Electrolytic capacitors

Abstract

This invention provides an electrolytic capacitor that can achieve even higher sound quality by adjusting it to the desired sound quality. [Solution] The electrolytic capacitor 1 comprises a capacitor element 2, a bottomed cylindrical metal case 3 that houses the capacitor element 2, and a sealing body 4 that seals the opening of the metal case 3. A conductive polymer layer 6, mainly composed of a conductive polymer, is formed on the outer or inner surface of a part of the metal case 3.

Description

Technical Field

[0001] The present invention relates to an electrolytic capacitor, particularly an electrolytic capacitor used in audio equipment.

Background Art

[0002] In audio equipment, electrolytic capacitors are used for applications such as power circuit filters, coupling of each circuit block, and decoupling. It is known that the sound quality reproduced varies depending on the materials used for such electrolytic capacitors for acoustics.

[0003] Patent Document 1 proposes an electrolytic capacitor in which a polymer resin layer is formed on the inner surface of a metal case for improving sound quality. In Patent Document 1, by enhancing the electrical insulation between the capacitor element and the metal case with a polymer resin layer having electrical insulation properties, the influence of electromagnetic waves generated from external components such as transformers on the capacitor element is reduced to achieve improved sound quality.

Prior Art Documents

Patent Document

[0004]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0005] According to the electrolytic capacitor described in Patent Document 1, by providing a polymer resin layer on the inner surface of the metal case housing the capacitor element, it is possible to completely insulate the metal case from the capacitor element using this polymer resin layer, thereby reducing the influence of electromagnetic waves generated from external components such as transformers on the capacitor element. In other words, it is necessary to provide a polymer resin layer over the entire inner surface of the metal case in order to achieve complete insulation. As a result, it is not possible to freely change the location or extent to which the polymer resin layer is formed, making it difficult to adjust to the desired sound quality depending on the size of the electrolytic capacitor and the type of audio equipment it is installed in.

[0006] The present invention aims to provide an electrolytic capacitor that can achieve further improvements in sound quality by adjusting it to a desired sound quality. [Means for solving the problem]

[0007] An electrolytic capacitor according to an embodiment of the present invention is an electrolytic capacitor comprising a capacitor element, a bottomed cylindrical metal case housing the capacitor element, and a sealing body sealing the opening of the metal case, characterized in that a conductive polymer layer mainly composed of a conductive polymer is formed on the outer or inner surface of a part of the metal case.

[0008] In this configuration, a conductive polymer layer is formed on a desired area of ​​the outer or inner surface of a portion of the metal case of the electrolytic capacitor. By changing the location and extent of the conductive polymer layer formation according to the size of the electrolytic capacitor and the type of audio equipment it is installed in, it is possible to adjust the sound quality to the desired level. This is thought to be because the conductive polymer layer disperses electromagnetic waves generated from external components such as transformers, thereby suppressing the influence of electromagnetic waves on the capacitor element. As a result, further improvements in sound quality can be achieved.

[0009] In an electrolytic capacitor according to an embodiment of the present invention, the side wall portion of the metal case has a constricted portion for fixing the sealing body to the metal case, and the conductive polymer layer does not need to be formed in the constricted portion of the metal case or in the portion of the metal case closer to the open end than the constricted portion.

[0010] This configuration allows for more reliable suppression of the effect of electromagnetic waves generated from external components on the capacitor element by the conductive polymer layer, thereby more reliably improving the sound quality reproduced in audio equipment using electrolytic capacitors.

[0011] Furthermore, in the electrolytic capacitor according to an embodiment of the present invention, the conductive polymer layer may be formed only on the outer or inner surface of the bottom of the metal case.

[0012] This configuration allows for a rich, full sound and excellent sound balance.

[0013] Furthermore, in the electrolytic capacitor according to an embodiment of the present invention, the conductive polymer layer may be formed on the outer or inner surface of the bottom of the metal case and on the outer or inner surface of a portion of the side wall of the metal case that is closer to the bottom than the constricted portion.

[0014] This configuration allows for superior sound quality with excellent extension, clarity, and spaciousness.

[0015] Furthermore, in the electrolytic capacitor according to an embodiment of the present invention, the conductive polymer layer may be formed in a region where the length of the side wall portion in the axial direction is less than half the total length of the side wall portion.

[0016] With this configuration, in addition to sound extension, clarity, and spaciousness, it is possible to obtain superior sound quality in terms of sound balance, richness, and reverberation. [Effects of the Invention]

[0017] According to the electrolytic capacitor of the present invention, by adjusting to a desired sound quality, further improvement in sound quality can be achieved.

Brief Description of the Drawings

[0018] [Figure 1] It is a cross-sectional view showing the configuration of an electrolytic capacitor according to an embodiment of the present invention. [Figure 2] It is a cross-sectional view showing the configuration of an electrolytic capacitor according to a modified example of an embodiment of the present invention. [Figure 3] It is a cross-sectional view showing the configuration of an electrolytic capacitor according to another modified example of an embodiment of the present invention.

Mode for Carrying Out the Invention

[0019] Hereinafter, an electrolytic capacitor 1 according to an embodiment of the present invention will be described with reference to FIG. 1.

[0020] The electrolytic capacitor 1 includes a capacitor element 2, a bottomed cylindrical metal case 3 in which the capacitor element 2 is housed, a sealing body 4 that seals the opening of the metal case 3, and a conductive polymer layer 6 formed on the outer surface of a part of the metal case 3. Further, the electrolytic capacitor 1 of the present embodiment is a chip-shaped electrolytic capacitor mounted on a substrate, and further includes a seat plate 5 having electrical insulation.

[0021] The capacitor element 2 is formed by winding an anode foil having an oxide film formed on a valve action metal foil such as aluminum foil and a cathode foil with a separator interposed therebetween. The anode foil and the cathode foil are respectively connected to an anode lead terminal 2a and a cathode lead terminal 2b. An insulating paper having electrical insulation is used for the separator, and the separator holds an electrolyte. The electrolyte held by the separator may be only an electrolytic solution, or both a solid electrolyte mainly composed of a conductive polymer and an electrolytic solution.

[0022] The anode lead terminal 2a and the cathode lead terminal 2b are respectively drawn out to the outside through holes 4a and 4b formed in the sealing body 4 and holes 5a and 5b formed in the seat plate 5. The anode lead terminal 2a and the cathode lead terminal 2b drawn out to the outside from the seat plate 5 are respectively bent and arranged along the seat plate 5.

[0023] The material of the metal case 3 is, for example, aluminum. The metal case 3 consists of a bottom portion 3a and a side wall portion 3b. The bottom portion 3a of the metal case 3 is circular. That is, the cross-sectional shape of the side wall portion 3b of the metal case 3 is circular. Note that the shape of the bottom portion 3a (the cross-sectional shape of the side wall portion 3b) of the metal case 3 may be elliptical. Also, the outer surface of the metal case 3 may be laminated with a polyethylene terephthalate resin or a nylon resin. The sealing body 4 is made of an elastic member such as rubber. The side wall portion 3b of the metal case 3 has a constriction portion 3b1 for fixing the sealing body 4. The constriction portion 3b1 has a shape that bulges toward the inside of the metal case 3 and is formed by a constriction process. In the axial direction of the cylinder of the metal case 3, the position of the end on the bottom portion 3a side of the sealing body 4 is closer to the bottom portion 3a than the end on the bottom portion 3a side of the constriction portion 3b1 in FIG. 1, but may be at the same position as the end on the bottom portion 3a side of the constriction portion 3b1. Also, the opening end portion of the metal case 3 is curled inward.

[0024] The conductive polymer layer 6 is formed on the outer surface of the bottom portion 3a of the metal case 3 and on a part of the outer surface of the side wall portion 3b of the metal case 3. The conductive polymer layer 6 is mainly composed of a conductive polymer. For the conductive polymer forming the conductive polymer layer 6, for example, polythiophenes such as poly(3,4-ethylenedioxythiophene) (PEDOT), polypyrrole, polyaniline or their derivatives are used. Hereinafter, the portion formed on the outer surface of the bottom portion 3a of the conductive polymer layer 6 is referred to as an outer bottom covering portion 6a, and the portion formed on a part of the outer surface of the side wall portion 3b of the conductive polymer layer 6 is referred to as an outer side wall covering portion 6b. The outer bottom covering portion 6a and the outer side wall covering portion 6b are connected. The outer bottom covering portion 6a is formed on the entire outer surface of the bottom portion 3a.

[0025] The length of the outer wall covering portion 6b of the metal case 3 in the cylindrical axis direction is constant in the circumferential direction. The outer wall covering portion 6b is formed on the outer surface of a portion of the side wall portion 3b2 that is closer to the bottom 3a than the constricted portion 3b1. In other words, the conductive polymer layer 6 is not formed on the constricted portion 3b1 of the metal case 3 or on the portion of the metal case 3 that is closer to the open end than the constricted portion 3b1. The outer wall covering portion 6b is formed on the outer surface of the side wall portion 3b in a region where the length in the cylindrical axis direction is less than half the total length of the side wall portion 3b. More specifically, the outer wall covering portion 6b is formed on the outer surface of a region 3b2 in which the length in the cylindrical axis direction is approximately half the total length of the region 3b2. In this specification, "approximately half" means half or nearly half.

[0026] In this embodiment of the electrolytic capacitor 1, a conductive polymer layer 6 is formed on a desired area of ​​the outer or inner surface of a part of the metal case 3. By changing the location and range where the conductive polymer layer 6 is formed according to the size of the electrolytic capacitor 1 and the type of audio equipment it is mounted on, it is possible to adjust the sound quality to the desired level. This is thought to be because the conductive polymer layer 6 disperses electromagnetic waves generated from external components such as transformers, thereby suppressing the influence of electromagnetic waves on the capacitor element 2. As a result, further improvements in sound quality can be achieved. In particular, it is possible to achieve sound quality with excellent sound information. Furthermore, because the sound quality is improved by the conductive polymer layer 6 provided on the metal case 3, sound quality can be improved while ensuring freedom in selecting the material of the capacitor element 2. The freedom in selecting the material of the capacitor element 2 refers to the freedom in selecting the separator, electrolyte, and electrode foil. In addition, by changing the location and range where the conductive polymer layer 6 is formed according to the size of the electrolytic capacitor 1 and the type of audio equipment it is mounted on, it is possible to adjust the sound quality to the desired level, thereby achieving further improvements in sound quality.

[0027] Furthermore, according to this embodiment, the conductive polymer layer 6 is formed on the outer surface of the bottom portion 3a of the metal case 3 and on the outer surface of a portion of the side wall portion 3b of the metal case 3. Therefore, it is possible to form the conductive polymer layer 6 while holding the remaining portion of the side wall portion 3b of the metal case 3. Thus, it is easier to form the conductive polymer layer 6 compared to the case where the conductive polymer layer 6 is formed on the entire outer surface of the metal case 3.

[0028] Next, a modified example of the electrolytic capacitor 1 of the embodiment will be described.

[0029] Components with the same configuration as electrolytic capacitor 1 are denoted by the same reference numerals and their explanations are omitted. Furthermore, the specific examples of conductive polymers forming the conductive polymer layer in the following modified examples are the same as the specific examples of conductive polymers forming the conductive polymer layer 6.

[0030] Although not shown in the diagram, the conductive polymer layer may be formed on the outer surface of the bottom 3a of the metal case 3 and on the outer surface of a portion of region 3b2 of the metal case 3, where the length in the axial direction of the cylinder is less than half the total length of region 3b2. For example, the conductive polymer layer may be formed on the outer surface of the bottom 3a of the metal case 3 and on the outer surface of a region of region 3b2 of the metal case 3 that is greater than one-third but less than half the total length of region 3b2.

[0031] Although not shown in the diagram, the conductive polymer layer may be formed on the outer surface of the bottom 3a of the metal case 3 and on the outer surface of a portion of region 3b2 of the metal case 3 in which the length in the axial direction of the cylinder is greater than half the total length of region 3b2. In this case, the conductive polymer layer may be formed on the outer surface of the bottom 3a of the metal case 3 and on the outer surface of a portion of region 3b2 of the metal case 3 in which the length in the axial direction of the cylinder is less than half the total length of the side wall portion 3b. Alternatively, the conductive polymer layer may be formed on the outer surface of the bottom 3a of the metal case 3 and on the outer surface of a portion of region 3b2 of the metal case 3 in which the length in the axial direction of the cylinder is greater than or equal to half the total length of the side wall portion 3b.

[0032] As shown in Figure 2(a) of the electrolytic capacitor 101, a conductive polymer layer 106 may be formed on the outer surface of the bottom 3a of the metal case 3 and on the outer surface of the entire region 3b2 of the metal case 3.

[0033] As shown in Figure 2(b) for the electrolytic capacitor 201, the conductive polymer layer 206 may be formed only on the outer surface of the bottom 3a of the metal case 3. In Figure 2(b), the conductive polymer layer 206 is formed on the entire outer surface of the bottom 3a, but it may also be formed on only a portion of the outer surface of the bottom 3a.

[0034] In the above embodiment, the conductive polymer layer 6 is formed on the outer surface of a portion of the metal case 3. However, as shown in Figures 3(a) to 3(c) for the electrolytic capacitors 301, 401, and 501, the conductive polymer layers 306, 406, and 506 may be formed on the inner surface of a portion of the metal case 3.

[0035] In Figures 3(a) and 3(b), the conductive polymer layers 306 and 406 are formed on the inner surface of the bottom portion 3a of the metal case 3 and on the inner surface of a portion of the side wall portion 3b of the metal case 3.

[0036] In Figure 3(a), the conductive polymer layer 306 is formed on the inner surface of the bottom 3a of the metal case 3 and on the inner surface of a portion of the side wall portion 3b of the metal case 3 that is closer to the bottom 3a than the constricted portion 3b1. Here, the portion of the conductive polymer layer 306 formed on the inner surface of the bottom 3a is referred to as the inner bottom coating portion 306a, and the portion of the conductive polymer layer 306 formed on the inner surface of a portion of the side wall portion 3b is referred to as the inner wall coating portion 306b. In Figure 3(a), the inner wall coating portion 306b is formed on the inner surface of a portion of the region 3b2 whose length in the axial direction of the cylinder is approximately half the total length of the region 3b2. In other words, the inner wall coating portion 306b is formed on the inner surface of a portion of the side wall portion 3b whose length in the axial direction of the cylinder is less than half the total length of the side wall portion 3b.

[0037] Although not shown in the diagram, the conductive polymer layer may be formed on the inner surface of the bottom 3a of the metal case 3 and on the inner surface of a portion of region 3b2 of the metal case 3, where the length in the axial direction of the cylinder is less than half the total length of region 3b2. For example, the conductive polymer layer may be formed on the inner surface of the bottom 3a of the metal case 3 and on the inner surface of a region of region 3b2 of the metal case 3 that is greater than one-third but less than half the total length of region 3b2.

[0038] Although not shown in the diagram, the conductive polymer layer may be formed on the inner surface of the bottom 3a of the metal case 3 and on the inner surface of a portion of region 3b2 of the metal case 3 in which the length in the axial direction of the cylinder is greater than half the total length of region 3b2. In this case, the conductive polymer layer may be formed on the inner surface of the bottom 3a of the metal case 3 and on the inner surface of a portion of region 3b2 of the metal case 3 in which the length in the axial direction of the cylinder is less than half the total length of the side wall portion 3b. Alternatively, the conductive polymer layer may be formed on the inner surface of the bottom 3a of the metal case 3 and on the inner surface of a portion of region 3b2 of the metal case 3 in which the length in the axial direction of the cylinder is greater than or equal to half the total length of the side wall portion 3b.

[0039] In Figure 3(b), the conductive polymer layer 406 is formed on the inner surface of the bottom 3a of the metal case 3 and on almost the entire inner surface of region 3b2 of the metal case 3. The conductive polymer layer may also be formed on the inner surface of the bottom 3a of the metal case 3 and on the entire inner surface of region 3b2 of the metal case 3.

[0040] In Figure 3(c), the conductive polymer layer 506 is formed only on the inner surface of the bottom 3a of the metal case 3. Although the conductive polymer layer 506 is formed on the entire inner surface of the bottom 3a in Figure 3(c), it may also be formed on only a portion of the inner surface of the bottom 3a.

[0041] When at least a portion of the conductive polymer layer is formed on the inner surface of a portion of the side wall portion 3b of the metal case 3, it is preferable that the conductive polymer layer be formed on at least a portion of the region 3b2 of the side wall portion 3b of the metal case 3 that is closer to the bottom portion 3a of the metal case 3 than to the sealing body 4. This prevents the conductive polymer layer from being peeled off by the sealing body 4 when the sealing body 4 is inserted into the metal case 3.

[0042] Although not shown in the illustrations, the conductive polymer layer may be formed on the inner surface of at least a portion of the bottom 3a of the metal case 3 and on the outer surface of a portion of the side wall 3b of the metal case 3. Although not shown in the illustrations, the conductive polymer layer may be formed on the outer surface of at least a portion of the bottom 3a of the metal case 3 and on the inner surface of a portion of the side wall 3b of the metal case 3. The size of the area on the outer or inner surface of the side wall 3b of the metal case 3 where the conductive polymer layer is formed may be the same as in any of the embodiments and modifications described above.

[0043] In the above embodiment, an example of applying the electrolytic capacitor of the present invention to a chip-type electrolytic capacitor was described, but the application of the present invention is not limited to chip-type electrolytic capacitors. The electrolytic capacitor of the present invention may also be applied to substrate-free electrolytic capacitors or leaded electrolytic capacitors, etc. [Examples]

[0044] The present invention will be described in more detail below with reference to examples. Electrolytic capacitors of Examples 1 to 6, a conventional example, and Comparative Examples 1 and 2 were fabricated and acoustic evaluation tests were conducted.

[0045] For Examples 1-6, the Conventional Example, and Comparative Examples 1 and 2, chip-type electrolytic capacitors with a diameter of 10 mm, a height of 10 mm, a rated voltage of 35 V, and a capacitance of 330 μF were fabricated. In the electrolytic capacitor of Example 1, a conductive polymer layer was formed on the outer surface of the bottom of the metal case and on approximately half of the outer surface of the side wall of the metal case, in the region closer to the bottom than the constricted portion (see region 3b2 in Figure 1), similar to electrolytic capacitor 1 shown in Figure 1. In the electrolytic capacitor of Example 2, a conductive polymer layer was formed on the outer surface of the bottom of the metal case and on almost the entire outer surface of the region closer to the bottom than the constricted portion of the side wall of the metal case, similar to electrolytic capacitor 101 shown in Figure 2(a). In the electrolytic capacitor of Example 3, a conductive polymer layer was formed only on the outer surface of the bottom of the metal case, similar to electrolytic capacitor 201 shown in Figure 2(b). In the electrolytic capacitor of Example 4, a conductive polymer layer was formed on the inner surface of the bottom of the metal case and on approximately half of the inner surface of the side wall of the metal case, closer to the bottom than the constricted portion, similar to the electrolytic capacitor 301 shown in Figure 3(a). In the electrolytic capacitor of Example 5, a conductive polymer layer was formed on the inner surface of the bottom of the metal case and on almost the entire inner surface of the side wall of the metal case, closer to the bottom than the constricted portion, similar to the electrolytic capacitor 401 shown in Figure 3(b). In the electrolytic capacitor of Example 6, a conductive polymer layer was formed only on the inner surface of the bottom of the metal case, similar to the electrolytic capacitor 501 shown in Figure 3(c). The conventional electrolytic capacitor had the same configuration as the electrolytic capacitors of Examples 1 to 6, except that a conductive polymer layer was not formed on the metal case. The electrolytic capacitor of Comparative Example 1 had the same configuration as Example 2, except that an electrically insulating polymer resin layer was formed instead of a conductive polymer layer. The electrolytic capacitor of Comparative Example 2 had the same configuration as Example 5, except that an electrically insulating polymer resin layer was formed instead of a conductive polymer layer. The metal cases of the electrolytic capacitors in Examples 1-6, the Conventional Example, and Comparative Examples 1 and 2 were made of aluminum, and the conductive polymer layer in Examples 1-6 was made of poly-3,4-ethylenedioxythiophene (PEDOT). In Comparative Examples 1 and 2, the polymer resin layer was made of epoxy resin.

[0046] In the acoustic evaluation test, electrolytic capacitors from Examples 1-6, the conventional example, and Comparative Examples 1 and 2 were tested as output coupling capacitors in a CD player, and their reproduced sound quality was evaluated. Two listeners evaluated nine items on a three-point scale: clarity, depth, spaciousness, reverberation, power, extension, information content, resonance, and balance. The results are shown in Table 1 below. The evaluation was rated as follows: × for worse than the conventional example, △ for the same as the conventional example, ○ for better than the conventional example, and ◎ for significantly better than the conventional example. The total score, when × is 0 points, △ is 1 point, ○ is 2 points, and ◎ is 3 points, is also shown in Table 1. Note that the information content of the sound refers to the smoothness of the pitch.

[0047] [Table 1]

[0048] Table 1 shows the following: The electrolytic capacitors in Examples 1 to 6 all showed improved sound quality compared to the conventional example and the electrolytic capacitors in Comparative Examples 1 and 2, and particularly superior sound quality in terms of information and power compared to the conventional example without a conductive polymer layer. Furthermore, Examples 1 to 3, in which the conductive polymer layer is formed on the outer surface of the metal case, and Examples 4 to 6, in which the conductive polymer layer is formed on the inner surface of the metal case, each achieved comparable sound quality.

[0049] In Examples 3 and 6, the electrolytic capacitors, in which a conductive polymer layer is formed only on the outer or inner surface of the bottom of the metal case, were able to achieve superior sound quality compared to conventional examples, not only in terms of the amount of sonic information and the power of the sound, but also in terms of the richness of the sound and the balance of the sound.

[0050] Examples 1, 2, 4, and 5, in which a conductive polymer layer is formed on the outer or inner surface of the bottom of the metal case and on the outer or inner surface of a portion of the side wall of the metal case, were able to obtain superior sound quality compared to the conventional example, not only in terms of the amount of sonic information and the power of the sound, but also in terms of sound extension, sound clarity, sound spread, and sound resonance.

[0051] In the electrolytic capacitors of Examples 1 and 4, in which a conductive polymer layer is formed on the outer or inner surface of the bottom of the metal case and on the outer or inner surface of a portion of the side wall of the metal case that is closer to the bottom than the constricted portion and whose length in the axial direction of the cylinder is less than half the total length of the side wall, all nine items were better than the conventional example, and in particular, it was possible to obtain sound quality that was excellent in terms of the amount of sound information, sound resonance, and sound balance.

[0052] Furthermore, the electrolytic capacitor of Comparative Example 1, in which an electrically insulating polymer resin was formed on the entire outer surface of the metal case instead of forming a conductive polymer layer, and the electrolytic capacitor of Comparative Example 2, in which an electrically insulating polymer resin was formed on the entire inner surface of the metal case, produced a muffled sound with less reverberation compared to the electrolytic capacitors of Examples 1 to 6, and it was found that the sound balance and other characteristics were inferior compared to the conventional electrolytic capacitors.

[0053] The above-described embodiment mentions the use of an electrolytic capacitor as an output coupling capacitor in a CD player. However, it is not limited to this, and good sound reproduction can also be achieved when applied to electrolytic capacitors used in applications such as power supply circuit filters, coupling and decoupling of various circuit blocks. [Explanation of Symbols]

[0054] 1, 101, 201, 301, 401, 501 Electrolytic capacitors 2 Capacitor elements 2a Anode lead terminal 2b Cathode lead terminal 3 Metal case 3a bottom 3b Side wall part 3b1 Aperture section 3b2 The region of the side wall that is closer to the bottom than the constricted portion. 4 Sealing body 5 Seat board 6, 106, 206, 306, 406, 506 conductive polymer layer

Claims

1. Capacitor element and A bottomed cylindrical metal case for housing the aforementioned capacitor element, A sealing body that seals the opening of the aforementioned metal case, An electrolytic capacitor equipped with, An electrolytic capacitor characterized in that a conductive polymer layer, mainly composed of a conductive polymer, is formed on the outer or inner surface of a part of the metal case.

2. The side wall portion of the metal case has a constricted portion for fixing the sealing body to the metal case. The electrolytic capacitor according to claim 1, characterized in that the conductive polymer layer is not formed in the constricted portion of the metal case or in the portion of the metal case closer to the open end than the constricted portion.

3. The electrolytic capacitor according to claim 1 or 2, characterized in that the conductive polymer layer is formed only on the outer or inner surface of the bottom of the metal case.

4. The electrolytic capacitor according to claim 2, characterized in that the conductive polymer layer is formed on the outer or inner surface of the bottom of the metal case and on the outer or inner surface of a portion of the side wall of the metal case that is closer to the bottom than the constricted portion.

5. The electrolytic capacitor according to claim 4, characterized in that the conductive polymer layer is formed in a region where the length of the side wall portion in the axial direction is less than half the total length of the side wall portion.

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