JP7054295B2 - Press molding method for tubular parts and O-bending die used for this - Google Patents

Description

The present invention relates to a method for press-molding a tubular part, and more particularly to a method for press-molding a tubular part having an irregular cross section.

As a method of forming a tubular part having a circular cross section, a metal plate is press-molded with a U-bending die to form an intermediate molded product having a U-shaped cross section, and then this intermediate molded product is press-molded with an O-bending die. A so-called UO bending molding method is known in which the ends of an intermediate molded product are butted against each other to form a tubular shape (see, for example, Patent Document 1 below).

Japanese Unexamined Patent Publication No. 2014-4626

However, it is very difficult to form a tubular part having a deformed (non-round) cross section by the conventional UO bending molding method as described above. For example, FIG. 9 shows an O-bending process when forming a tubular part having a substantially quadrangular cross section by the UO bending molding method. In this step, the intermediate molded product 201 having a U-shaped cross section is set on the lower mold 220 of the O bending die {see FIG. 9 (A)}, and the upper mold 210 is lowered to form the molding surface 210a of the upper mold 210. The end portions 201b of the pair of vertical wall portions 201a are pushed downward, and the tubular part 101 is formed while bending the pair of vertical wall portions 201a inward {see FIGS. 9 (B) to 9 (E)}.

However, depending on the shape of the product and the like, as shown in FIG. 9C, the end portion 201b of the pair of vertical wall portions 201a of the intermediate molded product 201 and a region in the vicinity thereof (hereinafter referred to as “end region”). ) P'may become parallel to each other at an early stage. When the upper mold 210 is further lowered from this state, as shown in FIG. 9D, the pair of vertical wall portions 201a of the intermediate molded product 201 is further bent, and the end region P'of the vertical wall portion 201a is formed. May tilt downward. In this case, as shown in FIG. 9E, the butt portion 101a of the tubular part 101 may be dented, which may hinder the subsequent welding work of the butt portion 101a.

From the above circumstances, it is an object of the present invention to prevent the butt portion of the tubular part from being dented when forming a tubular part having a deformed cross section by the UO bending molding method, and to perform the subsequent welding work without any trouble. And.

In order to solve the above problems, the present invention has a U-bending step of forming an intermediate molded product having a bottom portion and a pair of vertical wall portions rising from both ends of the bottom portion, and a bottom portion of the intermediate molded product on one mold. Is set, the other mold is brought close to the one mold, and the ends of the pair of vertical wall portions are held together while being bent by pressing the pair of vertical wall portions with the molding surface of the other mold. A method of press-molding a tubular part having an O-bending step of butt-molding into a tubular shape, which is provided on the molding surface of the other mold in a region facing the butt portion of the tubular part in a state at the time of completion of molding. Provided is a press forming method in which a relief portion is provided on the side separated from the tubular part.

Further, in order to solve the above-mentioned problems, the present invention is an O-bending die for pressing an intermediate molded product having a bottom portion and a pair of vertical wall portions rising from both ends of the bottom portion to form a tubular shape. One mold having a recess for setting the bottom of the intermediate molded product, and the other having a molding surface that can be approached and separated from the one mold and that presses and bends the vertical wall portion of the intermediate molded product. On the molding surface of the other mold, a relief portion is provided on the molding surface of the other mold in a region facing the butt portion of the tubular part and retracted to the side away from the tubular part. O bending mold is provided.

In the O-bending process, the intermediate molded product is molded along the molding surface of the mold, so the molding surface of the mold is usually the same as the shape of the tubular part that is the finished product, and when the molding is completed, the mold When the entire area of the molded surface is in close contact with the tubular part, the shape of the molded surface is transferred to the tubular part. On the other hand, in the present invention, the O-bending die's molding surface (specifically, the region facing the butt portion of the tubular part at the completion of molding) dares to retreat to the side away from the finished tubular part. A part was provided. In this case, when the end portion of the vertical wall portion of the intermediate molded product is pushed in by the molding surface of the mold, the amount of pushing the end portion of the intermediate molded product is reduced by the amount of the relief portion provided on the molding surface. As a result, the timing of bending the vertical wall portion of the intermediate molded product can be delayed, so that the situation where the end regions of the pair of vertical wall portions become parallel at an early stage is avoided, and the situation where the butt portion of the tubular part is dented is prevented. can do.

As described above, according to the present invention, when molding a tubular part having a deformed cross section, it is possible to prevent the butt portion of the tubular part from being dented, so that the subsequent welding work can be performed without any trouble.

(A) is a perspective view of a tubular part, (B) is a plan view, and (C) is a side view. (A) to (D) are sectional views showing the procedure of the molding method of a tubular part. (A) to (D) are sectional views showing an intermediate bending process. (A) to (E) are cross-sectional views showing an O bending process, and show the shape change of the intermediate molded product in the XX cross section of FIG. 1 (B). (A) to (E) are cross-sectional views showing an O bending process, and show the shape change of the intermediate molded article in the YY cross section of FIG. 1 (B). (A) to (E) are cross-sectional views showing the O bending process, and show the shape change of the intermediate molded article in the ZZ cross section of FIG. 1 (B). It is a bottom view of the upper die of an O bending die. (A) to (E) are cross-sectional views showing an O bending process according to another embodiment, and show the shape change of the intermediate molded product in the ZZ cross section of FIG. 1 (B). (A) to (E) are sectional views showing an O bending process which concerns on a comparative example.

Hereinafter, embodiments of the present invention will be described with reference to the drawings.

In this embodiment, a method of molding the tubular part 1 shown in FIG. 1 is shown. The tubular part 1 is formed into a tubular shape by abutting the ends of flat plate-shaped blank materials. In the following description, for convenience, the side where the butt portion 1a of the tubular component 1 is located is upward, the opposite side thereof is downward, and the longitudinal direction and the direction orthogonal to the vertical direction of the tubular component 1 are the width direction.

The tubular part 1 has a modified cross section (non-circular cross section). As shown in FIG. 1A, the tubular component 1 of the present embodiment has a substantially rectangular cross section, and the cross-sectional shape changes in the longitudinal direction. Specifically, the opening on one end side of the tubular part 1 forms a substantially circular shape in which the vertical dimension and the width direction dimension are substantially equal to each other, and the opening on the other end side of the tubular part 1 is larger than the vertical dimension. It has a flat shape with large dimensions in the width direction. The intermediate portion in the longitudinal direction of the tubular component 1 is smoothly continuous while gradually changing the cross-sectional shape from one end to the other end. The tubular component 1 has a bottom portion 1b, a pair of side portions 1c rising upward from both ends in the width direction of the bottom portion 1b, and a pair of upper portions 1d extending inward in the width direction from the upper ends of the pair of side portions 1c. The ends of the pair of upper parts 1d are butted against each other to form the butted portion 1a. A bent portion (hereinafter referred to as “bottom bent portion 1e”) is provided at the boundary between the bottom portion 1b and each side portion 1c, and a bent portion (hereinafter referred to as “bending portion”) is provided at the boundary between each side portion 1c and each upper portion 1d. Shoulder bending portion 1f ") is provided. The tubular component 1 of the present embodiment is curved in the longitudinal direction, and specifically, it is curved in both the plan view shown in FIG. 1 (B) and the side view shown in FIG. 1 (C).

As shown in FIG. 2, the tubular part 1 is manufactured through a U-bending step, an intermediate bending step, an O-bending step, and a welding step. Note that FIG. 2 shows only the shape change in the vicinity of the opening on the other end side of the tubular component 1 having a flat shape.

In the U-bending step, the blank material 2 shown in FIG. 2 (A) is pressed with a U-bending die (not shown) to obtain an intermediate molded product 3 having a substantially U-shaped cross section shown in FIG. 2 (B). The intermediate molded product 3 has a bottom portion 11 and a pair of vertical wall portions 12 rising upward from both ends in the width direction of the bottom portion 11. A bottom bent portion 13 is provided at the boundary between the bottom portion 11 and each vertical wall portion 12. The shape of the lower portion (bottom portion 11 and bottom bent portion 13) of the intermediate molded product 3 is substantially the same as the shape of the lower portion (bottom portion 1b and bottom bent portion 1e) of the tubular part 1 of the finished product. That is, the bottom portion 11 and the bottom bent portion 13 of the intermediate molded product 3 have a cross-sectional shape that changes in the longitudinal direction, and are curved in both the plan view and the side view.

In the intermediate bending step, the intermediate molded product 3 is pressed to form the shoulder bent portion 12a on the vertical wall portion 12 of the intermediate molded product 3 {see FIG. 2 (C)}. As shown in FIG. 3, the intermediate bending die 20 for performing an intermediate bending step on the intermediate molded product 3 is composed of an upper mold 21 and a lower mold 22 that can approach and separate from each other. In the present embodiment, the lower mold 22 is a fixed type, and the upper mold 21 is a movable type that is raised and lowered by a driving means (not shown). Note that FIG. 3 shows only the shape change of the portion of the intermediate molded product 3 corresponding to the vicinity of the opening on the other end side having a flat shape.

The upper die 21 is provided with a pair of inclined surfaces 21a inclined in a direction toward each other as they go upward, a pair of locking surfaces 21b extending downward from the upper end of each inclined surface 21a, and a pair of locking surfaces 21b below each inclined surface 21a. It has a pair of shoulder bent portion molding surfaces 21c. The pair of locking surfaces 21b are provided on the locking portion 21d provided integrally or separately from the main body portion 21e of the upper die 21. In the illustrated example, the main body portion 21e and the locking portion 21d are provided separately. The shoulder bent portion molded surface 21c is smoothly continuous with the lower end of the inclined surface 21a. The lower mold 22 is provided with a recess 22a in which the intermediate molded product 3 is set. The concave portion 22a has the same shape as the lower portion (bottom portion 11 and bottom bent portion 13) of the intermediate molded product 3.

In the intermediate bending step, first, as shown in FIG. 3A, the bottom portion 11 of the intermediate molded product 3 is set in the recess 22a of the lower mold 22. Then, by lowering the upper mold 21 and pushing the upper end 12d of the pair of vertical wall portions 12 of the intermediate molded product 3 downward with the upper mold 21, the pair of vertical wall portions as shown in FIG. 3 (B). The upper end 12d of the 12 is guided inward in the width direction along the inclined surface 21a of the upper die 21, the pair of vertical wall portions 12 are inclined inward in the width direction, and the upper end 12d abuts on the locking surface 21b of the upper die 21. As a result, the upper ends 12d of the pair of vertical wall portions 12 are further restricted from moving inward in the width direction (sides approaching each other) in a state of being separated in the width direction. In this state, when the upper die 21 is further lowered, the vertical wall portion 12 is compressed in the length direction to cause buckling, and the intermediate portion of the vertical wall portion 12 bulges outward {see FIG. 3 (C). }. When the upper mold 21 is further lowered, the bulging portion of the vertical wall portion 12 is pressed against the shoulder bending portion forming surface 21c of the upper mold 21, whereby the shoulder bending portion 12a is formed in the intermediate portion of the vertical wall portion 12. {See FIG. 3 (D)}.

The pair of vertical wall portions 12 of the intermediate molded product 3 thus formed include a shoulder bending portion 12a, a side portion 12b extending downward from the shoulder bending portion 12a, and an inclined portion 12c extending diagonally upward from the shoulder bending portion 12a. Is provided. The shoulder bending portion 12a has the same shape as a part of the shoulder bending portion 1f of the tubular part 1 of the finished product, and in the illustrated example, has an arc shape having the same curvature as the shoulder bending portion 1f of the tubular part 1. ..

In the O-bending step, the intermediate molded product 3 subjected to the intermediate bending step is further pressed, the ends of the intermediate molded product 3 are abutted against each other, and the tubular part 1 is formed {see FIG. 2 (D)}. As shown in FIG. 4, the O-bending mold 30 for performing the O-bending step on the intermediate molded product 3 is composed of an upper mold 31 and a lower mold 32 that can approach and separate from each other. In the illustrated example, the lower mold 32 is a fixed type, and the upper mold 31 is a movable type that is raised and lowered by a driving means (not shown). The lower mold 32 is provided with a recess 32a for setting the bottom portion 11 of the intermediate molded product 3. The upper mold 31 is provided with a molding surface 31a. The molded surface 31a of the upper mold 31 has the same shape as the upper portion (upper portion 1d and shoulder bent portion 1f) of the tubular part 1 which is a finished product, except for the relief portion 31b described later.

The shape change of the intermediate molded product 3 in the XX cross section of FIG. 1 (B) due to O-bending molding will be described with reference to FIG. When the intermediate molded product 3 is set in the recess 32a of the lower mold 32 and then the upper mold 31 is lowered, the molded surface 31a of the upper mold 31 abuts on the upper end 12d of the vertical wall portion 12 of the intermediate molded product 3 {FIG. 4 See (A)}. When the upper mold 31 is further lowered, the upper end 12d of the vertical wall portion 12 of the intermediate molded product 3 is pushed downward by the upper mold 31, and the shoulder bent portion 12a of the intermediate molded product 3 is further bent, so that the vertical wall portion 12 is further bent. The upper ends 12d of the above abut each other {see FIGS. 4 (B) and 4 (C)}. When the upper mold 31 is further lowered, the shoulder bending portion 12a of the vertical wall portion 12 is further bent and deformed so as to approach the molding surface 31a of the upper mold 31, and the end of the intermediate molded product 3 is formed immediately before the bottom dead center. Part regions P are substantially parallel to each other {see FIG. 4 (D)}. Then, by lowering the upper die 31 to the bottom dead center, the entire vertical wall portion 12 is pressed against the molding surface 31a of the upper die 31 in a state where the ends of the vertical wall portions 12 are stretched against each other, and the tubular component is formed. 1 is molded {see FIG. 4 (E)}.

By the way, in the tubular part 1, the flatter the upper surface {see FIG. 1 (A)} having the butt portion 1a, the earlier the end region P of the vertical wall portion 12 becomes parallel in the O bending step, and a recess is formed. It tends to be easy to do. In the present embodiment, the O-bending step is performed with the core inserted in the opening on the other end side where the upper surface is the flattest among the tubular parts 1. Specifically, the shape change of the intermediate molded product 3 due to O-bending molding in the YY cross section of FIG. 1B will be described with reference to FIG. First, the intermediate molded product 3 is set in the recess 32a of the lower mold 32, and then the core 33 is arranged on the inner circumference of the opening on the other end side of the intermediate molded product 3 {see FIG. 5 (A)}. After that, when the upper mold 31 is lowered, the end regions P of the intermediate molded product 3 become parallel to each other before the end portions 12d of the intermediate molded product 3 abut against each other {see FIG. 5 (B)}. When the upper mold 31 is further lowered, the end region P of the intermediate molded product 3 is rounded, and the end portion 12d is separated downward from the molding surface 31a of the upper mold 31 {see FIG. 5 (C)}. When the upper mold 31 is further lowered, the end portion 12d of the vertical wall portion 12 of the intermediate molded product 3 is supported from below by the core 33 {see FIG. 5 (D)}. When the upper mold 31 descends to the bottom dead center, the end region P of the intermediate molded product 3 is corrected by being sandwiched between the molding surface 31a of the upper mold 31 and the core 33 from above and below, and the tubular part 1 is desired. Is molded into the shape of {see FIG. 5 (E)}.

As described above, in the O-bending step, if the core is arranged inside the intermediate molded product 3, the shape can be corrected by sandwiching the end region P of the intermediate molded product 3, so that a recess is formed on the upper surface of the tubular component 1. Can be avoided from being formed. However, it is difficult to arrange the core 33 to the depth of the inner circumference of the tubular part 1, and in the present embodiment, the core 33 can be arranged only in a very small area near the opening. Therefore, in the longitudinal intermediate portion of the tubular component 1, the shape of the end region P of the intermediate molded product 3 cannot be corrected by the core 33.

Therefore, in the present embodiment, as shown in FIG. 6, a relief portion 31b is provided on the molding surface 31a of the upper die 31 of the O-bending die 30. The relief portion 31b is provided in a region facing the butt portion 1a of the tubular component 1 in the state at the time of completion of molding shown in FIG. 6 (E), and is a region retracted to the side away from the tubular component 1 (upper side in the figure). Is. In the illustrated example, the relief portion 31b is composed of a recess recessed upward with respect to the other region of the molding surface 31a. As shown in FIG. 7, the relief portion 31b extends along the abutting portion 1a of the tubular component 1 (indicated by a chain line). In the present embodiment, since the opening on one end side of the tubular part 1 is close to a circle, it can be molded into a desired shape without taking any special measures. On the other hand, although the opening on the other end side of the tubular component 1 is flat, it can be formed into a desired shape by arranging the core 33. Therefore, in the present embodiment, the relief portion 31b is provided in the region corresponding to the intermediate portion in the longitudinal direction of the molded surface 31a of the upper mold 31 excluding both ends in the longitudinal direction of the tubular component 1. In the illustrated example, the relief portion 31b is provided at a position unevenly distributed on the other end side (flat opening side) of the tubular component 1 in the longitudinal direction.

The shape change of the intermediate molded product 3 due to O-bending molding in the ZZ cross section of FIG. 1 (B) will be described with reference to FIG. After setting the intermediate molded product 3 in the recess 32a of the lower mold 32, the upper mold 31 is lowered so that the edge portion 31c at the lower end of the relief portion 31b of the molding surface 31a of the upper mold 31 is the intermediate molded product 3. It abuts on the inclined portion 12c of the vertical wall portion 12 {see FIG. 6 (A)}. If the molding surface 31a of the upper mold 31 is not provided with the relief portion 31b, the end portion 12d of the vertical wall portion 12 of the intermediate molded product 3 is pushed downward as shown by the dotted line in the figure. It becomes. That is, by providing the relief portion 31b on the molding surface 31a of the upper mold 31, the amount of pushing the end portion 12d of the vertical wall portion 12 of the intermediate molded product 3 downward is reduced, so that the vertical wall portion 12 of the intermediate molded product 3 is reduced. The timing of bending the shoulder bending portion 12a can be delayed.

After that, when the upper mold 31 is further lowered, the end region P of the intermediate molded product 3 is pushed downward by the edge portion 31c of the upper mold 31. At this time, as described above, the timing of bending the shoulder bending portion 12a of the intermediate molded product 3 is delayed by the amount provided with the relief portion 31b, so that it is possible to avoid a situation in which the pair of end regions P are parallel to each other at an early stage. See FIGS. 6 (B) to 6 (D)}. Immediately before the upper die 31 reaches the bottom dead center, the ends of the intermediate molded product 3 abut against each other, the vertical wall portion 12 is pressed against the molding surface 31a, and the tubular part 1 is molded {FIG. 6 See (E)}.

In the normal O-bending process, the molding surface of the mold has the same shape as the tubular part of the finished product, and when the molding is completed, the entire molding surface of the mold is in close contact with the tubular part, so that the shape of the molding surface is tubular. Transferred to the component {see, eg, FIG. 4 (E)}. In the present invention, the molding surface 31a of the upper die 31 is provided with a relief portion 31b separated from the tubular part 1 in the state when the molding is completed, so that the shoulder bending of the intermediate molded product 3 in the longitudinal region provided with the relief portion 31b. The timing of bending the portion 12a can be delayed. At this time, a relief portion provided on the molded surface 31a of the upper mold 31 so that the upper surface of the tubular component 1 is smoothly continuous at the butt portion 1a or the butt portion 1a on the upper surface of the tubular component 1 slightly protrudes upward. The position, shape, size (width direction dimension, longitudinal direction dimension), depth (vertical direction dimension), etc. of 31b are set. As described above, since the situation where the butt portion 1a of the tubular part 1 is dented can be prevented, the subsequent welding work to the butt portion 1a of the tubular part 1 is easy.

At the time of completion of molding shown in FIG. 6E, in the longitudinal region provided with the relief portion 31b, the periphery of the butt portion 1a of the tubular part 1 cannot be pressed from above by the molding surface 31a of the upper mold 31. The butted portion 1a of the tubular component 1 may be slightly displaced in the vertical direction. Even in such a case, since the butt portion 1a is not recessed, welding can be performed on the butt portion 1a without any trouble. That is, by providing the relief portion 31b, there may be a demerit that the accuracy of the butt portion 1a of the tubular part 1 is lowered, but there is a merit that welding can be reliably performed by avoiding the concave portion on the upper surface of the tubular part 1. Is larger.

The present invention is not limited to the above embodiment. Hereinafter, other embodiments of the present invention will be described, but the same points as those of the above-described embodiments will be omitted.

In the above embodiment, O-bending molding is performed without the intermediate molded product 3 coming into contact with the relief portion 31b (region excluding the edge portion 31c) of the upper mold 31. On the other hand, in the embodiment shown in FIG. 8, the depth of the relief portion 31b is made shallower than that in the embodiment shown in FIG. 6 so that the end portion 12d of the intermediate molded product 3 comes into contact with the relief portion 31b. .. Specifically, by setting the intermediate molded product 3 in the recess 32a of the lower mold 32 and then lowering the upper mold 31, the bottom surface (upper surface) of the relief portion 31b abuts on the end portion 12d of the intermediate molded product 3 { See FIG. 8 (A)}. Then, when the upper mold 31 is lowered, the end region P of the intermediate molded product 3 is pushed downward only on the bottom surface of the relief portion 31b, or on both the edge portion 31c and the bottom surface {see FIG. 8 (B)}.

After that, when the upper mold 31 is further lowered, the bottom surface of the relief portion 31b and the end portion 12d of the intermediate molded product 3 are separated from each other, and the end region P of the intermediate molded product 3 is lowered only by the edge portion 31c of the relief portion 31b. {See FIGS. 8 (C) and 8 (D)}. Then, by lowering the upper mold 31 to the bottom dead center, the tubular part 1 is molded {see FIG. 8 (E)}.

In this way, by pushing down the end portion 12d of the intermediate molded product 3 at the bottom surface of the relief portion 31b while lowering the upper mold 31, the intermediate molded product 3 by the upper mold 31 is compared with the embodiment shown in FIG. The amount of pushing of the end portion 12d of the vertical wall portion 12 becomes large, and the timing of bending the shoulder bending portion 12a of the intermediate molded product 3 becomes earlier. That is, by adjusting the depth of the relief portion 31b, the timing of bending the shoulder bending portion 12a of the intermediate molded product 3 can be adjusted, so that the shape accuracy of the butt portion 1a of the tubular part 1 can be improved. Become.

In the above embodiment, the case where the relief portion 31b is provided in the region of the molding surface 31a of the upper mold 31 of the O-bending die 30 facing the intermediate portion in the longitudinal direction of the butt portion 1a of the tubular component 1 is shown. , Not limited to this. For example, when the core 33 is not inserted into the end of the tubular part 1, it is located in the region of the molding surface 31a of the upper mold 31 of the O-bending die 30 facing the longitudinal end of the butt portion 1a of the tubular part 1. A relief portion 31b may be provided.

Further, in the above embodiment, in the O-bending step, the case where the core 33 is arranged at the other end of the flat tubular part 1 to assist the molding is shown, but further, it is substantially circular. A core may also be arranged at one end of the tubular component 1 formed of the above. Further, if it is not particularly necessary, the core may not be provided in the O-bending die 30. Further, in the intermediate bending step, a core may be arranged on the inner circumference of one or both ends of the intermediate molded product 3 to assist molding.

Further, in the above embodiment, the intermediate bending step of forming the shoulder bending portion 12a is provided on the vertical wall portion 12 of the intermediate molded product 3, but if there is no particular need, the intermediate bending step is omitted and the shape is substantially U-shaped. An O-bending step may be performed on the intermediate molded product 3 of the shape {see FIG. 2 (B)}. Alternatively, the intermediate bending step may be performed in multiple stages.

The above press molding method can be suitably applied to tubular parts having a deformed cross section, and is, for example, applied to the left and right trailing arms of the rear suspension of an automobile and the press molding of an intermediate beam connecting them. Can be done.

1 Tubular part 1a Butt part 2 Blank material 3 Intermediate molded product 11 Bottom part 12 Vertical wall part 20 Intermediate bending mold 21 Upper mold 22 Lower mold 30 O Bending mold 31 Upper mold (the other mold)
31a Molding surface 31b Relief part 31c Edge part 32 Lower mold (one mold)
32a recess 33 core P end area

Claims (2)

A U-bending step of forming an intermediate molded product having a bottom portion and a pair of vertical wall portions rising from both ends of the bottom portion, and setting the bottom portion of the intermediate molded product in one mold and using the other mold as the one. The end portions of the pair of vertical wall portions are brought close to the mold, and the pair of vertical wall portions are pressed and bent by the molding surface of the other mold without bending near the end portions of the pair of vertical wall portions. A method for press-molding a tubular part, which comprises an O-bending step of butt-butting each other to form the intermediate molded product into a tubular shape.
Of the molding surface of the other mold, in the longitudinal region where the core that supports the intermediate molded product from the inside is not provided, in the region facing the butt portion of the tubular part in the state at the time of molding completion. Provided with a retracted relief on the side away from the tubular part ,
A press molding method in which the end portion of a tubular part and the relief portion are not in contact with each other when molding is completed . An intermediate molded product having a bottom portion and a pair of vertical wall portions rising from both ends of the bottom portion is pressed, and the ends of the pair of vertical wall portions are abutted against each other without bending the vicinity of the pair of vertical wall portions. It is an O-bending mold that molds a molded product into a tubular shape.
It has one mold having a recess for setting the bottom of the intermediate molded product, and a molding surface that can be approached and separated from the one mold and that presses and bends the vertical wall portion of the intermediate molded product. Equipped with the other mold,
Of the molding surface of the other mold, in the longitudinal region where the core that supports the intermediate molded product from the inside is not provided, in the region facing the butt portion of the tubular part in the state at the time of molding completion. Provided with a retracted relief on the side away from the tubular part ,
An O-bending die in which the end portion of the tubular part and the relief portion are not in contact with each other when molding is completed .

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