A method of forming a composite material using dual-sided flexible airbag assisted RTM

Abstract

The application provides a method for forming a composite material by using a double-sided flexible air bag to assist RTM. The double-sided flexible air bag is used to replace a traditional rigid mold, and the air bag is in flexible and conformal contact with the surface of a preform. The air bag does not cause damage or scratches to fibers at curvature mutation positions such as circular arc corners, and the fiber structure integrity of the three-dimensional TFP preform is completely preserved, so that the strength of the reinforcing fibers is fully exerted. By filling compressed air or high-pressure oil into the double-sided air bag arranged on the inner and outer surfaces of the preform, the isotropic characteristics of the hydrostatic pressure are used to apply an absolutely uniform out-of-plane pressing force to the preform in all directions, so that the internal voids and defects caused by uneven pressure distribution of the traditional rigid mold are eliminated. By the isostatic pressure of the double-sided air bag, the resin can be forced to perfectly infiltrate the dense aramid sewing thread area, the trapped gas can be effectively discharged, the micro pores caused by the curvature mutation are eliminated, and the compactness and bulletproof performance of the component are significantly improved.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of fiber composite material preparation, and particularly relates to a method for forming a composite material by using a double-sided flexible air bag in an RTM mode. BACKGROUND

[0002] Tailored fiber placement (TFP) technology is an advanced fiber preform preparation method. A fiber placement path is designed by computer-aided design, and a fiber tows are sewn on a base material by a sewing thread to form a composite material component. The technology can realize continuous variable-angle placement of fibers, and the fiber orientation is highly matched with a service stress field of a product. The technology has shown a broad application prospect in the fields of aerospace structures, armor protection and the like.

[0003] However, when the TFP technology is applied to a three-dimensional preform such as a fiber preform having a complex curved surface profile, subsequent forming links face severe challenges. In a traditional process, a three-dimensional TFP preform is usually placed in a rigid mold for hot-pressing RTM (resin transfer molding) injection. Since it is difficult to realize conformal matching between the rigid mold and the three-dimensional preform, the fibers are easily damaged or scratched at curvature mutation positions such as circular corners during mold clamping. Meanwhile, the pressure distribution applied by the rigid mold to different regions of the curved surface is uneven. The pressure is concentrated in regions with large curvature, and the pressure is insufficient in regions with small curvature, which causes inconsistent advancement of resin flow fronts, and micro-pores and delamination defects are easily generated between the fiber tows, in the sewing thread dense area and in the curved surface transition area. These defects can significantly weaken the interlaminar bonding strength and impact resistance of the composite material, directly affecting the ballistic limit and safety protection level of the ballistic fiber preform, and become a core bottleneck restricting the engineering application of the three-dimensional TFP component.

[0004] In view of the technical problems that the existing three-dimensional TFP preform is damaged due to curvature mutation and internal voids are generated due to uneven pressure in the traditional rigid mold hot-pressing RTM injection process, it is necessary to improve the process. SUMMARY

[0005] In view of the defects in the prior art, the present application provides a method for forming a composite material by using a double-sided flexible air bag in an RTM mode. The present application realizes uniform compaction in all directions of a complex curved surface by using the hydrostatic pressure of a double-sided air bag through an innovative integrated process, fundamentally solves the problems of fiber damage and uneven pressure caused by a rigid mold, and obtains a composite material with excellent internal quality and outstanding mechanical properties.

[0006] In order to achieve the above-mentioned purpose, the present application adopts the following technical solutions:

[0007] In a first aspect, the present application provides a method for forming a composite material by using a double-sided flexible airbag in an RTM mode, comprising the following steps:

[0008] The first airbag is laid on the surface of the first mold;

[0009] Fiber preforms were prepared using a fiber placement method;

[0010] The fiber preform is laid on the first airbag;

[0011] A second airbag is laid on the surface of the fiber preform;

[0012] A second mold is placed on top of the second airbag;

[0013] Both the second mold and the second airbag are provided with resin injection holes;

[0014] Thermosetting resin is injected into the fiber preform through the resin injection hole opened on the second mold and the second airbag, and then cured to obtain a composite material.

[0015] During the injection and curing process of thermosetting resin, the pressure inside the first and second airbags is controlled to be 0.2~5.0 MPa.

[0016] Preferably, during the thermosetting resin injection process, the pressure inside the first and second airbags is controlled at 0.2-0.5 MPa for 0-20 minutes of injection, and the pressure inside the first and second airbags is controlled at 2.0-5.0 MPa for 20-60 minutes of injection. Finally, the resin is cured at a pressure of 2.0-5.0 MPa inside the first and second airbags to obtain the composite material.

[0017] Preferably, the curing temperature is 80~200℃ and the curing time is 1~6h.

[0018] Preferably, the method for preparing the fiber preform includes the following steps:

[0019] Water-soluble PVA nonwoven fabric is used as the substrate, and the substrate is adapted to the surface of the first mold;

[0020] The reinforcing fibers are laid on the surface of the substrate;

[0021] After laying, the substrate with reinforcing fibers is hot-pressed at 80~100℃ and 0.5~1.0MPa for 5~15 minutes to obtain the preform;

[0022] The preform is immersed in water at 60-80℃ for 10-20 minutes to dissolve and remove the substrate; then the remaining fiber layer is removed and dried to obtain the fiber preform.

[0023] Preferably, the reinforcing fiber is selected from one or more of aramid fiber, carbon fiber, glass fiber, and ceramic fiber;

[0024] Reinforcing fibers are laid on the surface of the substrate, wherein the laying density is 8 to 12 reinforcing fibers per centimeter, the number of layers is 3 to 5, and the laying angle between adjacent layers is 45° to 60°.

[0025] Preferably, the thermosetting resin includes at least one of epoxy resin system and bismaleimide resin system;

[0026] The epoxy resin system comprises the following components in parts by weight: 100 parts epoxy resin, 80-110 parts curing agent, 10-45 parts reactive diluent, 0.1-1.0 parts accelerator, and 0.1-0.5 parts defoamer; the epoxy resin is selected from one or more of bisphenol A type epoxy resin, bisphenol F type epoxy resin, and multifunctional epoxy resin.

[0027] The curing agent is selected from one or more of amine curing agents and acid anhydride curing agents;

[0028] The bismaleimide resin system comprises the following components in parts by weight: 100 parts of bismaleimide resin, 50-90 parts of allyl compound, and 10-90 parts of reactive diluent; the bismaleimide resin is selected from diphenylmethane bismaleimide; the allyl compound is selected from one or more of diallylbisphenol A and allylphenol.

[0029] Preferably, both the first airbag and the second airbag are provided with injection ports;

[0030] Compressed air or high-pressure oil is introduced into the first and second airbags through the injection port to control the pressure inside the first and second airbags.

[0031] Preferably, it also includes resin storage tanks;

[0032] The resin storage tank contains thermosetting resin;

[0033] The resin storage tank is connected to a resin pipe;

[0034] The resin pipe connects to the resin injection hole opened on the second mold and the second airbag;

[0035] The thermosetting resin stored in the resin storage tank is injected into the fiber preform through resin pipes and resin injection holes.

[0036] Preferably, the space formed by the first mold and the second mold is evacuated before injecting thermosetting resin into the fiber preform.

[0037] Thermosetting resin is injected into the fiber preform at a pressure of 0.5~1.0MPa.

[0038] Preferably, it further includes: a resin collection tank, on which a resin collection pipe is connected; the resin collection pipe passes through the second mold and the second air bladder and then connects to the fiber preform;

[0039] The resin collection tank is equipped with a vacuum pump, which is used to evacuate the resin collection tank.

[0040] The method for double-sided flexible airbag-assisted RTM molding of composite materials of the present invention has the following advantages over the prior art:

[0041] 1. The method of double-sided flexible airbag-assisted RTM molding of composite materials of the present invention, in the process of resin injection and curing, compressed air or high-pressure oil is introduced into the first airbag and the second airbag, and the expansion of the first airbag and the second airbag is used to apply out-of-plane clamping force on the inner and outer surfaces of the preform simultaneously; in the process of resin injection and curing, the pressure in the first airbag and the second airbag is adjusted in real time, and the hydrostatic pressure is used to apply uniform out-of-plane clamping force to each curved surface direction of the preform to achieve isobaric molding. The hydrostatic pressure of the double-sided airbags is used to achieve uniform compaction in each direction of the complex curved surface, which fundamentally solves the problems of fiber injury and uneven pressure caused by rigid mold, and obtains a composite material with excellent internal quality and excellent mechanical properties;

[0042] 2. The method of double-sided flexible airbag-assisted RTM molding of composite materials of the present invention uses double-sided flexible airbags to replace the traditional rigid mold. The airbags and the surface of the preform are in flexible conformal contact, which will not cause pinching or scratching to the fibers at the curvature abrupt changes such as arc corners, thus completely preserving the fiber structure integrity of the three-dimensional TFP preform and ensuring that the strength of the reinforcing fibers is fully utilized. By filling the double-sided airbags covering the inner and outer surfaces of the preform with compressed air or high-pressure oil, the isotropic properties of hydrostatic pressure are used to apply an absolutely uniform out-of-plane clamping force in all directions of the curved surface of the fiber preform, fundamentally eliminating the traditional rigid mold. The mold suffers from internal voids and defects due to uneven pressure distribution. The isostatic pressure of the double-sided airbags forces the resin to perfectly impregnate the dense aramid stitching area, effectively expelling trapped gas and eliminating all microscopic pores caused by abrupt curvature changes, significantly improving the density and ballistic performance of the component. This invention cleverly integrates 3D TFP technology with double-sided airbag-assisted RTM technology, allowing for direct airbag application and RTM injection after preform placement, avoiding the risk of deformation and damage during transfer. The process is simple and controllable, easily automating and scaling up production, possessing high industrial application value and market potential. Attached Figure Description

[0043] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0044] Figure 1 This is a schematic diagram of the apparatus used in the preparation method of the double-sided flexible airbag-assisted RTM molding composite material of the present invention. Detailed Implementation

[0045] The present invention will now be clearly and completely described with reference to specific embodiments. Obviously, the described embodiments are only a part of the embodiments of the present invention, and not all of them. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0046] To better understand the invention and not to limit its scope, all figures indicating amounts, percentages, and other numerical values ​​used in this application should, in all cases, be understood to be modified by the word "approximately." Therefore, unless specifically stated otherwise, the numerical parameters listed in the specification and appended claims are approximate values ​​and may vary depending on the desired properties being sought. Each numerical parameter should at least be considered as obtained based on reported significant figures and through conventional rounding methods.

[0047] It should be noted that the order of description of the following embodiments is not intended to limit the preferred order of embodiments. Furthermore, in the description of this application, the term "comprising" means "including but not limited to". Various embodiments of the present invention may exist in the form of a range; it should be understood that the description in the form of a range is merely for convenience and brevity and should not be construed as a rigid limitation on the scope of the invention; therefore, it should be considered that the range description has specifically disclosed all possible sub-ranges and single numerical values ​​within that range. For example, it should be considered that the range description from 1 to 6 has specifically disclosed sub-ranges, such as from 1 to 3, from 1 to 4, from 1 to 5, from 2 to 4, from 2 to 6, from 3 to 6, etc., and single digits within the range, such as 1, 2, 3, 4, 5, and 6, regardless of the range. Additionally, whenever a numerical range is referred to herein, it means including any referenced number (fraction or integer) within the referred range.

[0048] This invention provides a method for forming a double-sided flexible airbag-assisted RTM composite material, using... Figure 1 The apparatus shown is prepared by the following steps:

[0049] S1. Lay the first airbag 5 on the surface of the first mold 6;

[0050] S2. Fiber preform 4 is prepared by fiber placement method;

[0051] S3. Lay the fiber preform 4 onto the first airbag 5;

[0052] S4. Lay the second airbag 3 on the surface of the fiber preform 4;

[0053] S5. Cover the second mold 2 on the second airbag 3;

[0054] Both the second mold 2 and the second airbag 3 are provided with resin injection holes;

[0055] Thermosetting resin is injected into the fiber preform 4 through the resin injection holes opened on the second mold 2 and the second airbag 3, and cured to obtain a composite material.

[0056] During the injection and curing process of thermosetting resin, the pressure inside the first airbag 5 and the second airbag 3 is controlled to be 0.2~5.0 MPa.

[0057] The present invention discloses a method for double-sided flexible airbag-assisted RTM molding of composite materials. The method involves injecting thermosetting resin into a fiber preform through resin injection holes in a second mold and a second airbag, followed by curing. During the resin injection and curing process, the pressure inside the first and second airbags is controlled to be 0.2~5.0 MPa. During resin injection and curing, compressed air or high-pressure oil is introduced into the first and second airbags, and the expansion of the first and second airbags applies out-of-plane pressure to both the inner and outer surfaces of the preform simultaneously. During resin injection and curing, the pressure inside the first and second airbags is adjusted in real time, and hydrostatic pressure is used to apply uniform out-of-plane pressure to each curved surface direction of the preform, achieving isobaric molding. The hydrostatic pressure of the double-sided airbags achieves uniform compaction of complex curved surfaces in all directions, fundamentally solving the problems of fiber damage and uneven pressure caused by rigid molds, resulting in a composite material with excellent internal quality and superior mechanical properties.

[0058] The present invention relates to a method for double-sided flexible airbag-assisted RTM molding of composite materials. This method uses double-sided flexible airbags instead of traditional rigid molds. The airbags and the surface of the preform are in flexible, conformal contact, preventing fiber pinching or scratching at abrupt curvature changes such as arc corners. This fully preserves the fiber structure integrity of the three-dimensional TFP preform, ensuring the full utilization of the reinforcing fiber strength. By filling the double-sided airbags covering the inner and outer surfaces of the preform with compressed air or high-pressure oil, the isotropic properties of hydrostatic pressure are utilized to apply an absolutely uniform out-of-plane clamping force to all directions of the fiber preform's curved surface. This invention fundamentally eliminates the internal voids and defects caused by uneven pressure distribution in traditional rigid molds. Through the isostatic pressure effect of double-sided airbags, the resin can be forced to perfectly impregnate the dense aramid stitching area, effectively expelling trapped gas and eliminating all microscopic pores caused by abrupt curvature changes, significantly improving the density and ballistic performance of the component. This invention cleverly integrates three-dimensional TFP technology with double-sided airbag-assisted RTM technology. After laying, airbag covering and RTM injection can be performed directly. The process is simple and controllable, easy to automate and scale up production, and has high industrial application value and market prospects.

[0059] In some embodiments, during the thermosetting resin injection process, the airbag pressure is dynamically controlled. Specifically, during the injection process from 0 to 20 minutes, the pressure inside the first and second airbags is controlled at 0.2 to 0.5 MPa to facilitate resin flow and wetting; during the injection process from 20 to 60 minutes, the pressure inside the first and second airbags is increased to 2.0 to 5.0 MPa to compact the preform and expel air bubbles.

[0060] Finally, the material is cured under a pressure of 2.0~5.0 MPa inside the first and second airbags to obtain the composite material.

[0061] Specifically, in this invention, the pressure inside the first and second airbags can be independently adjusted, forming a controllable pressure gradient in the thickness direction of the preform, adapting to the process requirements of different resin systems, different fiber types, and different component shapes; at the same time, the dynamically adjustable airbag pressure can apply the optimal pressure at different stages such as injection, impregnation, and curing, achieving precise control of process parameters.

[0062] In some embodiments, the curing temperature is 80~200℃ and the curing time is 1~6h.

[0063] In some embodiments, the method for preparing the fiber preform includes the following steps:

[0064] Water-soluble PVA (polyvinyl alcohol) nonwoven fabric is used as the base material, with a basis weight of 30~60g / m². 2 Cut it to a size that matches the surface of the first mold (the shape and size are basically the same);

[0065] A fiber-laying method is used to lay reinforcing fibers on the surface of the substrate;

[0066] After the fiber reinforcement is laid, the substrate with the fiber reinforcement is placed in a hot press and hot-pressed at 80~100℃ and 0.5~1.0MPa for 5~15 minutes to obtain the preform.

[0067] Immerse the preform in water at 60-80℃ for 10-20 minutes to dissolve and remove the substrate; then remove the remaining fiber layer and dry it in an oven at 80-100℃ for 2-4 hours to obtain the fiber preform.

[0068] In some embodiments, the reinforcing fiber is selected from one or more of aramid fiber, carbon fiber, glass fiber, and ceramic fiber;

[0069] Reinforcing fibers are laid on the surface of the substrate, with a density of 8-12 reinforcing fibers per centimeter and 3-5 layers. The layup angle between adjacent layers is 45°-60°. Due to the high fiber density (8-12 fibers per centimeter, 3-5 layers cross-laid), sufficient mechanical interlocking and shape retention are formed between the fibers after hot pressing and shaping, so the three-dimensional shape of the preform is completely preserved after the PVA substrate is removed. Laying density: 8-12 fibers per centimeter, arranged along the direction perpendicular to the fibers, with 8-12 fibers within a 1-centimeter width. The higher the number, the denser the fibers, the thicker the preform, and the higher the ballistic performance. Laying layers: 3-5 layers, with adjacent layers intersecting at 45°-60°. After laying the first layer of curved, gradually spiral fibers, the machine is tilted at a different angle to lay the second layer, with the two layers offset by 45°-60°, for a total of 3-5 layers. With multiple layers intersecting, fibers are present in all directions, ensuring the helmet can withstand impacts from all directions and preventing one-way cracking.

[0070] In some embodiments, the fiber placement path is digitally programmed and optimized based on the stress distribution in different areas of the helmet. The fiber placement angle changes continuously along the meridian direction (transitioning from the equator to the pole, with the placement angle gradually changing from 0° to 90°) to achieve a high degree of matching between the fiber orientation and the service stress field. Specifically, the equator is the circumference around the helmet opening (the lower edge of the helmet, the circumference close to the neck when worn), and the pole is the highest point of the dome at the very top of the helmet. The placement angle gradually changes from 0° to 90°. At 0° (equator position), the fiber follows the circumference of the helmet (circling), encircling the helmet opening in circles, equivalent to circling the equator of a basketball, moving towards the top of the helmet (the pole). The fiber direction gradually rotates and deflects, and the angle continuously increases until it reaches the pole (the top of the helmet). At 90°, the fiber becomes radial along the meridian direction of the helmet, pointing from the lower edge to the top of the helmet.

[0071] In some embodiments, the layup is a 0° / 90° orthogonal grid layer, which is a different scheme from the gradient spiral above. 0° means circumferential around the helmet; 90° means radial from the bottom edge to the top. One layer of 0° and one layer of 90° orthogonal.

[0072] In some embodiments, the thermosetting resin includes at least one of an epoxy resin system and a bismaleimide resin system;

[0073] The epoxy resin system comprises the following components in parts by weight: 100 parts epoxy resin, 80-110 parts curing agent, 10-45 parts reactive diluent, 0.1-1.0 parts accelerator, and 0.1-0.5 parts defoamer; the epoxy resin is selected from one or more of bisphenol A type epoxy resin, bisphenol F type epoxy resin, and multifunctional epoxy resin; the reactive diluent is selected from one or more of butyl glycidyl ether, C12-C14 alkyl glycidyl ether, phenyl glycidyl ether, 1,4-butanediol diglycidyl ether, and polypropylene glycol diglycidyl ether; the accelerator is selected from tertiary amine accelerators or imidazole accelerators; the tertiary amine accelerator includes DMP-30 (2,4,6-tris(dimethyl)methacrylate... The following are selected from the following: aminomethylphenol, BDMA (benzyl dimethylamine), and triethanolamine; imidazole accelerators include 2-ethyl-4-methylimidazolium and 2-methylimidazolium; curing agents are selected from amine curing agents and acid anhydride curing agents; amine curing agents include diethylenetriamine, triethylenetetramine, isophorone diamine, 4,4'-diaminodiphenylmethane, diethylaminopropylamine, and low molecular weight polyamide; acid anhydride curing agents include liquid methyltetrahydrophthalic anhydride and methylhexahydrophthalic anhydride; defoamers are selected from silicone defoamers and non-silicone polymer defoamers; silicone defoamers include BYK-A 530, BYK-066N, and Tego Foamex N; non-silicone polymer defoamers include BYK-A 535 and Efka. ® One or more of SI 2723. Among them, the BYK series defoamers are manufactured by BYK-Chemie (Germany), TEGO Foame N is manufactured by Evonik's Digo division (Germany), and Efka... ® SI 2723 is manufactured by BASF’s Efka additives line.

[0074] The bismaleimide resin system comprises the following components in parts by weight: 100 parts of bismaleimide resin, 50-90 parts of allyl compound, and 10-90 parts of reactive diluent; the bismaleimide resin is selected from diphenylmethane bismaleimide; the allyl compound is selected from one or more of diallyl bisphenol A and allylphenol; the reactive diluent is selected from one or more of allyl p-cresol ether (AMPE), allyl cresol, diallyl bisphenol F, and hyperbranched polysiloxane.

[0075] In some embodiments, both the first airbag and the second airbag are provided with injection ports;

[0076] Compressed air or high-pressure oil is introduced into the first and second airbags through the injection port to control the pressure inside the first and second airbags.

[0077] In some embodiments, a resin storage tank 1 is also included;

[0078] Resin storage tank 1 contains thermosetting resin;

[0079] Resin storage tank 1 is connected to a resin pipe 12;

[0080] The resin pipe 12 connects to the resin injection hole opened on the second mold 2 and the second airbag 3;

[0081] The thermosetting resin stored in resin storage tank 1 is injected into the fiber preform through resin pipes and resin injection holes.

[0082] Specifically, a feeding pipe 11 is provided on the resin storage tank 1. Thermosetting resin is added to the resin storage tank 1 through the feeding pipe 11. The thermosetting resin stored in the resin storage tank 1 is injected into the fiber preform through the resin pipe 12 and the resin injection hole.

[0083] In some embodiments, the space formed by the first mold and the second mold is evacuated before the thermosetting resin is injected into the fiber preform.

[0084] Thermosetting resin is injected into the fiber preform at a pressure of 0.5~1.0MPa.

[0085] In some embodiments, it further includes: a resin collection tank 7, on which a resin collection pipe 71 is connected; the resin collection pipe 71 passes through the second mold 2 and the second airbag 3 and then connects to the fiber preform 4.

[0086] The resin collection tank 7 is equipped with a vacuum pump 8, which is used to evacuate the resin collection tank 7.

[0087] Specifically, the vacuum pump 8 is connected to the resin collection tank 7 through the connecting pipe 81. The vacuum pump 8 is used to create a vacuum in the resin collection tank 7, thereby creating a vacuum in the space enclosed by the first mold and the second mold. This allows the thermosetting resin stored in the resin storage tank 1 to be smoothly injected into the fiber preform and flow back to the resin collection tank 7.

[0088] In some embodiments, the first mold 6 is arc-shaped, the inner side of the first airbag 5 is adapted to the surface of the first mold 6, and the inner side of the first airbag 5 is laid on the surface of the first mold 6; the inner side of the fiber preform 4 matches the surface of the first mold 6, and the inner side of the fiber preform 4 is laid on the outer side of the first airbag 5; the inner side of the second airbag 3 is adapted to the outer side of the fiber preform 4, and the inner side of the second airbag 3 is laid on the outer side of the fiber preform 4; the inner side of the second mold 2 is adapted to the outer side of the second airbag 3, and the inner side of the second mold 2 covers the outer surface of the second airbag 3.

[0089] In some embodiments, both the first airbag and the second airbag are high-temperature resistant silicone flexible conformal airbags.

[0090] The following detailed embodiments further illustrate the method for double-sided flexible airbag-assisted RTM molding of composite materials according to the present invention. This section further describes the content of the present invention in conjunction with specific embodiments, but should not be construed as limiting the present invention. Unless otherwise specified, the technical means used in the embodiments are conventional means well known to those skilled in the art. Unless otherwise specified, the reagents, methods, and equipment used in the present invention are conventional reagents, methods, and equipment in the art.

[0091] Example 1

[0092] This embodiment provides a method for forming composite materials with double-sided flexible airbag-assisted RTM molding, including the following steps:

[0093] S1. Lay the first airbag on the surface of the first mold.

[0094] The first mold 6 is a convex round-head mold with a cross-section in the shape of an arc (i.e., hemispherical or spherical crown), an arc radius R of 120mm, and a central angle of 180° (i.e., hemispherical shell shape), used to form the shell of a bulletproof helmet; the first airbag 5 is laid on the convex outer surface of the first mold 6. The first airbag 5 is a high-temperature resistant silicone flexible conformal airbag, and its inner side conforms to the outer surface of the first mold 6.

[0095] S2. Provide a pre-prepared fiber preform and lay the fiber preform on the surface of the first airbag;

[0096] The method for preparing fiber preforms includes the following steps:

[0097] S21. Substrate Preparation: Water-soluble PVA (polyvinyl alcohol) nonwoven fabric is selected as the temporary substrate, with a basis weight of 45 g / m². 2 Cut it to a size that matches the surface of the first mold;

[0098] S22. Fiber Placement: Aramid fiber bundles (200 tex) are selected as reinforcing fibers; CNC fiber placement equipment is used to place the aramid fiber bundles on the surface of the water-soluble PVA nonwoven fabric substrate; the fiber placement angle continuously changes along the warp direction (transitioning from the equator to the poles, with the placement angle gradually changing from 0° to 90°) to achieve a high degree of matching between fiber orientation and service stress field. The placement density is 10 fiber bundles per centimeter, with 5 layers, and the placement angle between adjacent layers is 50° cross-over.

[0099] S23. Hot pressing and shaping: After the fabric is laid out, the PVA nonwoven fabric substrate with fiber layer is placed in a hot press and hot-pressed for 10 minutes at 90℃ and 0.8MPa to initially bond the fiber bundles and the fiber layer with the PVA substrate, forming a preform with a three-dimensional arc shape.

[0100] S24. Water washing to remove PVA substrate: Immerse the preform after hot pressing and shaping in 70℃ hot water for 15 minutes, stirring to completely dissolve and remove the water-soluble PVA nonwoven fabric; then remove the remaining fiber layer and dry it in a 100℃ oven for 2 hours to obtain a pure aramid fiber preform (i.e., fiber preform). Due to the high fiber bundle density (10 fibers per centimeter, 5 layers cross-laid), sufficient mechanical interlocking and shape retention capabilities have been formed between the fiber bundles after hot pressing and shaping, so the three-dimensional shape of the preform is completely preserved after removing the PVA substrate;

[0101] S25. Laying: The dried pure aramid fiber preform is directly laid on the outer surface of the first airbag, so that the concave surface of the preform conforms to the outer surface of the first airbag.

[0102] S3. Lay a second airbag on the surface of the fiber preform.

[0103] A second airbag is laid on the outer surface of the fiber preform. The second airbag is a high-temperature resistant silicone flexible conformal airbag, and its inner side conforms to the outer surface of the fiber preform.

[0104] S4. Cover the second mold onto the second airbag, inject resin and allow it to cure.

[0105] A second mold is placed on the second airbag, and the inner side of the second mold is adapted to the outer side of the second airbag; both the second mold and the second airbag are provided with resin injection holes.

[0106] Before injecting thermosetting resin into the fiber preform, the space enclosed by the first mold and the second mold is evacuated to a vacuum degree of ≤-0.095MPa.

[0107] An epoxy resin system was selected as the resin matrix for RTM injection. By weight, the epoxy resin system includes the following components: 100 parts of bisphenol A type epoxy resin (CYD-128 is the commercial brand name of bisphenol A type liquid epoxy resin produced by Yueyang Baling Petrochemical, also known in the industry as E-51 epoxy resin), 85 parts of liquid methyltetrahydrophthalic anhydride (curing agent), 20 parts of glycidyl ether reactive diluent (propylene oxide butyl ether), 0.5 parts of tertiary amine accelerator (DMP-30, 2,4,6-tris(dimethylaminomethyl)phenol), and 0.3 parts of defoamer (specifically BYK-A 530). All components were stirred evenly at 40°C, degassed, and then injected into the resin storage tank.

[0108] The epoxy resin system in the resin storage tank is injected into the fiber preform through the resin pipe, the resin injection hole on the second mold and the second air bladder at an injection pressure of 0.5 MPa.

[0109] During the thermosetting resin injection process, the airbag pressure is dynamically controlled: compressed air is introduced into the first and second airbags through the injection port. During the injection period of 0-20 minutes, the pressure inside the first and second airbags is controlled at 0.3 MPa to facilitate the full flow and wetting of the resin. During the injection period of 20-60 minutes, the pressure inside the first and second airbags is increased to 2.0 MPa to compact the preform and expel air bubbles.

[0110] After the resin injection is completed, the pressure inside the first air bladder 5 and the second air bladder is maintained at 2.0 MPa. The first mold and the second mold are then heated and cured at 120°C for 4 hours. During the curing process, excess resin flows into the resin collection tank 7 through the resin collection pipe.

[0111] After curing, the pressure inside the first and second airbags is released, and the second mold and second airbag are removed in sequence. The composite material product is then demolded and removed, yielding the aramid fiber reinforced epoxy resin-based bulletproof helmet shell composite material.

[0112] Example 2

[0113] This embodiment provides a method for forming a double-sided flexible airbag-assisted RTM composite material, which is the same as that in embodiment 1, except that carbon fiber (Toray T300 type 3K) is used instead of aramid fiber as the reinforcing fiber, bismaleimide resin is used as the resin system, and the injection pressure and dynamic control parameters of the airbag pressure are different from those in embodiment 1; the remaining process parameters are the same as those in embodiment 1.

[0114] The bismaleimide resin system, by weight, comprises the following components: 100 parts diphenylmethane bismaleimide (BDM), 65 parts diallyl bisphenol A (DABPA), and 25 parts styrene (reactive diluent). All components are stirred uniformly at 80°C, degassed, and then injected into a resin storage tank. The injection pressure is 0.8 MPa. Dynamic pressure control parameters for the air bladders are as follows: compressed air is introduced into the first and second air bladders through the injection port; during injection 0-20 minutes, the pressure in the first and second air bladders is controlled at 0.5 MPa; during injection 20-60 minutes, the pressure in the first air bladder (5) and the second air bladder (3) is increased to 3.0 MPa. After injection, curing is carried out while maintaining a pressure of 3.0 MPa in the first and second air bladders. The curing temperature is increased to 180°C, and the curing time is 3 hours. The resulting composite material exhibits higher rigidity and strength, making it suitable for aerospace structural components requiring higher rigidity and heat resistance.

[0115] Example 3

[0116] This embodiment provides a method for double-sided flexible airbag-assisted RTM molding of composite materials, similar to Embodiment 1, except that the fiber layup is a 0° / 90° orthogonal grid layup scheme. During the injection period of 0~20 min, the pressure inside the first airbag 5 and the second airbag 3 is controlled at 0.3 MPa to facilitate sufficient resin flow and wetting. During the injection period of 20~60 min, the pressure inside the first airbag 5 and the second airbag 3 is increased to 1.5 MPa to compact the preform and remove air bubbles. After the resin injection is completed, the material is cured under the condition that the pressure inside the first airbag 5 and the second airbag 3 is maintained at 1.5 MPa. The resulting composite material exhibits balanced mechanical properties in both the warp and weft directions, making it suitable for structural panels requiring bidirectional load-bearing capacity.

[0117] Comparative Example 1

[0118] This comparative example provides a method for conventional rigid mold RTM molding of composite materials, including the following steps:

[0119] S1. Mold Preparation

[0120] A rigid die system is used, consisting of a rigid punch and a rigid die. The rigid punch is a convex, round-headed die machined from metal, with a circular arc cross-section, a radius R of 120mm, and a central angle of 180° (i.e., hemispherical). It is used to form the shell of a bulletproof helmet. The inner surface of the rigid die matches the outer surface of the rigid punch, and the thickness of the mold cavity after mold closing is the design thickness of the composite material product. The forming surfaces of the rigid punch and rigid die are cleaned and uniformly coated with a release agent, then baked at 80℃ for 30 minutes to cure the release agent.

[0121] S2, Laying the fiber preform

[0122] Following the methods in steps S21-S24 of Example 1, a pure aramid fiber preform was prepared in advance (using water-soluble PVA nonwoven fabric as the substrate, laying aramid fiber bundles, hot pressing and shaping, washing to remove PVA, and drying). The prepared pure aramid fiber preform was laid on the outer surface of a rigid punch, so that the concave surface of the preform conformed to the outer surface of the rigid punch.

[0123] S3, Mold Closure

[0124] The rigid die and rigid punch are closed, and a closing force of 8 tons is applied by bolt locking or a hydraulic clamping device to ensure that the mold is completely closed and the mold cavity is sealed. After the mold is closed, the preform is held in the mold cavity between the rigid punch and the rigid die.

[0125] S4, Vacuuming

[0126] Vacuum the closed mold cavity to a vacuum level of ≤-0.095MPa through the evacuation port on the rigid die, and the evacuation time shall not be less than 30 minutes.

[0127] S5, Injection of adhesive

[0128] The same epoxy resin system as in Example 1 was used. All components were stirred evenly at 40°C, degassed, and then injected into a resin storage tank.

[0129] Using a traditional RTM injection pressure of 0.5 MPa, the epoxy resin system is injected into the mold cavity through a resin injection hole in a rigid die. The resin flows under pressure and impregnates the fiber preform. When the resin flows out from the vent of the rigid die, the injection is stopped and the vent is closed.

[0130] S6, Curing

[0131] After the resin injection is completed, the rigid punch and rigid die are heated and cured at 120°C for 4 hours (same curing conditions as in Example 1). During the curing process, the mold remains locked, and the pressure in the mold cavity is only the chemical shrinkage stress generated during the resin curing process, without any external compaction pressure.

[0132] S7, Demolding

[0133] After curing, the mold is opened, and the composite material product is demolded from the rigid mold to obtain the aramid fiber reinforced epoxy resin-based composite material product formed by traditional rigid mold RTM.

[0134] Comparative Example 2

[0135] This comparative example provides a method for single-sided airbag-assisted RTM molding of composite materials, including the following steps:

[0136] S1. Lay the first airbag on the surface of the first mold.

[0137] The same first mold as in Example 1 is used. A first airbag is laid on the convex outer surface of the first mold. The first airbag is a high-temperature resistant silicone flexible conformal airbag, and its inner side conforms to the outer surface of the first mold.

[0138] S2, Laying the fiber preform

[0139] Following the methods in steps S21-S24 of Example 1, a pure aramid fiber preform is prepared in advance. The prepared pure aramid fiber preform is then laid on the outer surface of the first airbag 5, so that the concave surface of the preform conforms to the outer surface of the first airbag.

[0140] S3. Install the second mold (without a second airbag).

[0141] A second mold (i.e., without a second airbag) is directly placed on the outer surface of the fiber preform. The second mold is a rigid concave mold, with a gap between its inner surface and the outer surface of the fiber preform (this gap is determined by the thickness of the preform and the subsequent compaction stroke after the airbag expands, and is usually 1~3mm). A resin injection hole is provided on the second mold. The second mold is locked to the first mold by bolts or a hydraulic locking device.

[0142] S4, Vacuuming

[0143] The space enclosed by the first and second molds is evacuated to a vacuum level of ≤-0.095MPa through the evacuation port opened on the second mold.

[0144] S5, Injection of adhesive

[0145] The same epoxy resin system as in Example 1 (formulation same as in Example 1) was selected, and each component was stirred evenly at 40°C. After degassing, the mixture was injected into the resin storage tank.

[0146] The epoxy resin system in the resin storage tank is injected into the fiber preform through the resin pipe and the resin injection hole on the second mold at an injection pressure of 0.5 MPa.

[0147] During the thermosetting resin injection process, compressed air is introduced into the first air bladder 5 only through the injection port (the second mold is a rigid cavity mold without an air bladder). During the injection period of 0~20min, the pressure inside the first air bladder is controlled at 0.3MPa to facilitate the full flow and wetting of the resin. During the injection period of 20~60min, the pressure inside the first air bladder is controlled to increase to 2.0MPa. The unilateral expansion of the first air bladder applies an out-of-plane pressing force from the inside to the outside to the preform (the outer surface of the preform is limited by the rigid cavity mold and can only withstand the unilateral pressure from the first air bladder).

[0148] S6, Curing

[0149] After the adhesive injection is completed, under the condition that the pressure inside the first airbag 5 is maintained at 2.0 MPa, the first mold 6 and the second mold 2 are heated and cured at 120°C for 4 hours (the same curing conditions as in Example 1). During the curing process, the first airbag continuously applies a one-way outward pressing force to the preform.

[0150] S7, Demolding

[0151] After curing, the pressure inside the first airbag 5 is released, and the second mold 2 is taken out in sequence to demold the composite material product, thus obtaining the aramid fiber reinforced epoxy resin-based composite material product formed by single-sided airbag-assisted RTM molding.

[0152] The properties of the composite materials prepared in Examples 1-3 and Comparative Examples 1-2 were tested, and the results are shown in Table 1 below.

[0153] The testing methods for each performance indicator are as follows:

[0154] (1) Fiber volume fraction: determined according to GB / T 3365-2008 "Test method for pore content and fiber volume content of carbon fiber reinforced plastics". After removing the resin matrix by chemical method, the fiber mass is weighed and the fiber volume fraction is calculated based on the density.

[0155] (2) Porosity: The porosity was determined according to the microscope scale method in GB / T 3365-2008. After polishing the sample, it was observed under a metallographic microscope, and the proportion of pore area was statistically analyzed by image analysis. At the same time, the micromorphology of the pores was observed by scanning electron microscopy (SEM).

[0156] (3) Tensile strength and tensile modulus: determined according to GB / T 1447-2005 "Test method for tensile properties of fiber reinforced plastics".

[0157] (4) Bending strength and bending modulus: determined according to GB / T 1449-2005 Test method for bending properties of fiber reinforced plastics (three-point bending method).

[0158] (5) Interlaminar shear strength: determined according to GB / T 1450.1-2005 "Test method for interlaminar shear strength of fiber reinforced plastics" (short beam method).

[0159] (6) Impact toughness: determined according to GB / T 1451-2005 "Test method for impact toughness of simply supported beam fiber reinforced plastic".

[0160] (7) Ballistic resistance V50: The ballistic limit V50 test was conducted using a 1.1g fragment simulated bullet, according to GB / T 32497-2016 "Test method for the elastic resistance of fiber reinforced composite materials against fragments V50".

[0161] Table 1 - Properties of the composite materials prepared in Examples 1-3 and Comparative Examples 1-2

[0162]

[0163] As shown in Table 1, compared with the traditional rigid mold RTM process (Comparative Example 1), the composite material prepared by the double-sided flexible airbag-assisted RTM process of the present invention (Example 1) has achieved comprehensive and significant improvements in various indicators, including fiber volume fraction (increased from 52% to 64%), porosity (decreased from 5% to <1%), tensile strength (increased by 37.4%), tensile modulus (increased by 30.3%), flexural strength (increased by 38.6%), flexural modulus (increased by 29.8%), interlaminar shear strength (increased by 27.5%), impact toughness (increased by 37.3%), and ballistic performance V50 (increased from 630m / s to 736m / s, an increase of 16.8%).

[0164] Compared with the single-sided airbag-assisted RTM process (Comparative Example 2), the double-sided airbag-assisted RTM process of the present invention (Example 1) has achieved further improvements in all performance indicators, with the fiber volume fraction increased by about 5 percentage points, the porosity reduced by about 75%, and the ballistic performance V50 increased by about 8.2%, which fully demonstrates the optimization effect of the isostatic pressing effect of the double-sided airbag on the internal quality and mechanical properties of the composite material.

[0165] Comparing Example 1 (aramid / epoxy, gradient helical layup) with Example 3 (aramid / epoxy, 0° / 90° orthogonal layup), the former is superior in ballistic performance V50 (736m / s vs 712m / s) and impact toughness (235kJ / m² vs 221.3kJ / m²), indicating that the helical layup path with continuously varying angles along the warp direction can better match the service stress field of helmet-like curved surfaces, improving impact resistance and ballistic performance; the latter has more balanced mechanical properties in both warp and weft directions, making it suitable for structural panels requiring bidirectional load-bearing capacity.

[0166] Comparing Example 1 (aramid / epoxy) and Example 2 (carbon fiber / BMI), it can be seen that the tensile modulus (135.2 GPa) and flexural modulus (112.5 GPa) of the carbon fiber / BMI system are significantly higher than those of the aramid / epoxy system, making it suitable for aerospace structural components with higher requirements for stiffness and heat resistance; while the aramid / epoxy system has advantages in impact toughness and ballistic performance, making it suitable for the field of armor protection.

[0167] The above results demonstrate that the isostatic pressing effect of the double-sided flexible airbag can apply absolutely uniform out-of-plane compaction force to the curved surface of the fiber preform in all directions, fundamentally eliminating the internal voids and defects caused by uneven pressure distribution in traditional rigid molds. Simultaneously, the double-sided airbag applies pressure from both the inner and outer surfaces, achieving completely uniform compaction in the thickness direction, further eliminating the interlayer residual porosity problem that is difficult to solve with single-sided pressure. Therefore, the double-sided flexible airbag-assisted RTM molding method for composite materials provided by this invention can significantly improve the density, mechanical properties, and ballistic performance of composite material products, and has broad application prospects in fields such as bulletproof helmets and aerospace structural components.

[0168] It is understood that the technical features of the above embodiments can be combined arbitrarily. For the sake of brevity, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0169] The above are merely preferred embodiments of this application, and only specifically describe the technical principles of this application. These descriptions are only for explaining the principles of this application and should not be construed as limiting the scope of protection of this application in any way. Based on this explanation, any modifications, equivalent substitutions, and improvements made within the spirit and principles of this application, as well as other specific embodiments of this application that can be conceived by those skilled in the art without creative effort, should be included within the scope of protection of this application.

Claims

1. A method for double-sided flexible airbag-assisted RTM molding of composite materials, characterized in that, Includes the following steps: The first airbag is laid on the surface of the first mold; Fiber preforms were prepared using a fiber placement method; The fiber preform is laid on the first airbag; A second airbag is laid on the surface of the fiber preform; A second mold is placed on top of the second airbag; Both the second mold and the second airbag are provided with resin injection holes; Thermosetting resin is injected into the fiber preform through the resin injection hole opened on the second mold and the second airbag, and then cured to obtain a composite material. During the injection and curing process of thermosetting resin, the pressure inside the first and second airbags is controlled to be 0.2~5.0 MPa.

2. The method for forming a double-sided flexible airbag-assisted RTM composite material as described in claim 1, characterized in that, During the thermosetting resin injection process, the pressure inside the first and second airbags is controlled at 0.2-0.5 MPa for 0-20 minutes of injection, and the pressure inside the first and second airbags is controlled at 2.0-5.0 MPa for 20-60 minutes of injection. Finally, the resin is cured at a pressure of 2.0-5.0 MPa inside the first and second airbags to obtain the composite material.

3. The method for forming a double-sided flexible airbag-assisted RTM composite material as described in claim 1, characterized in that, The curing temperature is 80~200℃ and the curing time is 1~6h.

4. The method for forming a double-sided flexible airbag-assisted RTM composite material as described in claim 1, characterized in that, The method for preparing the fiber preform includes the following steps: Water-soluble PVA nonwoven fabric is used as the substrate, and the substrate is adapted to the surface of the first mold; The reinforcing fibers are laid on the surface of the substrate; After laying, the substrate with reinforcing fibers is hot-pressed at 80~100℃ and 0.5~1.0MPa for 5~15 minutes to obtain the preform; The preform is immersed in water at 60-80℃ for 10-20 minutes to dissolve and remove the substrate; then the remaining fiber layer is removed and dried to obtain the fiber preform.

5. The method for forming a double-sided flexible airbag-assisted RTM composite material as described in claim 4, characterized in that, The reinforcing fiber is selected from one or more of aramid fiber, carbon fiber, glass fiber, and ceramic fiber; Reinforcing fibers are laid on the surface of the substrate, wherein the laying density is 8 to 12 reinforcing fibers per centimeter, the number of layers is 3 to 5, and the laying angle between adjacent layers is 45° to 60°.

6. The method for forming a double-sided flexible airbag-assisted RTM composite material as described in claim 1, characterized in that, The thermosetting resin includes at least one of epoxy resin system and bismaleimide resin system; The epoxy resin system comprises the following components in parts by weight: 100 parts epoxy resin, 80-110 parts curing agent, 10-45 parts reactive diluent, 0.1-1.0 parts accelerator, and 0.1-0.5 parts defoamer; the epoxy resin is selected from one or more of bisphenol A type epoxy resin, bisphenol F type epoxy resin, and multifunctional epoxy resin. The curing agent is selected from one or more of amine curing agents and acid anhydride curing agents; The bismaleimide resin system comprises the following components in parts by weight: 100 parts of bismaleimide resin, 50-90 parts of allyl compound, and 10-90 parts of reactive diluent; the bismaleimide resin is selected from diphenylmethane bismaleimide; the allyl compound is selected from one or more of diallylbisphenol A and allylphenol.

7. The method for forming a double-sided flexible airbag-assisted RTM composite material as described in claim 1, characterized in that, Both the first and second airbags are equipped with injection ports; Compressed air or high-pressure oil is introduced into the first and second airbags through the injection port to control the pressure inside the first and second airbags.

8. The method for forming a double-sided flexible airbag-assisted RTM composite material as described in claim 1, characterized in that, It also includes resin storage tanks; The resin storage tank contains thermosetting resin; The resin storage tank is connected to a resin pipe; The resin pipe connects to the resin injection hole opened on the second mold and the second airbag; The thermosetting resin stored in the resin storage tank is injected into the fiber preform through resin pipes and resin injection holes.

9. The method for forming a double-sided flexible airbag-assisted RTM composite material as described in claim 1, characterized in that, Before injecting thermosetting resin into the fiber preform, the space formed by the first mold and the second mold is evacuated. Thermosetting resin is injected into the fiber preform at a pressure of 0.5~1.0MPa.

10. The method for forming a double-sided flexible airbag-assisted RTM composite material as described in claim 1, characterized in that, It also includes: a resin collection tank, on which a resin collection pipe is connected; the resin collection pipe passes through the second mold and the second air bladder and then connects to the fiber preform; The resin collection tank is equipped with a vacuum pump, which is used to evacuate the resin collection tank.

Images