Fiber-reinforced resin article and production method for same and fusible core and production method for same

Abstract

The principal purpose of the present invention is to make a beneficial improvement on a method for producing a fiber-reinforced resin article that has a substantially closed cavity by heating and curing a reinforcing fiber pre-molded body that has been installed in a mold with a core that is made of wax.　This production method for a fiber-reinforced resin article includes: a molding step for installing a reinforcing fiber pre-molded body in a mold with a fusible core that is made of wax and heating inside the mold to obtain a cured product of the reinforcing fiber pre-molded body; and a core removal step for removing the fusible core from the cured object. The fusible core comprises at least two types of compatible waxes, the melting point difference between the wax that has the highest melting point and the wax that has the lowest melting point being at least 30°C.

Description

Fiber-reinforced resin article and method for manufacturing the same, and fusible core and method for manufacturing the same

[0001] This application claims priority to Japanese Patent Application No. 2024-7530, filed with the Japan Patent Office on January 22, 2024, the contents of which are incorporated herein by reference.

[0002] Fiber-reinforced plastics (FRP) are used in a variety of applications, including automotive reinforcing components. One proposed method for molding FRP articles having hollow portions or U-shaped cross sections is fusible core molding, which involves placing a prepreg preform together with a wax core in a mold, melting and expanding at least a portion of the core in the mold, thereby curing the prepreg preform under pressure, and then removing the core (Patent Document 1).

[0003] International Publication No. 2018 / 079824

[0004] The present invention has as its main object the provision of a valuable improvement in a method for producing a fiber-reinforced resin article having a substantially closed cavity, the method comprising placing a reinforcing fiber preform together with a wax core in a mold, and heating and curing the preform. The problems to be solved by each embodiment of the present invention may be explicitly or implicitly disclosed in the present specification.

[0005] Embodiments of the present invention include, but are not limited to, the following: [1] A method for producing a fiber-reinforced resin article, comprising: a molding step of placing a reinforcing fiber preform together with a fusible core made of wax in a mold, heating the reinforcing fiber preform in the mold, and obtaining a cured product of the reinforcing fiber preform; and a core removal step of removing the fusible core from the cured product, wherein the fusible core is made of at least two types of wax that are compatible with each other, and the difference in melting point between the wax with the highest melting point and the wax with the lowest melting point is 30°C or more. [2] The manufacturing method according to [1], wherein, in the molding step, a prepreg preform made of a prepreg impregnated with a thermosetting resin is placed in the mold together with the fusible core as the reinforcing fiber preform, and heated in the mold to obtain a cured product. [3] The manufacturing method according to [1], wherein, in the molding step, a dry fabric preform made of fabric not impregnated with a thermosetting resin is placed in the mold together with the fusible core as the reinforcing fiber preform, and a thermosetting liquid resin is injected into the mold to form a fabric resin-impregnated body, and heated in the mold to obtain a cured product. [4] The manufacturing method according to any one of [1] to [3], wherein the highest melting point Tm (°C) among the melting points of the waxes constituting the fusible core and the molding temperature Tf (°C) satisfy the relationship Tm≧Tf-30. [5] The manufacturing method according to any one of [1] to [4], wherein the highest melting point Tm (°C) among the melting points of the waxes constituting the fusible core and the heating temperature Td (°C) during core removal satisfy the relationship Td≧Tm. [6] The manufacturing method according to any one of [1] to [5], wherein the wax contains an organic compound having a polar group. [7] The manufacturing method according to any one of [1] to [6], wherein the wax contains a fatty acid compound. [8] The manufacturing method according to any one of [1] to [7], wherein the fusible core is substantially in contact with the reinforcing fiber preform. [9] The manufacturing method according to any one of [1] to [8], wherein the fusible core has a groove or depression on its surface.

[10] The manufacturing method according to any one of [1] to [9], wherein the fusible core has a through-hole.

[11] A fiber-reinforced resin article having a hollow structure obtained by fusible core molding, wherein the hollow structure is substantially free of an outer skin.

[12] A fiber-reinforced resin article having a hollow structure in which a cavity is formed and struts connecting wall surfaces of the cavity, wherein the joints between the wall surfaces and the struts are seamless and integrated.

[13] The fiber-reinforced resin article according to

[12] , wherein the struts are made of chopped fiber-reinforced resin.

[14] The fiber-reinforced resin article according to

[12] or

[13] , wherein the length of the struts is 5 to 100 mm and the minimum width of the struts is 1 to 10 mm.

[15] The fiber-reinforced resin article according to any one of

[12] to

[14] , wherein the fiber-reinforced resin article has a partial joint where wall surfaces within the cavity are partially joined.

[16] The fiber-reinforced resin article according to

[15] , wherein at least some of the wall surfaces of the partial joint are joined with an adhesive.

[17] The fiber-reinforced resin article according to any one of

[11] to

[16] , wherein the reinforcing fiber is continuous fiber.

[18] The fiber-reinforced resin article according to any one of

[11] to

[16] , wherein the reinforcing fiber is chopped fiber.

[19] The fiber-reinforced resin article according to

[18] , which contains a cured product of SMC.

[20] The fiber-reinforced resin article according to any one of

[11] to

[19] , wherein a metal part is integrated into the fiber-reinforced resin article.

[0006]

[21] A method for producing a fusible core, comprising filling a moldable semi-solid or clay-like compatible wax, the difference in melting points between the wax with the highest melting point and the wax with the lowest melting point being 30°C or more, and obtained by changing the temperature of at least two compatible waxes to a temperature 5°C to 25°C lower than the melting point of the wax with the highest melting point, into a mold, and then pressure-molding the semi-solid or clay-like compatible wax in the mold.

[22] A method for producing a fusible core, comprising filling a moldable semi-solid or clay-like compatible wax, the semi-solid or clay-like compatible wax being obtained by changing the melting point of the wax with the lowest melting point to a temperature 5°C to 25°C lower than the melting point of the wax with the lowest melting point. Ｌ When the temperature of the mold during pressure molding is Ｌ -20)℃ or higher (T Ｌ

[23] A method for producing a fusible core according to

[21] , wherein the difference in melting point between the wax with the highest melting point and the wax with the lowest melting point is 30°C or more, and the method comprises crushing a solid compatible wax consisting of at least two compatible waxes to obtain solid wax fragments, filling the solid wax fragments into a mold, and then pressure-molding the solid wax fragments in the mold.

[24] A method for producing a fusible core according to

[21] , wherein the difference in melting point between the wax with the highest melting point and the wax with the lowest melting point is 30°C or more, and the melting point of the wax with the lowest melting point is T Ｌ When the temperature of the mold during pressure molding is T Ｌ ℃ or more (T Ｌ +50°C or less.

[25] The manufacturing method according to

[23] or

[24] , wherein the average particle size of the crushed solid wax fragments is 0.5 to 5 mm.

[26] The manufacturing method according to any one of

[21] to

[25] , wherein the pressure during the pressure molding is 1 to 10 MPa.

[27] The manufacturing method according to any one of

[21] to

[26] , wherein the material of the mold is aluminum.

[28] A method for manufacturing a fusible core, comprising: pulverizing a solid compatible wax comprising at least two compatible waxes, the melting point difference between the wax with the highest melting point and the wax with the lowest melting point being 30°C or more, and introducing the resulting solid crushed wax fragments into a mold; and then pouring and filling the mold with the wax having the same composition as the melted wax, followed by cooling.

[29] The manufacturing method according to

[28] , wherein the average particle size of the crushed solid wax fragments is 5 mm or more.

[30] The manufacturing method according to

[28] or

[29] , wherein the filling rate when the crushed solid wax pieces are filled into the mold is 10 to 90 volume %.

[31] The manufacturing method according to any one of

[28] to

[30] , wherein the mold is made of aluminum.

[0007]

[32] A fusible core, comprising at least two types of waxes that are compatible with each other, wherein the difference in melting point between the wax with the highest melting point and the wax with the lowest melting point is 30°C or more, and when the temperature drop data of the fusible core is measured using a differential scanning calorimeter (DSC), the peak temperature of the highest melting point component is 3°C or more lower than the melting point temperature of the wax with the highest melting point alone.

[0008] A beneficial improvement is provided with respect to a method for manufacturing a fiber reinforced resin article that involves curing under pressure at least a reinforcing fiber preform with a core comprised of wax.

[0009]

[0023] Figure 1 is a flow diagram of a method for manufacturing a fiber-reinforced resin article according to an embodiment. Figure 2 is a cross-sectional view showing the structure of a fusible core that can be used in the method for manufacturing a fiber-reinforced resin article according to an embodiment. Figure 3 is a cross-sectional view showing a reinforcing fiber preform placed in a mold with a fusible core wrapped inside. Figure 4 is a cross-sectional view showing a reinforcing fiber preform having a partial bond placed in a mold with a fusible core wrapped inside. Figure 5 is a cross-sectional view showing a reinforcing fiber preform having a partial bond placed in a mold with a fusible core and adhesive wrapped inside. Figure 6 is a cross-sectional view showing a metal plate and a reinforcing fiber preform having a partial bond placed in a mold with a fusible core wrapped inside. Figure 7 is a cross-sectional view showing a reinforcing fiber preform having a partial bond placed in a mold with a fusible core wrapped inside, and a groove formed in the fusible core. Figure 8 is a cross-sectional view showing a reinforcing fiber preform having a partial bond placed in a mold with a fusible core wrapped inside, and a through hole formed in the fusible core. Fig. 9 is a cross-sectional view showing a fiber-reinforced resin article molded from a reinforcing fiber preform having the partial bond shown in Fig. 4. Fig. 10 is a cross-sectional view showing a fiber-reinforced resin article molded from a reinforcing fiber preform having the partial bond shown in Fig. 5. Fig. 11 is a cross-sectional view showing a fiber-reinforced resin article molded from a metal plate and a reinforcing fiber preform having the partial bond shown in Fig. 6. Fig. 12 is a cross-sectional view showing a fiber-reinforced resin article molded from a reinforcing fiber preform having the partial bond and protrusions shown in Fig. 7. Fig. 13 is a cross-sectional view showing a fiber-reinforced resin article molded from a reinforcing fiber preform having the partial bond and protrusions penetrating the cavity shown in Fig. 8.

[0010] 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. In the following description, for convenience of explanation, the method for manufacturing a fiber-reinforced resin article according to one embodiment will be divided into the following three steps as shown in the flow chart in Figure 1: (i) a core preparation step of preparing a fusible core made of wax; (ii) a molding step of placing a reinforcing fiber preform together with the fusible core in a mold and heating them in the mold to form a cured product; and (iii) a core removal step of removing the fusible core from the cured product.

[0011] The manufacturing method of a fiber-reinforced resin article according to the embodiment is preferably used for manufacturing a fiber-reinforced resin article having at least a portion thereof a structure consisting of a bent or curved wall surface. Typical examples of structures consisting of a bent or curved wall surface include a hollow structure, a cylindrical structure, a U-shaped cross-section structure, and an L-shaped cross-section structure. Various undercut portions often have bent or curved wall surfaces within their structures. Each step will be described in detail below.

[0012] (1) Core Preparation Step The core preparation step (i) is a step of preparing a fusible core made of wax. The fusible core is made of at least two types of wax that are compatible with each other, and the difference in melting point between the wax with the highest melting point and the wax with the lowest melting point is 30°C or more.

[0013] As an example, let us consider a case where the core is composed of two wax components. If the melting point of the low-melting wax is below the maximum core temperature during molding and the melting point of the high-melting wax is higher than the maximum core temperature during molding, the fusible core will be in a solid-liquid mixed state, i.e., a slurry state, during molding. When the core is in a slurry state during molding, the core will flow under pressure, resulting in a uniform internal pressure. Furthermore, since the wax does not completely liquefy, the likelihood of leakage of the wax from the mold is reduced. On the other hand, if the melting points of both waxes are higher than the maximum core temperature during molding, the fusible core will be solid during molding, resulting in an uneven internal core pressure, which is likely to result in defects in quality and appearance, which is undesirable. Furthermore, if the melting points of both waxes are below the maximum core temperature during molding, the fusible core will completely liquefy during molding, increasing the likelihood of the liquefied wax leaking from the mold, which is undesirable. The "maximum core temperature during molding" refers to the highest temperature the core surface reaches during in-mold heating and pressure molding.

[0014] The mass ratio of the low-melting point wax to the high-melting point wax in the fusible core is preferably 1:10 to 10:1, more preferably 1:5 to 5:1, and even more preferably 1:3 to 3:1. 1:3 to 1:1 is most preferred. The higher the ratio of the low-melting point wax, the easier it is to apply pressure to the reinforcing fiber preform during molding, liquefying the fusible core and removing the wax from the fiber-reinforced resin article. The higher the ratio of the high-melting point wax, the easier it is to prevent the liquefied wax from leaking from the mold during molding.

[0015] When the core is ejected, if the fusible core is heated to a temperature higher than that of the high-melting-point wax, it will be completely liquefied and can be ejected. The fusible core may be composed of two components, a low-melting-point wax and a high-melting-point wax, or may be composed of three or more wax components. In this case, the molding temperature should be set between the highest and lowest melting points of the waxes, so that the fusible core will become a slurry during molding.

[0016] A large difference between the maximum and minimum melting points of each wax is preferable because it allows the fusible core to maintain a slurry state for a long period of time during molding. The difference between the maximum and minimum melting points of each wax is 30°C or more, and preferably 50°C or more.

[0017] The maximum melting point Tm (°C) of the wax constituting the fusible core and the molding temperature Tf (°C) preferably satisfy the relationship Tm≧Tf−30, and more preferably Tm≧Tf−20. By satisfying this condition, it is easy to prevent the liquefied wax from leaking from the mold during molding.

[0018] The maximum melting point Tm (°C) of the wax constituting the fusible core and the heating temperature Td (°C) during core removal preferably satisfy the relationship Td ≥ Tm, and more preferably Td ≥ Tm + 5. By satisfying this condition, the fusible core can be liquefied and the wax can be removed from the fiber-reinforced resin article.

[0019] The wax used in the fusible core preferably contains an organic compound having a polar group, because it tends to have a relatively low viscosity when melted and discharged. Examples of "polar groups" include carboxyl groups and hydroxyl groups, with carboxyl groups being preferred. The wax used in the fusible core is preferably a fatty acid compound. Fatty acid ester compounds have a relatively low melting point, while fatty acid amide compounds have a relatively high melting point. The combination of fatty acid ester compounds and fatty acid amide compounds is preferred because they are well compatible.

[0020] Specifically, the aliphatic compound is preferably stearic acid, and the fatty acid ester compound is preferably stearic acid ester. The fatty acid amide compound is preferably, for example, stearic acid amide or oleic acid amide. The wax combination used in the fusible core is preferably a combination of stearic acid ester and stearic acid amide, or a combination of stearic acid ester and oleic acid amide.

[0021] When at least two types of waxes are compatible with each other, the following condition is met: When the temperature drop data of a molten mixture of at least two types of waxes is measured using a differential scanning calorimeter (DSC), the peak temperature of the component with the highest melting point is at least 3°C ​​lower than the peak temperature of the wax alone with the highest melting point.

[0022] As described herein, an embodiment of the present invention includes a fusible core that is made of at least two compatible waxes, wherein the difference in melting point between the wax with the highest melting point and the wax with the lowest melting point is 30°C or more, and when the temperature drop data of the fusible core is measured using a differential scanning calorimeter (DSC), the peak temperature of the highest melting point component is 3°C or more lower than the melting point temperature of the wax with the highest melting point alone.

[0023] For example, when molding a fiber-reinforced resin article having a hollow structure, a fusible core is prepared having substantially the same shape as the cavity in the hollow structure. Therefore, when the fiber-reinforced resin article to be molded is, for example, a hollow rectangular parallelepiped, a rectangular parallelepiped-shaped fusible core is prepared. Below, possible structures of the fusible core prepared in the core preparation step will be described using the case where the fusible core has a rectangular parallelepiped shape as an example.

[0024] FIG. 2 is a cross-sectional view of a rectangular parallelepiped fusible core 10 having a portion 12 made of wax.

[0025] The fusible core 10 shown in Figure 2 can be produced by molding, but is not limited to this. For example, it may be produced by cutting. Alternatively, the fusible core 10 may be produced by stacking multiple rectangular plates. When multiple plates are stacked, the plates may be fused to each other by heating, but this is not required.

[0026] Several examples of methods for producing a fusible core will be described below, but the method for producing a fusible core is not limited to these examples.

[0027] A fusible core may be obtained by filling a mold with a moldable semi-solid or clay-like compatible wax, which is obtained by changing the temperature of at least two compatible waxes, where the difference in melting points between the wax with the highest melting point and the wax with the lowest melting point is 30°C or more, to a temperature 5°C to 25°C lower than the melting point of the wax with the highest melting point, and then pressure-molding the semi-solid or clay-like compatible wax in the mold. A semi-solid state has the properties of both a liquid and a solid, is viscous, and is freely deformable. A viscous state is a state that is even more viscous than a semi-solid state, lacks fluidity, but is freely deformable.

[0028] By changing the temperature of at least two compatible waxes to a temperature 5°C to 25°C lower than the melting point of the wax with the highest melting point, a moldable semi-solid or clay-like compatible wax can be obtained. The fusible core obtained by pressure molding the semi-solid or clay-like compatible wax in a mold is less likely to develop porous defects and has improved homogeneity. If pores develop in the fusible core, molding materials such as resins may flow into the pores during the production of fiber-reinforced resin articles, potentially resulting in a structure that differs from the intended design. In contrast, a homogeneous fusible core with reduced porous defects contributes to the proper performance of fiber-reinforced resin articles. Additionally, since heating to a liquid state is not required, thermal degradation can be reduced. As a result, repeated use of the wax in the fusible core has less impact on quality. Productivity is also expected to improve.

[0029] The temperature of the mold when compressing the semi-solid or clay-like compatible wax is (T Ｌ -20)℃ or higher (T Ｌ +30° C. or less is preferable. When the temperature is within the above range, porous defects are less likely to occur in the fusible core.

[0030] Alternatively, the fusible core may be prepared by heat-molding solid wax fragments in a mold. That is, a solid compatible wax is crushed to obtain solid wax fragments, and the melting point difference between the highest and lowest waxes is 30° C. or more, and the solid compatible wax is composed of at least two compatible waxes. The solid wax fragments are then filled into a mold, and the solid wax fragments are then pressure-molded in the mold to obtain the fusible core.

[0031] Fusible cores obtained by crushing solid compatible wax into crushed pieces and then pressurizing the crushed pieces in a mold are also less likely to have porous defects and have improved homogeneity. A homogeneous fusible core with reduced porous defects contributes to the proper performance of fiber-reinforced resin articles. Additionally, since there is no need to heat the core until it becomes liquid, thermal degradation can be reduced. As a result, repeated use of the wax in the fusible core has less impact on quality. It is also expected that productivity will improve.

[0032] The temperature of the mold when the solid wax fragments are pressed into the mold is T Ｌ ℃ or more (T Ｌ +50°C or less is preferable. When the temperature is within the above range, porous defects are unlikely to occur in the fusible core. Ｌ is the melting point of the wax with the lowest melting point.

[0033] The average particle size of the solid wax fragments obtained by pulverizing the solid compatible wax is preferably 0.5 to 5 mm. When the average particle size of the wax fragments is within the above numerical range, porous defects are less likely to occur in the fusible core. The average particle size of the wax fragments is determined by extracting 30 pieces of each powder or fragment that are substantially close to the maximum diameter, measuring the total weight, and calculating the diameter equivalent to a sphere of equal volume, assuming a specific gravity of 1, from the average weight. However, powders or fragments with extremely uneven particle size distribution are excluded from the average particle size calculation.

[0034] Alternatively, the fusible core may be prepared by casting solid wax fragments and heated and melted wax in a mold. That is, a solid compatible wax consisting of at least two compatible waxes, the melting points of which differ by 30° C. or more between the highest and lowest melting points, is crushed and introduced into a mold to obtain solid wax fragments. The wax having the same composition as the compatible wax is then added and poured into the mold, followed by cooling to obtain the fusible core.

[0035] Fusible cores obtained by casting solid wax fragments and heated, molten wax in a mold are also less likely to have porous defects and have improved homogeneity. A homogeneous fusible core with reduced porous defects contributes to the proper performance of fiber-reinforced resin articles. Because the casting mold is not pressurized, a resin mold can be used in addition to a metal mold.

[0036] When crushed pieces of solid wax and heated, melted wax are cast in a mold, the average particle size of the crushed pieces of solid wax is preferably 5 mm or more. There is no particular upper limit, and crushed pieces of wax of a size that can fit inside the mold may be used.

[0037] When crushed pieces of solid wax and heated, molten wax are cast into a mold, the filling rate of the crushed pieces of solid wax in the mold is preferably 10 to 90 volume %. When the filling rate is within this range, porous defects are less likely to occur in the fusible core.

[0038] In the above-described method for producing a fusible core, the pressure during pressure molding is preferably 1 to 10 MPa. When the pressure is within this range, porous defects are less likely to occur in the fusible core.

[0039] The material of the mold or die used to obtain the fusible core is not particularly limited, but aluminum is preferred. However, by appropriately considering the temperature and pressure conditions, it is possible to obtain a suitable fusible core using a die made of various materials other than aluminum.

[0040] (2) Molding Step In the molding step (ii), the reinforcing fiber preform is placed in a mold together with the fusible core and heated to form a cured product (fiber-reinforced resin article). In the molding step (ii), the fusible core is substantially in contact with the reinforcing fiber preform. The phrase "the fusible core is substantially in contact with the reinforcing fiber preform" may include portions where the fusible core and the reinforcing fiber preform are partially separated from each other.

[0041] (Reinforcing fiber preform) There are two types of reinforcing fiber preform: a prepreg preform in which reinforcing fibers are impregnated with uncured resin, and a dry fabric preform in which reinforcing fibers are not impregnated with uncured resin. Each preform will be described in detail below.

[0042] (1. Prepreg Preform) Prepregs are composites impregnated with a fiber reinforcement material and used as intermediate materials in the manufacture of fiber-reinforced plastic (FRP) structures. The fiber reinforcement in prepregs can take various forms, including continuous fiber, chopped fiber, woven fabric, nonwoven fabric, and non-crimp fabric. Sheet-type prepregs containing multiple parallel continuous fiber bundles as the fiber reinforcement are called unidirectional prepregs (UD prepregs). Prepregs using woven fabrics composed of continuous fiber bundles as the fiber reinforcement are called cross prepregs. Prepregs using mats formed by stacking chopped fiber bundles as the fiber reinforcement are called SMCs. SMCs are preferred because they are easily deformed, even in complex structures. Prepregs using a single continuous fiber bundle as the reinforcement are called tow prepregs. Fibers used in prepregs include, for example, carbon fiber, glass fiber, aramid fiber, silicon carbide fiber, and metal fiber. The fibers may be used alone or in combination of two or more.

[0043] 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. One type of thermosetting resin may be used alone, or two or more types may be used in combination.

[0044] Various additives may be added to the thermosetting resin composition, such as reactive diluents, flame retardants, antifoaming agents, defoaming agents, mold release agents, particulate fillers, colorants, and silane coupling agents.

[0045] The content of the thermosetting resin composition in the prepreg is not limited, but is, for example, 15 to 60 mass%, and may be 15 to 20 mass%, 20 to 25 mass%, 25 to 40 mass%, 40 to 50 mass%, 50 to 60 mass%, etc.

[0046] The prepreg preform is produced by processing one or more prepreg sheets outside a mold so that they have a net shape, i.e., a shape that is substantially the same as the desired fiber-reinforced resin article. For example, when molding a fiber-reinforced resin article having a hollow structure, it is preferable to produce the prepreg preform so that the fusible core is enclosed inside from the beginning.

[0047] The prepreg preform may include a structure in which the same type of prepreg sheets are laminated, or a structure in which two or more different types of prepreg sheets are laminated. In one example, the prepreg preform may include a structure in which a prepreg using continuous fiber bundles, such as a unidirectional prepreg or a cross prepreg, and an SMC are laminated. Furthermore, a resin-impregnated prepreg sheet and a non-resin-impregnated fabric may be laminated.

[0048] The prepreg preform may also include a structure in which a metal plate and a prepreg sheet are laminated together. Examples of materials for the metal plate include alloys of iron, aluminum, magnesium, titanium, copper, nickel, etc. The prepreg preform may also include a structure in which a cured fiber composite plate and a prepreg sheet are laminated together.

[0049] Before placing the prepreg preform in the mold, it is preferable that the mold has already been heated to the same temperature as the molding temperature. The molding temperature is usually in the range of 120°C to 180°C, and preferably 130°C or higher. The higher the molding temperature, the shorter the time required for molding. After the prepreg preform is placed in the mold together with the fusible core, the mold is closed.

[0050] 3 is a cross-sectional view showing the prepreg preform 20, together with the fusible core 10 wrapped inside, placed in a mold 30 consisting of a lower mold 32 and an upper mold 34 to mold a hollow rectangular parallelepiped fiber-reinforced resin article. The detailed structure of the fusible core 10 is not shown. During the molding process, the fusible core 10 absorbs heat transferred from the mold 30 through the prepreg preform 20, causing it to expand and press the prepreg preform 20 against the inner surface of the mold 30. In other words, the fusible core 10 attempts to expand against the clamping force of the mold 30, generating internal pressure which is applied to the prepreg preform 20.

[0051] Immediately after the mold is closed, the volume of the fusible core 10 increases only slowly due to thermal expansion, and the pressure applied to the prepreg preform 20 is weak. When a part of the wax constituting the fusible core 10 begins to undergo a phase transition to a slurry state, i.e., a solid-liquid phase, the pressure applied to the prepreg preform 20 increases rapidly.

[0052] By setting the molding temperature (mold temperature during molding) so that the wax softens to a slurry state on the surface of the fusible core 10, the prepreg preform 20 can be sufficiently compressed. The wax becomes a slurry due to the melting of the low-melting-point components, which is accompanied by a large volume expansion. It is undesirable for all of the wax components to melt and become liquid during the molding process. This is because the wax that melts and flows into the gap between the prepreg preform and the mold does not contribute to the compression of the prepreg preform. Preferably, the molding temperature is set so that the high-melting-point wax does not melt and become liquid on the surface of the fusible core 10 until the molding process is completed.

[0053] If the fusible core is in a slurry state during molding of the prepreg preform, the fusible core can be used without an outer skin, but an outer skin can be placed around the fusible core to smooth the inner surface of the prepreg preform that comes into contact with the fusible core and to prevent wax from flowing into the gaps between the prepregs. The outer skin is sometimes called a barrier layer.

[0054] The preferred material for the outer shell is an organic material, and a polymer is particularly preferred. Specific examples include synthetic polymers such as polyolefin, polyamide, polyester, polyurethane, acrylic, silicone, fluororubber, polyvinyl alcohol, and ethylene vinyl alcohol, as well as elastomers made from these polymers. The thickness of the outer shell is not particularly limited as long as it achieves its purpose as a seal to prevent leakage of the molten wax. For example, it is 0.05 to 1 mm, and may be 0.5 mm or less, or even 0.1 mm or less.

[0055] In the molding step, it is preferable to preheat the fusible core 10 before the molding step (ii) so that the surface temperature of the fusible core 10 approaches the molding temperature in a short time after the mold is closed. This preheating must be performed so that the wax does not soften and become a slurry. This is because if the fusible core 10 expands significantly during the preheating step, it will be impossible to use the expansion of the fusible core 10 to pressurize the prepreg preform 20 in the molding step.

[0056] In the case of a fiber-reinforced resin article having a large flat surface or one requiring high rigidity at the flat surface, it is preferable to provide a protrusion such as a rib, or a boss-like or planar partial joint. For example, as shown in Fig. 4, a prepreg preform 20 is prepared, which includes a flat upper layer 20A and a lower layer 20B that is bent into a trapezoidal convex shape toward the upper layer 20A, with the tip surface of the convex shape contacting the upper layer 20A, with a fusible core 10 enclosed therein, and then placed in a mold 30 for curing. A lower mold 32 of the mold 30 is provided with a convex portion shaped to fit the trapezoidal bend of the lower layer 20B. In this case, as shown in Fig. 9, the fiber-reinforced resin article 40 obtained after curing has a partial joint 44 where the wall surfaces are partially joined within a cavity 43, such that the convexly bent lower surface 42 supports the upper surface 41.

[0057] In molding an example fiber-reinforced resin article 40 shown in FIG. 9 , there is no fusible core 10 between the upper layer 20A and the lower layer 20B of the prepreg preform 20 at the portion forming the partial joint 44, and the expansion pressure of the fusible core 10 cannot be utilized. Therefore, there is a possibility of poor appearance or voids such as delamination occurring at the partial joint 44 of the fiber-reinforced resin article 40. In contrast, as shown in the example shown in FIG. 5 , by bonding the upper layer 20A and the lower layer 20B of the prepreg preform 20 with an adhesive 60 at the portion forming the partial joint 44, molding defects at the partial joint 44 of the fiber-reinforced resin article 40 can be suppressed. In this case, as shown in FIG. 10 , the resulting fiber-reinforced resin article 40 has a partial joint 44 in which the wall surfaces within the cavity 43 are partially bonded, and the wall surfaces of the partial joint 44 are bonded together with the adhesive 60. Using a foaming adhesive as the adhesive 60 is more preferable because the foaming adhesive expands and hardens during molding, thereby increasing the bonding strength of the partial joint 44.

[0058] In the partial joint where the upper surface portion of the fiber-reinforced resin article is supported by the convex lower surface portion, the expansion pressure of the fusible core cannot be utilized during molding, even when the partial joint is located at the end portion of the upper surface portion in the planar direction. In this case, too, the joint portion between the upper layer 20A and the lower layer 20B of the prepreg preform 20 can be bonded with an adhesive during molding.

[0059] 4, at the end of the face direction of the upper layer 20A of the prepreg preform 20, the end of the lower layer 20B is bent toward the center in the face direction, and the upper surface of the bent portion of the lower layer 20B is in contact with the upper layer 20A. At the joint between the bent portion of the lower layer 20B and the upper layer 20A, the expansion pressure of the fusible core 10 can be utilized during molding, but this joint may also be joined using an adhesive.

[0060] As shown in an example in Figure 6, the upper layer 20A of the prepreg preform 20 may be replaced with a metal plate 20C. In this case, as shown in Figure 11, a fiber-reinforced resin article 40 is obtained in which the metal plate 20C is bonded to a lower surface portion 42, which is the cured product of the lower layer 20B of the prepreg preform 20. If the metal plate 20C is thin, for example, 1 mm or less in thickness, it is possible to simultaneously bend the metal plate at an angle by using the expansion pressure of the fusible core. As with bonding between cured products, an adhesive can be used in the bonding portion between the cured product and the metal plate in the prepreg preform, and a foam adhesive is preferably used.

[0061] Another example of a fiber-reinforced resin article having a large flat surface portion or a flat surface portion requiring high rigidity is a fiber-reinforced resin article having a reinforcing structure of protrusions such as ribs. That is, the fusible core preferably has grooves or depressions on its surface for forming protrusions such as ribs. Figure 7 is a cross-sectional view showing a fusible core 10 wrapped inside a prepreg preform 20, with grooves 11A and 11B on its surface. Except for the grooves 11A and 11B on the surface of the fusible core 10, this example has the same configuration as the example shown in Figure 4.

[0062] In this example, after the mold is closed, the fusible core 10 expands by absorbing heat transmitted from the mold 30 through the prepreg preform 20, and the prepreg preform 20 is pressed against the inner surface of the mold 30. At the same time, the uncured resin of the prepreg preform 20 and even some of the reinforcing fibers enter the grooves 11A, 11B of the fusible core 10 so as to fill them. In this case, the fiber-reinforced resin article 40 obtained after curing has ribs 45A, 45B on the wall surface of the lower surface portion 42 facing the cavity 43, which improve rigidity, as shown in Fig. 12 .

[0063] Fig. 8 is a cross-sectional view showing a fusible core 10 wrapped inside a prepreg preform 20, the fusible core 10 having through holes 12A and 12B. This example has the same configuration as the example shown in Fig. 4 , except that the fusible core 10 has through holes 12A and 12B. In this example, after clamping, the fusible core 10 expands by absorbing heat transferred from the mold 30 through the prepreg preform 20. The prepreg preform 20 is pressed against the inner surface of the mold 30, and simultaneously, the uncured resin of the prepreg preform 20 and some of the reinforcing fibers enter the through holes 12A and 12B of the fusible core 10. In this case, the fiber-reinforced resin article 40 obtained after curing has struts 46A and 46B connecting the wall surfaces of the upper surface portion 41 and the lower surface portion 42 within the cavity 43, as shown in Fig. 13 . The joints between the wall surfaces of the upper surface portion 41 and the lower surface portion 42 and the support columns 46A, 46B are seamless and integrated. In such a fiber-reinforced resin article, the rigidity can be significantly improved by increasing the number of support columns.

[0064] (2. Dry Fabric Preform) A dry fabric preform is a preform in which a laminate, in which multiple sheets of reinforcing fiber material, each having a heat-fusible binder disposed on the surface of a fabric such as a woven fabric, knitted fabric, nonwoven fabric, or non-crimp fabric, are laminated, is formed into a near net shape by thermal fusion of the heat-fusible binder. The dry fabric preform is placed in a mold, and a thermosetting liquid resin is injected into the mold by a method called Resin Transfer Molding (hereinafter referred to as "RTM") to impregnate the reinforcing fiber material.

[0065] Fibers used in the dry fabric include, for example, carbon fibers, glass fibers, aramid fibers, silicon carbide fibers, and metal fibers. One type of fiber may be used alone, or two or more types may be used in combination.

[0066] Examples of thermosetting liquid resins used in the RTM process include epoxy resins, urea resins, vinyl ester resins (also called epoxy acrylate resins), unsaturated polyesters, polyurethanes, and phenolic resins. Various additives may be added to the thermosetting liquid resin composition, such as reactive diluents, flame retardants, antifoaming agents, defoaming agents, mold release agents, particulate fillers, colorants, and silane coupling agents.

[0067] The dry fabric preform is typically fabricated by processing two or more dry fabric sheets outside a mold to a net shape. For example, when molding a fiber-reinforced resin article having a hollow structure, it is preferable to fabricate the dry fabric preform from the beginning so that the fusible core is enclosed within the preform.

[0068] The dry fabric preform may include a structure in which dry fabric sheets of the same type are laminated, or may include a structure in which two or more different types of dry fabric sheets are laminated. In one example, the dry fabric preform may include a structure in which dry fabrics using continuous fiber bundles, such as uniaxial or multiaxial non-crimp fabrics or cross fabrics, are laminated.

[0069] The dry fabric preform may also include a structure in which a metal plate and a dry fabric sheet are laminated together. Examples of the material for the metal plate include alloys of iron, aluminum, magnesium, titanium, copper, nickel, etc. The dry fabric preform may also include a structure in which an already cured fiber composite plate and a dry fabric sheet are laminated together.

[0070] Before placing the dry fabric preform in the mold, the mold is preferably already heated to the same temperature as the molding temperature. The molding temperature is usually in the range of 120°C to 180°C, and preferably 130°C or higher. The higher the molding temperature, the shorter the molding time. After the dry fabric preform is placed in the mold together with the fusible core, the mold is closed and a thermosetting liquid resin is injected into the mold to form a fabric resin-impregnated body.

[0071] 3 is a cross-sectional view showing the dry fabric preform 20', together with the fusible core 10 wrapped inside, placed in a mold 30 consisting of a lower mold 32 and an upper mold 34 to mold a hollow rectangular parallelepiped fiber-reinforced resin article. The detailed structure of the fusible core 10 is not shown. During the molding process, the fusible core 10 absorbs heat transmitted from the mold 30 through the resin-impregnated fabric, causing it to expand, and the resin-impregnated fabric is pressed against the inner surface of the mold 30. In other words, the fusible core 10 tries to expand against the clamping force of the mold 30, generating internal pressure, which is applied to the resin-impregnated fabric.

[0072] After the mold is closed, a thermosetting liquid resin is injected into the mold 30 through a resin injection hole (not shown), and the dry fabric preform 20' becomes a resin-impregnated fabric body, and a resin injection pressure is applied. Furthermore, when a part of the wax constituting the fusible core 10 starts to undergo a phase transition to a slurry state, i.e., a solid-liquid phase, the pressure applied to the resin-impregnated fabric body increases rapidly.

[0073] By setting the molding temperature (mold temperature during molding) so that the wax softens to a slurry state on the surface of the fusible core 10, the resin-impregnated fabric can be sufficiently compressed. The wax becomes a slurry due to the melting of the low-melting-point component, which is accompanied by a large volume expansion. It is undesirable for all wax components to melt and become liquid during the molding process. This is because the wax that melts and flows into the gap between the resin-impregnated fabric and the mold does not contribute to the compression of the resin-impregnated fabric. Preferably, the molding temperature is set so that the high-melting-point wax does not melt and become liquid on the surface of the fusible core 10 until the molding process is completed.

[0074] If the fusible core is in a slurry state during molding of the resin-impregnated fabric body, the fusible core can be used without an outer skin, but an outer skin can be placed around the fusible core to smooth the inner surface of the resin-impregnated fabric body that comes into contact with the fusible core and to prevent wax from flowing into the resin-impregnated fabric body.

[0075] The preferred material for the outer shell is an organic material, and a polymer is particularly preferred. Specific examples include synthetic polymers such as polyolefin, polyamide, polyester, polyurethane, acrylic, silicone, fluororubber, polyvinyl alcohol, and ethylene vinyl alcohol, as well as elastomers made from these polymers. The thickness of the outer shell is not particularly limited as long as it achieves its purpose as a seal to prevent leakage of the molten wax. For example, it is 0.05 to 1 mm, and may be 0.5 mm or less, or even 0.1 mm or less.

[0076] It is preferable to preheat the fusible core 10 before the molding step (ii) so that the temperature of the surface of the fusible core 10 approaches the molding temperature in a short time after the mold is closed. This preheating must be performed so that the wax does not soften and become a slurry. This is because if the fusible core 10 expands significantly during the preheating step, it will be impossible to use the expansion of the fusible core 10 to pressurize the resin-impregnated fabric body during the molding step.

[0077] For fiber-reinforced resin articles with large flat surfaces or requiring high rigidity at the flat surfaces, it is preferable to have protrusions such as ribs, or boss-like or planar joints. For example, as shown in Fig. 4, a dry fabric preform 20' is prepared, including a flat upper layer 20A' and a lower layer 20B' that is bent toward the upper layer 20A' into a trapezoidal convex shape with the tip of the convex portion in contact with the upper layer 20A', with the fusible core 10 wrapped around it. The dry fabric preform 20' is then placed in a mold 30 and cured. In the example shown in Fig. 4, at the end of the upper layer 20A' in the planar direction of the dry fabric preform 20', the end of the lower layer 20B' is bent toward the center in the planar direction, and the upper surface of the bent portion of the lower layer 20B' is in contact with the upper layer 20A'. The lower mold 32 of the mold 30 is provided with a convex portion shaped to fit the trapezoidal bend of the lower layer 20B'. 9, the fiber-reinforced resin article 40 obtained after curing has a partial joint 44 where the wall surfaces are partially joined together within the cavity 43 so that the convexly bent lower surface 42 supports the upper surface 41. In the RTM method, the space between the upper layer 20A' and the lower layer 20B' in the dry fabric preform 20' where the partial joint 44 is formed is filled with the injected thermosetting liquid resin, so that an adhesive is usually not required.

[0078] As shown in an example in Fig. 6, the dry fabric preform 20' may be replaced with a metal plate 20C. In this case, a fiber-reinforced resin article is obtained in which the cured resin-impregnated fabric body and the metal plate 20C are bonded together, as shown in Fig. 11. When the metal plate 20C is thin, for example, with a thickness of 1 mm or less, it is possible to simultaneously bend the metal plate at an angle by the expansion pressure of the fusible core.

[0079] Another example of a fiber-reinforced resin article having a large flat surface portion or a flat surface portion requiring high rigidity is a fiber-reinforced resin article having a reinforcing structure of protrusions such as ribs. That is, the fusible core preferably has grooves or depressions on its surface for forming protrusions such as ribs. Figure 7 is a cross-sectional view showing a fusible core 10 wrapped inside a dry fabric preform 20', which has grooves 11A and 11B on its surface. Except for the grooves 11A and 11B on the surface of the fusible core 10, this example has the same configuration as the example shown in Figure 4.

[0080] In this example, after the mold is closed and the resin is injected, the fusible core 10 expands by absorbing heat transmitted from the mold 30 through the resin-impregnated fabric, and the resin-impregnated fabric is pressed against the inner surface of the mold 30. At the same time, the uncured resin of the resin-impregnated fabric and even some of the reinforcing fibers enter the grooves 11A and 11B of the fusible core 10 so as to fill them. Then, as shown in Fig. 12, the fiber-reinforced resin article 40 obtained after curing has ribs 45A and 45B on the wall surface of the lower surface portion 42 facing the cavity 43 to improve rigidity.

[0081] Figure 8 is a cross-sectional view showing the fusible core 10 wrapped inside the dry fabric preform 20', in which the fusible core 10 has through holes 12A and 12B. This example has the same configuration as the example shown in Figure 4, except that the fusible core 10 has through holes 12A and 12B. In this example, after mold clamping and resin injection, the fusible core 10 expands by absorbing heat transmitted from the mold 30 through the resin-impregnated fabric. The resin-impregnated fabric is pressed against the inner surface of the mold 30, and at the same time, the uncured resin of the resin-impregnated fabric and some of the reinforcing fibers enter the through holes 12A and 12B of the fusible core 10. The fiber-reinforced resin article 40 obtained after curing has struts 46A and 46B connecting the wall surfaces of the upper surface portion 41 and the lower surface portion 42 within the cavity 43, as shown in Figure 13. The joints between the wall surfaces of the upper surface portion 41 and the lower surface portion 42 and the support columns 46A, 46B are seamless and integrated. In such a fiber-reinforced resin article, the rigidity can be significantly improved by increasing the number of support columns.

[0082] (3) Wax Removal Step In the wax removal step (iii), the fusible core 10 is removed from the fiber-reinforced resin article obtained by curing the prepreg preform or the resin-impregnated fabric body in the molding step (ii). For example, when the fiber-reinforced resin article has a closed cavity, the fiber-reinforced resin article is heated, for example, in an oven to melt the fusible core remaining inside the fiber-reinforced resin article, and the liquefied wax is discharged through holes made in the fiber-reinforced resin article using a drill or hole saw. It is desirable to heat the fiber-reinforced resin article so that the heat distortion temperature (deflection temperature under load) of the fiber-reinforced resin article is not reached.

[0083] The opening area of ​​the hole that discharges the liquid wax is 10 to 500 mm ２ When the opening area of ​​the holes is equal to or greater than the lower limit, the wax discharge time can be easily shortened. When the opening area of ​​the holes is equal to or less than the upper limit, a decrease in the strength of the fiber-reinforced resin article can be easily suppressed.

[0084] When the fiber-reinforced resin article has an undercut, the fusible core can be melted and removed, just as when the fiber-reinforced resin article has a closed cavity. When the fiber-reinforced resin article has neither a closed cavity nor an undercut, the fusible core may be removed without melting.

[0085] 2. Fiber-reinforced resin article Another embodiment of the present invention relates to a fiber-reinforced resin article. The fiber-reinforced resin article according to one embodiment includes at least a portion made of fiber-reinforced resin and has a hollow structure in which a cavity is formed. The cavity in the hollow structure is a substantially closed cavity. A "substantially closed cavity" refers to a cavity in which the ratio of the opening area to the area of ​​the surface in which an opening leading to the cavity is formed is small, and specifically refers to a cavity in which the opening ratio is 50% or less, preferably 30% or less.

[0086] Preferably, the fiber-reinforced resin article has a hollow structure that is substantially free of an outer skin. A "fiber-reinforced resin article substantially free of an outer skin" refers to a fiber-reinforced resin article that is molded without having an outer skin on a fusible core. Fiber-reinforced resin articles from which the outer skin has been removed after molding are not included in the term "fiber-reinforced resin article substantially free of an outer skin."

[0087] The fiber reinforcement contained in the fiber-reinforced resin article may take various forms, such as continuous fiber, chopped fiber, woven fabric, nonwoven fabric, and non-crimp fabric. Examples of the fiber include carbon fiber, glass fiber, aramid fiber, silicon carbide fiber, and metal fiber. One type of fiber may be used alone, or two or more types may be used in combination.

[0088] The fiber-reinforced resin article includes a cured product of a thermosetting resin. Examples of the thermosetting resin include epoxy resin, urea resin, vinyl ester resin (also called epoxy acrylate resin), unsaturated polyester, polyurethane, and phenolic resin. The thermosetting resin may be used alone or in combination of two or more.

[0089] A fiber-reinforced resin article according to another embodiment has a hollow structure portion in which a cavity is formed, and a partial joint portion where wall surfaces of the cavity are partially joined together. For example, a fiber-reinforced resin article 40 shown in Fig. 9 has a partial joint portion 44 where wall surfaces of a cavity 43 are partially joined together so that an upper surface portion is supported by a convex lower surface portion.

[0090] A fiber-reinforced resin article according to another embodiment has a hollow structure portion in which a cavity is formed, and a partial joint portion in which wall surfaces within the cavity are partially joined, and the wall surfaces of the partial joint portion are joined with an adhesive, preferably a foam adhesive. For example, there is a fiber-reinforced resin article 40 shown in Figure 10 in which the wall surfaces joined at the partial joint portion 44 are joined with an adhesive 60. Using a foam adhesive as the adhesive 60 is more preferable because the foam adhesive expands and hardens during molding, thereby increasing the joining strength of the partial joint portion 44.

[0091] In the fiber-reinforced resin article according to the embodiment, joints other than partial joints where wall surfaces are partially joined together within a cavity may also be joined with an adhesive. For example, as shown in the example of Figures 9 and 10, the joint between the folded portion of the end of the lower layer 20B and the upper layer 20A at the end of the upper layer 20A in the planar direction of the prepreg preform 20 may also be joined with an adhesive. In this case, a foam adhesive is also preferred.

[0092] A fiber-reinforced resin article according to another embodiment has a hollow structure having a cavity formed therein and a metal part integrated therewith. For example, a fiber-reinforced resin article 40 shown in FIG. 11 is exemplified, in which a lower surface 42, which is a cured product of a prepreg preform or a resin-impregnated fabric body, is joined to a metal plate 20C.

[0093] A fiber-reinforced resin article according to another embodiment has a hollow structure portion having a cavity formed therein and protrusions such as ribs and bosses provided on the wall surfaces of the cavity. For example, a fiber-reinforced resin article 40 having ribs 45A and 45B as illustrated in FIG. 12 can be given.

[0094] A fiber-reinforced resin article according to another embodiment includes a hollow structure having a cavity formed therein and struts connecting the wall surfaces of the cavity, and the joints between the wall surfaces and the struts are seamless and integrated. For example, a fiber-reinforced resin article 40 having struts 46A and 46B as shown in FIG. 13 is an example. The struts are preferably made of a cured product of chopped fiber-reinforced resin, i.e., SMC. This is because SMC is easy to flow and easily fills the through-holes of the fusible core during molding, making it easy to form the struts.

[0095] The cross-sectional shape of the strut is not particularly limited, and examples include a round shape, an elliptical shape, a square shape, a rectangular shape, and a triangular shape. The length of the strut is preferably 5 to 100 mm, and more preferably 10 to 50 mm. If the length of the strut is equal to or greater than the lower limit, the lightweight effect is excellent. If the length of the strut is equal to or less than the upper limit, the reinforcing effect of the strut is stable. The minimum width of the strut is preferably 1 to 10 mm, and more preferably 2 to 5 mm. If the minimum width of the strut is equal to or greater than the lower limit, the reinforcing effect of the strut is stable. If the minimum width of the strut is equal to or less than the upper limit, the lightweight effect of the fiber-reinforced resin article is excellent. The minimum width of the strut is the shortest diameter, as some distortion may occur even in the case of a round shape, and is the minor diameter of an elliptical shape or the length of the short side of a rectangular shape.

[0096] The fiber-reinforced resin article and the method for manufacturing the same described above can be preferably used for manufacturing various fiber-reinforced resin articles having a structure in which some of the wall surfaces are joined together, in addition to manufacturing rectangular parallelepiped articles having closed cavities as exemplified. The fiber-reinforced resin article referred to in the present invention naturally includes articles made only of FRP, but is not limited thereto, and also includes fiber-reinforced resin articles formed as composites in which FRP is combined with parts made of materials other than FRP, such as metal plates.

[0097] 3. Experimental Results The following are the results of experiments conducted by the present inventors.

[0098] 3.1 Experiment 1 A fiber-reinforced resin article made of FRP, measuring 120 mm x 120 mm x 10 mm, was fabricated using the following procedure. The article had an upper and lower surface portions partially joined at the center of the surface, forming a substantially closed cavity, as shown in Figure 9. Two SMC sheets (STR120N131, manufactured by Mitsubishi Chemical Corporation) with a fiber content of 53% by mass and a thickness of approximately 2 mm, each composed of a chopped carbon fiber mat impregnated with an epoxy acrylate resin, were cut into a predetermined shape. One sheet was folded to form a lower layer of a prepreg preform with a substantially net shape and a central convex portion, while the other sheet was folded to form a flat upper layer of a prepreg preform. A separately prepared, square-ring-shaped fusible core was placed inside the lower layer of the prepreg preform, and the upper layer of the prepreg preform was then placed on top of the lower layer of the prepreg preform to prepare the prepreg preform. The fusible core was prepared by heating and melting two components, a low-melting point wax (a fatty acid ester wax manufactured by Ito Oil Mills, Ltd., melting point 50°C) and a high-melting point wax (a fatty acid amide wax manufactured by Ito Oil Mills, Ltd., melting point 142°C) in a mixing ratio (mass ratio) of 1:2 to make them compatible, and then using a cast mold to produce a core with dimensions that would fit perfectly inside the prepreg preform.

[0099] The prepared prepreg preform, together with the fusible core, was placed in a mold preheated to the same temperature as the molding temperature and heated to harden. The molding temperature was 135°C, and the molding time was 10 minutes. The molding time here refers to the time from mold clamping to mold opening. The molded article removed from the mold was heated to 145°C in an oven to melt the wax that forms the fusible core, and then a 5 mm diameter discharge hole was drilled in the molded article to discharge the wax. The appearance of the obtained fiber-reinforced resin article was good, with no voids such as peeling in the cross section.

[0100] 3.2 Experiment 2 A fiber-reinforced resin article having a substantially closed cavity made of FRP and a metal plate was produced in the same manner as in Experiment 1, except that the upper layer of the prepreg preform used was changed from SMC to a metal plate of SUS430 and the thickness was changed to 0.4 mm. The appearance of the obtained fiber-reinforced resin article was good, with no voids such as peeling in the cross section.

[0101] 3.3. Experiment 3 A fiber-reinforced resin article having a substantially closed cavity made of FRP was produced in the same manner as in Experiment 1, except that a fusible core with multiple 2 mm diameter through-holes connecting the upper and lower layers of the prepreg preform was used. Observation of the cross section of the obtained fiber-reinforced resin article revealed that SMC had flowed into the through-holes in the fusible core, forming pillars that supported and reinforced the walls of the cavity. The article also had a good appearance, with no voids such as peeling.

[0102] 3.4. Experiment 4 As in Experiment 1, two components, a low-melting-point wax (melting point 50°C) and a high-melting-point wax (melting point 142°C), were mixed. These waxes were heated to melt and dissolve, and then cooled to 135°C to obtain a clay-like compatible wax. The clay-like compatible wax was solid enough to deform under its own weight. This clay-like compatible wax was placed in a mold at 50°C. The clay-like compatible wax was pressure-molded in the mold at 3 MPa to produce a clay-like fusible core. A fiber-reinforced resin article having a substantially closed cavity made of FRP was produced in the same manner as in Experiment 1, except for using the clay-like fusible core obtained in this manner. The appearance of the obtained fiber-reinforced resin article was good, with no voids such as peeling in the cross section.

[0103] 3.5. Experiment 5 As in Experiment 1, two components, a low-melting-point wax (melting point 50°C) and a high-melting-point wax (melting point 142°C), were mixed. These waxes were heated to melt and then solidified by cooling. The solid, compatible wax thus obtained was pulverized in a pulverizer to obtain solid wax fragments. The solid wax fragments had an average particle size of 1.5 mm. The solid wax fragments were placed in a mold at 80°C. The solid wax fragments were press-molded at 3 MPa in the mold to produce a powdered fusible core. A fiber-reinforced resin article having a substantially closed cavity made of FRP was produced in the same manner as in Experiment 4, except that the powdered fusible core thus obtained was used. The appearance of the obtained fiber-reinforced resin article was good, with no voids such as peeling in the cross section.

[0104] 3.6. Experiment 6 As in Experiment 1, two components, a low-melting-point wax (melting point 50°C) and a high-melting-point wax (melting point 142°C), were mixed. These waxes were heated to melt and then solidified by cooling. The resulting solid, compatible wax was pulverized in a pulverizer to obtain solid wax fragments. The solid wax fragments were placed in a mold at a filling rate of 60%. Separately, the same compatible wax was melted, and the molten compatible wax was poured into a mold, filling it and casting to produce a fusible core. A fiber-reinforced resin article having a substantially closed cavity made of FRP was produced in the same manner as in Experiment 1, except for using the fusible core obtained in this manner. The appearance of the resulting fiber-reinforced resin article was good, with no voids such as peeling in the cross section.

[0105] 3.7 Experiment 7 A fiber-reinforced resin article having a substantially closed cavity made of FRP was produced in the same manner as in Experiment 1, except that only one component, a low-melting-point wax (fatty acid ester-based, melting point 50°C), was used as the fusible core. The obtained fiber-reinforced resin article had defects in appearance due to wax leakage.

[0106] 3.8 Experiment 8 A fiber-reinforced resin article having a substantially closed cavity made of FRP was produced in the same manner as in Experiment 1, except that only one component, a high-melting-point wax (fatty acid amide-based, melting point 142°C), was used as the fusible core. The obtained fiber-reinforced resin article had a relatively good appearance, but it was difficult to expel the wax, and a hollow structure could not be formed.

[0107] While the present invention has been described above with reference to specific embodiments, these embodiments are presented as examples and do not limit the scope of the present invention. Each embodiment described in this specification can be modified in various ways without departing from the spirit of the invention, and can be combined with features described in other embodiments to the extent possible.

[0108] The invention disclosed herein can be preferably used for producing fiber-reinforced resin articles having a hollow structure and surface reinforcement. The invention disclosed herein can be preferably used for producing, but is not limited to, parts (including structural parts) for automobiles, ships, railroad cars, aircraft, and other transportation equipment, as well as various sporting goods including bicycle frames, tennis rackets, and golf shafts, from fiber-reinforced resin.

[0109] REFERENCE SIGNS LIST 10 Fusible core 11A, 11B Groove 12 Wax 12A, 12B Through-hole 20 Reinforced fiber preform (prepreg preform) 20A Upper layer 20B Lower layer 20' Reinforced fiber preform (dry fabric preform) 20A' Upper layer 20B' Lower layer 20C Metal plate 30 Mold 32 Lower mold 34 Upper mold 40 Fiber reinforced resin article 41 Upper layer 42 Lower layer 43 Cavity 44 Partial joint 45A, 45B Rib 46A, 46B Support 60 Adhesive

Claims

1. A method for manufacturing a fiber-reinforced resin article, comprising: placing a reinforced fiber preform in a mold together with a fusible core made of wax, heating the preform in the mold to obtain a cured product of the reinforced fiber preform, and a core removal step of removing the fusible core from the cured product, wherein the fusible core is composed of at least two kinds of waxes that are compatible with each other, and the melting point difference between the wax with the highest melting point and the wax with the lowest melting point is 30 ° C or more.

2. The manufacturing method according to claim 1, wherein in the molding step, as the reinforced fiber preform, a prepreg preform made of a prepreg impregnated with a thermosetting resin is placed in the mold together with the fusible core and heated in the mold to obtain a cured product.

3. The manufacturing method according to claim 1, wherein in the molding step, as the reinforced fiber preform, a dry fabric preform made of a fabric not impregnated with a thermosetting resin is placed in the mold together with the fusible core, a thermosetting liquid resin is injected into the mold to form a fabric resin impregnated body, and the fabric resin impregnated body is heated in the mold to obtain a cured product.

4. The manufacturing method according to claim 1, wherein the highest melting point Tm (° C) of the melting points of the waxes constituting the fusible core and the molding temperature Tf (° C) satisfy Tm ≧ Tf - 30.

5. The manufacturing method according to claim 1, wherein the highest melting point Tm (° C) of the melting points of the waxes constituting the fusible core and the heating temperature Td (° C) during core removal satisfy Td ≧ Tm.

6. The manufacturing method according to claim 1, wherein the wax contains an organic compound having a polar group.

7. The manufacturing method according to claim 1, wherein the wax contains a fatty acid-based compound.

8. The manufacturing method according to claim 1, wherein the fusible core is substantially in contact with the reinforced fiber preform.

9. The manufacturing method according to claim 1, wherein the fusible core has grooves or depressions on its surface.

10. The manufacturing method according to claim 1, wherein the fusible core has through holes.

11. A fiber-reinforced resin article having a hollow structure portion obtained by fusible core molding, wherein the hollow structure portion has substantially no outer skin.

12. A fiber-reinforced resin article having a hollow structure portion in which cavities are formed and struts connecting the wall surfaces of the cavities, and the joint portion between the wall surface and the strut is integrally formed without a joint.

13. The fiber-reinforced resin article according to claim 12, wherein the strut is made of chopped fiber-reinforced resin.

14. The fiber-reinforced resin article according to claim 12, wherein the length of the pillar is 5 to 100 mm and the minimum width of the pillar is 1 to 10 mm.

15. The fiber-reinforced resin article according to claim 12, which has a partial joint portion where the wall surfaces are partially joined within the cavity.

16. The fiber-reinforced resin article according to claim 15, wherein at least a part of the wall surfaces of the partial joint portion are joined with an adhesive.

17. The fiber-reinforced resin article according to claim 11 or 12, which contains continuous fibers as reinforcing fibers.

18. The fiber-reinforced resin article according to claim 11 or 12, which contains chopped fibers as reinforcing fibers.

19. The fiber-reinforced resin article according to claim 18, which contains a cured product of SMC.

20. The fiber-reinforced resin article according to claim 11 or 12, in which a metal part is integrated with the fiber-reinforced resin article.

21. A method for manufacturing a fusible core, comprising: filling a mold with a moldable semi-solid or clay-like compatible wax obtained by changing the temperature of at least two kinds of compatible waxes, which are compatible and have a melting point difference of 30°C or more between the wax with the highest melting point and the wax with the lowest melting point, to a temperature 5°C to 25°C lower than the melting point of the wax with the highest melting point, and then pressure-molding the semi-solid or clay-like compatible wax in the mold.

22. When the melting point of the wax with the lowest melting point is T Ｌ the temperature of the mold during pressure molding is (T Ｌ - 20)°C or higher and (T Ｌ + 30)°C or lower. The manufacturing method according to claim 21 23. A method for manufacturing a fusible core, comprising: filling a mold with solid wax fragments obtained by crushing a solid compatible wax composed of at least two kinds of compatible waxes, which are compatible and have a melting point difference of 30°C or more between the wax with the highest melting point and the wax with the lowest melting point, and then pressure-molding the solid wax fragments in the mold.

24. Let the melting point of the wax with the lowest melting point be T Ｌ When this is the case, the temperature of the mold during pressure molding is T Ｌ °C or higher (T Ｌ + 50)°C or lower. The manufacturing method according to claim 23 25. The manufacturing method according to claim 23, wherein the average particle size of the solid wax fragments is 0.5 to 5 mm.

26. The manufacturing method according to any one of claims 21 to 25, wherein the pressure during the pressure molding is 1 to 10 MPa.

27. The manufacturing method according to any one of claims 21 to 25, wherein the material of the mold is aluminum.

28. A method for manufacturing a fusible core, comprising: putting solid wax crushed pieces obtained by crushing a solid-phase compatible wax composed of at least two kinds of mutually compatible waxes with a melting point difference between the wax with the highest melting point and the wax with the lowest melting point of 30 °C or more into a mold, heating and melting the wax of the same composition, casting and filling the mold with the melted wax, and then cooling.

29. The manufacturing method according to claim 28, wherein the average particle size of the solid wax crushed pieces is 5 mm or more.

30. The manufacturing method according to claim 28, wherein the filling rate when filling the solid wax crushed pieces into the mold is 10 to 90% by volume.

31. The manufacturing method according to claim 28, wherein the material of the mold is aluminum.

32. A fusible core, comprising: composed of at least two kinds of mutually compatible waxes, with a melting point difference between the wax with the highest melting point and the wax with the lowest melting point of 30 °C or more, and when measuring the temperature drop data of the fusible core with a differential scanning calorimeter (DSC), the peak temperature of the highest melting point component is 3 °C or more lower than the melting point temperature of the single wax with the highest melting point.

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