should be ±1.5% of the full scale reading at -0.1Mpa.

▷ The pressure system of the autoclave should be equipped with a gas storage tank and devices such as automatic pressure adjustment, automatic recording and insurance.

▷The pressure fluctuation value in the autoclave should be ±0. 03Mpa.

▷ The compressed air entering the autoclave should be supplied by an air compressor (or oxygen generator) without oil lubrication, otherwise there should be degreasing and water removal measures. If possible, inert gas (C02 or N2) should be used as the pressurized medium.

▷The vacuum pressure system in the autoclave should be airtight, leak-free, and have a separate pipeline connected to the atmosphere.

 

2.3 Security controls

 

▷ The autoclave should be equipped with a light and a red emergency door opening handle to ensure the safety of the people who are mistakenly locked in the tank.

▷ Safety valves and pressure alarm devices should be calibrated once a year.

▷ The tank door and the pressurization mechanism should have an interlocking device, so that when the tank door is closed but not locked, the pressure in the tank cannot be added. When the pressure in the tank does not drop to zero, the door cannot be opened.

 

3、Molding materials

3.1 Prepreg

 

The raw materials of thermosetting composites are fibers and resins, while prepregs are impregnated with resin matrix under strictly controlled conditions to make a composition of resin matrix and reinforcement, which is an intermediate in the manufacture of composite materials.

 

3.2 Sandwich material

 

At present, the sandwich materials used are mainly aluminum honeycomb, NOMEX honeycomb, glass cloth honeycomb, and foam material.

 

3.3  Adhesive

 

The structural adhesives used in autoclave molding are divided into various forms such as adhesive film, coating glue, filling glue, styrofoam, etc., and high-temperature, medium-temperature and low-temperature adhesives can be selected according to the structural requirements. Major domestic manufacturers: Heilongjiang Petrochemical Research Institute, Beijing Institute of Aeronautical Materials, Chengdu Organic Silicon Research Center.

 

3.4  Ancillary Materials

 

Auxiliary materials are necessary materials to complete the autoclave molding process, and are the guarantee for the completion of autoclave molding, so auxiliary materials with stable performance, reliability and long storage period must be selected. The types of auxiliary materials and their use in the autoclave molding process are shown in the figure below and the table below.

 

 Table Auxiliary Materials, Uses and Materials

 

4、Mold

 

The first requirement of autoclave forming molds is that the mold material maintains proper performance at molding temperature and pressure. At the same time, other factors should be considered in the design and manufacture of molds, such as cost, life, accuracy, strength, weight, machinability, coefficient of thermal expansion, dimensional stability, surface treatment, and thermal conductivity. In the main process of design and manufacturing, the following aspects should be required:

 

▷Strength and stiffness: The strength and stiffness of the mold should be designed according to the requirements of the process, and in addition to vacuum and positive pressure, warping thermal deformation should also be considered. 

▷Maintenance: In addition to functional design, mold maintenance operability should be considered.

▷ Forming surface: surface finish and hardness meet the requirements of molding and demoulding, marking and positioning, and assembly and engraving.

▷ Fit and positioning: The assembly and positioning mechanism should be designed for the combination mold, and the assembly accuracy should be guaranteed to meet the accuracy requirements of the product.

▷Temperature field: The mold design should meet the requirements of the autoclave temperature field, so that the hot air flow is smooth, the temperature resistance is sufficient, and the heat capacity is small.

▷ Demoulding: When designing the mold, it should be considered that the product can be smoothly demoulded, and an auxiliary demoulding mechanism should be designed if necessary.

▷ Arrangement with the furnace: When necessary, the molding of the furnace specimen should be considered at the same time when the mold is designed.

▷Tolerance allocation: In the mold design, the tolerance allocation should be carried out according to the tolerance requirements of the product, the molding shrinkage and the thermal expansion coefficient of the mold material.

 

According to the material system of advanced composites and their molding process requirements, many materials are suitable as mold materials. In general, mold materials can be basically divided into the following categories: (1) resin matrix composites used in the low to medium temperature range; (2) Metal materials suitable for low temperature to high temperature range; (3) Ceramic and graphite materials suitable for ultra-high temperature. In addition, gypsum and other inexpensive materials that are easy to process and form can also be used as mold materials for the production of small batches or typical parts for experimental validation. Autoclave molds mainly include: aluminum, steel, invar, electroformed nickel, composite materials, rubber molds, integral graphite and ceramic molds.

 

5、Autoclave forming process

 

The whole production process of thermosetting composite parts for aerospace generally includes the following processes. The molding process for thermoset composites is often quite complex. Due to the precise forming process and the part, each of the above steps has detailed sub-steps. The complexity of the molding process depends on the complexity of the part and the precision of the molding process.

 

6、Defect detection technology

 

At present, the detection methods of composite defects can be divided into two categories: destructive testing methods and nondestructive testing methods. Destructive detection methods mainly include density method, water absorption method, microphotography method and pickling absorption method. Non-destructive testing methods mainly include ultrasonic testing, X-ray testing, infrared heat wave method, etc.

 

6.1  Ultrasonic testing technology

 

For fiber-reinforced composites, ultrasonic testing technology is currently the most widely used non-destructive testing technology. Due to the reflection and refraction of sound waves at the interface of different media, the energy loss and propagation speed of sound waves in the propagation direction are changed, and the energy loss and propagation speed change at the interface between solid and gas are particularly severe. Therefore, when the composite contains pores, the ultrasonic waves penetrating the composite will occur the above phenomenon, so that the porosity in the composite sample can be determined according to the energy attenuation of the ultrasonic wave and the change of propagation velocity. Ultrasonic has strong penetration, good directionality, high sensitivity, and is harmless to the human body, which is more suitable for the detection of internal defects of composite materials. At the same time, the porosity ultrasonic testing method can be used for on-site real-time detection, and the detection after calibration is fast and convenient, which is a particularly important and effective porosity detection method for composite materials. The advantage of this measurement method is that it can measure the entire area of the composite specimen, rather than the local area. However, it must be corrected by other methods, so the overall accuracy is not greater than ±0.5%. Ultrasonic method is a widely used method.

 

6.2 Radiographic detection

 

The principle of radiographic inspection is to use X-rays, γ-rays to take pictures of defects, or use radioactive detectors such as scintillation counting tubes to measure penetrating rays. Radiographic inspection can be used to detect inclusions, cracks, and holes in composite materials. For porosity defects, defects greater than 0.lmm can be detected, because the pore size in the carbon fiber composite material changes greatly in different porosity, when the porosity is less than 4%, there is a considerable part of the pore size is less than 0.lmm, so it is not very sensitive in detecting porosity, and the requirements for personal safety measures are higher. X-ray non-destructive testing is a common method for detecting damage in composite materials. This method is difficult to detect delamination defects, and cracks can only be detected when their planes are roughly parallel to the beam, so this method can usually only detect cracks perpendicular to the surface of the specimen, which can be complementary to the ultrasonic reflectometry method.

 

6.3 Infrared heat wave method

 

The working principle of infrared heat wave nondestructive testing is to obtain the uniformity information of the inspected object material and the characteristic information of its subsurface structure and the characteristic information of the structure and thermal properties under the surface of the inspected object through special algorithms and image processing, according to the interaction between the variable heat source and the medium material and its geometric structure, geometry and geometric structure, by controlling the thermal excitation and timely monitoring and recording the temperature field change on the surface surface, so as to achieve the purpose of detection and flaw detection. This detection method has the characteristics of non-contact, real-time, efficient and intuitive, and is divided into two types: active (active infrared) detection method and passive (passive infrared) detection method.