Non-autoclave forming device and forming process for resin-based composite material

Abstract

The invention discloses a non-autoclave forming device and a non-autoclave forming process for a resin-based composite material. The forming device comprises an air bag, a breathable block and a forming mold, a pure rubber area is arranged below the air bag, a rubber-poured metal net area is arranged above the air bag, and the breathable block is arranged in the center of the interior of the air bag and connected with a vacuum nozzle on the air bag; the air bag and the forming mold are connected and sealed through a sealing rubber strip, and a vacuum nozzle is further reserved in the forming mold. The forming process comprises the following steps: firstly, sequentially laying a non-porous release film and a medium-temperature cured epoxy resin prepreg in a forming mold workpiece area, and laying air guide wires at four corners of the prepreg; then a non-porous release film and a breathable material are sequentially laid on the prepreg, finally, the whole forming device is put into a drying oven, curing forming and demolding are conducted, and a product workpiece is obtained. According to the forming device and process, the laminated plate can be produced by using the prepreg, the low porosity of the product is equivalent to that of an autoclave forming process, the operation is simple, and the product quality is stable.

Description

Technical Field

[0001] The present invention relates to a non-autoclave molding device and molding process for resin matrix composites, belonging to the technical field of resin matrix composite molding. Background Art

[0002] Resin matrix composites have the characteristics of high strength and light weight, and are thus applied in industries such as aviation, automotive, and marine. Among them, high-performance composite products in fields such as aerospace are all prepared by autoclave molding process.

[0003] In existing patents such as CN118619564A A Modifying Method and Application of Quartz Fibers and CN113105714A A Continuous High Thermal Conductivity Asphalt-based Carbon Fiber Reinforced Epoxy Resin Composite Material and Its Preparation Method, autoclave molding is adopted for resin composites. The products prepared by autoclave molding process have advantages such as stable dimensions, good repeatability, reliable mechanical properties, low porosity of the products, uniform pressure and temperature distribution, and wide application range. However, the autoclave molding process also has disadvantages such as high equipment investment, expensive auxiliary materials, large energy consumption, high operating cost, high technical requirements, complex process, and product size limited by the autoclave size. This has become a "bottleneck" restricting the wide application of composites. Low-cost composite material technology was born under this background. European and American countries have proposed and implemented a number of low-cost composite material plans, including DMLCC, AFS, LCCP, ACT, CAI, ALCAS, and TANGO, etc. The implementation of these plans has promoted the development of composite material technology towards low cost. Summary of the Invention

[0004] To solve the above problems, the present invention proposes a non-autoclave molding device and molding process for resin matrix composites. The molding device and process can realize the production of laminates using prepregs. The laminates are not limited to composite products such as flat plates and shells with curvature. The low porosity of the products is comparable to that of the autoclave molding process, and the operation is simple and the product quality is stable.

[0005] To achieve the above invention purpose, the present invention provides the following technical solutions:

[0006] A non-autoclave molding device for resin matrix composites, including an airbag, a breather block, and a molding die. The shape and size of the airbag are designed according to the surface of the molding die, and the surface of the molding die is designed according to the actual requirements of the composite product;

[0007] The main material of the airbag is rubber, and it is divided into upper and lower regions. The size of the upper region is larger than that of the lower region, and the cross-section is oval. The airbag mainly plays the role of double vacuum and applying molding pressure to the workpiece;

[0008] The area below the airbag is designed as section A-B, the size of which is determined according to the product workpiece area. It is a pure rubber area with sufficient elasticity and an elongation rate ≥ 100% under normal temperature and low pressure (≤ -0.01 MPa). Rubbers with different heat resistances are customized according to the process requirements of composite products.

[0009] The area above the airbag is a metal mesh area filled with rubber, and its elongation rate ≤ 5% under an internal pressure ≥ 0.5 MPa and the product curing temperature. An air pump nozzle and a vacuum nozzle Ⅰ are reserved on the metal mesh area section of the airbag, and hanging rings are embedded outside the metal mesh area section of the airbag adjacent to section A-B. The number of hanging rings is related to the inflation pressure and the size of the airbag. The aperture of the metal mesh ≤ 2 mm, and it can withstand an internal pressure ≥ 0.5 MPa for a long time. When the pressure inside the airbag ≤ 0.2 MPa, there are at least 4 evenly distributed hanging rings. When 0.2 MPa < the pressure inside the airbag ≤ 0.5 MPa, for every 0.1 MPa increase in pressure, the number of hanging rings is distributed according to the number distributed with a hanging ring spacing ≤ 300 mm.

[0010] The air permeable block is placed at the center of the airbag, and its diameter is equal to or slightly smaller than the maximum inner diameter of the airbag. The shape and structure of the air permeable block are preferably the same as those of the airbag. The air permeable block is made of a material resistant to over 200 °C, specifically fiberglass fabric, nylon fabric or air permeable felt. By setting the air permeable block inside the airbag, the air path is ensured to be unobstructed when the airbag is evacuated.

[0011] The vacuum nozzle Ⅰ on the airbag is connected to the air permeable block and is used to evacuate the inside of the airbag. The air pump nozzle is used to introduce compressed air into the airbag to apply pressure to the product.

[0012] The molding die is used for workpiece molding and is made of metal. Hooks are arranged along the edge of the molding die. The airbag is placed above the molding die, and the hanging rings are hooked to the hooks, and the numbers of both match.

[0013] A sealing strip is used to seal the circumference between the molding die and the airbag. The sealing strip needs to be bonded to the metal mesh area section of the airbag and is located between the hanging ring and the partition of the upper and lower areas of the airbag.

[0014] A vacuum nozzle Ⅱ is also reserved on the molding die and is located inside the sealant.

[0015] Furthermore, the material of the molding die in the present invention is stainless steel or aluminum.

[0016] Furthermore, the present invention also provides a workpiece auxiliary material between the airbag and the forming surface of the forming mold to assist in forming the workpiece. The workpiece auxiliary material includes air guide wires and breathable materials. Among them, the breathable material is in a frame shape and covers the vacuum nozzle II on the forming mold. The breathable material is not limited to breathable felts, fiber fabrics, etc., and the shape of the breathable material matches that of the workpiece. The formed workpiece is placed within the breathable material frame. The air guide wires are laid at the corners of the breathable material and the workpiece, and both ends of the air guide wires are respectively connected to the workpiece and the breathable material. The air guide wires are equivalent to gas passages and are used to introduce the gas generated between the layers of the workpiece or during the curing process into the breathable material under the drive of vacuum pressure, which can ensure that the volatile components and small molecules generated by the reaction in the workpiece are smoothly discharged during the curing process, thereby reducing the porosity of the product.

[0017] Furthermore, the present invention also provides a process for forming a resin matrix composite material using the above device, which includes the following steps:

[0018] (1) Use a liquid coating agent to lay a layer of non-porous release film on the laying area of the workpiece on the forming surface of the forming mold, which can withstand a high temperature of ≥150 °C;

[0019] (2) Lay a medium-temperature curing epoxy resin prepreg on the above non-porous release film, and use a squeegee to scrape and fit the prepreg flat for each layer laid;

[0020] (3) Lay air guide wires at the four corners of the prepreg. One end of the air guide wire is placed on the prepreg and is 1 - 2 cm away from the edge of the prepreg, and the other end extends beyond the non-porous isolation film to be laid in the next step;

[0021] (4) Lay a layer of non-porous release film above the prepreg, and the non-porous release film extends 2 - 5 cm beyond the edge of the prepreg;

[0022] (5) Lay a piece of breathable material cut into a frame shape on the non-porous release film in step (4). The inner size of the breathable material frame is larger than the prepreg, the outer frame size is larger than the non-porous release film, and each corner is connected to the other end of the air guide wire; in addition, the breathable material should cover the vacuum nozzle II on the forming mold;

[0023] (6) Paste a circle of sealant strips at a suitable position beyond the pure rubber area of the airbag, and then attach the airbag to the forming surface of the forming mold through the sealant strips to obtain a forming device;

[0024] (7) Place the above forming device in an oven, close the air pump nozzle, and open the vacuum nozzle I on the airbag to evacuate to ≤ - 0.090 MPa;

[0025] (8) Heat up the oven, open the vacuum nozzle II on the forming mold, and control the vacuum degree in the workpiece area according to the selection of the prepreg;

[0026] The temperature range of 70°C to 110°C is used for the rheological test of the product, which has a relatively low viscosity. At the same time, the gel time at 110°C is ≥ 20 min, aiming to allow sufficient time for the gas between layers of the workpiece to escape.

[0027] (9) Heat up to 110°C ± 5°C, close the vacuum nozzle Ⅰ on the airbag, evacuate the vacuum nozzle Ⅱ on the forming mold to ≤ -0.090 MPa, and at the same time turn on the air pump nozzle to introduce compressed air until the pressure inside the airbag reaches 0.2 - 0.3 MPa; continue to heat up to 125°C ± 5°C and maintain the vacuum pressure and the pressure inside the airbag, and cure for ≥ 90 min; maintain the curing pressure and naturally cool down to ≤ 60°C to demold and obtain the product workpiece.

[0028] When the medium-temperature curing epoxy resin prepreg fabric in step (1) is in the form of 7781 and the resin content is 35%, in step (8), in the temperature range where the resin viscosity of the prepreg is lower than 5 Pa·s, turn on the vacuum nozzle Ⅱ on the forming mold, and control the vacuum degree so that the vacuum degree in the product workpiece area is ≥ -0.09 MPa and ≤ -0.06 MPa.

[0029] Compared with the prior art, the present invention has the following beneficial effects:

[0030] (1) The forming device of the present invention is simple and has low energy consumption, and is suitable for forming various shaped plate and shell composite products such as flat plates and shell with curvature.

[0031] (2) By using the forming device of the present invention and selecting corresponding forming process parameters, the resin-based composite product workpiece obtained has a low porosity, and its porosity, strength performance and hot press forming process are comparable, and the operation is simple and the product quality is stable. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] Figure 1 It is a schematic structural diagram of the forming device of the present invention.

[0033] Figure 2 It is a longitudinal plane sectional view of the forming device of the present invention.

[0034] Figure 3 It is a schematic diagram of the partial layout of the workpiece and its auxiliary materials in the forming device of the present invention.

[0035] In the figure, each label is: 1 airbag, 2 air pump nozzle, 3 vacuum nozzle Ⅰ, 4 hanging ring, 5 hook, 6 air permeable block, 7 sealant strip, 8 forming mold, 9 vacuum nozzle Ⅱ, 10 workpiece, 11 air permeable material, 12 air guiding wire. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0036] The present invention will be described in detail below in conjunction with specific embodiments and the accompanying drawings.

[0037] The present invention provides a non-autoclave forming device for resin matrix composites, including an airbag 1, a breather block 6 and a forming mold 8. The shape and size of the airbag are designed according to the surface of the forming mold, and the surface of the forming mold is designed according to the actual requirements of the composite material product.

[0038] The main material of the airbag 1 is rubber, and it is divided into upper and lower regions. The size of the upper region is larger than that of the lower region, and the cross-section is oval. The airbag mainly plays the role of double vacuum and applying forming pressure to the workpiece.

[0039] The lower region of the airbag is designed as section A-B, and its size is determined according to the product workpiece area. It is a pure rubber region, and has sufficient elasticity with an elongation rate ≥ 100% at normal temperature and low pressure (≤ -0.01 MPa). Rubbers with different heat resistances are customized according to the process requirements of the composite material product.

[0040] The upper region of the airbag is a metal mesh region filled with rubber, and the elongation rate at an internal pressure ≥ 0.5 MPa and the curing temperature of the product is ≤ 5%. An air pump nozzle 2 and a vacuum nozzle I 3 are reserved on the metal mesh region section of the airbag, and a hanging ring 4 is embedded outside the metal mesh region section of the airbag adjacent to section A-B. The number of hanging rings is related to the inflation pressure and the size of the airbag. The aperture of the metal mesh is ≤ 2 mm, and it can withstand an internal pressure ≥ 0.5 MPa for a long time. When the pressure in the airbag ≤ 0.2 MPa, there are at least 4 evenly distributed hanging rings. When 0.2 MPa < the pressure in the airbag ≤ 0.5 MPa, for every 0.1 MPa increase in pressure, the number of hanging rings is distributed according to the number of distributions with a hanging ring spacing ≤ 300 mm.

[0041] The breather block 6 is placed at the center of the airbag, and its diameter is equal to or slightly smaller than the maximum inner diameter of the airbag. The shape and structure of the breather block are preferably the same as those of the airbag. The breather block is made of a material resistant to temperatures above 200 °C, specifically glass fiber fabric, nylon fabric or breather felt. By setting a breather block inside the airbag, the air path is ensured to be unobstructed when the airbag is evacuated.

[0042] The vacuum nozzle I 3 on the airbag is connected to the breather block 6 for evacuating the inside of the airbag. The air pump nozzle is used to introduce compressed air into the airbag to apply pressure to the product.

[0043] The forming mold 8 is used for workpiece forming, and it is made of metal. Hooks 5 are arranged along the edge of the forming mold. The airbag 1 is placed above the forming mold 8, and the hanging ring 4 is hooked to the hook 5, and the number of both is matched.

[0044] A sealing rubber strip is used for circumferential sealing between the forming mold and the airbag. The sealing rubber strip needs to be bonded to the metal mesh region section of the airbag and is located between the hanging ring and the partition between the upper and lower regions of the airbag.

[0045] A vacuum nozzle II 9 is also reserved on the molding die 8 and is located inside the sealant 7.

[0046] Furthermore, the material of the molding die in the present invention is stainless steel or aluminum.

[0047] Furthermore, in the present invention, a part auxiliary material is also provided between the airbag and the molding surface of the molding die to assist in molding the product part. The part auxiliary material includes air guiding wires and breathable materials; among them, the breathable material is in a frame shape and covers the vacuum nozzle II on the molding die. The breathable material is not limited to breathable felts, fiber fabrics, etc.; the molded product part is placed inside the breathable material square; the air guiding wires are laid at the corners of the breathable material and the part, and both ends of the air guiding wires are respectively connected to the part and the breathable material. The air guiding wires are equivalent to gas passages and are used to introduce the gas generated between the layers of the part or during the curing process into the breathable material under the drive of vacuum pressure, which can ensure that the volatile components in the part and the small molecules generated by the reaction are smoothly discharged during the curing process, thereby reducing the porosity of the product.

[0048] Embodiment

[0049] The process of molding a resin matrix composite material by using the autoclave-free molding device of the present invention includes the following steps:

[0050] (1) Use a liquid coating agent to lay a layer of non-porous release film on the laying area of the product part on the molding surface of the molding die, which can withstand a high temperature of ≥150 °C;

[0051] (2) Lay 8 layers of medium-temperature curing epoxy resin prepreg on the above-mentioned non-porous release film, and use a squeegee to scrape and fit the prepreg flat for each layer;

[0052] The fabric form of the prepreg is 7781, and the resin content is 35%;

[0053] (3) Lay air guiding wires at the four corners on the prepreg. One end of the air guiding wire is placed on the prepreg and is 1-2 cm away from the edge of the prepreg, and the other end extends beyond the length of the prepreg to exceed the non-porous isolation film to be laid in the next step;

[0054] (4) Lay a layer of non-porous release film above the prepreg, and the non-porous release film extends 2-5 cm beyond the edge of the prepreg;

[0055] (5) Lay a piece of breathable material cut into a square shape on the non-porous release film described in step (4). The inner size of the breathable material frame is larger than the prepreg, and the outer frame size is larger than the non-porous release film, and each corner is connected to the other end of the air guiding wire; in addition, the breathable material should cover the vacuum nozzle on the molding die;

[0056] (6) Paste a circle of sealant strips at a suitable position beyond the pure rubber area of the airbag, and then paste the airbag on the molding surface of the molding die through the sealant strips to obtain the molding device;

[0057] (7) Place the above-mentioned forming device in an oven, close the air pump nozzle, and turn on the vacuum nozzle Ⅰ on the airbag to evacuate to ≤ -0.090 MPa.

[0058] (8) Heat up the oven. In the range of 70 °C to 110 °C, turn on the vacuum nozzle Ⅱ on the forming mold, and control the vacuum degree so that the vacuum degree in the product part area is ≥ -0.09 MPa and ≤ -0.06 MPa.

[0059] The temperature range of 70 °C to 110 °C is the temperature with relatively low viscosity for the rheological test of the product. At the same time, the gel time at 110 °C ≥ 20 min, the purpose is to allow sufficient time for the gas between the layers of the part to escape.

[0060] (9) Heat up to 110 °C ± 5 °C, close the vacuum nozzle Ⅰ on the airbag, evacuate the vacuum nozzle Ⅱ on the forming mold to ≤ -0.090 MPa, and at the same time turn on the air pump nozzle to introduce compressed air into the airbag until the pressure in the airbag reaches 0.2 - 0.3 MPa; continue to heat up to 125 °C ± 5 °C and maintain the vacuum pressure and the pressure in the airbag, cure for ≥ 90 min; maintain the curing pressure and naturally cool down to ≤ 60 °C to demold and obtain the product part.

[0061] Comparative Example

[0062] A autoclave molding process for resin matrix composites:

[0063] (1) Use a liquid coating agent to lay a layer of non-porous release film on the laying area of the product part on the forming mold, which can withstand a temperature of ≥ 150 °C.

[0064] The forming mold used is the same as the forming mold of the present invention.

[0065] (2) Lay up a medium-temperature curing epoxy resin prepreg (the form of the prepreg fabric is 7781, resin content 35%), 8 layers. Each time a layer is laid, use a squeegee to scrape the prepreg flat and fit it. The first layer needs to be pre-compacted, and then pre-compacted once every 3 layers laid. The pre-compaction pressure ≤ -0.06 MPa.

[0066] (3) Lay gas guiding wires at the four corners of the prepreg. One end of the gas guiding wire is placed on the prepreg and is 1 - 2 cm away from the edge of the prepreg, and the other end extends beyond the length of the prepreg to exceed the non-porous isolation film laid in the next step.

[0067] (4) Encapsulate the part according to the autoclave molding method.

[0068] (5) After checking that the vacuum degree ≤ -0.09 MPa, cure according to the following regime (the whole curing process is under vacuum) to obtain the product:

[0069] ① Heat from room temperature to 125°C ± 5°C at a heating rate of 1.5 ± 1°C / min, with a pressure of 0.35 ± 0.05 MPa;

[0070] ② Keep at 125°C for ≥ 90 min, with a pressure of 0.35 ± 0.05 MPa;

[0071] ③ Naturally cool to 60°C, and keep the pressure at 0.35 ± 0.05 MPa.

[0072] Products and components of 3 batches were respectively formed by the forming process of the embodiment of the present invention and the forming process of the comparative example, and the porosity, compressive strength, and compressive modulus of the components were tested. The test results are shown in Table 1.

[0073] Table 1 Performance test of formed components of the embodiment and the comparative example

[0074]

[0075] As can be seen from the data in Table 1, for the components obtained by using the forming process of the present invention, their porosity, strength performance are comparable to those of the autoclave forming process, and the operation is simple and the product quality is stable.

Claims

1. A resin matrix composite non-autoclave molding device, characterized in that, It includes an airbag (1), a breathable block (6) and a forming mold (8), wherein the shape and size of the airbag (1) are designed according to the surface of the forming mold, and the surface of the forming mold (8) is designed according to the actual requirements of the composite material product; The main material of the airbag (1) is rubber, and it is divided into upper and lower regions. The size of the upper region is larger than that of the lower region, and the cross-section is oval; The lower region of the airbag is designed as section A-B, and its size is determined according to the product part area. It is a pure rubber region, and the elongation rate at normal temperature and low pressure is ≥100%; the upper region of the airbag is a metal mesh region filled with rubber, and the elongation rate at an internal pressure of ≥0.5MPa and the curing temperature of the product is ≤5%; an air pump nozzle (2) and a vacuum nozzle I (3) are reserved on the metal mesh region section of the airbag, and a hanging ring (4) is embedded outside the metal mesh region section of the airbag adjacent to section A-B. The number of hanging rings (4) is related to the inflation pressure and the size of the airbag; The breathable block (6) is placed at the center inside the airbag (1), and its diameter is equal to or slightly smaller than the maximum inner diameter of the airbag (1) close to it. The breathable block is made of a material resistant to above 200°C; the breathable block (6) is connected to the vacuum nozzle I (3) on the airbag; The forming mold (8) is used for part forming, and it is made of metal; hooks (5) are arranged along the edge of the forming mold (8). The airbag (1) is placed above the forming mold (8), and the hanging ring (4) is hooked to the hook (5), and the number of both is matched; A sealing rubber strip (7) is used to seal the periphery between the forming mold (8) and the airbag (1). The sealing rubber strip (7) is bonded to the metal mesh region section of the airbag and is located between the hanging ring (4) and the partition between the upper and lower regions of the airbag; A vacuum nozzle II (9) is also reserved on the forming mold (8) and is located inside the sealant (7).

2. The autoclave-free forming device for a resin matrix composite material according to claim 1, characterized in that, The aperture of the metal mesh in the airbag is ≤2mm; When the pressure in the airbag ≤0.2MPa, there are at least 4 evenly distributed hanging rings; When 0.2MPa < the pressure in the airbag ≤0.5MPa, for every 0.1MPa increase in pressure, the number of hanging rings is distributed according to the distribution with a hanging ring spacing ≤300mm.

3. The autoclave-free forming device for a resin matrix composite material according to claim 1, characterized in that, The shape and structure of the breathable block are the same as those of the airbag.

4. The autoclave-free forming device for a resin matrix composite material according to claim 1, wherein, The material of the breathable block is fiberglass fabric, nylon fabric or breathable felt.

5. The autoclave-free molding device for a resin matrix composite material according to claim 1, wherein, The material of the forming mold is stainless steel or aluminum.

6. The autoclave-free forming device for a resin matrix composite material according to claim 1, wherein, A part auxiliary material is also provided between the airbag (1) and the forming mold (8) to assist in forming the product part. The part auxiliary material includes a gas guiding wire (12) and a breathable material (11); Among them, the breathable material (11) is in a frame shape and covers the vacuum nozzle II (9) on the forming mold. The formed product part (10) is placed in the frame of the breathable material (11); The gas guiding wire (12) is laid at the corners of the breathable material (11) and the part (10), and both ends of the gas guiding wire (12) are respectively connected to the part (10) and the breathable material (11).

7. The autoclave-free molding device for resin matrix composites according to claim 6, wherein The breathable material (11) is breathable felt or fiber fabric.

8. A process for molding a resin matrix composite material using the device according to any one of claims 1 to 7, characterized in that, It includes the following steps: (1) Use a liquid coating agent to lay a layer of non-porous release film on the laying area of the product part on the forming surface of the forming mold, resistant to a high temperature of ≥150°C; (2) Lay a medium-temperature curing epoxy resin prepreg on the above-mentioned hole-free release film, and use a squeegee to scrape and fit the prepreg flat for each layer laid. (3) Lay air guide wires at the four corners of the prepreg. One end of the air guide wire is placed on the prepreg and is 1 - 2 cm away from the edge of the prepreg, and the length of the other end extending out of the prepreg should exceed the hole-free isolation film laid in the next step. (4) Lay a layer of hole-free release film above the prepreg, and the hole-free release film extends 2 - 5 cm beyond the edge of the prepreg. (5) Lay a piece of breathable material cut into a frame shape on the hole-free release film described in step (4). The inner size of the breathable material frame is larger than the prepreg, the outer frame size is larger than the hole-free release film, and each corner is connected to the other end of the air guide wire; in addition, the breathable material should cover the vacuum nozzle II on the forming mold. (6) Paste a circle of sealant strips at a suitable position beyond the pure rubber area of the airbag, and then attach the airbag to the forming surface of the forming mold through the sealant strips to obtain a forming device. (7) Put the above-mentioned forming device into an oven, close the air pump nozzle, and open the vacuum nozzle I on the airbag to evacuate to ≤ -0.090 MPa. (8) Heat up the oven, open the vacuum nozzle II on the forming mold, and control the vacuum degree in the product part area according to the selection of the prepreg. (9) Heat up to the initial curing temperature of the prepreg resin DSC test, close the vacuum nozzle I on the airbag, evacuate the vacuum nozzle II on the forming mold to ≤ -0.090 MPa, and at the same time open the air pump nozzle to introduce compressed air until the pressure in the airbag reaches 0.2 - 0.3 MPa; continue to heat up to the curing peak temperature of the prepreg resin DSC test and maintain the vacuum pressure and the pressure in the airbag, and cure for ≥ 30 min; maintain the curing pressure and naturally cool down to ≤ 60 °C to demold to obtain the product part.

9. The process according to claim 8, characterized in that, When the fabric form of the medium-temperature curing epoxy resin prepreg described in step (1) is 7781 and the resin content is 35%, in the temperature range where the viscosity of the prepreg resin is lower than 5 Pa·s in step (8), open the vacuum nozzle II on the forming mold and control the vacuum degree so that the vacuum degree in the product part area is ≥ -0.09 MPa and ≤ -0.06 MPa.

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