Method for manufacturing honeycomb sandwich structure and said structure

Abstract

Provided is a novel method for manufacturing a honeycomb sandwich structure in which a honeycomb core is sandwiched between CFRP layers. The method comprises: (i) charging a mold for primary molding with a honeycomb core and at least two first prepregs; (ii) obtaining a primary molded article of a honeycomb sandwich structure in which the honeycomb core is sandwiched between two first CFRP layers by pressurizing and heating the charged mold for primary molding with a press machine; (iii) charging a mold for secondary molding with the primary molded article and at least two second prepregs; and (iv) obtaining a secondary molded article of the honeycomb sandwich structure in which the honeycomb core is sandwiched between the two first CFRP layers and second CFRP layers are stacked on the two respective first CFRP layers by pressurizing and heating the charged mold for secondary molding with the press machine.

Description

Method for manufacturing a honeycomb sandwich structure and the structure

[0001] The present invention relates to a method for manufacturing a honeycomb sandwich structure and the structure. This application claims priority based on Japanese Patent Application No. 2022-133293 filed on August 24, 2022, and Japanese Patent Application No. 2023-070926 filed on April 24, 2023, the contents of which are incorporated herein by reference.

[0002] In recent years, the development of unmanned aerial vehicles (UAVs) has been vigorously pursued, and there is a demand for them to be able to fly at higher altitudes, i.e., in spaces with lower air density. One way to achieve this is to rotate the wings at a higher speed to generate greater lift, which requires increasing the rigidity of the wings. On the other hand, because high-speed wing rotation increases noise, one possible solution to achieve the above objective is to thicken the wings to increase the lift per rotation speed. However, the increased weight associated with thickening the wings offsets the increased lift. Therefore, there is a demand for thicker wings without significantly increasing weight. To meet this demand, a honeycomb sandwich-structured wing has been proposed (Patent Document 1), in which a honeycomb core is sandwiched between layers of CFRP (Carbon Fiber Reinforced Plastics).

[0003] Japanese Patent Application Laid-Open No. 2021-112849

[0004] Patent Document 1 describes a method for manufacturing a laminate, which includes a first step of pressing a punching die from a direction intersecting the cell opening plane of a honeycomb material to manufacture the punched honeycomb structure, and a second step of laminating a first skin layer before hardening or solidification and a second skin layer before hardening or solidification on the honeycomb structure obtained by punching a honeycomb core, and curing or solidifying the first skin layer before hardening or solidification and the second skin layer before hardening or solidification. In this method, because the first skin layer and the second skin layer are laminated on a previously obtained honeycomb structure, depending on the shape of the honeycomb structure, wrinkles may occur in the first skin layer and the second skin layer during lamination, or the pressure on the surface of the laminate may not be uniform. Patent Document 1 also describes a method for manufacturing a laminate in which cured or solidified third and fourth skin layers and a punched honeycomb core are prepared in advance, and the honeycomb core, prepreg, third and fourth skin layers are charged into a mold, compressed, and heated to obtain a honeycomb sandwich structure. This method produces third and fourth skin layers with few surface voids, but voids generated from the prepreg cannot pass through the third or fourth skin layer and the prepreg, so voids tend to accumulate between the third or fourth skin layer and the prepreg. Furthermore, if the mold used in the honeycomb sandwich structure manufacturing process does not match the shapes of the third and fourth skin layers due to shrinkage during curing or solidification of the third and fourth skin layers, thickness variations may occur in the first and second skin layers. Furthermore, there are four separate processes, namely, a third skin layer manufacturing process, a fourth skin layer manufacturing process, a honeycomb core punching process, and a honeycomb sandwich structure manufacturing process, which require a lot of time and effort, resulting in poor productivity and high production costs.

[0005] The objective of this invention is to provide a novel method for manufacturing a honeycomb sandwich structure in which a honeycomb core is sandwiched between CFRP layers. Another objective is to provide a honeycomb sandwich structure with excellent rigidity. Another objective is to provide a honeycomb sandwich structure with good appearance.

[0006] One aspect of the present invention is a method for manufacturing a honeycomb sandwich structure, including: (i) charging a primary molding die with a honeycomb core and at least two first prepregs; (ii) applying pressure and heat to the charged primary molding die in a press to obtain a primary molded product having a honeycomb sandwich structure in which the honeycomb core is sandwiched between two first CFRP layers; (iii) charging a secondary molding die with the primary molded product and at least two second prepregs; and (iv) applying pressure and heat to the charged secondary molding die in a press to obtain a secondary molded product having a honeycomb sandwich structure in which the honeycomb core is sandwiched between the two first CFRP layers and a second CFRP layer is laminated on each of the two first CFRP layers.

[0007] Another aspect of the present invention is a honeycomb sandwich structure in which a honeycomb core is sandwiched between CFRP layers, and the value A obtained by the following measurement method is 0.01 or more and 0.10 or less. (Method for measuring value A) A cross section of the thickest part of the honeycomb sandwich structure cut parallel to the thickness direction of the honeycomb core is imaged. Using image processing software ImageJ, the outer surface coordinates of the honeycomb sandwich structure and the innermost surface coordinates of the CFRP layers are obtained for each pixel within a 20 mm range in a direction perpendicular to the thickness direction of the honeycomb core in the image of the cross section. For each point of the outer surface coordinates, the distance between the outer surface coordinates and the innermost surface coordinates located in the normal direction to the outer surface at the outer surface coordinates is measured, and the standard deviation D of the distance is calculated. Ａ and the average value of the distances D Ｂ is calculated, and the value A is calculated from the following formula (1): A = D Ａ / D Ｂ ...Equation (1) As described above, the value A is measured at the thickest part of the honeycomb sandwich structure. Even if the value A is outside the above range at parts other than the thickest part, it can be considered to fall under this embodiment as long as the value A at the thickest part satisfies the above range.

[0008] Another aspect of the present invention is the honeycomb sandwich structure described above, wherein the UD layer is disposed adjacent to the outer surface side of the honeycomb sandwich structure relative to the fabric layer, and the value B obtained by the following measurement method is 0.01 or more and 0.10 or less. (Method for measuring value B) A cross section of the thickest part of the honeycomb sandwich structure cut parallel to the thickness direction of the honeycomb core is imaged. Using image processing software ImageJ, the outer surface coordinates of the honeycomb sandwich structure and the boundary surface coordinates between the fabric layer and the UD layer are obtained for each pixel within a 20 mm range in the direction perpendicular to the thickness direction of the honeycomb core in the image of the cross section. For each point of the outer surface coordinates, the distance between the outer surface coordinates and the boundary surface coordinates located in the normal direction to the outer surface at the outer surface coordinates is measured, and the standard deviation D of the distance is calculated. Ｃ and the average value of the distances D Ｄ and calculate the value B from the following formula (2): B = D Ｃ / D Ｄ ...Equation (2) As described above, the value B is measured at the thickest part of the honeycomb sandwich structure. Even if the value B is outside the above range at parts other than the thickest part, it can be considered to correspond to this embodiment as long as the value B at the thickest part satisfies the above range.

[0009] Another aspect of the present invention is a honeycomb sandwich structure in which a honeycomb core is sandwiched between CFRP layers, the CFRP layers including woven fabric layers having a woven fiber reinforcement material, and the standard deviation of the distances between the woven fabric layers and the woven fabric yarns in each segment layer, as determined by the following measurement method, is 30 μm or less. (Method for measuring the standard deviation of the distances) When one of the orthogonal woven fabric yarns of the woven fabric is designated as yarn A and the other as yarn B, a cross section of the CFRP layer cut in the orthogonal direction of yarn B is imaged, and the cross section is binarized using image processing software ImageJ to calculate the position of the center of gravity of each of the yarns B. In each of the segment layers on the surface side of yarn A in the woven fabric layer and the segment layer on the honeycomb core side, the shortest distances from the position of the center of gravity to the outer surface of the honeycomb sandwich structure are determined for each of the five adjacent yarns B, and the standard deviation is calculated.

[0010] Another aspect of the present invention is a honeycomb sandwich structure in which a honeycomb core is sandwiched between CFRP layers, and at least one of the CFRP layers sandwiching the honeycomb core has a woven fabric layer having a woven fiber reinforcement material disposed in the CFRP layer closest to the honeycomb core, and the standard deviation of the distances between the woven fabric yarns of each dividing layer, as determined by the following measurement method for the woven fabric layer, is 30 μm or less. (Method for measuring the standard deviation of the distances) One of the orthogonal woven fabric yarns closest to the honeycomb core is designated as Thread A, and the other is designated as Thread B. An image of a cross section of the CFRP layer cut in the direction perpendicular to Thread B is captured, and the cross section is binarized using image processing software ImageJ, and the position of the center of gravity of the cross section of each Thread B is calculated. In each of the split layers on the surface side of the thread A in the fabric layer closest to the honeycomb core side and the split layer on the honeycomb core side, the shortest distance from the center of gravity position to the outer surface of the honeycomb sandwich structure is determined for each of the five adjacent threads B, and the standard deviation is calculated.

[0011] Another aspect of the present invention is a honeycomb sandwich structure having a honeycomb core sandwiched between CFRP layers, the CFRP layers including woven fabric layers having a woven fiber reinforcement, wherein the standard deviation of the angle measured by the following measurement method for the woven fabric yarns of the woven fabric layers is 1.5 degrees or less. (Method for measuring the standard deviation of the angle) When one of the orthogonal woven fabric yarns of the woven fabric is designated as Yarn A and the other as Yarn B, a cross section of the CFRP layer cut in the orthogonal direction of Yarn B is imaged, the cross section is binarized using image processing software ImageJ, and the position of the center of gravity of each Yarn B cross section is calculated. For each of 10 adjacent Yarns B in the woven fabric layers, the angle formed by the major axis direction of an ellipse-approximated cross section of Yarn B and a reference line that is parallel to the tangent to the outer surface of the honeycomb sandwich structure at the point of closest point to Yarn B on the outer surface curve and passes through the center of gravity of Yarn B is calculated, and the standard deviation is calculated.

[0012] A novel method for manufacturing a honeycomb sandwich structure in which a honeycomb core is sandwiched between CFRP layers is provided. The honeycomb sandwich structure also has excellent rigidity and a good appearance.

[0013] FIG. 1 is a cross-sectional view of a wing for an unmanned aerial vehicle that can be manufactured using a manufacturing method according to an embodiment. FIG. 2 shows a honeycomb core that can be used in the manufacturing method according to an embodiment. FIG. 2(A) is a perspective view, and FIG. 2(B) is a plan view. FIG. 3 is a cross-sectional view showing an example of charging a mold for primary molding with a honeycomb core and a first prepreg. FIG. 4 is a cross-sectional view showing an example of charging a mold for primary molding with a honeycomb core and a first prepreg. FIG. 5 is a cross-sectional view showing a example of charging a mold for secondary molding with a primary molded product and a second prepreg. FIG. 6 is a cross-sectional view showing a schematic view inside a mold during primary molding. FIG. 7 is a cross-sectional view showing a schematic view inside a mold during secondary molding. FIG. 8 is a cross-sectional view showing an example of a portable mold that can be used in the manufacturing method according to an embodiment. FIG. 9 is a cross-sectional view showing an example of a press that can be used in the manufacturing method according to an embodiment. FIG. 10 is a plan view showing the lower plate of the press. FIG. 11 is a plan view (viewed from below) showing the upper plate of the press. FIG. 12 is a cross-sectional view showing a state in which a charged portable mold is pressed by a press. FIG. 13 is a cross-sectional view showing an example of a press that can be used in the manufacturing method according to the embodiment. FIG. 14 is a cross-sectional view showing an example of a press that can be used in the manufacturing method according to the embodiment. FIG. 15 is an example of a cross-sectional view in which a cross section of a woven fabric layer of a honeycomb sandwich structure according to Experimental Example 1 has been binarized. FIG. 16 is an example of a cross-sectional view in which a cross section of a woven fabric layer of a honeycomb sandwich structure according to Experimental Example 2 has been binarized. FIG. 17 is an explanatory diagram showing the angle between the long axis direction of a fiber bundle cross section (cross section of yarn B) and a reference line. FIG. 18 is an explanatory diagram of a CFRP layer including a woven fabric layer.

[0014] 1. Honeycomb sandwich structure The manufacturing method according to the embodiment relates to a method for manufacturing a honeycomb sandwich structure, and can be preferably used, for example, to manufacture a wing for an unmanned aerial vehicle having a honeycomb sandwich structure. Figure 1 is a cross-sectional view of a wing for an unmanned aerial vehicle that can be manufactured using the manufacturing method according to the embodiment. The wing 1 has a honeycomb sandwich structure, and has a structure in which a honeycomb core 13 is sandwiched between two CFRP layers 10, each consisting of a first CFRP layer 11 and a second CFRP layer 12.

[0015] The honeycomb core 13 is processed into a predetermined shape by cutting or pressing as disclosed in Japanese Patent Application Laid-Open No. 2021-112849. The material of the metal foil constituting the honeycomb core 13 is an aluminum alloy (typically A5052, an Al-Mg aluminum alloy, or A3003, an Al-Mn aluminum alloy), but may also be other metals, such as stainless steel.

[0016] The cell size and thickness of the honeycomb core are defined as shown in Figure 2. The cell size d of the honeycomb core 13 is preferably 2 to 20 mm. The foil thickness of the metal foil that constitutes the honeycomb core 13 is preferably 20 to 120 µm. The maximum thickness C of the honeycomb core 13 is, for example, 1 to 50 mm, preferably 2 to 10 mm, and more preferably 2 to 7 mm.

[0017] The first CFRP layer 11 and the second CFRP layer 12 are made of CFRP. CFRP is a composite material in which a fiber reinforcement material containing carbon fibers is impregnated with a cured thermosetting resin composition. The honeycomb core 13 may penetrate into the first CFRP layer 11, in which case, cutting of the carbon fibers may occur at the penetrated portions. The honeycomb core 13 does not penetrate into the second CFRP layer 12, and cutting of the carbon fibers by the honeycomb core 13 does not occur in the second CFRP layer 12. The first CFRP layer 11 may contain more voids than the second CFRP layer 12. The second CFRP layer 12 does not contain voids, or if it does contain voids, the number of voids is small.

[0018] Honeycomb sandwich structures that can be manufactured using the manufacturing method according to the embodiment are not limited to wings for unmanned aerial vehicles. The manufacturing method according to the embodiment can also be preferably used to manufacture honeycomb sandwich structures used as components for space equipment, household appliances, automobiles, ships, etc., or as building materials. In honeycomb sandwich structures manufactured using the manufacturing method according to the embodiment, the specifications of the honeycomb core, such as the material, cell size, foil thickness, and maximum thickness, are determined appropriately depending on the application. Honeycomb cores can be metal honeycomb cores made of metals such as aluminum alloys and stainless steel, as well as paper honeycomb cores and fiber-reinforced plastics (FRP) honeycomb cores, such as aramid honeycomb cores made of aramid fiber paper impregnated with phenolic resin. The honeycomb core is preferably charged in a flat state with a first prepreg into a mold and then molded into the desired shape together with the first prepreg by pressing. The honeycomb core may also be pre-formed into the desired shape or a shape close to the desired shape, and then charged into the mold with the first prepreg.

[0019] As will be described later, in the honeycomb sandwich structure of this embodiment, the first prepreg is cured in the primary molding, and then the second prepreg is cured in the secondary molding. During the primary molding, the first prepreg is pressed against the mold by the honeycomb core, so there are some parts of the first prepreg that are pressed against the mold and some parts that are not. On the other hand, during the secondary molding, the second prepreg is pressed against the mold over the entire surface of the cured first molded product, so it is possible to reduce the disorder of the orientation of the fiber reinforcement material.

[0020] A honeycomb sandwich structure manufactured using a manufacturing method according to one embodiment has a honeycomb core sandwiched between CFRP layers, each of which includes a woven fabric layer having a woven fiber reinforcement material. The standard deviation of the distances between the woven fabric layers and the woven fabric yarns in each of the segment layers, as measured by the following method, is 30 μm or less. (Method for measuring the standard deviation of the distances) One of the orthogonal woven fabric yarns in the woven fabric is designated as yarn A, and the other is designated as yarn B. A cross section of the CFRP layer cut in the orthogonal direction of yarn B is imaged, and the cross section is binarized using image processing software ImageJ to calculate the center of gravity of each of the yarns B. In each of the segment layers on the surface side of yarn A in the woven fabric layer and the segment layer on the honeycomb core side, the shortest distances from the center of gravity to the outer surface of the honeycomb sandwich structure are determined for each of the five adjacent yarns B, and the standard deviation is calculated. In addition, it is usually not known which of the perpendicularly intersecting fabric threads in the CFRP layer is the warp thread and which is the weft thread, but either the warp thread or the weft thread may be designated as thread A or thread B, and in either case, the standard deviation of the distance is 30 μm or less.

[0021] When the CFRP layer includes multiple fabric layers, the standard deviation of the distances measured for the fabric yarns of each of the divided layers of each fabric layer is 30 μm or less. If the standard deviation of the distances is 30 μm or less, the fabric yarns are arranged along the outer surface of the honeycomb sandwich structure, and the strength of the honeycomb sandwich structure is likely to be uniform. From these viewpoints, the standard deviation of the distances is preferably 25 μm or less, and more preferably 20 μm or less.

[0022] The woven fabric layer may be present on at least one of the CFRP layers sandwiching the honeycomb core. The woven fabric layer is preferably present in the CFRP layer closest to the honeycomb core. The woven fabric layer placed in the CFRP layer closest to the honeycomb core is pressed against the mold in the thickness direction of the honeycomb core, which tends to reduce fiber disorder. Aside from the woven fabric layer closest to the honeycomb core, it is also preferable for the woven fabric layer to be present in the outermost layer of the CFRP layer. Placing the woven fabric layer in the outermost layer tends to improve the appearance. In a CFRP layer including a woven fabric layer, woven fiber reinforcement can also be combined with unidirectional (UD) fiber reinforcement or nonwoven fiber reinforcement. The appropriate materials can be selected depending on the desired physical properties.

[0023] Figure 18 is an explanatory diagram showing a CFRP layer including four woven fabric layers as an example of a CFRP layer including woven fabric layers. The divided layers of the woven fabric layers refer to the respective layers when each woven fabric layer is divided into layers A, with the yarn B on the surface side of the yarn A and the yarn B on the honeycomb core side of the yarn A. The woven fabric layers in Figure 18 are numbered 1, 2, 3, and 4 from the surface side, with "1A" representing layer A in layer 1 and "1B" representing layer B in layer 1, and similarly for 2A, 2B, 3A, 3B, 4A, and 4B.

[0024] (Detailed Description of the Method for Measuring the Standard Deviation of Distance) A small piece was cut from the honeycomb sandwich structure, allowing observation of the cross section of the structure including the woven fabric layer, perpendicular to the woven fabric yarn B, and embedded in epoxy resin. The cross section was then polished with a rotary polisher (Degramin-25, #4000, manufactured by Struers Corporation) to prepare a sample. The polished cross section was then imaged at 80x magnification using a laser microscope (VHX-8000, manufactured by Keyence Corporation). The image processing software ImageJ was used to obtain the distance per pixel from the scale bar of the cross section observation image, thereby converting the coordinates from pixel number to a length scale. The outer surface of the honeycomb sandwich structure was then approximated with a line or spline curve to obtain an outer surface curve. Next, the contour of the fiber bundle (yarn B) oriented in a direction perpendicular to the image plane was approximated with a spline curve, and the cross section of the fiber bundle (yarn B) was extracted by binarization processing. From the cross sections of all extracted yarns B, the standard deviation of the shortest distance from the centroid coordinates (center of gravity position) to the outer surface is obtained for five adjacent yarns B in each divided layer. The method of converting coordinates from pixel numbers to length scales using image processing software ImageJ and the method of obtaining the outer surface curve of the honeycomb sandwich structure are also adopted in the measurement method described later.

[0025] A honeycomb sandwich structure manufactured using a manufacturing method according to one example of the embodiment is a structure in which a honeycomb core is sandwiched between CFRP layers, and at least one of the CFRP layers sandwiching the honeycomb core has a fabric layer having a woven fiber reinforcement material disposed in the CFRP layer closest to the honeycomb core, and the standard deviation of the distances between the fabric yarns of each divided layer, as measured by the following method, is 30 μm or less. (Method for measuring the standard deviation of the distances) One of the orthogonal fabric yarns of the fabric closest to the honeycomb core is designated as Yarn A, and the other is designated as Yarn B. An image of a cross section of the CFRP layer cut in the direction perpendicular to Yarn B is captured, and the cross section is binarized using image processing software ImageJ, and the position of the center of gravity of the cross section of each Yarn B is calculated. In each of the divided layers on the surface side of the yarn A in the fabric layer closest to the honeycomb core side and the divided layer on the honeycomb core side, the shortest distances from the center of gravity position to the outer surface of the honeycomb sandwich structure are determined for each of the five adjacent yarns B, and the standard deviation is calculated. Either the warp yarn or the weft yarn may be yarn A or yarn B, and in either case, the standard deviation of the distances is 30 μm or less.

[0026] If the standard deviation of the distance obtained for the fabric yarns in the fabric layer closest to the honeycomb core is 30 μm or less, the fabric located in the fabric layer closest to the honeycomb core is uniformly arranged along the outer surface of the honeycomb sandwich structure, and the strength of the honeycomb sandwich structure is likely to be uniform. From these viewpoints, the standard deviation of the distance in the fabric layer closest to the honeycomb core is preferably 25 μm or less, and more preferably 20 μm or less.

[0027] A honeycomb sandwich structure manufactured using a manufacturing method according to one embodiment has a honeycomb core sandwiched between CFRP layers, the CFRP layers including fabric layers having woven fiber reinforcement, and the standard deviation of the angle measured for the fabric yarns of the fabric layers by the following measurement method is 1.5 degrees or less. When the CFRP layer includes multiple fabric layers, the standard deviation of the angle measured for the fabric yarns of each fabric layer is 1.5 degrees or less. If the standard deviation of the angle is 1.5 degrees or less, the fabric surface is arranged along the outer surface of the honeycomb sandwich structure, which makes it easier for the strength of the honeycomb sandwich structure to be uniform. From these perspectives, the standard deviation of the angle is preferably 1.2 degrees or less, and more preferably 1.0 degrees or less.

[0028] (Method for Measuring the Standard Deviation of Angle) When one of the orthogonal woven yarns of the woven fabric is designated as yarn A and the other as yarn B, a cross section of the CFRP layer cut in the orthogonal direction of yarn B is imaged, and the cross section is binarized using image processing software ImageJ to calculate the position of the center of gravity of each of the cross sections of yarn B. For each of 10 adjacent yarns B in the woven layer, as shown in Figure 17, the angle A formed by the major axis direction B of the cross section of yarn B (fiber bundle cross section 18) approximated as an ellipse and a reference line C parallel to the tangent to the outer surface of the honeycomb sandwich structure at the closest point to yarn B on the outer surface curve and passing through the center of gravity of yarn B is determined, and the standard deviation of the angle A is calculated. Note that either yarn A or yarn B may be used as the warp yarn or weft yarn, and in either case, the standard deviation of the angle is 1.5 degrees or less.

[0029] The honeycomb sandwich structure manufactured using the manufacturing method according to one example of the embodiment is a structure in which a honeycomb core is sandwiched between CFRP layers, and the standard deviation D of the distance between the outer surface of the honeycomb sandwich structure and the innermost surface of the CFRP layer is measured by the following measurement method. Ａ (μm) is the average value of the distance D ＢThe value A divided by (μm) is 0.01 or more and 0.10 or less. (Method for measuring the standard deviation and average value of the distance between the outer surface and the innermost surface) A cross section of the thickest part of the honeycomb sandwich structure cut parallel to the thickness direction of the honeycomb core is imaged, and the cross section is binarized using image processing software ImageJ. In the image of the cross section, the outer surface coordinates of the honeycomb sandwich structure and the innermost surface coordinates of the CFRP layer are obtained for each pixel within a range of 20 mm in a direction perpendicular to the thickness direction of the honeycomb core. For each point of the outer surface coordinates, the distance between the outer surface coordinates and the innermost surface coordinates located in the normal direction to the outer surface at the outer surface coordinates is measured, and the standard deviation D of the distance is calculated. Ａ and the average value of the distances D Ｂ Calculate.

[0030] By adopting a two-stage molding process consisting of primary molding and secondary molding (described below), the thickness of the prepreg in the first stage can be reduced. This makes it easier to maintain the straightness of the prepreg even when it is pressed by the honeycomb core, thereby reducing the value A. Since there is less variation in the thickness of the CFRP layers, the rigidity of the honeycomb sandwich structure is increased. The value A of 0.01 or more and 0.10 or less is a characteristic of two-stage molding.

[0031] A honeycomb sandwich structure manufactured using a manufacturing method according to an embodiment preferably includes a CFRP layer having woven fiber reinforcement (woven fabric layer) and a CFRP layer having unidirectional (UD) fiber reinforcement (UD layer). The inclusion of the woven fabric layer can prevent the honeycomb sandwich structure from cracking, and the inclusion of the UD layer can increase rigidity in a specific direction.

[0032] In the honeycomb sandwich structure manufactured by the manufacturing method according to one example of the embodiment, the UD layer is disposed adjacent to the outer surface side of the honeycomb sandwich structure relative to the fabric layer, and the standard deviation D of the distance from the outer surface of the honeycomb sandwich structure to the interface between the fabric layer and the UD layer is obtained by the following measurement method. Ｃ (μm) is the average value of the distance D ＤThe value B obtained by dividing the distance from the outer surface to the boundary surface by the distance (μm) is 0.01 or more and 0.10 or less. (Method for measuring the standard deviation of the distance from the outer surface to the boundary surface and the average value of the distance) A cross section of the thickest part of the honeycomb sandwich structure cut parallel to the thickness direction of the honeycomb core is imaged, and the cross section is binarized using image processing software ImageJ. Within a range of 20 mm in the direction perpendicular to the thickness direction of the honeycomb core in the image of the cross section, the outer surface coordinates of the honeycomb sandwich structure and the boundary surface coordinates between the fabric layer and the UD layer are obtained for each pixel. For each point of the outer surface coordinates, the distance between the outer surface coordinates and the boundary surface coordinates located in the normal direction to the outer surface at the outer surface coordinates is measured, and the standard deviation D of the distance is calculated. Ｃ and the average value of the distances D Ｄ Calculate.

[0033] The two-stage molding improves the linearity of each layer, thereby increasing the rigidity of the honeycomb sandwich structure. The value B of 0.01 to 0.10 is a characteristic of the two-stage molding.

[0034] 2. Manufacturing Method of Honeycomb Sandwich Structure A manufacturing method according to an embodiment will be described using the example of manufacturing a blade 1. (1) Primary Molding As materials, a honeycomb core, either in a flat plate form or processed into a predetermined shape by cutting or pressing (the method disclosed in JP 2021-112849 A), and at least two first prepregs are prepared. The first prepregs are sheets made of a carbon fiber reinforcement material and a thermosetting resin matrix. The resin matrix is ​​preferably an epoxy resin composition. Examples of the first prepregs include, but are not limited to, UD prepregs, woven fabric prepregs, nonwoven fabric prepregs, and sheet molding compounds (SMCs). Preferably, the at least two first prepregs include at least one of UD prepregs and woven fabric prepregs.

[0035] As shown in Figures 3 and 4, a primary molding die 20 is charged with a honeycomb core 13 and at least two sheets of first prepreg 14. In other words, the honeycomb core 13 and at least two sheets of first prepreg 14 are placed in a molding cavity formed when the primary molding die 20, consisting of a lower die 21 and an upper die 22, is closed. The honeycomb core 13 is placed in the molding cavity so that its thickness direction is the vertical direction, and at least one first prepreg 14 is placed on each of the top and bottom of the honeycomb core 13, i.e., on both honeycomb surfaces. Figure 3 shows an embodiment using a honeycomb core 13 that has been pre-processed into a desired shape or a state close to the desired shape, while Figure 4 shows an embodiment using a flat honeycomb core 13.

[0036] It is preferable to apply a mold release agent to the portion of the primary molding die 20 that forms the molding cavity. By applying a mold release agent, it becomes easier to remove the primary molded product from the primary molding die 20.

[0037] The number of first prepregs arranged above and below the honeycomb core is preferably one to five, and more preferably three or less. The fewer the number of first prepregs, the more likely it is that the prepregs in contact with the honeycomb core will be molded without significant undulation, resulting in higher rigidity. From this perspective, it is preferable that the number of first prepregs be three or less. On the other hand, when obtaining a large structure, a greater thickness is required compared to when obtaining a small structure, and therefore a greater number of first prepregs makes it easier to obtain rigidity and strength. From this perspective, it is therefore preferable that the number of first prepregs be five to 30, respectively.

[0038] The end of the first prepreg 14 disposed below the honeycomb core 13 may be wrapped around onto the honeycomb core 13. The end of the first prepreg 14 disposed above the honeycomb core 13 may be wrapped around underneath the honeycomb core 13. The thickness direction of the honeycomb core 13 refers to the height direction of the multiple hexagonal columnar cells provided in the honeycomb core 13.

[0039] The type and number of the first prepregs 14 placed under the honeycomb core 13 and the first prepregs 14 placed on top of the honeycomb core 13 may be the same or different. When two or more UD prepregs are stacked as the first prepregs 14 under the honeycomb core 13, they may be stacked so that the fiber directions are aligned, or they may be cross-ply or angle-ply stacked. The same applies when two or more UD prepregs are stacked as the first prepregs 14 on top of the honeycomb core 13.

[0040] An adhesive film may be disposed between the honeycomb core 13 and the first prepreg 14 .

[0041] The honeycomb core may include a portion where the number of first prepregs arranged above and below the honeycomb core is locally increased. By partially increasing the number of first prepregs 14, it becomes easier to increase the rigidity in a specific direction while minimizing the increase in weight of the honeycomb sandwich structure. The prepregs arranged in the portion are preferably UD prepregs, and it is preferable from the viewpoint of bending rigidity that the fiber direction of the UD prepregs be oriented in the long direction of the structure.

[0042] The charged primary molding die 20 is pressurized and heated using a press to harden the first prepreg and obtain a primary molded product. When a flat honeycomb core is used, as in the example shown in Figure 4, the charged primary molding die 20 is pressurized and heated using a press to harden the first prepreg while forming the honeycomb core into the desired shape, thereby obtaining a primary molded product. In this case, since the first prepreg can be positioned on the honeycomb core while it is still flat, the first prepreg is easy to position and is less likely to wrinkle. Furthermore, the honeycomb core can apply uniform pressure to the first prepreg.

[0043] The primary molded product is a honeycomb sandwich structure in which a honeycomb core is sandwiched between two first CFRP layers. The first CFRP layers are cured products of the first prepreg. To shorten the time required for curing the first prepreg, it is preferable to heat the press to the molding temperature before the primary molding die is completely charged. The molding temperature may be 110°C or higher and 120°C or lower, 120°C or higher and 135°C or lower, 135°C or higher and 145°C or lower, 145°C or higher and 150°C or lower, or 150°C or higher and 180°C or lower.

[0044] (2) Secondary Molding At least two sheets of second prepreg are further prepared for secondary molding. Like the first prepreg, the second prepreg is a sheet composed of a carbon fiber reinforcement and a thermosetting resin matrix. The resin matrix is ​​preferably an epoxy resin composition. Examples of the second prepreg include, but are not limited to, UD prepreg, woven fabric prepreg, nonwoven fabric prepreg, and sheet molding compound (SMC). Preferably, the at least two sheets of second prepreg include at least one of UD prepreg and woven fabric prepreg. The prepreg that will become the outermost layer is preferably a woven fabric prepreg because the fibers at the edge are less likely to become disordered.

[0045] As shown in Figure 5, a secondary molding die 30 is charged with the primary molded article 2 and at least two sheets of second prepreg 15. In other words, the primary molded article 2 and at least two sheets of second prepreg 15 are placed in a molding cavity formed when the secondary molding die 30, which is made up of a lower die 31 and an upper die 32, is closed. The secondary molding die 30 differs from the primary molding die 20 in that it is larger than the primary molding die 20 due to the thickness increased by the second prepreg to be laminated.

[0046] If a mold release agent is applied to the mold during primary molding, the mold release agent will adhere to the primary molded product, which will deteriorate the adhesion to the second prepreg, so it is necessary to remove the mold release agent from the primary molded product. Methods for removing the mold release agent include polishing by sandblasting or degreasing.

[0047] The primary molded product 2 is placed in the molding cavity so that the thickness direction of the honeycomb core 13 included in the primary molded product 2 is the vertical direction, and at least one second prepreg 15 is placed above and below the primary molded product 2. The end of the second prepreg 15 placed below the primary molded product 2 may wrap around onto the primary molded product 2. The end of the second prepreg 15 placed above the primary molded product 2 may wrap around below the primary molded product 2. In this way, the top and bottom of the primary molded product 2 are connected by the fiber reinforcing material of the second prepreg 15, which increases the strength of the molded product and provides a good appearance with no boundary between the top and bottom.

[0048] The type and number of the second prepregs 15 placed below the primary molded article 2 and the second prepregs 15 placed above the primary molded article 2 may be the same or different. When two or more UD prepregs are stacked as the second prepregs 15 below the primary molded article 2, they may be laminated so that the fiber directions are aligned, or they may be cross-ply laminated or angle-ply laminated. The same applies when two or more UD prepregs are stacked as the second prepregs 15 above the primary molded article 2.

[0049] The charged secondary molding die 30 is pressurized and heated using a press to cure the second prepreg and obtain a secondary molded product, i.e., the desired molded product. The secondary molded product is a honeycomb sandwich structure in which a honeycomb core 13 is sandwiched between two first CFRP layers 11 and second CFRP layers 12 laminated thereon. The second CFRP layers are cured products of the second prepreg. To shorten the time required to cure the second prepreg, it is preferable to heat the press to the molding temperature before the secondary molding die is completely charged. The molding temperature can be 110°C to 130°C, 130°C to 135°C, 135°C to 145°C, 145°C to 150°C, or 150°C to 180°C, etc.

[0050] (3) Effects During primary molding, as shown in FIG. 6 (left), the first prepreg 14 is placed between the honeycomb core 13 and the mold 20. When the mold 20 is compressed by a press, the honeycomb core 13 presses the first prepreg 14 against the surface of the mold 20. However, as shown in FIG. 5 (right), only the edges of the metal foil constituting the honeycomb core 13 contact the first prepreg 14, so the entire first prepreg 14 is not uniformly pressed against the surface of the mold 20. As a result, the shape of the mold 20 (the shapes of the surfaces 21a and 22a of the lower mold 21 and the upper mold 22, respectively) is not fully reflected in the surface shape of the first CFRP layer 11. Furthermore, because the first prepreg 14 is not sufficiently compressed, voids are likely to form inside the first CFRP layer 11. Furthermore, as shown in FIG. 6 (right), the honeycomb core 13 may bite into the first CFRP layer 11, resulting in cuts in the carbon fibers.

[0051] During secondary molding, as shown in FIG. 7 (left), the second prepreg 15 is placed between the primary molded product 2 and the mold 30. As shown in FIG. 7 (right), when the mold 30 is pressed by a press, the second prepreg 15 is pressed against the surface of the mold 30 by the first CFRP layer 11 of the primary molded product 2. Because the entire surface of the first CFRP layer 11 contacts the second prepreg 15, the entire second prepreg 15 is uniformly pressed. As a result, the shape of the mold 30 (the shapes of the surfaces 31 a, 32 a of the lower mold 31 and the upper mold 32, respectively) is fully reflected in the surface shape of the second CFRP layer 12. Furthermore, voids are unlikely to form inside the second CFRP layer 12. Furthermore, the second CFRP layer 12 does not experience cutting of the carbon fibers due to the honeycomb core biting into it.

[0052] According to the manufacturing method of one example of the embodiment, a honeycomb sandwich structure can be obtained in two steps, enabling efficient production. Furthermore, since the second prepreg can be uniformly pressed between the cured first CFRP layer and the mold, thickness variations and distortion of the second CFRP layer can be reduced, and rigidity can be increased.

[0053] 3. Preferred Embodiments (1) Use of Portable Mold In the manufacturing method described in 2. above, it is preferable to use a portable mold at least in the secondary molding, and preferably in both the primary and secondary molding. The portable mold can be charged outside the press, for example, in a room at a temperature of 17 to 28°C. The temperature of the portable mold when charging is preferably the same as the temperature of the working environment.

[0054] For example, a portable mold heated for drying purposes after cleaning may be charged before its temperature drops to the working temperature. However, even in this case, it is preferable to wait until the temperature of the portable mold drops to 40°C or below before charging in order to prevent wrinkles from forming in the prepreg during handling. It is particularly important to place UD prepregs and woven prepregs in the portable mold in a manner that prevents wrinkles from forming. This is because wrinkles can disrupt the orientation of the fiber reinforcement, resulting in reduced mechanical properties after curing. If the temperature of the portable mold is low, such as 40°C or below, or even 28°C or below, the temperature of the prepreg will not increase and its surface tackiness will not increase, even if charging takes a long time, making the prepreg less likely to wrinkle.

[0055] When a portable mold is used, it is preferable to preheat the charged portable mold before applying pressure and heat with a press. This shortens the time it takes to heat the portable mold with the press, allowing for an increase in the number of moldings that can be performed per day with a single press. Preheating is performed within a range that does not significantly reduce the viscosity of the resin matrix of the prepreg. Specifically, the temperature of the portable mold after preheating is preferably 40°C or higher and 80°C or lower, and more preferably 60°C or higher and 80°C or lower. If the temperature of the portable mold is 80°C or lower, it can be touched with gloved hands, preventing any problems during transportation, etc.

[0056] (2) Portable Mold The material of the portable mold may be carbon steel or alloy steel (steel with added elements such as chromium, molybdenum, tungsten, or vanadium) that have traditionally been used as steel materials for plastic molding. However, since the portable mold does not have an internal heat source and is heated by a heat source provided in the press, more preferred materials are aluminum alloy or beryllium copper. These alloys have a high thermal conductivity of 100 W / (m·K) or more at 20°C.

[0057] Aluminum alloys are particularly preferred because they have high thermal conductivity and are lightweight. The specific gravity of aluminum alloys at 20°C is usually 2.7 to 2.8, while that of steel is around 7.9 and that of beryllium copper is over 8. Examples of aluminum alloys include, but are not limited to, 7000 series aluminum alloys (Al-Zn-Mg alloys), which are considered to have the highest strength among aluminum alloys, and 2000 series aluminum alloys (Al-Cu alloys), which have strength comparable to that of steel.

[0058] The thermal conductivity of aluminum alloys varies depending on the tempering, but it is said that the thermal conductivity at 20°C is 130-190 W / (m·K) for A2017 (duralumin), 150 W / (m·K) for A7003, and 130 W / (m·K) for A7075 (super super duralumin).In contrast, among steel materials used in plastic molding, even chromium molybdenum steel, which has a relatively high thermal conductivity, has a thermal conductivity of approximately 60 W / (m·K) at 20°C, and the thermal conductivity of carbon steel is even lower.

[0059] The weight of the portable mold may preferably be 2 kg or more and 30 kg or less. A weight within this range is convenient for transporting the portable mold. However, the weight of the portable mold is not limited to this range.

[0060] The portable mold may be composed of a bottom plate 40, a top plate 50, and an intermediate plate 60, as shown in an example in Figure 8. The number of intermediate plates is not limited to three, and may be two or less, or four or more. The planar shape of each plate is, for example, rectangular. If necessary, a handle can be attached to the side of any plate. All plates have the same outer shape and dimensions in plan view, and can be stacked tightly on top of each other.

[0061] When the bottom plate 40, the intermediate plate 60, and the top plate 50 are stacked so as to closely overlap one another, molding cavities 70 are formed between the bottom plate 40 and the intermediate plate 60 immediately above it, between adjacent intermediate plates 60, and between the top plate 50 and the intermediate plate 60 immediately below it. In one example, one and the other of the adjacent plates may be provided with an alignment protrusion and an alignment recess, respectively, so that the two plates will fit together only when they are stacked so as to closely overlap one another.

[0062] The lower surface of the bottom plate and the upper surface of the top plate are both entirely flat. When all the plates are stacked, the lower surface of the bottom plate and the upper surface of the top plate are parallel to each other. In one example, multiple molding cavities may be formed between adjacent plates. In one example, at least some of the plates constituting the portable mold may have an insert structure and be composed of a mother mold and a nesting mold.

[0063] (3) Decompression In the manufacturing method of 2. above, it is preferable to use a portable mold at least in the secondary molding, and to decompress the space around the portable mold when heating and pressurizing the portable mold with a press machine. An example of a press machine that can decompress the space around the portable mold when heating and pressurizing the portable mold is shown in Figure 9.

[0064] The press 100 includes a lower plate 110, an upper plate 120 disposed above the lower plate 110 and facing the lower plate 110, and a pressure mechanism (only a cylinder 132 of the pressure mechanism is shown in FIG. 9 ) that moves the lower plate 110 and the upper plate 120 toward each other. The lower plate 110 is made up of a horizontally disposed lower substrate 111 and a lower heating plate 112 attached to the upper surface of the lower substrate 111. The upper plate 120 is made up of a horizontally disposed upper substrate 121 and an upper heating plate 122 attached to the lower surface of the upper substrate 121. A frame that secures the upper plate 120 and other components is not shown in FIG. 9 .

[0065] The pressure mechanism is preferably hydraulic because it is easy to control the pressure. The cylinder 132 of the pressure mechanism may be arranged to act on the upper plate 120 instead of the lower plate 110. The lower heating plate 112 and the upper heating plate 122 each have a built-in heater 131. Examples of the heater 131 include, but are not limited to, a steam heater, an oil heater, an electric heater, and an electromagnetic induction heater.

[0066] An annular lower wall member 113 is fixed to the upper surface of the lower heating platen 112. As shown in the plan view of Figure 10, the lower wall member 113 has a structure in which four vertical walls, each perpendicular to the upper surface of the lower heating platen 112, are connected to form a rectangular ring, and is positioned slightly inward from the peripheral surface of the lower heating platen 112. The lower heating platen 112 and the lower wall member 113 may be integrated without any seams. The lower wall member 113 has a groove formed around its outer periphery, and a vacuum sealing ring 114 is fitted into the groove.

[0067] An annular upper wall member 123 is fixed to the underside of the upper heating platen 122. As shown in the plan view (viewed from below) in Figure 11, the upper wall member 123 has a structure in which four vertical walls, each perpendicular to the underside of the upper heating platen 122, are connected to form a rectangular ring and are arranged along the periphery of the upper heating platen 122. The upper heating platen 122 and the upper wall member 123 may be integrated without any seams. The upper wall member 123 has an exhaust hole 124 that penetrates one of the vertical walls. The exhaust hole 124 is connected to a vacuum pump 133.

[0068] 12, when a charged portable mold is placed on the lower heating platen and the pressure mechanism is operated to bring the lower platen 110 close to the upper platen 120, the lower wall member 113 and the upper wall member 123 are joined via the vacuum sealing ring 114, forming a closed space S surrounded by the lower heating platen 112, the lower wall member 113, the upper heating platen 122, and the upper wall member 123. The pressure in this closed space S can be reduced by operating a vacuum pump 133.

[0069] The closed space S is depressurized by operating a vacuum pump 133 so that the pressure after depressurization is preferably 0.1 Pa or less, more preferably 0.01 Pa or less. When applied to the above-mentioned secondary molding, depressurizing the closed space S not only prevents bubbles generated between the first CFRP layer and the second prepreg from becoming voids after molding, but also prevents void formation within the second CFRP layer because the resin matrix of the second prepreg is degassed. When multiple second prepregs are laminated, it is possible to prevent bubbles generated between adjacent second prepregs from becoming voids after molding, making it easier to obtain a good appearance.

[0070] In a modified press, as shown in FIG. 13, the lower wall member 113 may be omitted, and the lower heating platen 112 and the upper wall member 123 may be joined together via a vacuum sealing ring 114.

[0071] In another modification of the press, as shown in Fig. 14, a lower wall member 113 and an upper wall member 123, which are joined together via vacuum sealing rings, may be fixed to the upper surface of the lower substrate 111 and the lower surface of the upper substrate 121, respectively. The configuration of the press in which a closed space that can be depressurized is formed between the lower heating platen and the upper heating platen is not limited to the above-mentioned example.

[0072] In Figures 12 to 14, the portable mold is shown as one consisting only of a lower mold (lower plate 40) and an upper mold (upper plate 50) without an intermediate plate, but this can be replaced with a portable mold having an intermediate plate.

[0073] 4. Summary of Embodiments [Embodiment 1] A method for manufacturing a honeycomb sandwich structure, comprising: (i) charging a mold for primary molding with a honeycomb core and at least two first prepregs; (ii) applying pressure and heat to the charged mold for primary molding in a press to obtain a primary molded product having a honeycomb sandwich structure in which the honeycomb core is sandwiched between two first CFRP layers; (iii) charging a mold for secondary molding with the primary molded product and at least two second prepregs; and (iv) applying pressure and heat to the charged mold for secondary molding in a press to obtain a secondary molded product having a honeycomb sandwich structure in which the honeycomb core is sandwiched between the two first CFRP layers and a second CFRP layer is laminated on each of the two first CFRP layers. [Embodiment 2] A method for manufacturing a honeycomb sandwich structure according to Embodiment 1, wherein at least two first prepregs are arranged on each of the honeycomb surfaces of the honeycomb core. [Embodiment 3] The manufacturing method according to embodiment 2, wherein the number of first prepregs arranged on each of both honeycomb surfaces of the honeycomb core is 1 to 5. [Embodiment 4] The manufacturing method according to embodiment 2, wherein the number of first prepregs arranged on each of both honeycomb surfaces of the honeycomb core is 5 to 30. [Embodiment 5] The manufacturing method according to any one of embodiments 1 to 4, wherein the honeycomb core includes a portion where the number of first prepregs is greater than one. [Embodiment 6] The manufacturing method according to any one of embodiments 1 to 5, wherein the honeycomb core is formed into a desired shape by pressing with a press. [Embodiment 7] The manufacturing method according to any one of embodiments 1 to 6, wherein the secondary molding die is a portable die. [Embodiment 8] The manufacturing method according to embodiment 7, wherein the portable die is made of an aluminum alloy. [Embodiment 9] The manufacturing method according to embodiment 7 or 8, wherein the temperature of the secondary molding die is 40°C or less when charging the secondary molding die. [Embodiment 10] The manufacturing method according to embodiment 9, wherein the secondary molding die is charged indoors at a temperature of 17 to 28°C. [Embodiment 11] A manufacturing method according to any one of embodiments 7 to 10, wherein the mold for primary molding is a portable mold.[Embodiment 12] The manufacturing method according to embodiment 11, wherein the material of the portable mold is an aluminum alloy. [Embodiment 13] The manufacturing method according to embodiment 11 or 12, wherein the temperature of the primary molding mold is 40°C or less when charging the primary molding mold. [Embodiment 14] The manufacturing method according to embodiment 13, wherein the charging of the primary molding mold is performed indoors at a temperature of 17 to 28°C. [Embodiment 15] The manufacturing method according to any of embodiments 7 to 14, wherein the space around the secondary molding mold is depressurized when the charged secondary molding mold is pressurized and heated in a press. [Embodiment 16] The manufacturing method according to embodiment 15, wherein the space around the secondary molding mold is depressurized when the charged primary molding mold is pressurized and heated in a press. [Embodiment 17] A honeycomb sandwich structure having a honeycomb core sandwiched between CFRP layers, wherein a value A obtained by the following measurement method is 0.01 or more and 0.10 or less. (Method for measuring value A) A cross section of the thickest part of the honeycomb sandwich structure is cut parallel to the thickness direction of the honeycomb core, and an image is taken. Using image processing software ImageJ, the outer surface coordinates of the honeycomb sandwich structure and the innermost surface coordinates of the CFRP layer are obtained for each pixel within a 20 mm range in the image of the cross section in a direction perpendicular to the thickness direction of the honeycomb core. For each point of the outer surface coordinates, the distance between the outer surface coordinates and the innermost surface coordinates located in the normal direction to the outer surface at the outer surface coordinates is measured, and the standard deviation D of the distances is calculated. Ａ and the average value of the distances D Ｂ is calculated, and the value A is calculated from the following formula (1): A = D Ａ / D Ｂ...Equation (1) [Embodiment 18] A honeycomb sandwich structure according to embodiment 17, wherein the CFRP layer includes a woven fabric layer, which is a CFRP layer having woven fiber reinforcement, and a UD layer, which is a CFRP layer having unidirectional (UD) fiber reinforcement. [Embodiment 19] A honeycomb sandwich structure according to embodiment 18, wherein the UD layer is disposed adjacent to the woven fabric layer on the outer surface side of the honeycomb sandwich structure, and wherein the value B obtained by the following measurement method is 0.01 to 0.10. (Method for measuring value B) An image of a cross section obtained by cutting the thickest part of the honeycomb sandwich structure parallel to the thickness direction of the honeycomb core is captured. Using image processing software ImageJ, the coordinates of the outer surface of the honeycomb sandwich structure and the coordinates of the interface between the woven fabric layer and the UD layer are obtained for each pixel within a 20 mm range in the image of the cross section in a direction perpendicular to the thickness direction of the honeycomb core. For each point of the outer surface coordinates, the distance between the outer surface coordinates and the boundary surface coordinates located in the normal direction to the outer surface at the outer surface coordinates is measured, and the standard deviation D of the distance is calculated. Ｃ and the average value of the distances D Ｄ and calculate the value B from the following formula (2): B = D Ｃ / D Ｄ... Equation (2) [Embodiment 20] A honeycomb sandwich structure having a honeycomb core sandwiched between CFRP layers, the CFRP layers including woven fabric layers having a woven fiber reinforcement, wherein the standard deviation of the distances for the woven fabric yarns in each segment layer, as determined by the following measurement method for the woven fabric layers, is 30 μm or less. (Method for measuring the standard deviation of the distances) When one of the orthogonal woven fabric yarns of the woven fabric is designated as yarn A and the other as yarn B, an image of a cross section of the CFRP layer cut in the orthogonal direction of yarn B is captured, and the cross section is binarized using image processing software ImageJ to calculate the position of the center of gravity of each of the cross sections of yarn B. In each of the segment layers on the surface side of yarn A in the woven fabric layer and the segment layer on the honeycomb core side, the shortest distances from the position of the center of gravity to the outer surface of the honeycomb sandwich structure are determined for each of five adjacent yarns B, and the standard deviation is calculated. [Embodiment 21] A honeycomb sandwich structure in which a honeycomb core is sandwiched between CFRP layers, at least one of the CFRP layers sandwiching the honeycomb core has a woven fabric layer having a woven fiber reinforcement material disposed in the CFRP layer closest to the honeycomb core, and the standard deviation of the distances between the woven fabric yarns of each divided layer, as determined for the woven fabric layer by the following measurement method, is 30 μm or less. (Method for measuring the standard deviation of the distances) One of the orthogonal woven fabric yarns closest to the honeycomb core is designated as Thread A, and the other is designated as Thread B. An image of a cross section of the CFRP layer cut in the direction perpendicular to Thread B is captured, and the cross section is binarized using image processing software ImageJ, and the position of the center of gravity of the cross section of each Thread B is calculated. In each of the divided layers on the surface side of the thread A in the fabric layer closest to the honeycomb core and the divided layer on the honeycomb core side, the shortest distance from the center of gravity position to the outer surface of the honeycomb sandwich structure is determined for each of five adjacent threads B, and the standard deviation is calculated. [Embodiment 22] A honeycomb sandwich structure having a honeycomb core sandwiched between CFRP layers, the CFRP layers including a fabric layer having a woven fiber reinforcement material, and the standard deviation of the angle obtained by the following measurement method for the fabric threads of the fabric layer is 1.5 degrees or less.(Method for measuring standard deviation of angle) When one of the orthogonal woven yarns of the woven fabric is designated as yarn A and the other as yarn B, a cross section of the CFRP layer cut in the orthogonal direction of yarn B is imaged, and the cross section is binarized using image processing software ImageJ to calculate the position of the center of gravity of each of the cross sections of yarn B. For each of 10 adjacent yarns B in the woven fabric layer, the angle formed by the major axis direction of an elliptical approximation of the cross section of yarn B and a reference line that is parallel to the tangent to the outer surface at the closest point to yarn B on the outer surface curve of the honeycomb sandwich structure and passes through the center of gravity of yarn B is determined, and the standard deviation of the angle is calculated.

[0074] 5. Experimental Results The results of the experiments conducted by the present inventors are described below.

[0075] (Experimental Example 1) An aluminum alloy honeycomb core and eight sheets of woven fabric prepreg were prepared. A primary molding die was charged with the honeycomb core and first prepregs, which were woven fabric prepregs arranged one above and one below the honeycomb core. The first prepregs were hardened by applying pressure and heat using a press to obtain a primary molded product. Next, a secondary molding die was charged with the primary molded product and second prepregs, which were woven fabric prepregs arranged three above and three below the primary molded product. The second prepregs were hardened by applying pressure and heat using a press to obtain a honeycomb sandwich structure.

[0076] (Experimental Example 2) An aluminum alloy honeycomb core and eight sheets of woven fabric prepreg were prepared. A secondary molding die was charged with the honeycomb core and four sheets of woven fabric prepreg arranged above and below the honeycomb core. The woven fabric prepreg was hardened by applying pressure and heat using a press machine to obtain a honeycomb sandwich structure.

[0077] For the textile yarns in each textile layer of the honeycomb sandwich structures obtained in Experimental Examples 1 and 2, the standard deviation of the shortest distance from the center of gravity of Yarn B to the outer surface of the honeycomb sandwich structure and the standard deviation of the angle formed by the long axis direction of an ellipse-approximated cross section of Yarn B and a reference line were determined using the above-mentioned method. The results are shown in Tables 1 and 2. In Table 1, 1A to 4B respectively indicate the surface-side divided layer (Layer A) and the honeycomb core-side divided layer (Layer B) in each of the four textile layers. In Table 2, 1 to 4 indicate each of the four textile layers. Furthermore, Figs. 15 and 16 show binarized cross-sectional views of the textile layers used to determine the standard deviation of the shortest distance from the center of gravity of Yarn B to the outer surface of the honeycomb sandwich structure and the standard deviation of the angle formed by the long axis direction of an ellipse-approximated cross section of Yarn B and a reference line.

[0078]

[0079]

[0080] (Experimental Example 3) An aluminum alloy honeycomb core, four woven fabric prepregs, and two UD prepregs were prepared. A primary molding die was charged with the honeycomb core and first prepregs, which were UD prepregs arranged one above and one below the honeycomb core. The first prepregs were hardened by applying pressure and heat using a press to obtain a primary molded product. Next, a secondary molding die was charged with the primary molded product and second prepregs, which were woven fabric prepregs arranged two above and two below the primary molded product. The second prepregs were hardened by applying pressure and heat using a press to obtain a honeycomb sandwich structure.

[0081] (Experimental Example 4) An aluminum alloy honeycomb core, 46 UD prepregs, and 6 woven fabric prepregs were prepared. A primary molding die was charged with the honeycomb core and a first prepreg, consisting of 21 UD prepregs and two woven fabric prepregs arranged above and below the honeycomb core. The first prepregs were hardened by applying pressure and heat using a press to obtain a primary molded product. Next, a secondary molding die was charged with the primary molded product and a second prepreg, consisting of two UD prepregs and one woven fabric prepreg arranged above and below the primary molded product. The second prepreg was hardened by applying pressure and heat using a press to obtain a honeycomb sandwich structure.

[0082] (Experimental Example 5) An aluminum alloy honeycomb core, four woven fabric prepregs, and two UD prepregs were prepared. A molding die was placed around the honeycomb core, with one UD prepreg placed above and below the honeycomb core, and two woven fabric prepregs placed above and below the honeycomb core. The prepregs were hardened by applying pressure and heat using a press, resulting in a honeycomb sandwich structure.

[0083] For the honeycomb sandwich structures obtained in Experimental Examples 3 to 5, the standard deviation D of the distance between the outer surface of the honeycomb sandwich structure and the innermost surface of the CFRP layer was measured by the above-mentioned method. Ａ The average value of the distances D Ｂ The value A obtained by dividing by 1 / 2 was calculated and the results are shown in Table 3. Furthermore, for the honeycomb sandwich structures obtained in Experimental Examples 3 to 5, the standard deviation D of the distance between the outer surface of the honeycomb sandwich structure and the boundary surface between the woven fabric layer and the UD layer was measured by the above-mentioned method. Ｃ the average value of the distances D Ｄ The value B obtained by dividing by is also shown in Table 3.

[0084]

[0085] While the present invention has been described above with reference to specific embodiments, these embodiments are presented as examples and do not limit the scope of the present invention. Each embodiment described in this specification can be modified in various ways within the scope of the effects of the invention, and can be combined with features described in other embodiments within the scope of feasibility.

[0086] 1 Wing (honeycomb sandwich structure) 2 Primary molded product 10 CFRP layer 11 First CFRP layer 12 Second CFRP layer 13 Honeycomb core 14 First prepreg 15 Second prepreg 16 Honeycomb core side 17 Outer surface of honeycomb sandwich structure 18 Fiber bundle cross section (cross section of thread B) 20 Mold for primary molding 21 Lower mold 22 Upper mold 30 Mold for secondary molding 31 Lower mold 32 Upper mold 40 Lower mold (lowest plate) 50 Upper mold (upper plate) 60 Intermediate plate 70 Molding cavity 100 Press machine 110 Lower plate portion 111 Lower substrate 112 Lower heating plate 113 Lower wall member 114 Vacuum sealing ring 120 Upper plate portion 121 Upper substrate 122 Upper heating platen 123 Upper wall member 131 Heater 132 Cylinder of pressure mechanism 133 Vacuum pump A Angle B Long axis direction of fiber bundle C Reference line

Claims

1. (i) charging a primary molding die with a honeycomb core and at least two first prepregs; (ii) pressurizing and heating the charged primary molding die with a press machine to obtain a primary molding product having a honeycomb sandwich structure in which the honeycomb core is sandwiched between two first CFRP layers; (iii) charging a mold for secondary molding with the primary molded article and at least two second prepregs; (iv) applying pressure and heat to the charged secondary molding die with a press machine to obtain a secondary molded product having a honeycomb sandwich structure in which the honeycomb core is sandwiched between the two first CFRP layers and a second CFRP layer is laminated on each of the two first CFRP layers; and A method for manufacturing a honeycomb sandwich structure comprising:

2. The manufacturing method according to claim 1 , wherein the at least two first prepregs are disposed on each of the honeycomb surfaces of the honeycomb core.

3. The manufacturing method according to claim 2 , wherein the number of first prepregs arranged on each of both honeycomb surfaces of the honeycomb core is 1 to 5.

4. The method according to claim 2 , wherein the number of first prepregs arranged on each of the honeycomb surfaces of the honeycomb core is 5 to 30.

5. The manufacturing method according to any one of claims 1 to 4, comprising a portion in which the number of the first prepregs is partially larger.

6. The method according to any one of claims 1 to 4, wherein the honeycomb core is pressed with a press to obtain a desired shape.

7. The method according to any one of claims 1 to 4, wherein the mold for secondary molding is a portable mold.

8. The manufacturing method according to claim 7 , wherein the material of the portable mold is an aluminum alloy.

9. The method according to claim 7, wherein the temperature of the secondary molding die when the secondary molding die is charged is 40°C or less.

10. The method according to claim 9, wherein the charging of the mold for secondary forming is carried out in a room at a temperature of 17 to 28°C.

11. The method according to claim 7 , wherein the mold for the primary molding is a portable mold.

12. The manufacturing method according to claim 11, wherein the material of the portable mold is an aluminum alloy.

13. The method according to claim 11, wherein the temperature of the primary molding die when the primary molding die is charged is 40°C or less.

14. The method according to claim 13, wherein the charging of the mold for the primary molding is carried out in a room at a temperature of 17 to 28°C.

15. The method according to claim 7, wherein the space around the secondary molding die is decompressed when the charged secondary molding die is pressurized and heated by a press machine.

16. The method according to claim 15, wherein the space around the secondary molding die is decompressed when the charged primary molding die is pressurized and heated by a press machine.

17. A structure in which a honeycomb core is sandwiched between CFRP layers, A honeycomb sandwich structure having a value A determined by the following measurement method of 0.01 or more and 0.10 or less. (Method of measuring value A) A cross section of the thickest part of the honeycomb sandwich structure is cut parallel to the thickness direction of the honeycomb core, and an image is taken. Using image processing software ImageJ, the outer surface coordinates of the honeycomb sandwich structure and the innermost surface coordinates of the CFRP layer are obtained for each pixel within a range of 20 mm in a direction perpendicular to the thickness direction of the honeycomb core in the image of the cross section. For each point of the outer surface coordinates, the distance between the outer surface coordinates and the innermost surface coordinates located in the normal direction to the outer surface at the outer surface coordinates is measured, and the standard deviation D of the distance is calculated. Ａ and the average distance D Ｂ is calculated, and the value A is calculated from the following formula (1). A = D Ａ / D Ｂ ・・・Formula (1)

18. 18. The honeycomb sandwich structure of claim 17, wherein the CFRP layers include woven layers that are CFRP layers having woven fiber reinforcement and unidirectional (UD) layers that are CFRP layers having UD fiber reinforcement.

19. The honeycomb sandwich structure according to claim 18, wherein the UD layer is arranged adjacent to the outer surface side of the honeycomb sandwich structure relative to the woven fabric layer, and a value B obtained by the following measurement method is 0.01 or more and 0.10 or less. (Method of measuring value B) A cross section of the thickest part of the honeycomb sandwich structure is cut parallel to the thickness direction of the honeycomb core, and an image is taken. Using image processing software ImageJ, the outer surface coordinates of the honeycomb sandwich structure and the boundary surface coordinates between the fabric layer and the UD layer are obtained for each pixel within a range of 20 mm in the direction perpendicular to the thickness direction of the honeycomb core in the image of the cross section. For each point of the outer surface coordinates, the distance between the outer surface coordinates and the boundary surface coordinates located in the normal direction to the outer surface at the outer surface coordinates is measured, and the standard deviation D of the distance is calculated. Ｃ and the average distance D Ｄ is calculated, and the value B is calculated from the following formula (2). B = D Ｃ / D Ｄ ・・・Formula (2)

20. A honeycomb sandwich structure, comprising a honeycomb core sandwiched between CFRP layers, the CFRP layers including a woven fabric layer having a woven fiber reinforcement material, and the standard deviation of the distance for the woven fabric yarns of each dividing layer, as determined by the following measurement method, is 30 μm or less. (Method of measuring standard deviation of distance) When one of the orthogonal textile threads of the woven fabric is designated as thread A and the other as thread B, a cross section of the CFRP layer cut in the orthogonal direction of thread B is imaged, and the cross section is binarized using image processing software ImageJ to calculate the center of gravity position of each of the cross sections of thread B. In each of the divided layer on the surface side of thread A in the woven fabric layer and the divided layer on the honeycomb core side, the shortest distance from the center of gravity position to the outer surface of the honeycomb sandwich structure is determined for each of five adjacent threads B, and the standard deviation is calculated.

21. A honeycomb sandwich structure comprising a honeycomb core sandwiched between CFRP layers, wherein at least one of the CFRP layers sandwiching the honeycomb core has a woven fabric layer having a woven fiber reinforcement material disposed in the CFRP layer closest to the honeycomb core side, and wherein the standard deviation of the distance for the woven fabric yarns of each dividing layer, as determined by the following measurement method for the woven fabric layer, is 30 μm or less. (Method of measuring standard deviation of distance) When one of the orthogonal textile yarns of the woven fabric closest to the honeycomb core side is designated as yarn A and the other as yarn B, a cross section of the CFRP layer cut in the orthogonal direction of yarn B is imaged and binarized using image processing software ImageJ to calculate the center of gravity position of each of the cross sections of yarn B. In each of the divided layer on the surface side of yarn A in the woven fabric layer closest to the honeycomb core side and the divided layer on the honeycomb core side, the shortest distance from the center of gravity position to the outer surface of the honeycomb sandwich structure is determined for each of five adjacent yarns B, and the standard deviation is calculated.

22. A honeycomb sandwich structure, comprising a honeycomb core sandwiched between CFRP layers, the CFRP layers including a woven fabric layer having a woven fiber reinforcement material, and the standard deviation of an angle measured by the following measurement method for the woven fabric yarns of the woven fabric layer is 1.5 degrees or less. (Method of measuring standard deviation of angle) When one of the orthogonal textile yarns of the woven fabric is designated as yarn A and the other as yarn B, a cross section of the CFRP layer cut in the orthogonal direction of yarn B is imaged, and the cross section is binarized using image processing software ImageJ to calculate the position of the center of gravity of each of the cross sections of yarn B. For each of ten adjacent yarns B in the woven fabric layer, an angle is determined between the long axis direction of an ellipse approximation of the cross section of yarn B and a reference line that is parallel to the tangent to the outer surface at the closest point to yarn B on the outer surface curve of the honeycomb sandwich structure and passes through the center of gravity of yarn B, and the standard deviation is calculated.