A variable stiffness sound-absorbing metamaterial and its preparation method

Abstract

This invention discloses a variable stiffness sound-absorbing metamaterial and its preparation method. A three-dimensional model of the sound-absorbing metamaterial is constructed using a computer. The model is then segmented using the interfaces between different materials forming the sound-absorbing metastructure as boundaries, and assembled and reassembled. Materials with different stiffnesses are selected, and different processing techniques are used to process each part of the sound-absorbing metamaterial separately. According to different processing methods, the different parts of the materials are combined and assembled until the interfaces between the different parts are completely adhered to each other and formed a whole. After sufficient settling time, the variable stiffness sound-absorbing metamaterial is obtained. This invention employs multi-material heterogeneous additive manufacturing and casting processes. By adjusting the material stiffness, the sound absorption frequency band is broadened, the sound absorption effect is optimized, and the material utilization rate is improved. The thickness of the metamaterial is significantly reduced, meeting the design and manufacturing requirements of sound-absorbing and sound-insulating materials in any frequency band.

Description

Technical Field

[0001] This invention belongs to the field of sound-absorbing functional materials, specifically relating to flexible variable stiffness composite sound-absorbing metamaterials and their preparation methods. Background Technology

[0002] Noise frequencies in daily life are diverse, especially low-frequency noise, which, due to its long propagation distance and strong penetrating power, can cause serious damage to the physical and mental health of people if it exceeds a certain limit. Sound-absorbing materials penetrate sound insulation materials, and their performance and size are often limited by the law of mass density and the wavelength of sound waves. This results in better sound-absorbing materials having relatively large densities and volumes, thus not being able to effectively absorb noise pollution. Therefore, to absorb noise pollution as much as possible, the volume and density of the material must be as small as possible for wider application. Therefore, exploring the design and fabrication methods of small-sized variable stiffness sound-absorbing materials is essential.

[0003] Fused Deposition Modeling (FDM) is currently the most widely used block-rate prototyping technology. It utilizes a heated block to melt filaments, which are then deposited layer by layer through a nozzle, from point to line, line to surface, and surface to volume, to obtain the desired part shape. Materials that can be molded include low-melting-point metals, polymers, and composite materials that are easily deposited and molded. FDM technology offers advantages such as low cost, short processing cycle, support for small-batch direct production, the ability to create complex geometries and cavities that are impossible to manufacture with other technologies, and excellent stability and repeatability. However, research on printing heterogeneous combinations of materials with different elastic moduli is limited; the most common approach is the fabrication of structures based on a single material, and methods that combine material properties with structural functionality are even rarer.

[0004] Photopolymer additive manufacturing (SLM) is a widely used additive manufacturing technology in recent years. Due to its ultra-high manufacturing precision and fast processing speed, it is frequently used to process parts with high precision requirements or to prepare molds for complex, high-precision models, achieving manufacturing through casting. The forming principles of SLM are mainly divided into two categories: a. Spot forming technology (SLA) and b. Distributed linear forming technology (DLP). However, regardless of the forming method, existing processing methods all involve single-material processing, and there is a lack of research and case studies on multi-material heterogeneous SLM printing. Research on molded devices from a functional structure perspective is even rarer. Summary of the Invention

[0005] This invention addresses the shortcomings of existing design methods and fabrication techniques. The purpose of this invention is to provide variable stiffness acoustic metamaterials and their fabrication methods. This invention employs additive manufacturing, casting, or injection molding processes using heterogeneous multimaterials. By adjusting the stiffness of the materials, the sound absorption frequency band is broadened, the sound absorption effect is optimized, and the material utilization rate is improved. The thickness of the metamaterial is significantly reduced, thus meeting the design and manufacturing requirements of sound-absorbing and sound-insulating materials for any frequency band.

[0006] This invention is achieved through the following technical solution;

[0007] This invention provides a method for preparing a variable stiffness sound-absorbing metamaterial, comprising:

[0008] A three-dimensional model of the sound-absorbing metamaterial is constructed using a computer. The three-dimensional model constituting the sound-absorbing metastructure is then divided into segments with different material bonding surfaces as boundaries, and then assembled and recombined with different material bonding surfaces as boundaries.

[0009] Materials with different stiffnesses were selected, and different processing techniques were used to process each part of the different materials in the sound-absorbing metamaterial separately;

[0010] Depending on the different process methods, different parts of different materials are combined and assembled or integrally molded. After the different parts are completely adhered to each other to form a whole, and after a sufficient period of settling time, a variable stiffness sound-absorbing metamaterial is obtained.

[0011] As a preferred method, constructing a three-dimensional model of the sound-absorbing metamaterial using a computer includes:

[0012] Based on the target sound absorption frequency band, determine the three-dimensional structure of the sound-absorbing metamaterial that can cover the target frequency band, then determine the local structural stiffness of different materials, and select different flexible materials to meet the local structural stiffness.

[0013] The three-dimensional model is divided into sections using the interface between different materials as the boundary, and then the divided model is assembled back into the original three-dimensional structure.

[0014] Preferably, materials with different stiffnesses include flexible polymers, flexible resin materials, mixtures of prepolymers and photoinitiators, and high-strength, high-stiffness materials.

[0015] Flexible polymers include styrene-based thermoplastic elastomers (SBS), ethylene-isoprene-styrene (SIS), ethylene propylene diene monomer (EPDM), thermoplastic ethylene propylene diene monomer (TPV), polyolefin thermoplastic elastomers (TPO), thermoplastic polyurethane elastomers (TPU), thermoplastic polyester elastomers (TPEE), and polypropylene polyethylene (PPE).

[0016] Flexible resin materials include phthalic unsaturated polyester resin, waterborne acrylic resin, copolymer petroleum resin, or epoxy resin.

[0017] The prepolymer and photoinitiator mixture is a mixture of prepolymer and photoinitiator at a mass ratio of (96.5-97):(3-3.5);

[0018] Prepolymers include epoxy acrylate (EA), polyurethane acrylate (PUA), polyester acrylate (PEA), polyether acrylate, or vinyl resin;

[0019] Photoinitiators include hyperbranched polyurethane acrylate HBP2UA-HMPP, benzophenone, or thioxanthone.

[0020] High-strength and stiffness materials include ABS, polylactic acid (PLA), or polyether ether ketone (PEEK).

[0021] As a preferred option, additive manufacturing and casting processes are used to process different parts of the sound-absorbing metamaterial separately, based on the different material bonding surfaces.

[0022] Additive manufacturing includes fused deposition modeling (FDM) or photopolymerization.

[0023] Preferably, the interface between different materials is an irregular contact surface, and variable stiffness sound-absorbing metamaterials are prepared using a melt deposition extrusion molding process, including:

[0024] S1: Perform 3D modeling of acoustic metamaterials, combine slicing in 3D slicing software, determine the planned path, and ensure that different material structures do not interfere with each other;

[0025] S2: Select thermoplastic polymer materials with similar melting points and stiffness that meet design requirements;

[0026] S3: Use a multi-nozzle fused deposition extrusion machine to extrude different materials using different nozzles, and appropriately modify the printing G-code at the interface of different materials;

[0027] S4: After the fused deposition process is completed, the temperature of the preparation cavity environment during the post-processing should be kept 60-150°C below the lowest melting point of the flexible material, and left to stand for at least 30 minutes.

[0028] S5: After the sample has slowly cooled to room temperature, remove the prepared structure from the equipment.

[0029] Preferably, the interface between different materials is an irregular contact surface, and variable stiffness sound-absorbing metamaterials are prepared using a photopolymerization molding process, including:

[0030] S1: Construct a multi-material three-dimensional model, select resins with different elastic moduli among flexible photosensitive resins, and prepare the model by mixing different resins according to the design requirements;

[0031] S2: Heterogeneous composite printing is achieved by changing materials during the photopolymerization process. The surface of the workpiece before material change is cleaned with a cleaning solvent.

[0032] S3: After changing the resin material, extend the exposure and curing time of the first layer of the different resin bonding surfaces by 2s-5s to ensure that the resins with different stiffness have good interfacial bonding performance.

[0033] S4: And so on, completing the structural preparation of different materials sequentially from top to bottom.

[0034] As a preferred method, the photopolymerization molding process is used to prepare variable stiffness sound-absorbing metamaterials. For the processing of series variable stiffness sound-absorbing metamaterials, different photosensitive resins are sequentially introduced into the molding tank by marking the position and height of different materials, so as to realize the integrated preparation of series variable stiffness sound-absorbing metamaterials from bottom to top.

[0035] Preferably, the interfaces of different materials can form a connected castable cavity, and variable stiffness sound-absorbing metamaterials can be prepared using additive manufacturing and casting processes, including:

[0036] The boundaries of different materials in the acoustic metamaterial structure are prepared by additive manufacturing. After obtaining the structural boundary contour, the corresponding materials are cast into the prepared cavity using a casting process and allowed to solidify.

[0037] As a preferred method, different parts of different materials are combined and assembled, and the joint surfaces of different parts are completely adhered to each other. The assembly is then allowed to stand for 2-6 hours, and the standing environment should be maintained at room temperature of 15-25℃.

[0038] In another aspect, the present invention provides a variable stiffness sound-absorbing metamaterial prepared by the method described above.

[0039] Inspired by additive manufacturing technology, this invention is applicable to all materials that can be flexibly processed using additive manufacturing, casting, injection molding, and other techniques. It primarily utilizes the different physical properties of various materials and the theoretically arbitrary shaping capabilities of additive manufacturing to achieve functional allocation of different materials within a sound-absorbing metamaterial in the required spatial locations. This not only maximizes the sound absorption frequency band and optimizes the sound absorption effect through material properties, but also significantly reduces the thickness of the metamaterial due to its extremely high material utilization rate. This method for designing and fabricating variable stiffness sound-absorbing metamaterials can meet the design and manufacturing needs of sound-absorbing and sound-insulating materials across any frequency band.

[0040] Compared with the prior art, the present invention has the following advantages:

[0041] 1. Compared with existing single-material, single-stiffness sound-absorbing metamaterials, this invention fully utilizes material properties and increases space utilization during the design process. While improving acoustic performance, it significantly reduces the design thickness and size of the acoustic metamaterial.

[0042] 2. This invention is prepared directly or indirectly through additive manufacturing technology. Compared with traditional composite, foamed, porous, and textile sound-absorbing materials, it has higher precision and therefore better stability and consistency in acoustic performance.

[0043] 3. Traditional sound-absorbing and sound-insulating materials are large and bulky, and the manufacturing process is complex and complicated. The variable stiffness sound-absorbing metamaterial proposed in this invention is small in size, has a simple manufacturing process, produces no intermediate waste, is low in cost, and is environmentally friendly. Attached Figure Description

[0044] The accompanying drawings, which are included to provide a further understanding of the invention and form part of this application, do not constitute an undue limitation of the invention. In the drawings:

[0045] Figures 1(a) and (b) are schematic diagrams of the construction and segmentation of the computer-generated three-dimensional model of the sound-absorbing metamaterial;

[0046] Figure 2 It is a structure consisting of parallel heterogeneous combinations of flexible materials with different elastic moduli;

[0047] Figure 3 It is a structure of flexible materials with different elastic moduli connected in series and heterogeneous;

[0048] Figure 4 It is a complex heterogeneous structure combining flexible materials with different elastic moduli;

[0049] Figure 5 This is a schematic diagram of a multi-material fused deposition process;

[0050] Figure 6 This is a schematic diagram of the molding process for multi-material photocurable heterogeneous materials;

[0051] Figures 7(a)-(c) are schematic diagrams of the preparation method combining additive manufacturing technology and casting technology.

[0052] In Figures 1-7, different cross-sectional lines represent materials and structures with different stiffnesses. 1, 2, 3, and 4 represent different stiffness materials constituting the variable stiffness sound-absorbing metamaterial; 5, 6, 7, and 8 represent different materials used in the fused deposition modeling (FDM) process; 9 represents the variable stiffness sound-absorbing metamaterial processed by the FDM process; 10 represents the base plate of the photocuring equipment; 11 represents the variable stiffness sound-absorbing metamaterial processed by the photocuring process; 12 represents the photocuring resin molding tank; 13 represents the photosensitive resin; and 14 represents the UV display screen. Detailed Implementation

[0053] The present invention will now be described in detail with reference to the accompanying drawings and specific embodiments. The illustrative embodiments and descriptions of the present invention are used to explain the present invention, but are not intended to limit the present invention.

[0054] The present invention relates to a design method and fabrication process for variable stiffness sound-absorbing metamaterials, the specific implementation of which is as follows:

[0055] Step 1, Construction and segmentation of metamaterial structures

[0056] A three-dimensional model of the sound-absorbing metamaterial structure is constructed using a computer. Since the structure of the sound-absorbing metamaterial is composed of different materials, it is necessary to determine the structure of each material composition. Based on the target sound absorption frequency band, a three-dimensional structure of the sound-absorbing metamaterial that can cover the target frequency band is determined. Through computer-aided design, the various parts of the metamaterial are bounded by their interface, and the local structural stiffness of each part of the three-dimensional structure of different materials is determined. Different flexible materials are selected to meet the local structural stiffness. The three-dimensional model is then segmented using the interface of the different materials as the boundary, and the segmented model is then assembled back into the original three-dimensional structure in the computer.

[0057] According to one embodiment, as shown in Figures 1(a) and 1(b), the metamaterial is divided into half, and then divided into four independent models. Different stiffness materials 1, 2, 3, and 4 that constitute the variable stiffness sound-absorbing metamaterial are constructed in sequence and then assembled together by computer.

[0058] Step 2: Based on the metamaterial design structure, select an appropriate processing technology.

[0059] This invention provides three acoustic metamaterials with different structures, which are then processed and prepared using appropriate processing methods.

[0060] Materials with different stiffnesses are used to make flexible materials with different elastic moduli, including flexible polymers, flexible resin textiles, mixtures of prepolymers and photoinitiators (photosensitive resins), and high-strength rigid materials.

[0061] The flexible polymers include styrene-based thermoplastic elastomers (SBS), ethylene-isoprene-styrene (SIS), ethylene propylene diene monomer (EPDM), thermoplastic ethylene propylene diene monomer (TPV), polyolefin thermoplastic elastomers (TPO), thermoplastic polyurethane elastomers (TPU), thermoplastic polyester elastomers (TPEE), polypropylene (PPE), or flexible resin materials.

[0062] Flexible resin materials include: phthalic unsaturated polyester resin, waterborne acrylic resin, copolymer petroleum resin or epoxy resin.

[0063] The prepolymer-photoinitiator mixture is a mixture of prepolymer and photoinitiator in a mass ratio of (96.5-97):(3-3.5).

[0064] Prepolymers include: epoxy acrylate (EA), polyurethane acrylate (PUA), polyester acrylate (PEA), polyether acrylate, or vinyl resin.

[0065] Photoinitiators include: hyperbranched polyurethane acrylate HBP2UA-HMPP, benzophenone, or thioxanthone.

[0066] High-strength, rigid materials include rigid polymers such as ABS, polylactic acid (PLA), and polyetheretherketone (PEEK).

[0067] After modeling and analyzing the designed acoustic metamaterial, the design structure is compared with the Z-axis of the model. Figure 2-4 The structure in the middle, if it only contains Figure 2 The structure can be manufactured using either additive manufacturing or casting processes. Additive manufacturing includes fused deposition modeling (FDM) or photopolymerization. If it contains... Figure 3 , Figure 4 The structure in the middle can be prepared using melt extrusion, but if a photocuring process is required, the process steps need to be designed in advance to facilitate casting.

[0068] Figure 2 As shown, the interface between different materials is an irregular contact surface, and variable stiffness sound-absorbing metamaterials are prepared by fused deposition extrusion molding or photopolymerization molding.

[0069] In Example 1, as Figure 2 As shown, the method for preparing variable stiffness sound-absorbing metamaterials using fused deposition modeling (FDM) extrusion molding includes the following steps:

[0070] S1: Perform 3D modeling of the designed acoustic metamaterial. If the structure is composed of two or more materials, including flexible polymers and flexible resins, it is necessary to model the structures of different materials separately, then combine and slice them in 3D slicing software, and check the slicing path (G-code) to avoid interference between different material structures and the phenomenon of heterogeneous materials being mixed together.

[0071] S2: Based on the design requirements, select polymer materials with similar melting points and stiffness that meet the design requirements, and obtain the ideal heterogeneous material bonding effect by adjusting the environmental and process parameters during the preparation process.

[0072] S3: The designed variable stiffness sound-absorbing metamaterial is prepared using a multi-nozzle melt deposition extrusion machine, such as... Figure 5As shown, different materials were extruded using printheads 5, 6, 7, and 8. By appropriately modifying the G-code at the interface between different materials, the adhesion between the contact surfaces of the different materials was increased. Under the premise of meeting the physical properties of the material design, high process precision and preparation quality were achieved.

[0073] The specific changes to the G-code are as follows: The main changes to the G-code at the interface are: 1. The extrusion amount E at the interface will be increased to 120%-150% based on material properties to enhance the interfacial bonding between different materials. 2. The routing path at the interface will be adjusted to a cross-shaped routing pattern to avoid poor interfacial bonding due to a single path. 3. The routing spacing at the interface interface will be adjusted to between 0.7 and 0.9 times the nozzle diameter.

[0074] Because it is a top-down layered molding process, it can achieve integrated manufacturing processes that traditional processing techniques cannot accomplish, such as encapsulation and embedding of heterogeneous materials.

[0075] S4: After fused deposition modeling (FDM) is completed, to avoid uneven cooling of different materials leading to varying shrinkage and compromised adhesion strength at the bonding surfaces, the ambient temperature of the preparation cavity must be maintained at 60-150°C below the minimum melting point of the flexible material during post-processing. The material should be allowed to stand for at least 30 minutes to prevent shrinkage from affecting acoustic and mechanical properties. After the FDM process is complete, the temperature of the substrate should be allowed to decrease slowly and uniformly.

[0076] S5: After the sample cools to room temperature, remove the prepared structure from the equipment to obtain the variable stiffness sound-absorbing metamaterial 9 processed by fused deposition modeling.

[0077] In Example 2, as Figure 2 As shown, the method for preparing variable stiffness sound-absorbing metamaterials using photopolymerization molding process includes the following steps:

[0078] S1: Construct a multi-material 3D model, select resins with different elastic moduli among flexible photosensitive resins, and use a mixture of prepolymer and photoinitiator to prepare different resins according to the design requirements to obtain different resins with the required elastic moduli.

[0079] S2: Analyze the designed acoustic metamaterial structure; if it is... Figure 2 The series structure allows for material replacement during the photopolymerization process, enabling heterogeneous composite printing. The first layer of resin for the processed part adheres to the base plate 10 of the photopolymerization equipment through UV light irradiation, and then the layers are sequentially deposited from top to bottom. When a material replacement is required during the molding process, the printing platform needs to be raised to the highest point that the printing equipment can be raised. The processed part is then cleaned with an organic cleaning solvent (such as ethanol) to ensure that there is no residual resin on the surface of the part before replacing the resin in the resin tank.

[0080] The specific implementation method is as follows: at the interface where different materials are bonded, after one of the materials is processed and formed, the surface of the part is cleaned with a cleaning solvent to ensure that there is no uncured resin residue inside the part. Then, the resin in the resin tank is replaced, and the remaining structure of the part is processed.

[0081] Different rigid and flexible photosensitive resins are sequentially introduced into the photocuring resin molding tank 12 to realize the integrated preparation of the series variable stiffness sound-absorbing metamaterial 11 from bottom to top.

[0082] S3: Replace resin material Figure 6 After applying the photosensitive resin 13, the exposure and curing time of the first layer at the interface between different resins needs to be extended by 2-5 seconds. This is mainly achieved by controlling the exposure time and the exposed image on the bottom UV display screen 14. This ensures that resins with different stiffnesses have good interfacial bonding performance.

[0083] S4: Following this pattern, after completing the structural preparation of different materials from top to bottom, the product is obtained.

[0084] Figure 3 , Figure 4 As shown, different material bonding surfaces can form a connected castable cavity. Variable stiffness sound-absorbing metamaterials are prepared using additive manufacturing and casting processes.

[0085] Specifically, in Example 3, a method combining additive manufacturing and casting is used, including:

[0086] Analyze the design of acoustic metamaterial structures; if it is Figure 3 , Figure 4 For parallel or complex structures, it is necessary to prepare the boundaries of different materials using additive manufacturing processes. Flexible resin and high-strength rigidity materials are selected to prepare the boundaries. After obtaining the shell of the boundary in Figure 7(a), different materials are cast into the prepared cavity in Figure 7(b) using a casting process and allowed to cure.

[0087] During the pouring process, try to select materials that have good adhesion to the boundary, and pour and assemble them from the inside to the outside to get the material structure shown in Figure 7(c).

[0088] Step 4: Assemble the different parts made of different materials:

[0089] Once the different parts are fully bonded together to form a whole, allow them to stand for at least 2-3 hours. The standing environment should be maintained at room temperature of 15-25℃ to obtain a variable stiffness sound-absorbing metamaterial.

[0090] This invention addresses the problem that single-material sound-absorbing materials have relatively large density and volume, thus failing to effectively address noise pollution. The small-size variable stiffness sound-absorbing material of this invention employs a multi-material heterogeneous combination photopolymerization printing process, which broadens the sound absorption frequency band and optimizes the sound absorption effect. At the same time, it improves the material utilization rate and significantly reduces the thickness of the metamaterial, thus meeting the design and manufacturing needs of sound-absorbing and sound-insulating materials in any frequency band.

[0091] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method of making a variable-stiffness acoustic metamaterial, comprising: The application relates to a variable-rigidity sound-absorbing metamaterial. A three-dimensional model of the sound-absorbing metamaterial is built through a computer, different-material combination surfaces are taken as boundaries to segment the three-dimensional model of the sound-absorbing metamaterial, and assembly recombination is carried out; Different-rigidity materials are selected, different processing technologies are used, and different parts of the sound-absorbing metamaterial are processed respectively; According to different processing methods, different parts of the materials are combined and assembled or integrally formed, the different parts are completely adhered to each other to form an integral whole, and the variable-rigidity sound-absorbing metamaterial is obtained after sufficient time is allowed to stand; The three-dimensional model of the sound-absorbing metamaterial is built through a computer, including: According to a target sound-absorbing frequency band, a three-dimensional structure of the sound-absorbing metamaterial covering the target frequency band is determined, the rigidity of each local structure of the three-dimensional structure of different materials is determined, different flexible materials are selected to meet the rigidity of the local structure; The three-dimensional model is segmented according to the combination surfaces of different materials, and the segmented model is assembled back to the original three-dimensional structure; The different-rigidity materials include flexible polymers, flexible resin materials, pre-polymer and photoinitiator mixtures and high-rigidity materials; The flexible polymers include styrene thermoplastic elastomers SBS, styrene-isoprene-styrene SIS, ethylene-propylene-diene rubber EPDM, thermoplastic ethylene-propylene-diene dynamic vulcanized rubber TPV, polyolefin thermoplastic elastomer TPO, main thermoplastic polyurethane elastomer rubber TPU, thermoplastic polyester elastomer TPEE or polypropylene PPE; The flexible resin materials include ortho-phthalate unsaturated polyester resin, water-based acrylic resin, copolymer petroleum resin or epoxy resin; The pre-polymer and photoinitiator mixture is a mixture of pre-polymer and photoinitiator in a mass ratio of (96.5-97):(3-3.5); The pre-polymer includes epoxy acrylate EA, polyurethane acrylate PUA, polyester acrylate PEA, polyether acrylate or vinyl resin; The photoinitiator includes hyperbranched polyurethane acrylate HBP2UA-HMPP, benzophenone or thioxanthone; The high-rigidity material includes ABS, polylactic acid PLA or polyether ether ketone PEEK; According to the combination surfaces of different materials, additive manufacturing, reverse mold pouring processing technology is used to process different parts of the sound-absorbing metamaterial respectively; The additive manufacturing includes a fused deposition extrusion molding process or a light curing molding process.

2. The method of claim 1, wherein the method further comprises: The different-material combination surfaces are irregular contact surfaces, and the variable-rigidity sound-absorbing metamaterial is prepared by using the fused deposition extrusion molding process, including: S1: three-dimensional modeling of the acoustic metamaterial is carried out, combined slicing is carried out in three-dimensional slicing software, a planned path is determined, and different material structures do not interfere with each other; S2: a thermoplastic polymer material with a similar melting point and a rigidity meeting the design requirement is selected; S3: a multi-nozzle fused deposition extruder is used to extrude different materials by using different nozzles, and the printing G-code is appropriately changed at the different-material combination surfaces; S4: after the fused deposition processing is completed, the internal cavity environment temperature is required to be kept at 60-150 DEG C lower than the minimum melting point of the flexible material during the post-processing process, and the sample is allowed to stand for at least 30 minutes; S5: after the sample is slowly reduced to room temperature, the prepared structure is taken out from the equipment, and the variable-rigidity sound-absorbing metamaterial is obtained.

3. The method of claim 2, wherein the method further comprises: The parts of different materials are assembled in combination, the combination of different parts is completely adhered to each other, and the combination is placed at 15-25℃ for 2-6h.

4. The method of claim 1, wherein the method further comprises: The combination of different materials is an irregular contact surface, and the variable stiffness sound absorption metamaterial is prepared by using a light curing molding process, comprising: S1: constructing a multi-material three-dimensional model, selecting resins with different elastic moduli in flexible photosensitive resin, and mixing different resins according to design requirements to prepare; S2: changing materials to realize heterogeneous composite printing in the light curing molding process, and using a cleaning solvent to clean the surface of the processed part before changing the material; S3: after changing the resin material, the exposure and curing time of the first layer of the combination surface of different resins is extended by 2s-5s, so as to ensure that the resins with different stiffnesses have good interface bonding performance; S4: in this way, the structures of different materials are sequentially prepared from top to bottom.

5. The method of claim 4, wherein the method further comprises: The variable stiffness sound absorption metamaterial prepared by the light curing molding process is used for processing the series variable stiffness sound absorption metamaterial, different photosensitive resins are sequentially introduced into the molding groove by marking the position and height of different materials, and the series variable stiffness sound absorption metamaterial is integrally prepared from bottom to top.

6. The method of claim 1, wherein the variable stiffness acoustic metamaterial is prepared by the steps of: The combination of different materials forms a connected pourable cavity, and the variable stiffness sound absorption metamaterial can be prepared by using additive manufacturing and reverse mold pouring process, comprising: The boundaries of the acoustic metamaterial structure of different materials are prepared by additive manufacturing process, and after obtaining the structure boundary profile, the corresponding material is poured into the prepared cavity by using reverse mold pouring process, and then it is solidified.

7. A variable stiffness sound absorption metamaterial prepared by the method of any one of claims 1-6.

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