Method for manufacturing a retaining member

Abstract

To provide a production method of a holding member which enables production of a holding member having a predetermined shape and exhibiting compression and recovery characteristics sufficient to hold a to-be held object in a high temperature environment.SOLUTION: A production method of a holding member comprises cutting a portion of a sheet made of inorganic fiber.SELECTED DRAWING: Figure 3

Description

[Technical Field]

[0001] This invention relates to a method for manufacturing a retaining member. [Background technology]

[0002] Inorganic fiber retaining members are sometimes used to hold components placed inside the exhaust pipe of an automobile. Because the inside of an automobile's exhaust pipe becomes hot, the retaining members are required to be durable in addition to retaining their shape. As a technology used for such applications, Patent Document 1 discloses a heat-resistant gasket made of inorganic fibers. [Prior art documents] [Patent Documents]

[0003] [Patent Document 1] Japanese Patent Application Publication No. 10-60412 [Overview of the project] [Problems that the invention aims to solve]

[0004] The gasket described in Patent Document 1 is obtained by preparing an aqueous slurry containing inorganic fibers and forming it using a papermaking machine. This papermaking process forms the gasket into a predetermined shape. However, the gasket manufactured according to Patent Document 1 does not exhibit good compression recovery characteristics, and there was a possibility that the object being held would detach from the gasket when used in high-temperature environments such as inside the exhaust pipe of an automobile.

[0005] The present invention was made to solve the above problems, and aims to provide a method for manufacturing a retaining member that can produce a retaining member having sufficient compression recovery characteristics for holding an object in a high-temperature environment and having a predetermined shape. [Means for solving the problem]

[0006] The present invention's method for manufacturing a retaining member is characterized by including a step of cutting a portion of a sheet made of inorganic fibers.

[0007] In the method for manufacturing the retaining member of the present invention, a retaining member having a predetermined shape is obtained by cutting a portion of a sheet made of inorganic fibers. By preparing a sheet made of inorganic fibers that has sufficient compression recovery properties, and then shaping this sheet into a predetermined form by cutting, it is possible to manufacture a holding member that has sufficient compression recovery properties to hold an object in a high-temperature environment and has a predetermined shape. Furthermore, because the density of inorganic fibers varies at the machined areas, it is expected that the material will more easily follow the thermal expansion when the object being held, such as a heater plate, is heated, thereby suppressing deformation of the object being held. Furthermore, while the inorganic fiber fragments generated during cutting may be removed, if some of the inorganic fiber fragments adhere to the machined surface or the inorganic fiber surface inside the machined surface, it can increase the frictional resistance between the object being held and the inorganic fibers, as well as between the inorganic fibers themselves, thereby improving the remaining surface pressure.

[0008] In the method for manufacturing the retaining member of the present invention, it is preferable that the cutting process is a milling process or an end milling process. If the cutting method is milling or end milling, the retaining member can be formed into a predetermined shape without reducing the compression recovery characteristics of the retaining member.

[0009] In the method for manufacturing the retaining member of the present invention, it is preferable that the retaining member is a ring-shaped gasket. Furthermore, it is preferable that the above cutting process involves forming a groove on the inner circumference of the ring.

[0010] The present invention's method for manufacturing a retaining member is suitable for manufacturing a ring-shaped gasket as a retaining member because it allows a sheet made of inorganic fibers to be processed into a ring shape by cutting. Furthermore, since the method for manufacturing the retaining member of the present invention also allows for the formation of grooves on the inner circumference of the ring by machining, it is also suitable as a method for manufacturing a ring-shaped gasket having grooves on the inner circumference of the ring.

[0011] In the method for manufacturing the retaining member of the present invention, the sheet made of the inorganic fibers is preferably a paper-formed mat produced by molding a slurry containing inorganic fibers by a papermaking method and then pressurizing and drying it. If the sheet made of inorganic fibers is a paper-making mat produced by forming a slurry containing inorganic fibers using a paper-making method and then pressurizing and drying it, the residual surface pressure of the paper-making mat is high, making it possible to manufacture a retaining member with high residual surface pressure.

[0012] In the method for manufacturing the retaining member of the present invention, the sheet made of the inorganic fibers is preferably a needle mat manufactured by the needling method. If the sheet made of inorganic fibers is a needle mat manufactured by the needling method, the residual surface pressure of the needle mat is high, making it possible to manufacture a retaining member with high residual surface pressure.

[0013] In needle mats, bundles of needles, formed by the entanglement of inorganic fibers, are created on the opposite side of the mat surface through needlering. However, on machined surfaces, these needle bundles protrude from the machined surface. This protrusion of the needle bundles from the machined surface increases the frictional resistance between the object being held and the machined surface, thereby suppressing the displacement of the object. Furthermore, the protruding length of the needle bundle can be adjusted by changing the shape of the needles, thereby increasing or decreasing the fiber density of the needle bundle. The protruding length of the needle bundle is preferably between 0.01 mm and 0.5 mm. If it is less than 0.01 mm, the effect of increasing frictional resistance due to the protrusion of the needle bundle will not be sufficiently obtained, and if it is greater than 0.5 mm, a gap will be created between the machined surface and the object to be held, and the effect of increasing frictional resistance will not be sufficiently obtained. [Brief explanation of the drawing]

[0014] [Figure 1] FIG. 1 is a perspective view schematically showing an example of a gasket. [Figure 2] FIG. 2 is a top view schematically showing a sheet made of inorganic fibers and the position of punching. [Figure 3] FIG. 3 is a perspective view schematically showing an example of cutting for forming a groove in a ring-shaped gasket. [Figure 4] FIG. 4 is a cross-sectional view schematically showing an example of using the gasket in an exhaust gas purification device. <00,00090>

MODE FOR CARRYING OUT THE INVENTION

[0015] Hereinafter, embodiments of the present invention will be specifically described. However, the present invention is not limited to the following embodiments, and can be appropriately changed and applied without changing the gist of the present invention.

[0016] <00,00098>[Method for manufacturing a holding member] The method for manufacturing the holding member of the present invention will be described. The method for manufacturing the holding member of the present invention is characterized by including a step of cutting a part of a sheet made of inorganic fibers.

[0017] [Sheet made of inorganic fibers] The inorganic fibers constituting the sheet made of inorganic fibers are not particularly limited, but it is desirable that they are composed of at least one selected from the group consisting of alumina fibers, silica fibers, alumina-silica fibers, mullite fibers, bio-soluble fibers, and glass fibers. When the inorganic fibers are at least one of alumina fibers, silica fibers, alumina-silica fibers, and mullite fibers, they have excellent heat resistance, so even when the holding member is exposed to a sufficiently high temperature, no deterioration or the like occurs, and the function as a holding member can be sufficiently maintained. Further, when the inorganic fibers are bio-soluble fibers, when manufacturing an exhaust gas purification device using the holding member, even if the scattered inorganic fibers are inhaled, etc., they dissolve in the living body, so they do not harm the health of the operator.

[0018] Alumina fibers may contain additives other than alumina, such as calcia, magnesia, or zirconia. The composition ratio of the alumina silica fibers is preferably Al2O3:SiO2 = 60:40 to 80:20 by weight, and more preferably Al2O3:SiO2 = 70:30 to 74:26.

[0019] The average fiber diameter and average fiber length of the inorganic fibers are not particularly limited, but if the sheet made of inorganic fibers is obtained from a papermaking mat described later, the average fiber diameter is preferably 2.0 to 15.0 μm and the average fiber length is preferably 0.05 to 3.0 mm. If the sheet made of inorganic fibers is obtained from the needle mat described later, it is preferable that the average fiber diameter is 3.0 to 10.0 μm and the average fiber length is 1.0 to 150 mm.

[0020] The sheet made of inorganic fibers preferably has a residual surface pressure of 120 kPa or more. The residual surface pressure of a sheet made of inorganic fibers is the surface pressure value remaining after 2500 compression and release cycles at high temperatures, and this can be measured as follows. A sheet made of inorganic fibers has a GBD (bulk density based on fiber weight) of 0.550 g / cm³. 3 After compressing for 10 minutes, the upper and lower plates were heated to 950°C at a rate of 95°C / min, resulting in a GBD of 0.529 g / cm². 3 (Open side) GBD = 0.550 g / cm 3 The compression process is repeated 2500 times until the (compression side) is reached, and at the 2500th compression, the GBD = 0.529 g / cm². 3 The surface pressure value on the open side is defined as the residual surface pressure.

[0021] Furthermore, the bulk density of sheets made of inorganic fibers is 0.050 to 0.300 g / cm³. 3 Preferably, the bulk density of the sheet made of inorganic fibers is 0.050 g / cm³. 3If the bulk density is less than 0.300 g / cm³, the entanglement of the inorganic fibers is weak, and the inorganic fibers are easily peeled off, making it difficult to maintain the shape of the sheet made of inorganic fibers in a predetermined form. On the other hand, if the bulk density of the sheet made of inorganic fibers is 0.300 g / cm³ 3 Beyond a certain point, the inorganic fiber sheet hardens, reducing its ability to attach to the target component, and making it more prone to tearing.

[0022] The sheet made of inorganic fibers is preferably a paper-formed mat manufactured by molding a slurry containing inorganic fibers using a papermaking method and then pressurizing and drying it. If the sheet made of inorganic fibers is a paper-making mat produced by forming a slurry containing inorganic fibers using a paper-making method and then pressurizing and drying it, the residual surface pressure of the paper-making mat is high, making it possible to manufacture a retaining member with high residual surface pressure.

[0023] Papermaking mats can be manufactured, for example, by following the procedure below. (1) Method for manufacturing paper mats by papermaking (1-1) Mixed liquid preparation process A binder solution is prepared by dissolving an organic binder, such as an acrylic resin, and an inorganic binder in water. Next, the inorganic fibers, binder solution, and water are mixed and stirred with a stirrer to prepare a mixture. Dispersants, flocculants, etc., may be added to the mixture as needed. Alternatively, inorganic fibers may be crushed using a pulper to produce short fibers, thereby obtaining a solution containing defibrated inorganic fibers.

[0024] (1-2) Dehydration process Next, the mixed liquid is poured into a molding machine with a filtration mesh formed on its bottom, and the water in the mixed liquid is dewatered through the mesh to produce the raw material sheet.

[0025] (1-3) Pressurized drying process Next, the raw material sheets are pressure-dried to produce papermaking mats. When drying, the drying temperature is preferably between 100 and 250°C. Also, the raw material sheet can be pressed using a press machine or the like. The pressure for pressing is determined such that the bulk density of the resulting formed mat becomes a predetermined value. For example, if the target bulk density of the formed mat after pressing and drying is 0.161 g / cm 3 when it is, it is compressed so that the bulk density during pressing becomes 0.214 g / cm 3 When the pressure is released, the raw material mat swells due to the repulsive force of the fibers, so the bulk density decreases (the thickness increases), and as a result, a formed mat with a target bulk density (about 0.161 g / cm 3 is obtained. Also, by using the press plate of the press machine as a hot plate and setting the temperature of the hot plate to a high temperature (for example, 100 to 250 °C), drying may be performed in combination with pressing. In this case, it is not necessary to raise the temperature inside the system during pressing, and the pressing and drying process can be performed by hot pressing.

[0026] By performing the pressing and drying process, the residual surface pressure of the formed mat can be increased.

[0027] The formed mat can be manufactured through the above steps. In the above method, in the mixed solution preparation step, the binder solution was added to the mixed solution. However, without using the binder solution in the “(1-1) mixed solution preparation step”, after the “(1-3) pressing and drying step”, the binder solution may be adhered to the formed mat and then dried, so that an inorganic binder containing an acrylic resin and crystal water is adhered to the inorganic fibers of the formed mat.

[0028] Also, the sheet made of inorganic fibers is preferably a needle mat manufactured by the needle punching method. When the sheet made of inorganic fibers is a needle mat manufactured by the needle punching method, since the residual surface pressure of the needle mat is high, a holding member with a high residual surface pressure can be manufactured.

[0029] The needle mat can be manufactured, for example, by the following procedure. (2) Method for manufacturing a needle mat by the needle punching method (2-1) Process for producing inorganic fiber precursors A spinning mixture containing raw materials for inorganic fibers, such as alumina and silica, is spun using a blowing method to produce inorganic fiber precursors.

[0030] (2-2) Firing process Next, the inorganic fiber precursor is compressed to produce a continuous sheet of a predetermined size, which is then subjected to needle punching, and subsequently fired to prepare an inorganic fiber aggregate made of inorganic fibers. The density of needle punching ranges from 0.1 to 30 needles / cm³. 2 It is preferable that this be the case.

[0031] (2-3) Binder application process Next, an organic binder such as an acrylic resin and an inorganic binder are dissolved in water to prepare a binder solution. The preparation of the binder solution can be done in the same way as described in the method for manufacturing papermaking mats. Then, the binder solution is applied to the inorganic fiber aggregate and subsequently dried. Through the above process, a needle mat can be manufactured.

[0032] In the present invention's method for manufacturing a retaining member, the retaining member is manufactured by cutting a portion of the inorganic fiber sheet described above. As an example of a retaining member obtained by cutting a sheet made of inorganic fibers, a gasket will be described.

[0033] [gasket] Figure 1 is a schematic perspective view showing an example of a gasket. The gasket 1 shown in Figure 1 is made of inorganic fibers. The gasket 1 is ring-shaped. The ring shape can be a circular ring, an elliptical ring, a polygonal ring, etc., but a circular ring is preferred. When the gasket is annular, the outer diameter (indicated by the double arrow R1 in Figure 1) is preferably 75 to 230 mm. The inner diameter (indicated by the double arrow R2 in Figure 1) is preferably 70 to 90 mm. Furthermore, the ratio of the inner diameter to the outer diameter (R2 / R1) when the gasket is annular is preferably 0.4 to 0.9.

[0034] The width of the gasket ring (indicated by the double arrow W1 in Figure 1) is preferably between 4 and 69 mm. The gasket thickness (indicated by the double arrow T1 in Figure 1) is preferably 6 to 20 mm.

[0035] The gasket preferably has a groove on the inner circumference of the ring. If the gasket has grooves on the inner circumference of the ring, the object to be held, such as a heater plate, can be fitted into the grooves and held in place, preventing the object from shifting and improving the holding force.

[0036] The gasket 1 shown in Figure 1 has a groove 10 on the inner circumference of the ring. The width of the groove 10 (indicated by the double arrow W2 in Figure 1) is preferably 1 to 55 mm. Furthermore, the ratio of the groove width to the gasket ring width (W2 / W1) is preferably 0.2 to 0.8. The depth of the groove 10 (indicated by the double arrow T2 in Figure 1) is preferably 0.6 to 10.0 mm. Furthermore, the ratio of the groove depth to the thickness of the gasket ring (T2 / T1) is preferably 0.1 to 0.5.

[0037] The groove 10 is shaped to follow the inner circumference of the ring, and is preferably a circular groove. Furthermore, it is not necessary for a part of the circle to be a groove; a groove that is part of a circle is also acceptable.

[0038] In gaskets, it is preferable that grooves are formed by machining, and the surface has both a machined surface and an unmachined surface. Machining methods include milling and end milling, which utilize mechanical tools. Machining is preferably performed using a machining device with NC (Numerical Control) control.

[0039] The gasket 1 shown in Figure 1 has a machined surface 20 and an unmachined surface 30. The surface of the groove 10 is the machined surface 20, and the surface of the parts other than the groove 10 is the unmachined surface 30. Since the machined surface has traces of contact with the cutting tool, while the unmachined surface does not, the machined and unmachined surfaces can be distinguished visually. If a gasket has both a machined surface and an unmachined surface, the inorganic fibers on the machined surface become unstable due to the cutting process. However, since the machined surface is in contact with the object being held, there is no problem of the inorganic fibers being eroded by the outside air. Rather, the frictional resistance between the machined surface and the object being held increases due to the crushed minute inorganic fibers, thereby improving the remaining surface pressure. On the other hand, the inorganic fibers on the unmachined surface remain in a stable state, so there is no need for contact with the object being held, and the surface condition can be maintained even when exposed to the outside air.

[0040] Furthermore, the gasket may have machined portions in areas other than the groove that follows the shape of the inner circumference of the ring. For example, it may have a groove that connects from the groove that follows the shape of the inner circumference of the ring to the outer circumference of the ring. Such grooves can be used as a passage for wiring to supply power to the heater ring when the heater ring, described later, is fitted into the gasket.

[0041] [Machining process for manufacturing gaskets] Next, a gasket is manufactured by cutting a sheet made of inorganic fibers. Figure 2 is a schematic top view showing the positions of the sheet made of inorganic fibers and the die-cutting process. Figure 2 shows a sheet 40 made of inorganic fibers. A ring-shaped gasket 50 is obtained by punching out the inorganic fiber sheet 40 at the positions indicated by the ring-shaped lines in Figure 2 using a ring-shaped punching blade. For the ring-shaped punching blade, a die-cutting blade such as a Thomson blade can be used.

[0042] Although punching is a machining process, it is not a cutting process. If a ring-shaped gasket is obtained by punching and the gasket manufacturing process is completed without performing any cutting, the method of manufacturing the gasket is not included in the method of manufacturing the retaining member of the present invention.

[0043] The manufacturing method for the retaining member of the present invention is carried out by forming a groove on the inner circumference of the ring-shaped gasket 50 by machining, thereby obtaining the retaining member to be manufactured by the manufacturing method for the retaining member of the present invention. For cutting processes, machining using machine tools such as milling or end milling can be used.

[0044] Figure 3 is a schematic perspective view showing an example of a machining process to form a groove in a ring-shaped gasket. Figure 3 shows the machining process being performed on the inner circumference of a ring-shaped gasket 50 using a cutting tool 60. The rotation of the cutting tool 60 is indicated by arrows around the cutting tool 60, and the direction in which the cutting tool 60 moves is indicated by arrows aligned with the inner circumference of the gasket 50. A groove 10 is formed in the gasket 50 by machining, and after the groove 10 is formed all the way around the inner circumference of the gasket 50, it becomes the gasket 1 shown in Figure 1.

[0045] As an example of the cutting conditions for forming grooves, when using an end mill as the cutting tool, the end mill rotation speed can be set to 1000-5000 rpm and the feed rate to 50-300 mm / min. Furthermore, the materials used for the cutting tools can include carbon steel, alloy tool steel, heat-treated steel, aluminum alloy, and the like. Through the above steps, a gasket can be manufactured as a retaining member using the method for manufacturing a retaining member of the present invention.

[0046] Furthermore, instead of using a ring-shaped punching blade on the inorganic fiber sheet 40, the inorganic fiber sheet may be processed by cutting processes such as milling or end milling to obtain a ring-shaped gasket. In the case where a ring-shaped gasket is obtained in this way, even if grooves are not formed on the ring-shaped gasket, cutting processes have been performed on the inorganic fiber sheet, so the method for manufacturing this gasket is included in the method for manufacturing the retaining member of the present invention.

[0047] [Examples of using gaskets as retaining members] A gasket manufactured by the method for manufacturing a retaining member of the present invention can be used to hold a heater plate in an exhaust gas purification device. The following diagram illustrates an example of gasket usage. Figure 4 is a schematic cross-sectional view showing an example of a gasket being used in an exhaust gas purification system. The exhaust gas purification device 100 shown in Figure 4 is equipped with an exhaust gas treatment body 120, and a retaining seal material 130 is wrapped around the exhaust gas treatment body 120 and housed in a casing 140. Exhaust gas (in Figure 4, exhaust gas is indicated by G and the flow of exhaust gas is indicated by arrows) emitted from the internal combustion engine flows into the exhaust gas purification device 100.

[0048] A heater plate 110 is provided on the upstream side of the exhaust gas flow direction relative to the exhaust gas treatment body 120, and the heater plate 110 is housed in the casing 140 by being held between two gaskets 1. In other words, the gasket 1 is used to hold the heater plate 110 within the exhaust gas purification device 100. Furthermore, an L-shaped welded member 150 is provided adjacent to the gasket 1, and the welded member 150 is welded to the casing 140 and fixed in a position that sandwiches the gasket 1.

[0049] The heater plate 110 has a disc shape, and it is fitted into a groove 10 in the gasket 1. By fitting the heater plate 110 into the groove 10, displacement of the heater plate 110 is prevented, and the heater plate 110 is held securely within the exhaust gas purification device 100.

[0050] The heater plate 110 is a heating element that generates heat when an electric current is applied, and exhaust gas can pass through its interior. As the exhaust gas passes through the heated heater plate, its temperature increases, and the high-temperature exhaust gas flows into the exhaust gas treatment unit 120 located downstream. The exhaust gas treatment unit 120 supports a catalyst for exhaust gas purification, and since the activity of the catalyst improves as the temperature of the exhaust gas treatment unit 120 increases, it is preferable to raise the temperature of the exhaust gas treatment unit 120. For these reasons, the heater plate 110 is provided in the exhaust gas purification device 100 in order to raise the temperature of the exhaust gas treatment unit 120.

[0051] The exhaust gas treatment unit 120 is configured such that exhaust gas flows in through a cell opening on one end face, is purified by a catalyst supported within the cell, and then flows out from the other end face. The exhaust gas treatment body may be made of non-oxide porous ceramics such as silicon carbide or silicon nitride, or it may be made of oxide porous ceramics such as alumina, cordierite, or mullite.

[0052] As the catalyst supported on the exhaust gas treatment unit 120, precious metals such as platinum, palladium, and rhodium are preferred, with platinum being the most preferred among these. Other catalysts that can be used include alkali metals such as potassium and sodium, and alkaline earth metals such as barium. These catalysts may be used individually or in combination of two or more. When these catalysts are supported, PM (particulate matter) can be more easily burned off, and toxic exhaust gases can also be purified.

[0053] The casing 140 has a substantially cylindrical shape. Its material is not particularly limited, but it is preferably made of stainless steel.

[0054] [Retaining members other than gaskets] The retaining member manufactured by the manufacturing method of the present invention is not limited to the gasket described above. Examples of retaining members other than gaskets include retaining seal materials for holding an exhaust gas treatment body. An example of a retaining seal material for holding an exhaust gas treatment body is the retaining seal material 130 shown in Figure 4 for holding the exhaust gas treatment body 120. The retaining seal material 130 is wrapped around the cylindrical exhaust gas treatment body 120 and housed in the casing 140. The retaining seal material 130 to be manufactured by the retaining member manufacturing method of the present invention can be obtained by cutting a predetermined shape from a sheet made of inorganic fibers by cutting processes such as milling or end milling. Alternatively, the retaining seal material to be manufactured by the retaining member manufacturing method of the present invention can also be obtained by punching out a sheet made of inorganic fibers to form a mat material of a predetermined shape, and then performing cutting processes such as forming grooves on the surface of the mat material.

[0055] This specification discloses the following:

[0056] (1) This disclosure is a method for manufacturing a retaining member, characterized by including a step of cutting a part of a sheet made of inorganic fibers.

[0057] Disclosure (2) is a method for manufacturing a retaining member according to Disclosure (1), wherein the cutting process is a milling process or an end milling process.

[0058] Disclosure (3) is a method for manufacturing a retaining member according to Disclosure (1) or (2), wherein the retaining member is a ring-shaped gasket.

[0059] This disclosure (4) is a method for manufacturing a retaining member according to this disclosure (3), wherein the cutting process is a process that forms a groove on the inner circumference of the ring.

[0060] Disclosure (5) is a method for manufacturing a retaining member according to any one of Disclosures (1) to (4), wherein the sheet made of inorganic fibers is a paper-formed mat manufactured by forming a slurry containing inorganic fibers by a papermaking method and then pressurizing and drying it.

[0061] This disclosure (6) is a method for manufacturing a retaining member according to any one of this disclosure (1) to (4), wherein the sheet made of inorganic fibers is a needle mat manufactured by a needling method. [Explanation of symbols]

[0062] 1 Gasket 10 grooves 20 Machining surface 30 Unmachined surface 40 Sheets made of inorganic fibers 50 Ring-shaped gaskets 60 Cutting tools 100 Exhaust gas purification device 110 Heater Plate 120 Exhaust gas treatment unit 130 Retaining sealant 140 Casing 150 Welding components G exhaust gas

Claims

1. A method for manufacturing a retaining member, characterized by including the steps of punching out a ring shape from a sheet made of inorganic fibers and cutting a portion of the punched-out sheet.

2. The method for manufacturing a retaining member according to claim 1, wherein the cutting process is a milling process or an end milling process.

3. The method for manufacturing a retaining member according to claim 1 or 2, wherein the retaining member is a ring-shaped gasket.

4. The method for manufacturing a retaining member according to claim 3, wherein the cutting process is a process that forms a groove on the inner circumference of the ring.

5. The method for manufacturing a retaining member according to claim 1 or 2, wherein the sheet made of inorganic fibers is a paper-formed mat manufactured by molding a slurry containing inorganic fibers by a papermaking method and then pressurizing and drying it.

6. The method for manufacturing a retaining member according to claim 1 or 2, wherein the sheet made of inorganic fibers is a needle mat manufactured by a needling method.

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