Hollow molded article

Abstract

The present invention addresses the problem of providing a hollow molded article that can be suitably applied to propeller blades for UAMs and drones, is lightweight, and has exceptional impact resistance. In order to solve the aforementioned problem, a hollow molded article comprises: a skin part (skin part (A)) composed of a composite material that contains a resin and reinforcing fibers, the skin part forming the outer surface of the hollow molded article; and a hollow part and a porous body in the internal space of the skin part (A), the hollow molded article being characterized in that, in regard to three planes (plane a, plane b, and plane c, in order from the plane with the greatest area) forming a rectangular parallelepiped that circumscribes the hollow molded article and has the smallest volume, the ratio (α / β) is 1.5-150, where α (cm) is the length of the long side of the plane a and β (cm) is the arithmetic average value of the lengths of the long and short sides of the plane c, and in that conditions 1 and 2 are satisfied.　Condition 1: The porous body has a specific gravity of 0.3 or lower, and at least one porous body is in contact with all or part of the inner surface of the skin part (A) forming a ridge line from either or both of a point and a surface in contact with the plane b to the plane c in top view seen from the plane a.　Condition 2: In all cross-sections perpendicular to the longitudinal direction at positions where the hollow molded article is equally divided into four sections in the longitudinal direction, the area occupied by the hollow part is 50-95%, where 100% is the sum of the area occupied by the hollow part and the area occupied by the porous body.

Description

Hollow molded products

[0001] The present invention relates to a hollow molded article, and particularly to a hollow molded article that can be suitably used for Urban Air Mobility (UAM) and propeller blades for drones.

[0002] In recent years, new forms of aerial use, such as UAM and drones, have been developed. These aircraft often have numerous propeller blades mounted on them that rotate using power to generate lift and thrust.

[0003] Propeller blades have a propeller shape and rotate around the drive shaft to push air backward, generating thrust. To efficiently push air backward, propeller blades have a complex shape in which the angle of attack and chord length change from the root to the tip.

[0004] Propeller blades for UAM and drones must be lightweight to increase range and payload, and weight reduction at the gram level is often valued.

[0005] For this reason, propeller blades are often made hollow internally, or if a core material is used, it is filled with a porous material, such as carbon fiber composite materials. However, due to the complex shape, molding carbon fiber composite materials is very difficult and time-consuming, making them unsuitable for mass production.

[0006] On the other hand, for safety reasons, propeller blades must also be resistant to bird strikes and collisions with flying stones, etc. In other words, they must be lightweight and impact resistant, and it is important to achieve a good balance between these properties.

[0007] JP 2009-74421 A JP 2017-172403 A JP 2001-165033 A

[0008] In recent years, the development of UAM and drones has accelerated, and the development of aircraft with many propeller blades is the norm. To increase range and payload, the more propeller blades are attached to a rotating shaft, the greater the need for lightweight propeller blades. Fiber-reinforced composite materials, which are lightweight and have excellent strength and rigidity, are therefore used for propeller blades. To further enhance lightness and strength and rigidity, fiber-reinforced composite materials may be made hollow or filled with a core material such as a porous body. When filled with a core material such as a porous body, the core material often has the effect of improving impact resistance.

[0009] Patent Document 1 shows a wind turbine blade made of a fiber-reinforced composite material with a porous core such as urethane foam filled inside, and proposes a structure that can shorten the molding time and maintain strength by devising a lamination method for the composite material. However, since the core material is filled entirely inside, it does not anticipate the design or use of a hollow structure, which is the most efficient way to reduce weight, and it is thought that further performance improvements are needed to reduce weight.

[0010] Patent Document 2 shows a lightweight rotor blade with excellent rigidity, in which a reinforcing layer of glass fiber fabric or the like is formed on the outside of a hollow body formed from a porous body in which resin fibers are entangled and bonded. However, there are no specifications regarding the positional relationship between the hollow structure and the porous body or the proportion of use, and it is thought that proposals that anticipate further effects in terms of simplifying molding and reducing weight are needed.

[0011] Patent Document 3 describes a lightweight and durable propeller blade for a wind power generator, which has a main beam with a hollow structure inside and reinforcing materials such as urethane foam before and after it. However, this document also does not specify the positional relationship between the hollow structure and the porous material or the proportion of use, and it is thought that proposals that anticipate further effects in terms of simplifying molding and reducing weight are needed.

[0012] Against this background, an object of the present invention is to provide a hollow molded product that is lightweight and has excellent impact resistance, and that can be particularly suitably applied to propeller blades for UAMs and drones.

[0013] The present invention employs the following means to solve the above problems: [1] A hollow molded article comprising a skin portion (skin portion (A)) made of a composite material containing a resin and reinforcing fibers and constituting the outer surface of the hollow molded article, and a hollow portion and a porous body in the internal space of the skin portion (A), wherein the hollow molded article has three faces (face a, face b, and face c, in order from the largest area) that circumscribe the hollow molded article and form a rectangular parallelepiped having the smallest volume, where α (cm) is the length of the long side of face a and β (cm) is the arithmetic mean value of the lengths of the long side and the short side of face c, the ratio (α / β) is 1.5 to 150, and the following conditions 1 and 2 are satisfied:

[0014] Condition 1: The porous body has a specific gravity of 0.3 or less, and at least one porous body is in contact with all or part of the inner surface of the skin portion (A) that forms a point of contact with surface b or a ridge line extending from either or both of the surfaces toward surface c when viewed from above from surface a.

[0015] Condition 2: In all cross sections perpendicular to the longitudinal direction at positions dividing the hollow molded article into four equal parts in the longitudinal direction, the area occupied by the hollow portions is 50 to 95% when the sum of the area occupied by the hollow portions and the area occupied by the porous body is taken as 100%. [2] The blow molded article according to the above item [1], further satisfying the following condition 3:

[0016]

[0013] [Condition 3]: In all of the cross sections, the area occupied by the porous body in contact with the inner surface of the skin portion (A) forming the ridge line is 50 to 100% of the area occupied by the porous body in the cross section. [3] The blow-molded article according to [1] or [2], characterized in that, in a portion from the surface having the opening to 50% of the length of the long side of surface a in the longitudinal direction, the blow-molded article has a region (undercut region) occupied by a surface excluding the portion overlapping with cross section Sa in a cross section perpendicular to the longitudinal direction from cross section Sa to the tip, when viewed in the longitudinal direction, where cross section Sa is the cross section at which the length of the line segment between the points where a line passing through the center of gravity of the cross section in a cross section perpendicular to the longitudinal direction intersects with the outer edge of the cross section is the shortest, and the porous body accounts for 30% by volume or more of the undercut region. [4] The hollow molded product according to any one of [1] to [3], wherein the hollow portion of the hollow molded product has a structure that can be formed by removing the core from the opening. [5] The hollow molded product according to any one of [1] to [4], wherein the composite material forming the skin portion (A) is molded into a propeller shape, and when the blade length of the propeller shape is taken as 100%, a cross section of a portion between 5 and 20% of the blade length as seen from the base of the propeller shape has a cross section in which a porous body is present on the inner surface of the skin portion (A) forming the leading edge of the propeller-shaped blade, and a porous body is not present on the inner surface of the skin portion (A) forming the trailing edge of the propeller-shaped blade. [6] The hollow molded article according to any one of [1] to [5], wherein the composite material forming the skin portion (A) is molded into a propeller shape, and when the blade length of the propeller shape is taken as 100%, a porous body is present on the inner surface of the skin portion (A) forming the leading edge of the propeller-shaped blade and the inner surface of the skin portion (A) forming the trailing edge of the propeller-shaped blade in a cross section of 20 to 80% of the blade length as seen from the base of the propeller shape. [7] The hollow molded article according to any one of [1] to [6], wherein the composite material forming the skin portion (A) is molded into a propeller shape, and when the blade length of the propeller shape is taken as 100%, a porous body is not present on the inner surface of the skin portion (A) forming the trailing edge of the propeller-shaped blade in a cross section of 80 to 100% of the blade length as seen from the base of the propeller shape. [8] The hollow molded article according to any one of [1] to [7], wherein the reinforcing fiber is carbon fiber.[9] The hollow molded product according to any one of [1] to [8], wherein the porous body is selected from the group consisting of urethane foam, acrylic foam, polyolefin foam, phenol foam, and polymethacrylimide foam.

[0017] According to the present invention, a hollow molded article that is lightweight and has excellent impact resistance can be provided. The hollow molded article of the present invention is particularly suitable for use as a propeller blade for an aircraft such as a UAM or a drone.

[0018] 1 is a perspective view of an example of a hollow molded product and a rectangular parallelepiped circumscribing the hollow molded product. FIG. 2 is a top view, a side view, and a front view of the hollow molded product of FIG. 1 and a rectangular parallelepiped circumscribing the hollow molded product. FIG. 3 is a perspective view of an example of a propeller blade. FIG. 4 is a top view and a front view of the blade root side of the propeller blade of FIG. 3. FIG. 5 is an example of a cross section of a propeller blade perpendicular to the blade span direction. FIG. 6 is a cross section of a propeller blade cut along a plane that passes through the tip and leading edge of the skin portion (A) and includes the leading edge and the trailing edge. FIG. 7 is a cross section of a propeller blade for explaining an undercut region. FIG. 8 is a cross section of a propeller blade showing an example in which a porous body is arranged so as to contact the inner surface of the skin portion (A) that forms the leading edge of the blade and the inner surface of the skin portion (A) that forms the trailing edge of the blade. FIG. 9 is a cross section of a propeller blade showing an example in which a porous body is arranged so as to contact the inner surface of the skin portion (A) that forms the leading edge of the blade and the inner surface of the skin portion (A) that forms the trailing edge of the blade. FIG. 10 is a cross section of a propeller blade showing an example in which a porous body is arranged so as to contact the inner surface of the skin portion (A) that forms the leading edge of the blade and the inner surface of the skin portion (A) that forms the trailing edge of the blade. FIG. 11 is a cross section of a propeller blade in the vicinity of the tip, (b) a cross section of the center, and (c) a cross section of a propeller blade in the vicinity of the root. (a) A cross-sectional view of the propeller blade of Example 1 taken along a plane passing through the tip and leading edge of the skin portion (A) and including the leading edge and trailing edge, (b) an A-A' cross-sectional view, (c-1) a B-B' cross-sectional view, (c-2) a C-C' cross-sectional view, and (c-3) a D-D' cross-sectional view.

[0019] The hollow molded article according to the present invention is a hollow molded article comprising a skin portion (skin portion (A)) made of a composite material containing a resin and reinforcing fibers and forming the outer surface of the hollow molded article, and a hollow portion and a porous body in the internal space of the skin portion (A), i.e., the space enclosed by the skin portion (A). The hollow molded article has three faces (surface a, face b, and face c, in order from the largest area) that circumscribe the hollow molded article and form a rectangular parallelepiped having the smallest volume, where the length of the long side of face a is α (cm) and the arithmetic mean value of the lengths of the long side and the short side of face c is β (cm), and the ratio (α / β) is 1.5 or more and 150 or less, and satisfies the following conditions 1 and 2, and in a more preferred embodiment, condition 3.

[0020] Condition 1: The porous body has a specific gravity of 0.3 or less, and at least one porous body contacts all or part of the inner surface of the skin portion (A) that forms a ridge line extending from either or both of the points contacting surface b or surface b toward surface c when viewed from above from surface a.

[0021] Condition 2: In all cross sections perpendicular to the longitudinal direction at positions where the hollow molded article is divided into four equal parts in the longitudinal direction, the area occupied by the hollow portions is 50 to 95%, when the sum of the area occupied by the hollow portions and the area occupied by the porous body is 100%.

[0022] Condition 3: In all of the cross sections, the area occupied by the porous body in contact with the inner surface of the skin portion (A) forming the ridge line is 50 to 100% of the area occupied by the porous body.

[0023] Regarding condition 1, when viewed from above from face a, if there are multiple points or faces that are in contact with one face b, the multiple points or faces are also considered to be edges. The reason for the phrase "either or both" is that there are two faces b in one rectangular parallelepiped, so two edges are assumed (see Figure 2).

[0024] In the hollow molded article of the present invention, the ratio (α / β) of the three faces circumscribing the hollow molded article and forming a rectangular parallelepiped with the smallest volume, where α (cm) is the length of the long side of face a and β (cm) is the arithmetic mean of the lengths of the long and short sides of face c, is 1.5 or more and 150 or less. The lower limit of α / β is preferably 2 or more, more preferably 3 or more, and the upper limit is preferably 120 or less, even more preferably 100 or less. Hollow molded articles that are long in one direction are often used while being moved in a direction perpendicular to the longitudinal direction, raising concerns about collisions with other objects during movement. The present invention can effectively absorb impacts even in such collisions. Here, the rectangular parallelepiped is defined as a rectangular parallelepiped that circumscribes the hollow molded article, as shown in Figures 1 and 2, and is the rectangular parallelepiped with the smallest volume among such rectangular parallelepipeds. However, attempts to intentionally change the shape of the rectangular parallelepiped by adding parts such as runners (branches) that are unnecessary when the hollow molded article is used during production of the hollow molded article are not permitted within the meaning of the present invention. Furthermore, from a practical standpoint, it is preferable that the length of the long side of face a be 1.5 times or more the length of the long side of face c. Furthermore, if face c is square, the areas of faces a and b will be equal, and in that case, the face that satisfies condition 1 will be defined as face a.

[0025] The hollow molded article of the present invention preferably has a propeller shape (sometimes referred to as an "airfoil shape") and is molded into a propeller shape and attached to a drive shaft to rotate about the drive shaft, or is fixed to an aircraft body to increase its traveling speed, thereby generating lift and thrust. The hollow molded article of the present invention is made of a composite material containing a resin and reinforcing fibers and has a skin portion (skin portion (A)) that forms the outer surface of the hollow molded article. Here, "forming the outer surface" means that 90% or more, preferably 100%, of the outer surface of the hollow molded article is exposed, and this does not prevent a portion of the outer surface of the hollow molded article from being covered with another member such as a metal member.

[0026] Here, the composite material constituting the skin portion (A) refers to a material in which short fiber-like reinforcing fibers are dispersed in a resin, and a material in which a woven or nonwoven fabric of long or short fiber reinforcing fibers, or a reinforcing fiber array in which long or short fiber reinforcing fiber cords are arranged in a uniaxial or multiaxial direction, is impregnated with resin. Generally, the reinforcing fibers can be in a form in which they are continuously reinforced in one direction, in a form like a woven fabric or braid, or in a form in which discontinuous reinforcing fibers are randomly oriented, and there are no particular limitations on this, but when high mechanical properties are required, it is preferable for them to be in the form of continuous reinforcing fibers.

[0027] Although glass fiber, organic fiber, metal fiber, etc. can be used as the reinforcing fiber, carbon fiber is preferred because it can make the propeller blade lighter and have excellent strength and rigidity. The strength and elastic modulus of the carbon fiber used can be selected depending on the application and required characteristics. Among these, polyacrylonitrile-based carbon fiber is preferred.

[0028] The resin may be either a thermosetting resin or a thermoplastic resin. Examples of thermosetting resins that can be used include epoxy resin, vinyl ester resin, and phenol resin. Examples of thermoplastic resins that can be used include polypropylene resin, polyamide resin, polyphenylene sulfide resin, polyether ether ketone resin, polyether ketone ketone resin, and polyaryl ether ketone resin.

[0029] The hollow molded article of the present invention also includes a hollow portion and a porous body within the space inside the skin portion (A). A typical method for imparting a shape to the skin portion (A) is to place a mold inside the skin portion (A) and apply pressure from the outside to form the molded article. Since the porous body can also serve as this mold, the upper and lower skin portions can be placed around the porous body and applied pressure from the outside to form the molded article. Alternatively, a method can be employed in which a core is placed in place of all or part of the porous body to form the hollow portion, and the core is removed after molding to form the hollow portion. Therefore, in the hollow molded article of the present invention, the hollow portion is preferably structured so that the core can be removed without damaging the skin portion (A) or the porous body. To remove the core, the hollow portion must be connected to the external space, i.e., the hollow molded article must have an opening. Possible methods for making the core easier to remove include dividing the core, tapering the core, softening the core surface material, or reducing the friction coefficient of the core surface.

[0030] It is important that the porous body used in the present invention has a specific gravity of 0.3 or less. With a specific gravity within this range, the weight of the molded product can be reduced, and the presence of such a porous body in the core portion is advantageous in terms of impact absorption ability during a bird strike compared to when only a hollow portion is present. From the viewpoint of weight reduction, the specific gravity of the porous body is preferably 0.2 or less, and more preferably 0.1 or less. When multiple porous bodies are included, the specific gravity of the porous body is determined by calculating the specific gravity of the entire porous body from the specific gravity of each porous body and the volume used. On the other hand, the hollow portion is the portion within the space surrounded by the skin portion (A), and corresponds to the portion where no other material is present. There is no particular restriction on the lower limit of the specific gravity of the porous body, but from the viewpoint of impact absorption ability, it is preferably 0.001 or more.

[0031] Furthermore, various foam materials can be used as the porous body from the viewpoints of light weight and impact absorption. For example, urethane foam, acrylic foam, polyolefin foam, phenolic foam, and polymethacrylimide foam can be used. Among them, urethane foam is characterized by flexibility and excellent impact resistance, while phenolic foam can also be imparted with effects such as flame retardancy. The foam material can be selected depending on the performance required for the propeller blade.

[0032] Furthermore, the hollow molded product of the present invention may contain components other than the porous body and hollow portion within the skin portion (A). For example, in the case of a propeller blade, which is an example of a molded product of the present invention, a shear web is preferably employed because it connects the inner surface of the skin portion (A) on the upper surface of the wing and the inner surface of the skin portion (A) on the lower surface of the wing and serves as a structure supporting the skin portion (A) to reinforce or stiffen the propeller blade. As shown in Figure 6 and other figures, the shear web can enhance its reinforcing effect by extending in the wing span direction, but it does not necessarily have to be linear in top view. Furthermore, if necessary, a rib extending from the leading edge to the trailing edge (also called the trailing edge) of the wing can also be provided.

[0033] Explaining with reference to FIG. 5, this embodiment has a porous body 4, a hollow portion 3, and a shear web 5 within a space surrounded by a skin portion (A) 2.

[0034] Next, conditions 1 and 2 will be explained with reference to the drawings, but the parts that have already been explained will be omitted.

[0035] In the present invention, one or more porous bodies may be contained within the skin portion (A), with at least one porous body being in contact with the inner surface (9 in Figure 8) of the skin portion (A) that forms the leading edge of the blade. The inner surface may be nearly linear. Placing a porous body in contact with such a surface can significantly enhance the impact absorption effect in the event of a bird strike. This is because, as the propeller blade rotates, the area where birds strike or flying stones strike is concentrated on the leading edge of the blade. By arranging a porous body in a position corresponding to this area, the impact of a bird strike or flying stones is transmitted to the porous body, enhancing the impact absorption effect. Furthermore, the volume of the hollow portion can be increased, which is advantageous in terms of weight reduction. Whether or not a porous body is in contact with the inner surface of the skin portion (A) that forms the leading edge of the blade can be confirmed by cutting out a cross section of the propeller blade and observing it.

[0036] In the hollow molded product of the present invention, when the sum of the area occupied by the hollow portion and the area occupied by the porous body is 100%, the area occupied by the hollow portion is 50 to 95% in all cross sections perpendicular to the longitudinal direction at positions dividing the hollow molded product into four equal parts in the longitudinal direction. The longitudinal direction refers to the long side direction of the surface a, and in the case of a propeller blade, it is the direction connecting the blade root and the blade tip. One embodiment of the hollow molded product of the present invention used as a propeller blade will be described with reference to Figures 3 and 4. By dividing the blade length L of propeller blade 101 into four equal parts, three points, point b, point c, and point d, excluding the root a and tip e, are determined. Corresponding to these points, three cross sections, cross sections 6, cross section 7, and cross section 8, are obtained as cross sections perpendicular to the blade length direction. The symbol Ro indicates the propulsion direction of the propeller blade. In the present invention, in all of Cross Sections 6, 7, and 8, when the sum of the area occupied by the hollow portions and the area occupied by the porous body is taken as 100%, the area occupied by the hollow portions is 50 to 95%. When the sum of the area occupied by the hollow portions and the area occupied by the porous body is taken as 100%, the area occupied by the hollow portions is more preferably 60 to 90%, and even more preferably 75 to 90%. To ensure light weight, a large proportion of hollow portions is preferable, but to exhibit impact resistance, a reasonable amount of porous body is preferable, and the above ratio is preferred, taking into account the balance between the effects of both. Figure 5 shows Cross Section 7. In Cross Section 7, the skin portion (A) 2, hollow portions 3, porous body 4, and shear web 5 can be seen. The areas occupied by the hollow portions 3 and the porous body 4 in the cross section are each calculated, and the percentage of the area occupied by the hollow portions is calculated. The proportion of the area occupied by the hollow portion in the cross section can be determined by performing the same procedure on cross sections 6 and 8. A specific measurement method involves cutting out the cross section and analyzing the optically observed image.

[0037] Furthermore, in a propeller blade, which is an example of the hollow molded product of the present invention, in all of the cross sections (referring to the three cross sections), the area occupied by the porous body in contact with the inner surface of the skin portion (A) forming the blade leading edge is preferably 50 to 100% of the area occupied by the porous body in the cross section. This is because a porous body arranged in contact with the inner surface of the skin portion (A) forming the blade leading edge can more effectively absorb impacts such as bird strikes compared to a porous body not arranged in contact with the inner surface. Specifically, referring to FIG. 8 , the porous body 15 is arranged in contact with the inner surface 16 of the skin portion (A) forming the blade trailing edge (trailing edge side). Although such a porous body contributes to absorbing impacts on the trailing end of the blade, since events such as bird strikes occur mainly on the leading edge side of the blade, its contribution to the impact resistance of the entire propeller blade is smaller than that of the porous body 4 in contact with the inner surface of the skin portion (A) forming the blade leading edge. From this viewpoint, in all of the cross sections, the area occupied by the porous body in contact with the inner surface of the skin portion (A) forming the blade leading edge is more preferably 70 to 99%, and even more preferably 80 to 95%, of the area occupied by the entire porous body.

[0038] Furthermore, the hollow molded product of the present invention has an opening in one of the surfaces opposite surface c described above, i.e., in one of the two surfaces of the rectangular parallelepiped opposite surface c (the surface opposite surface c is the surface with the largest area when projected perpendicularly to surface c), and in a portion extending from the surface having the opening to 50% of the length of the long side of surface a in the longitudinal direction, the region is occupied by a surface excluding the portion overlapping with cross section Sa when viewed in the longitudinal direction in a cross section perpendicular to the longitudinal direction from cross section Sa to the tip, where Sa is the cross section where the length of the line segment between the points where a line passing through the center of gravity of the cross section in a cross section perpendicular to the longitudinal direction intersects with the outer edge of the cross section is the shortest (for convenience, this region is referred to as an "undercut region"; in other words, it can be said to be a region within the internal space of the hollow molded product excluding a space in the shape of a right column (for example, a cylindrical shape when cross section Sa is circular) with cross section Sa as the bottom surface), and the porous body occupies 30% or more by volume of the undercut region. To explain this with reference to the drawings, taking a propeller blade, which is one embodiment of the hollow molded product of the present invention, as an example, the propeller blade of FIG. 3 has an opening on the base side of the propeller shape, and the portion up to half the length of the blade length L on the side of point a (i.e., the portion between a and c) corresponds to the portion up to 50% of the length of the long side of surface a in the longitudinal direction from the surface having the opening. Referring to FIG. 7 , the cross section Sa at the base of the blade, which is perpendicular to the blade length direction and in which the length of the line segment between the points where a line passing through the center of gravity of the cross section intersects with the outer edge of the cross section, is the cross section that gives the length 13 (for convenience, in this example, the length indicated by the reference numeral 13 is assumed to be the length at which the length of the line segment between the points where a line passing through the center of gravity of the cross section intersects with the outer edge of the cross section). In this cross section, the area other than the area sandwiched between the line 10 on the blade tip side as viewed from the cross section Sa (the line 10 is a tangent to the cross section Sa) is understood to be the undercut region 11.

[0039] In the case of a propeller blade, which is an example of a hollow molded product of the present invention, the skin portion (A) is molded into a propeller shape by contacting a sheet of composite material containing resin and reinforcing fibers that will form the skin portion (A) with the surface of a propeller-shaped mold material, shaping the sheet, and, if necessary, performing a curing treatment. In this case, a core is preferably used as the mold material in the area corresponding to the hollow portion, as a component to be removed after molding. Materials for the core include metal, plastic, and sand, and are shaped to fit the shape of the hollow portion. To facilitate removal after molding, the core may be divided into multiple pieces. Furthermore, using a material that can be crumbled and removed after molding increases the design flexibility of the hollow portion. The core is typically removed from an opening at the base after molding. Meanwhile, a porous body is used as part of the mold material, filling the area inside the skin portion (A) where it should be placed. Furthermore, materials for other components such as shear webs, which are provided as needed, can also be used as part of the molding material that will remain inside the skin portion (A) after molding.

[0040] When producing a propeller blade using a mold material, the process of removing the core from the base is often very time-consuming. This is because, after molding, the resin of the skin portion (A) and the core may adhere to each other, or the difference in linear expansion between the skin portion (A) and the core may cause them to adhere to each other, making removal difficult. In particular, when the cross section of the core portion (A) at the tip side of the propeller blade is larger than the cross section of the opening near the base, i.e., when a so-called undercut region exists, removing the core after molding with a core inserted to make this core portion hollow requires removing a large core from a small opening at the base. This often requires considerable effort and time, such as assembling and using small cores that can be placed through the opening at the base, and then removing them. Therefore, it is preferable to use a porous body, which does not require removal, as the mold material for the portion corresponding to the undercut region 11, rather than using a core. Specifically, it is preferable to place a porous body in at least 30 volume % of the undercut region, and more preferably at least 50 volume %. The upper limit is preferably 95% by volume or less from the viewpoint of weight reduction.

[0041] In a propeller blade, which is an example of the hollow molded product of the present invention, it is preferable that one or more of the following a) to c) be satisfied when the blade length is taken as 100%.

[0042] a) From the viewpoint of lightness, a propeller blade, which is an example of a hollow molded product of the present invention, preferably has a cross section in which only the hollow portion extending from the base of the rotation center of the propeller shape toward the blade tip is free of porous material in a portion between 80 and 100% from the rotation center. In other words, other than the skin portion (A), it is preferable that the cross section of only the hollow portion exists between 80 and 100% of the blade length as seen from the base (see Figure 9(a)).

[0043] b) From the viewpoints of impact resistance, light weight, and molding workability, a propeller blade, which is an example of the hollow molded product of the present invention, preferably has a cross section in which a porous body is present on the inner surface of the skin portion (A) forming the leading edge of the propeller-shaped blade and on the inner surface of the skin portion (A) forming the trailing edge of the blade, in a portion between 20 and 80% of the blade length as viewed from the base of the propeller shape (see Figure 9 (b)).

[0044] c) From the viewpoint of lightness, a propeller blade, which is an example of the hollow molded product of the present invention, preferably has a cross section in which a porous body is present on the inner surface of the skin portion (A) forming the leading edge of the propeller-shaped blade in a portion between 5 and 20% of the blade length as seen from the root portion of the propeller shape, and no porous body is present on the inner surface of the skin portion (A) forming the trailing edge of the propeller-shaped blade (see Figure 9 (c)).

[0045] The propeller blade, which is an example of the hollow molded article of the present invention, will be described in more detail below with reference to Examples, although the present invention should not be construed as being limited to the description in the Examples section.

[0046] The obtained propeller blades were subjected to the following evaluations, and the results are summarized in Tables 1 and 2. It goes without saying that the outer shape of the propeller blades in each example and comparative example was the same, and therefore the length, width, and thickness of the propeller blades in each example were the same.

[0047] (Arrangement of porous bodies in propeller blades) The cross sections of the propeller blades were observed, and each cross section was classified into the following four levels: A: There are cross sections where a porous body is present on the inner surface of the skin portion (A) that forms the leading edge of the blade, and no porous body is present on the inner surface of the skin portion (A) that forms the trailing edge of the blade. B: There are cross sections where a porous body is present on both the inner surface of the skin portion (A) that forms the leading edge of the blade and the inner surface of the skin portion (A) that forms the trailing edge of the blade. C: There are cross sections where no porous body is present. D: Does not fall into any of the above A to C.

[0048] (Lightweightness Evaluation) Using the weight of the propeller blade of Comparative Example 1, 400 g, as the standard, the propeller blades were evaluated on the following five-point scale. A, B, and C were judged to be excellent in lightness, with A being judged to be the most excellent in lightness. A: 45 g or more lighter than the propeller blade of Comparative Example 1. B: 35 g or more and less than 45 g lighter than the propeller blade of Comparative Example 1. C: 25 g or more and less than 35 g lighter than the propeller blade of Comparative Example 1. D: 0 g or more and less than 25 g lighter than the propeller blade of Comparative Example 1. E: Heavier than the propeller blade of Comparative Example 1.

[0049] (Impact resistance evaluation, gelatin collision test) The obtained propeller blade was molded into a cylindrical shape with a diameter of 100 mm and a weight of 500 g. ３ The gelatin was struck at 500 km / h from the leading edge of the propeller blade, 25% of the total length of the propeller from the tip, in the chord direction, and the extent of damage was measured. Damage was judged visually, with minor damage rated A, moderate damage rated B, and major damage rated C, with ratings A and B being considered effective results in terms of impact resistance.

[0050] (Molding workability) When producing a propeller blade, the time required to remove the core was measured and rated on the following four-point scale. A and B were judged to have excellent molding workability, with A being judged to have the best molding workability. A: The work time to remove the core was less than 15 minutes B: The work time to remove the core was 15 minutes or more but less than 30 minutes C: The work time to remove the core was 30 minutes or more but less than 45 minutes D: The work time to remove the core was 45 minutes or more.

[0051] Example 1: A machined polymethacrylimide hard foam ROHACELL® 110 IG-F manufactured by Polyplaevonik Corporation was combined with a resin core to form the shape of a propeller blade (for convenience, this will be referred to as the "core material"). Figure 10 shows the specific arrangement of the resin core and porous body. A porous body 4 was placed in a portion of the undercut region 11 in Figure 10 , and a resin core was placed in the portion indicated by Sa in the cross-sectional view and in the portion corresponding to the hollow portion 3. This resin core was formed by combining a part corresponding to cross-section Sa with a part corresponding to the hollow portion 3. In this case, the ratio of the area occupied by the hollow portion and the area occupied by the porous body in a cross-section perpendicular to the blade length direction was 50%. Furthermore, in a cross-section perpendicular to the blade length direction, the area occupied by the porous body 4 relative to the undercut region 11 was 30%. In this case, the porous body was placed at a position corresponding to the position where it contacts the inner surface of the skin on the leading edge side of the blade, and the contact surface between the resin core and the porous body was aligned so that it was parallel to the blade length direction.

[0052] Next, P707AG-15 carbon fiber unidirectional prepreg manufactured by Toray Composite Materials America, Inc. was prepared and cut into the base material shape of the propeller blade skin. During this cutting, a prepreg (prepreg 1) in which the carbon fibers were oriented in a direction that was 0° relative to the blade length direction, and a prepreg (prepreg 2) in which the carbon fibers were oriented in a direction that was 90° relative to the blade length direction were prepared.

[0053] Next, the prepreg 1 and prepreg 2 that would form the skin portion (A) were laminated on the surface of the core material in a configuration of [prepreg 1 / prepreg 2 / prepreg 1 / prepreg 2 / prepreg 1 / prepreg 2 / prepreg 1 / prepreg 2 / prepreg 1 / prepreg 2 / prepreg 1 / prepreg 2] 2s, and the laminate was set in a mold that resembled the outer shape of a propeller blade. The mold was then closed and heated to 150°C for 2 hours for hardening, followed by demolding. After molding, the resin core was removed from the opening at the base of the propeller blade. Specifically, the part corresponding to cross section Sa was first removed, and then the part corresponding to hollow portion 3 was removed, thereby producing a propeller blade. The porous body was not removed, but was left in the propeller blade. The evaluation results of the resulting propeller blade are shown in Table 1.

[0054] (Examples 2 to 7, Comparative Examples 1 to 3) Propeller blades were produced in the same manner as in Example 1, except that the proportion of the area occupied by the hollow portion when the sum of the area occupied by the hollow portion and the area occupied by the porous body is taken as 100%, the proportion of the area of ​​the porous body in contact with the inner surface of the skin (A) on the leading edge side when the area occupied by the porous body in the cross section is taken as 100%, and the proportion of the porous body in the undercut region were changed as shown in the tables. Evaluation results of the obtained propeller blades are shown in Table 1 or Table 2.

[0055] Example 8 A propeller blade was produced in the same manner as in Example 1, except that a polyurethane foam block made by foaming Puff Pure Ace (registered trademark) LG5010 polyurethane rigid foam manufactured by Nippon Paftem Co., Ltd. to a specific gravity of 0.11 was used instead of the polymethacrylimide rigid foam ROHACELL (registered trademark) 110 IG-F, and the proportion of the area occupied by the hollow portion when the sum of the area occupied by the hollow portion and the area occupied by the porous body was taken as 100%, and the proportion of the area of ​​the porous body present in the leading edge portion of the blade when the area occupied by the porous body in the cross section was taken as 100%, were changed as shown in the table. Table 1 shows the evaluation results of the obtained propeller blade.

[0056] (Comparative Example 4) A polyurethane foam block was prepared by foaming Puff Pure Ace (registered trademark) LG5010, a polyurethane hard foam manufactured by Nippon Paftem Co., Ltd., to a specific gravity of 0.4, and a propeller blade was prepared in the same manner as in Example 6, except that this polyurethane foam block was used as the porous body.

[0057] The results in Tables 1 and 2 show that in Examples 1 to 6, which used a porous body with a specific gravity of 0.11, propeller blades were obtained that were both lightweight and impact-resistant, while in Example 2, in which the ratio of the hollow portion to the porous body was adjusted to a more preferred range, the lightness was improved while maintaining impact resistance, and in Example 3, in which the ratio of the hollow portion to the porous body was adjusted to an even more preferred range, the lightness was further improved. Furthermore, in Example 4, the impact resistance was further improved, and in Example 5, the molding workability was also improved.

[0058] On the other hand, Comparative Example 1 did not have a hollow portion, resulting in significantly poorer lightness. Comparative Example 2 did not use a porous body, resulting in significantly reduced impact resistance. Comparative Example 3, in which the specific gravity of the porous body was 0.4, resulted in significantly poorer lightness. Comparative Example 4, which did not satisfy Condition 2, also resulted in poorer lightness.

[0059]

[0060]

[0061]

[0062] The annotations in Tables 1 and 2 are as follows: *1: In the cross section corresponding to each point, [area occupied by the hollow portion] / ([area occupied by the hollow portion] + [area occupied by the porous body]) × 100 (%) *2: In the cross section corresponding to each point, [area occupied by the porous body in contact with the inner surface of the skin portion (A) forming the leading edge of the blade] / ([area occupied by the porous body in contact with the inner surface of the skin portion (A) forming the leading edge of the blade] + [area occupied by the porous body not in contact with the inner surface of the skin portion (A) forming the leading edge of the blade]) × 100 (%) *3: [Volume of the porous body present in the undercut region] / ([Volume of the porous body present in the undercut region] + [Volume of the porous body not present in the undercut region]) × 100 (%)

[0063] The hollow molded article according to the present invention is lightweight and has excellent impact resistance, and can be suitably used for propeller blades for UAMs, drones, etc.

[0064] 101: Propeller blade 2: Skin portion (A) 3: Hollow portion 4: Porous body 5: Shear web 6: Cross section perpendicular to the blade length direction of the propeller blade at position b 7: Cross section perpendicular to the blade length direction of the propeller blade at position c 8: Cross section perpendicular to the blade length direction of the propeller blade at position d 9: Inner surface of the leading edge of the blade of skin portion (A) 10: Tangent to cross section Sa 11: Undercut region 12: Near the root of the propeller blade 13: At the root portion, the length of the line segment between the points where a straight line passing through the center of gravity of the cross section intersects with the outer edge of the cross section is the shortest 15: Porous body 16: Inner surface of the trailing edge of the blade of skin portion (A) a: Blade root b: Point that divides the blade shape into four equal parts in the blade length direction c: Point that divides the blade shape into four equal parts in the blade length direction d: Point that divides the blade shape into four equal parts in the blade length direction e: Blade tip f, f': point tangent to surface b or ridge line from surface b to surface c w: wingspan

Claims

1. A hollow molded article comprising a skin portion (hereinafter sometimes referred to as "skin portion (A)") made of a composite material containing resin and reinforcing fibers and forming the outer surface of the hollow molded article, and a hollow portion and a porous body within the internal space of the skin portion (A), wherein the hollow molded article has three faces (faces a, b, and c, in order from largest to smallest area) that circumscribe the hollow molded article and form a rectangular parallelepiped with the smallest volume, where α (cm) is the length of the long side of face a and β (cm) is the arithmetic mean of the lengths of the long and short sides of face c, the ratio (α / β) is 1.5 to 150, and the following conditions 1 and 2 are satisfied: Condition 1: The porous body has a specific gravity of 0.3 or less, and at least one porous body is in contact with all or part of the inner surface of the skin portion (A) that forms a ridgeline extending from either or both of the points contacting face b or face c in a top view from face a. Condition 2: In all cross sections perpendicular to the longitudinal direction at positions where the hollow molded article is divided into four equal parts in the longitudinal direction, the area occupied by the hollow portions is 50 to 95%, when the sum of the area occupied by the hollow portions and the area occupied by the porous body is 100%.

2. The hollow molded article according to claim 1, further satisfying the following condition 3: Condition 3: In all of the cross sections, the area occupied by the porous body in contact with the inner surface of the skin portion (A) forming the ridge line is 50 to 100% of the area occupied by the porous body in the cross section.

3. The hollow molded product according to claim 1 or 2, characterized in that the hollow molded product has an opening in one of the faces opposite face c, and in a portion from the face having the opening in the longitudinal direction up to 50% of the length of the long side of face a, when cross section Sa is defined as the cross section where the length of the line segment between the points where a straight line passing through the center of gravity of the cross section in a cross section perpendicular to the longitudinal direction intersects with the outer edge of the cross section is the shortest, the hollow molded product has a region (for convenience, this region is referred to as an "undercut region") occupied by a surface excluding the portion overlapping with cross section Sa when viewed in the longitudinal direction in a cross section perpendicular to the longitudinal direction from cross section Sa to the tip, and the porous body occupies 30% or more by volume of the undercut region.

4. A hollow molded product according to claim 3, wherein the hollow portion of the hollow molded product has a structure that can be formed by removing a core from the opening.

5. A hollow molded product according to claim 1 or 2, wherein the composite material forming the skin portion (A) is molded into a propeller shape, and when the blade length of the propeller shape is taken as 100%, in a cross section of a portion between 5 and 20% of the blade length as seen from the base of the propeller shape, a porous body is present on the inner surface of the skin portion (A) forming the leading edge of the propeller-shaped blade, and a porous body is not present on the inner surface of the skin portion (A) forming the trailing edge of the propeller-shaped blade.

6. A hollow molded product according to claim 1 or 2, wherein the composite material forming the skin portion (A) is molded into a propeller shape, and when the blade length of the propeller shape is taken as 100%, a porous body is present on the inner surface of the skin portion (A) forming the leading edge of the propeller-shaped blade and on the inner surface of the skin portion (A) forming the trailing edge of the blade in a cross section of a portion between 20 and 80% of the blade length as seen from the base of the propeller shape.

7. A blow molded product according to claim 1 or 2, wherein the composite material constituting the skin portion (A) is molded into a propeller shape, and when the blade length of the propeller shape is taken as 100%, the cross section has no porous body in a portion between 80 and 100% of the blade length as seen from the base of the propeller shape.

8. The hollow molded product according to claim 1 or 2, wherein the reinforcing fibers are carbon fibers.

9. The blow molded product according to claim 1 or 2, wherein the porous body is selected from the group consisting of urethane foam, acrylic foam, polyolefin foam, phenol foam, and polymethacrylimide foam.

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