JPH06262282A - Bent pipe and its manufacture - Google Patents

Abstract

PURPOSE:To provide a bent pipe, in which the radius of curvature is the pipe diameter or smaller and the curvature is uniform and the roundness in the cross section of a passage is high, in a low cost and its manufacturing method. CONSTITUTION:By bending the pipe 2, a bent pipe intermediate body 2' provided with a first and a second straight pipe parts 2a, 2b and a bent part 2c is manufactured. Successively, by using a first punch 8 and a first core metal 9, sprue working is applied to the first straight pipe part 2a side to form a flange part 11, and also the half part of the bent part 2c at the first straight pipe part side is formed. Successively, by using a second punch 13 and a second core metal 14, bulging work is applied to the second straight pipe part 2b to form a bead part 15, and also the half part of the bent part 2c at the second straight pipe part side is formed into the bent pipe part 2d having the radius of curvature in the pipe diameter or smaller. In such a way, the bent pipe P, in which the radius of curvature is the pipe diameter or smaller and the curvature is almost uniform and the roundness in the cross section of the passage is high, is manufactured from the simple straight pipe 2 at a low cost.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a bent pipe provided with a bent pipe portion having a radius of curvature equal to or smaller than a pipe diameter, and a method for manufacturing the bent pipe.

[0002]

2. Description of the Related Art Generally, in a pipeline system for transporting a liquid or gas, a joint is arranged at a portion where the direction of the pipeline is changed. A bent pipe (elbow) in which a pipe portion is formed and a curved pipe portion is formed between both straight pipe portions is often used. Further, such a bent pipe is usually manufactured by bending a straight pipe. However, such bending cannot reduce the radius of curvature of the curved tube portion so much. This is because if the radius of curvature is made small, the bent pipe part cannot be made to have a uniform curvature due to bending when performing bending work, and the roundness of the passage cross section becomes extremely poor. .

However, in general, there are many cases in which the pipe must be bent sharply. For example, in a blow-by gas passage for guiding blow-by gas of an automobile engine to an intake system, one end of the blow-by gas is inserted downward into a cylinder head cover, while the other end is required to have a side opening. Since the bonnet is arranged just above the head cover, such a joint should turn 90 ° at a position as close as possible to the upper end surface of the cylinder head cover in order to avoid interference with the bonnet, that is, extremely rapidly 90 °. It is required to change direction. Therefore, when a bent pipe is used as such a joint, its radius of curvature is required to be extremely small, for example, the radius of curvature is required to be equal to or smaller than the pipe diameter. However, it is extremely difficult to manufacture a bent pipe having such a small radius of curvature by bending.

Therefore, when it is necessary to change the direction rapidly by 90 °, the following joint is generally used. (1) Two straight pipes, each of which is cut obliquely in a direction forming an angle of 45 ° with respect to the axis, are arranged so that their cut surfaces come into contact with each other, and the contact portions are brazed. (2) A hole in which a side wall of a straight pipe-shaped first pipe is formed, and a straight pipe-shaped second pipe is brazed to the side wall around the hole so as to be orthogonal to the first pipe. (3) A hollow connecting member having two side walls which are orthogonal to each other, with holes formed in both side walls, and straight pipes are brazed to the side walls around both holes so as to be orthogonal to the side walls.

[0005]

[Problems to be Solved by the Invention]
Each of the conventional joints shown in (1) to (3) has a problem that the manufacturing cost is high because a plurality of parts are required to manufacture one joint. Further, since each part has to be brazed, there is a problem that the brazing work is troublesome, which further increases the manufacturing cost. The present invention has been made to solve the above-mentioned conventional problems, and has a very small radius of curvature of a curved pipe portion, a uniform curvature of the curved pipe portion, and a high circularity of a passage cross section. It is an object of the present invention to provide a bent pipe at low cost or a manufacturing method thereof.

[0006]

In order to achieve the above object, the first aspect of the present invention is such that straight pipe portions are formed on both end sides, and a radius of curvature between both straight pipe portions is equal to or smaller than a pipe diameter. Provided is a bent pipe characterized in that a bent pipe portion is formed, and a flange portion for press-fitting the pipe is integrally formed at a predetermined position of one straight pipe portion.

According to a second aspect of the present invention, in the bent pipe according to the first aspect of the present invention, the collar portion is formed near a boundary portion between the straight pipe portion and the curved pipe portion where the collar portion is formed. A characteristic bent pipe is provided.

According to a third aspect of the present invention, in the bent pipe according to the second aspect, the curved pipe portion is bent such that the axis of one straight pipe portion and the axis of the other straight pipe portion are orthogonal to each other. To provide a bent pipe.

According to a fourth aspect of the present invention, straight pipe portions are formed on both end sides, curved pipe portions having a radius of curvature equal to or smaller than the pipe diameter are formed between the straight pipe portions, and the first straight pipe is formed. A method for manufacturing a bent pipe in which a flange portion for press-fitting the pipe is integrally formed at a predetermined position of the portion, (1) Corresponding to the outer peripheral surface of the bent pipe portion of the bent pipe to be finally obtained Prepare a mold having a cavity defined by the inner peripheral surface of the shape, and (2) bend the straight tubular metal pipe,
A bent pipe intermediate having a curved portion formed in the middle while leaving straight pipes at both ends and having a shape that can be housed in the cavity of the molding die is manufactured. 1 straight pipe part is projected to the outside of the mold by a predetermined length, and the tip part of the second straight pipe part is projected to the outside of the mold so as to be arranged in the cavity part of the mold, (4) The first straight pipe portion has a predetermined shape in the hollow portion of the bent pipe intermediate body.
By press-fitting the insertion member of, the bent pipe intermediate body is expanded radially outward, and the outer peripheral portion of the curved portion on the side of the first straight pipe portion is molded into a shape corresponding to the inner peripheral surface of the molding die. 5) First
A pressing member that fits with the first straight pipe portion is pressed against the tip portion of the straight pipe portion of the first straight pipe portion while leaving a predetermined gap between the mold and the molding die, thereby molding the first straight pipe portion and the pressing member. The collar portion is formed by bulging the portion located in the gap between the die and the outside in the radial direction, and (6) a predetermined shape is formed from the second straight pipe portion side to the hollow portion of the bent pipe intermediate body. The second insertion member having the same is press-fitted, and thereby the bent pipe intermediate body is radially outwardly expanded to form the outer peripheral portion of the curved portion on the second straight pipe portion side into a shape corresponding to the inner peripheral surface of the molding die. The present invention provides a method for producing a bent pipe, characterized in that the bent pipe is obtained.

[0010]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A method for manufacturing a bent pipe according to the present invention will be specifically described below with reference to FIGS. 1 (a) to 1 (g). (1) First step (bending) As shown in FIG. 1 (a), a straight metal pipe 2 having a hollow portion 1 and having a predetermined size is bent to form a first straight pipe portion 2a. A bent pipe intermediate body 2 ′ including the second straight pipe portion 2b and the curved portion 2c is manufactured. In this bending process, a force in the bending direction is applied to a substantially central portion of the pipe 2 in the axial direction, so that the pipe 2 is orthogonal to the axis of the first straight pipe portion 2a and the axis of the second straight pipe portion 2b. After being bent like this, the curved portion 2c is subjected to restrike (reworking),
However, it is preferably deformed into a shape that can be accommodated in the cavity 6 of the mold 5 described later. The first and second
The straight pipe portions 2a and 2b are the straight pipes 2 which are not deformed at the corresponding end portions and are left as straight pipes.

Here, since the curved portion 2c is in a sharply bent state, the inner curved surface 3 of the curved portion 2c is a pipe that is more curved than the inner curved surface of the curved pipe portion of the bending pipe to be finally obtained. It is dented toward the center side, and also the outer curved surface 4 of the curved portion 2c.
Also, the bent pipe portion of the bent pipe to be finally obtained is dented toward the pipe center side from the outer curved surface. In addition, the dimensions of the straight pipe 2 are, for example, a pipe length of 80 mm, an outer diameter of 17.3 mm,
The inner diameter is 14.5 mm and the wall thickness is 1.4 mm. Also, pipe 2
The material of is STKM11A, for example.

(2) Second Step (First Stage of Spool Processing) As shown in FIG. 1 (b), the bent pipe intermediate 2'is placed (clamped) in the cavity 6 of the forming die 5. Although not shown in detail, the molding die 5 is a split die including an upper die and a lower die.
(Split die), the bent pipe intermediate 2'is put in the cavity of the lower mold, and then the upper mold is fixed on the lower mold. The hollow portion 6 is defined by an inner peripheral surface having a shape corresponding to a predetermined portion (a large part of the second straight pipe portion and the curved pipe portion) of the outer peripheral surface of the bending pipe to be finally obtained. ing. Here, the radius of curvature of the hollow portion 6 (inner peripheral surface of the molding die) is set to be equal to or smaller than the outer diameter of the pipe 2. The molding die 5 has a shape such that almost all of the first straight pipe portion 2a projects out of the molding die 5 and the tip of the second straight pipe portion 2b projects slightly out of the molding die 5. Has been done.

Then, the first molding device 7 is arranged at a position facing the tip end surface of the first straight pipe portion 2a. The first forming device 7 is provided with a substantially cylindrical first punch 8 having a cylindrical space portion 8a and a first cored bar 9. The diameter of the cylindrical space portion 8a is such that the cylindrical space portion 8a is the first straight pipe portion 2a.
The depth of the cylindrical space portion 8a is set to be slightly shorter than the axial length of the first straight pipe portion 2a. On the other hand, the diameter of the first cored bar 9 is
The core metal 9 is preferably set so that it can be fitted in the hollow portion 1 of the first straight pipe portion 2a, and the shape of the tip of the first core metal 9 is
It has a shape corresponding to the inner peripheral surface of the portion on the first straight pipe portion side of the bent pipe portion of the bent pipe to be finally obtained. Here, the first punch 8 and the first cored bar 9 can be moved in the arrow X ₁ direction with a predetermined pressing force. The first punch 8 and the first cored bar 9 respectively correspond to the “pressing member” and the “first inserting member” described in the claims.

(3) Third step (second step of spool processing) As shown in FIG. 1 (c), the first punch 8 and the first cored bar 9 are directed toward the molding die side, that is, as shown in FIG. b) Move it in the direction of arrow X ₁ in the figure. Here, the cylindrical space portion 8a of the first punch 8 is fitted with the first straight pipe portion 2a, but a clearance portion (gap) having a predetermined length α is provided between the first punch 8 and the molding die 5. Is left. At this time, the first cored bar 9 is press-fitted into the hollow portion 1 of the first straight pipe portion 2a (bent pipe intermediate 2 ').

(4) Fourth Step (Third Stage of Spool Processing) As shown in FIG. 1 (d), the first punch 8 is further moved toward the molding die side with a strong pressing force to move the first punch. 8
Is brought into contact with the molding die 5. At this time, the first straight pipe portion 2a is compressed in the axial direction, and the portion of the first straight pipe portion 2a located in the clearance portion between the first punch 8 and the molding die 5 is radially outside. Bulge in the direction, which results in the collar 1
1 is formed. Here, notches are provided in the end surface of the molding die 5 and the end surface of the first punch 8 which face each other,
The flange portion 11 is formed by plastically flowing the wall portion of the first straight pipe portion 2a into this cutout portion. In addition, this collar part 1
Reference numeral 1 is provided to make the insertion depth of the first straight pipe portion 2a into another member (for example, a cylinder head cover) constant.

Further, at this time, the first cored bar 9 is pressed deeper into the hollow portion 1, and the tip end of the first cored bar 9 reaches almost the center of the curved portion 2c. At this time, the first of the bending portion 2c
The straight pipe portion-side half portion is expanded by the first cored bar 9, and the outer peripheral portion thereof is brought into pressure contact with the inner peripheral surface of the molding die 5 (cavity portion 6), and this portion is the bent pipe to be finally obtained. It has the same shape as the outer peripheral portion of the curved pipe section. Further, as described above, the tip end portion of the first cored bar 9 has a shape corresponding to the inner peripheral surface of the portion on the first straight pipe portion side of the bent pipe portion of the bent pipe to be finally obtained. Therefore, the inner peripheral portion of the half portion of the curved portion 2c on the side of the first straight pipe portion also has the same shape as the inner peripheral portion of the bent pipe portion of the bent pipe to be finally obtained. That is, the half portion (first straight pipe portion side half portion) of the curved pipe portion having the outer peripheral surface and the inner peripheral surface of a predetermined shape and thus having a high circularity in the passage cross section is completed.

(5) Fifth Step (First Stage of Bulge Processing) As shown in FIG. 1 (e), the second molding device 12 is arranged at a position facing the tip end surface of the second straight pipe portion 2b. . The second molding device 12 is provided with a substantially cylindrical second punch 13 having a circular dish-shaped recess 13 a and a second core bar 14. The diameter of the recess 13a of the second punch 13 is set to be slightly larger than the outer diameter of the second straight pipe portion 2b.
The depth of 3a is set to be slightly shorter than the protruding length of the portion of the second straight pipe portion 2b protruding from the molding die 5. On the other hand, the second
The diameter of the cored bar 14 is preferably set so that the second cored bar 14 can fit into the hollow portion 1 of the second straight pipe portion 2b, and the shape of the tip of the second cored bar 14 is finally obtained. The bent pipe has a shape corresponding to the inner peripheral surface of the portion on the second straight pipe portion side of the bent pipe portion. Here, the second punch 13 and the second cored bar 14 can be moved by a predetermined pressing force in the arrow X ₂ direction. The second core metal 14 corresponds to the "second insertion member" described in the claims.

(6) Sixth step (second step of bulging) As shown in FIG. 1 (f), the second punch 13 and the second core bar 14
And toward the mold side, that is, arrow X ₂ in FIG. 1 (e)
Move in the direction. At this time, the second core metal 14 is press-fitted into the hollow portion 1 of the second straight pipe portion 2b (bent pipe intermediate 2 ').

(7) Seventh Step (Third Stage of Bulge Processing) As shown in FIG. 1 (g), the second punch 13 is further moved toward the forming die side with a strong pressing force, and the second punch 13 is moved. Is brought into contact with the molding die 5. Here, the second punch 13
As a result, the portion of the second straight pipe portion 2b protruding from the molding die 5 is compressed, and the wall portion of this portion is filled in the recess 13a of the second punch 13 by plastic flow, whereby the tip of the second straight pipe portion 2b. A bead portion 15 is formed on the portion. The bead portion 15 is provided to prevent the hose from coming off when the hose is connected to the second straight pipe portion 2b.

At this time, the second cored bar 14 is pressed deeper into the hollow portion 1, and the tip of the second cored bar 14 reaches almost the center of the curved portion 2c. At this time, the half portion of the curved portion 2c on the second straight pipe portion side is expanded by the second core metal 14,
The outer peripheral portion is pressed against the inner peripheral surface of the molding die 5 (cavity portion 6), and this portion has the same shape as the outer peripheral portion of the bent pipe portion of the bending pipe to be finally obtained. Also, as mentioned above,
The tip portion of the second cored bar 14 has a shape corresponding to the inner peripheral surface of the portion of the bent pipe portion of the bending pipe to be finally obtained, which is on the second straight pipe portion side. The inner peripheral portion of the half portion on the second straight pipe portion side also has the same shape as the inner peripheral portion of the bent pipe portion of the bent pipe to be finally obtained. At this point, a curved tube portion 2d having a predetermined shape of the outer peripheral surface and the inner peripheral surface, having a small radius of curvature (equal to or less than the tube diameter) and having a high roundness of the passage cross section is completed. In this way, the first straight pipe portion 2a, the second straight pipe portion 2b, the curved pipe portion 2d, the collar portion 11 and the bead portion 15 are provided as shown in FIGS. 2 (a) to 2 (d). The bent pipe P is completed. A taper portion 16 is formed at the tip of the first straight pipe portion 2a by post-processing. In the production of the bent pipe P, the second step to the fourth step (spool processing),
Of course, the fifth to seventh steps (bulge processing) may be performed in parallel. By doing so, the manufacturing time can be shortened.

According to the method for manufacturing the bent pipe P,
Simply bending, spooling and bulging a single piece (straight tubular pipe), therefore at very low cost, the radius of curvature is less than or equal to the pipe diameter, the curvature is almost uniform and the passage It is possible to manufacture a bent pipe P having a high circularity in cross section.

The shape characteristics of the bent pipe P manufactured by the manufacturing method according to the present invention will be described below. Figure 3
An example of a bent pipe P actually manufactured from a straight metal pipe having an outer diameter of 17.3 mm, an inner diameter of 14.5 mm, a wall thickness of 1.4 mm and a material STKM11A is shown in FIG. As is clear from FIG. 3, in this bent pipe P, the bent pipe portion 2d is curved with a substantially uniform curvature. The radius of curvature R ₂ of the curved pipe portion 2d is 12 mm, that is, approximately 0.7 times the outer diameter (17.3 mm) of the pipe. In this way, since the radius of curvature of the curved pipe portion 2d is very small, the direction of the curved pipe P is changed extremely rapidly by 90 ° like the joint of the blow-by gas passage of the automobile engine. It can be used as a joint in situations where there is a need. Further, the radius R ₃ outer curved surface of the curved pipe portion 2d also is curved in a substantially uniform curvature and has a 18.4 mm. In this way, since the outer curved surface has a substantially uniform curvature, the curved pipe portion 2d
The roundness of the cross section of the passage becomes extremely high. Also, the curved pipe section 2
The inner curved surface of d is extremely sharply curved, its radius R ₁ (not measured) is very small, and it can be considered that the inner curved surface is bent substantially at a right angle.

The wall thickness of the bent pipe P at the positions t ₁ to t ₁₂ is as follows. t ₁ : 1.18mm t ₇ : 1.02mm t ₂ : 1.12mm t ₈ : 1.06mm t ₃ : 1.10mm t ₉ : 1.03mm t ₄ : 0.87mm t ₁₀ : 1.22mm t ₅ : 0.92mm t ₁₁ : 1.21mm t ₆ : 0.90mm t ₁₂ : 1.31mm As described above, sufficient thickness can be obtained at any part of the bent pipe portion 2d and the second straight pipe portion 2b of the bent pipe P. Since it is secured,
Sufficient strength of the bent pipe P is secured.

Bending pipe P is shown in FIGS. 4, 5 and 6, respectively.
A of the first straight pipe section 2a of one prototype (No. 1 work) of
The radial cross section of the passage cross section at the B and C positions should be 1000
The cross-sectional shape characteristic obtained by doubling is shown. Further, in FIGS. 7, 8 and 9, respectively, in the radial direction of the passage cross section at the A, B and C positions of the first straight pipe portion 2a of the other prototype (No. 2 work) of the bent pipe P, respectively. The cross-sectional shape characteristic obtained by enlarging the unevenness 1000 times is shown. The circularity of the cross-sectional shape shown in FIGS. 4 to 9 is as follows. (No.1 work) (No.2 work) Figure 4: 12.4 μm Figure 7: 11.0 μm Figure 5: 12.7 μm Figure 8: 12.8 μm Figure 6: 12.2 μm Figure 9: 14.8 μm In addition, in the first straight pipe portion 2a of the bent pipe P, sufficient roundness is ensured.

As described above, according to the present invention, the radius of curvature of the curved pipe portion 2d is equal to or smaller than the pipe diameter, the curvature of the curved pipe portion 2d is substantially uniform, and the bent pipe P having a high circularity in the passage cross section. Can be obtained at low cost.

[0026]

According to the first aspect of the present invention, since the radius of curvature of the bent pipe portion of the bent pipe is equal to or smaller than the pipe diameter, the bent pipe is inexpensive even in a situation where the pipe line needs to be bent extremely rapidly. Can be used, and the cost of the pipeline can be reduced.

According to the second invention, basically, the same operation and effect as those of the first invention can be obtained. Further, since the curved pipe portion is close to the flange portion, when the straight pipe portion is inserted into another member up to the flange portion, the curved pipe portion is curved at a position very close to the surface of the member. Therefore, even when another member is arranged close to the member, the other member and the bent pipe do not interfere with each other.

According to the third invention, basically, the same operation and effect as those of the second invention can be obtained. Furthermore, since the straight pipe portion on one side and the straight pipe portion on the other side are orthogonal to each other, it is possible to easily sharply bend the pipeline by 90 ° by using the bent pipe.

According to the fourth aspect of the invention, a single straight pipe is bent to manufacture a bent pipe intermediate, and the bent pipe intermediate is then subjected to the first and second insertion members and the pressing member. By simply performing simple processing (spool processing, bulging processing) using and, at a very low cost, the radius of curvature of the curved pipe section is less than or equal to the pipe diameter, and the curvature of the curved pipe section is almost uniform. In addition, it is possible to manufacture a bent pipe having a high circularity in the passage cross section.

[Brief description of drawings]

1 (a) to 1 (g) are views showing a manufacturing process of a bent pipe according to the present invention, and show first to seventh processes, respectively.

2 (a) to (d) are respectively a top plan view, an elevation view, a side view and a bottom plan view of a bent pipe according to the present invention.

FIG. 3 is an elevational view of a bent pipe according to the present invention.

FIG. 4 One prototype of a bent pipe according to the present invention
It is a figure which shows the cross-sectional shape characteristic obtained by enlarging 1000 times the radial concavity and convexity of the passage cross section in the A position in FIG. 3 of (No. 1 work).

5 is a diagram showing cross-sectional shape characteristics obtained by enlarging the radial unevenness of the passage cross-section at position B in FIG. 3 of the No. 1 workpiece by 1000 times.

6 is a view showing a cross-sectional shape characteristic obtained by enlarging the radial unevenness of the passage cross-section at position C in FIG. 3 of the No. 1 workpiece by 1000 times.

7 is a cross-sectional shape characteristic obtained by enlarging the radial unevenness of the passage cross-section at position A in FIG. 3 of another prototype of the bent pipe according to the present invention (No. 2 work) by 1000 times. FIG.

8 is a diagram showing cross-sectional shape characteristics obtained by enlarging the radial unevenness of the passage cross section at position B in FIG. 3 of the No. 2 workpiece by 1000 times.

9 is a diagram showing a cross-sectional shape characteristic obtained by enlarging the radial unevenness of the passage cross section at position C in FIG.

[Explanation of symbols]

 P ... Bending pipe 1 ... Hollow part 2 ... Pipe 2 '... Bending pipe intermediate body 2a ... 1st straight pipe part 2b ... 2nd straight pipe part 2c ... Curved part 2d ... Bending pipe part 5 ... Mold 6 ... Cavity part 8 ... 1st punch 9 ... 1st core metal 11 ... Collar part 14 ... 2nd core metal

Claims (4)

[Claims] 1. A straight pipe portion is formed on each of both end sides, a curved pipe portion having a radius of curvature equal to or smaller than a pipe diameter is formed between the both straight pipe portions, and a predetermined position of one straight pipe portion. A bent pipe characterized in that a flange portion for press-fitting the pipe is integrally formed on the. 2. The bent pipe according to claim 1, wherein the collar portion is formed near a boundary portion between the straight pipe portion and the curved pipe portion where the collar portion is formed. Bent pipe. 3. The bending pipe according to claim 2, wherein the bending pipe portion is bent so that an axis line of one straight pipe portion and an axis line of the other straight pipe portion are orthogonal to each other. Bending pipe to do. 4. A straight pipe portion is formed on each of both end sides, a curved pipe portion having a radius of curvature equal to or smaller than a pipe diameter is formed between the both straight pipe portions, and a predetermined straight pipe portion is formed. A method for manufacturing a bent pipe in which a flange portion for press-fitting the pipe is integrally formed at a position, the inner peripheral surface having a shape corresponding to the outer peripheral surface of the bent pipe portion of the bent pipe to be finally obtained. Prepare a mold with a cavity defined by, and bend a straight metal pipe to form a curved part in the middle part while leaving a straight pipe part at both ends, and molding A bent pipe intermediate body having a shape that can be accommodated in the cavity of the mold is manufactured, and the bent pipe intermediate body has a first straight pipe portion protruding out of the molding die by a predetermined length and a second straight pipe portion. The first straight pipe is arranged so that the tip of the From the side, the first insertion member having a predetermined shape is press-fitted into the hollow portion of the bent pipe intermediate body, and thereby the bent pipe intermediate body is radially outwardly expanded so that the first straight pipe portion side of the curved portion is provided. The outer peripheral portion is molded into a shape corresponding to the inner peripheral surface of the molding die, and a pressing force is fitted to the first straight pipe portion at the tip portion of the first straight pipe portion while leaving a predetermined gap between the die and the molding die. The member is pressed, whereby the portion of the first straight pipe portion located in the gap between the pressing member and the molding die is bulged radially outward to form the collar portion, and the second straight pipe portion is formed. The second straight member of the curved portion by press-fitting a second insertion member having a predetermined shape into the hollow portion of the bent pipe intermediate body from the side of the bent portion to expand the bent pipe intermediate body radially outward. The bending part is characterized in that a bent pipe is obtained by molding the outer peripheral part of the mold into a shape corresponding to the inner peripheral surface of the mold. Method for producing a flop.