Method for manufacturing fiber-reinforced resin articles, and fiber-reinforced resin articles

Abstract

The present invention provides a beneficial improvement relating to a method for manufacturing a fiber-reinforced resin article having at least partially a structure comprising a bent or curved wall surface, such as a hollow structural portion or a U-shaped cross-sectional structural portion.　This method for manufacturing a fiber-reinforced resin article comprises: a forming step in which a prepreg preformed body is heated in a press die together with a core made of a wax material to obtain a cured product; and a core removal step in which the core is removed from the cured product. The core has an expansion portion made of a first wax material, and a stamper portion made of a second wax material incompatible with the first wax material. The stamper portion is disposed on the surface of the core, and a section mirroring the surface shape of the stamper portion is formed on the surface of the cured product.

Description

Technical Field

[0001] The present invention mainly relates to a method for manufacturing a fiber-reinforced resin article and a fiber-reinforced resin article. This application claims priority based on Japanese Patent Application No. 2021-069190 filed with the Japan Patent Office on April 15, 2021, and Japanese Patent Application No. 2021-106583 filed with the Japan Patent Office on June 28, 2021, and incorporates the contents herein by reference.

Background Art

[0002] Fiber Reinforced Plastic (FRP) is lightweight and has excellent mechanical properties, so it is used in various applications including reinforcement members for automobiles, and its importance has been increasing in recent years. A method has been proposed for manufacturing a fiber-reinforced resin article having a hollow structure portion or a cross-sectional U-shaped structure portion by heating and curing a prepreg preform together with a core made of a wax material in a press mold (Patent Document 1).

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] The main object of the present invention is to provide a useful improvement regarding a method for manufacturing a fiber-reinforced resin article having at least a part of a structure composed of a bent or curved wall surface such as a hollow structure portion or a cross-sectional U-shaped structure portion by heating and curing a prepreg preform together with a core made of a wax material in a press mold. An object of the present invention is to provide a technique for controlling the surface shape of the portion of a fiber-reinforced resin article that was in contact with the core during molding, manufactured by the above method. Problems addressed by each embodiment of the present invention may be disclosed explicitly or implicitly herein. [Means for solving the problem]

[0005] According to one aspect of the present invention, a method for manufacturing a fiber-reinforced resin article is provided, comprising a molding step of heating a prepreg premolded body together with a core made of a wax material in a press die to obtain a cured product, and a core removal step of removing the core from the cured product, wherein the core has an expanded portion made of a first wax material and a stamper portion made of a second wax material incompatible with the first wax material, the stamper portion is arranged on the surface of the core, and a portion reflecting the surface shape of the stamper portion is formed on the surface of the cured product. According to another aspect of the present invention, a fiber-reinforced resin article is provided having a hollow structure in which a cavity is formed and ribs formed on the wall surface of the cavity. According to yet another aspect of the present invention, a fiber-reinforced resin article is formed having a hollow structure with a cavity formed therein and a rib pattern formed on the wall surface of the cavity. According to yet another aspect of the present invention, a fiber-reinforced resin article is formed having a hollow structure with a cavity formed therein and a boss formed on the wall surface of the cavity. [Effects of the Invention]

[0006] A useful improvement is provided to a method for manufacturing a fiber-reinforced resin article having at least part of a structure consisting of bent or curved wall surfaces, such as a hollow structure or a U-shaped cross-section structure, by placing a prepreg premolded body together with a core made of wax material in a press die and curing it. [Brief explanation of the drawing]

[0007] [Figure 1]Figure 1 is a flowchart of a fiber-reinforced resin article manufacturing method according to an embodiment. [Figure 2] Figure 2 is a schematic diagram of an FRP article that can be manufactured by the fiber-reinforced resin article manufacturing method according to the embodiment, where Figure 2(a) is a perspective view and Figure 2(b) is a cross-sectional view. [Figure 3] Figure 3 is a schematic diagram of a core that may be used in the fiber-reinforced resin article manufacturing method according to the embodiment, where Figure 3(a) is a perspective view and Figure 3(b) is a cross-sectional view. [Figure 4] Figure 4 is a cross-sectional view of a prepreg premolded body fabricated with a core positioned inside. [Figure 5] Figure 5 is a cross-sectional view showing the prepreg premolded body shown in Figure 4 being placed into the mold. [Figure 6] Figure 6 shows a cross-sectional view of the cured product obtained from the prepreg premolded body shown in Figure 4, before the core is removed from the cavity. [Figure 7] Figure 7 is a cross-sectional view of an FRP article obtained from the prepreg premolded body shown in Figure 4. [Figure 8] Figure 8 is a schematic diagram of a core that may be used in a fiber-reinforced resin article manufacturing method according to an embodiment, where Figure 8(a) is a perspective view and Figure 8(b) is a cross-sectional view. [Figure 9] Figure 9 is a schematic diagram of a core that may be used in the FRP article manufacturing method according to the embodiment, where Figure 9(a) is a perspective view and Figure 9(b) is a cross-sectional view. [Figure 10] Figure 10 is a cross-sectional view showing an example of the core manufacturing procedure shown in Figure 9. [Figure 11] Figure 11 is a schematic diagram of a core that may be used in a fiber-reinforced resin article manufacturing method according to an embodiment, where Figure 11(a) is a perspective view and Figure 11(b) is a cross-sectional view. [Figure 12] Figure 12 is a schematic diagram of a core that may be used in a fiber-reinforced resin article manufacturing method according to an embodiment, where Figure 12(a) is a perspective view and Figure 12(b) is a cross-sectional view. [Figure 13] Figure 13 is a cross-sectional view of a prepreg premolded body that has been fabricated with a core positioned inside. [Figure 14] Figure 14 is a cross-sectional view of a fiber reinforced resin article obtained using the core shown in Figure 12. [Figure 15] Figure 15 is a cross-sectional view showing a portion where the surface of the core shown in Figure 3 is covered with a barrier film. [Figure 16] Figure 16 is a cross-sectional view showing a state where a prepreg preform is placed in a mold together with a core whose surface is covered with a barrier film.

Mode for Carrying Out the Invention

[0008] 1. Method for manufacturing a fiber reinforced resin article One embodiment of the present invention relates to a method for manufacturing a fiber reinforced resin article (hereinafter also referred to as an FRP article). The method for manufacturing an FRP article according to the embodiment consists of the following two steps, as shown in the flow in Figure 1. (i) A molding step of heating a prepreg preform together with a core made of a wax material in a press mold to obtain a cured product. (ii) A core removal step of removing the core from the cured product. The above core has an expansion part made of a first wax material and a stamper part made of a second wax material that is incompatible with the first wax material. The stamper part is disposed on the surface of the core, and a portion reflecting the surface shape of the stamper part is formed on the surface of the cured product obtained in the molding step. Hereinafter, the method for manufacturing an FRP article according to the embodiment will be described in detail while referring to the drawings.

[0009] 1.1. Molding step The molding step is a step of heating a prepreg preform together with a core made of a wax material in a press mold to obtain a cured product.

[0010] (Prepreg preform) The prepreg preform is prepared outside the press mold in advance using a prepreg such as a prepreg sheet or a tow prepreg as a main material so as to have a near net shape. When prepreg sheets are used, two or more prepreg sheets may be laminated in part or all of the prepreg premolded body. The portion in which multiple prepreg sheets are laminated may include two or more prepreg sheets of the same type, two or more prepreg sheets of different types, or both. A prepreg premolded body using a prepreg sheet may have a portion reinforced with tow prepreg. The prepreg premolded body may be formed in part or entirely from tow prepreg.

[0011] The fiber reinforcements used in prepregs come in a variety of forms, including continuous fibers, chopped fibers, woven fabrics, nonwoven fabrics, and non-crimped fabrics. A prepreg sheet having a fiber-reinforced material made up of multiple continuous fiber bundles arranged in one direction is called a unidirectional prepreg (UD prepreg). A prepreg sheet that uses a fabric made of continuous fiber bundles as a fiber reinforcement material is called a cross prepreg. Prepreg sheets that use a mat formed by depositing chopped fiber bundles as a fiber reinforcement material are called SMC (sheet molding compound). Tow prepreg is a prepreg that uses a single continuous fiber bundle as a reinforcing material. Examples of fibers used in fiber reinforcement materials include carbon fibers, glass fibers, aramid fibers, silicon carbide fibers, and metal fibers. Two or more types of fibers may also be used in combination.

[0012] Examples of thermosetting resins used in prepregs include epoxy resins, urea resins, vinyl ester resins (also called epoxy acrylate resins), unsaturated polyesters, polyurethanes, and phenolic resins. Two or more thermosetting resins may be used in mixture form. The content of the thermosetting resin composition in the prepreg is not limited, but is, for example, 15 to 60% by mass. This content may be 15 to 20% by mass, 20 to 25% by mass, 25 to 40% by mass, 40 to 50% by mass, 50 to 60% by mass, etc. Various additives may be added to thermosetting resin compositions. Examples include curing agents, thickeners, reactive diluents, flame retardants, defoaming agents, defoaming agents, mold release agents, particulate fillers, colorants, silane coupling agents, and antibacterial agents.

[0013] (core) The core is provided with an expansion section made of a first wax material and a stamper section made of a second wax material that is incompatible with the first wax material. The stamper section is positioned on the surface of the core. Details of the first and second wax materials will be described later. The overall shape of the core is, for example, if the goal is to manufacture an FRP article having a hollow structure, then it is to be approximately the same as the cavity formed in that hollow structure.

[0014] Here, we will describe an example of the arrangement of the expansion section and stamper section, using as a model the core used when manufacturing a hollow FRP article 1 in the shape of a rectangular prism, as shown in Figure 2, where the top and bottom surfaces are square and the four sides are rectangular. The overall shape of this core is a rectangular prism, with a square top and bottom surface and rectangular sides, similar to the cavity in FRP article 1. In one example, as shown in the cross-sectional view in Figure 3, the core 10 may have a rectangular prism-shaped expansion portion 12 and two plate-shaped stamper portions 14 arranged to cover its upper and lower surfaces, respectively. The expansion portion 12 is exposed on the side surface of the core 10.

[0015] In the core 10, the centrally located expansion portion 12 may be a single mass of the first wax material, or it may be a stack of multiple plates made of the first wax material. The core 10 can be obtained by separately manufacturing the expansion section 12 and multiple stamper sections 14, and then assembling them. The expansion section 12 and the stamper sections 14 do not need to be bonded to each other.

[0016] (hardening) When manufacturing an FRP article having a hollow structure, a prepreg premolded body is first created such that a core is positioned to correspond to the cavity formed in the hollow structure. Next, the prepreg premolded body, along with the core positioned inside it, is fed into a press die. Figure 4 is a cross-sectional view of a prepreg premolded body prepared for manufacturing the FRP article 1 shown in Figure 2. The prepreg premolded body 20 has a rectangular prism shape, and a core 10 having the structure shown in Figure 3 is located inside.

[0017] In this specification, the temperature of the press die used to cure the prepreg preform is referred to as the molding temperature. The molding temperature should be such that the prepreg premolded body can be cured in preferably 1 hour or less, more preferably 40 minutes or less, and even more preferably 20 minutes or less. The molding temperature can be, for example, 100°C or higher, and may be 120°C or higher, or even 130°C or higher. The higher the molding temperature, the shorter the time required for the prepreg premolding to harden. When the molding temperature exceeds 160°C, the selection of first and second wax materials becomes limited. From the viewpoint of shortening the time required to heat the press die and reducing energy consumption, the molding temperature can be set to 150°C or lower, and even 140°C or lower.

[0018] The temperature of the press die is maintained at the molding temperature by the temperature control mechanism typically provided by the press molding machine, even before the prepreg premolding body is introduced. Figure 5 shows the prepreg premolded body 20, as shown in Figure 4, being fed together with the core 10 into a press die 100 consisting of a lower die 102 and an upper die 104. Since the press die 100 is pre-heated to the molding temperature, the core 10 begins to heat up and expand immediately after being placed into the press die 100 along with the prepreg pre-molded body 20. As the core 10 expands, the prepreg preform 20 is pressed against the inner surface of the press die 100. In other words, the pressure generated within the press die 100 as the core 10 attempts to expand against the clamping force is applied to the prepreg preform 20. By curing the prepreg under pressure, FRP with fewer voids is produced.

[0019] The first and second wax materials are described below. The first wax material has a lower softening temperature than the second wax material. There is a sufficient difference between the softening temperatures of the first wax material and the second wax material so that the second wax material does not soften in the stamper section before the first wax material softens in the expansion section. This means that the pressure generated within the press die is primarily due to the expansion of the expansion section. When the first wax material begins to soften in a portion of the expansion section, the pressure inside the press die rises rapidly.

[0020] Furthermore, it is necessary that expansion due to the softening of the first wax material begins before the plasticity of the prepreg premolded body is lost due to the progression of hardening. One way to accelerate the softening of the first wax material is to lower its softening temperature. Another way to accelerate the softening of the first wax material is to preheat the core before placing it in the press die, thereby bringing its temperature closer to the softening temperature of the first wax material. When preheating, the core, with its internal structure inside the prepreg premolded body, should be placed in an oven along with the prepreg premolded body. These two methods can be employed simultaneously.

[0021] It is preferable that the time from when the first wax material begins to soften until it melts and becomes a highly fluid liquid is long. When this time is sufficiently long, the prepreg premolded body hardens before the first wax material melts, so the molten first wax material does not flow into the gap between the prepreg premolded body and the press die.

[0022] Polyolefin waxes are preferred as primary wax materials due to their long softening and melting time. Typical examples of polyolefin waxes are polyethylene wax and polypropylene wax. Preferred examples of polyolefin waxes include pyrolysis-type polyethylene wax, which is a pyrolysis product of polyethylene resin, and pyrolysis-type polypropylene wax, which is a pyrolysis product of polypropylene resin.

[0023] It is preferable, but not essential, that the second wax material does not soften at the molding temperature. Since heat from the press die is transferred to the stamper section via the prepreg premolded body, it takes time for the temperature of the stamper section to reach the molding temperature after the press die is closed. Therefore, if the first wax material softens sufficiently quickly, the pressure inside the press die may build up before the second wax material softens in the stamper section, even if the softening temperature of the second wax material is lower than the molding temperature.

[0024] Because the pressure inside the press die builds up before the prepreg premolded body loses its plasticity and before the second wax material softens in the stamper area, the surface of the prepreg premolded body deforms to conform to the surface of the stamper area in the portion in contact with it. As a result, a portion of the cured product is formed that reflects the surface shape of the stamper area.

[0025] For example, when the prepreg premolded body 20 shown in Figure 4 is cured in a press die, as shown in Figure 6, the inner surface of the cured product 30 becomes flatter in the portion in contact with the stamper portion 14 compared to the portion in contact with the expansion portion 12. The reason why the flatness of the inner surface of the hardened product 30 is poor in the portion that contacts the expanded portion 12 of the core 10 is that the pressure increase inside the press die 100 during the molding process is mainly caused by the softening of the surface of the expanded portion 12. Even when the softened surface of the expanded portion 12 is pressed against the surface, the shape of the inner surface of the prepreg premolded body 20 does not change significantly.

[0026] 1.2. Core Removal Process The core removal step (ii) is a step in which the core used in the molding step is removed from the cured product obtained in the molding step. If the cured product has a hollow structure, the first wax material and the second wax material, which are the core materials, are removed from inside the cavity formed in that hollow structure. While not limited to this method, one example involves first heating the cured material in an oven to melt the first and second wax materials within the cavity, and then using a drilling tool to create discharge holes in the hollow structure. Through these discharge holes, the melted first and second wax materials can be discharged to the outside of the cavity. The core can be removed by melting the first wax material and the second wax material, not only when the cured product has a hollow structure, but also when the cured product has an undercut. When the cured product has neither a hollow structure nor an undercut, the core may be removed without melting.

[0027] Figure 7 shows a cross-section of the FRP article 1 obtained by removing the core 10 from the cavity of the cured material 30 shown in Figure 6. As can be seen from Figure 7, the use of a core 10 consisting of an expansion section 12 and a stamper section 14 flattens the inner surface (cavity wall) of the FRP article 1, which is equivalent to the wall thickness t of the FRP article 1. W This contributes to improving uniformity. Wall thickness t W Improving the uniformity of the material can contribute to reducing variations in the mechanical properties and strength of the FRP article 1. This effect is particularly pronounced when the prepreg premolded body 20 contains SMC. This is because, among the various types of prepreg sheets, SMC has a large range of in-plane thickness variation.

[0028] Since the first wax material and the second wax material are incompatible with each other, they can be easily separated and reused. When there is a sufficient difference in the melting points of the first wax material and the second wax material, they can be separated by melting only the one with the lower melting point. When the specific gravities of the molten first wax material and the molten second wax material are different, they can be separated by utilizing the difference in specific gravity.

[0029] Waxes primarily composed of hydrocarbons and waxes made of organic compounds containing polar groups are often incompatible with each other. Polar groups are functional groups (excluding ether groups) that contain a carbon-oxygen bond or a carbon-nitrogen bond, such as hydroxyl groups, amino groups, amide groups, carbonyl groups, carboxyl groups, and ester groups. When a polyolefin wax is incorporated into the first wax material, preferably, a wax containing an organic compound having a polar group is incorporated into the second wax material. Typical examples of waxes containing organic compounds with polar groups include waxes containing one or more organic compounds selected from hydroxy fatty acid amides, fatty acid amides, hydroxy fatty acid esters, and fatty acid esters.

[0030] 1.3. Various Embodiments The core used when manufacturing the FRP article 1 shown in Figure 2 is not limited to having the structure shown in Figure 3. In a preferred example, the core may have a structure in which, as shown in Figure 8, the top and bottom surfaces of the rectangular prism-shaped expansion portion 12, as well as the four sides, are covered with plate-shaped stamper portions 14. Since the core 10 shown in Figure 8 does not have any portion of the expansion portion 12 exposed on its surface, the flatness of the entire inner surface is improved in the FRP article 1 manufactured using this core by the method described above.

[0031] The core 10 shown in Figure 9 is a modified version of the core shown in Figure 8, and the entire surface of the expansion portion 12 is covered by an integrally formed stamper portion 14. The core 10 shown in Figure 9 can be manufactured by following the procedure shown in Figure 10. First, as shown in Figure 10(a), a mold M with injection / discharge holes H is prepared, having an internal dimension slightly larger than the external dimension of the core 10 to be manufactured. As shown in Figure 10(b), the molten second wax material WM2 is injected into the mold M through the injection / discharge hole H. If the mold M is cooled to an appropriate temperature at this time, a portion of the second wax material will solidify on the wall surface of the mold M. The remaining second wax material that has not yet solidified is discharged from the mold M through the injection / discharge hole, thereby forming the stamper portion 14 as shown in Figure 10(c).

[0032] Next, the molten first wax material WM1 is injected into the mold M through the injection / discharge hole H, as shown in Figure 10(d). At this time, the temperature of the first wax material WM1 is set so that the second wax material does not melt in the stamper section 14. After the first wax material WM1 solidifies and the expanded portion 12 is formed, the core 10 is removed from the mold M as shown in Figure 10(e). Finally, the core 10 is completed by filling the remaining holes in the stamper section 14 with the second wax material, as shown in Figure 10(f). This core fabrication method is also useful when the core has a complex external shape or when the core has curved sections on its surface.

[0033] The core 10 shown in Figure 11 is another variation of the core shown in Figure 8, with a rectangular prism-shaped low-expansion section 16 made of a second wax material positioned at its center. Multiple plate-shaped expansion sections 12 are arranged to cover the top, bottom, and four sides of the low-expansion section 16, and multiple plate-shaped stamper sections 14 are positioned further outside of these. The expansion sections 12 are not exposed on the surface of the core 10. In the core 10 shown in Figure 11, the reason why the low-expansion portion 16 is located in the center is to reduce the volume ratio of the expansion portion 12 to the core 10, thereby preventing the core 10 from expanding too much during the molding process.

[0034] The core 10 shown in Figure 12 is yet another modification of the core shown in Figure 8, in which cross grooves G are provided on the surface of the stamper portion 14 on both the upper and lower surfaces. The purpose of this cross groove G is to form a cross-shaped rib on the wall surface inside the cavity of the FRP article 1 to be manufactured. As shown in Figure 13, the prepreg premolded body 20 with the core shown in Figure 12 positioned inside does not have a shape that follows the inner surface of the cross groove G, and its upper and lower surfaces are flat.

[0035] During the molding process, a portion of the prepreg material constituting the prepreg premolded body 20 flows into the cross groove G. In particular, when the prepreg premolded body 20 contains SMC, the inside of the groove is more easily filled with the prepreg material. In one example, during the process of manufacturing the prepreg premolded body 20, prepreg material prepared separately from the prepreg premolded body 20 may be filled into the cross groove G in advance. The FRP article 1 shown in Figure 13 is manufactured using the core 10 shown in Figure 12, and has ribs R formed in a cross shape on the ceiling and bottom surfaces of the cavity.

[0036] The groove pattern on the core 10 shown in Figure 12 is merely one example. There are no limitations to the groove patterns that can be formed on the stamper surface to provide ribs to the FRP article; various patterns are possible, such as rectangular grid patterns, triangular grid patterns, radial patterns, and striped patterns. Multiple regions with different groove patterns can also be provided on the stamper surface. The depth of the grooves may be the same or different between different grooves. It is not necessary for the depth of each groove to be constant. By forming ribs, it is possible to reduce the wall thickness while maintaining rigidity, thereby making FRP products lighter. In addition to grooves for rib formation, recesses for forming bosses on the walls of the cavities of the FRP article to be manufactured may be provided on the surface of the stamper portion.

[0037] Various additives can be added to the core as needed. For example, by adding particles of a material that generates heat under a high-frequency electromagnetic field to the first wax material, the expansion portion can be inductively heated. Examples of such materials include ferromagnetic materials, ferrimagnetic materials, and conductive materials. Examples of ferromagnetic materials include iron, nickel, cobalt, iron alloys, nickel alloys, cobalt alloys, permalloy, and many types of steel. Examples of ferrimagnetic materials include magnetite, nickel-zinc ferrite, manganese-zinc ferrite, and copper-zinc ferrite. Examples of conductive materials include copper, aluminum, and brass.

[0038] In another example, the expansion portion can be microwave-heated by adding particles of a material that has the property of absorbing microwaves and generating heat to the first wax material. Examples of such materials include silicon carbide, ferrite, barium titanate, anatase titanium oxide, graphite, and carbon black. Induction heating or microwave heating of the expansion section can preferably be performed when preheating the expansion section outside the press die. For example, particles of a material that generates heat under a high-frequency electromagnetic field or particles of a material that generates heat by absorbing microwaves may be added to both the first and second wax materials so that the entire core can be heated by induction or microwave. Then, induction heating or microwave heating can be used to melt the entire core in the core removal process.

[0039] For example, a coloring agent such as a pigment or dye can be used to make the first wax material and the second wax material visually distinguishable by their color difference. The coloring agent may be added to either the first wax material or the second wax material, or to both. By making the first wax material and the second wax material different in color, it is possible to prevent, for example, mixing up the expansion part and the stamper part when assembling the core. In the process of separating the first and second wax materials after the core removal process, it is also advantageous if the two materials have different colors.

[0040] In one example, the core may be used after its surface is covered with a barrier film. Figure 15 shows the core surface shown in Figure 3 covered with a barrier film. The barrier film 18 is in close contact with the core 10 and conforms to the surface of the core 10. Figure 16 shows the prepreg premolded body 20 being fed into the press die 100 along with the core 10 covered with a barrier film 18. The purpose of the barrier film 18 is to prevent the first wax material, which forms the expansion portion 12, from leaking into the gap between the prepreg preform 20 and the press die when the first wax material melts before the prepreg preform 20 hardens.

[0041] To prevent fracture due to core expansion, the barrier film is made of a material that can undergo tensile deformation at the molding temperature. This tensile deformation may be elastic, plastic, or possess both properties. Therefore, preferred materials for barrier films are, but are not limited to, organic materials, and in particular, resin materials. Preferred materials for barrier films include synthetic polymers such as polyolefins, polyamides, polyesters, polyurethanes, silicones, and fluororubber, and furthermore, elastomers made of these polymers. The thickness of the barrier film is not limited to any range in which the above objectives are achieved, but for example, it may be 0.05 to 1 mm, 0.5 mm or less, or even 0.1 mm or less.

[0042] In one example, the barrier film can be formed by thermally shrinking a shrinkable polymer tube around the core. In another example, the barrier film can be formed by applying liquid rubber to the surface of the core and allowing it to dry. In yet another example, the barrier film can also be formed from a UV-curable elastomer. The liquid barrier film material can preferably be applied to the core surface by spraying.

[0043] 1.4. Scope of Application The scope of application of the FRP article manufacturing method according to the embodiment is not limited to the manufacture of FRP articles of the specific shapes exemplified in the above description. The FRP article manufacturing method according to the embodiment can be applied to the manufacture of any FRP article having at least a part of its structure consisting of bent or curved wall surfaces. Typical examples of structures consisting of bent or curved wall surfaces include hollow structures, cylindrical structures, U-shaped cross-section structures, and L-shaped cross-section structures. Various undercut sections also often have bent or curved wall surfaces within the structure.

[0044] The FRP article manufacturing method according to this embodiment is also applicable when manufacturing large FRP articles from two or more partial prepreg premolded bodies. In an FRP article manufactured by the FRP article manufacturing method according to the embodiment, the FRP may be molded in such a way that a composite is formed between it and a part made of a material other than FRP, such as metal.

[0045] 1.5. Summary of Embodiments Embodiments of the present invention include, but are not limited to, the following. [Embodiment 1] A method for manufacturing a fiber-reinforced resin article, comprising a molding step of heating a prepreg premolded body together with a core made of wax material in a press die to obtain a cured product, and a core removal step of removing the core from the cured product, wherein the core has an expansion portion made of a first wax material and a stamper portion made of a second wax material incompatible with the first wax material, the stamper portion is arranged on the surface of the core, and a portion reflecting the surface shape of the stamper portion is formed on the surface of the cured product. [Embodiment 2] A manufacturing method according to Embodiment 1, wherein the expanded portion is not exposed on the surface of the core. [Embodiment 3] A manufacturing method according to Embodiment 1 or 2, wherein the core further comprises a low-expansion portion made of the second wax material. [Embodiment 4] A manufacturing method according to any one of Embodiments 1 to 3, wherein the core has grooves for forming ribs provided on the surface of the stamper portion. [Embodiment 5] A manufacturing method according to any one of Embodiments 1 to 3, wherein the core has a groove pattern for forming a rib pattern provided on the surface of the stamper portion. [Embodiment 6] The manufacturing method according to Embodiment 5, wherein the groove pattern includes one or more selected from a rectangular grid pattern, a triangular grid pattern, a radial pattern, and a striped pattern. [Embodiment 7] A manufacturing method according to any one of Embodiments 1 to 6, wherein the core has a recess for forming a boss provided on the surface of the stamper portion. [Embodiment 8] A manufacturing method according to any one of Embodiments 1 to 3, wherein the surface of the stamper portion is flat. [Embodiment 9] A manufacturing method according to any one of Embodiments 1 to 3, wherein the core has a projection or recess provided on the surface of the stamper portion. [Embodiment 10] A manufacturing method according to any one of Embodiments 1 to 9, wherein the surface of the core is covered with a barrier film before the molding step. [Embodiment 11] A manufacturing method according to any one of Embodiments 1 to 10, wherein the prepreg premolded body includes SMC. [Embodiment 12] A manufacturing method according to any one of Embodiments 1 to 11, wherein the core is preheated before the molding step. [Embodiment 13] A manufacturing method according to any one of Embodiments 1 to 12, wherein the first wax material is blended with polyolefin wax. [Embodiment 14] A manufacturing method according to Embodiment 13, wherein polypropylene wax is blended as the polyolefin wax in the first wax material. [Embodiment 15] A manufacturing method according to Embodiment 13 or 14, wherein polyethylene wax is blended as the polyolefin wax in the first wax material. [Embodiment 16] A manufacturing method according to any one of Embodiments 13 to 15, wherein the second wax material contains an organic compound having a polar group. [Embodiment 17] A manufacturing method according to any one of Embodiments 1 to 16, wherein one or both of the first wax material and the second wax material contain a coloring agent in order to make the first wax material and the second wax material visually distinguishable. [Embodiment 18] A manufacturing method according to any one of Embodiments 1 to 17, wherein the expansion portion contains particles of a material that has the property of generating heat under a high-frequency electromagnetic field. [Embodiment 19] A manufacturing method according to any one of Embodiments 1 to 17, wherein the expansion portion contains particles of a material that has the property of absorbing microwaves and generating heat. [Embodiment 20] A fiber-reinforced resin article manufactured using a manufacturing method according to any one of Embodiments 5 to 7. [Embodiment 21] A fiber-reinforced resin article having a hollow structure in which a cavity is formed and ribs formed on the wall surface of the cavity. [Embodiment 22] A fiber-reinforced resin article having a hollow structure in which a cavity is formed and a rib pattern formed on the wall surface of the cavity. [Embodiment 23] A fiber-reinforced resin article according to Embodiment 22, wherein the rib pattern includes one or more selected from a rectangular grid pattern, a triangular grid pattern, a radial pattern, and a striped pattern. [Embodiment 24] A fiber-reinforced resin article according to any one of embodiments 21 to 23, having a boss formed on the wall surface of the cavity. [Embodiment 25] A fiber-reinforced resin article having a hollow structure in which a cavity is formed and a boss formed on the wall surface of the cavity.

[0046] 2. Experimental Results The following are the results of experiments conducted by the inventors. 2.1. Experiment 1 A hollow rectangular prism with external dimensions of 300mm x 100mm x 30mm was fabricated as an FRP (fiber-reinforced plastic) item. A prepreg premolding body with nearly the net shape was produced by stacking two sheets of SMC (STR120N131, manufactured by Mitsubishi Chemical Corporation), which is made by impregnating chopped carbon fiber mat with epoxy acrylate resin, has a fiber content of 53% by mass, and a thickness of approximately 2 mm, cutting it into a predetermined shape, and then folding it. When fabricating the prepreg premolded body, a separately prepared rectangular core was placed inside, with its surface covered by a barrier film. The core was made using a pyrolysis-type polypropylene wax with a melting point of 123°C [Viscol®, manufactured by Sanyo Chemical Industries, Ltd.], and was fabricated using a mold to a size that would fit perfectly inside the prepreg premolded body when its surface was covered with the barrier film. A nylon stretch film [Stretchlon® 800, manufactured by Airtech] was used as the barrier film.

[0047] A prepreg premolded body with a core inside was placed in a mold that had been preheated to the molding temperature, and then heated and cured. The molding temperature was 140°C, and the molding time was 10 minutes. The cured material removed from the mold was heated to 150°C in an oven to melt the wax forming the core, and then a 10mm diameter discharge hole was drilled into the cured material to discharge the wax. The resulting hollow rectangular prism had a good appearance, and its shape and dimensions were as designed. Using a surface roughness measuring instrument [Surfcom® 1400LCD, manufactured by Tokyo Seimitsu Co., Ltd.], the arithmetic mean roughness (reference length 50 mm) at the center of the cavity wall (approximately 300 mm x 100 mm) was measured and found to be 45 μm.

[0048] 2.2. Experiment 2 A hollow rectangular prism made of FRP was fabricated in the same manner as in Experiment 1, except that the core used was changed. The core used in Experiment 2 was fabricated according to the procedure shown in Figure 10, and had a structure in which the entire surface of the expanded portion, which was made of pyrolytic polypropylene wax, was densely covered with a stamper layer made of another wax that was incompatible with this wax. The pyrolysis-type polypropylene wax was the same as the one used in Experiment 1. For forming the stamper portion, a synthetic wax with a melting point of 142°C, mainly composed of N,N'-ethylene-bis-12-hydroxystearylamide (ITOWAX J-530, manufactured by Ito Oil Co., Ltd.), was used. The thickness of the stamper portion was approximately 2 mm. The external dimensions of the core were almost the same as those of the one fabricated in Experiment 1. The resulting hollow rectangular prism had a good appearance, and its shape and dimensions were as designed. The arithmetic mean roughness of the cavity wall surface was measured in the same manner as in Experiment 1 and was found to be 12 μm.

[0049] 2.3. Experiment 3 A hollow rectangular parallelepiped made of FRP was fabricated in the same manner as in Experiment 2, except that the wax used in the expansion section of the core was changed from a pyrolytic polypropylene wax to a pyrolytic polyethylene wax with a melting point of 107°C [Sunwax (registered trademark) 151-P manufactured by Sanyo Chemical Industries, Ltd.]. The resulting hollow rectangular prism had a good appearance, and its shape and dimensions were as designed. The arithmetic mean roughness of the cavity wall surface was measured in the same manner as in Experiment 1 and was found to be 10 μm.

[0050] 2.4. Experiment 4 A hollow rectangular parallelepiped made of FRP was fabricated in the same manner as in Experiment 2, except that the core expansion section was formed using a wax material blended in a weight ratio of 1:1 between pyrolytic polypropylene wax [Viscol (registered trademark) manufactured by Sanyo Chemical Industries, Ltd.] and pyrolytic polyethylene wax [Sunwax 151-P manufactured by Sanyo Chemical Industries, Ltd.]. The resulting hollow rectangular prism had a good appearance, and its shape and dimensions were as designed. The walls of the cavities were flatter than those of the FRP articles fabricated in Experiment 1.

[0051] Although the present invention has been described above with reference to specific embodiments, each embodiment is presented as an example and does not limit the scope of the present invention. Each embodiment described herein can be modified in various ways within the scope in which the effects of the invention are achieved, and can be combined with features described in other embodiments to the extent that is feasible. [Industrial applicability]

[0052] The inventions disclosed herein can be preferably used, for example, when manufacturing parts (including structural parts) for automobiles, ships, railway vehicles, aircraft, unmanned aerial vehicles and other transportation equipment, as well as various sporting goods, including bicycle frames, tennis rackets and golf shafts, using fiber-reinforced resin. [Explanation of Symbols]

[0053] 1. Fiber-reinforced plastic articles (FRP articles) 10 cores 12 Expansion section 14 Stamper section 16 Low expansion section 18 Barrier film 20 Prepreg preforms 30 Cured product 100 press molds 102 Lower mold 104 Upper mold

Claims

1. A method for manufacturing a fiber-reinforced resin article, comprising a molding step of heating a prepreg premolded body together with a core made of wax material in a press die to obtain a cured product, and a core removal step of removing the core from the cured product, wherein the core has an expansion portion made of a first wax material and a stamper portion made of a second wax material incompatible with the first wax material, the stamper portion is arranged on the surface of the core, and a portion reflecting the surface shape of the stamper portion is formed on the surface of the cured product.

2. The manufacturing method according to claim 1, wherein the expanded portion is not exposed on the surface of the core.

3. The manufacturing method according to claim 1 or 2, wherein the core further comprises a low-expansion portion made of the second wax material.

4. The manufacturing method according to any one of claims 1 to 3, wherein the core has grooves for forming ribs provided on the surface of the stamper portion.

5. The manufacturing method according to any one of claims 1 to 3, wherein the core has a groove pattern for forming a rib pattern provided on the surface of the stamper portion.

6. The manufacturing method according to claim 5, wherein the groove pattern includes one or more selected from a rectangular grid pattern, a triangular grid pattern, a radial pattern, and a striped pattern.

7. The manufacturing method according to any one of claims 1 to 6, wherein the core has a recess for forming a boss provided on the surface of the stamper portion.

8. The manufacturing method according to any one of claims 1 to 3, wherein the surface of the stamper portion is flat.

9. The manufacturing method according to any one of claims 1 to 3, wherein the core has a projection or recess provided on the surface of the stamper portion.

10. The manufacturing method according to any one of claims 1 to 9, wherein the surface of the core is covered with a barrier film before the molding step.

11. The manufacturing method according to any one of claims 1 to 10, wherein the prepreg premolded body includes SMC.

12. The manufacturing method according to any one of claims 1 to 11, wherein the core is preheated before the molding step.

13. The manufacturing method according to any one of claims 1 to 12, wherein the first wax material is blended with polyolefin wax.

14. The manufacturing method according to claim 13, wherein polypropylene wax is blended as the polyolefin wax in the first wax material.

15. The manufacturing method according to claim 13 or 14, wherein polyethylene wax is blended as the polyolefin wax in the first wax material.

16. The method for producing the product according to any one of claims 13 to 15, wherein the second wax material contains an organic compound having a polar group.

17. The manufacturing method according to any one of claims 1 to 16, wherein, in order to make the first wax material and the second wax material visually distinguishable, either one or both of the first wax material and the second wax material contain a coloring agent.

18. The manufacturing method according to any one of claims 1 to 17, wherein the expanded portion contains particles of a material having the property of generating heat under a high-frequency electromagnetic field.

19. The manufacturing method according to any one of claims 1 to 17, wherein the expansion portion contains particles of a material having the property of absorbing microwaves and generating heat.

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