JP5821121B2 - Self-drilling rivet die - Google Patents

Description

  The present invention relates to a die used in a self-piercing rivet fastening device that fastens a plurality of members to be fastened using a self-piercing rivet having a large-diameter head and hollow legs hanging from the head. In particular, the present invention relates to a die for use in fastening a member to be fastened made of a poorly spreadable material such as a die cast material.

The self-piercing type rivet has an advantage that the member to be joined can be easily joined simply by driving in the rivet without processing a hole for inserting a bolt or the like for fastening to the member to be fastened in advance.
FIG. 1 is an enlarged view of a portion for fastening a self-piercing rivet of a conventional self-piercing rivet fastening device. The self-piercing rivet fastening device 1 clamps two fastened members 41 and 42 with a strong force (see arrows) by the die 20 and the nose 3. The self-piercing rivet 10 has a large-diameter head 11 and hollow cylindrical legs 12 depending from the head 11.

The die 20 has a cavity 21 on the top surface, and the bottom surface 22 of the cavity is substantially flat. The self-piercing rivet 10 is driven into the fastened members 41 and 42 arranged on the die 20 by the punch 4. The leg tip 13 of the self-piercing rivet 10 passes through the punched member 41 adjacent to the punch 4 and the receiving member 42 adjacent to the die 20 does not penetrate but remains in it. The front end of the leg portion 12 of the self-piercing rivet 10 is deformed so as to expand radially outward by the die 20. The fastened members 41 and 42 are connected to each other by the leg 12 and the head 11 that are open in the fastened member 42.
In FIG. 1, the bottom surface 22 of the cavity 21 is formed substantially flat. Unlike this shape, some of the cavities 21 are provided with a protrusion protruding toward the punch at the center.

  Such a self-piercing rivet is suitable for connecting aluminum body panels that are not suitable for welding. The weight of automobile bodies has been reduced, and aluminum bodies have also been adopted, and the demand for self-drilling rivets is increasing. In particular, the self-drilling rivet penetrates the clamped member 41 on the punch side, but is driven so that the clamped member 42 on the receiving side adjacent to the die 20 does not penetrate and stays therein. No rivet hole is formed on the surface of the fastened member 42. Therefore, there is an advantage that the sealing performance to the receiving side fastened member 42 is not impaired and the appearance is maintained as it is.

  When such a conventional self-drilling rivet fastening device using the die 20 is used to fasten the fastened members 41 and 42 having poor spreadability such as die-cast material, the self-drilling rivet 10 to be driven by a punch. When the tip of the leg portion 12 is pierced and pushed into the fastened members 41 and 42 and deformed by the cavity 21 of the die 20, the fastened members 41 and 42 cannot withstand plastic deformation and cracks occur. There is a case. In particular, the receiving-side fastened member 42 is often cracked.

Patent document 1 discloses joining a member to be joined made of a low ductility material using a self-piercing (self-piercing type) rivet. Patent Document 1 discloses that when a self-piercing rivet is driven, the member to be bonded is heated around the rivet driving point to prevent occurrence of defects such as cracks due to rivet driving even when the member to be bonded is a low ductility material. I can do it.
However, the bonding method of Patent Document 1 requires a heating device for heating the member to be bonded, and also has a drawback that it takes a long time to bond since the member to be bonded is heated.

Patent Document 2 discloses a method for fastening a thin plate material by a self-piercing rivet. In the method of Patent Document 2, a through-hole is formed in at least one fastening portion of the members to be joined or partially thinned to facilitate fastening with rivets.
However, in the fastening method of Patent Document 2, it is necessary to process the member to be fastened in advance, such as making a through hole in the joint portion of the member to be fastened, or thinly processing the countersunk hole. Met. In Patent Document 2, the advantage of the self-drilling rivet that the hole to be fastened does not have to be made in advance is impaired.

In this way, the self-piercing rivet can easily join the member to be fastened without drilling the member to be fastened, but the member to be fastened made by die-casting or the like is not self-piercing. When fastening with a mold rivet, cracks often occur in the fastening coupling member on the die side.
In the joining method using the self-piercing rivets of Patent Documents 1 and 2, it is difficult for the member to be fastened to crack, but additional work is required for fastening, and the fastening work that is originally simple is complicated. There is.
For this reason, there has been a demand for a self-piercing rivet fastening device and a fastening method for preventing cracks in the fastened member when fastening a fastened member having poor spreadability with a self-piercing rivet.

JP 2006-7266 A Japanese National Patent Publication No. 10-501744

  An object of the present invention is to provide a die for a self-piercing rivet fastening device that can fasten a fastened member having poor spreadability easily so as not to be cracked by a self-piercing rivet.

  To achieve this goal, the inventors have reduced the cavity depth of the die and formed the cavity surface with a single R concave spherical surface, or formed the cavity with a bottom surface and a surrounding ramp. It has been found that by forming the angle between the inclined surface and the bottom surface at 165 ° to 173 °, deformation of the fastened member can be suppressed to the minimum, and the fastened member becomes difficult to break.

In one aspect of the present invention, a self-piercing rivet having a large-diameter head and a hollow leg hanging from the head is fastened on a die by a punch of a self-piercing rivet fastening device. A self-piercing rivet die configured to be driven into a member,
A cavity is formed on the upper surface of the die to receive a portion of the fastened member projected by the self-piercing rivet driven by the punch, and the cavity has a center on a central axis of the cavity. A die formed of a single radius spherical surface.

  If the cavity is formed with a spherical surface with a single radius, the bottom surface of the cavity will continue smoothly from the center to the periphery, so that when the member to be fastened is received by the cavity, the member to be fastened will be bent at a steep angle. The deformation of the fastened member can be kept small. As a result, the occurrence of cracks in the fastened member is reduced.

According to another aspect of the present invention, a self-piercing rivet having a large-diameter head and hollow legs hanging from the head is placed on a die placed on a die by a punch of a self-piercing rivet fastening device. A self-drilling rivet die configured to be driven into a fastening member,
A cavity is formed on the upper surface of the die to receive a portion of the fastened member protruded by the self-piercing rivet driven by the punch, and the cavity has a circular bottom surface at the center of the cavity. And a slope formed between the bottom surface and the top surface of the die, and the slope of the slope from the top surface of the die is 7 to 15 degrees.

  If the cavity is formed with a circular bottom surface and a slope with a gentle slope around it, the bottom surface and the slope continue with a gentle obtuse angle, so that when the member to be fastened is received by the cavity, the fastened member suddenly It is not bent at an angle, and the deformation of the fastened member can be kept small.

When the self-piercing rivet is driven into the member to be fastened, the leg portion penetrates the member to be fastened on the punch side, and the front end of the leg portion faces down the member to be fastened adjacent to the die. And the die receives the fastened member on the receiving side and deforms the tip of the leg so as to expand radially outward, and the tip of the leg is fastened on the receiving side adjacent to the die. It is preferable that the plurality of members to be fastened are fastened to each other by the leg portion and the head portion which remain in the hole without passing through the head and have an enlarged diameter.
The die acts to open the leg portion of the self-piercing rivet and deform the leg end so as to remain in the receiving member to be fastened.

The upper surface of the die and the upper end portion of the cavity are preferably continuous at an obtuse angle.
Deformation of the portion of the fastened member that hits the boundary between the upper surface of the die and the upper end portion of the cavity can be minimized.

The depth from the upper surface of the die to the center of the cavity is preferably 0.5 to 1.5 mm, and more preferably 0.5 to 0.9 mm.
If the cavity is shallower than 0.5 mm, the self-piercing rivet cannot be sufficiently driven into the fastened member, and it is difficult to fasten the fastened member. When the cavity becomes deeper than 1.5 mm, the deformation of the fastened member increases.

  The diameter of the cavity on the upper surface of the die is preferably 10 to 18 mm, and more preferably 11 to 15 mm. If the diameter of the cavity is smaller than 10 mm, it is difficult to increase the diameter of the legs of the self-drilling rivet within the cavity. When the diameter of the cavity is larger than 18 mm, it is not possible to support the portion around the portion into which the leg portion of the self-piercing rivet is driven, and it is difficult to fasten with the self-piercing rivet.

According to the present invention, the deformation of the fastened member can be reduced and the crack of the fastened member having poor spreadability can be reduced as compared with the case where the self-piercing rivet is fastened using the conventional die.
As a result, it is possible to provide a die for a self-piercing rivet fastening device that can easily fasten a fastened member having poor spreadability by the self-piercing rivet.

It is an enlarged view of the part which fastens the self-drilling type rivet of the conventional self-drilling type rivet fastening device. 1 is a cross-sectional view of a die according to a first embodiment of the present invention. FIG. 3 is an enlarged cross-sectional view of a chain line portion A in FIG. 2. It is an expanded sectional view before fastening a to-be-fastened member by a self-drilling type rivet using the conventional die. It is an expanded sectional view after fastening a to-be-fastened member by a self-drilling type rivet using the conventional die. It is an expanded sectional view before fastening a to-be-fastened member by a self-piercing type rivet using the die of a 1st embodiment of the present invention. It is an expanded sectional view after fastening a member to be fastened with a self-drilling type rivet using a die of a 1st embodiment of the present invention. It is the figure which plotted the relationship of the diameter (phi) of the cavity of the die | dye of the 1st Embodiment of this invention, the depth D, and the radius R of a spherical surface. It is an expanded sectional view of the chain line part A of Drawing 2 of the die of a 2nd embodiment of the present invention. (a) is the schematic of the apparatus used for the measurement of a load and a displacement amount, (b) is a graph which shows the relationship between a load and a displacement amount. (a) is sectional drawing of the cavity part of the conventional die used for experiment, (b) is sectional drawing of the cavity part of the die | dye of the 1st Embodiment of this invention.

  Hereinafter, a first embodiment of a self-piercing rivet die according to the present invention will be described with reference to FIGS. The self-piercing rivet fastening device uses the die 30 of the first embodiment of the present invention instead of the conventional die 20. The self-piercing rivet 10 is driven by the punch 4 into the fastened members 41 and 42 arranged on the die 30. The other points are the same as those of the conventional self-piercing rivet fastening device shown in FIG.

FIG. 2 is a sectional view of the die 30 used in the self-piercing rivet fastening device 1 according to the first embodiment of the present invention. The die 30 is symmetrical around the central axis, and has a cylindrical base portion 33 and a processed portion 34 having a cylindrical shape with an outer diameter larger than that of the base portion 33 thereon. The upper part of the processing part 34 is made of a hard material such as high-speed tool steel so as to deform the leg part 12 of the rivet 10. A cavity 31 for deforming the leg portion 12 of the self-piercing rivet 10 is formed on the upper surface of the processing portion.
The material of the self-piercing rivet 10 is a material that can be easily bent, such as boron steel or chromium molybdenum steel, and the outer diameter of the leg portion 12 is 3 to 5.5 mm.

FIG. 3 is an enlarged cross-sectional view of a chain line portion A of the die 30 of FIG. The cavity 31 is symmetric with respect to the central axis l, and is formed of a concave spherical surface having a single radius R and having a center on the central axis l. The diameter φ of the cavity 31 on the upper surface of the processed part 34 is 10 to 18 mm, and the depth D from the upper surface of the processed part 34 to the lowest point of the bottom surface of the cavity 31 is 0.5 to 1.5 mm. The radius R of the spherical surface is 9 to 82 mm.
If the cavity 31 is relatively shallow, deformation of the fastened members 41 and 42 can be suppressed to a small level, and as a result, cracking of the fastened members is less likely to occur.

  The diameter of the cavity 31 of the die 30 is preferably 7 mm or more larger than the outer diameter of the legs 12 of the self-piercing rivet. When the outer diameter of the leg 12 of the self-piercing rivet 10 is 3 mm and the diameter of the cavity 31 is smaller than 10 mm, it is difficult to receive the self-piercing rivet 10 in the cavity and expand the diameter of the leg 12. If the diameter of the cavity is larger than 18 mm, it is difficult to fasten the fastened members 41 and 42 around the portion into which the self-piercing rivet 10 is driven, so that fastening is difficult.

With reference to FIGS. 4A and 4B, a description will be given of a case in which fastened members 41 and 42 are fastened by a self-drilling rivet 10 using a die 20 having a cavity 21 with a flat bottom surface 22 according to the related art. 5A and 5B, a comparison is made on the case where the fastened members 41 and 42 are fastened by using the die 30 having the cavity 31 formed of the spherical surface having a single radius R according to the first embodiment of the present invention. To explain.
FIG. 4A is an enlarged cross-sectional view of the self-drilling rivet fastening device before fastening the fastened members 41 and 42 with the self-drilling rivet 10 using the conventional die 20. The die 20 has a cavity 21 on the upper surface. The bottom surface 22 of the cavity 21 is substantially flat and the side surface 23 is substantially cylindrical. The self-piercing rivet 10 has a large-diameter head 11 and hollow cylindrical legs 12 depending from the head 11. The members to be fastened 41 and 42 are arranged on the upper surface of the die 20, and the periphery of the fastening part is pressed by a nose (not shown), and the leg part tip 13 of the self-piercing rivet 10 contacts the fastening part.

  FIG. 4B is an enlarged cross-sectional view after the fastened members 41 and 42 are fastened by the self-piercing rivet 10 using the die 20. When the self-piercing rivet 10 is driven by the punch 4, the leg portion 12 penetrates the punch-side fastened member 41 by the punch 4 and plastically deforms the receiving-side fastened member 42. The fastened member 42 is protruded downward by the leg 12, and the protruding portion of the fastened member 42 is received in the cavity 21, and a part of the lower surface of the fastened member 42 hits the bottom surface 22 of the cavity 21. Since the fastened member 42 cannot move any further downward, the leg portion 12 of the self-piercing rivet 10 is deformed so as to expand radially outward while pushing the fastened member 42 radially outward. The leg tip 13 remains in the receiving member 42 adjacent to the die 20 without penetrating it. The fastened members 41 and 42 are connected to each other by the leg 12 and the head 11 developed in the fastened member 42.

  Since the curvature of the boundary between the bottom surface 22 and the side surface 23 of the die 20 changes greatly, the portion indicated by 42a of the fastened member 42 has a large amount of deformation, so that cracking is likely to occur. Further, the boundary portion between the upper surface of the die 20 and the side surface 23 of the cavity 21 is substantially perpendicular, and it is considered that cracks are likely to occur also in the portion 42b of the fastened member 42 that hits here.

Next, with reference to FIGS. 5A and 5B, a case where the fastened members 41 and 42 are fastened by the self-piercing rivet 10 using the die 30 according to the first embodiment of the present invention will be described.
FIG. 5A is an enlarged sectional view before the fastened members 41 and 42 are fastened by the self-piercing rivet 10 using the die 30 according to the first embodiment of the present invention. The die 30 has a cavity 31 formed of a concave spherical surface having a single radius R. The fastened members 41 and 42 are set on the die 30, the outer periphery of the fastened part is pressed with a nose (not shown), and the leg tip 13 of the self-piercing rivet 10 is brought into contact with the surface of the fastened member 41.

  FIG. 5B is an enlarged cross-sectional view after the fastened members 41 and 42 are fastened by the self-piercing rivet 10 using the die 30. When the self-piercing rivet 10 is driven by the punch 4, the leg portion 12 penetrates the punch-side fastened member 41 by the punch 4 and plastically deforms the receiving-side fastened member 42. The fastened member 42 is protruded downward by the legs 12, and the protruding portion of the fastened member 42 is received in the cavity 31 of the die 30, and a part of the lower surface of the fastened member 42 is the bottom surface 32 of the cavity 31. It hits. Since the fastened member 42 cannot move any further downward, the leg portion 12 of the self-piercing rivet 10 is deformed so as to expand radially outward while pushing the fastened member 42 radially outward. The leg tip 13 remains in the receiving member 42 adjacent to the die 30 without penetrating it. The fastened members 41 and 42 are connected to each other by the leg 12 and the head 11 developed in the fastened member 42.

  In the die 30 according to the first embodiment of the present invention, since the bottom surface 32 of the cavity 31 is a single R spherical surface, there is no boundary between the bottom surface 32 and the side surface of the cavity 31, and the die 30 is smoothly continuous. Further, the cavity 31 is shallow at the portion where the leg portion 12 of the rivet 10 bites into the fastened member 42. Therefore, the portion indicated by 42a ′ of the fastened member 42 is not abruptly bent and the amount of deformation is small. Therefore, it is difficult for cracks to occur. The boundary between the upper surface of the die 30 and the cavity 31 is an obtuse angle. For this reason, the portion 42b ′ of the fastened member 42 that hits the boundary between the upper surface of the die 30 and the cavity 31 is less likely to crack.

In order to obtain the optimum shape of the cavity 31 of the die 30 according to the first embodiment of the present invention, the radius R of the spherical surface is determined by the diameter φ and the depth D of the cavity 31 of the single R cavity 31 of the die 30. I examined whether it would change.
The radius R of the spherical surface of the cavity 31 was calculated when the diameter φ of the cavity 31 was changed at intervals of 1 mm in the range of 10 to 18 mm and the depth D was changed at intervals of 0.1 mm in the range of 0.5 to 1.5 mm.
FIG. 6 is a graph plotting the calculation results of the radius R when the diameter φ is changed in the range of 10 to 18 mm and the depth D in the range of 0.5 to 1.5 mm. The radius R increases as the diameter Φ increases, and the radius R increases as the depth D increases.

  Next, the die | dye 35 of the 2nd Embodiment of this invention is demonstrated. FIG. 7 shows the die 35 of the second embodiment, and is an enlarged cross-sectional view of the chain line portion A of the processed portion 34 ′ of the die 35 of FIG. The cavity 36 is axisymmetric with respect to the central axis l ′, and is formed by a circular bottom surface 37 parallel to the top surface of the die 35 and the inclined surface 38 around the bottom surface 37, with the center axis l ′ as the center. . The slope 38 is frustoconical. The outer diameter φ of the inclined surface 38 of the cavity 36 on the upper surface of the die 35 is 11 to 15 mm, and the depth D from the upper surface of the die 35 to the bottom surface 37 of the cavity 31 is 0.5 to 0.9 mm. The inclination θ of the slope 38 from the upper surface of the die 35 is 7 to 15 ° (the angle between the bottom surface 37 and the slope 38 is 165 to 173 °).

  In the die 35 according to the second embodiment of the present invention, the bottom surface 37 of the cavity 36 is a circular plane and is connected to the surrounding inclined surface 38 at an obtuse angle. Therefore, the portion of the fastened member 42 that is in contact with the vicinity of the boundary between the bottom surface 37 and the slope 38 is not abruptly bent and the amount of deformation is small. Therefore, it is difficult for cracks to occur. The boundary between the upper surface of the die 35 and the slope 38 is also obtuse. Therefore, cracks are unlikely to occur in the portion of the fastened member 42 that hits the boundary between the upper surface of the die 35 and the slope 38.

(Measurement of displacement when the fastened member breaks)
A test was performed to determine the amount of displacement when the receiving-side fastened member 42 is cracked by the diameter φ of the cavity 31 of the die 30 of the first embodiment of the present invention. The material of the rivet 10 was boron steel, and the outer diameter of the leg 12 was 3.35 mm.
A 0.65 mm thick mild steel plate (SCGA270-45) was prepared as the clamped member 41 on the punch side, and an unheat-treated aluminum die cast material with a thickness of 2.4 mm was prepared as the clamped member 42 on the receiving side.
FIG. 8A is a schematic diagram illustrating a method for measuring a load and a displacement amount. As shown in FIG. 8 (a), the fastened members 41 and 42 are placed on the die 20 and the rivet 10 is placed at a position on the cavity 21 of the conventional die 20. Position punch 4 on rivet 10. In this test, a conventional die 20 having a flat bottom surface was used, and the depth of the die was set sufficiently deeper than the depth at which the fastened member 42 was broken so that the amount of displacement when the die was broken could be measured. The punch 4 was lowered at a speed of 1 mm / min, and the load applied to the punch 4 at that time and the displacement amount of the punch 4 were measured.

  FIG. 8B is a graph showing the relationship between load and displacement. When the load decreases as indicated by point B, it is estimated that a crack has occurred in the fastened member 42, and the pressing of the punch is stopped. The deformation amount of the receiving side fastening member 42 at that time was measured with a micrometer.

  Five types of dies having different diameters φ of the cavities 31 of the dies 30 were prepared, the number of samples n = 5, the above-described test was performed, and the amount of displacement of the fastened member 42 when a crack occurred was measured. The results are shown in Table 1. The displacement amount of the fastened member 42 represents an average value of five samples. The depth D of the cavity 31 of the die 30 is set to 0.6 mm, which is the same as the depth of the conventional die, because the shallower one can suppress the deformation of the fastened members 41 and 42.

Table 1

  From the test results, it can be seen that if the diameter φ of the cavity 31 of the die 30 is in the range of 7.0 to 8.0 mm, the amount of displacement until the fastened member 42 breaks does not change much.

(A crack test by fastening a conventional die and the die according to the first embodiment of the present invention)
Next, a die 20 having a cavity 21 consisting of a conventional cylindrical side surface 23 and a flat bottom surface 22, and a die having a cavity 31 in which the bottom surface 32 of the first embodiment of the present invention is formed of a single R. 30 was used to fasten the fastened member with the self-drilling rivet 10 and test whether the fastened members 41 and 42 would crack.

  9A is a cross-sectional view of the cavity 21 portion of the conventional die 20 used in the experiment, and FIG. 9B is a cross-sectional view of the cavity 31 portion of the die 30 of the first embodiment of the present invention. It is. The depths of the cavities 21 and 31 were both 0.60 mm. A soft steel plate (SCGA270-45, plate thickness 0.65 mm) was fastened as the punched member 41 and an aluminum die-cast material (plate thickness 2.4 mm) was fastened by the self-piercing rivet 10 as the fastened member 42 on the receiving side. The self-piercing rivet 10 was the same as that used in the test of Table 1 above.

  When the conventional bottom surface 22 shown in FIG. 9 (a) is fastened using a flat die 20, the receiving-side fastened member 42 is not cracked at the portion in contact with the bottom surface 22 of the die 20. Cracks occurred in the vicinity of the boundary between the bottom surface 22 and the side surface 23 of the die 20. Since the curvature of the boundary portion between the bottom surface 22 and the side surface 23 of the die 20 changes greatly, the amount of deformation of the fastened member 42 is large at this portion. Further, since the boundary portion between the upper surface of the die 20 and the side surface 23 of the cavity 21 is substantially perpendicular, it is considered that cracking is likely to occur because the fastened member 42 hits this portion.

  When the bottom surface 32 of the first embodiment of the present invention shown in FIG. 9 (b) is fastened by using a die 30 formed of a single R, no crack is generated in the receiving member 42 to be fastened. It was. Since the bottom surface 32 of the cavity 31 is a single R spherical surface, there is no boundary between the bottom surface and the side surface of the cavity 31. In addition, the cavity 31 is shallow at the periphery. For this reason, it is considered that the amount of deformation of the fastened member 42 is small and no cracks occurred. The boundary between the upper surface of the die 30 and the cavity 31 is an obtuse angle. Therefore, it is considered that cracks did not occur in the portion corresponding to the boundary between the upper surface of the die 30 and the cavity 31.

  When using a die according to an embodiment of the present invention to fasten a fastened aluminum die-cast fastening member with a self-piercing rivet, deformation of the fastened member can be reduced, and the fastened member is cracked. Is less likely to occur.

1 Self-piercing rivet fastening device
3 Nose
4 punch
10 Self-drilling rivets
11 head
12 legs
13 Leg tip
20 Conventional die
21 cavity
22 Bottom
23 Side
30 Die of the first embodiment of the present invention
31 cavity
32 Cavity bottom
33 Base
34 Machining part
35 Die of second embodiment of the present invention
36 cavity
37 Bottom
38 slopes
41 (Punch side) Fastened member
42 (Receiving side) Fastened member

Claims (5)

A self-piercing rivet having a large-diameter head and hollow legs hanging from the head is driven by a punch of a self-piercing rivet fastening device into a member to be fastened on a die. Self-drilling rivet die,
A cavity is formed on the upper surface of the die to receive a portion of the fastened member projected by the self-piercing rivet driven by the punch, and the cavity has a circular outer periphery at the center of the cavity. A die comprising: a bottom surface; and a slope formed between the bottom surface and the top surface of the die, wherein the slope of the slope from the top surface of the die is 7 to 15 °.   2. The die according to claim 1, wherein when the self-piercing rivet is driven into the fastened member, the leg portion penetrates the fastened member on the punch side, and a tip end of the leg portion is adjacent to the die. The member to be fastened on the side is pushed downward, and the die receives the member to be fastened on the receiving side, and deforms the distal end of the leg so as to expand radially outward. The plurality of members to be fastened are fastened to each other by the leg portions and the heads of the self-piercing rivets that are expanded in diameter without staying through the receiving member to be fastened adjacent thereto. Die to make. The die according to claim 1 or 2 , wherein the upper surface of the die and the upper end portion of the cavity are continuous at an obtuse angle. The die according to any one of claims 1 to 3 , wherein a depth from an upper surface of the die to a central portion of the cavity is 0.5 to 1.5 mm. The die according to any one of claims 1 to 4 , wherein the diameter of the cavity on the top surface of the die is 10 to 18 mm.

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