Hot pressing forming method for reinforced thin-wall carbon fiber shell

Abstract

The invention belongs to the technical field of material forming, and discloses a reinforced thin-wall carbon fiber shell hot-press forming method which comprises the following steps: laying carbon fiber prepreg in a mold from inside to outside, firstly laying ribs, and then laying skin to form a prepreg blank; continuously and sequentially laying a carbon fiber pad with demolding cloth, a silicon rubber pad, an isolating membrane, a breathable felt and a vacuum bag membrane on the surface of the mold, integrally sealing by using a sealing rubber strip, vacuumizing and compacting, and curing in an autoclave; and after curing is finished, cooling, opening the tank, taking out, demolding after cooling to room temperature, and removing the mold and auxiliary materials to obtain the reinforced thin-wall carbon fiber shell. According to the invention, the silicon rubber pad is added on the carbon fiber pad, so that the carbon fiber participating in hot pressing is thickened, and the external soft silicon rubber pad is used as the substrate, so that the pressure transmission of the reinforced structure to the non-reinforced side surface is fundamentally counteracted, and the convex mark defect is thoroughly eliminated. The method is suitable for manufacturing the reinforced thin-wall carbon fiber shell in the aerospace industry.

Description

Technical Field

[0001] This invention belongs to the field of material forming technology and discloses a hot pressing forming method for reinforced thin-walled carbon fiber shells. Background Technology

[0002] With the advancement of materials technology and the continuous improvement of aircraft performance requirements, the application and demand for advanced composite materials in aerospace are increasing. Among them, stiffened panels are gradually being used as the main load-bearing components in aircraft structures. Their forming process involves the forming of ribs and panels, and the manufacturing process is more complex than that of ordinary laminated panel structures, resulting in a variety of surface quality problems.

[0003] Carbon fiber reinforced resin matrix composites, with their superior properties such as high specific strength, high specific modulus, fatigue resistance, and corrosion resistance, have become one of the core materials for lightweight structures in high-end equipment fields such as aerospace and launch vehicles. Among them, stiffened thin-walled carbon fiber shells, due to their dual advantages of lightweight structure and load-bearing capacity, are widely used in the manufacture of key components such as rocket bodies and spacecraft modules.

[0004] The molding of such reinforced thin-walled carbon fiber shells mostly employs autoclave molding technology. The core process involves laying prepreg layers onto the mold surface according to the design, then integrating the reinforcing structural layer with the thin-walled shell layer. The resin melts, impregnates, and cures under the high temperature and pressure environment of the autoclave, ultimately forming an integral load-bearing structure. However, in the actual autoclaving process of curved reinforced thin-walled carbon fiber shells, a pressing technical challenge has consistently hindered the improvement of product surface quality and structural performance.

[0005] Due to the thinness of the shell wall and its overall curved shape, during the hot-pressing curing stage, the prepreg on the reinforced side undergoes resin flow and fiber compaction deformation under high temperature and pressure. The rigid support of the reinforced structure creates localized pressure concentration on the other side of the thin-walled shell (the unreinforced side), causing uneven compaction and distribution of the resin and fibers on the unreinforced side during pressure transmission. Specifically, the unreinforced side surface will develop convex imprints corresponding to the location of the reinforced structure, mirroring the shape of the ribs, resulting in an uneven shell surface and severely compressing the smoothness of the curved shell surface.

[0006] Such surface irregularities not only affect the appearance quality of the shell but also hinder subsequent assembly processes, increasing the workload and cost of post-processing steps such as surface polishing and repair. Furthermore, uneven distribution of fibers and resin in certain areas can lead to stress concentration, reducing the overall load-bearing capacity and fatigue life of the shell, posing a potential threat to the flight reliability of rockets and other equipment.

[0007] Currently, existing technologies for improving molding defects in reinforced composite structures mainly focus on optimizing hot pressing process parameters (such as heating rate, holding pressure, and curing time) or improving the prepreg application method. For example, segmented pressurization and gradient heating can be used to slow down the resin flow rate, or vacuum-assisted molding combined with autoclave processes can be used to improve resin impregnation uniformity. However, these methods have very limited effectiveness in improving the problem of embossing on the unreinforced side surfaces of curved thin-walled shells caused by pressure transmission from the reinforced structure.

[0008] By fundamentally blocking the pressure transmission from the reinforced structure to the unreinforced side surface, it is difficult to completely eliminate the convex mark defect.

[0009] The patent "A composite reinforced board laying and forming device and its usage method" discloses a composite reinforced board laying and forming device and its usage method. However, it adopts the method of first curing the ribs and then curing the panel a second time. The process is relatively complicated and time-consuming. Moreover, the device it uses is only suitable for forming thin-walled reinforced flat plates, and is not suitable for forming reinforced boards with curvature.

[0010] Therefore, in order to address the technical challenge of embossed defects on the unreinforced side surface caused by pressure transmission from the reinforcing structure during the hot pressing process of curved stiffened thin-walled carbon fiber shells, it is urgent to develop a molding method or auxiliary structure that can effectively resist local pressure concentration and ensure the flatness of the shell surface, so as to improve the molding quality and structural reliability of stiffened thin-walled carbon fiber shells. Summary of the Invention

[0011] To address the above problems, this invention provides a method for hot-pressing a reinforced thin-walled carbon fiber shell, comprising the following steps: Carbon fiber prepreg is laid up from the inside out in the mold according to the layup design. First, the reinforcing bars are laid up, and then the skin is laid up to form a prepreg blank. A carbon fiber pad with a release cloth, a silicone rubber pad, a release film, a breathable felt and a vacuum bag film are laid sequentially on the surface of the prepreg blank. The whole thing is then sealed with sealing strips, vacuumed and compacted, and then placed in a thermostatic jar for curing. After curing is complete and the material is cooled, the entire shell is removed from the can. After cooling to room temperature, the shell is demolded and the coverings that were sequentially laid on the surface of the prepreg blank are removed to obtain a reinforced thin-walled carbon fiber shell.

[0012] Preferably, the surface layer of the prepreg blank is composed of one or more layers, and the surface layers of adjacent prepreg blanks are bent outward to form a mating surface. The gap formed at the bend of the mating surface is filled with twisted yarn composed of carbon fiber and resin.

[0013] Preferably, the vacuum compaction time is 20 minutes, and the vacuum degree is ≤-90KPa.

[0014] Preferably, the vacuum compaction process further includes leak detection. When the vacuum degree is between -90 KPa and -100 KPa and the value remains stable, stop vacuum pumping and start leak detection. If the vacuum drop value is ≤ 2 Kpa within 5 minutes, the leak detection is qualified.

[0015] Preferably, the carbon fiber prepreg is a carbon fiber twill fabric epoxy prepreg formed by compounding T700 grade carbon fiber and epoxy resin.

[0016] Preferably, the carbon fiber twill fabric epoxy prepreg is laid in layers according to the sequence of first circumferential and then longitudinal or first longitudinal and then circumferential. The warp and weft directions of the laid carbon fiber fabric include two categories of [0°, 90°] and [-45°, 45°].

[0017] Preferably, during laying, it is divided into 8 layers for both the rib and the skin. The warp and weft directions of the carbon fiber fabric laid inside the 8 layers are sequentially [0°, 90°], [0°, 90°], [-45°, 45°], [0°, 90°], [0°, 90°], [-45°, 45°], [0°, 90°], [0°, 90°].

[0018] Preferably, the number of laying layers forming the surface layer of the prepreg blank is 4 layers.

[0019] Preferably, it further includes performing vacuum pre-compaction for 20 minutes respectively after the first layer of the rib and the skin is laid, performing vacuum pre-compaction before and after filling the filling yarn and after the rib laying is completed, and then performing vacuum pre-compaction every four layers until the laying process is completed.

[0020] Compared with the prior art, a hot pressing forming method for a stiffened thin-walled carbon fiber shell provided by the present invention has the following advantages: By adding a silicone rubber pad on the carbon fiber pad, the carbon fiber participating in the hot pressing becomes thicker, and with the softer silicone rubber pad on the outside as a substrate, the pressure transfer of the stiffening structure to the non-stiffened side surface is fundamentally offset, and the convex imprint defect is completely eliminated. There is no convex imprint consistent with the shape of the rib on the position of the non-stiffened side surface corresponding to the stiffening structure. Especially, the flatness of the curved surface is higher than expected. This one-time integral forming process is simple and easy to implement. The appearance quality of the stiffened thin-walled plate prepared by this method is good, and at the same time, it can meet the bearing performance requirements. Description of the Drawings

[0021] Figure 1 It is a schematic diagram of sequentially laying an auxiliary hot pressing layer on the surface of the prepreg blank in an embodiment of the present invention; Figure 2 It is a schematic diagram of the filling position of the filling yarn in an embodiment of the present invention; Figure 3 It is a schematic diagram of the structure of a stiffened thin-walled carbon fiber shell in an embodiment of the present invention; Among them, 1-prepreg blank; 2-carbon fiber pad with release cloth; 3-silicone rubber pad; 4-release film; 5-breathable felt; 6-vacuum bag film; 7-twisted yarn. Detailed Implementation

[0022] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be described below with reference to the accompanying drawings. The described embodiments are only some embodiments of the present invention, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without innovative effort are within the scope of protection of the present invention.

[0023] Please see Figure 1-3 The specific steps of an embodiment of a hot pressing forming method for reinforced thin-walled carbon fiber shells are as follows: Material requisition and assembly: The cut T700 carbon fiber prepreg is ready for use. Check the angle, quantity and surface quality of the material sheets according to the process documents. Mold cleaning: Clean the mold surface thoroughly, and roughen the bag-making area with sandpaper; Apply release agent: Apply three coats of release agent to the contact surfaces between the blocks and the mold, as well as to the entire forming surface; First layer of reinforcement: There are 3 longitudinal reinforcement bars and 3 circumferential reinforcement bars. The longitudinal reinforcement bars are laid first, followed by the circumferential ones. After laying all the longitudinal reinforcement bars, the circumferential reinforcement bars are laid, and the circumferential reinforcement bars are broken at the points where they meet the longitudinal reinforcement bars.

[0024] The semi-finished products after the tiling begins are collectively referred to as parts. After each tiling, use a scraper to press the fibers, especially the contact points between the prepreg and the sides of the groove, to avoid bridging. Vacuum pre-compression: After the first layer of reinforcing strips is laid, lay the release film and breathable felt on its surface in sequence, and then use sealing strips to attach the vacuum bag to the mold bag-making area. When vacuuming, use a scraper to press down the radius corners of the vacuum bag to avoid bridging. Vacuum degree ≤ -90KPa, pre-compression time 20min; The second layer of reinforcing bars is laid in a "circular first, then longitudinal" sequence; the third layer is laid in a "longitudinal first, then circular" sequence; and the fourth layer is laid in a "circular first, then longitudinal" sequence. After the fourth layer of reinforcing bars is laid, vacuum pre-compression is performed. Twisted yarn filling: The surfaces of adjacent prepreg blanks are bent outwards to form a butt joint. The gap formed at the bend is filled with twisted yarn synthesized from carbon fiber and resin. Ten 600mm long carbon fiber filaments are selected from the uncut prepreg fabric; twisting the carbon fiber filaments is not allowed. First, place the twisted yarn 7 along the left side and press it down with a wedge or the back of a knife to ensure it adheres firmly to the reinforcing strip. Then, place it along the right side and press it down with a wedge or the back of a knife. After filling six twisted yarns in this way, check the filling effect and make any necessary adjustments. After filling, roll the twisted yarn with a roller. Inspection method: Every 20mm along the length of the twisted yarn, press the narrow end of a steel ruler against the end face of the twisted yarn and observe the gap between the end face of the twisted yarn and the ruler. If there is a gap, compensate with twisted yarn 7. Check the filling effect again after each compensation. The final filled twisted yarn 7 can be 0~0.3mm higher than the two reinforcing strips. After the twisted wire 7 is filled, a vacuum pre-compression is performed; The fifth to eighth layers of reinforcing bars are laid as follows: the fifth layer is laid longitudinally first, then circumferentially; the sixth layer is laid circumferentially first, then longitudinally; the seventh layer is laid longitudinally first, then circumferentially; and the eighth layer is laid circumferentially first, then longitudinally. After the eighth layer of reinforcing bars is filled, a vacuum pre-compression is performed. Rib Compensation: After vacuum pre-compression, check the height difference between the ribs and the mold. Every 20mm along the length of the rib, use the narrow end of a steel ruler, pressing half of it onto the mold surface and the other half onto the rib surface, and observe the height difference between the ribs and the mold. If it is lower than the mold, compensate with twisted wire. After each compensation, check the filling effect again. The final filled ribs can be 0~0.3mm higher than the mold.

[0025] First layer of skin installation: Before installing the skin, place glass fiber air guide wires within a range of 0~10mm on all four sides of the part, with 3 wires on each side. After the first layer of skin is installed, vacuum pre-press.

[0026] The second to fifth layers of skin are laid: start laying along the outer edge of the ribs, and vacuum pre-compression is performed after the fifth layer of skin is laid.

[0027] The sixth to eighth layers of skin are laid: the skin is laid starting from the outer edge of the rib. After the eighth layer of skin is laid, vacuum pre-pressing is performed to form the prepreg blank 1.

[0028] Place the sealant strip: Place the sealant strip 5mm~10mm away from the edge of the prepreg blank. The side of the sealant strip with the release film should face the part. The placed sealant strip should be in a U-shape. The sealant strip is allowed to overlap.

[0029] Encapsulation: Place the carbon fiber pad 2 with release cloth on one side in contact with the prepreg blank 1, place a silicone rubber pad 3 on the other side of the carbon fiber pad 2 with release cloth, and fix it; then cover the surface with the release film 4 and the breathable felt 5 in sequence; finally put the vacuum bag on, and press the sealing strip onto the vacuum bag film 6 to seal the bag making area.

[0030] Leak test and vacuuming: Keep the surface breathable felt and vacuum bag flat. When the vacuum degree is between -90KPa and -100KPa and the value is stable, stop vacuuming and start leak test. The vacuum drop value is ≤2KPa within 5 minutes to be qualified. Leak test before loading: Place the sealed parts into the autoclave; after loading, check that the vacuum bag fits snugly to the parts, and that the vacuum bag is free of bridging. Evacuate the autoclave; once the vacuum level reaches -90KPa to -100KPa, stabilize the pressure for 10 minutes, then turn off the vacuum for 5 minutes. If the vacuum drop is ≤2KPa, the parts can be placed in the autoclave for curing. Unpacking: Unpack the bag after the surface temperature has dropped to 50℃ or below. Remove the vacuum bag, breathable felt, release film, non-adhesive release cloth, and adhesive strip. Demolding is not allowed. Demolding: Install the set screws on the segment, and use a wrench to tighten the set screws on the segment to remove the part from the mold; Grinding: After demolding, the edges of the panel are ground according to the dimensions to remove the glue that overflowed during curing, and finally a qualified reinforced thin-walled composite material part is obtained.

[0031] Those skilled in the art will understand that embodiments of the present invention can be provided as methods, systems, or computer program products. Therefore, the present invention can take the form of a completely hardware embodiment, a completely software embodiment, or an embodiment combining software and hardware aspects. Furthermore, the present invention can take the form of a computer program product embodied on one or more computers or available storage media (including, but not limited to, disk storage, CD-ROM, optical storage, etc.) containing computer-usable program code.

[0032] This invention is described with reference to flowchart illustrations and / or block diagrams of methods, apparatus (systems), and computer program products according to embodiments of the invention. It will be understood that each block of the flowchart illustrations and / or block diagrams, and combinations of blocks in the flowchart illustrations and / or block diagrams, can be implemented by computer program instructions. These computer program instructions can be provided to a processor of a general-purpose computer, special-purpose computer, embedded processor, or other programmable data processing apparatus to produce a machine, such that the instructions, which execute via the processor of the computer or other programmable data processing apparatus, generate instructions for implementing the flowchart illustrations and / or block diagrams. Figure 1 One or more processes and / or boxes Figure 1 A device that provides the functions specified in one or more boxes.

[0033] These computer program instructions may also be stored in a computer-readable storage medium that can direct a computer or other programmable data processing device to function in a particular manner, such that the instructions stored in the computer-readable storage medium produce an article of manufacture including instruction means, which are implemented in a process Figure 1 One or more processes and / or boxes Figure 1 The function specified in one or more boxes.

[0034] These computer program instructions may also be loaded onto a computer or other programmable data processing equipment to cause a series of operational steps to be performed on the computer or other programmable equipment to produce a computer-implemented process, thereby providing instructions that execute on the computer or other programmable equipment for implementing the process. Figure 1 One or more processes and / or boxes Figure 1 The steps of the function specified in one or more boxes.

[0035] Finally, it should be noted that the above descriptions are merely preferred embodiments of the present invention and are not intended to limit the present invention. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art can still modify the technical solutions described in the foregoing embodiments or make equivalent substitutions for some of the technical features. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for hot-pressing a reinforced thin-walled carbon fiber shell, characterized in that, It includes the following steps: Lay the carbon fiber prepreg layer by layer from the inside to the outside in the mold according to the layup design. First, lay the ribs and then the skin to form a prepreg blank; Lay a carbon fiber mat with a release cloth, a silicone rubber mat, an isolation film, a breather felt and a vacuum bag film on the surface of the prepreg blank in sequence. Then, seal it integrally with a sealant strip, evacuate and compact it, and then put it into an autoclave for curing; After the curing is completed, cool down and then take out the whole from the autoclave. After cooling to room temperature, demold and remove the covering parts laid on the surface of the prepreg blank in sequence to obtain a stiffened thin-walled carbon fiber shell; 2. The hot pressing forming method for a reinforced thin-walled carbon fiber shell according to claim 1, characterized in that, The prepreg blank surface layer is composed of one or more layers of layup. The adjacent prepreg blank surface layers are bent outward to form a butt joint surface. The gap formed at the bending part of the butt joint surface is filled with a twist yarn made of carbon fiber and resin; 3. The hot pressing forming method for a reinforced thin-walled carbon fiber shell according to claim 2, characterized in that, The evacuation and compaction time is 20 minutes, and the vacuum degree ≤ -90 KPa; 4. The hot pressing forming method for a reinforced thin-walled carbon fiber shell according to claim 3, characterized in that, The evacuation and compaction process also includes leak detection. When the vacuum degree is between -90 KPa and -100 KPa and the value remains stable, stop evacuating and start leak detection. If the vacuum drop value ≤ 2 KPa within 5 minutes, the leak detection is qualified; 5. The hot pressing forming method for a reinforced thin-walled carbon fiber shell according to claim 4, characterized in that, The carbon fiber prepreg is a carbon fiber twill fabric epoxy prepreg formed by compounding T700 grade carbon fiber and epoxy resin; 6. The hot pressing forming method for a reinforced thin-walled carbon fiber shell according to claim 5, characterized in that, The carbon fiber twill fabric epoxy prepreg is laid up according to the sequence of first circumferential and then longitudinal and first longitudinal and then circumferential. The warp and weft directions of the laid-up carbon fiber fabric include two types: [0°, 90°] and [-45°, 45°]; 7. The hot pressing forming method for a reinforced thin-walled carbon fiber shell according to claim 6, characterized in that, When laying up, there are 8 layers for both the ribs and the skin. The warp and weft directions of the carbon fiber fabric laid up inside the 8 layers are [0°, 90°], [0°, 90°], [-45°, 45°], [0°, 90°], [0°, 90°], [-45°, 45°], [0°, 90°], [0°, 90°] in sequence; 8. The hot pressing forming method for a reinforced thin-walled carbon fiber shell according to claim 7, characterized in that, The number of layers of the layup constituting the prepreg blank surface layer is 4 layers; 9. The hot pressing forming method for a reinforced thin-walled carbon fiber shell according to claim 8, characterized in that, It also includes evacuating and pre-compacting for 20 minutes respectively after the first layer of the ribs and the skin are laid up, evacuating and pre-compacting before and after filling the twist yarn and after the ribs are laid up, and then evacuating and pre-compacting once every four layers until the layup process is completed.

Images