EXTRUSION MOLDING METHOD AND EXTRUSION MOLDING APPARATUS OF A DIFFERENTIAL THICKNESS PIPE HAVING A SOLID PART

Description

TECHNICAL FIELD

The present invention relates to an extrusion molding method and an extrusion molding apparatus of a differential thickness pipe having a solid part.

BACKGROUND ART

In the art, for example, a differential thickness pipe (which may be referred to as a “butted pipe” or “butted tube”, etc.) in which a thick-walled part is formed in a part in a direction of an axis of the pipe for the purpose of attaining a weight reduction in a thin-walled part (part other than the thick-walled part) while attaining desired mechanical strength in the thick-walled part, etc. has been known. Such a differential thickness pipe which is hollow over its total length can be integrally molded from a cylindrical raw pipe by extrusion molding as disclosed in Patent Literature 1 (PTL1, Japanese Patent No. 6933762), for example.

Moreover, although there are needs to which a differential thickness pipe is applied also in in fields, such as shafts, for example, it may be demanded to prepare a solid part with a predetermined length in a direction of its axis on its one end part side, rather than making the pipe hollow over its total length. For example, from a viewpoint of reducing a manufacturing cost, etc., it is also desirable to mold such a differential thickness pipe having a solid part by extrusion molding. However, such a differential thickness pipe having a solid part, cannot be molded by the above-mentioned methods.

On the other hand, in the art, a method for manufacturing a bottomed hollow metal product that is a differential thickness pipe having a solid part at its one end part by extrusion molding has been known, although the length of the solid part is short. For example, in Patent Literature 2 (PTL2, Japanese Examined Patent Publication (kokoku) No. S49-035497), a method for manufacturing a bottomed hollow metal product by a combination of a boring-and-compressing process in which an intermediate product having a hollow hole is formed by thrusting a punch into an approximately columnar solid material and the following process in which a hollow part (differential thickness part) and a solid part (bottom part) are formed by extruding the whole intermediate product while pressing a bottom part of the hollow hole with another punch is disclosed.

Moreover, in Patent Literature 3 (PTL3, Japanese Examined Patent Publication (kokoku) No. S58-048264), a method for forming a hollow part (differential thickness part) and a solid part (and a bottom part) by forming a material having a flange part and a hollow hole (bottom part) in a predetermined positional relation previously in a forge process, forming an intermediate material by sharpening a corner of a bottom of the hollow hole with a tip of a center punch in the following process, and pressing an end part opposite to the bottom part of the intermediate material with a sleeve punch in the following process is disclosed.

However, in any of the above-mentioned methods, it is necessary to prepare a material having a comparatively complicated shape for satisfying specific requirements. Moreover, the bottomed hollow metal products manufactured by these methods are what is called “bottomed cylindrical members”, and do not correspond to a differential thickness pipe having a solid part with a predetermined length in an axis direction as mentioned above. Furthermore, in the latter method, a crack is likely to occur at a boundary between the bottom part and side wall part of the material in the process in which the corner of the bottom of the hollow hole is sharpened with the tip of the center punch, there is a problem that pressing force by the center punch cannot be set to be strong.

CITATION LIST

Patent Literature

• • [PTL1] Japanese Patent No. 6933762
  • [PTL2] Japanese Examined Patent Publication (kokoku) No. S49-035497
  • [PTL3] Japanese Examined Patent Publication (kokoku) No. S58-048264

SUMMARY OF INVENTION

Technical Problem

Taking into consideration the problem in a prior art as mentioned above, the present inventor attempted an experiment in which a differential thickness pipe having a solid part is extruded by inserting a mandrel (core bar) into a hollow hole of a material which has a hollow part and a solid part and pushing the material into a container (dice) which has a small inner diameter part in its tip end side to perform a diameter contraction, based on the technology described in Patent Literature 1 (PTL1). However, it has been confirmed that a crack over the entire circumference easily occurs on the boundary between a bottom part and a side wall part of the hollow hole, as exemplified by a thick solid line in (a) of FIG. 6, when pressing the bottom part of the hollow hole (topmost part of the solid part) of the material to the extrusion direction by the tip of the mandrel.

On the other hand, when carrying out extrusion molding while maintaining a space between the tip of the mandrel and the bottom part of the hollow hole in order to prevent occurrence of a crack as mentioned above, it has been confirmed that the stuff (forming material) of the material causes a plastic flow toward the space to generate stuff accumulation (material accumulation) over the entire circumference inside the space in association with compression of the material toward the inner side in the radial direction, as exemplified by black semi-circles in (b) of FIG. 6, and therefore a desired hollow shape cannot be realized.

Namely, in the art, there is a demand for a manufacturing method and manufacturing apparatus of a differential thickness pipe having a solid part in which occurrence of defects such as a crack and/or stuff accumulation as mentioned above can be reduced without requiring a material having a complicated shape.

Solution to Problem

Based on the above-mentioned knowledge, as a result of a further diligent research, the present inventor has found out that the above-mentioned subject can be solved by controlling appropriately a positional relation between the tip of the mandrel and the bottom part of the hollow hole of the material as well as movements of a sleeve and the mandrel.

Specifically, an extrusion-molding method of a differential thickness pipe having a solid part (which may be referred to as a “present invention method” hereafter) is an extrusion-molding method for molding the differential thickness pipe having a solid part from a material having a predetermined shape by extrusion processing in an extrusion-molding apparatus. The extrusion-molding apparatus comprises a mandrel having a predetermined shape, a sleeve having a predetermined shape, a container in which a container hole that is a penetration hole having a predetermined shape is formed, and a drive mechanism configured so as to push the mandrel and the sleeve into the container hole.

The material is a member which comprises a first hollow part and a first solid part and has a circular columnar outer shape having a first outer diameter that is a predetermined outer diameter as a whole. The first hollow part is a circular cylindrical part which has a first hollow hole that is a circular columnar space opened at an end surface on a base end side that is an upstream side in an extrusion direction and having a first inner diameter that is a predetermined inner diameter formed therein and has a first thickness that is a predetermined thickness. The first solid part is a circular columnar part which is located between an end surface on a tip end side that is a downstream side in the extrusion direction and the first hollow part.

The differential thickness pipe is a member comprising a second hollow part, a third hollow part, a fourth hollow part and a second solid part. The second hollow part is a circular cylindrical part which has the first outer diameter and the first thickness. The third hollow part is a cylindrical part which is located adjacent to the second hollow part on the tip end side and has an outer diameter changing from the first outer diameter to a second outer diameter that is a predetermined outer diameter smaller than the first outer diameter and a thickness changing from the first thickness to a second thickness that is a predetermined thickness smaller than the first thickness from the base end side toward the tip end side. The fourth hollow part is a circular cylindrical part which is located adjacent to the third hollow part on the tip end side and has the second outer diameter and the second thickness. The second solid part is a circular columnar part which is located between an end part on the tip end side and the fourth hollow part and has the second outer diameter. Furthermore, a second hollow hole that is a circular columnar space opened at an end surface on the base end side and having the first inner diameter is formed continuously from the second hollow part to the fourth hollow part.

The mandrel is a member internally fitted in the sleeve coaxially and slidably in an axial direction and having a circular columnar shape which has a third outer diameter that is a predetermined outer diameter corresponding to the first inner diameter. The sleeve is a member externally fitted with the mandrel coaxially and slidably in an axial direction and having a circular cylindrical shape which has the first outer diameter and a second inner diameter that is a predetermined inner diameter corresponding to the third outer diameter.

The container hole comprises a large inner diameter part, a small inner diameter part, and an inner diameter decreasing part. The large inner diameter part is a part formed on the base end side and having a third inner diameter that is an inner diameter corresponding to the first outer diameter. The small inner diameter part is a part formed on the tip end side and having a fourth inner diameter that is an inner diameter corresponding to the second outer diameter. The inner diameter decreasing part is a part formed between the large inner diameter part and the small inner diameter part and having an inner diameter decreasing from the third inner diameter to the fourth inner diameter as approaching from the large inner diameter part to the small inner diameter part.

The present invention method includes first to third processes which will be listed below.

The first process is a process in which the material is inserted in the large inner diameter part of the container hole to bring an end part on the tip end side of the material into contact with the inner diameter decreasing part of the container hole, the sleeve is brought into contact with an end part on the base end side of the material, the mandrel is inserted into the first hollow hole of the material, and a tip-to-tip distance that is a relative distance in the extrusion direction between an end part on the tip end side of the sleeve and an end part on the tip end side of the mandrel is fixed at a first distance that is a predetermined distance.

The second process is a process in which extrusion processing is performed by advancing the sleeve and the mandrel toward the extrusion direction while maintaining the tip-to-tip distance at the first distance to push the material into the small inner diameter part through the inner diameter decreasing part of the container hole, and the advance of the sleeve and the mandrel is continued until a first time point that is a time point when the end part on the tip end side of the mandrel arrives at an end part on the base end side of the inner diameter decreasing part of the container hole.

The third process is a process in which the sleeve and the mandrel are advanced toward the extrusion direction while maintaining the tip-to-tip distance at the first distance after the first time point.

Moreover, the present invention relates also to an extrusion-molding apparatus of a differential thickness pipe having a solid part (which may be referred to as a “present invention apparatus” hereafter) for molding the differential thickness pipe having a solid part by carrying out the above-mentioned present invention method.

Advantageous Effects of Invention

By carrying out the present invention method including the above-mentioned first to third processes in the present invention apparatus which has the above-mentioned configuration, a differential thickness pipe having a solid part can be molded accurately and easily from a material which has a simple structure. Namely, in accordance with the present invention, it is possible to provide a manufacturing method and manufacturing apparatus of a differential thickness pipe having a solid part in which occurrence of defects such as a crack and/or stuff accumulation can be reduced without requiring a material having a complicated shape.

Other objectives, other features, and accompanying advantages of the present invention will be easily understood from the following explanation about respective embodiments of the present invention which will be described referring to drawings.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic sectional view for showing examples of configurations of a raw material as a source of a material used in an extrusion-molding method of a differential thickness pipe having a solid part according to a first embodiment of the present invention (first method), the material and a differential thickness pipe having a solid part molded from the material.

FIG. 2 is a schematic sectional view for showing examples of configurations of a mandrel, a sleeve and a container used in the first method.

FIG. 3 is a flow chart for exemplifying a flow of first to third processes carried out in the first method.

FIG. 4 is a schematic sectional view for showing examples of shapes of the material and the differential thickness pipe as well as positional relations of the mandrel, the sleeve and the container with the material and the differential thickness pipe at an end time of the first process as well as a start time and an end time of the third process carried out in the first method.

FIG. 5 is a schematic sectional view for showing an example of a positional relation a bottom part of a first hollow hole of the material and the mandrel in the vicinity of the bottom part of the first hollow hole at a starting time point of the second process included in the extrusion-molding method of a differential thickness pipe having a solid part according to a second embodiment of the present invention (second method).

FIG. 6 is a schematic sectional view for exemplifying problems observed when attempting an experiment in which a differential thickness pipe having a solid part is made by extrusion molding based on a conventional technology.

DESCRIPTION OF EMBODIMENTS

First Embodiment

Hereafter, an extrusion-molding method of a differential thickness pipe having a solid part according to a first embodiment of the present invention (which will be referred to as a “first method” hereafter) will be explained referring to drawings.

<Configuration>

The first method is an extrusion-molding method for molding the differential thickness pipe having a solid part from a material having a predetermined shape by extrusion processing in an extrusion-molding apparatus. The extrusion-molding apparatus comprises a mandrel having a predetermined shape, a sleeve having a predetermined shape, a container in which a container hole that is a penetration hole having a predetermined shape is formed, and a drive mechanism configured so as to push the mandrel and the sleeve into the container hole. Although a detailed explanation of a fundamental configuration of such an extrusion-molding apparatus is omitted since it is well known to a person skilled in the art, and components including the mandrel, the sleeve and the container are constituted by stuff having properties (for example, mechanical strength and durability, etc.) which can withstand processing conditions, such as load which acts on the components in extrusion processing that will be mentioned later, for example. Moreover, the drive mechanism for pushing the mandrel and the sleeve into the container hole can be suitably chosen from various drive mechanisms well-known in the art depending on properties (for example, mechanical strength and hardness, etc.) of the stuff which constitutes the material subjected to extrusion processing. Typically, a pressing machine, such as a hydraulic pressing machine, are adopted as the drive mechanism, for example.

(a), (b) and (c) of FIG. 1 are schematic sectional views for showing examples of configurations of a raw material 11 as a source of a material used in the first method, the material 21 and a differential thickness pipe 31 having a solid part molded from the material 21, respectively. As exemplified in (b) of FIG. 1, the material 21 is a member which comprises a first hollow part PH1 and a first solid part PS1 and has a circular columnar outer shape having a first outer diameter DO1 that is a predetermined outer diameter as a whole. The first hollow part PH1 is a circular cylindrical part which has a first hollow hole HH1 that is a circular columnar space opened at an end surface on a base end side that is an upstream side in an extrusion direction and having a first inner diameter DI1 that is a predetermined inner diameter formed therein and has a first thickness T1 that is a predetermined thickness. The first solid part PS1 is a circular columnar part which is located between an end surface on a tip end side that is a downstream side in the extrusion direction and the first hollow part PH1.

Since the material 21 has a comparatively simple structure as mentioned above, it is possible to manufacture the material 21 easily by forming the first hollow hole HH1 in the raw material 11 that is a circular columnar member having the first outer diameter DO1 as exemplified in (a) of FIG. 1. Although specific techniques for forming the first hollow hole HH1 in the raw material 11 is not limited in particular, the first hollow hole HH1 can be easily formed in the raw material 11 with a technique, such as machining (cutting), for example.

Stuff which constitutes the raw material 11 is not limited in particular as long as it is possible to mold it into a desired shape by the plastic deformation in extrusion processing. Typically, the stuff which constitutes the raw material 11 are metal including lead, tin, aluminum, copper, zirconium, titanium, molybdenum, vanadium, niobium and steel, etc., for example.

As exemplified in (c) of FIG. 1, the differential thickness pipe 31 is a member comprising a second hollow part PH2, a third hollow part PH3, a fourth hollow part PH4 and a second solid part PS2. The second hollow part PH2 is a circular cylindrical part which has the first outer diameter DO1 and the first thickness T1. The third hollow part PH3 is a cylindrical part which is located adjacent to the second hollow part PH2 on the tip end side and has an outer diameter changing from the first outer diameter DO1 to a second outer diameter DO2 that is a predetermined outer diameter smaller than the first outer diameter DO1 and a thickness changing from the first thickness T1 to a second thickness T2 that is a predetermined thickness smaller than the first thickness T1 from the base end side toward the tip end side. The fourth hollow part PH4 is a circular cylindrical part which is located adjacent to the third hollow part PH3 on the tip end side and has the second outer diameter DO2 and the second thickness T2. The second solid part PS2 is a circular columnar part which is located between an end part on the tip end side and the fourth hollow part PH4 and has the second outer diameter DO2. Furthermore, a second hollow hole HH2 that is a circular columnar space opened at an end surface on the base end side and having the first inner diameter DI1 is formed continuously from the second hollow part PH2 to the fourth hollow part PH4.

In the differential thickness pipe 31 exemplified in (c) of FIG. 1, a rate of change of the outer diameter of the third hollow part PH3 becomes larger from the base end side to the tip end side, and an outline of the outer diameter of the third hollow part PH3 is a curve convex toward the outside in a radial direction. However, the pattern of change of the outer diameter of the third hollow part PH3 from the first outer diameter DO1 to the second outer diameter DO2 is not limited to this. For example, the outline of the outer diameter of the third hollow part PH3 may be a curve concave toward the outside in the radial direction, or the rate of change of the outer diameter of the third hollow part PH3 is constant from the end part on the base end side to the end part on the tip end side and the outline of the outer diameter of the third hollow part PH3 may be a straight line.

In the differential thickness pipe 31 exemplified in (c) of FIG. 1, the bottom part (end part on the tip end side) of the second hollow hole HH2 has a conical shape. However, the shape of the bottom part of the second hollow hole HH2 is not limited to this, and can be made into various shapes depending on the use of the differential thickness pipe 31, etc., for example. For example, the shape of the bottom part of the second hollow hole HH2 may be a plane perpendicular to an axial direction of the differential thickness pipe 31, and may be a curved surface (for example, a spherical surface, etc.) convex toward the tip end side. Such a shape of the bottom part of the second hollow hole HH2 can be attained by making shape of the end part on the tip end side of the mandrel into a shape corresponding to the shape of the bottom part of the second hollow hole HH2, for example.

(a) and (b) of FIG. 2 are schematic sectional views for showing examples of configurations of a mandrel 41, a sleeve 51 and a container 61 used in the first method, respectively. As exemplified in (a) of FIG. 2, the mandrel 41 is a member internally fitted in the sleeve 51 coaxially and slidably in an axial direction and having a circular columnar shape which has a third outer diameter DO3 that is a predetermined outer diameter corresponding to the first inner diameter DI1 that is the inner diameter of the first hollow part PH1 of the material 21. The sleeve 51 is a member externally fitted with the mandrel 41 coaxially and slidably in an axial direction and having a circular cylindrical shape which has the first outer diameter DO1 that is the outer diameter of the material 21 and a second inner diameter DI2 that is a predetermined inner diameter corresponding to the third outer diameter DO3 that is the outer diameter of the mandrel 41.

In addition, although not illustrated in FIG. 2, parts on the base end side of the mandrel 41 and the sleeve 51 can have a structure and/or mechanism suitable for being driven by a drive mechanism which the extrusion-molding apparatus comprises and/or being attached to and detached from the drive mechanism, etc.

As exemplified in (b) of FIG. 2, the container hole HC1 formed in the container 61 comprises a large inner diameter part PDIL, a small inner diameter part PDIS, and an inner diameter decreasing part PDIT. The large inner diameter part PDIL is a part formed on the base end side and having a third inner diameter DI3 that is an inner diameter corresponding to the first outer diameter DO1 that is the outer diameter of the material 21. The small inner diameter part PDIS is a part formed on the tip end side and having a fourth inner diameter DI4 that is an inner diameter corresponding to the second outer diameter DO2 that is the outer diameter of the fourth hollow part PH4 and the second solid part PS2 of the differential thickness pipe 31. The inner diameter decreasing part PDIT is a part formed between the large inner diameter part PDIL and the small inner diameter part PDIS and having an inner diameter decreasing from the third inner diameter DI3 to the fourth inner diameter DI4 as approaching from the large inner diameter part PDIL to the small inner diameter part PDIS.

In the container 61 exemplified in (b) of FIG. 2, the large inner diameter part PDIL is constituted by two members, the inner diameter decreasing part PDIT and a part on the most base end side of the small inner diameter part PDIS are constituted integrally by one member, and the remaining part of the small inner diameter part PDIS is constituted by two components. Namely, the container 61 exemplified in (b) of FIG. 2 is constituted by five members as a whole. However, the configuration of the container 61 may not be limited to this, and may be constituted by one member as a whole, or each of the large inner diameter part PDIL, the small inner diameter part PDIS and the inner diameter decreasing part PDIT may be constituted by one member and the entire container 61 may be constituted by three members.

The first method includes first to third processed listed below so as exemplified in the flow chart of FIG. 3.

The first process carried out in Step S01 is a process in which the material is inserted in the large inner diameter part of the container hole to bring an end part on the tip end side of the material into contact with the inner diameter decreasing part of the container hole, the sleeve is brought into contact with an end part on the base end side of the material, the mandrel is inserted into the first hollow hole of the material, and a tip-to-tip distance that is a relative distance in the extrusion direction between an end part on the tip end side of the sleeve and an end part on the tip end side of the mandrel is fixed at a first distance that is a predetermined distance. Namely, the material, the mandrel, the sleeve and the container are set to predetermined positions in the first process.

The second process carried out in Step S02 is a process in which extrusion processing is performed by advancing the sleeve and the mandrel toward the extrusion direction while maintaining the tip-to-tip distance at the first distance to push the material into the small inner diameter part through the inner diameter decreasing part of the container hole, and the advance of the sleeve and the mandrel is continued until a first time point that is a time point when the end part on the tip end side of the mandrel arrives at an end part on the base end side of the inner diameter decreasing part of the container hole. Namely, in the second process, the sleeve and the mandrel are advanced toward the extrusion direction while maintaining the tip-to-tip distance at the first distance by cooperative driving of the mandrel and the sleeve until the end part on the tip end side of the mandrel arrives at the end part on the base end side of the inner diameter decreasing part of the container hole. Thereby, extrusion processing from the first solid part of the material to the second solid part of the differential thickness pipe is performed.

The third process carried out in Step S03 is a process in which the sleeve and the mandrel are advanced toward the extrusion direction while maintaining the tip-to-tip distance at the first distance after the first time point. Namely, also in the third process that is a process carried out after the first time point, the sleeve and the mandrel are advanced toward the extrusion direction while maintaining the tip-to-tip distance at the first distance by cooperative driving of the mandrel and the sleeve. Thereby, extrusion processing from a part on the tip end side of the first hollow part of the material to the third hollow part and the fourth hollow part of the differential thickness pipe is performed. In addition, a part which was not subjected to the extrusion processing in the third process in the first hollow part of the material becomes the second hollow part of the differential thickness pipe.

(a) of FIG. 4 is a schematic sectional view for showing examples of a shape of the material 21 as well as a positional relation of the mandrel 41, the sleeve 51 and the container 61 with the material and the differential thickness pipe at an end time of the first process. In FIG. 4, in order to make the drawing brief, only a part of the reference signs given to respective parts shown in FIG. 1 and FIG. 2 is displayed. However, since the reference signs shown in FIG. 1 and FIG. 2 are used in order to ensure accuracy in the following explanation about FIG. 4, please refer to FIG. 1 and FIG. 2 when needed.

As exemplified in (a) of FIG. 4, the material 21 is inserted in the large inner diameter part PDIL of the container hole HC1 and an end part on the tip end side of the material 21 is in contact with the inner diameter decreasing part PDIT of the container hole HC1. Thereby, the material 21 is held at a predetermined position in the inside of the container hole HC1. Moreover, the sleeve 51 is in contact with an end part on the base end side of the material 21, and the mandrel 41 is inserted into the first hollow hole HH1 formed in the material 21. Furthermore, the sleeve 51 and the mandrel 41 are arranged such that a tip-to-tip distance DT that is a relative distance in the extrusion direction between an end part on the tip end side of the sleeve 51 and an end part on the tip end side of the mandrel 41 is a first distance D1 that is a predetermined distance (namely, DT=D1).

Thereafter, although not illustrated, in the second process following the first process, the mandrel 41 and the sleeve 51 are advanced toward the extrusion direction (downward direction in FIG. 4) while maintaining the tip-to-tip distance DT at the first distance D1 by the cooperative driving of the mandrel 41 and the sleeve 51. Thereby, the first solid part PS1 of the material 21 is pushed into the small inner diameter part PDIS through the inner diameter decreasing part PDIT of the container hole HC1 from the tip end side, and extrusion processing from the first solid part PS1 of the material 21 to the second solid part PS2 of the differential thickness pipe 31 is performed.

The second outer diameter DO2 that is an outer diameter of the second solid part PS2 of the differential thickness pipe 31 is smaller than the first outer diameter DO1 that is an outer diameter of the first solid part PS1 of the material 21 (DO2<DO1). Namely, the cross sectional area of the second solid part PS2 of the differential thickness pipe 31 is smaller than the cross sectional area of the first solid part PS1 of the material 21. Therefore, the length in the extrusion direction of the second solid part PS2 of the differential thickness pipe 31 formed from the first solid part PS1 of the material 21 by extrusion processing is larger than the length in the extrusion direction of the first solid part PS1 the material 21 (which will be mentioned later in detail).

By the way, from a viewpoint of avoiding problems, such as occurrence of a crack as exemplified in (a) of FIG. 6, for example, it is preferable that the bottom part of the first hollow hole HH1 is not pressed by the mandrel 41 whereas the end part on the base end side of the material 21 is pressed by the sleeve 51 toward the extrusion direction at least at the second time point that is a starting time point of the above-mentioned extrusion processing in the second process. Such a state can be attained by preventing the bottom part of the first hollow hole HH1 of the material 21 from being brought into contact with the mandrel 41 or providing a gap having a predetermined size between the bottom part of the first hollow hole HH1 of the material 21 and the end part on the tip end side of the mandrel 41 at the second time point, etc. (which will be mentioned later in detail), for example.

However, although not illustrated, for example, depending on the stuff constituting the material, the mandrel, the sleeve and/or the container as well as conditions of extrusion processing such as pressing load and/or pressing speed, for example, there is a possibility that what is called “upsetting phenomenon” may occur in association with progress of the above-mentioned extrusion processing. In this case, the material expands toward the outside in the radial direction and contracts in the axial direction due to a plastic flow of the stuff constituting the material and/or elastic deformation of the container, etc., and the bottom part of the first hollow hole formed in the material is slightly displaced to the base end side. Therefore, even when the gap exists between the bottom part of the first hollow hole of the material and the end part on the tip end side of the mandrel at the second time point, there is a possibility that the gap may disappear or contract due to the above-mentioned upsetting phenomenon.

In association with the progress of the above-mentioned extrusion processing, the end part on the tip end side of the mandrel 41 arrives at the end part on the base end side of the inner diameter decreasing part PDIT of the container hole HC1 soon. The time period until the first time point that is a time point when the end part on the tip end side of the mandrel 41 arrives at the end part on the base end side of the inner diameter decreasing part PDIT of the container hole HC1 in this way corresponds to the second process, and the time period after the first time point corresponds to the third process.

(b) of FIG. 4 is a schematic sectional view for showing examples of a shape of the material 21 as well as positional relations of the mandrel 41, the sleeve 51 and the container 61 with the material 21 immediately after the end of the second process, i.e., immediately after the start of the third process. In the third process, the advance toward the extrusion direction of the sleeve 51 and the mandrel 41 is continued also after the first time point while maintaining the tip-to-tip distance DT at the first distance D1. Thereby, the first hollow part PH1 of the material 21 is pushed into the small inner diameter part PDIS through the inner diameter decreasing part PDIT of the container hole HC1 from the tip end side, and extrusion processing from a part on the tip end side of the first hollow part PH1 of the material 21 to the fourth hollow part PH4 and the third hollow part PH3 of the differential thickness pipe 31 is performed. In addition, as mentioned above, a part which was not subjected to the extrusion processing in the third process in the first hollow part PH1 of the material 21 becomes the second hollow part PH2 of the differential thickness pipe 31.

The second outer diameter DO2 that is an outer diameter of the fourth hollow part PH4 of the differential thickness pipe 31 is smaller than the first outer diameter DO1 that is an outer diameter of the first hollow part PH1 of the material 21 (DO2<DO1). Moreover, the inner diameter of the second hollow hole HH2 of the differential thickness pipe 31 is maintained by the mandrel 41 at the first inner diameter DI1 that is an inner diameter of the first hollow part PH1 of the material 21 as it is. Namely, the cross sectional area of the fourth hollow part PH4 of the differential thickness pipe 31 is smaller than the cross sectional area of the first hollow part PH1 of the material 21. On the other hand, whereas the outer diameter of the third hollow part PH3 of the differential thickness pipe 31 changes from the first outer diameter DO1 to the second outer diameter DO2 smaller than the first outer diameter DO1 from the base end side toward the tip end side as mentioned above, the inner diameter of the third hollow part PH3 of the differential thickness pipe 31 is also maintained by the mandrel 41 at the first inner diameter DI1. Namely, the cross sectional area of the third hollow part PH3 of the differential thickness pipe 31 is also smaller than the cross sectional area of the first hollow part PH1 of the material 21. Therefore, the length in the extrusion direction of the fourth hollow part PH4 and the third hollow part PH3 of the differential thickness pipe 31 formed from the first hollow part PH1 of the material 21 by extrusion processing becomes larger than the length in the extrusion direction of the first hollow part PH1 the material 21 subjected to the extrusion processing (which will be mentioned later in detail).

Therefore, as shown in the region surrounded by a thick broken line in (b) of FIG. 4, when the extrusion processing from first hollow part PH1 of the material 21 to the fourth hollow part PH4 and the third hollow part PH3 of the differential thickness pipe 31 is started in the third process, the bottom part of the first hollow hole HH1 of the material 21 and the end part on the tip end side of the mandrel 41 begin to separate from each other.

(c) of FIG. 4 is a schematic sectional view for showing examples of a shape of the differential thickness pipe 31 as well as a positional relation of the mandrel 41, the sleeve 51 and the container 61 with the differential thickness pipe 31 at an end time of the third process. As exemplified in (c) of FIG. 4, by carrying out the above-mentioned first to third processed, the differential thickness pipe 31 which comprises the second hollow part PH2, the third hollow part PH3, the fourth hollow part PH4 and the second solid part PS2 having expected shapes can be easily molded from the material 21 which has a simple shape.

In the second process included in the first method, as mentioned above, by advancing the sleeve and the mandrel toward the extrusion direction while maintaining the tip-to-tip distance DT that is a relative distance in the extrusion direction between an end part on the tip end side of the sleeve 51 and an end part on the tip end side of the mandrel 41 at the first distance D1 that is a predetermined distance, the first solid part PS1 of the material 21 is pushed into the small inner diameter part PDIS through the inner diameter decreasing part PDIT of the container hole HC1 from the tip end side and the extrusion processing from the first solid part PS1 of the material 21 to the second solid part PS2 of the differential thickness pipe 31 is performed.

On the other hand, in the third process, as mentioned above, the bottom part of the first hollow hole HH1 of the material 21 and the end part on the tip end side of the mandrel 41 are separating from each other. Namely, in the third process, the mandrel 41 does not contribute to the extrusion processing, but undertakes a function to maintain the shape of the cross section of the second hollow hole HH2 formed in the differential thickness pipe 31 identical to that of the first hollow hole HH1 formed in the material 21 in a course in which a part on the tip end side of the first hollow part PH1 of the material 21 changes to the fourth hollow part PH4 and the third hollow part PH3 of the differential thickness pipe 31.

Here, the lengths in the extrusion direction (which is the same as the axial direction of the material 21) of respective parts of the material 21 and the differential thickness pipe 31 which was referred previously will be explained in detail below. In the following explanation, for the purpose of easy understanding, it is premised that both shapes of the bottom parts of the first hollow hole HH1 formed in the material 21 and the second hollow hole HH2 formed in the differential thickness pipe 31 are planes perpendicular to the axial direction. Therefore, when the shapes of these bottom parts are not planes perpendicular to the axial direction, it is needless to say that correction according to the shapes is required.

The second hollow part PH2, the third hollow part PH3 and the fourth hollow part PH4 of the differential thickness pipe 31 are molded from first hollow part PH1 of the material 21, and the second solid part PS2 of the differential thickness pipe 31 is molded from the first solid part PS1 of the material 21. Therefore, relations listed below are established between the lengths in the axial direction (which may be referred simply as “lengths” hereafter) of respective parts of the material 21 and the lengths in the axial direction of respective parts of the differential thickness pipe 31.

First, the length of the second hollow part PH2 of the differential thickness pipe 31 (LPH2) is a the length of the part which remains in its unprocessed state of the material 21 as it is without being subjected to the extrusion processing at the time point when the third process is ended. Therefore, as long as the LPH2 is smaller than the length of the first hollow part PH1 of the material 21 (LPH1), the LPH2 can be determined to be an arbitrary length by the timing when the third process is ended.

Next, the length of the third hollow part PH3 of the differential thickness pipe 31 (LPH3) is determined naturally according to the length of the inner diameter decreasing part PDIT (LPDIT) of the container hole HC1 formed in the container 61. Therefore, the LPH3 can be determined to be an arbitrary length by adjusting the LPDIT in the container hole HC1 formed in the container 61.

Next, the length of fourth hollow part PH4 of the differential thickness pipe 31 (LPH4) can be calculated by subtracting a volume of the stuff constituting the second hollow part PH2 and the third hollow part PH3 of the differential thickness pipe 31 (VPH2+VPH3) from a volume of the stuff constituting the first hollow part PH1 of the material 21 (VPH1) to obtain a remaining volume and dividing the remaining volume by an area of the annular cross section of the fourth hollow part PH4 (APH4). Therefore, in order to obtain the expected LPH4, it is necessary to configure the material 21, the differential thickness pipe 31 and the container hole HC1 formed in the container 61 such that the following formula (1) is established materialized. In addition, the VPH1, VPH2 and APH4 can be expressed by the following formulas (2) to (4). However, since the VPH3 changes according to patterns of change in the outer diameter (namely, shapes of the outline) of the third hollow part PH3, it is necessary to calculate the VPH3 according to the shape of the third hollow part PH3.

LPH ⁢ 4 = { VPH ⁢ 1 - ( VPH ⁢ 2 + VPH ⁢ 3 ) } / APH ⁢ 4 ( 1 ) VPH ⁢ 1 = π ⁢ { ( DO ⁢ 1 / 2 ) 2 - ( DI ⁢ 1 / 2 ) 2 } × LPH ⁢ 1 ( 2 ) VPH ⁢ 2 = π ⁢ { ( DO ⁢ 1 / 2 ) 2 - ( DI ⁢ 1 / 2 ) 2 } × LPH ⁢ 2 ( 3 ) APH ⁢ 4 = π ⁢ { ( DO ⁢ 1 / 2 ) 2 - ( DI ⁢ 1 / 2 ) 2 } ( 4 )

A relation between the length of the first solid part PS1 of the material 21 (LPS1) and the length of the second solid part PS2 of the differential thickness pipe 31 (LPS2) can be expressed by the following formula (5).

LPS ⁢ 1 : LPS ⁢ 2 = ( DO ⁢ 2 / 2 ) 2 : ( DO ⁢ 1 / 2 ) 2 ( 5 )

Therefore, in order to obtain the expected LPS2, it is necessary to define the length of the first solid part PS1 of the material 21 (LPS1) according to the first outer diameter DO1 of the material 21 and the second outer diameter DO2 of the differential thickness pipe 31 such that the following formula (6) is established.

LPS ⁢ 1 = LPS ⁢ 2 × ( DO ⁢ 2 / DO ⁢ 1 ) 2 ( 6 )

<Effect>

As explained above, by carrying out the first method including the above-mentioned first to third processes in the above-mentioned extrusion-molding apparatus comprising the mandrel, the sleeve, the container and the drive mechanism having the above-mentioned configurations, a differential thickness pipe having a solid part can be molded accurately and easily from a material having a simple structure. Namely, the first method is a manufacturing method of a differential thickness pipe having a solid part, in which occurrence of defects such as a crack and/or stuff accumulation can be reduced without requiring a material having a complicated shape.

Such a differential thickness pipe having a solid part is useful as a part required to attain a weight reduction in a thin-walled part (parts other than a thick-walled part) while attaining desired mechanical strength in the thick-walled part. Moreover, for example, when a differential thickness pipe having a solid part as mentioned above is adopted as a part for which an oil stop is required among parts of a shaft system, a plug for an oil stop becomes unnecessary and design flexibility can be raised and/or a manufacturing cost can be reduced.

Second Embodiment

Hereafter, the extrusion-molding method of a differential thickness pipe of having a solid part according to a second embodiment of the present invention (which may be referred to as a “second method” hereafter) will be explained, referring to drawings.

As mentioned above, in the first method, extrusion processing is performed by advancing the sleeve and the mandrel toward the extrusion direction while maintaining the tip-to-tip distance that is a relative distance in the extrusion direction between the end part on the tip end side of the sleeve and the end part on the tip end side of the mandrel at the first distance that is a predetermined distance to push the material into the small inner diameter part through the inner diameter decreasing part of the container hole. As a result, in the third process in which extrusion processing from a part on the tip end side of the first hollow part PH1 of the material 21 to the fourth hollow part PH4 and the third hollow part PH3 of the differential thickness pipe 31 is performed, the bottom part of the first hollow hole HH1 of the material 21 and the end part on the tip end side of the mandrel 41 separate from each other. Namely, in the third process, the mandrel 41 does not contribute to the extrusion processing, but undertakes a function to maintain the shape of the cross section of the second hollow hole HH2 formed in the differential thickness pipe 31 in a course in which the first hollow part PH1 of the material 21 formed in the material 21 changes to the second hollow part HH2 formed in the differential thickness pipe 31.

As a result, the differential thickness pipe having a solid part can be molded accurately and easily from the material having a simple structure. Namely, in accordance with the first method, the differential thickness pipe having a hollow part can be manufactured easily without requiring a material having a complicated shape while reducing occurrence of defects such as a crack and/or stuff accumulation.

However, as mentioned above, for example, depending on the stuff constituting the material, the mandrel, the sleeve and/or the container as well as conditions of extrusion processing such as pressing load and/or pressing speed, for example, there is a possibility that what is called “upsetting phenomenon” may occur at a time point when the material begins to be pushed into the small inner diameter part through the inner diameter decreasing part of the container hole formed in the container (namely, a starting time point of extrusion processing). In this case, the material expands toward the outside in the radial direction and contracts in the axial direction due to a plastic flow of the stuff constituting the material and/or elastic deformation of the container, etc., and the bottom part of the first hollow hole formed in the material is slightly displaced to the base end side. As a result, there is a possibility that the end part on the tip end side of the mandrel may be pressed toward the base end side by the bottom part of the first hollow hole, stress may act on the bottom part of the first hollow hole due to the reaction, and a crack may arise on the boundary between the bottom part and the side wall part of the first hollow hole.

<Configuration>

Then, the second method is the above-mentioned first method, wherein the bottom part of the first hollow hole of the material is not in contact with the mandrel at least at a second time point that is a starting time point of the extrusion processing in the second process.

FIG. 5 is a schematic sectional view for showing an example of a positional relation the bottom part of the first hollow hole of the material and the mandrel in the vicinity of the bottom part of the first hollow hole at a starting time point of the second process included in the second method, and is an enlargement of the region surrounded by a thick broken line in (a) of FIG. 4. As exemplified in FIG. 5, in the second method, at least at a time point when the advance of the sleeve 51 and the mandrel 41 (not shown) toward the extrusion direction is started (the second time point), the bottom part of the first hollow hole HH1 of the material 21 is not in contact with the mandrel 41. Specifically in the example shown in FIG. 5, a gap G is formed between the bottom part of the first hollow hole HH1 of the material 21 and the mandrel 41.

Thereby, even in a case where the upsetting phenomenon occurs and the bottom part of the hollow hole formed in the material is displaced to the base end side as mentioned above at the starting time point of the extrusion processing in the second process, a possibility that the bottom part of the first hollow hole may press the tip of the mandrel can be reduced. As a result, a possibility that a crack may arise on the boundary between the bottom part and the side wall part of the first hollow hole due to the reaction of the pressing to the tip of the mandrel by the bottom part of the first hollow hole can be reduced.

In the material 21 exemplified in FIG. 5, the bottom part (end part on the tip end side) of the first hollow hole HH1 has a conical shape. For this reason, in the example shown in FIG. 5, the distance between the average position in the axial direction of the bottom part of the first hollow hole HH1 of the material 21 and the tip end surface of the mandrel 41 is adopted as a gap G between the bottom part of the first hollow hole HH1 of the material 21 and the mandrel 41. However, the shape of the bottom part of first hollow hole HH1 is not limited to this, and it can be made into various shapes, for example, according to the use of the differential thickness pipe 31 and/or a shape of the end part on the tip end side of the mandrel 41, etc. For example, the shape of the bottom part of the first hollow hole HH1 may be a plane perpendicular to the axial direction of the material 21, or may be a curved surface convex toward the tip end side (for example, a spherical surface, etc.).

<Effect>

As mentioned above, in accordance with the second method, even in a case where the upsetting phenomenon occurs and the bottom part of the first hollow hole formed in the material is displaced to the base end side as mentioned above at the starting time point of the extrusion processing in the second process, a possibility that the bottom part of the first hollow hole may press the tip of the mandrel can be reduced. As a result, a possibility that a crack may arise on the boundary between the bottom part and the side wall part of the first hollow hole due to the reaction of the pressing to the tip of the mandrel by the bottom part of the first hollow hole can be reduced.

Third Embodiment

Hereafter, the extrusion-molding method of a differential thickness pipe of having a solid part according to a third embodiment of the present invention (which may be referred to as a “third method” hereafter) will be explained, referring to drawings.

As mentioned above, in the second method, at least at a time point when the advance of the sleeve and the mandrel toward the extrusion direction is started (the second time point), the bottom part of the first hollow hole of the material is not in contact with the mandrel. Therefore, in accordance with the second method, even in a case where the upsetting phenomenon occurs and the bottom part of the first hollow hole formed in the material is displaced to the base end side as mentioned above at the starting time point of the extrusion processing in the second process, a possibility that the bottom part of the first hollow hole may press the tip of the mandrel can be reduced. As a result, a possibility that a crack may arise on the boundary between the bottom part and the side wall part of the first hollow hole due to the reaction of the pressing to the tip of the mandrel by the bottom part of the first hollow hole can be reduced.

However, when the gap G between the bottom part of the first hollow hole of the material and the mandrel at the second time point (initial gap) is too small, there is a possibility that a crack may arise on the boundary between the bottom part and the side wall part of the first hollow hole cannot sufficiently reduced. Therefore, in order to sufficiently reduce the possibility that a crack may arise on the boundary between the bottom part and the side wall part of the first hollow hole, it can be thought that it is desirable to prepare the initial gap large enough. However, as a result of the further research by the present inventor, it has been found that, for example, depending on the stuff constituting the material, the mandrel, the sleeve and/or the container as well as conditions of extrusion processing such as pressing load and/or pressing speed, for example, the possibility that a crack may arise on the boundary between the bottom part and the side wall part of the first hollow hole may be unable to be reduced sufficiently. Namely, there is a suitable range in the initial gap for sufficiently reducing the possibility that a crack may arise on the boundary between the bottom part and the side wall part of the first hollow hole.

<Configuration>

Then, the third method is the above-mentioned second method wherein an initial length that is a length in the extrusion direction of an initial gap is a predetermined first length or more and less than a predetermined second length longer than the first length. The initial gap is a gap between the bottom part of the first hollow hole of the material and the end part on the tip end side of the mandrel at the second time point.

The first length can be determined based on a magnitude of a dimensional change of the material in a time period from the second time point to a third time point that is a time point when the end part on the tip end side of the material begins to enter the inner diameter decreasing part of the container hole. Preferably, the first length can be determined based on a magnitude of the displacement to the base end side of the bottom part of the first hollow hole in the time period from the second time point to the third time point. A magnitude of a dimensional change of the material, such as displacement to the base end side of the bottom part of the first hollow hole like this can be specified by verification based on previous preliminary experiment and/or by computer simulation such as flow analysis, etc.

On the other hand, the second length can be determined to be a predetermined length not more than the maximum length at which the stuff constituting the material does not flow into a first gap that is a gap between the bottom part of the first hollow hole of the material and the end part on the tip end side of the mandrel in a time period from the second time point to a fourth time point (that is a predetermined time point at or after a time point when the end part on the tip end side of the mandrel pass the inner diameter decreasing part of the container hole). Such a second length can be specified by verification based on previous preliminary experiment and/or by computer simulation such as flow analysis, etc., too.

<Effect>

As mentioned above, in the third method, the initial gap is set in a range suitable for sufficiently reducing the possibility that a crack may arise on the boundary between the bottom part and the side wall part of the first hollow hole at a time point when the advance of the sleeve and the mandrel toward the extrusion direction is started (the second time point). As a result, in accordance with the third method, the possibility that a crack may arise on the boundary between the bottom part and the side wall part of the first hollow hole of the material can be reduced more certainly.

Fourth Embodiment

By the way, as mentioned in the beginning of the present specification, the present invention relates not only to the extrusion-molding method of a differential thickness pipe having a solid part including the above-mentioned first to third methods, but also to an extrusion-molding apparatus of a differential thickness pipe having a solid part. Therefore, extrusion-molding apparatuses of a differential thickness pipe having a solid part according to various embodiments of the present invention will be explained below.

First, an extrusion-molding apparatus of a differential thickness pipe according to a fourth embodiment of the present invention (which may be referred to as a “fourth apparatus” hereafter) will be explained.

<Configuration>

The fourth apparatus is an extrusion-molding apparatus of a differential thickness pipe having a solid part, which comprises a mandrel having a predetermined shape, a sleeve having a predetermined shape, a container in which a container hole that is a penetration hole having a predetermined shape is formed, and a drive mechanism configured so as to push the mandrel and the sleeve into the container hole. As mentioned above, a fundamental configuration of such an extrusion-molding apparatus is well known to a person skilled in the art, and components including the mandrel, the sleeve and the container are constituted by stuff having properties (for example, mechanical strength and durability, etc.) which can withstand processing conditions, such as load which acts on the components in extrusion processing, for example.

The fourth apparatus is configured so as to mold the differential thickness pipe having a solid part from a material having a predetermined shape through extrusion processing by carrying out first to third processes listed below.

The first process is a process in which the material is inserted in the large inner diameter part of the container hole to bring an end part on the tip end side of the material into contact with the inner diameter decreasing part of the container hole, the sleeve is brought into contact with an end part on the base end side of the material, the mandrel is inserted into the first hollow hole of the material, and a tip-to-tip distance that is a relative distance in the extrusion direction between an end part on the tip end side of the sleeve and an end part on the tip end side of the mandrel is fixed at a first distance that is a predetermined distance.

The second process is a process in which extrusion processing is performed by advancing the sleeve and the mandrel toward the extrusion direction while maintaining the tip-to-tip distance at the first distance to push the material into the small inner diameter part through the inner diameter decreasing part of the container hole, and the advance of the sleeve and the mandrel is continued until a first time point that is a time point when the end part on the tip end side of the mandrel arrives at an end part on the base end side of the inner diameter decreasing part of the container hole.

The third process is a process in which the sleeve and the mandrel are advanced toward the extrusion direction while maintaining the tip-to-tip distance at the first distance after the first time point.

Since details of the first to third processes had been already mentioned in the explanation about the above-mentioned first method, explanation thereof is omitted here.

As already explained referring to FIG. 1, the material 21 is a member which comprises a first hollow part PH1 and a first solid part PS1 and has a circular columnar outer shape having a first outer diameter DO1 that is a predetermined outer diameter as a whole. The first hollow part PH1 is a circular cylindrical part which has a first hollow hole HH1 that is a circular columnar space opened at an end surface on a base end side that is an upstream side in an extrusion direction and having a first inner diameter DI1 that is a predetermined inner diameter formed therein and has a first thickness T1 that is a predetermined thickness. The first solid part PS1 is a circular columnar part which is located between an end surface on a tip end side that is a downstream side in the extrusion direction and the first hollow part PH1.

Moreover, the differential thickness pipe 31 is a member comprising a second hollow part PH2, a third hollow part PH3, a fourth hollow part PH4 and a second solid part PS2. The second hollow part PH2 is a circular cylindrical part which has the first outer diameter DO1 and the first thickness T1. The third hollow part PH3 is a cylindrical part which is located adjacent to the second hollow part PH2 on the tip end side and has an outer diameter changing from the first outer diameter DO1 to a second outer diameter DO2 that is a predetermined outer diameter smaller than the first outer diameter DO1 and a thickness changing from the first thickness T1 to a second thickness T2 that is a predetermined thickness smaller than the first thickness T1 from the base end side toward the tip end side. The fourth hollow part PH4 is a circular cylindrical part which is located adjacent to the third hollow part PH3 on the tip end side and has the second outer diameter DO2 and the second thickness T2. The second solid part PS2 is a circular columnar part which is located between an end part on the tip end side and the fourth hollow part PH4 and has the second outer diameter DO2. Furthermore, a second hollow hole HH2 that is a circular columnar space opened at an end surface on the base end side and having the first inner diameter DI1 is formed continuously from the second hollow part PH2 to the fourth hollow part PH4.

As already explained referring to FIG. 2, the mandrel 41 is a member internally fitted in the sleeve 51 coaxially and slidably in an axial direction and having a circular columnar shape which has a third outer diameter DO3 that is a predetermined outer diameter corresponding to the first inner diameter DI1 that is the inner diameter of the first hollow part PH1 of the material 21. The sleeve 51 is a member externally fitted with the mandrel 41 coaxially and slidably in an axial direction and having a circular cylindrical shape which has the first outer diameter DO1 that is the outer diameter of the material 21 and a second inner diameter DI2 that is a predetermined inner diameter corresponding to the third outer diameter DO3 that is the outer diameter of the mandrel 41.

Moreover, the container hole HC1 formed in the container 61 comprises a large inner diameter part PDIL, a small inner diameter part PDIS, and an inner diameter decreasing part PDIT. The large inner diameter part PDIL is a part formed on the base end side and having a third inner diameter DI3 that is an inner diameter corresponding to the first outer diameter DO1 that is the outer diameter of the material 21. The small inner diameter part PDIS is a part formed on the tip end side and having a fourth inner diameter DI4 that is an inner diameter corresponding to the second outer diameter DO2 that is the outer diameter of the fourth hollow part PH4 and the second solid part PS2 of the differential thickness pipe 31. The inner diameter decreasing part PDIT is a part formed between the large inner diameter part PDIL and the small inner diameter part PDIS and having an inner diameter decreasing from the third inner diameter DI3 to the fourth inner diameter DI4 as approaching from the large inner diameter part PDIL to the small inner diameter part PDIS.

As mentioned above, the fourth apparatus is an extrusion-molding apparatus of a differential thickness pipe having a solid part, which corresponds to the above-mentioned first method. Therefore, since the differential thickness pipe molded by the fourth apparatus, the material as a source for molding the differential thickness pipe and a raw material as a source of the material, as well as the mandrel, sleeve and container constituting the fourth apparatus are clear from the explanation about the above-mentioned first method, explanation thereof is omitted here.

<Effect>

By carrying out the first to third processed in the fourth apparatus having the configuration as mentioned above, a differential thickness pipe having a solid part can be molded accurately and easily from a material having a simple structure. Namely, the fourth apparatus is a manufacturing apparatus of a differential thickness pipe having a solid part, by which occurrence of defects such as a crack and/or stuff accumulation can be reduced without requiring a material having a complicated shape.

Fifth Embodiment

Next, the extrusion-molding apparatus of a differential thickness pipe of having a solid part according to a fifth embodiment of the present invention (which may be referred to as a “fifth apparatus” hereafter) will be explained.

<Configuration>

The fifth apparatus is the above-mentioned fourth apparatus wherein the bottom part of the first hollow hole of the material is not in contact with the mandrel at least at the second time point that is a starting time point of the extrusion processing in the second process.

As mentioned above, the fifth apparatus is an extrusion-molding apparatus of a differential thickness pipe having a solid part, which corresponds to the above-mentioned second method. Therefore, since a configuration and operation of the fifth apparatus are clear from the explanation about the above-mentioned second method, explanation thereof is omitted here.

<Effect>

In accordance with the fifth apparatus, even in a case where the upsetting phenomenon occurs and the bottom part of the first hollow hole formed in the material is displaced to the base end side as mentioned above at the starting time point of the extrusion processing in the second process, a possibility that the bottom part of the first hollow hole may press the tip of the mandrel can be reduced. As a result, a possibility that a crack may arise on the boundary between the bottom part and the side wall part of the first hollow hole due to the reaction of the pressing to the tip of the mandrel by the bottom part of the first hollow hole can be reduced.

Sixth Embodiment

Next, the extrusion-molding apparatus of a differential thickness pipe of having a solid part according to a sixth embodiment of the present invention (which may be referred to as a “sixth apparatus” hereafter) will be explained.

<Configuration>

The sixth apparatus is the above-mentioned fifth apparatus wherein an initial length that is a length in the extrusion direction of an initial gap is a predetermined first length or more and less than a predetermined second length longer than the first length. The initial gap is a gap between the bottom part of the first hollow hole of the material and the end part on the tip end side of the mandrel at the second time point. The first length can be determined based on a magnitude of a dimensional change of the material in a time period from the second time point to a third time point that is a time point when the end part on the tip end side of the material begins to enter the inner diameter decreasing part of the container hole. The second length can be determined to be a predetermined length not more than the maximum length at which the stuff constituting the material does not flow into a first gap that is a gap between the bottom part of the first hollow hole of the material and the end part on the tip end side of the mandrel in a time period from the second time point to a fourth time point (that is a predetermined time point at or after a time point when the end part on the tip end side of the mandrel pass the inner diameter decreasing part of the container hole).

As mentioned above, the sixth apparatus is an extrusion-molding apparatus of a differential thickness pipe having a solid part, which corresponds to the above-mentioned third method. Therefore, since a configuration and operation of the sixth apparatus are clear from the explanation about the above-mentioned third method, explanation thereof is omitted here.

<Effect>

In the sixth apparatus, the initial gap is set in a range suitable for sufficiently reducing the possibility that a crack may arise on the boundary between the bottom part and the side wall part of the first hollow hole at a time point when the advance of the sleeve and the mandrel toward the extrusion direction is started (the second time point). As a result, in accordance with the sixth apparatus, the possibility that a crack may arise on the boundary between the bottom part and the side wall part of the first hollow hole of the material can be reduced more certainly.

Although some embodiments which have specific configurations have been explained, sometimes referring to accompanying drawings, as the above, for the purpose of explaining the present invention, it should not be interpreted that the scope of the present invention is limited to these exemplary embodiments, and it is needless to say that modifications can be properly added within the limits of the matter described in the claims and the specification.

REFERENCE SIGNS LIST

• • 11: Raw Material, 21: Material, 31: Differential Thickness Pipe, 41: Mandrel, 51: Sleeve, 61: Container,
  • DO1: First Outer Diameter, DO2: Second Outer Diameter, DO3: Third Outer Diameter,
  • DI1: First Inner Diameter, DI2: Second Inner Diameter, DI3: Third Inner Diameter, DI4: Fourth Inner Diameter,
  • HH1: First Hollow Hole, HH2: Second Hollow Hole,
  • T1: First Thickness, T2: Second Thickness,
  • PH1: First Hollow Part, PH2: Second Hollow Part, PH3: Third Hollow Part,
  • PH4: Fourth Hollow Part,
  • PS1: First Solid Part, PS2: Second Solid Part,
  • HC1: Container Hole,
  • PDIL: Large Inner Diameter Part, PDIS: Small Inner Diameter Part, PDIT: Inner Diameter Decreasing Part, and
  • G: Gap.