An in-situ heating, curing and cooling process for composite materials

Abstract

The present application belongs to the technical field of composite material curing process, and relates to a composite material in-situ heating curing and cooling process. After the in-situ heating method is used to heat and cure the composite material to form a product, when there is no bag outer pressure curing, the natural cooling rate of the composite material is lower than the required cooling rate of the material, the in-situ heat source is directly separated from the energy supply device, and the natural cooling is performed under the vacuum in the bag. When there is bag outer pressure curing, after the natural cooling rate of the composite material is lower than the required cooling rate of the material, the high-pressure gas in the curing device is directly released, the in-situ heat source is separated from the energy supply device, the composite material is moved to outside of the curing device, and the natural cooling is performed under the vacuum in the bag. Until the temperature of the composite material is reduced to the required temperature of the material. The present application significantly reduces the occupancy of the curing device under the premise of ensuring the curing quality of the composite material, and provides important technical support for the industrial application of the in-situ heating curing method of the composite material.

Description

Technical Field

[0001] This invention relates to the field of composite material curing technology, and more particularly to an in-situ heating curing process for composite materials, specifically an in-situ heating curing and cooling process for composite materials. Background Technology

[0002] Advanced composite materials possess numerous advantages, including high specific strength, high specific modulus, corrosion resistance, good fatigue resistance, and excellent designability. They have become the preferred material for weight reduction and efficiency improvement in high-end equipment and are widely used in aerospace, automotive, and shipbuilding industries. The composite material manufacturing process generally consists of four steps: shaping, curing, processing, and assembly. Among these, heat curing is a crucial step in the manufacturing of composite components. Currently, the curing and molding of composite components primarily employs autoclave curing and oven curing methods. The heating principle involves first heating a gas, then transferring heat to the composite material through heat conduction and convection. However, this gas-medium heat transfer method inevitably suffers from long curing cycles, high energy consumption, and high costs, failing to meet the demands of high-performance composite material equipment development and energy conservation and environmental protection. With the development of materials technology, in-situ heating-based curing methods for composite materials have attracted widespread attention from researchers both domestically and internationally in recent years. These methods mainly include curing the composite material itself as a heat source, such as microwave curing, self-resistance electrothermal curing, electromagnetic induction curing, and radio frequency curing; and curing where a heat source is attached to the composite material to directly heat it, such as mold curing, electrothermal film curing, and microwave indirect curing. In in-situ heating-based curing methods, the heat generated by the heat source directly heats and cures the composite material, eliminating the need for long-distance heat transfer through a gaseous medium, which is generally not heated. Therefore, these methods offer significant advantages such as high heating efficiency and low curing energy consumption, making them a potential method for achieving efficient and low-cost manufacturing of composite material components.

[0003] However, during in-situ heating and curing, the composite material is primarily heated while the surrounding gas remains near room temperature. This significant temperature difference causes a large amount of heat from the composite material to dissipate into the surrounding gas, easily leading to uneven temperature distribution within the composite material. To prevent heat dissipation, an insulation layer is typically placed around the composite material and the mold to insulate the internal heat. However, this method of covering with an insulation layer results in an extremely slow cooling rate during the curing process, far less than the cooling rate limit imposed by the curing process (typically no higher than 1.5℃ / min). For traditional autoclave curing processes of composite materials with external pressure, on the one hand, heat is transferred to the composite material through high-pressure gas. If the pressure inside the autoclave is released from 0.6 MPa to 0, according to the ideal gas law PV = nRT (where P is pressure, V is gas volume, T is temperature, n is the amount of substance of the gas, and R is the molar gas constant) and the compressed air parameters, the temperature inside the autoclave will drop sharply from 200℃ to 90℃. This rapid cooling will cause severe deformation of the composite material parts. On the other hand, the piping hardware and accessories of the curing equipment cannot withstand high-temperature, high-pressure gas. This means that the high-temperature, high-pressure gas inside the autoclave cannot be directly released; a pressure-holding and cooling design must be adopted. The pressure inside the autoclave can only be released when the temperature of the composite material drops below a certain value. Existing pressure curing processes of composite materials based on in-situ heating directly follow the traditional pressure-holding and cooling approach of autoclave curing, which results in a very slow cooling rate, long cooling time, and extremely high equipment occupancy.

[0004] For traditional composite material oven curing processes without external pressure, heat is transferred to the composite material via gas. If the composite material is directly removed from the oven during cooling, its temperature will drop sharply, resulting in severe deformation. Furthermore, the high-temperature gas inside the oven makes it difficult to remove the composite material. These factors necessitate that the composite material be cooled within the oven until its temperature drops below a certain threshold before removal. Existing composite material curing processes based on in-situ heating directly follow the inherent principle of oven curing requiring furnace cooling, which also results in excessively slow cooling rates, long cooling times, and extremely high equipment occupancy rates.

[0005] To address the aforementioned issues, this invention considers the boundary condition and premise that the gaseous medium in in-situ heating and curing methods for composite materials is generally not heated. It breaks away from the traditional understanding that high-temperature, high-pressure gases in autoclave curing processes cannot be directly released and must be cooled under pressure, and the traditional thinking that oven curing processes require in-furnace cooling. This invention proposes an in-situ heating and curing cooling process for composite materials. After the composite material heating and curing is completed, when the natural cooling rate of the composite material is lower than the required cooling rate, the in-situ heat source is directly disconnected from the power supply equipment or the composite material is removed from the curing equipment, allowing for natural cooling under vacuum inside the bag. At this point, the curing equipment and power supply equipment can be directly shut off or used to cure other composite material parts. The cycle time of the composite material in the equipment will be significantly shortened compared to existing in-situ heating and curing processes based on pressure-holding cooling and in-furnace cooling approaches, and the equipment occupancy rate will be significantly reduced. This provides important technical support for the industrial application of in-situ heating and curing methods for composite materials. Summary of the Invention

[0006] The purpose of this invention is to address the problem of high equipment occupancy in existing in-situ heating and curing processes, which directly adopt the traditional process design of autoclave curing with pressure holding and cooling and oven curing with furnace cooling. This invention provides an in-situ heating and curing cooling process for composite materials, which can significantly reduce the equipment occupancy of in-situ heating and curing processes.

[0007] To achieve the above objectives, the present invention adopts the following technical solution:

[0008] A composite material in-situ heating, curing, and cooling process is described, in which the composite material is heated and cured in situ, followed by a cooling process during the cooling stage:

[0009] When the composite material is cured without external pressure, after the natural cooling rate of the composite material is lower than the required cooling rate, the in-situ heat source is directly disconnected from the power supply equipment, so that the composite material can be naturally cooled under vacuum inside the bag.

[0010] When there is external pressure for curing, after the natural cooling rate of the composite material is lower than the required cooling rate, the high-pressure gas in the curing equipment is directly released to separate the in-situ heat source from the power supply equipment, and the composite material is moved outside the curing equipment and self-heated for cooling under vacuum inside the bag.

[0011] Until the temperature of the composite material drops to the temperature required for the material curing process.

[0012] The in-situ heating method for the composite material includes the composite material itself acting as a heat source to generate heat to heat the composite material itself, or a heat source being attached to the composite material to generate heat to directly heat the composite material.

[0013] The in-situ heating method of the composite material itself as a heat source includes one of self-resistance electric heating, microwave heating, electromagnetic induction heating and radio frequency heating. The in-situ heating method of the heat source being attached to the composite material includes at least one of mold heating, flexible electric heating film heating and flexible microwave absorbing material / structure microwave heating.

[0014] An insulation layer is applied around the composite material and the mold. The method of application is to tightly wrap the insulation layer around the outside of the composite material and the mold on the outer surface of the vacuum bag or inside the vacuum bag.

[0015] The insulation layer applied around the composite material and the mold is made of a flexible material with an extremely low heat transfer coefficient.

[0016] The flexible material with extremely low heat transfer coefficient includes one or any combination of aerogel felt, glass fiber felt, glass wool felt, and composite silicate felt. When in-situ heating methods such as microwave heating, electromagnetic induction heating, or radio frequency heating are used, the insulation layer applied to the periphery of the composite material and the mold is made of a microwave-transparent material.

[0017] The thickness of the insulation layer applied around the composite material and the mold should ensure that the natural cooling rate of the composite material after heating and curing is lower than the required cooling rate of the material. At this time, the in-situ heat source is directly disconnected from the power supply equipment during the cooling stage, so that the composite material can be cooled naturally.

[0018] When the thickness of the insulation layer is limited and it cannot be guaranteed that the natural cooling rate of the composite material after heating and curing is lower than the required cooling rate of the material, the power supply equipment should first apply power during the cooling stage so that the cooling rate of the composite material is lower than the required cooling rate of the material. After the natural cooling rate of the composite material is lower than the required cooling rate of the material, the in-situ heat source should be disconnected from the power supply equipment so that the composite material can be naturally cooled.

[0019] The external pressure refers to the pressure applied outside the vacuum bag, which is applied by high-pressure gas inside the pressure vessel.

[0020] The vacuum inside the bag refers to the vacuum applied inside the vacuum bag. During the curing process of the composite material, the vacuum level inside the bag must be maintained at no more than -0.06 MPa.

[0021] Composite materials prepared using the above-mentioned in-situ heating, curing, and cooling process and their applications in aerospace, automotive, and shipbuilding industries.

[0022] Compared with the prior art, the present invention has the following beneficial effects:

[0023] The in-situ heating, curing, and cooling process for composite materials provided by this invention combines the principle that gas is not heated in in-situ heating and curing methods. It breaks the original assumptions of traditional thermosetting processes and the inherent understanding that pressure holding and cooling with the furnace must be used. After the composite material is heated and cured, when the natural cooling rate of the composite material is lower than the required cooling rate, the in-situ heat source is directly removed from the power supply equipment or the composite material is removed from the curing equipment. Natural cooling is then carried out under vacuum inside the bag. This can significantly reduce the equipment occupancy rate of the in-situ heating and curing process, providing important technical support for the industrial application of in-situ heating and curing technology for composite materials. Attached Figure Description

[0024] Figure 1 These are actual temperature and pressure data curves of carbon fiber reinforced epoxy resin matrix composite flat plate components cured by microwave pressure curing process of composite materials; wherein, (a) is the actual temperature and pressure data curve of microwave pressure curing process of composite materials based on the cooling process of the present invention, and (b) is the actual temperature and pressure data curve of microwave pressure curing process of composite materials based on the traditional pressure holding and cooling approach.

[0025] Figure 2 The diagram shows the mold dimensions and the placement of the L-shaped component; (a) shows the dimensions of the L-shaped mold; and (b) shows the placement of the L-shaped component made of carbon fiber reinforced epoxy resin.

[0026] Figure 3 The results are statistical measurements of the deformation and springback angle of L-shaped carbon fiber reinforced epoxy resin composite material components cured by the cooling process of this invention and the microwave pressure curing process of composite materials based on the traditional pressure holding and cooling approach.

[0027] Figure 4 The actual temperature curves of carbon fiber reinforced bismaleimide resin matrix composite laminates cured using the microwave pressure curing process based on the cooling process of this invention are shown.

[0028] Figure 5 The results are the degree of curing test results of carbon fiber reinforced bismaleimide resin matrix composite laminates cured by microwave pressure curing process and autoclave curing process based on the cooling process of this invention.

[0029] Figure 6 These are pore distribution diagrams of carbon fiber reinforced bismaleimide resin-based composite laminates; wherein, (a) is a pore distribution diagram of the composite laminate cured by microwave pressure curing process based on the cooling process of the present invention; and (b) is a pore distribution diagram of the composite laminate cured by autoclave curing process.

[0030] Figure 7The results show the carbon fiber content of carbon fiber reinforced bismaleimide resin-based composite laminates cured by microwave pressure curing and autoclave curing processes based on the cooling process of this invention.

[0031] Figure 8 The results are based on dynamic thermomechanical analysis of carbon fiber reinforced bismaleimide resin matrix composite laminates cured by microwave pressure curing and autoclave curing processes using the cooling process of this invention. Detailed Implementation

[0032] To address the high equipment occupancy rate of existing in-situ heating and curing processes, which directly adopt the traditional design approach of autoclave curing with pressure holding and cooling and oven curing with furnace cooling, this invention proposes an in-situ heating and curing cooling process for composite materials, thereby significantly reducing the equipment occupancy rate of in-situ heating and curing processes. The technical solution of this invention will be clearly and thoroughly described and explained below with reference to the accompanying drawings and specific embodiments.

[0033] A composite material in-situ heating, curing, and cooling process is characterized by the following cooling process after the composite material has been heated, cured, and molded in situ:

[0034] When the composite material is cured without external pressure, after the natural cooling rate of the composite material is lower than the required cooling rate, the in-situ heat source is directly disconnected from the power supply equipment, so that the composite material can be naturally cooled under vacuum inside the bag.

[0035] When there is external pressure for curing, after the natural cooling rate of the composite material is lower than the required cooling rate, the high-pressure gas in the curing equipment is directly released to separate the in-situ heat source from the power supply equipment, and the composite material is moved outside the curing equipment and self-heated for cooling under vacuum inside the bag.

[0036] Until the temperature of the composite material drops to the temperature required for the material curing process.

[0037] In this invention, the insulation layer is composed of a flexible material with an extremely low heat transfer coefficient, including one or any combination of aerogel felt, glass fiber felt, glass wool felt, and composite silicate felt. As a specific embodiment, the insulation layer is described using chopped glass fiber felt, but the insulation layer material is by no means limited to chopped glass fiber felt. The thickness of the insulation layer is determined using the finite element method, specifically calculated using heat transfer simulation software.

[0038] Example 1

[0039] This embodiment utilizes a microwave pressure curing process for composite materials based on the cooling process provided by this invention to cure carbon fiber reinforced epoxy resin matrix composite flat panel components. The thickness of the carbon fiber reinforced epoxy resin matrix composite prepreg used is 0.1 mm. To meet the requirements of subsequent mechanical property testing, two composite material flat panel components with dimensions of 300 mm × 300 mm (length × width) were cured and molded, with a layup sequence of [0° / 45° / 90° / -45°]. 3S [0° / 45° / 90° / -45°] 4S Two types.

[0040] Before curing begins, prepreg sheets of the appropriate size are cut and stacked according to the layup design of the flat panel component. Then, release cloth and the pre-laid composite flat panel component are placed on the upper surface of a steel mold (300mm×300mm×2mm). Simultaneously, sealing strips are placed along the edges of the flat panel component and aluminum foil is attached to prevent arcing. Subsequently, release cloth, an electromagnetic resonance structure, a porous isolation membrane, a non-porous isolation membrane, and chopped fiberglass mat with good wave transmission properties are sequentially placed on the upper surface of the composite flat panel component. Three fluorescent fiber optic temperature sensors are then used to monitor the actual curing temperature. The electromagnetic resonance structure consists of copper foil with a thickness of 18μm and a periodically perforated square unit shape, and a 0.25mm thick polyimide film. The chopped fiberglass mat serves both as a breathable layer and as an insulation layer. Based on the thermal conductivity and heat transfer simulation model of the chopped fiberglass mat, the number of layers is set to 7, ensuring that the natural cooling rate of the composite material is lower than the required cooling rate. In addition, to prevent heat dissipation, the same number of layers of chopped fiberglass mat are placed underneath the mold. Finally, the mold is sealed in a vacuum bag, evacuated, and placed in a microwave high-pressure curing equipment for microwave pressure curing.

[0041] During the curing process, the microwave pressure curing process for composite materials incorporates the cooling process of this invention into the recommended curing process for carbon fiber reinforced epoxy resin matrix composites. Specifically, before microwave heating, the pressure inside the microwave high-pressure curing equipment is increased to 0.6 MPa. After pressurization, the temperature is increased to 180°C at a rate of 1.5°C / min and held for 180 minutes. After holding, the pressure inside the microwave curing equipment is released to 0 and the door is opened. Then, the well-insulated composite material component is removed from the curing equipment and allowed to cool naturally to below 60°C under vacuum inside the bag at a rate lower than 1.5°C / min, thus completing the curing process.

[0042] The actual curing temperature and pressure data curves of the microwave pressure curing process for composite materials based on the cooling method of this invention are shown in the figure below. Figure 1As shown in (a), the actual temperature of the composite material plate component does not drop sharply when the gas pressure inside the microwave high-pressure curing equipment is directly depressurized from 0.6 MPa to 0 after the 180°C heat preservation period, meeting the requirement that the cooling rate does not exceed 1.5°C / min. This indicates that the process meets the requirements of practical industrial applications. Furthermore, due to the good heat preservation effect, even after all microwave sources are turned off after the heat preservation period, the cooling rate throughout the entire cooling process will never exceed 1.5°C / min, and the cooling rate will become even slower as the temperature gradually decreases. Since the composite material component can be moved outside the curing equipment after the depressurization is completed, and naturally cooled to below 60°C under vacuum inside the bag, the actual curing cycle inside the curing equipment for the microwave pressure curing process based on the cooling method of this invention is 295 minutes.

[0043] A universal testing machine was used to conduct seven mechanical property tests on carbon fiber reinforced epoxy resin composite plate components cured under the microwave pressure curing process based on the cooling process of this invention. These tests were conducted according to the mechanical property testing standards ASTM D3039, ASTM D6641, ASTM D7264, and ASTM D2344. The mechanical properties included tensile strength, tensile modulus, compressive strength, room temperature flexural strength, 150°C flexural strength, room temperature interlaminar shear strength, and 150°C interlaminar shear strength. For each mechanical property test, five test specimens were used, and the average value was calculated. The test results are shown in Table 1. The tensile strength of the composite plate component cured under the microwave pressure curing process based on the cooling process of this invention was 102.6 MPa, the tensile modulus was 8.6 GPa, the compressive strength was 1330.0 MPa, the room temperature flexural strength was 1637 MPa, the 150°C flexural strength was 1391.5 MPa, the room temperature interlaminar shear strength was 124.0 MPa, and the 150°C interlaminar shear strength was 86.8 MPa.

[0044] Comparative Example 1

[0045] This comparative example utilizes a microwave pressure curing process based on the traditional pressure-holding and cooling approach to cure carbon fiber reinforced epoxy resin composite flat panel components. In this comparative example, the experimental materials and setup, including the carbon fiber reinforced epoxy resin composite prepreg, auxiliary materials, insulation layer and its placement sequence, and temperature measurement methods, are consistent with those in Example 1. During the curing process, the microwave pressure curing process based on the traditional pressure-holding and cooling approach follows the recommended curing process for carbon fiber reinforced epoxy resin composites. This involves applying a pressure of 0.6 MPa, followed by heating after pressurization. Heating is performed at a rate of 1.5 °C / min to reach 180 °C and holding at that temperature for 180 min. After holding, the temperature is lowered to below 60 °C at a rate lower than 1.5 °C / min. The high-pressure gas inside the microwave high-pressure curing equipment is then released, and the composite component is removed from the curing equipment.

[0046] The actual curing temperature and pressure data curves of the microwave pressure curing process for composite materials based on the traditional pressure holding and cooling approach are shown below. Figure 1 As shown in (b), due to the good heat preservation effect, after the heat preservation is completed and all microwave sources are turned off, the cooling rate of the entire cooling process in this process will never exceed 1.5℃ / min, and the cooling rate will become slower as the temperature gradually decreases. Since the existing microwave pressure curing process based on the traditional pressure holding and cooling approach requires the high-pressure gas in the microwave high-pressure curing equipment to be released and the door opened only after the temperature drops below 60℃, the curing cycle of the composite material component in the equipment is 885min.

[0047] A universal testing machine was used to conduct seven mechanical property tests on carbon fiber reinforced epoxy resin composite plate components cured under two microwave pressure curing processes, according to the mechanical property testing standards ASTM D3039, ASTM D6641, ASTM D7264, and ASTM D2344. These tests included tensile strength, tensile modulus, compressive strength, room temperature flexural strength, 150℃ flexural strength, room temperature interlaminar shear strength, and 150℃ interlaminar shear strength. Five test specimens were used for each mechanical property test, and the average value was calculated. The test results are shown in Table 1. The composite plate component cured under the traditional pressure-holding and cooling method for composite materials exhibited a tensile strength of 106.0 MPa, a tensile modulus of 8.5 GPa, a compressive strength of 1304.0 MPa, a room temperature flexural strength of 1554 MPa, a 150℃ flexural strength of 1341.0 MPa, a room temperature interlaminar shear strength of 123.0 MPa, and a 150℃ interlaminar shear strength of 87.7 MPa.

[0048] Table 1. Mechanical property test results of carbon fiber reinforced epoxy resin composite flat plate components cured by microwave pressure curing process based on the cooling process of this invention and microwave pressure curing process based on traditional pressure holding and cooling approach.

[0049]

[0050] Comparing the results of Example 1 and Comparative Example 1, compared with the existing microwave pressure curing process for composite materials based on the traditional pressure holding and cooling approach, the microwave pressure curing process for composite materials based on the cooling process of this invention shortens the curing cycle in the microwave curing equipment by 66.7%, that is, the equipment occupancy rate is reduced by 66.7%. This shows that the present invention effectively reduces the equipment occupancy rate in the existing in-situ heating and curing process for composite materials.

[0051] In addition, by comparing the mechanical property test results of Example 1 and Comparative Example 1 in Table 1, the composite material plate component cured by the microwave pressure curing process of the present invention based on the cooling process of the present invention is comparable to the existing composite material microwave pressure curing process based on the traditional pressure holding and cooling approach in all seven mechanical properties mentioned above. The maximum mechanical property test deviation of the composite material plate component does not exceed ±5%.

[0052] Example 2

[0053] Example 2 utilizes a microwave pressure curing process for composite materials based on the cooling process of the present invention to cure a carbon fiber reinforced epoxy resin matrix composite female mold L-shaped component. The experimental setup and arrangement of the carbon fiber reinforced epoxy resin matrix composite prepreg, auxiliary materials, insulation layer and their placement order, temperature measurement method and corresponding curing process are the same as in Example 1.

[0054] In this embodiment, the mold used is an L-shaped steel mold with a thickness of 5mm, and the other dimensions are as follows. Figure 2 As shown in (a), the prepreg is cut to dimensions of 200mm × 30mm × 0.1mm, with a [0° / 90°] symmetrical layup and a layer thickness of 1mm. The placement of the carbon fiber reinforced epoxy resin composite female mold L-shaped component on the mold is as follows. Figure 2 As shown in (b). After three carbon fiber reinforced epoxy resin matrix composite female mold L-shaped components were cured and molded using the microwave curing process based on the cooling technology of this invention, the springback angle of the L-shaped components was measured by side profile scanning, and the average value was taken from multiple measurements. The statistical results of the deformation springback angle measurement of the carbon fiber reinforced epoxy resin matrix composite female mold L-shaped components are as follows. Figure 3 As shown, the average springback angle of the L-shaped component of the female mold cured by the microwave pressure curing process of the composite material based on the cooling process of the present invention is about 1.533°.

[0055] In addition, this embodiment uses the same microwave pressure curing process to cure and mold carbon fiber reinforced epoxy resin matrix composite male mold L-shaped component and flat plate component respectively. The L-shaped mold is the same as the L-shaped mold used above (…). Figure 2 (a) is consistent with the prepreg cut dimensions of 200mm×30mm×0.1mm and 160mm×30mm×0.1mm for the L-shaped male mold component and the flat component, respectively. Both are [0° / 90°] symmetrical layup with a layup thickness of 1mm. For the flat component, the flat area of ​​the L-shaped mold is used as the curing mold. The placement position of the L-shaped male mold component on the male side of the mold is consistent with... Figure 2 (b) The positions of the L-shaped components correspond. After curing three male L-shaped components and six flat components using the microwave pressure curing process based on the cooling method of this invention, the deformation of both types of components was measured. The deformation measurement results are shown in Table 2. As can be seen from the table, the average deformation of the flat components cured using the microwave pressure curing process based on the cooling method of this invention is approximately 0.137 mm. For the male L-shaped components, the average springback angle of the L-shaped components cured using the microwave pressure curing process based on the cooling method of this invention is approximately 4.058°.

[0056] Comparative Example 2

[0057] Comparative Example 2 utilizes a microwave pressure curing process for composite materials based on the traditional pressure-holding and cooling approach to cure L-shaped components of carbon fiber reinforced epoxy resin matrix composite female molds. The experimental setup and arrangement, including the mold, carbon fiber reinforced epoxy resin matrix composite prepreg, auxiliary materials, insulation layer and their placement order, and temperature measurement method, are consistent with Example 2. The curing process is the same as Comparative Example 1. The deformation and springback angle measurements of the three L-shaped components of carbon fiber reinforced epoxy resin matrix composite female molds cured using the traditional pressure-holding and cooling approach are as follows: Figure 3 As shown, the average springback angle of the L-shaped component cured by the microwave pressure curing process based on the traditional pressure holding and cooling approach is approximately 3.133°.

[0058] In addition, Comparative Example 2 used a microwave pressure curing process based on the traditional pressure-holding and cooling approach (pressure-holding and cooling microwave pressure curing process) to cure carbon fiber reinforced epoxy resin matrix composite male mold L-shaped components and flat plate components, respectively, and measured the deformation of the corresponding components. The experimental setup and arrangement were the same as in Example 2, and the curing process was the same as in Comparative Example 1. The deformation measurement results of the three male mold L-shaped components and six flat plate components cured using the traditional pressure-holding and cooling approach are shown in Table 2. The table shows that the average deformation of the flat plate components cured using the existing microwave pressure curing process based on the traditional pressure-holding and cooling approach is approximately 0.183 mm. For the male mold L-shaped components, the average springback angle of the male mold L-shaped components cured using the existing microwave pressure curing process based on the traditional pressure-holding and cooling approach is approximately 4.375°.

[0059] Table 2 shows the statistical results of deformation measurements of carbon fiber reinforced epoxy resin composite flat plate components and male mold L-shaped components cured using the cooling process of this invention (pressure relief and cooling microwave pressure curing process) and the composite material microwave pressure curing process based on the traditional pressure holding and cooling approach (pressure holding and cooling microwave pressure curing process).

[0060]

[0061] Comparing the results of Example 2 and Comparative Example 2, it can be seen that, compared with the existing microwave pressure curing process for composite materials based on the traditional pressure holding and cooling approach, the rebound angle of the carbon fiber reinforced epoxy resin matrix composite female mold L-shaped component cured by the microwave pressure curing process based on the cooling process of the present invention is reduced by about 51%, and for the male mold L-shaped component, the rebound angle is reduced by about 7%, and the deformation of the flat plate component is reduced by about 25%. These results indicate that the microwave pressure curing process for composite materials based on the cooling process of the present invention not only does not increase the curing deformation of the composite material component, but also alleviates the curing deformation of the component to a certain extent.

[0062] Example 3

[0063] This embodiment employs a microwave pressure curing process based on the cooling technology proposed in this invention and a conventionally recommended pressure-holding and cooling curing process (i.e., autoclave curing process) to cure and form carbon fiber reinforced bismaleimide resin-based composite laminates. The carbon fiber reinforced bismaleimide resin-based composite prepreg used is model ZT7H / QY9611, with a single-layer prepreg thickness of 0.125 mm. To meet the requirements of subsequent laminate performance testing, both curing processes are used to cure and form two composite laminates with dimensions of 300 mm × 300 mm × 2.5 mm (length × width × thickness) and 300 mm × 300 mm × 3.0 mm (length × width × thickness), respectively, with layup patterns of [0° / 45° / 90° / -45°]5 and [0° / 45° / 90° / -45°]6, using a symmetrical layup. In the microwave pressure curing process based on the cooling process of this invention, the arrangement of the mold, composite laminate, and auxiliary materials is the same as in Example 1. In the autoclave curing process, the order of arrangement of the laminate and auxiliary materials is as follows: release cloth, composite laminate, release cloth, porous release film, non-porous release film, and breathable felt are placed sequentially on the steel mold, and then sealed with a vacuum bag.

[0064] The manufacturer's recommended curing process for the ZT7H / QY9611 prepreg used is to apply a pressure of (0.6±0.2) MPa, and then begin heating after pressurization. During heating, the temperature is increased from room temperature to (125±5)℃ at a rate of (0.5~2)℃ / min and held for (60~70) min, then increased to (185±5)℃ at a rate of (0.5~2)℃ / min and held for (60~70) min, then increased to (200±5)℃ at a rate of (0.5~2)℃ / min and held for (300~305) min, and finally cooled to below 70℃ at a rate of less than 1.5℃ / min, then the pressure is released and the product is removed from the can. To eliminate the influence of temperature and process factors, during the curing process of the composite laminate, the curing heating rate for both the microwave pressure curing process and the autoclave curing process based on the cooling method of this invention is 1.5℃ / min. The process involves holding at 125℃ for 60 minutes, at 185℃ for 60 minutes, at 200℃ for 300 minutes, and then cooling down to 70℃ at a rate lower than 1.5℃ / min. For the microwave pressure curing process based on the cooling process of this invention, after holding at 200℃, the high-pressure gas in the microwave high-pressure curing equipment is released to 0 MPa. Then, the well-insulated composite laminate and mold are removed from the curing equipment, and the laminate is allowed to cool naturally under vacuum in a bag at a rate lower than 1.5℃ / min. For the autoclave curing process, it is strictly carried out according to the manufacturer's recommended process. After holding at 200℃ for 300 minutes, the temperature is reduced at a rate lower than 1.5℃ / min until the temperature inside the autoclave is below 70℃ before depressurization. Then, the composite laminate is opened and removed.

[0065] The temperature curve of the actual curing process of the microwave pressure curing process based on the cooling process proposed in this invention is as follows: Figure 4 As shown, during microwave pressure curing, after holding at 200℃ for 300 minutes, the high-pressure gas in the microwave high-pressure curing equipment is rapidly released. The actual temperature of the composite laminate remains almost unaffected, with no significant temperature fluctuations. At this point, all microwave sources are directly shut off, resulting in an actual cooling rate of approximately 0.7℃ / min, far lower than the manufacturer's recommended cooling rate of 1.5℃ / min. Furthermore, the cooling rate decreases further as the temperature drops. The cooling process does not require microwave energy to meet the cooling rate requirement of less than 1.5℃ / min, allowing the microwave curing equipment to be shut down directly, further saving energy. Compared to the manufacturer's recommended pressure-holding and cooling curing process, equipment occupancy is reduced by nearly 10 hours.

[0066] The thickness of the laminates cured by the two methods was measured using vernier calipers. Five measurement points were taken on each side of the laminates, and the average value was calculated. The test results showed that the thicknesses of the two laminates cured by the microwave pressure curing process based on the cooling process of this invention were 2.42±0.06mm and 2.90±0.08mm, respectively, while the thicknesses of the laminates cured by the autoclave curing process were 2.40±0.06mm and 2.90±0.10mm, respectively. The thicknesses of the composite laminates cured by the two processes were almost identical. Based on this, the degree of cure of the composite laminates cured by the two methods was measured using differential scanning calorimetry. The measurement results are as follows: Figure 5 As shown, the heat released per unit mass of ZT7H / QY9611 prepreg after complete curing is 79.74 J / g. However, no obvious exothermic peak was observed in the composite material samples cured by the microwave pressure curing process based on the cooling process of this invention and the autoclave curing process. This indicates that the laminates cured by the two curing processes have been completely cured. Furthermore, the degree of curing by the microwave pressure curing process based on the cooling process of this invention is comparable to that of the autoclave curing process. Therefore, the microwave pressure curing process based on the cooling process of this invention will not affect the degree of curing of CFRP.

[0067] The porosity of composite laminates cured by microwave pressure curing and autoclave curing processes based on the cooling process of this invention was measured using water immersion ultrasonic C-scan method. The ultrasonic C-scan results are as follows: Figure 6 As shown, Figure 6 (a) and Figure 6(b) Scanning results of laminates prepared by microwave pressure curing and autoclave curing processes based on the cooling technology of this invention are shown respectively. No obvious pore defects were found in any of the four laminates, and the porosity of the composite laminates cured by both processes was less than 1.0%, meeting the porosity requirements (porosity not exceeding 1.0%) for composite component curing in the aerospace field. Simultaneously, the carbon fiber volume fraction of the composite laminates formed by the two curing processes was measured using a thermogravimetric analyzer. This method utilizes the decomposition of resin under high temperature conditions; the carbon fiber content can be determined by the weight loss of the sample before and after the experiment. The test results of the fiber volume fraction are as follows: Figure 7 As shown, the fiber mass fraction of the laminate cured by the microwave pressure curing process based on the cooling method of this invention is 79.9521%, while the fiber mass fraction of the laminate cured by the autoclave curing process is 80.1376%. The fiber content in the composite laminate cured by the two processes differs by only about 0.2%. Furthermore, the inflection point and midpoint of mass loss during high-temperature decomposition of the samples cured by both processes are the same, both around 445℃ and 454℃. Therefore, compared to the existing autoclave curing process, the microwave pressure curing process based on the cooling process proposed in this invention does not affect the fiber volume fraction of the cured composite component. In addition, dynamic thermomechanical analysis was used to quantitatively characterize the dynamic modulus (storage modulus and loss modulus), mechanical damping loss factor (tanδ), and glass transition temperature (Tg) of the composite laminates cured by the microwave pressure curing process based on the cooling process of this invention and the existing autoclave curing process. The test results are as follows: Figure 8 As shown in the figure, it can be clearly seen that the energy storage modulus E' curve of the laminate cured by the microwave pressure curing process based on the cooling process of the present invention is slightly higher than that of the cured sample by the autoclave curing process. This may be because the microwave cured sample has a stronger interface transfer effective load from the matrix to the fiber.

[0068] Furthermore, the loss modulus value E” of the sample cured by the microwave pressure curing process based on the cooling process of this invention is also slightly higher than that of the autoclave curing process, possibly due to the increased internal friction, which enhances energy dissipation. The ratio of loss modulus to storage modulus is defined as the mechanical loss factor tanδ (or loss tangent), which represents the decay rate of structural vibration energy. It can be noted from the test results that, compared with the composite laminate sample cured by the autoclave curing process (0.16), the microwave pressure curing process based on the cooling process of this invention exhibits smaller damping in the glass transition transition region (…). The temperature difference was reduced by approximately 12.5% ​​(0.14). Meanwhile, the glass transition temperature (Tg) of the composite material cured by the microwave pressure curing process based on the cooling process of this invention (defined as the starting temperature at which the storage modulus drops sharply) was 219°C, comparable to the Tg of the autoclave curing process (221°C), differing by only 0.9%. Therefore, compared to the autoclave curing process, the composite laminate samples cured by the microwave pressure curing process based on the cooling process of this invention showed comparable storage modulus, loss modulus, mechanical damping loss factor, and glass transition temperature (Tg), with no significant differences.

[0069] Based on the aforementioned physicochemical property tests, a universal testing machine was used to conduct tests on the carbon fiber reinforced bismaleimide resin-based composite laminates cured under two curing processes, according to the mechanical property testing standards ASTM D3039, ASTM D6641, ASTM D7264, and ASTM D2344. These tests covered seven mechanical properties specified by aerospace enterprises, including tensile strength, tensile modulus, compressive strength, room temperature flexural strength, 150°C flexural strength, room temperature interlaminar shear strength, and 150°C interlaminar shear strength. For each mechanical property test, five test specimens were used, and the average value was calculated. The test results are shown in Table 3. It can be seen that the seven mechanical properties of the composite laminate cured by the microwave pressure curing process based on the cooling process of this invention are almost identical to those of the currently mainstream autoclave curing process, with a deviation of ±2.0%. This indicates that the microwave pressure curing process based on the cooling process proposed in this invention does not reduce the mechanical properties of the composite component.

[0070] Table 3. Mechanical property test results of carbon fiber reinforced bismaleimide resin matrix composite laminates cured by microwave pressure curing and autoclave curing processes based on the cooling process of this invention.

[0071]

[0072] Example 4

[0073] Example 4 follows the same experimental setup and arrangement as Example 3. A carbon fiber reinforced epoxy resin composite unidirectional laminate (curing temperature 180℃) was cured using a self-resistance electrothermal pressure curing process based on the cooling process proposed in this invention (pressure relief and cooling self-resistance electrothermal curing process) and a conventionally recommended pressure-holding and cooling curing process (i.e., autoclave curing process). In the self-resistance electrothermal pressure curing process based on the cooling process proposed in this invention, the number of chopped glass fiber mat layers was reduced from 7 to 3, meaning the insulation layer thickness was reduced. When the carbon fiber reinforced epoxy resin composite unidirectional laminate finished its 180℃ insulation period and began to cool, its natural cooling rate was 1.8℃ / min, higher than the required cooling rate (1.5℃ / min). At this point, a certain amount of power was continued to be applied to the carbon fiber reinforced epoxy resin composite unidirectional laminate to maintain its cooling rate below 1.5℃ / min. When the temperature of the one-way laminate drops to 150℃, its natural cooling rate decreases to 1.4℃ / min. At this point, power is stopped from being applied to the one-way laminate, the high-pressure gas in the curing equipment is released directly, the one-way laminate is then disconnected from the power supply equipment, and the well-insulated one-way laminate is removed from the curing equipment. It is then allowed to cool naturally to below 60℃ under vacuum inside the bag, at which point curing is complete.

[0074] In addition, a carbon fiber reinforced epoxy resin matrix composite unidirectional laminate was cured using an autoclave curing process as Comparative Example 4, with the experimental setup and arrangement consistent with Example 3. After curing, nine mechanical properties of the carbon fiber reinforced epoxy resin matrix composite unidirectional laminate cured under both curing processes were tested according to the mechanical property testing standards ASTM D3039, ASTM D6641, ASTM D7264, and ASTM D2344. These properties included 90° tensile strength, 90° tensile modulus, 0° compressive strength, 0° compressive modulus, 90° compressive strength, 90° compressive modulus, flexural strength, flexural modulus, and interlaminar shear strength. For each mechanical property test, five test specimens were used to calculate the average value. The test results are shown in Table 4. It can be seen that the nine mechanical properties of the composite laminate cured by the self-resistance electrothermal curing process based on the cooling process of this invention are comparable to those of the current mainstream autoclave curing process, indicating that the composite pressure curing process based on the cooling process proposed in this invention does not reduce the mechanical properties of the composite component.

[0075] Table 4. Mechanical property test results of carbon fiber reinforced epoxy resin composite unidirectional laminates cured by the self-resistance electrothermal pressure curing process (pressure relief cooling self-resistance electrothermal curing process) and autoclave curing process based on the cooling process of this invention.

[0076]

[0077] Example 5

[0078] Example 5 follows the same experimental setup and arrangement as Example 3. Carbon fiber reinforced epoxy resin composite unidirectional laminates were cured using a mold heating and pressure curing process based on the cooling process proposed in this invention (pressure relief and cooling mold heating and curing process) and a conventionally recommended pressure holding and cooling curing process (i.e., autoclave curing process, as Comparative Example 5). After curing, nine mechanical properties of the carbon fiber reinforced epoxy resin composite unidirectional laminates cured under the two curing processes were tested according to the mechanical property testing standards ASTM D3039, ASTM D6641, ASTM D7264, and ASTM D2344. These properties included 90° tensile strength, 90° tensile modulus, 0° compressive strength, 0° compressive modulus, 90° compressive strength, 90° compressive modulus, flexural strength, flexural modulus, and interlaminar shear strength. For each mechanical property test, five test specimens were used to calculate the average value. The test results are shown in Table 5. It can be seen that the nine mechanical properties of the composite laminate cured by the mold heating curing process based on the cooling process of the present invention are comparable to those of the current mainstream autoclave curing process. This indicates that the pressure curing process of composite materials based on the cooling process proposed in this invention will not reduce the mechanical properties of composite material components.

[0079] Table 5. Mechanical property test results of carbon fiber reinforced epoxy resin matrix composite unidirectional laminates cured by the mold heating and pressure curing process (pressure relief and cooling mold heating and curing process) and the autoclave curing process based on the cooling process of this invention.

[0080]

[0081]

[0082] Example 6

[0083] Example 6 followed the same experimental setup and arrangement as Example 3. Carbon fiber reinforced epoxy resin composite unidirectional laminates were cured using both the electrothermal film heating pressure curing process (pressure relief and cooling electrothermal film heating curing process) based on the cooling process proposed in this invention and the existing recommended pressure holding and cooling curing process (i.e., autoclave curing process, as Comparative Example 6). After curing, nine mechanical properties of the carbon fiber reinforced epoxy resin composite unidirectional laminates cured under the two curing processes were tested according to the mechanical property testing standards ASTM D3039, ASTM D6641, ASTM D7264, and ASTM D2344. These properties included 90° tensile strength, 90° tensile modulus, 0° compressive strength, 0° compressive modulus, 90° compressive strength, 90° compressive modulus, flexural strength, flexural modulus, and interlaminar shear strength. For each mechanical property test, five test specimens were used to calculate the average value. The test results are shown in Table 6. It can be seen that the nine mechanical properties of the composite laminate cured by the electrothermal film heating curing process based on the cooling process of the present invention are comparable to those of the current mainstream autoclave curing process. This indicates that the composite pressure curing process based on the cooling process proposed in this invention will not reduce the mechanical properties of the composite component.

[0084] Table 6. Mechanical property test results of carbon fiber reinforced epoxy resin matrix composite unidirectional laminates cured by the electrothermal film heating pressure curing process (pressure relief and cooling electrothermal film heating curing process) and autoclave curing process based on the cooling method of the present invention.

[0085]

[0086]

[0087] Example 7

[0088] This embodiment uses the microwave indirect heating curing process based on the cooling process proposed in this invention and the existing recommended oven-cooled curing process (i.e., oven curing process, as Comparative Example 7) to cure carbon fiber reinforced epoxy resin matrix composite laminates. The experimental setup and arrangement in this embodiment are basically the same as in Example 3. In the microwave indirect heating curing process based on the cooling process of this invention, the microwave absorbing material is placed between the porous and non-porous isolation membranes, and the heat generated by its absorption of microwaves is directly used to heat and cure the composite material. In this process, no external pressure is applied. After the carbon fiber reinforced epoxy resin matrix composite laminate is heated and cured, it is directly removed from the microwave curing equipment (i.e., the composite material is separated from the power supply equipment as a heat source) and allowed to cool naturally outside the equipment. In the oven curing process, the arrangement of the mold, composite laminate, and auxiliary materials is the same as in the autoclave curing process, but no external high pressure is applied. After the carbon fiber reinforced epoxy resin matrix composite laminate is heated and cured, it is cooled by oven-cooling. After the temperature of the laminate drops to 70°C, it is removed from the oven, and the curing is complete. Subsequently, according to the mechanical property testing standards ASTM D3039, ASTM D6641, ASTM D7264, and ASTM D2344, four mechanical properties of the carbon fiber reinforced epoxy resin composite laminate cured under the two curing processes were tested, including tensile strength, compressive strength, flexural strength, and interlaminar shear strength. For each mechanical property test, five test specimens were used to calculate the average value. The test results are shown in Table 7. It can be seen that the four mechanical properties of the composite laminate cured by the microwave indirect heating curing process based on the cooling process of this invention are comparable to those of the traditional oven curing process (Comparative Example 7), indicating that the composite curing process based on the cooling process proposed in this invention does not reduce the mechanical properties of the composite component.

[0089] Table 7. Mechanical property test results of carbon fiber reinforced epoxy resin matrix composite laminates cured by microwave indirect heating curing process and oven curing process based on the cooling process of this invention.

[0090]

[0091] It should be noted that the in-situ heating and curing process of composite materials can be achieved using existing in-situ heating and curing processes. This invention is an improvement based on existing in-situ heating and curing processes. All parts not covered in this invention are existing technologies or implemented using other existing technologies.

[0092] The above are merely preferred embodiments of the present invention; however, the scope of protection of the present invention is not limited thereto. Any equivalent substitutions or modifications made by those skilled in the art within the scope of the technology disclosed in the present invention, based on the technical solution and its improved concept, should be covered within the scope of protection of the present invention.

Claims

1. A composite material in-situ heating, curing, and cooling process, characterized in that, After the composite material is heated and cured in situ, the following cooling process is used during the cooling stage: Apply an insulation layer around the composite material and the mold; When the composite material is cured without external pressure, after the natural cooling rate of the composite material is lower than the required cooling rate, the in-situ heat source is directly disconnected from the power supply equipment, so that the composite material can be naturally cooled under vacuum inside the bag. When there is external pressure for curing, after the natural cooling rate of the composite material is lower than the required cooling rate of the material, the high-pressure gas in the curing equipment is directly released, the in-situ heat source is separated from the power supply equipment, and the composite material is moved outside the curing equipment and naturally cooled under vacuum inside the bag. Until the temperature of the composite material drops to the temperature required for the material curing process; The thickness of the insulation layer applied around the composite material and the mold should ensure that the natural cooling rate of the composite material after heating and curing is lower than the required cooling rate of the material. At this time, the in-situ heat source is directly disconnected from the power supply equipment during the cooling stage, so that the composite material can be cooled naturally. When the thickness of the insulation layer is limited and it cannot be guaranteed that the natural cooling rate of the composite material after heating and curing is lower than the required cooling rate of the material, the power supply equipment should first apply power during the cooling stage so that the cooling rate of the composite material is lower than the required cooling rate of the material. After the natural cooling rate of the composite material is lower than the required cooling rate of the material, the in-situ heat source should be disconnected from the power supply equipment so that the composite material can be naturally cooled.

2. The in-situ heating, curing, and cooling process for composite materials according to claim 1, characterized in that, The in-situ heating method for the composite material includes the composite material itself acting as a heat source to generate heat to heat the composite material itself, or a heat source being attached to the composite material to generate heat to directly heat the composite material.

3. The in-situ heating, curing, and cooling process for composite materials according to claim 2, characterized in that, The in-situ heating method where the composite material itself acts as a heat source includes one of self-resistance electric heating, microwave heating, electromagnetic induction heating, and radio frequency heating. The in-situ heating method where the heat source is attached to the composite material includes at least one of mold heating and flexible electric heating film heating.

4. The in-situ heating, curing, and cooling process for composite materials according to claim 1, characterized in that, The insulation layer applied around the composite material and mold is made of a flexible material with an extremely low thermal conductivity. The flexible material with extremely low heat transfer coefficient includes one or any combination of aerogel felt, glass fiber felt, glass wool felt and composite silicate felt. When in-situ heating methods such as microwave heating, electromagnetic induction heating or radio frequency heating are used, the insulation layer applied to the periphery of the composite material and the mold is made of a wave-transparent material.

5. The in-situ heating, curing, and cooling process for composite materials according to claim 1, characterized in that, The external pressure refers to the pressure applied outside the vacuum bag, which is applied by high-pressure gas inside the pressure vessel.

6. The in-situ heating, curing, and cooling process for composite materials according to claim 1, characterized in that, The vacuum inside the bag refers to the vacuum applied inside the vacuum bag. During the curing process of the composite material, the vacuum level inside the bag must be maintained at no more than -0.06 MPa.

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