TWI675052B - Isotropically reinforced sheet, FRP shaped body, and method for producing FRP shaped body - Google Patents

Abstract

In order to provide: a quasi-isotropic reinforcing sheet that is not only excellent in strength but can be formed into a variety of shapes, and an FRP molded body that uses the quasi-isotropic reinforcing sheet as a main material and has protrusions that are not easily bent. And an efficient manufacturing method of the FRP molded body. A plurality of UD sheets (P ･) composed of a reinforcing fiber material (F) and a thermoplastic resin (M), which is a quasi-isotropic reinforcing sheet (S), which is composed of continuous fibers in a unidirectional arrangement. P ...), in a state where the fiber direction of each sheet is randomly aligned and the sheets are arranged in a layered manner so as to overlap each other, to form a sheet body, and as the aforementioned UD sheet (P 薄片 P ...), it is used A quadrangular shape with a width of 2mm to 20mm, a length of 5mm to 70mm, and a ratio of width to length of 1: 1 to 1: 6.

Description

Quasi-isotropic reinforcing sheet, FRP formed body, and method for manufacturing FRP formed body

The present invention relates to the improvement of prepreg sheets, and in particular, it relates to a quasi-isotropic reinforcing sheet excellent in strength and shapeability, and an FRP having a slender protrusion using the quasi-isotropic reinforcing sheet as a main material. A molded body and an efficient manufacturing method of the FRP molded body.

In recent years, fiber-reinforced plastics (FRP) composed of reinforcing fiber materials such as carbon fibers and glass fibers, and matrix resins have been used in various applications based on their excellent performance. In addition, as the FRP, a continuous fiber as a reinforcing fiber material is vertically or horizontally aligned, or a continuous fiber is plain woven, and a matrix resin is impregnated and formed, and it is widely known.

However, as for the FRP using the conventional continuous fibers, when the laminated prepreg sheet is formed into an arbitrary shape by hot pressing, the long continuous fiber extending across the entire sheet becomes an obstacle to the FRP. The main cause of the followability of the stamper is the deterioration of the formability, and as a result, there is a problem that the formable shape is limited.

In addition, when the conventional prepreg sheet is used, in order to form elongated stripe (rib) protrusions, it is necessary to heat-seal or adhere the individually formed protrusions and the main body. When this method is used, At the base of the protruding portion, the reinforcing fiber of the protruding portion and the reinforcing fiber of the main body portion are separated, and there is a problem that the base portion of the protruding portion is easily bent.

On the other hand, conventionally, a prepreg sheet formed by continuously cutting a UD sheet (unidirectional sheet) formed by continuous fibers in one direction and arranging them in a two-dimensional direction and integrating them with a matrix resin has been developed. (Refer to Patent Document 1), however, in Document 1, there is no mention of a technique for manufacturing an FRP molded body having an elongated protrusion using such a prepreg sheet.

[Patent Document 1] Japanese Patent Laid-Open No. 2016-27956

[Problems to be solved by the invention]

The present invention has been developed in view of the above-mentioned problems, and an object thereof is to provide a quasi-isotropic reinforcing sheet that is not only excellent in strength but can be formed into various shapes and has excellent shaping properties, and the quasi-isotropic reinforcing sheet is An FRP molded body mainly composed of protrusions that are not easily bent, and an efficient manufacturing method of the FRP molded body. [Technical means to solve the problem]

The means adopted by the inventors to solve the above problems will be described below with reference to the attached drawings.

That is, the quasi-isotropic reinforcing sheet of the present invention is a quasi-isotropic reinforcing sheet S composed of a reinforcing fiber material F and a thermoplastic resin M, and is characterized in that continuous fibers are bonded in a unidirectional state. The plurality of integrated UD sheets P ･ P ... are formed in a state where the fiber directions of the sheets are randomly aligned and the sheets are arranged in a layered manner so as to overlap each other to form a sheet body. P ･ P ... is a quadrangular shape with a width of 2mm to 20mm, a length of 5mm to 70mm, and a ratio of width to length of 1: 1 to 1: 6.

In the present invention, the reinforcing fiber material F is made of carbon fiber, and the thermoplastic resin M is made of polyamide resin, polypropylene, fluorine resin, or polyphenylene sulfide. The fiber volume content of the sheet body is 20% ~ 70%, and a plurality of sheet bodies are laminated and integrated to form a rupture strain of 1.10 to 1.75% when the thickness is 2.0 mm, thereby forming a quasi-isotropic reinforcing sheet with excellent formability and ductility.

On the other hand, in the present invention, in the FRP molded body mainly composed of the above-mentioned laminated body of the quasi-isotropic reinforcing sheet S, a width of 0.5 mm or more and a height of 5 mm (preferably 20 mm, more preferably) is formed on the surface. 50 mm) or more stripe-shaped protrusions 12 and the reinforcing fiber material F is allowed to exist at the base portion B of the protrusions 12 to connect the main body portion 11 and the protrusions 12, thereby forming the protrusions having a small width and a large height as It is not easily bent with respect to the main body portion.

In addition, the front end portion 12a and the base end portion 12b of the protruding portion 12 are formed to have the same width. With respect to the bending strength of the front end portion 12a, the bending strength of the base end portion 12b is − With a size of 10% to + 30%, a protrusion 12 having a small difference in strength at different locations can be formed.

In addition, in the present invention, a manufacturing method that can be used when manufacturing the above-mentioned FRP formed article is a method in which a laminated body of the quasi-isotropic reinforcing sheet S is hot-pressed with a mold to form a width on the surface of the body portion 11 of the formed article Stripe-shaped protrusions 12 having a height of 0.5 mm or more and a height of 5 mm or more. [Inventive effect]

According to the present invention, a plurality of UD sheets are aligned in a state in which the fiber directions of the sheets are randomly aligned, and the sheets are arranged in a layered manner so as to overlap each other to form a quasi-isotropic reinforcing sheet, which is also used as a UD sheet. Those who use a square shape of a predetermined size can improve the shapeability of the prepreg sheet.

In addition, as long as the quasi-isotropic reinforcing sheet is laminated and hot-pressed, it is easy to form elongated stripe-shaped protrusions on the surface of the main body of the FRP molded body, and the base of the formed protrusions becomes The state where the reinforcing fiber material is disposed so as to connect the main body portion and the protruding portion allows the elongated stripe-shaped protruding portion to be formed so as not to be easily bent at the base portion.

Therefore, according to the present invention, it is possible to provide a pseudo-isotropic reinforcing sheet that can be formed into a variety of shapes by hot press forming, and an FRP molding having protrusions that uses the pseudo-isotropic reinforcing sheet as a main material and has excellent durability. Therefore, the practical use value of the present invention is extremely high.

[First Embodiment] The first embodiment of the present invention will be described with reference to Figs. 1 to 3. In the figure, the symbol S represents a quasi-isotropic reinforcing sheet, and the symbol P represents a UD sheet. A symbol F indicates a reinforcing fiber material, and a symbol M indicates a thermoplastic resin.

"Composition of quasi-isotropic reinforcement sheet" [1] About the basic structure of quasi-isotropic reinforcement sheet First, the basic structure of the quasi-isotropic reinforcement sheet S will be described. In this embodiment, as shown in FIG. 1, a plurality of UD sheets P ･ P... Are randomly aligned in the fiber direction of each sheet (the orientation of the sheet in the fiber direction is scattered) and the sheets are arranged with each other In a state of being stacked in a layered manner (along the two-dimensional direction), the quasi-isotropic reinforcing sheet S is formed by performing the next step.

In addition, as shown in FIG. 2, the quasi-isotropic reinforcing sheet S is impregnated with a thermoplastic resin M as a matrix resin in the reinforcing fiber material F, and each of the UDs constituting the quasi-isotropic reinforcing sheet S is impregnated. As shown in FIG. 3, the sheet P is formed by continuous fibers (carbon fibers in the present embodiment) aligned in a unidirectional manner and then integrated. Thereby, a quasi-isotropic reinforcing sheet S having excellent strength and shapeability can be formed.

[2] Reinforcing fiber material Next, the constituent elements of the aforementioned quasi-isotropic reinforcing sheet S will be described. First, although the above-mentioned reinforcing fiber material F is carbon fiber in this embodiment, as long as it is a high-strength fiber material, fibers other than carbon fiber can also be used, such as glass fiber, polyamide fiber, ceramic fiber, Inorganic fibers such as metal fibers, organic fibers such as polyarylate fibers and PBO (polyparaphenylene benzodioxazole) fibers.

[3] About the fiber volume fraction (Vf) In addition, regarding the fiber volume fraction (Vf) of the reinforcing fiber material in the quasi-isotropic reinforcing sheet S described above, sufficient strength cannot be obtained when the content of the reinforcing fiber is too low In addition, when the content of the reinforcing fiber is too high, the proportion of the resin decreases and it is difficult to obtain sufficient forming properties. Therefore, it is preferable to adjust it in the range of Vf 20% to 70%.

[4] Thermoplastic resin In addition to the above-mentioned thermoplastic resin M, although a polyamide resin (nylon resin) is used in this embodiment, as long as it is a thermoplastic resin (acrylic resin) that has impregnation with reinforcing fiber materials, , PET, polyethylene, polystyrene, polycarbonate, ABS resin, polyetherimide, fluororesin, polyphenylene sulfide, etc.), other materials can also be used, of which polyfluorene is preferred Amine resin, polypropylene, fluorine resin or polyphenylene sulfide.

[5] Method of impregnating thermoplastic resin In addition, the method of impregnating thermoplastic resin M with reinforcing fiber material F, in this embodiment, a thin film-like heat is laminated on UD sheets P ･ P ... which are arranged in layers. The thermoplastic resin M is hot-pressed with the thermoplastic resin M and the UD sheet P ･ P ..., so that the thermoplastic resin M is impregnated in the reinforcing fiber material F, but the UD sheet P ･ P ... can also be laminated in granular form Or short-fiber thermoplastic resin M is impregnated. In addition, as for the impregnation rate of the thermoplastic resin M, it is not necessary to completely impregnate the reinforcing fibers F of all the UD sheets P, as long as they are impregnated to such an extent that each of the UD sheets P ･ P...

[6] Regarding the impregnation properties of the thermoplastic resin In the above-mentioned impregnation method of the thermoplastic resin M, when a method in which a film is laminated and hot-pressed on a UD sheet P ･ P ... is preferably a polyamide-based system. Resin, more preferably low viscosity. In addition, polypropylene and ETFE (tetrafluoroethylene, copolymer of ethylene), which is inferior to the polyamide resin, can also be used. For reference, the test results of the impregnation properties of the thermoplastic resin are shown in Table 1 below.

[7] Regarding the shape of the UD sheet In addition, regarding the UD sheet P shown in FIG. 3, although a width W5mm × length L20mm is used in this embodiment, considering the strength and shapeability, a width W2mm to 20mm ( It is preferably a quadrangular shape having a length L5mm to 70mm (preferably 10mm to 30mm) and a ratio of the width W to the length L of 1: 1 to 1: 6. In this specification, a direction parallel to the longitudinal direction of the reinforcing fiber material is referred to as a longitudinal direction of the UD sheet P, and a direction perpendicular thereto is referred to as a width direction.

[8] Regarding the thickness of the quasi-isotropic reinforcement sheet and the UD sheet In addition, the thickness of the quasi-isotropic reinforcement sheet S is preferably designed in the range of 0.3 mm to 1.0 mm, and the thickness of the UD sheet P The thickness is preferably designed in a range of 40 μm to 100 μm. In addition, regarding the average number of UD sheets P ･ P, which are laminated in the quasi-isotropic reinforcing sheet S, in the thickness direction, although this embodiment is designed in the range of 5.0 to 8.0, the strength is considered The impregnation property of the resin is preferably designed in the range of 2.0 to 15.0 pieces.

[9] About the physical properties of the quasi-isotropic reinforcing sheet In addition, the quasi-isotropic reinforcing sheet S is preferably designed so that the breaking strain at a thickness of 2.0 mm becomes 1.10 to 1.75%. Regarding the quasi-isotropic reinforcing sheet S of this embodiment, when a plurality of sheets were laminated to form a thickness of 2.0 mm, a burst strain test was performed, and a plain sheet of carbon fiber was laminated to impregnate a resin with a conventional sheet. The laminated product is 0.71%, which is equivalent to this, and the laminated product of this embodiment is 1.53%. The comparison of other physical property values is shown in Table 2 below. The values of "rupture strain, tensile strength, and tensile modulus" in the table are calculated in accordance with JISK7164, the values of "flexural fracture strength, flexural modulus" are calculated in accordance with JISK7074, and the "compressive strength" is Calculated based on JISK7076. In addition, the number of test bodies of each laminated product is n = 6, and the values in the table are average values.

[Second Embodiment] [Configuration of FRP molded article] [1] Basic configuration of FRP molded article Next, a second embodiment of the present invention will be described with reference to FIGS. 4 to 5. In this embodiment, a laminated body having a quasi-isotropic reinforcing sheet S (other conditions are the same as the first embodiment) of Vf54% is formed by using a UD sheet having a width of W5mm × length L20mm, as shown in FIG. 4. The surface of the main body portion 11 of the FRP molded body 1 whose main material is this laminate is provided with stripe-shaped protrusions 12.

The stripe-shaped protrusions 12 can be designed to have a width of 0.5 mm or more and a height of 5 mm (preferably 20 mm, more preferably 50 mm). In addition, as shown in FIG. 5, a reinforcing fiber material F that is bent so as to connect the main body portion 11 and the protruding portion 12 exists in the base portion B of the protruding portion 12. Thereby, it is possible to form the elongated rib-shaped protrusions 12 so that they are less likely to be bent than in the past.

[2] Regarding the shape of the protruding portion In addition to the above-mentioned protruding portion 12, in this embodiment, as shown in FIG. 5, the front end portion 12a and the base end portion 12b of the protruding portion 12 have the same width, and There is not much difference in fiber volume fraction (Vf). As a result, the flexural strength of the base portion 12b is -10% to + 30% relative to the flexural strength of the front portion 12a, and the protrusions 12 having small differences in strength at different locations are formed.

｣Method of manufacturing FRP molded body 体 In addition, when manufacturing the FRP molded body 1 of this embodiment, the laminated body of the quasi-isotropic reinforcing sheet S can be hot-pressed with a mold, so that the main body of the molded body can be easily formed. Stripe-shaped protrusions 12 having a width of 1.0 mm to 6.0 mm and a height of 20 mm or more are formed on the surface of the portion 11. In addition, by adopting this manufacturing method, a reinforcing fiber material F for connecting the main body portion 11 and the protrusion portion 12 can be made to the base portion B of the protrusion portion 12. [Example]

[Verification Test of Effect (i)] Next, a verification test (i) of the effect of the present invention will be described. First, in this test, the type of the quasi-isotropic reinforcing sheet as the main material of the FRP molded body and the size of the stripe-shaped protrusions formed on the FRP molded body were changed to perform hot press molding. The results were used to evaluate the shapeability.

"Example 1" The pseudo-isotropic reinforcing sheet (reinforcing fiber material: carbon fiber, thermoplastic resin: polyamide resin, UD sheet) used in the FRP molded body of the second embodiment was adopted. Dimensions: width 5mm × length 20mm, fiber content: Vf54%), and using a mold having a recess with a width of 0.5mm and a depth of 45mm, the laminated body of this sheet was hot-pressed. As a result, an FRP molded body having protrusions having a width of 0.5 mm and a height of 45 mm on the surface of the main body portion can be formed.

"Example 2" The pseudo-isotropic reinforcing sheet (reinforcing fiber material: carbon fiber, thermoplastic resin: polyamide resin, UD sheet) used in the FRP molded body of the second embodiment was used. Dimensions: width 5mm × length 20mm, fiber content rate: Vf54%), and using a mold having a recess with a width of 1.0mm and a depth of 45mm, the laminated body of this sheet was hot-pressed. As a result, an FRP molded body having protrusions having a width of 1.0 mm and a height of 45 mm on the surface of the main body portion can be formed.

"Example 3" The pseudo-isotropic reinforcing sheet (reinforcing fiber material: carbon fiber, thermoplastic resin: polyamide resin, UD sheet) used in the FRP molded body of the second embodiment was used. Dimensions: width 5mm × length 20mm, fiber content rate: Vf54%), and using a mold having a concave portion with a width of 2.0mm and a depth of 45mm, the laminated body of this sheet was hot-pressed. As a result, an FRP molded body having protrusions having a width of 2.0 mm and a height of 45 mm on the surface of the main body portion can be formed.

"Example 4" The pseudo-isotropic reinforcing sheet (reinforcing fiber material: carbon fiber, thermoplastic resin: polyamide resin, UD sheet) used in the FRP molded body of the second embodiment was used. Dimensions: width 5mm × length 20mm, fiber content rate: Vf54%), and using a die having a concave portion having a width of 3.0mm and a depth of 45mm, the laminated body of the sheet was subjected to hot pressing. As a result, an FRP molded body having protrusions having a width of 3.0 mm and a height of 45 mm on the surface of the main body portion can be formed.

"Example 5" A pseudo-isotropic reinforcing sheet (reinforcing fiber material: carbon fiber, thermoplastic resin: polyamide resin, UD sheet) used in the FRP molded body of the second embodiment was used. Dimensions: width 5mm × length 20mm, fiber content rate: Vf54%), and using a mold having a concave portion with a width of 5.0mm and a depth of 45mm, the laminated body of this sheet was hot-pressed. As a result, an FRP molded body having protrusions having a width of 5.0 mm and a height of 45 mm on the surface of the main body portion can be formed.

"Comparative Example 1" A quasi-isotropic reinforcing sheet formed by laminating flat-woven sheets of reinforcing fiber material and impregnating the resin (reinforcing fiber material: carbon fiber, thermoplastic resin: polyamide resin, fiber content rate: Vf45%) The laminated body of this sheet was hot-pressed using a mold having a recessed portion having a width of 1.0 mm and a depth of 45 mm. As a result, only a protrusion having a height of 3 mm or less can be formed on the surface of the main body portion.

"Comparative Example 2" A quasi-isotropic reinforcing sheet formed by laminating flat-woven sheets of reinforcing fiber material and impregnating the resin (reinforcing fiber material: carbon fiber, thermoplastic resin: polyamide resin, fiber content rate: Vf45%) The laminated body of this sheet was hot-pressed by using a mold having a concave portion having a width of 2.0 mm and a depth of 45 mm. As a result, only a protrusion having a height of 3 mm or less can be formed on the surface of the main body portion.

"Comparative Example 3" A quasi-isotropic reinforcing sheet formed by laminating flat-woven sheets of reinforcing fiber material and impregnating the resin (reinforcing fiber material: carbon fiber, thermoplastic resin: polyamide resin, fiber content rate: Vf45%) The laminated body of this sheet was hot-pressed by using a mold having a concave portion having a width of 3.0 mm and a depth of 45 mm. As a result, only a protrusion having a height of 3 mm or less can be formed on the surface of the main body portion.

"Comparative Example 4" A quasi-isotropic reinforcing sheet formed by laminating flat-woven sheets of reinforcing fiber material and impregnating the resin (reinforcing fiber material: carbon fiber, thermoplastic resin: polyamide resin, fiber content rate: Vf45%) The laminated body of this sheet was hot-pressed by using a mold having a concave portion having a width of 5.0 mm and a depth of 45 mm. As a result, only a protrusion having a height of 3 mm or less can be formed on the surface of the main body portion.

"Comparative Example 5" The quasi-isotropic reinforcing sheet (reinforcing fiber material: carbon fiber, thermoplastic resin: polyamide resin, UD sheet) used in the FRP molded body of the second embodiment was used. Dimensions: width 5 mm × length 20mm, fiber content rate: Vf54%), and using a mold having a recess with a width of 0.1mm and a depth of 45mm, the laminated body of this sheet was hot-pressed. As a result, beautiful protrusions cannot be formed on the surface of the main body portion.

<Summary of test results> From the results of Examples 1 to 5 above, it was confirmed that by using the quasi-isotropic reinforcing sheet of the present invention for hot pressing, a width of 0.5 mm to 5.0 mm and a height can be formed without any problem. 45mm stripe-shaped protrusions. In addition, it is confirmed from the results of Comparative Examples 1 to 4 that it is difficult to form a stripe-shaped protrusion with a width of 1.0 mm to 5.0 mm and a height of 45 mm using a conventional quasi-isotropic reinforcing sheet. In addition, from the results of Comparative Example 5, it was confirmed that even if the quasi-isotropic reinforcing sheet of the present invention is used for hot press molding, it is still difficult to form protrusions having a width of 0.1 mm or less.

[Verification Test of Effect (ii)] Next, a verification test (ii) of the effect of the present invention will be described. In this test, a quasi-isotropic reinforcing sheet (reinforcing fiber material: carbon fiber, thermoplastic resin: polyamide resin, and UD sheet) used in the FRP molded body of the second embodiment was used. Dimensions: width 5mm × length 20mm, fiber content rate: Vf54%), the laminated body of this sheet was hot-pressed using a mold having a recess with a width of 2.0mm and a depth of 60mm to produce a width of 2.0 on the surface of the main body as shown in FIG. 6 FRP molded body with a projection of 60 mm in height and 60 mm in height.

Next, a plate-shaped test piece (size: width 15 mm × length 120 mm × thickness 2 mm) was cut out from each of the front, center, and base end portions of the protruding portion of the FRP molded body, and the test piece was trimmed to a length At 100 mm, three test pieces at each location (nine in total, n = 3) were prepared, and a four-point bending test was performed in accordance with JIS K7074 to calculate the average value. As shown in FIG. 6, the test pieces on the front end side and the base end side were cut out at a distance of 5.5 mm from the edges of the front end and the base end of the protrusion. In addition, the test pieces in each part were tested. The sheets were cut out at intervals of 2.0 mm from each other.

<Test results> As a result of performing a 4-point bending test on each of the test pieces at the front end side, the center portion, and the base end side, it was confirmed that the value of the flexural rupture strength of each part was within ± 10%, and there was not much between the test pieces. The UD sheet can be uniformly dispersed in the protrusions without bias. In this way, it can be seen that the forming method of the present invention is effective for forming not only flat plates but also three-dimensional shapes. Table 3, which summarizes the data of this test, is shown below.

1: FRP molded body 11: Main body portion 12: Protruded portion 12a: Front end side portion 12b: Base end side portion S: Quasi-isotropic reinforcing sheet P: UD sheet F: Reinforced fiber material M: Thermoplastic resin B: Base

FIG. 1 is an enlarged plan view showing a quasi-isotropic reinforcing sheet according to a first embodiment of the present invention. 2 is a cross-sectional explanatory view showing a cross-sectional structure of a quasi-isotropic reinforcing sheet according to a first embodiment of the present invention. FIG. 3 is a perspective view of the entire UD sheet according to the first embodiment of the present invention. FIG. 4 is an overall perspective view showing an FRP molded body according to a second embodiment of the present invention. Fig. 5 is an explanatory view showing an X-X 'cross-section of a cross-sectional structure of an FRP molded article according to a second embodiment of the present invention. FIG. 6 is an explanatory diagram for explaining a method for verifying an effect.

Claims (4)

A quasi-isotropic reinforcing sheet is a quasi-isotropic reinforcing sheet composed of a reinforcing fiber material (F) and a thermoplastic resin (M), which is characterized by continuous integration of continuous fibers in a unidirectional arrangement state and subsequent integration The plurality of UD sheets (P) (P) ... formed in the state where the fiber directions of the sheets are randomly aligned and the sheets are arranged in a layered manner so as to overlap each other to form a sheet body as The UD sheet (P) is a quadrangular shape having a width of 2 mm to 20 mm, a length of 5 mm to 70 mm, and a ratio of width to length of 1: 1 to 1: 6. The reinforcing fiber material (F) is a carbon fiber, and the thermoplastic The resin (M) is made of polyamide resin, polypropylene, fluororesin or polyphenylene sulfide. The fiber volume content of the sheet body is 20% to 70%, and a plurality of sheet bodies are laminated and integrated. When the thickness is 2.0 mm, the breaking strain is 1.10 to 1.75%. An FRP formed body is an FRP formed body comprising a laminated body of the quasi-isotropic reinforcing sheet (S) as described in claim 1 as a main material, characterized in that a width of 0.5 is formed on the surface of the main body portion (11). The stripe-shaped protrusions (12) with a height of 5 mm or more and a height of 5 mm or more are further reinforced with a reinforcing fiber at the base (B) of the protrusions (12) to connect the main body portion (11) and the protrusions (12). The dimension material (F) exists. The FRP molded article according to claim 2, wherein the front end portion (12a) and the base end portion (12b) of the protrusion (12) are formed to have the same width as the front end portion (12a) ), The bending strength of the base (12b) is -10% ~ + 30%. A method for manufacturing an FRP molded article, characterized in that a laminated body of the quasi-isotropic reinforcing sheet (S) as described in claim 1 is hot-pressed with a mold, and a main body portion (11) of the molded article is formed. A stripe-shaped protrusion (12) having a width of 0.5 mm or more and a height of 5 mm or more is formed on the surface.