Extrusion molding apparatus for differential thickness pipe and extrusion molding method for differential thickness pipe

Abstract

To provide a technique which can make the shape of the outer contour of a cross-section of a small-diameter part non-circular in extrusion processing in which a mandrel is inserted into a cylindrical tubular element tube and the element tube is pushed into a die bore by a drive mechanism so that the tip side of the element tube is drawn and diameter-reduced to form a small-diameter part and thereby mold a differential thickness pipe.SOLUTION: An inner peripheral surface of a connection part formed between a large inner-diameter part formed on a base-end side of a die bore and having a circular cross-section, and a small inner-diameter part formed on a tip-end side of the die bore and having a non-circular cross-section is constituted by a substantially truncated conical side surface coaxial with the large inner-diameter part and the small inner-diameter part. An intersection line between a columnar side surface with the non-circular cross-section being the shape of the internal space of the small inner-diameter part, and the substantially truncated conical side surface being the shape of the internal space of the connection part is defined as a boundary line between the small inner-diameter part and the connection part.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present invention relates to an extrusion molding device for a pipe with different thicknesses and a method for extruding the same, and more particularly to an extrusion molding device for a pipe with different thicknesses and a long small-diameter portion whose cross-sectional outer contour is non-circular, and a method for extruding the same. [Background technology]

[0002] In the technical field, a differential thickness pipe (also called a "butted pipe" or "butted tube") is known in which a thick-walled portion is formed in a portion of the axial direction of the pipe in order to achieve a desired mechanical strength in the thick-walled portion while reducing the weight in the thin-walled portion (portion other than the thick-walled portion).

[0003] As a method for forming such a differential thickness pipe, for example, Patent Document 1 (Japanese Patent No. 6933762) discloses an extrusion molding method in which a tubular material is inserted into a die and stretched and reduced in diameter, while increasing the wall thickness at the tip. In this molding method, by leaving the end of the tubular material on the base end side unprocessed, a differential thickness pipe can be obtained that has a large diameter portion that is the unprocessed part, a long small diameter portion that has been stretched and reduced in diameter, and a thick portion formed at the tip of the small diameter portion and having an increased wall thickness.

[0004] In the differential thickness pipe obtained by the above-mentioned forming method, the cross-sectional shapes of the large diameter portion, the small diameter portion, and the thick-walled portion are all circular. However, for example, in applications such as shafts constituting a vehicle drive train, there are cases where a differential thickness pipe having a long small diameter portion with a non-circular (e.g., polygonal) cross-sectional outer contour is required.

[0005] In this technical field, Patent Document 2 (JP 2000-140980 A) proposes a method of forming a small-diameter portion having a non-circular cross section by extruding a material having a circular cross section. This method involves inserting a mandrel having a diameter smaller than that of a tubular material having a small aspect ratio (length / width ratio) and a through hole formed therein, and then forcing the tubular material into a die hole having a desired non-circular cross section to reduce the diameter and form a small-diameter portion, while also making the outer contour of the cross section of the small-diameter portion non-circular.

[0006] According to the above-mentioned method, by leaving the unprocessed tubular material, it is possible to form a pipe with a thickness difference, which has a large diameter portion with a circular cross-sectional outer contour at the base end and a small diameter portion with a non-circular cross-sectional outer contour at the tip end. However, the above-mentioned method is a simple extrusion molding, and it is difficult to obtain a long small diameter portion as in the extrusion molding method according to the invention disclosed in the above-mentioned Patent Document 1.

[0007] On the other hand, Patent Document 3 (JP 2009-160608 A) proposes a method for preventing overshoot in a small diameter portion having a circular cross section by forming a convex portion midway along the tapered portion of a die hole in a tool having a tapered portion for forcing a hollow cylindrical member or a solid rod-shaped member into a die hole to reduce its diameter, thereby making the cross section of the die hole non-circular, thereby inducing a difference in processing hardness.

[0008] However, the cross-sectional shape of the small diameter portion finally obtained by the above-mentioned method is circular, and it is not possible to form a small diameter portion having a non-circular cross section. In other words, even with this method, it is not possible to obtain a pipe with a different thickness by extending the small diameter portion, which is a portion having a non-circular outer cross-sectional contour.

[0009] As described above, in this technical field, there is a demand for a method that can make the shape of the outer contour of the cross section of a small diameter portion non-circular (particularly polygonal) by extrusion processing that involves stretching the small diameter portion, such as the extrusion molding method according to the invention disclosed in the above-mentioned Patent Document 1. [Prior art documents] [Patent documents]

[0010] [Patent Document 1] Patent No. 6933762 [Patent Document 2] Japanese Patent Application Laid-Open No. 2000-140980 [Patent Document 3] Japanese Patent Application Laid-Open No. 2009-160608 Summary of the Invention [Problem to be solved by the invention]

[0011] As described above, there is a need in the technical field for a method capable of forming a noncircular (particularly polygonal) outer contour shape of the cross section of a small diameter portion by extrusion processing that involves elongating the small diameter portion, such as the extrusion molding method according to the invention disclosed in the aforementioned Patent Document 1. Therefore, one object of the present invention is to provide a technology capable of forming a noncircular outer contour shape of the cross section of a small diameter portion in an extrusion processing in which, with a mandrel inserted into or passing through a cylindrical mother tube, the mother tube is forced into a die hole by a drive mechanism, and the leading end of the mother tube is elongated and reduced in diameter to form a small diameter portion and form a differential thickness pipe.

[0012] Therefore, the inventors conducted an experiment to make the shape of the outer contour of the cross section of the long small diameter section non-circular by making the cross section of the first small inner diameter section, which is the tip side part of the first die hole, which is a through hole formed in the first die used in the extrusion molding method according to the invention disclosed in Patent Document 1, non-circular.

[0013] However, as will be described in detail later, when an attempt was made to form a long small-diameter portion having a non-circular cross-sectional outer contour at the tip end of the blank tube by forcing a tubular blank tube with a mandrel inserted into the die hole and performing an extrusion process, problems such as cracking and / or deformation sometimes occurred in the small-diameter portion. [Means for solving the problem]

[0014] In view of the above problems, the present inventors conducted extensive research and found that the above problems can be solved by configuring the inner surface of a connecting portion formed between a large inner diameter portion, which is formed at the base end of the die hole and has a circular cross section, and a small inner diameter portion, which is formed at the tip end of the die hole and has a non-circular cross section, with a substantially truncated conical side surface that is coaxial with the large inner diameter portion and the small inner diameter portion, and by defining a continuum line, which is the intersection of the cylindrical side surface having a non-circular cross section, which is the shape of the internal space of the small inner diameter portion, and the substantially truncated conical side surface, which is the shape of the internal space of the connecting portion, as the boundary line between the small inner diameter portion and the connecting portion.

[0015] Specifically, the extrusion molding apparatus for a differential thickness pipe according to the present invention (hereinafter, sometimes referred to as the "apparatus of the present invention") includes a mandrel having a predetermined shape, a sleeve which is a cylindrical member arranged coaxially with the mandrel, a die having a die hole which is a through hole having a predetermined shape, and a drive mechanism which drives at least the sleeve and the die so as to approach each other in the axial direction. The apparatus of the present invention is also configured to perform an extrusion process in which, with the mandrel inserted into or passing through a blank tube which is a member having a predetermined shape, the blank tube is forced into the die hole by the drive mechanism to perform an extrusion process, thereby forming a differential thickness pipe having a predetermined shape.

[0016] The mother tube is a cylindrical member having a first outer diameter which is a predetermined outer diameter, a first inner diameter which is a predetermined inner diameter, and a first wall thickness which is a predetermined wall thickness.

[0017] The differential thickness pipe includes a large diameter portion, a small diameter portion, and a first reduced outer diameter portion. The large diameter portion is a cylindrical portion formed at the base end, which is upstream in the extrusion direction, which is the direction in which the blank tube is forced into the die hole during the extrusion process, and has a second outer diameter equal to the first outer diameter, a second inner diameter that is a predetermined inner diameter equal to or smaller than the first inner diameter, and a predetermined second thickness. The small diameter portion is formed at the tip end, which is downstream in the extrusion direction, and has a non-circular cross-sectional outer contour that is smaller than the circular cross-sectional outer contour of the large diameter portion and the same second inner diameter as the large diameter portion. The first reduced outer diameter portion is formed between the large diameter portion and the small diameter portion, has the same second inner diameter as the large diameter portion, and is a portion that connects the large diameter portion and the small diameter portion, with the outer diameter decreasing at a constant rate from the second outer diameter as it approaches the small diameter portion.

[0018] The mandrel includes a large outer diameter portion that is a cylindrical portion formed on the tip side and has a third outer diameter that is an outer diameter corresponding to the second inner diameter.

[0019] The sleeve includes a pressing portion that is a cylindrical portion formed at the tip end and has a fourth outer diameter equal to the first outer diameter, a third inner diameter equal to the second inner diameter, and a third thickness that is a predetermined thickness.

[0020] The die hole includes a large inner diameter portion, a small inner diameter portion, a connecting portion, and a transition portion. The large inner diameter portion is formed on the base end side and defines a cylindrical space having a fourth inner diameter corresponding to the first outer diameter. The small inner diameter portion is formed on the tip end side and defines a cylindrical space having a non-circular cross section corresponding to the outer contour of the non-circular cross section of the small inner diameter portion. The connecting portion is formed between the large inner diameter portion and the small inner diameter portion and has an inner diameter that decreases at a constant rate from the fourth inner diameter as it approaches the small inner diameter portion. The transition portion is formed between the large inner diameter portion and the connecting portion and has an inner circumferential surface that is a curve that continuously connects the inner circumferential surface of the large inner diameter portion and the inner circumferential surface of the connecting portion in a cross section taken along a plane including the central axis of the die hole.

[0021] Furthermore, in the die hole, the boundary line between the small inner diameter portion and the connecting portion is formed by a line where the inner peripheral surface of the small inner diameter portion and the inner peripheral surface of the connecting portion intersect.

[0022] In addition, the device of the present invention is configured such that during the extrusion process, the tip end of the mandrel is located more distal than the base end of the small inner diameter portion of the die hole, at least during the period from the first point onwards, which is the point at which the material constituting the mother tube begins to flow distally of the tip end of the large inner diameter portion of the die hole due to the extrusion process.

[0023] As stated at the beginning of this specification, the present invention relates not only to the above-mentioned apparatus for extrusion molding a pipe with different thicknesses according to the present invention (apparatus of the present invention), but also to a method for extrusion molding a pipe with different thicknesses. As will be described in detail later, the method for extrusion molding a pipe with different thicknesses according to the present invention (method of the present invention) is a method for extruding a pipe with different thicknesses, having a long small-diameter portion whose cross-sectional outer contour is non-circular, from a blank tube, which is a cylindrical member, using the apparatus of the present invention. [Effects of the Invention]

[0024] As described above, in the extrusion molding device for differential thickness pipes according to the present invention (the device of the present invention), the boundary line between the small inner diameter portion, which is formed at the tip end of the die hole and defines a columnar space having a non-circular cross section corresponding to the outer contour of the non-circular cross section of the small diameter portion, and the large inner diameter portion, which is formed at the base end and defines a cylindrical space, is formed by a continuum line where the inner surface of the small inner diameter portion intersects with the inner surface of the connecting portion.

[0025] The above-described configuration can reduce problems such as cracking and / or deformation that occurred in the small diameter portion when an attempt was made to form a long small diameter portion having a noncircular cross-sectional outer contour shape at the tip end of a mother tube in the aforementioned trial. That is, according to the present invention, the small diameter portion can be made to have a noncircular cross-sectional outer contour shape by extrusion processing that involves elongation of the small diameter portion.

[0026] Other objects, other features and attendant advantages of the present invention will be readily apparent from the following description of the embodiments of the present invention which will be given with reference to the drawings. [Brief explanation of the drawings]

[0027] [Figure 1] 1 is a schematic diagram showing an example of the configuration of a blank pipe to be subjected to extrusion processing carried out in an extrusion processing step by an extrusion molding apparatus (first apparatus) for a differential thickness pipe according to a first embodiment of the present invention. FIG. [Figure 2] 3 is a schematic diagram showing an example of the configuration of a pipe with different thicknesses formed by extrusion processing carried out by a first device in an extrusion processing step. FIG. [Figure 3] 3A to 3C are schematic cross-sectional views of the variable thickness pipe illustrated in FIG. 2 at a plurality of locations. [Figure 4] FIG. 2 is a schematic cross-sectional view showing an example of the configuration of a mandrel, a sleeve, and a die provided in the first device. [Figure 5] FIG. 2 is a schematic perspective view of a die provided in the first device, the die being divided by a plane including the central axis. [Figure 6] 6 is a schematic diagram illustrating the difference between the apex and flat portion in the path along which material flows from the large inner diameter portion of the die provided in the first device illustrated in FIG. 5 through the transition portion and the connection portion to the small inner diameter portion. [Figure 7] FIG. 3 is a schematic cross-sectional view showing an example of a change in the shape of a raw tube resulting from the execution of extrusion processing by a first device. [Figure 8] 1 is a schematic cross-sectional view showing an example of a change in the shape of a mother tube that accompanies extrusion processing in a first method for forming a differential thickness pipe having an inner diameter smaller than the inner diameter of the mother tube. FIG. [Figure 9] 10 is a schematic cross-sectional view illustrating the minimum and maximum wall thickness values ​​of a small diameter portion of a differential thickness pipe. FIG. [Figure 10] FIG. 10 is a schematic diagram illustrating the configuration of a die provided in an extrusion molding device (third device) for a differential thickness pipe according to a third embodiment of the present invention. [Figure 11] FIG. 10 is a schematic diagram illustrating the shapes of the inner surfaces of the large inner diameter portion, transition portion, connection portion, and small inner diameter portion that constitute the die hole in a cross section taken along a plane including the central axis of the die provided in the extrusion molding device (fourth device) for a differential thickness pipe according to the fourth embodiment of the present invention. [Figure 12]FIG. 10 is a schematic diagram showing an example of the configuration of a pipe with different thicknesses formed by extrusion processing carried out in an extrusion processing step by an extrusion molding device (fifth device) for a pipe with different thicknesses according to a fifth embodiment of the present invention. [Figure 13] 13A to 13C are schematic cross-sectional views of a plurality of locations of the variable thickness pipe illustrated in FIG. 12. [Figure 14] FIG. 10 is a schematic cross-sectional view showing an example of the configuration of a mandrel and a sleeve provided in a fifth device. [Figure 15] FIG. 10 is a schematic cross-sectional view showing an example of a change in the shape of a raw tube resulting from the execution of extrusion processing by a fifth device. [Figure 16] 10 is a schematic diagram showing an example of elastic deformation occurring in a small diameter portion whose cross-sectional outer contour shape is to be a regular octagon. FIG. [Figure 17] FIG. 10 is a schematic diagram showing an example of the configuration of a die provided in an extrusion molding device (sixth device) for a differential thickness pipe according to a sixth embodiment of the present invention. [Figure 18] FIG. 10 is a schematic diagram showing an example of the configuration of a small diameter portion of a pipe with a different thickness formed by extrusion processing performed by a sixth device. [Figure 19] 18A is a photograph illustrating the appearance of a pipe with a different thickness formed by the sixth device equipped with the die D6 illustrated in FIG. 17, and FIG. 18B is a photograph illustrating the appearance of a pipe with a different thickness formed by the first device. [Figure 20] 10 is a flowchart showing an example of the flow of each step included in the extrusion processing step executed in the extrusion molding method (first method) for a differential thickness pipe according to the seventh embodiment of the present invention. [Figure 21] 1 is a schematic perspective view showing one example of the configuration of a differential thickness pipe formed from a cylindrical base pipe by an extrusion molding method using an extrusion molding device for a differential thickness pipe according to an embodiment of the present invention. FIG. [Figure 22] FIG. 10 is a schematic perspective view showing another example of the configuration of a differential thickness pipe formed from a cylindrical base pipe by an extrusion molding method using an extrusion molding device for a differential thickness pipe according to an embodiment of the present invention. [Figure 23]FIG. 1 is a schematic perspective view illustrating the vicinity of a die connection portion divided by a plane including the central axis of a first die hole, which is a tip-side portion of a first small inner diameter portion that is a through hole formed in a first die used in an extrusion molding method according to the invention disclosed in Patent Document 1, in an attempt to make the outer contour shape of the cross section of a long small diameter portion non-circular. [Figure 24] 10 is a schematic diagram illustrating the difference in thickness between the apex of the small diameter portion and the flat portion, and the resulting cracks. FIG. [Figure 25] 24 is a schematic diagram illustrating the difference between the apex and flat portions in the path along which material flows from the large inner diameter portion through the transition portion and connection portion to the small inner diameter portion of the two types of dies illustrated in FIG. 23. DETAILED DESCRIPTION OF THE INVENTION

[0028] First Embodiment Below, we will explain the extrusion molding device for differential thickness pipes (hereinafter sometimes referred to as the ``first device'') related to the first embodiment of the present invention with reference to the drawings.However, before explaining the first device, we will explain the above-mentioned trial in more detail.

[0029] As described above, the present inventors have conducted an experiment to make the outer contour shape of the cross section of the long small diameter portion noncircular by making the cross section of the first small inner diameter portion, which is the tip side portion of the first die hole, a through hole formed in the first die used in the extrusion molding method according to the invention disclosed in Patent Document 1, noncircular. This experiment will be described in detail below.

[0030] In the above attempt, first, the cross section of a connecting portion, which is formed between a large inner diameter portion having a circular cross section formed at the base end of the die hole and a small inner diameter portion having a noncircular cross section as described above and which connects the large inner diameter portion and the small inner diameter portion, was made noncircular so as to be similar to the cross section of the small inner diameter portion. Specifically, for example, as illustrated in Figure 23, when the cross section of the small inner diameter portion PSDI is polygonal (a regular octagon in Figure 23), the inner peripheral surface of the connecting portion PC was configured by arranging the surfaces (hereinafter sometimes referred to as "connecting surfaces SC") that connect each side ED that defines the cross section at the base end of the small inner diameter portion PSDI to the inner peripheral surface of the large inner diameter portion PLDI in a petal shape.

[0031] Figure 23 is a schematic perspective view illustrating the vicinity of the connection portion PC of a die divided by a plane including the central axis of the die hole. Figure 23(a) is a schematic view illustrating the configuration of the connection portion PC when the connection surface SC is configured as a plane. Therefore, in the die Da illustrated in Figure 23(a), the boundary line between the large inner diameter portion PLDI and the connection portion PC is the intersecting line between the side surface (pyramidal surface) of a regular truncated pyramid (a regular octagonal truncated pyramid in Figure 23) whose apex is the cross section at the end of the base end side (upper side in Figure 23) of the small inner diameter portion PSDI of the die hole DHa and the inner peripheral surface of the large inner diameter portion PLDI.

[0032] 23(b) is a schematic diagram illustrating the configuration of the connecting portion PC when the boundary between the inner peripheral surface of the large inner diameter portion PLDI and the inner peripheral surface of the connecting portion PC is defined as the intersection line (i.e., a circle) between a plane perpendicular to the central axis of the die hole and the inner peripheral surface of the large inner diameter portion PLDI. Therefore, in the die Db illustrated in FIG. 23(b), each connecting surface SC is neither a flat surface nor a simple curved surface with a constant curvature, but is composed of a gradually changing surface with a complex shape that smoothly connects each side ED that defines the cross section at the base end of the small inner diameter portion PSDI to the inner peripheral surface of the large inner diameter portion PLDI.

[0033] 23(a) and 23(b), a transition portion PT was provided between the large inner diameter portion PLDI and the connecting portion PC to smoothly connect their inner peripheral surfaces. That is, the transition portion PT was configured so that the inner peripheral surface of the transition portion PT connecting the inner peripheral surface of the large inner diameter portion PLDI and the inner peripheral surface of the connecting portion PC formed a smooth curve in a cross section taken along a plane including the central axis of the die hole.

[0034] However, as described above, when an attempt is made to form a long small-diameter portion having a non-circular outer cross-sectional shape at the tip end of a blank tube by forcing a tubular blank tube with a mandrel inserted into the die hole and performing an extrusion process, problems such as cracks and / or deformation may occur in the small-diameter portion. Specifically, in the example shown in Fig. 23, problems such as cracks and / or deformation may occur near the vertices (hereinafter sometimes referred to as "vertices") of the polygonal cross-sectional shape of the small-diameter portion and near the midpoints of the sides (hereinafter sometimes referred to as "flat portions").

[0035] One of the causes of the above problem is thought to be the difference in thickness between the apex and the flat portion. Figure 24 is a schematic diagram illustrating the difference in thickness between the apex and the flat portion of the small diameter portion and the resulting cracks. Figure 24(a) is a schematic diagram showing an example of a cross section of the small diameter portion, and Figure 24(b) is an enlarged view of the area surrounded by the thick dashed line in Figure 24(a). As illustrated in Figure 24(b), the thickness Ta at the apex of the small diameter portion is greater than the thickness Te at the flat portion (Ta > Te). Therefore, the flow rate of the material extruded from the large inner diameter portion PLDI of the die hole through the connecting portion PC to the small inner diameter portion PSDI is faster at the flat portion than at the apex of the small diameter portion. As a result, as illustrated in Figure 24(c), problems such as cracks (see the solid arrows) and / or deformation may occur around the boundary between the apex and the flat portion of the small diameter portion, such as at the tip end of the small diameter portion.

[0036] In addition to the difference in material flow rate due to the difference in thickness between the apex and flat portions as described above, the path along which the material flows from the large inner diameter portion PLDI of the die hole through the transition portion PT and the connection portion PC to the small inner diameter portion PSDI is different between the apex and flat portions, which is also thought to be one of the causes of the above problem.

[0037] 25(a) and 25(b) are schematic diagrams illustrating the difference between the apex and the flat portion in the path along which material flows from the large inner diameter portion PLDI through the transition portion PT and the connecting portion PC to the small inner diameter portion PSDI of the two types of dies Da and Db illustrated in FIGS. 23(a) and 23(b). Specifically, FIGS. 25(a) and 25(b) are schematic diagrams illustrating the shapes of the inner peripheral surfaces of the large inner diameter portion PLDI, the transition portion PT, the connecting portion PC, and the small inner diameter portion PSDI that constitute the die holes DHa and DHb in a cross section taken along a plane including the central axes of the dies Da and Db. In FIG. 25, for the purpose of facilitating understanding of the following description, the line representing the inner peripheral surface of the connecting portion PC and the line representing the inner peripheral surface of the small inner diameter portion PSDI are drawn as if they directly intersect at a predetermined angle. However, in reality, a portion can be provided between the connecting portion PC and the small inner diameter portion PSDI that smoothly connects the inner surface of the connecting portion PC and the inner surface of the small inner diameter portion PSDI, similar to the transition portion PT described above.

[0038] In the die Da illustrated in Fig. 23(a), as described above, the boundary line between the large inner diameter portion PLDI and the connecting portion PC is the intersecting line between the side surface of a regular truncated pyramid having an apex at the cross section at the base end of the small inner diameter portion PSDI of the die hole DHa and the inner peripheral surface of the large inner diameter portion PLDI. Therefore, as illustrated in Fig. 23(a), the boundary line in the die hole DHa is a curve in which the base end end of the inner peripheral surface of the small inner diameter portion PSDI, which forms the apex of the small diameter portion, is located closer to the tip (lower side in Fig. 23) than the base end end of the inner peripheral surface of the small inner diameter portion PSDI, which forms the flat portion of the small diameter portion.

[0039] 25(a), in the die hole DHa, the position where the tip end of the cylindrical material is extruded from the large inner diameter portion PLDI to the transition portion PT and the connecting portion PC and starts to reduce in diameter is closer to the tip at the apex (dashed line) than at the flat portion (solid line), and the timing at which the diameter reduction starts is later at the apex than at the flat portion. On the other hand, the position where the tip end of the cylindrical material reaches the small inner diameter portion PSDI and ends to reduce in diameter is closer to the outside in the radial direction (to the right in FIG. 25) at the apex (dashed line) than at the flat portion (solid line).

[0040] As described above, in the die hole DHa, the position and timing at which diameter reduction begins and the radial position at which diameter reduction ends are different between the flat portion and the apex portion, so the path along which the material flows from the large inner diameter portion PLDI through the transition portion PT and the connection portion PC to the small inner diameter portion PSDI is different between the apex portion (solid line) and the flat portion (dashed line). As a result, distortion occurs in the circumferential direction of the part that forms the small diameter portion after diameter reduction and elongation, which is thought to increase the likelihood of problems such as cracking and / or deformation occurring in the resulting small diameter portion.

[0041] On the other hand, in the die Db illustrated in Fig. 23(b), as described above, the intersection line (i.e., the circle) between the plane perpendicular to the central axis of the die hole and the inner peripheral surface of the large inner diameter portion PLDI forms the boundary between the inner peripheral surface of the large inner diameter portion PLDI and the inner peripheral surface of the connecting portion PC. Therefore, in the die hole DHb, as illustrated in Fig. 23(b), the position where the tip end of the cylindrical blank starts to narrow as it is extruded from the large inner diameter portion PLDI to the transition portion PT and the connecting portion PC is the same for both the flat portion and the apex, and the timing at which the narrowing starts is also the same for both the flat portion and the apex.

[0042] However, even in the die hole DHb, as illustrated in (b) of Figure 25, the position where the tip end of the cylindrical material reaches the small inner diameter portion PSDI and the reduction in diameter ends is located radially outward (to the right as you face Figure 25) at the apex (dashed line) rather than at the flat portion (solid line).

[0043] As described above, in the die hole DHb, the position and timing at which diameter reduction begins are the same at both the flat and apex portions. However, the radial position at which diameter reduction ends is different between the flat and apex portions. Therefore, the path along which the material flows from the large inner diameter portion PLDI through the transition portion PT and the connection portion PC to the small inner diameter portion PSDI is different between the apex portion (solid line) and the flat portion (dashed line). Therefore, the path and length of time from when diameter reduction begins to when the material reaches the same position in the extrusion direction within the small inner diameter portion PSDI are also different between the apex portion (solid line) and the flat portion (dashed line). As a result, in the die hole DHb, distortion occurs in the circumferential direction of the portion undergoing diameter reduction, which is thought to increase the likelihood of problems such as cracking and / or deformation in the resulting small diameter portion.

[0044] <composition> As a result of extensive research in light of the above-mentioned problems, the inventors have found that the above-mentioned problems can be solved by configuring the inner surface of a connecting portion formed between a large inner diameter portion, which is formed at the base end of the die hole and has a circular cross section, and a small inner diameter portion, which is formed at the tip end of the die hole and has a non-circular cross section, with a substantially truncated conical side surface that is coaxial with the large inner diameter portion and the small inner diameter portion, and by defining a continuum line, which is the intersection of the cylindrical side surface having a non-circular cross section, which is the shape of the internal space of the small inner diameter portion, and the substantially truncated conical side surface, which is the shape of the internal space of the connecting portion, as the boundary line between the small inner diameter portion and the connecting portion.

[0045] More specifically, the extrusion molding device (first device) for a differential thickness pipe according to the first embodiment of the present invention comprises a mandrel having a predetermined shape, a sleeve which is a cylindrical member arranged coaxially with the mandrel, a die having a die hole which is a through hole having a predetermined shape, and a drive mechanism which drives at least the sleeve and the die to approach each other in the axial direction.

[0046] As described above, the drive mechanism may be configured to be capable of driving at least the sleeve and the die so as to approach each other in the axial direction. That is, the drive mechanism may be configured to be capable of driving only the sleeve and the die so as to approach each other in the axial direction, or may be configured to be capable of driving both the mandrel and the sleeve and the die so as to approach each other in the axial direction.

[0047] The mandrel may be slidably inserted into the sleeve, and when the drive mechanism drives only the sleeve and the die to move closer to each other in the axial direction, the mandrel that is not driven by the drive mechanism may be retracted toward the tip side as the tip side of the mother tube is reduced in diameter and stretched by the extrusion process to form a small diameter portion, which is a portion having a non-circular outer cross-sectional contour of the desired differential thickness pipe.

[0048] On the other hand, when the drive mechanism drives both the mandrel and the sleeve and the die to approach each other in the axial direction, the mandrel and the sleeve may be driven integrally or independently. When the mandrel and the sleeve are driven integrally, the mandrel and the sleeve may be formed integrally or separately. When the mandrel and the sleeve are driven independently, the mandrel and the sleeve must naturally be formed separately.

[0049] Furthermore, the drive mechanism may be configured to move only the sleeve or both the mandrel and the sleeve closer to the fixed die, or may be configured to move the die closer to only the fixed sleeve or both the mandrel and the sleeve, or may be configured to move only the sleeve or both the mandrel, sleeve, and die closer to each other. In the following description, for the purpose of easily understanding the present invention, a case will be described in which the drive mechanism is configured to move only the sleeve or both the mandrel and the sleeve closer to the fixed die, but the configuration of the drive mechanism is not limited to the following description.

[0050] The first device is configured to perform an extrusion process, which is a process of forming a differential thickness pipe having a predetermined shape by inserting or passing a mandrel through a blank tube, which is a component having a predetermined shape, and forcing the blank tube into a die hole using a drive mechanism to perform extrusion processing.

[0051] The first device having the above-described configuration can be configured using an extrusion molding device known to those skilled in the art. The basic configuration of a typical extrusion molding device is well known to those skilled in the art, so a detailed description will be omitted. However, components such as the die, sleeve, and mandrel are made of materials having properties (e.g., mechanical strength and durability) that can withstand processing conditions such as the load acting on the components in the extrusion process. Furthermore, the drive mechanism provided in the first device can be appropriately selected from various drive mechanisms known in the art, depending on the properties (e.g., mechanical strength and hardness) of the material constituting the blank tube to be extruded in the extrusion process. Typically, a press, such as a hydraulic press, is used as the drive mechanism.

[0052] FIG. 1 is a schematic diagram showing an example of the configuration of a mother tube to be extruded by a first device in the extrusion process. (a) of FIG. 1 is a side view of the mother tube W1, with the inner circumferential surface defining the internal space of the mother tube W1, which would normally be invisible, depicted by a dashed line. (b) and (c) of FIG. 1 are arrow views Ar and Af of the mother tube W1, respectively. (d) of FIG. 1 is a cross-sectional view of the mother tube W1 taken along a plane including the central axis AX shown in (a) of FIG. 1, and (e) of FIG. 1 is a cross-sectional view of the mother tube W1 taken along a plane including the line segment PP depicted in (a) of FIG. 1 and perpendicular to the central axis AX.

[0053] As shown in Fig. 1, the mother tube W1 is a cylindrical member having a predetermined outer diameter DO1, a predetermined inner diameter DI1, and a predetermined wall thickness T1. Note that AX shown in Fig. 1(a) and 1(d) is the central axis of the mother tube W1, and this axis AX is the central axis common to the mandrel and the die hole during the extrusion process.

[0054] The material for the mother pipe is not particularly limited as long as it can be formed into a desired shape by plastic deformation during extrusion. Typical materials for the mother pipe include metals such as lead, tin, aluminum, copper, zirconium, titanium, molybdenum, vanadium, niobium, and iron, or alloys containing two or more of these metals.

[0055] Figure 2 is a schematic diagram showing an example of the configuration of a pipe with different thicknesses formed by extrusion processing performed by the first device in the extrusion processing step. Figure 2(a) is a side view of the pipe with different thicknesses P1, and the inner peripheral surface that defines the internal space of the pipe with different thicknesses P1, which would normally be invisible, is depicted by a dashed line. Figure 2(b) and (c) are arrow views Ar and Af of the pipe with different thicknesses P1, respectively.

[0056] 3A is a schematic cross-sectional view of the pipe P1 at several locations shown in FIG. 2. FIG. 3A is a cross-sectional view of the pipe P1 along a plane including the central axis AX shown in FIG. 2A, the line segment PP shown in FIG. 2B, and the line segment QQ shown in FIG. 2C. FIG. 3B, FIG. 3C, and FIG. 3D are cross-sectional views of the pipe P1 along a plane including the line segments RR, SS, and TT shown in FIG. 3A and perpendicular to the central axis AX. In the following description, for accuracy, the reference numerals used in FIG. 1 will be used to refer to the unillustrated pipe W1, so please refer to FIG. 1 as well as other parts as needed.

[0057] 2 and 3, the differential thickness pipe P1 includes a large diameter portion PLD, a small diameter portion PSD, and a first reduced outer diameter portion PdDO1. The large diameter portion PLD is formed at the base end (upper side in FIGS. 2 and 3) on the upstream side in the extrusion direction, which is the direction in which the mother tube W1 is forced into the die hole during the extrusion process. It is a cylindrical portion having a second outer diameter DO2 (DO2 = DO1) equal to the first outer diameter DO1 of the mother tube W1, a second inner diameter DI2 (DI2 ≦ DI1) that is a predetermined inner diameter equal to or smaller than the first inner diameter DI1 of the mother tube W1, and a predetermined second wall thickness T2. The second inner diameter DI1, which is the inner diameter of the differential thickness pipe P1, only needs to be equal to or smaller than the first inner diameter DI1, which is the inner diameter of the mother tube W1 (DI2 ≦ DI1). That is, the second inner diameter DI1, which is the inner diameter of the differential thickness pipe P1, may be equal to the first inner diameter DI1, which is the inner diameter of the mother pipe W1 (DI2=DI1), or may be less than the first inner diameter DI1 (DI2 <DI1)。

[0058] The small diameter portion PSD is formed on the tip side, i.e., downstream in the extrusion direction, and has a non-circular outer cross-sectional contour that is smaller than the circular outer cross-sectional contour of the large diameter portion PLD and the same second inner diameter DI2 as the large diameter portion PLD. The first outer diameter reducing portion PdDO1 is formed between the large diameter portion PLD and the small diameter portion PSD and has the same second inner diameter DI2 as the large diameter portion PLD. The first outer diameter reducing portion PdDO1 is formed between the large diameter portion PLD and the small diameter portion PSD and has the same second inner diameter DI2 as the large diameter portion PLD. The first outer diameter reducing portion PdDO1 connects the large diameter portion PLD and the small diameter portion PSD, and the outer diameter decreases at a constant rate from the second outer diameter DO2 as it approaches the small diameter portion PSD. That is, the inner contours of the cross sections of the large diameter portion PLD, the first outer diameter reducing portion PiDO1, and the small diameter portion PSD are all circular, with the second inner diameter DI2 corresponding to the third outer diameter DO3, which is the outer diameter of the large outer diameter portion PLDO of the mandrel M1.

[0059] The cross-sectional outer contour of the small diameter portion PSD of the variable thickness pipe P1 shown in Figures 2 and 3 is a regular octagon. Therefore, the width across flats WAF, which is the distance between two parallel opposing sides, is the minimum outer diameter of the cross-sectional outer contour, and the distance between two opposing corners on a diagonal line passing through the center of the cross-section is the maximum outer diameter of the cross-sectional outer contour. However, the cross-sectional outer contour of the small diameter portion PSD of the variable thickness pipe P1 is not limited to a regular octagon and can be selected from a wide variety of non-circular shapes, including, for example, other polygons or circles or ellipses in which a portion of the outer contour is cut out to have two parallel opposing sides, depending on the application of the variable thickness pipe P1.

[0060] 4A is a schematic cross-sectional view showing an example of the configuration of the mandrel and sleeve provided in the first device. As shown in FIG. 4A, the mandrel M1 includes a large outer diameter portion PLDO, which is a cylindrical portion formed on the tip side and has a third outer diameter DO3 that corresponds to the second inner diameter DI2 of the differential thickness pipe P1.

[0061] The sleeve SL includes a pressing portion PP which is a cylindrical portion formed at the tip end and has a fourth outer diameter DO4 (DO4 = DO1) equal to the first outer diameter DO1 of the base tube W1 and the differential thickness pipe P1, a third inner diameter DI3 (DI3 = DI2) equal to the second inner diameter DI2, and a predetermined third thickness T3.

[0062] Fig. 4(b) is a schematic cross-sectional view showing an example of the configuration of a die provided in the first device. Fig. 5(a) is a schematic perspective view of the die divided by a plane including the central axis, and Fig. 5(b) is an enlarged view of the vicinity of the connection portion and transition portion of the die hole shown in Fig. 5(a). Note that in Fig. 4(b), the transition portion formed between the large inner diameter portion and the connection portion is omitted.

[0063] As illustrated in Figures 4(b) and 5, the die hole DH1 formed in the die D1 includes a large inner diameter portion PLDI, a small inner diameter portion PSDI, a connection portion PC, and a transition portion PT. The large inner diameter portion PLDI is formed at the base end and defines a cylindrical space having a fourth inner diameter DI4 that corresponds to the first outer diameter DO1 of the mother tube W1 and the differential thickness pipe P1. The small inner diameter portion PSDI is formed at the tip end and defines a cylindrical space having a non-circular cross section that corresponds to the outer contour of the non-circular cross section of the small diameter portion PSD of the differential thickness pipe P1. The connection portion PC is formed between the large inner diameter portion PLDI and the small inner diameter portion PSDI and defines a portion whose inner diameter decreases at a constant rate from the fourth inner diameter DI4 as it approaches the small inner diameter portion PSDI. The transition portion PT is a portion formed between the large inner diameter portion PLDI and the connection portion PC, and whose inner surface in a cross section taken along a plane including the central axis AX of the die hole DH1 is a curve that continuously connects the inner surface of the large inner diameter portion PLDI and the inner surface of the connection portion PC.

[0064] Although not shown in Figures 4 and 5, the mandrel M1, sleeve SL, and die D1 naturally have the necessary components to be connected to and driven by a drive mechanism provided in the extrusion molding device (not shown).

[0065] Furthermore, in the die hole DH1, the boundary between the small inner diameter portion PSDI and the connecting portion PC is defined by a continuum line where the inner peripheral surface of the small inner diameter portion PSDI intersects with the inner peripheral surface of the connecting portion PC. In other words, the boundary between the small inner diameter portion PSDI and the connecting portion PC is defined by a continuum line where the side surface of a column having a noncircular cross section, which is the shape of the internal space of the small inner diameter portion PSDI, intersects with the side surface of a substantially truncated cone, which is the shape of the internal space of the connecting portion PC. In yet other words, the boundary between the small inner diameter portion PSDI and the connecting portion PC is defined by a continuum line where an imaginary column obtained by extending the inner peripheral surface of the small inner diameter portion PSDI intersects with an imaginary cone obtained by extending the inner peripheral surface of the connecting portion PC.

[0066] Therefore, as illustrated in Figure 5, the boundary line in the die hole DH1 is a curve in which the base end of the inner surface of the small inner diameter portion PSDI, which forms the vertex of the small diameter portion (near the vertex of the polygon that is the cross-sectional shape of the small diameter portion), is located closer to the base end (upper side in Figure 5) than the base end of the inner surface of the small inner diameter portion PSDI, which forms the flat portion of the small diameter portion (near the midpoint of the side of the polygon that is the cross-sectional shape of the small diameter portion).

[0067] On the other hand, as illustrated in Figure 5, the boundary line between the connection portion PC and the transition portion PT, which are formed between the large inner diameter portion PLDI and the small inner diameter portion PSDI of the die hole DH1 and whose inner diameter decreases at a constant rate as it approaches from the large inner diameter portion PLDI to the small inner diameter portion PSDI, and the boundary line between the transition portion PT and the large inner diameter portion PLDI are intersections (i.e., circles) between a plane perpendicular to the central axis AX of the die hole DH1 and the inner surfaces of these portions.

[0068] FIG. 6 is a schematic diagram illustrating the difference between the apex and the flat portion in the path along which material flows from the large inner diameter portion PLDI of the die D1 provided in the first apparatus illustrated in FIG. 5 through the transition portion PT and the connection portion PC to the small inner diameter portion PSDI. Specifically, similar to FIG. 25 referred to in the description of the trial, FIG. 6 is a schematic diagram illustrating the shapes of the inner surfaces of the large inner diameter portion PLDI, the transition portion PT, the connection portion PC, and the small inner diameter portion PSDI that constitute the die hole DH1 in a cross section taken along a plane including the central axis AX of the die D1. Note that, similar to FIG. 25, in FIG. 6, for the purpose of facilitating understanding of the following description, a line representing the inner surface of the connection portion PC and a line representing the inner surface of the small inner diameter portion PSDI are drawn as if they were directly adjacent to each other at a predetermined angle. However, in reality, a portion that smoothly connects the inner surfaces of the connection portion PC and the small inner diameter portion PSDI can be provided between the connection portion PC and the small inner diameter portion PSDI, similar to the transition portion PT described above.

[0069] As illustrated in Figure 6, in the die hole DH1 of the die D1 provided in the first device, the position and timing at which the leading end of the cylindrical material is extruded from the large inner diameter portion PLDI to the transition portion PT and the connection portion PC and begins to reduce in diameter are the same for both the flat portion (solid line) and the apex portion (dashed line). On the other hand, the position at which the leading end of the cylindrical material reaches the small inner diameter portion PSDI and ends in diameter reduction is located radially outward (to the right as viewed in Figure 6) and closer to the base end in the axial direction (extrusion direction) than the flat portion (solid line). However, in the die hole DH1 of the die D1 provided in the first device, the path and period from the position and timing at which the leading end of the cylindrical material begins to reduce in diameter to the position and timing at which the apex portion reaches the small inner diameter portion PSDI and ends in diameter reduction are the same for both the flat portion (solid line) and the apex portion (dashed line). Therefore, in the die hole DH1 of the die D1 provided in the first device, the difference between the peak and flat portions in the path along which the material flows from the large inner diameter portion PLDI of the die hole DH1 through the transition portion PT and the connection portion PC to the small inner diameter portion PSDI is smaller than in the prior art and the aforementioned trial. Therefore, the difference between the peak (solid line) and flat portions (dashed line) in the path and the length of the period from the point when the diameter reduction starts to the point when the material reaches the same position in the extrusion direction within the small inner diameter portion PSDI is also smaller than in the prior art and the aforementioned trial.

[0070] As a result, in the die hole DH1 of the die D1 provided in the first device, the circumferential distortion of the portion that forms the small diameter portion through diameter reduction and elongation is reduced. That is, the first device reduces the possibility of problems such as cracking and / or deformation occurring in the resulting small diameter portion, and can reduce problems such as cracking and / or deformation occurring in the resulting small diameter portion.

[0071] In addition, the first device is configured so that during the extrusion process, the tip end of the mandrel is located more distal than the base end of the small inner diameter portion of the die hole, at least during the period from a first point onwards, which is the point at which the material constituting the mother tube begins to flow distally of the tip end of the large inner diameter portion of the die hole due to the extrusion process.

[0072] Figure 7 is a schematic cross-sectional view showing an example of changes in the shape of a blank tube resulting from the execution of extrusion processing by the first device. In order to simplify the drawing, the reference numerals assigned to the respective parts of the respective components shown in Figures 1 to 5 are omitted in Figure 7, and only the reference numerals of the respective components are shown. However, in the following explanation of Figure 7, the reference numerals shown in Figures 1 to 5 will be used for accuracy, so please refer to Figures 1 to 5 as necessary.

[0073] As shown in Figure 7(a) , in the extrusion process performed by the first device, first, a mother tube W1 is inserted into the large inner diameter portion PDIL of the die hole DH1 formed in the die D1, and then a mandrel M1 is inserted into the mother tube W1. The pressing portion PP of the sleeve SL abuts against the base end of the mother tube W1. When the pressing portion PP of the sleeve SL is further driven forward from this state, the material constituting the mother tube W1 begins to flow forward beyond the tip end of the large inner diameter portion PLDI of the die hole DH1. Therefore, Figure 7(a) can be said to illustrate the positions of the mandrel M1, sleeve SL, die D1, and mother tube W1, as well as the shape of the mother tube W1, at a first point in time.

[0074] As the extrusion process progresses, the pressing portion PP of the sleeve SL is driven further toward the tip, and the material constituting the blank tube W1 is extruded from the large inner diameter portion PLDI of the die hole DH1 through the transition portion PT and the connecting portion PC to the small inner diameter portion PSDI, thereby forming the small diameter portion PSD of the differential thickness pipe P1. At this time, the outer contour of the cross section of the small diameter portion PSD must be defined by the inner circumferential surface of the small inner diameter portion PSDI of the die hole DH1, and the inner contour of the cross section of the small diameter portion PSD must be defined by the outer circumferential surface of the large outer diameter portion PLDO of the mandrel M1.

[0075] As described above, in the first device, during the extrusion process, at least during the period from the first point in time onwards, which is the point at which the material constituting the blank tube W1 begins to flow further towards the tip side than the tip side end of the large inner diameter portion PLDI of the die hole DH1 due to the extrusion process, the tip side end of the mandrel M1 is configured to be at a position further towards the tip side than the base side end of the small inner diameter portion PSDI of the die hole DH1 (the position indicated by the dashed line L1 in Figure 7).

[0076] Therefore, when the material for the mother tube W1 is extruded from the large inner diameter portion PLDI of the die hole DH1 through the transition portion PT and the connecting portion PC to the small inner diameter portion PSDI, the material for the mother tube W1 is extruded toward the tip side of the base end of the small inner diameter portion PSDI of the die hole DH1 through the gap between the outer peripheral surface of the large outer diameter portion PLDO of the mandrel M1 and the base end of the small inner diameter portion PSDI of the die hole DH1. That is, the outer contour of the cross section of the small diameter portion PSD of the differential thickness pipe P1 is noncircular because it is determined by the inner peripheral surface of the small inner diameter portion PSDI of the die hole DH1, and the inner contour of the cross section of the small diameter portion PSD is circular because it is determined by the outer peripheral surface of the large outer diameter portion PLDO of the mandrel M1.

[0077] Thereafter, by stopping the advancement of the pressing portion PP of the sleeve SL and terminating the extrusion process before the tip end of the pressing portion PP of the sleeve SL reaches the tip end of the large inner diameter portion PSDI of the die hole DH1, it is possible to form the large diameter portion PLD at the base end of the differential thickness pipe P1, as illustrated in (b) of Figure 7. In addition, the above-mentioned first outer diameter reduced portion PdDO1 can be formed between the large diameter portion PLD and the small diameter portion PSD of the differential thickness pipe P1.

[0078] The differential thickness pipe P1 formed as described above has a configuration as shown in Figures 2 and 3. That is, the outer contours of the cross sections of the large diameter portion PLD and first outer diameter reduced portion PdDO1 formed proximal to the dashed line L1 (upstream in the extrusion direction indicated by the open arrow) in Figure 7 are circular, while the outer contour of the cross section of the small diameter portion PSD formed distal to the dashed line L1 (downstream in the extrusion direction indicated by the filled arrow) is non-circular. Furthermore, the inner contours of the cross sections of the large diameter portion PLD, first outer diameter reduced portion PiDO1, and small diameter portion PSD are all circular, with a second inner diameter DI2 corresponding to the third outer diameter DO3, which is the outer diameter of the large outer diameter portion PLDO of the mandrel M1.

[0079] In the example shown in FIG. 7, the mandrel M1 and the sleeve SL are driven by a drive mechanism (not shown) while maintaining a constant positional relationship between them. When the mandrel M1 and the sleeve SL are driven integrally in this manner, as described above, the mandrel M1 and the sleeve SL may be integrally formed. Furthermore, as long as the first device is configured so that the distal end of the mandrel M1 is located further forward than the proximal end of the small inner diameter portion PSDI of the die hole DH1 during the period from the first time point onward, the mandrel M1 and the sleeve SL may be driven separately. Furthermore, the distal end of the mandrel M1 may be fixed at a position further forward than the proximal end of the small inner diameter portion PSDI of the die hole DH1, and only the sleeve SL may be driven.

[0080] 7, the second inner diameter DI1 of the differential thickness pipe P1 is equal to the first inner diameter DI1 of the mother pipe W1 (DI2=DI1). However, as described above, the second inner diameter DI1 of the differential thickness pipe P1 may be equal to or less than the first inner diameter DI1 of the mother pipe W1 (DI2≦DI1). In other words, the second inner diameter DI1 of the differential thickness pipe P1 may be equal to the first inner diameter DI1 of the mother pipe W1 (DI2=DI1), or may be less than the first inner diameter DI1 (DI2 <DI1)。

[0081] FIG. 8 is a schematic cross-sectional view showing an example of a change in the shape of a blank tube during extrusion in a first method for forming a differentially-thickened pipe P1 having a second inner diameter DI2 that is smaller than a first inner diameter DI1 that is the inner diameter of the blank tube W1. The third outer diameter DO3, which is the outer diameter of the large outer diameter portion PLDO of the mandrel M1 illustrated in FIG. 8, is smaller than the third outer diameter DO3, which is the outer diameter of the large outer diameter portion PLDO of the mandrel M1 illustrated in FIG. 7. In this case, in the extrusion process performed by the first device, as illustrated in FIG. 8, the first inner diameter DI1, which is the inner diameter of the blank tube W1, is reduced to a second inner diameter DI1 that is smaller than the first inner diameter DI1 in the differentially-thickened pipe P1. As with FIG. 7, in FIG. 8, for simplicity, the reference numerals assigned to the components shown in FIGS. 1 to 5 are omitted, and only the reference numerals of the components are shown. Therefore, please refer to FIGS. 1 to 5 as necessary.

[0082] <effect> As described above, in the first device, the inner surface of the connecting portion formed between the large inner diameter portion, which is formed on the base end side of the die hole and has a circular cross section, and the small inner diameter portion, which is formed on the tip end side of the die hole and has a non-circular cross section, is composed of a substantially truncated conical side surface coaxial with the large inner diameter portion and the small inner diameter portion, and the boundary line between the small inner diameter portion and the connecting portion is the intersecting line between the cylindrical side surface having a non-circular cross section, which is the shape of the internal space of the small inner diameter portion, and the substantially truncated conical side surface, which is the shape of the internal space of the connecting portion.

[0083] With the above-described configuration, the first device, in an extrusion process for forming a differential-thickness pipe, involves inserting a mandrel into or through a blank tube, which is a component having a predetermined shape, and forcing the blank tube into a die hole using a drive mechanism to elongate and reduce the diameter of the tip end of the blank tube, thereby forming a small-diameter portion, it is possible to reduce problems such as cracking and / or deformation that may occur between the apex and flat portion of the small-diameter portion, while making it possible to form a non-circular outer contour shape of the cross section of the small-diameter portion.

[0084] Second Embodiment An extrusion molding device for a pipe with different thicknesses according to a second embodiment of the present invention (hereinafter, sometimes referred to as a "second device") will be described below with reference to the drawings.

[0085] In the trial described above with reference to Figure 24, when a cylindrical mother tube with a mandrel inserted was forced into a die hole and extruded to form a long small-diameter portion having a non-circular cross-sectional outer contour at the tip end of the mother tube, problems such as cracks and / or deformation sometimes occurred between the apex and flat portions of the small-diameter portion. One cause of this problem is thought to be the difference in wall thickness between the apex and flat portions.

[0086] In an extrusion molding apparatus for a differential thickness pipe according to the present invention (apparatus of the present invention), such as the first apparatus described above, the inner circumferential surface of a connecting portion formed between a large inner diameter portion, which is a portion formed near the base end of a die hole and has a circular cross section, and a small inner diameter portion, which is a portion formed near the tip end of the die hole and has a noncircular cross section, is configured with a substantially frusto-conical side surface coaxial with the large inner diameter portion and the small inner diameter portion, and the boundary between the small inner diameter portion and the connecting portion is a line of intersection between the cylindrical side surface having a noncircular cross section, which represents the shape of the internal space of the small inner diameter portion, and the substantially frusto-conical side surface, which represents the shape of the internal space of the connecting portion. With this configuration, in an extrusion process in which a mandrel is inserted into or passed through a mother tube, which is a member having a predetermined shape, and the mother tube is forced into the die hole by a drive mechanism to elongate and reduce the diameter of the mother tube at its tip end to form a small diameter portion, problems such as cracking and / or deformation occurring between the apex and flat portion of the small diameter portion can be reduced, while the outer contour of the cross section of the small diameter portion can be made noncircular.

[0087] However, from the viewpoint of more reliably reducing problems such as cracks and / or deformations that occur between the apex of the small diameter portion and the flat portion, it is preferable to suitably adjust the wall thickness at the apex and flat portion of the small diameter portion of the differential thickness pipe to be formed in the device of the present invention as well.

[0088] <composition> Therefore, the second device is the above-mentioned first device, which is an extrusion molding device for a differential thickness pipe, configured so that the ratio of the maximum value to the minimum value of the wall thickness of the small diameter portion of the differential thickness pipe molded by the second device is 2 or less.

[0089] Fig. 9 is a schematic cross-sectional view illustrating the minimum and maximum wall thicknesses of a small diameter portion of a pipe with a different thickness. Fig. 9(a) is a cross-sectional view of a small diameter portion having a regular octagonal cross-sectional outer contour, such as those illustrated in Figs. 2 and 3 referred to in the description of the first device above. Fig. 9(b) is a cross-sectional view of a small diameter portion having a substantially circular cross-sectional outer contour, with a portion of the outer contour cut out so as to have two parallel opposing sides.

[0090] In the example shown in Figure 9(a), the minimum thickness (Tmin) of the cross section of the small diameter portion taken along a plane that includes the central axis of the small diameter portion and passes through the midpoints of the sides of the outer contour of the regular octagonal cross section is the minimum wall thickness, and the maximum thickness (Tmax) of the cross section of the small diameter portion taken along a plane that includes the central axis of the small diameter portion and passes through the vertices of the regular octagonal cross section is the maximum wall thickness. In contrast, in the example shown in Figure 9(b), the minimum thickness (Tmin) of the cross section of the small diameter portion taken along a plane that includes the central axis of the small diameter portion and is perpendicular to the two opposing parallel sides is the minimum wall thickness, and the maximum thickness (Tmax) of the cross section of the small diameter portion taken along a plane that includes the central axis of the small diameter portion and passes through the corners (vertices) located near the ends of the two opposing parallel sides is the maximum wall thickness. To form such a small diameter portion, the gap between the inner peripheral surface of the small inner diameter portion of the die hole and the outer peripheral surface of the large outer diameter portion of the mandrel must correspond to the above-described shape of the small diameter portion of the differential thickness pipe.

[0091] In other words, the second apparatus is the above-mentioned first apparatus, which is an extrusion molding apparatus for a differential thickness pipe, in which the wall thickness ratio, which is the ratio of the maximum wall thickness to the minimum wall thickness, is greater than 1 and not greater than 2. The minimum wall thickness is half the value obtained by subtracting the third outer diameter, which is the outer diameter of the large outer diameter portion of the mandrel, from the minimum value of the inner diameter of the small inner diameter portion of the die hole, and the maximum wall thickness is half the value obtained by subtracting the third outer diameter, which is the outer diameter of the large outer diameter portion of the mandrel, from the maximum value of the inner diameter of the small inner diameter portion of the die hole.

[0092] As a result of further research, the inventors have found that in order to more reliably obtain the effect of reducing problems such as cracks and / or deformation that occur between the apex of the small diameter portion and the flat portion, it is preferable that the thickness ratio be 2 or less. Therefore, the thickness ratio of the second device is 2 or less.

[0093] On the other hand, the wall thickness ratio becomes 1 only when the cross section of the small inner diameter portion of the die hole is circular so that the outer contour of the cross section of the small diameter portion of the differential thickness pipe is circular. Therefore, the wall thickness ratio in the second device, which is an extrusion molding device for a differential thickness pipe whose outer contour of the cross section of the small diameter portion is non-circular, will never be 1. Furthermore, as mentioned above, the wall thickness ratio is the ratio of the maximum wall thickness to the minimum wall thickness, so the wall thickness ratio in the second device will never be less than 1.

[0094] From the above, in the second device, the thickness ratio, which is the ratio of the maximum thickness to the minimum thickness, is greater than 1 and not greater than 2. Preferably, the thickness ratio is greater than 1 and not greater than 1.8. More preferably, the thickness ratio is greater than 1 and not greater than 1.6.

[0095] <effect> As described above, in the second device, the thickness ratio, which is the ratio of the maximum wall thickness to the minimum wall thickness, is greater than 1 and not greater than 2. As a result, the ratio of the maximum wall thickness to the minimum value of the wall thickness of the small diameter portion of the differential thickness pipe formed by the second device is not greater than 2. As a result, the second device can reduce problems such as cracks and / or deformation that occur between the apex and flat portions of the small diameter portion due to the difference in wall thickness between the apex and flat portions of the small diameter portion.

[0096] Third Embodiment Hereinafter, an extrusion molding device for a pipe with different thicknesses according to a third embodiment of the present invention (hereinafter, sometimes referred to as "third device") will be described with reference to the drawings.

[0097] In the first device described above, the inner peripheral surface of the connecting portion connecting the large inner diameter portion and the small inner diameter portion of the die hole is formed by a substantially truncated cone side surface, and the boundary between the small inner diameter portion and the connecting portion is defined by a continuum line where the inner peripheral surfaces of the small inner diameter portion and the connecting portion intersect. This reduces problems such as cracks and / or deformation that may occur between the apex and flat portion of the small diameter portion, while allowing the outer contour of the cross section of the small diameter portion to have a non-circular shape.

[0098] Furthermore, in the second device described above, the thickness ratio, which is the ratio of the maximum thickness to the minimum thickness, is greater than 1 and not greater than 2. As a result, the ratio of the maximum thickness to the minimum value of the wall thickness of the small diameter portion of the differential thickness pipe formed by the second device is not greater than 2. As a result, the second device can reduce problems such as cracks and / or deformation that occur between the apex and flat portions of the small diameter portion due to the difference in wall thickness between the apex and flat portions of the small diameter portion.

[0099] However, as a result of further research, the inventors discovered that by keeping the die half angle, which is the angle between the inner surface of the connection portion of the die hole and the central axis, within a predetermined range, problems such as cracks and / or deformation that occur between the apex of the small diameter portion and the flat portion can be more reliably reduced.

[0100] <composition> Therefore, the third device is the above-mentioned first or second device, and is an extrusion molding device for a differential thickness pipe in which, in a cross section taken along a plane including the central axis of the die hole, the die half angle, which is the angle between the inner surface of the connection part of the die hole and the central axis, is 45° or more and less than 70°.

[0101] Fig. 10 is a schematic diagram illustrating the configuration of a die provided in the third device. Fig. 10(a) is a plan view of the die D3 provided in the third device when observed from the base end side (the side of the large inside diameter portion PLDI), and Fig. 10(b) is a cross-sectional view of the die D3 and the mother tube W1 along a plane including the line segment UU and the central axis AX depicted in Fig. 10(a).

[0102] As illustrated in Figure 10, in the die hole DH3 formed in the die D3 provided in the third device, in a cross section taken along a plane including the central axis AX of the die hole DH3, the die half angle θ, which is the angle formed between the inner surface of the connection portion PC of the die hole DH3 and the central axis AX, is greater than or equal to 45° and less than 70° (45°≦θ≦70°).

[0103] If the die half angle θ is less than 45°, the material flows excessively smoothly from the large inner diameter portion PLDI through the transition portion PT and the connection portion PC toward the small inner diameter portion PSDI, resulting in a large difference in the material flow velocity between the vertices (vertices) of the polygonal cross section of the small inner diameter portion PSDI and the midpoints of the sides (flat portions), increasing the risk of problems such as cracking and / or deformation occurring between the vertices and the flat portions.

[0104] On the other hand, if the die half angle θ is greater than 70°, the flow of material from the large inner diameter portion PLDI through the transition portion PT and the connection portion PC to the small inner diameter portion PSDI will suddenly change direction at the connection portion PC, and the processing load will become excessive when the tip side of the mother tube W1 is stretched and reduced in diameter by extrusion processing to form the small diameter portion.

[0105] <effect> Therefore, as described above, in the die hole formed in the die provided in the third device, the die half angle, which is the angle between the inner peripheral surface of the connecting portion of the die hole and the central axis, is set to be equal to or greater than 45° and less than 70°. As a result, the third device can more reliably reduce problems such as cracking and / or deformation that occur between the apex and flat portion of the small diameter portion while reducing the increase in processing load when forming the small diameter portion.

[0106] Fourth Embodiment An extrusion molding device for a pipe with different thicknesses according to a fourth embodiment of the present invention (hereinafter, sometimes referred to as "fourth device") will be described below with reference to the drawings.

[0107] As described above, in the extrusion molding apparatus for producing a pipe with different thicknesses according to the present invention (the apparatus of the present invention), including the first to third apparatuses, a blank tube with a mandrel inserted or passing through it is forced from the large inner diameter portion of the die hole through the transition section and the connection section to the small inner diameter portion, thereby stretching and reducing the diameter of the tip side of the blank tube to form the small diameter portion.

[0108] The reduced diameter portion formed by the extrusion process described above is pressed from the base end side by the material being further forced into the small inner diameter portion, and progresses toward the tip end side of the small inner diameter portion. As the extrusion process progresses, the contact area between the outer circumferential surface of the formed small diameter portion and the inner circumferential surface of the small inner diameter portion of the die hole increases, and the processing load increases due to friction between them.

[0109] <composition> Therefore, the fourth device is an extrusion molding device for a differential thickness pipe that is any one of the above-mentioned first to third devices, in which a land is formed in a predetermined range at the end on the base end side of the small inner diameter section of the die hole, and the outer contour of the cross section of the columnar space defined by the portion distal to the land is larger than the outer contour of the cross section of the columnar space defined by the land. In other words, in the die hole formed in the die provided in the fourth device, the outer contour of the cross section of the portion distal to the land in the small inner diameter section is larger than the outer contour of the cross section of the portion where the land is formed.

[0110] 11 is a schematic diagram illustrating the shapes of the inner peripheral surfaces of the large inner diameter portion, transition portion, connecting portion, and small inner diameter portion that constitute the die hole in a cross section taken along a plane including the central axis of the die provided in the fourth apparatus. As with FIG. 6 referred to in the description of the first apparatus described above, in FIG. 11, for the purpose of facilitating understanding of the following description, a line representing the inner peripheral surface of the connecting portion PC and a line representing the inner peripheral surface of the small inner diameter portion PSDI are drawn as if they were directly adjacent to each other at a predetermined angle. However, in reality, a portion that smoothly connects the inner peripheral surfaces of the connecting portion PC and the small inner diameter portion PSDI can be provided between the connecting portion PC and the small inner diameter portion PSDI, similar to the transition portion PT described above.

[0111] 11, the cross section passing near the vertices (vertices) of the polygonal cross section of the small inner diameter portion PSDI is indicated by dashed lines, and the cross section passing near the midpoints of the sides of the polygon (flat portions) is indicated by solid lines. Furthermore, the direction of material flow along the inner surface of the connecting portion PC is indicated by hollow arrows. At both the vertices and flat portions, the path of material flowing along the inner surface of the die hole from the large inner diameter portion PLDI through the transition portion PT (not shown) and the connecting portion PC to the small inner diameter portion PSDI is the same as the path illustrated in FIG. 6.

[0112] However, in the die hole DH4 formed in the die provided in the fourth apparatus, as described above, a land is formed in a predetermined range at the base end of the small inside diameter portion PSDI. In the example shown in Fig. 11, a land La is formed on the apex portion and a land Le is formed on the flat portion in the range from the base end of the small inside diameter portion PSDI to a predetermined position Pe on the tip side. The outer contour of the cross section of the columnar space defined by the portion on the tip side (lower side in Fig. 11) of these lands La and Le is larger than the outer contour of the cross section of the columnar space defined by these lands La and Le.

[0113] <effect> As described above, in the fourth device, a land is formed in a predetermined range at the base end of the small diameter portion of the die hole, and the outer contour of the cross section of the columnar space defined by the portion distal to the land is larger than the outer contour of the cross section of the columnar space defined by the land. Therefore, the fourth device can reduce the increase in processing load caused by the increase in contact friction between the outer surface of the small diameter portion and the inner surface of the small diameter portion of the die hole as the extrusion process progresses.

[0114] Fifth Embodiment Hereinafter, an extrusion molding device for a pipe with different thicknesses according to a fifth embodiment of the present invention (hereinafter, sometimes referred to as "fifth device") will be described with reference to the drawings.

[0115] In the above explanation of the first to fourth devices, a small diameter portion is formed at the tip end of the differential thickness pipe, and the inner contours of the cross sections of the large diameter portion, the first outer diameter reducing portion and the small diameter portion are consistently circular with a second inner diameter corresponding to the third outer diameter, which is the outer diameter of the large outer diameter portion of the mandrel.

[0116] However, by using a mandrel that further has a cylindrical portion with a small outer diameter at the tip end, such as the mandrel used in the extrusion molding method disclosed in Patent Document 1 (Japanese Patent No. 6933762), it is possible to form a thick-walled portion with a smaller inner diameter and a larger wall thickness at the tip end of the differential thickness pipe molded by the device of the present invention, i.e., closer to the tip than the small diameter portion.

[0117] <composition> That is, the fifth device is any of the first to fourth devices described above, and is an extrusion molding device for a differential thickness pipe, equipped with a mandrel having a cylindrical portion at the tip end thereof, the cylindrical portion having an outer diameter smaller than the large outer diameter portion, as described above.

[0118] Figure 12 is a schematic diagram showing an example of the configuration of a differential thickness pipe formed by extrusion processing performed by the fifth device in the extrusion processing step. Figure 12(a) is a side view of the differential thickness pipe P2, and the inner circumferential surface that defines the internal space of the differential thickness pipe P2, which would normally be invisible, is depicted by a dashed line. Figure 12(b) and (c) are arrow views Ar and Af of the differential thickness pipe P2, respectively.

[0119] Also, Fig. 13 is a schematic cross-sectional view of multiple locations of the differential thickness pipe P2 illustrated in Fig. 12. Fig. 13(a) is a cross-sectional view of the differential thickness pipe P2 taken along a plane including the central axis AX shown in Fig. 12(a), the line segment VV shown in Fig. 12(b), and the line segment WW shown in Fig. 12(c). Furthermore, Fig. 13(b), (c), and (d) are cross-sectional views of the differential thickness pipe P2 taken along a plane including the line segments XX, YY, and ZZ drawn in Fig. 13(a) and perpendicular to the central axis AX.

[0120] 12 and 13, the differential-thickness pipe P2 includes, in addition to the large-diameter portion PLD, small-diameter portion PSD, and first reduced-outer-diameter portion PdDO1, a thick-walled portion PLT and a reduced-inner-diameter portion PdDI. The thick-walled portion PLT is formed at the tip end and has the same noncircular cross-sectional outer contour as the small-diameter portion PSD and a fifth inner diameter DI5 that is a predetermined inner diameter smaller than the second inner diameter DI2 that is the inner diameter of the small-diameter portion PSD. The reduced-inner-diameter portion PdDI is formed between the small-diameter portion PSD and the thick-walled portion PLT and has an inner diameter that decreases from the second inner diameter DI2 to the fifth inner diameter DI5 as it approaches the thick-walled portion PLT.

[0121] FIG. 14 is a schematic cross-sectional view showing an example of the configuration of the mandrel and sleeve provided in the fifth apparatus. As illustrated in FIG. 14, the mandrel M2 provided in the fifth apparatus further includes a small outer diameter portion PSDO and a second reduced outer diameter portion PdDO2 located distally of the large outer diameter portion PLDO. The small outer diameter portion PSDO is a cylindrical portion formed distally of the large outer diameter portion PLDO and having a fifth outer diameter DO5, which corresponds to the fifth inner diameter DI5, which is the inner diameter of the thick-walled portion PLT of the differential thickness pipe, and is smaller than the third outer diameter DO3, which is the outer diameter of the large outer diameter portion PLDO. The second reduced outer diameter portion PdDO2 is formed between the large outer diameter portion PLDO and the small outer diameter portion PSDO, and its outer diameter decreases from the third outer diameter DO3 to the fifth outer diameter DO5 as it approaches the small outer diameter portion PSDO from the large outer diameter portion PLDO. The sleeve SL has a configuration similar to that of the sleeve SL provided in the first to fourth apparatuses described above.

[0122] In order to form a variable-thickness pipe P2 having the configuration illustrated in Figures 12 and 13 using a fifth device equipped with a mandrel M2 having the configuration illustrated in Figure 14, during the period in which the material constituting the mother tube W1 is extruded from the large inner diameter portion PLDI of the die hole through the transition portion PT and the connection portion PC to the small inner diameter portion PSDI and the thick-walled portion PLT is formed at the tip end of the variable-thickness pipe P2 by performing the extrusion process, the outer contour of the cross-section of the thick-walled portion PLT must be defined by the inner surface of the small inner diameter portion PSDI of the die hole, and the inner contour of the cross-section of the thick-walled portion PLT must be defined by the outer surface of the small outer diameter portion PSDO of the mandrel M2. Furthermore, thereafter, during the period in which the small diameter portion PSD of the differential thickness pipe P2 is being formed, the outer contour of the cross section of the small diameter portion PSD must be defined by the inner surface of the small inner diameter portion PSDI of the die hole DH1, and the inner contour of the cross section of the small diameter portion PSD must be defined by the outer surface of the large outer diameter portion PLDO of the mandrel M2.

[0123] Therefore, in the fifth device, during the extrusion process, the positional relationship between the mandrel M2 and the sleeve SL and the die is controlled so that the base end of the small outer diameter section PSDO of the mandrel M2 is located more proximal than the base end of the small inner diameter section PSDI of the die hole at a second point in time when the material constituting the mother tube W1 begins to flow toward the tip of the base end of the small inner diameter section PSDI of the die hole.Furthermore, in the fifth device, the positional relationship between the mandrel M2 and the sleeve SL and the die is controlled so that the tip end of the large outer diameter section PLDO of the mandrel M2 is located more distal than the base end of the small inner diameter section PSDI of the die hole from a third point in time onward, which is a predetermined point in time after the second point in time.

[0124] Note that the phrase "the positional relationship between the mandrel M2 and the sleeve SL and the die is controlled" does not mean that the present invention is limited to a mode in which each component is driven individually and linked to satisfy the above requirements. In other words, as long as the above requirements can be satisfied, the configuration, arrangement, and driving method of the mandrel M2, the sleeve SL, and the die are not particularly limited. For example, the shapes of the mandrel M2, the sleeve SL, and / or the die may be designed so that the above requirements can be satisfied even if the mandrel M2 and the sleeve SL are driven integrally and moved closer to the die.

[0125] FIG. 15 is a schematic cross-sectional view showing an example of the change in shape of a blank tube resulting from extrusion processing by the fifth apparatus. Note that, for simplicity, the reference numerals assigned to the components shown in FIGS. 1, 5, and 12 to 14 are omitted in FIG. 15 , and only the reference numerals of the components are shown. However, in the following description of FIG. 15 , the reference numerals shown in FIGS. 1, 5, and 12 to 14 will be used for accuracy, so please refer to FIGS. 1, 5, and 12 to 14 as needed. The following description will exemplify a case in which the fifth apparatus is equipped with the die D1 provided in the first apparatus described above. However, the die provided in the fifth apparatus may be any of the dies provided in the second to fourth apparatuses described above.

[0126] As shown in Figure 15(a) , in the extrusion process performed by the fifth apparatus, similar to the extrusion process performed by the first apparatus illustrated in Figure 7 , first, a mother tube W1 is inserted into the large inner diameter portion PDIL of the die hole DH1 formed in the die D1, and then a mandrel M2 is inserted into the mother tube W1. The pressing portion PP of the sleeve SL abuts against the base end of the mother tube W1. When the pressing portion PP of the sleeve SL is further driven forward from this state, the material constituting the mother tube W1 begins to flow forward beyond the tip end of the large inner diameter portion PLDI of the die hole DH1. Therefore, Figure 15(a) can be said to illustrate the positions of the mandrel M2, sleeve SL, die D1, and mother tube W1, as well as the shape of the mother tube W1, at a first point in time.

[0127] As the extrusion process progresses, the pressing portion PP of the sleeve SL is driven further toward the tip, and the material constituting the mother tube W1 begins to be extruded from the large inner diameter portion PLDI of the die hole DH1 through the transition portion PT and the connecting portion PC toward the small inner diameter portion PSDI. That is, the second point described above is reached. At this time, the small outer diameter portion PSDO of the mandrel M2 is located at a position radially opposite the base end of the small inner diameter portion PSDI (the position indicated by the dashed line L1 in FIG. 15). Therefore, the material constituting the mother tube W1 is extruded toward the tip of the base end of the small inner diameter portion PSDI of the die hole DH1 through the gap between the outer peripheral surface of the small outer diameter portion PSDO of the mandrel M2 and the base end of the small inner diameter portion PSDI of the die hole DH1.

[0128] That is, the outer contour of the cross section of the thick-walled portion PLT of the variable-thickness pipe P2 is determined by the inner peripheral surface of the small inner diameter portion PSDI of the die hole DH1 and is non-circular, while the inner contour of the cross section of the thick-walled portion PLT is determined by the outer peripheral surface of the small outer diameter portion PSDO of the mandrel M2 and is circular, smaller than the inner contour of the cross section of the small diameter portion PSD. In this way, the thick-walled portion PLT with a smaller inner diameter and a larger wall thickness can be formed at the tip end of the variable-thickness pipe P2 formed by the fifth device, i.e., closer to the tip than the small diameter portion PSD.

[0129] Subsequently, as the extrusion process progresses, the mandrel M2 and the pressing portion PP of the sleeve SL are driven further toward the tip, and the tip end of the large outer diameter portion PLDO of the mandrel M2 reaches a position (dashed line L1) where it faces the base end of the small inner diameter portion PSDI in the radial direction. That is, the above-mentioned third point in time is reached. From the third point in time onwards, the material constituting the mother tube W1 is extruded forward beyond the tip end of the small inner diameter portion PSDI of the die hole DH1 through the gap between the outer peripheral surface of the large outer diameter portion PLDO of the mandrel M2 and the base end of the small inner diameter portion PSDI of the die hole DH1. That is, the outer contour of the cross section of the small diameter portion PSD of the differential thickness pipe P2 is determined by the inner surface of the small inner diameter portion PSDI of the die hole DH1 and is non-circular, while the inner contour of the cross section of the small diameter portion PSD is determined by the outer surface of the large outer diameter portion PLDO of the mandrel M2 and is circular, larger than the inner contour of the cross section of the thick-walled portion PLT.

[0130] Thereafter, similar to the extrusion process executed by the first device illustrated in Fig. 7, the advancement of the pressing portion PP of the sleeve SL is stopped and the extrusion process is terminated before the tip end of the pressing portion PP of the sleeve SL reaches the tip end of the large inner diameter portion PSDI of the die hole DH1, thereby forming the large diameter portion PLD at the base end of the differential thickness pipe P2, as illustrated in Fig. 15(b). In addition, the above-mentioned first outer diameter reduced portion PdDO1 can be formed between the large diameter portion PLD and the small diameter portion PSD of the differential thickness pipe P1.

[0131] The differential thickness pipe P2 formed as described above has a configuration as shown in Figures 12 and 13. Specifically, the outer contours of the cross sections of the large diameter portion PLD and the first outer diameter reduced portion PdDO1 formed proximal to the dashed line L1 (upstream in the extrusion direction indicated by the open arrow) are circular, while the outer contour of the cross section of the small diameter portion PSD formed distal to the dashed line L1 (downstream in the extrusion direction indicated by the filled arrow) is non-circular. Furthermore, the inner contours of the cross sections of the large diameter portion PLD, the first outer diameter reduced portion PiDO1, and the small diameter portion PSD are all circular, with a second inner diameter DI2 corresponding to the third outer diameter DO3, which is the outer diameter of the large outer diameter portion PLDO of the mandrel M2. The inner contour of the cross section of the thick-walled portion PLT is circular with a fifth inner diameter DI5 corresponding to the fifth outer diameter DO5, which is the outer diameter of the small outer diameter portion PSDO of the mandrel M2.

[0132] As described in the description of the first device, in the fifth device as well, the configuration and drive control of the mandrel M2 and sleeve SL are not particularly limited as long as the above-mentioned requirements are met.

[0133] <effect> As described above, the mandrel provided in the fifth device further includes a small outer diameter portion and a second outer diameter reduced portion distal to the large outer diameter portion. Furthermore, in the fifth device, during the extrusion process, the positional relationship between the mandrel, the sleeve, and the die is controlled so that the base end of the small outer diameter portion of the mandrel is located proximal to the base end of the small inner diameter portion of the die hole at a second point in time, when the material for the blank tube begins to flow distally from the base end of the small inner diameter portion of the die hole. Additionally, in the fifth device, the positional relationship between the mandrel, the sleeve, and the die is controlled so that the tip end of the large outer diameter portion of the mandrel is located distal to the base end of the small inner diameter portion of the die hole from a third point in time onward, which is a predetermined point in time after the second point in time. As a result, a thick-wall portion with a smaller inner diameter and a greater wall thickness can be formed at the distal end of the differential-wall pipe formed by the fifth device, i.e., distal to the small diameter portion.

[0134] Sixth Embodiment An extrusion molding device for a pipe with different thicknesses according to a sixth embodiment of the present invention (hereinafter, sometimes referred to as "sixth device") will be described below with reference to the drawings.

[0135] As described above, in the extrusion molding devices for differential thickness pipes according to the present invention (devices of the present invention), including the first to fifth devices, the inner surface of the connecting portion formed between the large inner diameter portion, which is formed at the base end of the die hole and has a circular cross section, and the small inner diameter portion, which is formed at the tip end of the die hole and has a non-circular cross section, is composed of approximately truncated conical side surfaces coaxial with the large inner diameter portion and the small inner diameter portion, and the boundary line between the small inner diameter portion and the connecting portion is the intersecting line between the cylindrical side surface having a non-circular cross section, which is the shape of the internal space of the small inner diameter portion, and the approximately truncated conical side surface, which is the shape of the internal space of the connecting portion.

[0136] With the above-described configuration, the apparatus of the present invention, in an extrusion process for forming a differential-thickness pipe, involves inserting a mandrel into or through a blank tube, which is a component having a predetermined shape, and forcing the blank tube into a die hole using a drive mechanism to elongate and reduce the diameter of the front end of the blank tube, thereby forming a small-diameter portion, it is possible to reduce problems such as cracking and / or deformation that may occur between the apex and flat portion of the small-diameter portion, while also making it possible to form the outer contour shape of the cross section of the small-diameter portion into a non-circular shape.

[0137] However, depending on the material constituting the mother tube and / or the processing conditions (e.g., the rate of diameter reduction from the large inner diameter portion to the small inner diameter portion of the die hole, the cross-sectional shape of the small inner diameter portion and / or the speed at which the mother tube is forced into the die hole), the dimensional accuracy of the small diameter portion of the differential thickness pipe formed by forcing the mother tube into the small inner diameter portion of the die hole may be deteriorated due to elastic deformation after extrusion.

[0138] For example, Fig. 16 is a schematic diagram showing an example of elastic deformation occurring in a small diameter section whose cross-sectional outer contour shape is to be a regular octagon (similar to the differential thickness pipe P1 illustrated in Fig. 3). Fig. 16(a) is a schematic cross-sectional view of the small diameter section corresponding to Fig. 3(c), and Fig. 16(b) is a schematic enlarged view of the part surrounded by the thick solid line in Fig. 16(a). In Fig. 16(b), the outer contour of the cross-section of the small diameter section having the desired regular octagonal shape is depicted by a solid line, and the outer contour that has become a shape different from the desired shape due to elastic deformation is depicted by a dashed line.

[0139] As shown by the solid line in Figure 16(b), the outer contour of the cross section of the small-diameter portion of the differential-thickness pipe should ideally be a regular octagon when the blank tube is pressed into the small-diameter portion of the die hole. However, due to the elastic deformation described above, the outer contour of the cross section of the small-diameter portion of the differential-thickness pipe may expand radially outward, resulting in a shape that differs from the intended shape, as shown by the dashed line in Figure 16(b). In other words, the dimensional accuracy of the small-diameter portion of the differential-thickness pipe formed by the method of the present invention may be deteriorated by elastic deformation after extrusion.

[0140] <composition> Therefore, as a result of extensive research, the present inventors have found that the deformation can be reduced by providing a recess, such as an axially extending groove, at a location where the unintended deformation of the outer contour of the cross section of the small-diameter portion of the differential thickness pipe due to the elastic deformation described above occurs. One way to provide such a recess in the outer contour of the cross section of the small-diameter portion of the differential thickness pipe is to form a corresponding protrusion on the inner circumferential surface of the small-diameter portion of the die hole.

[0141] That is, the sixth device is any one of the first to fifth devices described above, and is an extrusion molding device for a differential thickness pipe, in which one or more convex portions extending in the extrusion direction are formed on the inner peripheral surface of the small inner diameter portion of the die hole. As a result, groove-like concave portions extending in the extrusion direction are formed on the outer peripheral surface of the small diameter portion of the differential thickness pipe at positions corresponding to the convex portions formed on the inner peripheral surface of the small inner diameter portion of the die hole. As a result, unintended deformation due to elastic deformation of the portion of the outer peripheral surface of the small diameter portion of the differential thickness pipe adjacent in the circumferential direction to the concave portions is reduced.

[0142] As described above, the small inner diameter portion of the die hole defines a cylindrical space having a noncircular cross section corresponding to the noncircular outer contour of the small diameter portion of the differential thickness pipe, and the cross section of the space defined by the small inner diameter portion of the die hole can have a polygonal shape such as a regular octagon. In this case, it goes without saying that the outer contour of the cross section of the small diameter portion of the differential thickness pipe formed by an extrusion molding apparatus equipped with a die having such a small inner diameter portion will have a polygonal shape such as a regular octagon.

[0143] When the cross-sectional shape of the space defined by the small inner diameter portion of the die hole and the outer contour of the small diameter portion of the pipe with different thicknesses are polygonal as described above, unintended deformation of the outer contour of the small diameter portion of the pipe with different thicknesses due to elastic deformation occurs significantly along the edges of the polygonal cross-sectional shape, as described above with reference to Figure 16(b). Therefore, from the viewpoint of effectively reducing such deformation, it is preferable to provide the convex portions at positions corresponding to the edges rather than the vertices of the polygonal cross-sectional shape of the small inner diameter portion of the die hole. In this case, the concave portions formed on the outer surface of the small diameter portion of the pipe with different thicknesses are formed at positions corresponding to the edges of the polygonal cross-sectional shape of the small diameter portion.

[0144] That is, in a preferred embodiment of the sixth device, the cross-sectional shape of the space defined by the small inner diameter portion of the die hole and the cross-sectional outer contour shape of the small diameter portion of the differential thickness pipe are polygonal, and the convex portion formed on the inner surface of the small inner diameter portion of the die hole and the concave portion formed on the outer surface of the small diameter portion of the differential thickness pipe are located at positions corresponding to the sides of the polygon.

[0145] More preferably, the convex portion is provided near the midpoint of a side of the polygon that defines the cross section of the small inner diameter portion of the die hole. In this case, the concave portion formed on the outer peripheral surface of the small diameter portion of the differential thickness pipe is formed near the midpoint of a side of the polygon that defines the outer contour of the cross section of the small diameter portion. That is, in a more preferred embodiment of the sixth device, the convex portion formed on the inner peripheral surface of the small inner diameter portion of the die hole and the concave portion formed on the outer peripheral surface of the small diameter portion of the differential thickness pipe are located closer to the midpoint of the side of the polygon than to the end.

[0146] Fig. 17 is a schematic diagram showing an example of the configuration of a die provided in the sixth apparatus. Fig. 17(a) is a schematic cross-sectional view of a die D6 provided in the sixth apparatus along a plane including the central axis AX, and Fig. 17(b) is a cross-sectional view of the small inner diameter portion PSDI of the die D6 along a plane including a line segment VV drawn in Fig. 17(a) and perpendicular to the central axis AX. Fig. 17(c) is a schematic enlarged view of the area surrounded by the thick solid line in Fig. 17(b). One or more (eight in the example shown in Fig. 17) protrusions B extending in the extrusion direction (i.e., the direction of the central axis AX) are formed on the inner peripheral surface of the small inner diameter portion PSDI of the die hole DH6 formed in the die D6 illustrated in Fig. 17, as illustrated in Figs. 17(b) and 17(c).

[0147] The height (amount of protrusion from the inner peripheral surface of the small inner diameter portion PSDI), width (size in the circumferential direction), and number of the convex portions B are determined so that the amount of deformation of the outer contour of the cross section of the small diameter portion of the differential thickness pipe due to elastic deformation does not deviate from the dimensional accuracy required for the intended use of the differential thickness pipe. Specifically, the height, width, and number of the convex portions B can be determined as appropriate based on, for example, the results of preliminary experiments in which the height, width, and number of the convex portions B are variously changed and / or simulation analysis using the finite element method.

[0148] The extent of the extension of the protrusion B in the extrusion direction is also determined so that the deformation of the outer contour of the cross section of the small-diameter portion of the differential-thickness pipe due to elastic deformation does not deviate from the dimensional accuracy required for the intended use of the differential-thickness pipe. For example, the protrusion B may be formed only in a predetermined area at the base-end end of the inner circumferential surface of the small inner diameter portion PSDI of the die hole DH6, or may be formed over the entire inner circumferential surface of the small inner diameter portion PSDI of the die hole DH6 in the extrusion direction. When a land is formed at the base-end end of the inner circumferential surface of the small inner diameter portion PSDI of the die hole DH6 as described above, the protrusion B may be formed only in a predetermined area at the base-end end of the land, or may be formed over the entire land in the extrusion direction.

[0149] Fig. 18 is a schematic diagram showing an example of the configuration of a small diameter portion of a pipe with a different thickness formed by extrusion processing performed by the sixth device. Fig. 18(a) is a schematic cross-sectional view of the small diameter portion corresponding to Fig. 16(a), and Fig. 18(b) is a schematic enlarged view of the part surrounded by the thick solid line in Fig. 18(a). In Fig. 18(b), the outer contour of the cross section of the small diameter portion, which has the desired regular octagonal shape, is depicted by a solid line, and the outer contour that has become different from the desired shape due to elastic deformation is depicted by a dashed line.

[0150] As shown by the solid lines in Fig. 18(b), groove-like recesses G extending in the extrusion direction (i.e., the direction of the central axis AX) are formed on the outer peripheral surface of the small diameter portion of the differential thickness pipe formed by the sixth apparatus at positions corresponding to the protrusions B formed on the inner peripheral surface of the small inner diameter portion PSDI of the die hole DH6 (illustrated in Fig. 17). As a result, if the protrusions B had not been formed on the inner peripheral surface of the small inner diameter portion PSDI of the die hole DH6, the recesses G divide the edges connecting adjacent vertices of a regular octagon in the circumferential direction.

[0151] As a result, even if deformation of the outer contour of the cross section of the small diameter portion PSDI of the differential thickness pipe occurs due to elastic deformation, as depicted by the dashed lines in Figure 18(b), relatively small deformation occurs individually in the portions of the sides divided by the recesses G and in each of the recesses G, compared to the case where no recesses G are formed as illustrated in Figure 16. In other words, the sixth device can reduce deterioration in the dimensional accuracy of the small diameter portion PSD of the differential thickness pipe due to elastic deformation after extrusion.

[0152] Furthermore, the small inner diameter portion PSDI formed in the die D6 illustrated in Figure 17 satisfies the requirements of the small inner diameter portion PSDI formed in the die D6 provided in the sixth device related to the more preferred embodiment described above, and the small diameter portion PSD of the differential thickness pipe illustrated in Figure 18 satisfies the requirements of the small diameter portion PSD of the differential thickness pipe formed by the sixth device related to the more preferred embodiment described above.

[0153] That is, the cross-sectional shape of the space defined by the small inner diameter portion PSDI of the die hole DH6 illustrated in Fig. 17 and the cross-sectional outer contour shape of the small diameter portion PSD of the variable thickness pipe illustrated in Fig. 18 are polygonal. Furthermore, the convex portion B formed on the inner surface of the small inner diameter portion PSDI of the die hole DH6 illustrated in Fig. 17 and the concave portion G formed on the outer surface of the small diameter portion PSD of the variable thickness pipe illustrated in Fig. 18 are located at positions corresponding to the sides of the polygon. Furthermore, the convex portion B formed on the inner surface of the small inner diameter portion PSDI of the die hole DH6 illustrated in Fig. 17 and the concave portion G formed on the outer surface of the small diameter portion PSD of the variable thickness pipe illustrated in Fig. 18 are located at positions closer to the midpoints of the sides of the polygon (i.e., near the midpoints) than the ends of the sides.

[0154] 19A and 19B are photographs illustrating the appearance of a pipe formed by the sixth apparatus equipped with the die D6 illustrated in FIG. 17, respectively. The small diameter portion PSD of the pipe P6 illustrated in FIG. 19A has axially extending recesses G near the midpoints of each side of the outer contour of the regular octagonal cross section, as illustrated in FIG. 18. The small diameter portion PSD of the pipe P1′ illustrated in FIG. 19B has axially extending recesses G near the midpoints of each side of the outer contour of the regular octagonal cross section, as illustrated in FIG. 16. As a result, the degree of deterioration in the dimensional accuracy of the outer contour of the cross section of the small diameter portion PSD of the pipe P6 due to elastic deformation after extrusion is smaller than that of the pipe P1′.

[0155] However, the configuration of the small inner diameter portion PSDI formed in the die D6 of the sixth apparatus and the configuration of the small diameter portion PSD of the differential thickness pipe formed by the sixth apparatus are not limited to the configurations exemplified in Figures 17 and 18. That is, when the cross-sectional shape of the space defined by the small inner diameter portion PSDI of the die hole DH6 and the outer contour shape of the cross-section of the small diameter portion PSD of the differential thickness pipe are polygonal, the convex portion B formed on the inner surface of the small inner diameter portion PSDI of the die hole DH6 and the concave portion G formed on the outer surface of the small diameter portion PSD of the differential thickness pipe do not necessarily have to be located near the midpoints of the sides of the polygon, as long as they are located at the sides of the polygon. Furthermore, the cross-sectional shape of the space defined by the small inner diameter portion PSDI of the die hole DH6 and the outer contour shape of the cross-section of the small diameter portion PSD of the differential thickness pipe do not necessarily have to be polygonal, as long as they are non-circular.

[0156] In the sixth apparatus, as described above, recesses G are formed in the outer contour of the cross section of the small diameter portion PSDI of the variable thickness pipe at positions corresponding to the protrusions B formed on the inner peripheral surface of the small inner diameter portion PSDI of the die hole DH6. Therefore, the shape of the outer contour of the cross section of the small diameter portion PSDI of the variable thickness pipe formed by the sixth apparatus can be considered to have low dimensional accuracy at the positions where the recesses G are formed, when taken as the original shape when the protrusions B are not formed on the inner peripheral surface of the small inner diameter portion PSDI of the die hole DH6. From this perspective, it is preferable that the positions where the protrusions B are formed on the inner peripheral surface of the small inner diameter portion PSDI of the die hole DH6 are positions that do not require high dimensional accuracy in the shape of the outer contour of the cross section of the small diameter portion PSDI of the variable thickness pipe formed by the sixth apparatus.

[0157] <effect> As described above, in the sixth device, one or more convex portions extending in the extrusion direction are formed on the inner peripheral surface of the small-diameter portion of the die hole, and groove-like concave portions extending in the extrusion direction are formed on the outer peripheral surface of the small-diameter portion of the pipe with different thicknesses at positions corresponding to the convex portions formed on the inner peripheral surface of the small-diameter portion of the die hole. Therefore, the outer contour of the cross section of the small-diameter portion of the pipe with different thicknesses formed by the sixth device is divided by the concave portions. As a result, even if the outer contour of the cross section of the small-diameter portion of the pipe with different thicknesses is deformed due to elastic deformation, the deformation of each of the divided portions of the outer contour and each of the concave portions is relatively small compared to when no concave portions are formed. In other words, the sixth device can reduce deterioration in the dimensional accuracy of the small-diameter portion of the pipe with different thicknesses due to elastic deformation after extrusion.

[0158] Seventh Embodiment As stated at the beginning of this specification, the present invention relates not only to an extrusion molding apparatus for a differential thickness pipe having a long, small-diameter portion whose cross-sectional outer contour is non-circular, but also to a method for extrusion molding such a differential thickness pipe.

[0159] Hereinafter, a method for extrusion molding a pipe with different thicknesses according to a seventh embodiment of the present invention (hereinafter, sometimes referred to as "first method") will be described with reference to the drawings.

[0160] <composition> The first method is a method for extrusion molding a differential thickness pipe, including the above-mentioned extrusion step performed in the above-mentioned first apparatus. That is, the first method is a method for extrusion molding a differential thickness pipe, including an extrusion step in an extrusion molding apparatus including a mandrel having a predetermined shape, a sleeve which is a cylindrical member arranged coaxially with the mandrel, a die having a die hole which is a through hole having a predetermined shape, and a drive mechanism which drives at least the sleeve and the die to approach each other in the axial direction, in which, with the mandrel inserted into or passing through a blank tube having a predetermined shape, the blank tube is forced into the die hole by the drive mechanism to perform extrusion, thereby forming a differential thickness pipe having a predetermined shape.

[0161] Fig. 20 is a flowchart showing an example of the flow of each step included in the extrusion process performed in the first method. In the flowchart shown in Fig. 20, the extrusion process performed in the first method includes a first step and a second step.

[0162] The first step, performed in step S10, is to insert and set a combination of a mother tube having a predetermined shape, with the mandrel inserted or threaded through it, into the large inner diameter portion of a die hole formed in the die. The second step, performed in step S20, is to extrude the combination of the mother tube and mandrel set in the large inner diameter portion of the die hole in the first step by forcing it into the die hole with a drive mechanism, thereby forming a pipe with a predetermined thickness.

[0163] However, the flow of each step included in the extrusion processing step performed in the first method is not limited to the above, and is not particularly limited as long as it is possible to form a differential thickness pipe having a predetermined shape by inserting or passing a mandrel through a blank tube having a predetermined shape, forcing the blank tube into a die hole using a drive mechanism, and performing extrusion processing.

[0164] The mother tube is a cylindrical member having a first outer diameter which is a predetermined outer diameter, a first inner diameter which is a predetermined inner diameter, and a first wall thickness which is a predetermined wall thickness.

[0165] The differential thickness pipe includes a large diameter portion, a small diameter portion, and a first reduced outer diameter portion. The large diameter portion is a cylindrical portion formed at the base end, which is upstream in the extrusion direction, which is the direction in which the blank tube is forced into the die hole during the extrusion process, and has a second outer diameter equal to the first outer diameter, a second inner diameter that is a predetermined inner diameter equal to or smaller than the first inner diameter, and a predetermined second thickness. The small diameter portion is formed at the tip end, which is downstream in the extrusion direction, and has a non-circular cross-sectional outer contour that is smaller than the circular cross-sectional outer contour of the large diameter portion and the same second inner diameter as the large diameter portion. The first reduced outer diameter portion is formed between the large diameter portion and the small diameter portion, has the same second inner diameter as the large diameter portion, and is a portion that connects the large diameter portion and the small diameter portion, with the outer diameter decreasing at a constant rate from the second outer diameter as it approaches the small diameter portion.

[0166] The mandrel includes a large outer diameter portion that is a cylindrical portion formed on the tip side and has a third outer diameter that is an outer diameter corresponding to the second inner diameter.

[0167] The sleeve includes a pressing portion that is a cylindrical portion formed at the tip end and has a fourth outer diameter equal to the first outer diameter, a third inner diameter equal to the second inner diameter, and a third thickness that is a predetermined thickness.

[0168] The die hole includes a large inner diameter portion, a small inner diameter portion, a connecting portion, and a transition portion. The large inner diameter portion is formed on the base end side and defines a cylindrical space having a fourth inner diameter corresponding to the first outer diameter. The small inner diameter portion is formed on the tip end side and defines a cylindrical space having a non-circular cross section corresponding to the outer contour of the non-circular cross section of the small inner diameter portion. The connecting portion is formed between the large inner diameter portion and the small inner diameter portion and has an inner diameter that decreases at a constant rate from the fourth inner diameter as it approaches the small inner diameter portion. The transition portion is formed between the large inner diameter portion and the connecting portion and has an inner circumferential surface that is a curve that continuously connects the inner circumferential surface of the large inner diameter portion and the inner circumferential surface of the connecting portion in a cross section taken along a plane including the central axis of the die hole.

[0169] Furthermore, in the die hole, the boundary line between the small inner diameter portion and the connecting portion is formed by a line where the inner peripheral surface of the small inner diameter portion and the inner peripheral surface of the connecting portion intersect.

[0170] The configurations of the mother pipe, the differential thickness pipe, the mandrel, the sleeve, and the die hole have been described in detail in the above description of the first apparatus with reference to FIGS. 1 to 6, so a detailed description thereof will be omitted here.

[0171] In addition, in the extrusion process included in the first method, the tip end of the mandrel is configured to be located more distal than the base end of the small inner diameter portion of the die hole, at least during the period from a first point onwards, which is the point at which the material constituting the mother tube begins to flow distally of the tip end of the large inner diameter portion of the die hole due to the extrusion process.

[0172] The change in shape from a blank tube to a differential-thickness pipe during the extrusion process included in the first method, as well as the movements and positional relationships of the mandrel, sleeve, and die hole, have been described in detail above with reference to Figures 7 and 8 in the description of the first apparatus, so further description will be omitted here.

[0173] <effect> In the extrusion molding device (first device) for a differential thickness pipe used in the first method, as described above, the inner surface of the connecting portion formed between the large inner diameter portion, which is formed on the base end side of the die hole and has a circular cross section, and the small inner diameter portion, which is formed on the tip end side of the die hole and has a non-circular cross section, is composed of a substantially truncated conical side surface coaxial with the large inner diameter portion and the small inner diameter portion, and the boundary line between the small inner diameter portion and the connecting portion is the intersecting line between the cylindrical side surface having a non-circular cross section, which is the shape of the internal space of the small inner diameter portion, and the substantially truncated conical side surface, which is the shape of the internal space of the connecting portion.

[0174] According to a first method using a first apparatus having the above-described configuration, in an extrusion process for forming a differential-thickness pipe, in which a mandrel is inserted into or passed through a blank tube, which is a component having a predetermined shape, and a drive mechanism is used to force the blank tube into a die hole and elongate and reduce the diameter of the front end of the blank tube to form a small-diameter portion, the shape of the outer contour of the cross section of the small-diameter portion can be made noncircular while reducing problems such as cracks and / or deformation that occur between the apex and flat portion of the small-diameter portion.

[0175] Eighth Embodiment An extrusion molding method for a pipe with different thicknesses according to an eighth embodiment of the present invention (hereinafter, sometimes referred to as the "second method") will be described below.

[0176] <composition> The second method is a method for extrusion molding a pipe with different thicknesses, including the extrusion process described above, which is carried out in the second apparatus described above. That is, the extrusion molding apparatus for a pipe with different thicknesses (the second apparatus) used in the second method is the first apparatus described above, but in which the wall thickness ratio, which is the ratio of the maximum wall thickness to the minimum wall thickness, is greater than 1 and not greater than 2. The minimum wall thickness is half the value obtained by subtracting the third outer diameter, which is the outer diameter of the large outer diameter portion of the mandrel, from the minimum value of the inner diameter of the small inner diameter portion of the die hole, and the maximum wall thickness is half the value obtained by subtracting the third outer diameter, which is the outer diameter of the large outer diameter portion of the mandrel, from the maximum value of the inner diameter of the small inner diameter portion of the die hole.

[0177] The thickness ratio was described in detail with reference to FIG. 9 in the description of the second device, and so a detailed description thereof will be omitted here.

[0178] <effect> In the second device used in the second method, the wall thickness ratio, which is the ratio of the maximum wall thickness to the minimum wall thickness, is greater than 1 and not greater than 2. As a result, the ratio of the maximum wall thickness to the minimum value of the wall thickness of the small diameter portion of the differential thickness pipe formed by the second device is not greater than 2. As a result, the second method can reduce problems such as cracking and / or deformation that occur between the apex and flat portions of the small diameter portion due to the difference in wall thickness between the apex and flat portions of the small diameter portion.

[0179] Ninth Embodiment A method for extrusion molding a pipe with different thicknesses according to a ninth embodiment of the present invention (hereinafter, sometimes referred to as the "third method") will be described below.

[0180] <composition> The third method is a method for extrusion molding a pipe with different thicknesses, including the extrusion process described above, which is carried out in the third apparatus described above. That is, the extrusion molding apparatus for a pipe with different thicknesses (third apparatus) used in the third method is the first or second apparatus described above, in which the die half angle, which is the angle between the inner peripheral surface of the connecting part of the die hole and the central axis in a cross section taken along a plane including the central axis of the die hole, is 45° or more and less than 70°.

[0181] The die half angle has been described in detail with reference to FIG. 10 in the description of the third apparatus, and so a detailed description thereof will be omitted here.

[0182] <effect> In the die hole formed in the die provided in the third device used in the third method, the die half angle, which is the angle between the inner peripheral surface of the connecting portion of the die hole and the central axis, is set to be equal to or greater than 45° and less than 70°. As a result, according to the third method, it is possible to more reliably reduce problems such as cracking and / or deformation that occur between the apex and flat portion of the small diameter portion while reducing the increase in processing load when forming the small diameter portion.

[0183] Tenth Embodiment Hereinafter, a method for extrusion molding a pipe with different thicknesses according to a tenth embodiment of the present invention (hereinafter, sometimes referred to as the "fourth method") will be described with reference to the drawings.

[0184] <composition> A fourth method is a method for extrusion molding a pipe with different thicknesses, including the extrusion process performed by the fourth apparatus. Specifically, the extrusion molding apparatus for a pipe with different thicknesses (the fourth apparatus) used in the fourth method is any one of the first to third apparatuses, in which a land is formed in a predetermined range at the end of the base end of the small inner diameter section of the die hole, and the outer contour of the cross section of the columnar space defined by the portion distal to the land is larger than the outer contour of the cross section of the columnar space defined by the land. In other words, in the die hole formed in the die of the fourth apparatus, the outer contour of the cross section of the portion distal to the land in the small inner diameter section is larger than the outer contour of the cross section of the portion where the land is formed.

[0185] The configuration of the lands and the effects achieved by the lands have been described in detail with reference to FIG. 11 in the description of the fourth device, and so a detailed description thereof will be omitted here.

[0186] <effect> As described above, in the fourth device used in the fourth method, a land is formed in a predetermined range at the base end of the small diameter portion of the die hole, and the outer contour of the cross section of the columnar space defined by the portion distal to the land is larger than the outer contour of the cross section of the columnar space defined by the land. Therefore, the fourth method can reduce the increase in processing load caused by the increase in contact friction between the outer surface of the small diameter portion and the inner surface of the small diameter portion of the die hole as the extrusion process progresses.

[0187] Eleventh Embodiment Hereinafter, an extrusion molding method for a pipe with different thicknesses according to an eleventh embodiment of the present invention (hereinafter, sometimes referred to as the "fifth method") will be described with reference to the drawings.

[0188] <composition> The fifth method is a method for extrusion molding a pipe with different thicknesses, including the extrusion process described above performed in the fifth apparatus described above. That is, the extrusion molding apparatus for a pipe with different thicknesses (fifth apparatus) used in the fifth method is any one of the first to fourth apparatuses described above.

[0189] The differential thickness pipe formed by the fifth method further includes a thick-walled portion and an inner diameter reduced portion in addition to the large diameter portion, small diameter portion, and first outer diameter reduced portion. The thick-walled portion is formed at the end portion near the tip and has the same non-circular cross-sectional outer contour as the small diameter portion and a predetermined inner diameter of a fifth inner diameter that is smaller than the second inner diameter that is the inner diameter of the small diameter portion. The inner diameter reduced portion is formed between the small diameter portion and the thick-walled portion and has an inner diameter that decreases from the second inner diameter to the fifth inner diameter as it approaches the thick-walled portion from the small diameter portion.

[0190] The mandrel provided in the fifth device further includes a small outer diameter portion and a second reduced outer diameter portion located distally of the large outer diameter portion. The small outer diameter portion is a cylindrical portion formed distally of the large outer diameter portion and having a fifth outer diameter that corresponds to the fifth inner diameter, which is the inner diameter of the thick-walled portion of the differential thickness pipe, and is smaller than the third outer diameter, which is the outer diameter of the large outer diameter portion. The second reduced outer diameter portion is formed between the large outer diameter portion and the small outer diameter portion and has an outer diameter that decreases from the third outer diameter to the fifth outer diameter as it approaches the small outer diameter portion.

[0191] The configuration of the differential thickness pipe and mandrel has been described in detail in the description of the fifth device with reference to FIGS. 12 to 14, and therefore will not be described here.

[0192] Furthermore, in the fifth method, in the extrusion step, the positional relationship between the mandrel, sleeve, and die is controlled so that the base end of the small outer diameter portion of the mandrel is located more proximal than the base end of the small inner diameter portion of the die hole at a second point in time when the material for the blank tube begins to flow distally from the base end of the small inner diameter portion of the die hole during the extrusion process. In addition, the fifth device controls the positional relationship between the mandrel, sleeve, and die during a period from a third point in time onward, which is a predetermined point in time after the second point in time, so that the distal end of the large outer diameter portion PLDO of the mandrel M2 is located more distal than the base end of the small inner diameter portion PSDI of the die hole.

[0193] The change in shape from a blank tube to a differential-thickness pipe during the extrusion process included in the fifth method, as well as the movements and positional relationships of the mandrel, sleeve, and die hole, were described in detail above with reference to Figure 15 in the explanation of the fifth apparatus, so further explanation will be omitted here.

[0194] <effect> As described above, the mandrel provided in the fifth apparatus used in the fifth method further includes a small outer diameter portion and a second outer diameter reduced portion distal to the large outer diameter portion. Furthermore, in the fifth method, the positional relationship between the mandrel, sleeve, and die is controlled so that the base end of the small outer diameter portion of the mandrel is located proximal to the base end of the small inner diameter portion of the die hole at a second point in time, when the material for the blank tube begins to flow distally from the base end of the small inner diameter portion of the die hole. Additionally, in the fifth method, the positional relationship between the mandrel, sleeve, and die is controlled so that the tip end of the large outer diameter portion of the mandrel is located distal to the base end of the small inner diameter portion of the die hole from a third point in time onward, which is a predetermined point in time after the second point in time. As a result, a thick-wall portion with a smaller inner diameter and a larger wall thickness can be formed at the distal end of the differential thickness pipe formed by the fifth method, i.e., distal to the small diameter portion.

[0195] Twelfth Embodiment Hereinafter, a method for extrusion molding a pipe with different thicknesses according to a twelfth embodiment of the present invention (hereinafter, sometimes referred to as "sixth method") will be described with reference to the drawings.

[0196] <composition> The sixth method is a method for extrusion molding a pipe with different thicknesses, including the extrusion process described above, which is performed in the sixth apparatus described above. That is, the extrusion molding apparatus for a pipe with different thicknesses (sixth apparatus) used in the sixth method is any one of the first to fifth apparatuses described above, in which one or more convex portions extending in the extrusion direction are formed on the inner peripheral surface of the small inner diameter portion of the die hole. As a result, groove-shaped concave portions extending in the extrusion direction are formed on the outer peripheral surface of the small inner diameter portion of the pipe with different thicknesses at positions corresponding to the convex portions formed on the inner peripheral surface of the small inner diameter portion of the die hole.

[0197] In addition, in a sixth method according to a preferred embodiment, the cross-sectional shape of the space defined by the small inner diameter portion of the die hole and the cross-sectional outer contour shape of the small diameter portion of the differential thickness pipe are polygonal, and the convex portion formed on the inner surface of the small inner diameter portion of the die hole and the concave portion formed on the outer surface of the small diameter portion of the differential thickness pipe are located at positions corresponding to the sides of the polygon.

[0198] In a sixth method according to a more preferred embodiment, the convex portion is provided near the midpoint of a side of a polygon that defines the cross section of the small inner diameter portion of the die hole. In this case, the concave portion formed on the outer peripheral surface of the small diameter portion of the differential thickness pipe is formed near the midpoint of a side of the polygon that defines the outer contour of the cross section of the small diameter portion. That is, in the sixth method according to a more preferred embodiment, the convex portion formed on the inner peripheral surface of the small inner diameter portion of the die hole and the concave portion formed on the outer peripheral surface of the small diameter portion of the differential thickness pipe are located closer to the midpoint of the side of the polygon than to the end of the side.

[0199] The configuration of the above-mentioned convex and concave portions and the effects achieved by the convex and concave portions have been described in detail with reference to FIGS. 16 to 19 in the description of the sixth device, and therefore a description thereof will be omitted here.

[0200] <effect> As described above, in the sixth device used in the sixth method, one or more protrusions extending in the extrusion direction are formed on the inner peripheral surface of the small inner diameter portion of the die hole, and groove-like recesses extending in the extrusion direction are formed on the outer peripheral surface of the small diameter portion of the differential thickness pipe at positions corresponding to the protrusions formed on the inner peripheral surface of the small inner diameter portion of the die hole. As a result, the sixth method can reduce deterioration in the dimensional accuracy of the small diameter portion of the differential thickness pipe due to elastic deformation after extrusion. [Example]

[0201] Examples of the extrusion molding apparatus (apparatus of the present invention) and extrusion molding method (method of the present invention) for producing pipes with different thicknesses according to the present invention will be described below with reference to the drawings. However, the examples described below are merely illustrative and do not limit the scope of the present invention. In the following description, reference symbols used in the drawings referred to in the above-mentioned description of various embodiments of the present invention may also be used, so please refer to these drawings as necessary.

[0202] 21 and 22 are schematic perspective views illustrating the configuration of a differential thickness pipe formed from a cylindrical mother tube by an extrusion molding method using an extrusion molding apparatus for a differential thickness pipe according to an embodiment of the present invention. Note that both of Fig. 21 and Fig. 22 are perspective views of the differential thickness pipe observed from the tip side, which is the downstream side in the extrusion direction (indicated in the figures by a thick solid arrow).

[0203] 21 includes a large diameter portion PLD, which is a cylindrical (flange-shaped) portion formed at the base end, which is the upstream side in the extrusion direction, a small diameter portion PSD, which is a portion formed at the tip end, which is the downstream side in the extrusion direction, and which has a non-circular outer cross-sectional contour that is smaller than the circular outer cross-sectional contour of the large diameter portion PLD, and a first outer diameter reducing portion PdDO1, which is formed between the large diameter portion PLD and the small diameter portion PSD, and which connects the large diameter portion PLD and the small diameter portion PSD, and whose outer diameter decreases at a constant rate as it approaches the large diameter portion PLD toward the small diameter portion PSD. The cross section of the small diameter portion PSD of the small diameter portion PSD of the variable thickness pipe PWK1 has a substantially regular octagonal outer contour and a circular inner contour, as shown in FIG. 9(a), for example.

[0204] 22 also includes a large diameter portion PLD, which is a cylindrical portion formed at the end of the base end, which is upstream in the extrusion direction, a small diameter portion PSD, which is formed at the tip end, which is downstream in the extrusion direction, and which has a non-circular outer cross-sectional contour that is smaller than the circular outer cross-sectional contour of the large diameter portion PLD, and a first outer diameter reducing portion PdDO1, which is formed between the large diameter portion PLD and the small diameter portion PSD, and which connects the large diameter portion PLD and the small diameter portion PSD, and whose outer diameter decreases at a constant rate as it approaches the large diameter portion PLD toward the small diameter portion PSD. The cross section of the small diameter portion PSD of the variable thickness pipe PWK2 has a substantially circular outer contour, with a portion of the outer contour cut out to have two parallel, opposing sides, and a circular inner contour, as shown in FIG. 9(b), for example.

[0205] When attempting to form either of the differential thickness pipes PWK1 and PWK2 using extrusion processing according to conventional technology, problems such as cracks and / or deformation tend to occur between the vicinity of the corners (vertices) and the vicinity of the midpoints of the sides (flat surfaces) in the outer contour of the cross section of the small diameter portion.

[0206] However, as described above, in the device of the present invention, the inner peripheral surface of the connecting portion formed between the large inner diameter portion, which is formed at the base end of the die hole and has a circular cross section, and the small inner diameter portion, which is formed at the tip end of the die hole and has a non-circular cross section, is composed of a substantially truncated conical side surface that is coaxial with the large inner diameter portion and the small inner diameter portion, and the boundary line between the small inner diameter portion and the connecting portion is the intersecting line between the cylindrical side surface having a non-circular cross section, which is the shape of the internal space of the small inner diameter portion, and the substantially truncated conical side surface, which is the shape of the internal space of the connecting portion.

[0207] With the above-described configuration, the apparatus and method of the present invention can reduce the above-described problems in an extrusion process for forming a differential-thickness pipe including a long, small-diameter portion having a noncircular outer cross-sectional contour, by forcing a cylindrical mother tube, with a mandrel inserted into or passing through the mother tube, into a die hole using a drive mechanism and stretching and reducing the diameter of the front end of the mother tube to form a differential-thickness pipe.

[0208] Furthermore, as described above, by designing the cross-sectional shape of the small diameter portion so that the thickness ratio, which corresponds to the ratio of the maximum wall thickness to the minimum wall thickness in the small diameter portion, falls within a suitable range (specifically, greater than 1 and less than 2), or by designing the shape of the die so that the die half angle falls within a suitable range (specifically, greater than 45° and less than 70°), it is possible to more reliably reduce the above problems and form a differential thickness pipe including a long small diameter portion having a non-circular outer cross-sectional contour.

[0209] Furthermore, by using a mandrel that further includes a small outer diameter portion and a second outer diameter reducing portion further forward than the large outer diameter portion and by appropriately controlling the positional relationship between the mandrel, sleeve, and die during the extrusion process, it is possible to form a thick-walled portion with a smaller inner diameter and a larger wall thickness at the tip end of the differential thickness pipes PWK1 and PWK2, i.e., further forward than the small diameter portion PSD.

[0210] In the small diameter portion PSD of the differential thickness pipe PWK1 shown in Figure 21, groove-like recesses extending in the axial direction are formed near the midpoints of each side of the outer contour of the cross section having a regular octagonal shape. That is, the differential thickness pipe PWK1 is a differential thickness pipe formed by the sixth method using the sixth device described above. Therefore, in the differential thickness pipe PWK1, as described in the explanation of the sixth device and the sixth method described above, deterioration in the dimensional accuracy of the small diameter portion of the differential thickness pipe due to elastic deformation after extrusion is reduced.

[0211] In the differential thickness pipe PWK1 shown in Fig. 21, cylindrical portions having a smaller cross-sectional outer contour than the large diameter portion PLD are formed proximal to the large diameter portion PLD and distal to the small diameter portion PSD. In addition, in the differential thickness pipe PWK2 shown in Fig. 22, a cylindrical portion having a smaller cross-sectional outer contour than the small diameter portion PSD is formed distal to the small diameter portion PSD.

[0212] A differential thickness pipe having the above-described shape can be formed, for example, by carrying out the method of the present invention in the apparatus of the present invention using a blank pipe having a portion at the base end and / or tip end that has an outer cross-sectional contour smaller than the outer cross-sectional contour of the large diameter portion PLD and / or small diameter portion PSD of the differential thickness pipe to be formed.

[0213] As described above, the apparatus and method of the present invention enable reliable extrusion of a cylindrical mother tube, in which a mandrel is inserted into or passed through the mother tube, by forcing the mother tube into a die hole using a drive mechanism to elongate and reduce the diameter of the front end of the mother tube. This process reduces problems such as cracking and / or deformation that may occur between the apex and flat portion of the minor diameter portion, and thereby enables reliable formation of a differential-thickness pipe having a long minor diameter portion and a noncircular outer cross-sectional contour.

[0214] For the purpose of explaining the present invention, several embodiments and examples having specific configurations have been described above, sometimes with reference to the accompanying drawings. However, the scope of the present invention should not be construed as being limited to these exemplary embodiments and examples, and it goes without saying that appropriate modifications can be made within the scope of the claims and the matters described in the specification. [Explanation of symbols]

[0215] AX…Central axis, W1...Main pipe, DO1…1st outer diameter DI1...First inner diameter T1...First thickness P1, P1', P2, P6, PWK1, PWK2...Different thickness pipes PLD: Large diameter section DO2…2nd outer diameter DI2...Second inner diameter T2…Second wall thickness PSD…Small diameter section WAF: Width across flats G...Concave PdDO1...first reduced outer diameter part PLT…thick part PdDI…Reduced inner diameter part M1, M2... Mandrel PLDO…Large outer diameter part DO3…3rd outer diameter PSDO…Small outer diameter part DO5…5th outer diameter PdDO2…Second reduced outer diameter part SL...Sleeve PP...Pressing part DO4…4th outer diameter DI3...Third inner diameter T3…Third wall thickness D1, D3, D6...Dice DH1, DH3, DH4, DH6...Die holes PLDI…Large inner diameter part DI4...Fourth inner diameter PSDI…Small inner diameter part B...Convex part DI5...5th inner diameter PC…Connection PT...Transition section PdDI2…Second inner diameter reduced part La,Le…Land

Claims

1. a mandrel having a predetermined shape, a sleeve which is a cylindrical member arranged coaxially with the mandrel, a die having a die hole which is a through hole having a predetermined shape, and a drive mechanism which drives at least the sleeve and the die so as to approach each other in the axial direction; The extrusion process is a process of forming a pipe with a different thickness and a predetermined shape by forcing a mother tube having a predetermined shape into the die hole with the drive mechanism while the mandrel is inserted into or passed through the mother tube. An extrusion molding device for a pipe with different thicknesses, the blank tube is a cylindrical member having a first outer diameter that is a predetermined outer diameter, a first inner diameter that is a predetermined inner diameter, and a first wall thickness that is a predetermined wall thickness, the differential thickness pipe includes: a large diameter portion which is a cylindrical portion formed at an end portion on a base end side which is upstream in an extrusion direction, which is a direction in which the blank tube is forced into the die hole in the extrusion process, and which has a second outer diameter equal to the first outer diameter, a second inner diameter which is a predetermined inner diameter equal to or smaller than the first inner diameter, and a second thickness which is a predetermined thickness; a small diameter portion which is formed at a tip end side which is downstream in the extrusion direction, and which has a non-circular outer cross-sectional contour which is smaller than the circular outer cross-sectional contour of the large diameter portion and the second inner diameter which is the same as the large diameter portion; and a first outer diameter reducing portion which is formed between the large diameter portion and the small diameter portion, has the same second inner diameter as the large diameter portion, and whose outer diameter decreases from the second outer diameter at a constant rate as it approaches the small diameter portion from the large diameter portion to the small diameter portion, and which connects the large diameter portion and the small diameter portion. the mandrel includes a large outer diameter portion that is a cylindrical portion formed on the tip side and has a third outer diameter that is an outer diameter corresponding to the second inner diameter, the sleeve includes a pressing portion that is a cylindrical portion formed at an end portion on the tip side and that has a fourth outer diameter equal to the first outer diameter, a third inner diameter equal to the second inner diameter, and a third thickness that is a predetermined thickness, the die hole includes: a large inner diameter portion formed on the base end side and defining a cylindrical space having a fourth inner diameter corresponding to the first outer diameter; a small inner diameter portion formed on the tip end side and defining a cylindrical space having a noncircular cross section corresponding to the outer contour of the noncircular cross section of the small diameter portion; a connecting portion formed between the large inner diameter portion and the small inner diameter portion, the inner diameter of which decreases from the fourth inner diameter at a constant rate as the diameter approaches the small inner diameter portion; and a transition portion formed between the large inner diameter portion and the connecting portion, the inner circumferential surface of which, in a cross section taken along a plane including the central axis of the die hole, is a curve that continuously connects the inner circumferential surface of the large inner diameter portion and the inner circumferential surface of the connecting portion, In the die hole, a boundary line between the small inner diameter portion and the connecting portion is formed by a line of intersection between an inner peripheral surface of the small inner diameter portion and an inner peripheral surface of the connecting portion, In the extrusion process, the distal end of the mandrel is located at a position distal to the base end of the small inner diameter portion of the die hole at least during a period from a first time point onward, which is a time point at which the material constituting the mother tube begins to flow distally beyond the distal end of the large inner diameter portion of the die hole due to the extrusion. Extrusion molding equipment for pipes with different thicknesses.

2. The extrusion molding device for a pipe with different thicknesses according to claim 1, a wall thickness ratio, which is the ratio of a minimum wall thickness, which is half of a value obtained by subtracting the third outer diameter of the mandrel from the minimum value of the inner diameter of the small inner diameter portion of the die hole, to a maximum wall thickness, which is half of a value obtained by subtracting the third outer diameter of the mandrel from the maximum value of the inner diameter of the small inner diameter portion of the die hole, is greater than 1 and not greater than 2; Extrusion molding equipment for pipes with different thicknesses.

3. The extrusion molding device for a pipe with different thicknesses according to claim 1 or 2, a die half angle, which is the angle between the inner peripheral surface of the connection portion of the die hole and the central axis in a cross section taken along a plane including the central axis of the die hole, is 45° or more and less than 70°; Extrusion molding equipment for pipes with different thicknesses.

4. The extrusion molding device for a pipe with different thicknesses according to claim 1 or 2, a land is formed in a predetermined range at an end portion on the base end side of the small inner diameter portion of the die hole, and an outer contour of a cross section of a columnar space defined by a portion of the die hole closer to the tip end than the land is larger than an outer contour of a cross section of a columnar space defined by the land; Extrusion molding equipment for pipes with different thicknesses.

5. The extrusion molding device for a pipe with different thicknesses according to claim 1 or 2, The differential thickness pipe further includes a thick-walled portion formed at the end portion on the tip side, the thick-walled portion having the same non-circular cross-sectional outer contour as the small-diameter portion and a fifth inner diameter that is a predetermined inner diameter smaller than the second inner diameter, and an inner diameter decreasing portion formed between the small-diameter portion and the thick-walled portion, the inner diameter decreasing from the second inner diameter to the fifth inner diameter as the small-diameter portion approaches the thick-walled portion, the mandrel further includes: a small outer diameter portion that is a cylindrical portion formed on the tip side of the large outer diameter portion and has a fifth outer diameter that corresponds to the fifth inner diameter and is a predetermined outer diameter smaller than the third outer diameter; and a second outer diameter decreasing portion that is formed between the large outer diameter portion and the small outer diameter portion and whose outer diameter decreases from the third outer diameter to the fifth outer diameter as the large outer diameter portion approaches the small outer diameter portion, In the extrusion process, the positional relationship between the mandrel, the sleeve, and the die is controlled so that at a second point in time, which is a point in time when the material constituting the blank tube begins to flow from the base end of the small inner diameter portion of the die hole toward the tip end, the base end of the small outer diameter portion of the mandrel is located closer to the base end than the base end of the small inner diameter portion of the die hole, and from a third point in time onward, which is a predetermined point in time after the second point in time, the tip end of the large outer diameter portion of the mandrel is located closer to the tip end than the base end of the small inner diameter portion of the die hole. Extrusion molding equipment for pipes with different thicknesses.

6. The extrusion molding device for a pipe with different thicknesses according to claim 1 or 2, one or more protrusions extending in the extrusion direction are formed on the inner circumferential surface of the small inner diameter portion of the die hole, a groove-shaped recess extending in the extrusion direction is formed on the outer peripheral surface of the small diameter portion of the differential thickness pipe at a position corresponding to the protrusion formed on the inner peripheral surface of the small inner diameter portion of the die hole; Extrusion molding equipment for pipes with different thicknesses.

7. The extrusion molding device for a pipe with different thicknesses according to claim 6, the cross-sectional shape of the space defined by the small inner diameter portion of the die hole and the cross-sectional outer contour shape of the small diameter portion of the differential thickness pipe are polygonal, the convex portion formed on the inner peripheral surface of the small inner diameter portion of the die hole and the concave portion formed on the outer peripheral surface of the small diameter portion of the differential thickness pipe are located at positions corresponding to the sides of the polygon. Extrusion molding equipment for pipes with different thicknesses.

8. The extrusion molding device for a pipe with different thicknesses according to claim 7, the convex portion formed on the inner peripheral surface of the small inner diameter portion of the die hole and the concave portion formed on the outer peripheral surface of the small diameter portion of the differential thickness pipe are located at positions closer to midpoints of the sides of the polygon than to the ends of the sides, Extrusion molding equipment for pipes with different thicknesses.

9. 1. A method for extrusion molding a differential thickness pipe, comprising: an extrusion molding apparatus including a mandrel having a predetermined shape; a sleeve which is a cylindrical member disposed coaxially with the mandrel; a die having a die hole which is a through hole having a predetermined shape; and a drive mechanism which drives at least the sleeve and the die to approach each other in the axial direction; and, in a state in which the mandrel is inserted into or passed through a blank tube having a predetermined shape, the drive mechanism is used to push the blank tube into the die hole to perform an extrusion process, thereby forming a differential thickness pipe having a predetermined shape, the blank tube is a cylindrical member having a first outer diameter that is a predetermined outer diameter, a first inner diameter that is a predetermined inner diameter, and a first wall thickness that is a predetermined wall thickness, the differential thickness pipe includes: a large diameter portion which is a cylindrical portion formed at an end portion on a base end side which is upstream in an extrusion direction, which is a direction in which the blank tube is forced into the die hole in the extrusion process, and which has a second outer diameter equal to the first outer diameter, a second inner diameter which is a predetermined inner diameter equal to or smaller than the first inner diameter, and a second thickness which is a predetermined thickness; a small diameter portion which is formed at a tip end side which is downstream in the extrusion direction, and which has a non-circular outer cross-sectional contour which is smaller than the circular outer cross-sectional contour of the large diameter portion and the second inner diameter which is the same as the large diameter portion; and a first outer diameter reducing portion which is formed between the large diameter portion and the small diameter portion, has the same second inner diameter as the large diameter portion, and whose outer diameter decreases from the second outer diameter at a constant rate as it approaches the small diameter portion from the large diameter portion to the small diameter portion, and which connects the large diameter portion and the small diameter portion. the mandrel includes a large outer diameter portion that is a cylindrical portion formed on the tip side and has a third outer diameter that is an outer diameter corresponding to the second inner diameter, the sleeve includes a pressing portion that is a cylindrical portion formed at an end portion on the tip side and that has a fourth outer diameter equal to the first outer diameter, a third inner diameter equal to the second inner diameter, and a third thickness that is a predetermined thickness, the die hole includes: a large inner diameter portion formed on the base end side and defining a cylindrical space having a fourth inner diameter corresponding to the first outer diameter; a small inner diameter portion formed on the tip end side and defining a cylindrical space having a noncircular cross section corresponding to the outer contour of the noncircular cross section of the small diameter portion; a connecting portion formed between the large inner diameter portion and the small inner diameter portion, the inner diameter of which decreases from the fourth inner diameter at a constant rate as the diameter approaches the small inner diameter portion; and a transition portion formed between the large inner diameter portion and the connecting portion, the inner circumferential surface of which, in a cross section taken along a plane including the central axis of the die hole, is a curve that continuously connects the inner circumferential surface of the large inner diameter portion and the inner circumferential surface of the connecting portion, In the die hole, a boundary line between the small inner diameter portion and the connecting portion is formed by a line of intersection between an inner peripheral surface of the small inner diameter portion and an inner peripheral surface of the connecting portion, in the extrusion process, the tip end of the mandrel is located closer to the tip than the base end of the small inner diameter portion of the die hole at least during a period from a first time point onwards, which is a time point at which the material constituting the mother tube begins to flow towards the tip side beyond the tip end of the large inner diameter portion of the die hole due to the extrusion process. A method for extrusion molding pipes with different thicknesses.

10. The method for extrusion molding a pipe with different thicknesses according to claim 9, a wall thickness ratio, which is the ratio of a minimum wall thickness, which is half of a value obtained by subtracting the third outer diameter of the mandrel from the minimum value of the inner diameter of the small inner diameter portion of the die hole, to a maximum wall thickness, which is half of a value obtained by subtracting the third outer diameter of the mandrel from the maximum value of the inner diameter of the small inner diameter portion of the die hole, is greater than 1 and not greater than 2; A method for extrusion molding pipes with different thicknesses.

11. The method for extrusion molding a differential thickness pipe according to claim 9 or 10, a die half angle, which is the angle between the inner peripheral surface of the connection portion of the die hole and the central axis in a cross section taken along a plane including the central axis of the die hole, is 45° or more and less than 70°; A method for extrusion molding pipes with different thicknesses.

12. The method for extrusion molding a differential thickness pipe according to claim 9 or 10, a land is formed in a predetermined range at an end portion on the base end side of the small inner diameter portion of the die hole, and an outer contour of a cross section of a columnar space defined by a portion of the die hole closer to the tip end than the land is larger than an outer contour of a cross section of a columnar space defined by the land; A method for extrusion molding pipes with different thicknesses.

13. The method for extrusion molding a differential thickness pipe according to claim 9 or 10, The differential thickness pipe further includes a thick-walled portion formed at the end portion on the tip side, the thick-walled portion having the same non-circular cross-sectional outer contour as the small-diameter portion and a fifth outer diameter that is a predetermined inner diameter smaller than the second inner diameter, and an inner diameter decreasing portion formed between the small-diameter portion and the thick-walled portion, the inner diameter decreasing from the second inner diameter to the fifth inner diameter as the small-diameter portion approaches the thick-walled portion, the mandrel further includes: a small outer diameter portion that is a cylindrical portion formed on the tip side of the large outer diameter portion and has a fifth outer diameter that corresponds to the fifth inner diameter and is a predetermined outer diameter smaller than the third outer diameter; and a second outer diameter decreasing portion that is formed between the large outer diameter portion and the small outer diameter portion and whose outer diameter decreases from the third outer diameter to the fifth outer diameter as the large outer diameter portion approaches the small outer diameter portion, In the extrusion process, the positional relationship between the mandrel, the sleeve, and the die is controlled so that at a second point in time, which is a point in time when the material constituting the blank tube begins to flow from the base end of the small inner diameter portion of the die hole toward the tip end, the base end of the small outer diameter portion of the mandrel is located closer to the base end than the base end of the small inner diameter portion of the die hole, and from a third point in time onward, which is a predetermined point in time after the second point in time, the tip end of the large outer diameter portion of the mandrel is located closer to the tip end than the base end of the small inner diameter portion of the die hole. A method for extrusion molding pipes with different thicknesses.

14. The method for extrusion molding a differential thickness pipe according to claim 9 or 10, one or more protrusions extending in the extrusion direction are formed on the inner circumferential surface of the small inner diameter portion of the die hole, a groove-shaped recess extending in the extrusion direction is formed on the outer peripheral surface of the small diameter portion of the differential thickness pipe at a position corresponding to the protrusion formed on the inner peripheral surface of the small inner diameter portion of the die hole; A method for extrusion molding pipes with different thicknesses.

15. The method for extrusion molding a pipe with different thicknesses according to claim 14, the cross-sectional shape of the space defined by the small inner diameter portion of the die hole and the cross-sectional outer contour shape of the small diameter portion of the differential thickness pipe are polygonal, the convex portion formed on the inner peripheral surface of the small inner diameter portion of the die hole and the concave portion formed on the outer peripheral surface of the small diameter portion of the differential thickness pipe are located at positions corresponding to the sides of the polygon. A method for extrusion molding pipes with different thicknesses.

16. The method for extrusion molding a pipe with different thicknesses according to claim 15, the convex portion formed on the inner peripheral surface of the small inner diameter portion of the die hole and the concave portion formed on the outer peripheral surface of the small diameter portion of the differential thickness pipe are located at positions closer to midpoints of the sides of the polygon than to the ends of the sides, A method for extrusion molding pipes with different thicknesses.

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