Composite material structure and method of manufacturing thereof

Abstract

The present application provides a composite structure and a manufacturing method thereof. The composite structure includes an outer layer, an inner layer, and an intermediate layer. The outer layer includes a metallic material. The inner layer includes a fibrous material and a resin material. The outer layer has a first thickness, the inner layer has a second thickness, and the first thickness is different from the second thickness. The intermediate layer includes an adhesive material and is disposed between the outer layer and the inner layer. Opposite surfaces of the intermediate layer directly contact the outer layer and the inner layer, respectively.

Description

Technical Field

[0001] This invention relates to a composite material structure and its manufacturing method. Background Technology

[0002] To achieve a metallic appearance, composite material structures often consist of metal and resin layers. However, the manufacturing process of composite structures frequently employs thermal processes. This can lead to warping and deformation of the metal and resin layers, which have significantly different coefficients of thermal expansion, due to thermal stress. Furthermore, to address this issue, composite structures are often fabricated as symmetrical mirror structures (films with the same thickness, density, and / or modulus are manufactured at symmetrical locations). However, symmetrical mirror structures significantly increase manufacturing costs. Summary of the Invention

[0003] This invention provides a composite material structure and its manufacturing method, which can effectively balance thermal stress while reducing manufacturing costs.

[0004] The present invention discloses a composite material structure comprising an outer layer, an inner layer, and an intermediate layer. The outer layer comprises a metallic material. The inner layer comprises a fiber material and a resin material. The outer layer has a first thickness, and the inner layer has a second thickness, wherein the first thickness and the second thickness are different. The intermediate layer comprises an adhesive material and is disposed between the outer layer and the inner layer. Two opposing surfaces of the intermediate layer are in direct contact with the outer layer and the inner layer, respectively.

[0005] A method for manufacturing a composite material structure according to the present invention includes at least the following steps: providing an outer layer comprising a metallic material; providing an inner layer comprising a fiber material and a resin material, wherein the outer layer has a first thickness and the inner layer has a second thickness, and the first thickness and the second thickness are different; providing an intermediate layer comprising an adhesive material; and bonding the outer layer and the inner layer through the intermediate layer.

[0006] Based on the above, the composite material structure of the present invention can effectively reduce the use of materials and simplify the process by designing an asymmetrical structure between the outer and inner layers (such as different thicknesses). Furthermore, by selecting the inner layer material, the difference in thermal expansion coefficient between it and the outer layer can be effectively reduced. Therefore, the composite material structure of the present invention can effectively balance thermal stress while reducing manufacturing costs.

[0007] To make the above features and advantages of the present invention more apparent and understandable, specific embodiments are described below in conjunction with the accompanying drawings. Attached Figure Description

[0008] Figure 1A , Figure 1B This is a partially exploded schematic diagram of a composite material structure according to some embodiments of the present invention;

[0009] Figure 1CThis is a cross-sectional schematic diagram of a composite material structure according to an embodiment of the present invention;

[0010] Figure 1D and Figure 1E These are photographs of the inner layer of an optical microscope according to some embodiments of the present invention;

[0011] Figures 2A to 2D This is a schematic diagram of a method for manufacturing a composite material structure according to some embodiments of the present invention;

[0012] Figures 3A to 3D This is a schematic diagram of a method for manufacturing a composite material structure according to some embodiments of the present invention;

[0013] Figures 4A to 4D This is a schematic diagram of a method for manufacturing a composite material structure according to some embodiments of the present invention. Detailed Implementation

[0014] Reference will now be made in detail to exemplary embodiments of the invention, examples of which are illustrated in the accompanying drawings. Wherever possible, the same element references are used in the drawings and description to denote the same or similar parts.

[0015] The preferred embodiments of the present invention will be described in detail below with reference numerals and illustrated with accompanying drawings. Where possible, unnecessary components are omitted from the drawings for clarity. Furthermore, the dimensions of the components in the drawings are drawn for ease of explanation and do not represent actual component size proportions.

[0016] The directional terms used in this article (e.g., up, down, right, left, front, back, top, bottom) are for reference only and are not intended to imply absolute orientation.

[0017] It should be understood that although the terms "first," "second," "third," etc., may be used herein to describe various elements, components, regions, layers, and / or parts, these elements, components, regions, and / or parts should not be limited by these terms. These terms are used only to distinguish one element, component, region, layer, or part from another element, component, region, layer, or part.

[0018] Unless otherwise defined, all terms used herein (including technical and scientific terms) have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains.

[0019] Figure 1A , Figure 1B This is a partially exploded schematic diagram of a composite material structure according to some embodiments of the present invention. Figure 1C This is a cross-sectional schematic diagram of a composite material structure according to an embodiment of the present invention. Figure 1D and Figure 1EThese are photographs of the inner layer of an optical microscope according to some embodiments of the present invention.

[0020] Please refer to Figures 1A to 1B In this embodiment, the composite material structure 100 includes an outer layer 110, an inner layer 120, and an intermediate layer 130. The intermediate layer 130 is disposed between the outer layer 110 and the inner layer 120, and its two opposing surfaces (such as the upper surface 130t and the lower surface 130b) are in direct contact with the outer layer 110 and the inner layer 120, respectively. Furthermore, the outer layer 110 includes a metallic material, allowing the composite material structure 100 to exhibit a metallic appearance. The inner layer 120 includes a fiber material and a resin material, while the intermediate layer 130 includes an adhesive material.

[0021] Furthermore, the first thickness 110D of the outer layer 110 is different from the second thickness 120D of the inner layer 120. Accordingly, the composite material structure 100 of this embodiment, through the asymmetrical structural design between the outer layer 110 and the inner layer 120, can effectively reduce the use of materials and simplify the process. Moreover, by selecting the material of the inner layer 120, the difference in thermal expansion coefficient between it and the outer layer 110 can be effectively reduced. Therefore, the composite material structure of the present invention can effectively balance thermal stress while reducing manufacturing costs.

[0022] Here, since the coefficient of thermal expansion of resin materials is greater than that of metal materials, while the coefficient of thermal expansion of fiber materials (such as high-rigidity materials) is between that of resin materials and metal materials, when the inner layer material is a combination of resin and fiber materials, the fiber material can suppress the thermal expansion and contraction of the resin material, reducing the difference in coefficients of thermal expansion between the resin and metal materials. This reduces residual stress and effectively improves warping deformation. Here, the inner layer 120 may not have any other metal film layers on its surface compared to the outer layer 110.

[0023] In some embodiments, the first thickness 110D is smaller than the second thickness 120D, which can achieve a better balance of thermal stress. For example, the range of the first thickness 110D can be greater than or equal to 0.1 mm and less than or equal to 1 mm (e.g., 0.1 mm, 0.3 mm, 0.5 mm, 0.7 mm, 1 mm or any value within the range of 0.1 mm to 1 mm), and the range of the second thickness can be greater than 0.1 mm and less than or equal to 5 mm (e.g., 2 mm, 3 mm, 4 mm, 5 mm or any value within the range of 0.1 mm to 5 mm), but the present invention is not limited thereto.

[0024] In some embodiments, the outer layer 110 has a first density and a first modulus, and the inner layer 120 has a second density and a second modulus. The first density and the second density are different, and the first modulus and the second modulus are different; for example, the first density is greater than the second density, and the first modulus is greater than the second modulus, but the invention is not limited thereto. It should be noted that the density and modulus will vary depending on the metal material, fiber material, and resin material selected; that is, the values ​​of density and modulus can be inferred based on the selection of the aforementioned materials in the actual design.

[0025] In some embodiments, the metallic materials include aluminum (Al), magnesium (Mg), lithium (Li), titanium (Ti), iron (Fe), or alloys thereof (such as magnesium-aluminum alloys, magnesium-lithium alloys, titanium alloys, or stainless steel (SUS)); the fiber materials include carbon fibers (which may be any suitable carbon fibers), glass fibers, plant fibers, or combinations thereof; and the resin materials include thermoplastic resins and thermosetting resins, wherein the thermoplastic resins include polycarbonate (PC), polyethylene terephthalate (PET), polymethyl methacrylate (PMMA), polyethylene (PE), propylene-butadiene-styrene resin (ABS), polypropylene (PP), polystyrene (PS), and polyamide (PA), and the thermosetting resins include epoxy resins and phenolic resins, but the invention is not limited thereto.

[0026] In some embodiments, the inner layer 120 can be more easily recycled when only one type of fiber material is used, but the invention is not limited thereto.

[0027] In some embodiments, the fiber material may be a discontinuous fiber formed by cutting long fibers into multiple segments of short fibers (the length of the fiber material is, for example, between 5 mm and 25 mm), but the invention is not limited thereto.

[0028] In some embodiments, the weight percentage of the fiber material in the inner layer 120 is between 10 wt% and 60 wt% (e.g., 10 wt%, 20 wt%, 40 wt%, 60 wt%, or any value within the range of 10 wt% to 60 wt%). For example, the inner layer 120 may consist solely of fiber material and resin material. Thus, when the weight percentage of the fiber material is 10 wt%, the weight percentage of the resin material is 90 wt%; when the weight percentage of the fiber material is 40 wt%, the weight percentage of the resin material is 60 wt%; and when the weight percentage of the fiber material is 60 wt%, the weight percentage of the resin material is 40 wt%. With the above-mentioned ratio of fiber material to resin material, the ratio of the X-axis flexural modulus to the Y-axis flexural modulus (X-axis flexural modulus / Y-axis flexural modulus) of the inner layer 120 can be effectively controlled to be less than 200% (measured according to ASTM D790), while also reducing the probability of problems such as poor impregnation bonding (e.g., ...). Figure 1D In wet carbon fiber mat (fiber area weight, FAW): 68 g / m² 2 In this case, the amount of resin material (PC) added is 72 wt%, and the amount of carbon fiber added is 28 wt%, as shown in the figure. Figure 1E In dry carbon fiber felt (FAW: 159g / m) 2 In the process of producing the fiber material, good impregnation effects are observed when the resin material content is 69 wt% and the carbon fiber content is 31 wt%, but the present invention is not limited thereto. Here, during the fabrication of the fiber material, the orientation of the fibers (corresponding to the water flow direction in the reprocessing process) can be controlled to allow the longitudinal direction (machine direction, MD) and the transverse direction (transverse direction, TD) to have different moduli, where the longitudinal direction can be the X-axis and the transverse direction is the Y-axis.

[0029] Furthermore, since the fiber materials are arranged in the same direction (e.g., all arranged in the X-axis (strong axis) direction), it is easy to cause breakage in the Y-axis (weak axis). However, in this embodiment, the weight ratio of the fiber materials is controlled between 10wt% and 60wt%. Even when the arrangement ratio is greatly different, such as when all are arranged in the X-axis (strong axis) direction, the ratio of the X-axis bending modulus to the Y-axis bending modulus can still be controlled to be less than 200%. That is, under this ratio, the Y-axis breakage is less likely to occur. However, the present invention is not limited to this. In other embodiments, the fiber materials can also use non-isotropic fiber arrangement, such as some arranged in the X-axis direction and some arranged in the Y-axis direction. In this way, the probability of Y-axis breakage can be further reduced.

[0030] In some embodiments, the fiber material accounts for a weight ratio of 20wt% to 40wt% of the inner layer 120, and the weight of the inner layer 120 is equal to the total weight of the fiber material and the resin material, which can have a better control effect, but the present invention is not limited thereto.

[0031] In some embodiments, the amount of fiber material added and the coefficient of thermal expansion of the inner layer 120 exhibit a non-linear inverse relationship, but the present invention is not limited thereto.

[0032] exist Figure 1A In this embodiment, the inner layer 120 is only a single layer structure, but the present invention is not limited thereto. In other embodiments, such as Figure 1C As shown, the inner layer 120 can consist of multiple sublayers ( Figure 1C The invention schematically illustrates a structure consisting of three layers (or two, four, or more), wherein each sublayer may have the same composition and thickness to further enhance the thermal stability of the composite material structure, but the invention is not limited thereto.

[0033] In some embodiments, the intermediate layer 130 comprises an adhesive film selected from epoxy resin, polyether polyol, polyurethane (PU), or combinations thereof, but the invention is not limited thereto, and the intermediate layer 130 may be any suitable adhesive material.

[0034] In some embodiments, the third thickness 130D of the intermediate layer 130 is between 0.01 mm and 0.3 mm (e.g., 0.01 mm, 0.05 mm, 0.1 mm, 0.3 mm or any value within the range of 0.01 mm to 0.3 mm mentioned above), but the invention is not limited thereto.

[0035] In some embodiments, the glass softening temperature of the intermediate layer 130 is between 65°C and 180°C (e.g., 65°C, 80°C, 140°C, 180°C or any value within the range of 65°C to 180°C), so that it can be effectively softened when a low-temperature hot pressing process is subsequently used, but the present invention is not limited thereto.

[0036] In some embodiments, the glass softening temperature of the intermediate layer 130 is between 65°C and 90°C, which can be closer to the low-temperature process, but the present invention is not limited thereto.

[0037] In some embodiments, such as Figure 1BAs shown by the dotted lines, structural member 140 can optionally be formed on inner layer 120. The material of structural member 140 can be the same fiber and resin type as inner layer 120 to facilitate subsequent recycling. For example, if inner layer 120 is carbon fiber felt impregnated with PC resin, the injection-molded structural member 140 is preferably made of PC and carbon fiber injection molding material, but the invention is not limited thereto. Here, structural member 140 can have different shapes and forms depending on actual product requirements, and the invention is not limited thereto. For example… Figure 1B The state of the electronic casing is shown.

[0038] The following will describe in more detail specific embodiments of the composite material structure of the present invention. However, it should be noted that although the following embodiments are described, the details of the materials used and the process, etc., may be appropriately changed without departing from the scope of the present invention, and the present invention should not be interpreted as limiting based on the embodiments described below.

[0039] As shown in Table 1, when PC resin and carbon fiber are used as the inner layer 120, their coefficients of thermal expansion can be effectively reduced to those of the outer layer 110, even at different thicknesses. Therefore, it is not necessary to form an additional matching metal layer on the other surface of the inner layer 120 relative to the outer layer 110, which effectively reduces material usage and simplifies the process. Furthermore, in Table 1, the weight concentration of carbon fiber added is 30 wt% (all arranged in the X-axis direction), and the weight concentration of PC resin added is 70 wt%. According to ASTM D790, the X-axis flexural modulus is 19.5 GPa, and the Y-axis flexural modulus is 9.93 GPa. Therefore, the ratio of the X-axis flexural modulus to the Y-axis flexural modulus (X-axis flexural modulus / Y-axis flexural modulus) is approximately 196%, which can be effectively controlled below 200%.

[0040] Table 1 presents the coefficient of thermal expansion along the Y-axis, measured using a TMA (Thermomechanical Analysis) instrument of model TA Q400-EM. Other relevant values ​​are omitted to clearly illustrate the invention, and these values ​​should be those that can be measured by extending the scope of the invention as disclosed herein by those skilled in the art.

[0041] Table 1

[0042] Material condition Coefficient of thermal expansion (μm / m·℃) Thickness (mm) aluminum alloy 25℃～160℃ 25.55 0.3 carbon fiber and PC resin 25℃～160℃ 24.86 0.7 PC resin 25℃～160℃ 70.2 0.04

[0043] It should be noted that the first thickness 110D, the second thickness 120D and the third thickness 130D mentioned above can be uniform thicknesses, that is, the distance between the two ends of the vertical line connecting each position of the outer layer 110, the inner layer 120 and the intermediate layer 130 is the same. However, the present invention is not limited to this. In the embodiment not shown, the first thickness, the second thickness and the third thickness can also be the maximum thickness among the outer layer, the inner layer and the intermediate layer.

[0044] The main manufacturing process of composite material structures according to some embodiments of the present invention is described below with reference to the accompanying drawings. It should be noted that the component reference numerals and some contents of the above embodiments are used below, wherein the same or similar reference numerals are used to represent the same or similar components, and the description of the same technical content is omitted. For the description of the omitted parts, please refer to the above embodiments, and will not be repeated here.

[0045] Figures 2A to 2D This is a schematic diagram of a method for manufacturing a composite material structure according to some embodiments of the present invention.

[0046] Please refer to Figure 2A In one embodiment, an outer layer 110 comprising a metallic material is first provided; an inner layer 120 comprising a fiber material and a resin material is provided; and an intermediate layer 130 comprising an adhesive material is provided. Then, the outer layer 110 and the inner layer 120 are bonded together via the intermediate layer 130 (e.g., through a low-temperature hot-press bonding process, wherein the temperature is, for example, below 100°C). Here, the outer layer 110, the intermediate layer 130, and the inner layer 120 may have the same width, and the outer layer 110 may completely cover the surfaces of the intermediate layer 130 and the inner layer 120. However, the invention is not limited to this; in another embodiment, the intermediate layer 130 and the inner layer 120 have the same width, but the width of the inner layer 120 is smaller than the width of the outer layer 110.

[0047] Please refer to Figure 2B The mold 10 is used to further hot press the joined structure, wherein the hot pressing temperature is, for example, greater than the glass transition temperature (Tg) of the resin material in the inner layer 120, such as a temperature greater than 100°C.

[0048] Please refer to Figure 2C Remove the mold 10 and perform a trimming process so that the edges 110e of the outer layer 110, the edges 130e of the middle layer 130 and the edges 120e of the inner layer 120 can be cut flush. However, the invention is not limited to this, and this step can be performed optionally.

[0049] Please refer to Figure 2D and Figure 1B The structural part 140 is formed by injection molding using mold 20. Then, removing mold 20 yields a product such as... Figure 1B The composite material structure 100 shown.

[0050] Figures 3A to 3D This is a schematic diagram of a method for manufacturing a composite material structure according to some embodiments of the present invention.

[0051] Please refer to Figure 3A and Figure 3B In one embodiment, an outer layer 110 comprising a metallic material is first provided; an inner layer 120 comprising a fiber material and a resin material and an intermediate layer 130 comprising an adhesive material are provided, wherein before joining the outer layer 110 and the inner layer 120, the outer layer 110 is subjected to a stamping process, on the one hand to form the desired shape and the space S (e.g., space S) that can subsequently cover the inner layer 120. Figure 3A (As shown), on the other hand, the inner layer 120 and the intermediate layer 130 can be joined and a preheating process can be performed using a suitable heating device 30 (e.g. Figure 3B As shown), the inner layer 120 can be a thermoplastic resin. Furthermore, the preheating temperature is, for example, greater than the glass transition temperature of the resin material in the inner layer 120, such as a temperature greater than 100°C.

[0052] Please refer to Figure 3C The stamped outer layer 110 is placed into the mold 40 for positioning, and the preheated inner layer 120 and intermediate layer 130 are placed between the upper and lower halves of the mold 40. Then, the inner layer 120 and intermediate layer 130 are pressed into the mold 40 for hot pressing forming, and then the mold 40 is removed.

[0053] Please refer to Figure 3D and Figure 1B After removing mold 40, injection molding is performed using mold 50 (similar to mold 20) to form structural part 140. Then, removing mold 50 allows for the formation of... Figure 1B The composite material structure 100 shown.

[0054] Figures 4A to 4D This is a schematic diagram of a method for manufacturing a composite material structure according to some embodiments of the present invention.

[0055] In one implementation, please refer to Figures 4A to 4D and Figure 1B Provides an inner layer 120 and an intermediate layer 130 comprising fiber material and resin material (e.g., Figure 4A As shown), before joining the outer layer 110 and the inner layer 120, the inner layer 120 and the intermediate layer 130 are subjected to a hot pressing forming process using a mold 60 (e.g., Figure 4B (As shown), then, remove mold 60 and perform a trimming process (as shown). Figure 4CAs shown), the edges of the outer layer 110 and the middle layer 130 can be trimmed, but the invention is not limited thereto, and this step can be performed optionally. Then, an injection molding process is performed using a mold 70 (similar to molds 20 and 50) to form the structural part 140. Next, the mold 70 is removed.

[0056] On the other hand, an outer layer 110 comprising a metallic material is provided; wherein, before joining the outer layer 110 and the inner layer 120, the outer layer 110 is subjected to a stamping process to form the desired shape and the space S that can subsequently cover the inner layer 120, such as... Figure 3A As shown. Then, after removing the mold 70, the formed inner layer 120 and intermediate layer 130 are joined as shown. Figure 3A The outer layer 110 formed can be heat-pressed to form a shape like... Figure 1B The composite material structure 100 shown.

[0057] It should be noted that those skilled in the art to which the above-mentioned molds 10, 20, 40, 50, 60, and 70 pertain, can design and select them according to the actual process and the required product under the spirit of this invention. This invention does not impose any limitations and will not be elaborated upon here.

[0058] In summary, the composite material structure of the present invention can effectively reduce material usage and simplify the process by designing an asymmetrical structure between the outer and inner layers (such as different thicknesses). Furthermore, the selection of the inner layer material can effectively reduce the difference in thermal expansion coefficient between it and the outer layer. Therefore, the composite material structure of the present invention can effectively balance thermal stress while reducing manufacturing costs.

[0059] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A composite material structure, characterized in that, include: The outer layer includes metallic materials; The inner layer comprises fiber material and resin material, wherein the outer layer has a first thickness, the inner layer has a second thickness, the first thickness and the second thickness are different, and the ratio of the X-axis flexural modulus to the Y-axis flexural modulus of the inner layer is less than 200%; as well as An intermediate layer, comprising an adhesive material and disposed between the outer layer and the inner layer, wherein two opposing surfaces of the intermediate layer are in direct contact with the outer layer and the inner layer, respectively.

2. The composite material structure according to claim 1, characterized in that, The first thickness is less than the second thickness.

3. The composite material structure according to claim 1, characterized in that, The first thickness range is greater than or equal to 0.1 mm and less than or equal to 1 mm, and the second thickness range is greater than 0.1 mm and less than or equal to 5 mm.

4. The composite material structure according to claim 1, characterized in that, The outer layer has a first density and a first modulus, and the inner layer has a second density and a second modulus, wherein the first density and the second density are different, and the first modulus and the second modulus are different.

5. The composite material structure according to claim 1, characterized in that, The edges of the outer layer, the middle layer, and the inner layer are aligned.

6. The composite material structure according to claim 1, characterized in that: The outer layer, the middle layer, and the inner layer have the same width; or The intermediate layer and the inner layer have the same width, and the width of the inner layer is smaller than the width of the outer layer.

7. The composite material structure according to claim 1, characterized in that, The length of the fiber material is between 5 mm and 25 mm.

8. The composite material structure according to claim 1, characterized in that, The fiber material accounts for between 10 wt% and 60 wt% of the weight of the inner layer.

9. The composite material structure according to claim 1, characterized in that, The metallic material includes aluminum, magnesium, lithium, titanium, iron, or alloys thereof.

10. The composite material structure according to claim 1, characterized in that, The fiber material includes carbon fiber, glass fiber, plant fiber, or a combination thereof.

11. The composite material structure according to claim 1, characterized in that, The resin material includes polycarbonate, polyethylene terephthalate, polymethyl methacrylate, polyethylene, propylene-butadiene-styrene resin, polypropylene, polystyrene, polyamide, epoxy resin, or phenolic resin.

12. The composite material structure according to claim 1, characterized in that, The inner layer is composed of one or more sub-layers.

13. The composite material structure according to claim 1, characterized in that, The intermediate layer comprises a film selected from epoxy resin, polyether polyol, polyurethane, or combinations thereof.

14. The composite material structure according to claim 1, characterized in that, The intermediate layer has a third thickness, which is between 0.01 mm and 0.3 mm.

15. The composite material structure according to claim 1, characterized in that, The glass softening temperature of the intermediate layer is between 65°C and 180°C.

16. A method for manufacturing a composite material structure, characterized in that, include: Provides an outer layer including metallic materials; An inner layer comprising fiber material and resin material is provided, wherein the outer layer has a first thickness, the inner layer has a second thickness, the first thickness and the second thickness are different, and the ratio of the X-axis flexural modulus to the Y-axis flexural modulus of the inner layer is less than 200%; Provides an intermediate layer including adhesive material; as well as The outer layer and the inner layer are joined by the intermediate layer.

17. The manufacturing method according to claim 16, characterized in that, The joined structure is then subjected to a hot pressing forming process.

18. The manufacturing method according to claim 16, characterized in that, Before joining the outer layer and the inner layer, the outer layer is stamped, and the inner layer and the intermediate layer are joined and preheated.

19. The manufacturing method according to claim 16, characterized in that, Before joining the outer layer and the inner layer, the outer layer is stamped, and the inner layer and the intermediate layer are hot-pressed.

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