GB2266485A

GB2266485A - Process of producing one-end-closed double pipe

Info

Publication number: GB2266485A
Application number: GB9308590A
Authority: GB
Inventors: Makoto Ito; Toshihiro Imai; Tokuo Shirai; Seiji Utsunomiya
Original assignee: NipponDenso Co Ltd
Current assignee: Denso Corp
Priority date: 1992-04-28
Filing date: 1993-04-26
Publication date: 1993-11-03
Anticipated expiration: 2013-04-26
Also published as: DE4313802A1; JP2586436B2; DE4313802B4; JPH05305381A; GB2266485B; GB9308590D0

Abstract

The process produces a one-end-closed double pipe (31) comprising an outer tube (32) and an inner tube (33). The gap between the tubes is closed at one end (34) of the pipe. The process comprises the steps of: providing a die (37, 38) having a molding cavity (39), a tubular punch (42), and a centre pin (40) disposed along the centerline of the punch; placing a piece of metallic material (35) in the molding cavity of the die; and pressing the metallic material with the tubular punch to force the metallic material to plastically flow separately into both a first space between the die and the tubular punch and a second space between the tubular punch and the centre pin to form the outer and inner tubes. The relative quantities of the separate flows of the metallic material into the first and second spaces are determined by the shape of the front portion of the tubular punch (42), thereby producing a desired ratio of the lengths of the outer and inner tubes. For example, the front surface (46, 47) of the tubular punch (42) may be tapered and/or side lands (43, 44) may be provided. These characteristics are adjusted to produce a smooth forging action. <IMAGE>

Description

PROCESS OF PRODUCING ONE-END-CLOSED DOUBLE PIPE The present invention relates to a process of producing a one-end-closed double pipe composed of an outer tube and an inner tube, the latter being disposed inside the former with a gap therebetween closed at one end of the pipe, by forging a single piece of metallic material.

Figure 1 shows a one-end-closed double pipe 21 suitably used as a component of a receiver unit for a refrigerating cycle. The conventional one-end-closed double pipe 21 is fabricated by staking a piece of inner tube 24 with another piece of outer tube 22 at the bottom portion of the latter.

This staking operation not only consumes time and labor, causing reduction in productivity, but also involves occurrence of staking defects inversely affecting the reliability of the bonding between the outer and inner tubes.

Thus, Japanese Unexamined Utility Model Publication (Kokai) No. 64-5067 suggested the use of a one-end-closed double pipe fabricated from a single piece of material.

Two possible ways for forming a one-end-closed double tube from a single piece of metallic material are casting (mold casting) and forging. Casting, however, is not suitable for producing a one-end-closed double tube because it does not provide good mechanical properties such as strength.

Forging, on the other hand, provides excellent mechanical properties such as strength in comparison with casting and is considered a suitable process for producing one-end-closed double pipes with a thin wall, but there is not established a technology or a die which enables a one-end-closed double pipe having outer and inner tubes to be fabricated as a monolithic body from a single piece of metallic material.

The present inventors conducted many experiments and tried to produce a one-end-closed double pipe with outer and inner tubes as a monolithic body by forging a single piece of metallic material. During the experiments, problems arose in that the length ratio between the outer and inner tubes deviates from a designed value because the plastic flow of the metallic material is biased to either one of the outer and inner tubes during forging. Thus, the conventional art merely provides an outer tube as a single or monolithic piece to which another piece of inner tube must be bonded by staking. This unavoidably causes reduction in productivity and bonding strength, as described herein above.

The object of the present invention is to provide a process of producing a one-end-closed-double pipe by forging a piece of metallic material at high efficiency while ensuring a desired ratio between lengths of the outer and inner tubes of the pipe.

To achieve the object according to the present invention, there is provided a process of producing a one-end-closed double pipe composed of an outer tube and an inner tube, the latter being disposed inside the former with a gap therebetween closed at one end of the pipe, by forging a single piece of metallic material, said process comprising the steps of: preparing a die having a molding cavity, a tubular punch, and a center pin disposed on the centerline of the punch; placing a single piece of metallic material in the molding cavity of the die; and pressing the metallic material with the tubular punch, thereby forcing the metallic material to plastically flow separately into both a first space between the molding cavity of the die and the tubular punch and a second space between the tubular punch and the center pin to form the outer and inner tubes of the pipe so that the separate flows of the metallic material into the first and second spaces are controlled by means of the front portion of the tubular punch that has a shape designed to provide selected quantities of the separate flows, thereby imparting the outer and inner tubes with a desired ratio of lengths thereof.

According to present invention, there is also provided a process of producing a one-end-closed double pipe composed of an outer tube and an inner tube, the latter being disposed inside the former with a gap therebetween closed at one end of the pipe, by forging a single piece of metallic material, said process comprising the steps of: preparing a die having a molding cavity, a tubular punch, and a center pin disposed on the centerline of the punch; placing a single piece of metallic material in the molding cavity of the die; and pressing the metallic material with the tubular punch, thereby forcing the metallic material to plastically flow separately into both a first space between the molding cavity of the die and the tubular punch and a second space between the tubular punch and the center pin to form the outer and inner tubes of the pipe so that the separate flows of the metallic material into the first and second spaces are controlled by means of the pressing front face of the tubular punch that has a tapered surface with a selected gradient and with an apex located at a selected position of the pressing front face; the gradient and the position being so selected as to provide desired quantities of the separate flows, thereby imparting the outer and inner tubes with a desired ratio of lengths thereof.

According to the present invention, there is further provided a process of producing a one-end-closed double pipe composed of an outer tube and an inner tube, the latter being disposed inside the former with a gap therebetween closed at one end of the pipe, by forging a single piece of metallic material, said process comprising the steps of: preparing a die having a molding cavity, a tubular punch, and a center pin disposed on the centerline of the punch; placing a single piece of metallic material in the molding cavity of the die; and pressing the metallic material with the tubular punch, thereby forcing the metallic material to plastically flow separately into both a first space between the molding cavity of the die and the tubular punch and a second space between the tubular punch and the center pin to form the outer and inner tubes of the pipe so that the separate flows of the metallic material into the first and second spaces are controlled by means of the front portion of the tubular punch that has outer and inner lands radially protruding outward and inward from the outer and inner side wall surfaces thereof, respectively; the lands having a ratio of lengths thereof so selected as to provide selected quantities of the separate flows, thereby imparting the outer and inner tubes with a desired ratio of lengths thereof.

In the process of the preset invention, a piece of metallic material is placed in the mold cavity of a forging die and is then pressed by a tubular punch, thereby forcing the metallic material to plastically flow separately into a first space between the die and the tubular punch and a second space between the tubular punch and a center pin to form outer and inner tubes which are continuous with each other at one end thereby composing a one-end-closed double pipe as a monolithic body.

Preferably, forging is carried out by using a tubular punch provided with a pressing front face having a tapered surface with a selected gradient and with an apex located at a selected position thereof, thereby controlling the separate flows of the metallic material into the first and second spaces, i.e., the apex location and the taper gradient of the front portion determine the position of a dividing ridge of the plastic flow of the metallic material into separate flows toward the first and second spaces for the outer and inner tubes, respectively. As the position of the dividing ridge determines the ratio of the lengths of the outer and inner tubes, a desired ratio of the outer and inner tube lengths is obtained by suitably selecting the apex position and the taper gradient.

Preferably, forging is carried out by using a tubular punch provided with a front portion having outer and inner lands radially protruding outward and inward from the outer and inner side wall surfaces thereof, respectively, the lands having a ratio of lengths thereof, thereby controlling the separate flows of the metallic material into the first and second spaces, i.e., the length ratio (or friction area ratio) of the outer and inner lands of the front portion determine the position of the division of the plastic flow of the metallic material into separate flows toward the first and second spaces for the outer and inner tubes, respectively.As the position of the division determines the ratio of the lengths of the outer and inner tubes, a desired ratio of the outer and inner tube lengths is obtained by suitably selecting the length ratio of the outer and inner lands.

It will be recognized that the tubular punch may be provided with both the tapered surface and the outer and inner lands so that the land length ratio and the apex location and taper gradient may be so selected as to control the position of the flow division, thereby controlling the length ratio between the outer and inner tubes of a product pipe.

Typically, the metallic material flows through the first and second spaces with a flow front free from mechanical constraint.

Non-limiting embodiments will now be described with reference to the accompanying drawings, in which: Figure 1 shows a conventional one-end-closed double pipe, in cross sectional view; Fig. 2 shows a one-end-closed double pipe produced by forging according to the present invention, in cross sectional view; Fig. 3 shows an example of metallic material to be forged into a one-end-closed double pipe according to the present invention, in perspective view; Figs. 4A and 4B show an arrangement of forging machine according to the present invention before and during forging, respectively, in cross sectional view; Fig. 5 shows the forging machine shown in Figs. 4A and 4B in the portion around the pressing front face of a tubular punch, at an enlarged scale; Fig. 6 is a graph showing the length ratio of the inner and outer tubes as a function of the inner land length;; Fig. 7 shows an example of the front portion of the tubular punch according to the present invention, in cross sectional view; Fig. 8 is a graph showing the length ratio of the inner and outer tubes as a function of the taper angle of the tapered surface; Fig. 9 shows another example of the front portion of the tubular punch according to the present invention, in cross sectional view; Fig. 10 shows a receiver in which a one-end-closed double pipe according to the present invention is used as the main body, in cross sectional view; Fig. 11 is a top plan view of the receiver shown in Fig. 10; Fig. 12 shows another example of the front portion of the tubular punch according to the present invention, in sectional view; Fig. 13 shows another example of the front portion of the tubular punch according to the present invention, in cross sectional view;; Fig. 14 shows another arrangement of forging machine according to the present invention, in cross sectional view; Fig. 15 shows another arrangement of forging machine according to the present invention, in cross sectional view; and Fig. 16 shows another example of metallic material to be forged into a one-end-closed double pipe according to the present invention, in perspective view.

Figure 2 shows a one-end-closed double pipe produced by a process according to the present invention, in sectional view. A one-end-closed double pipe 31 is a forged monolithic body composed of a thin wall outer tube 32 and a thin wall inner tube 33 with a closed end 34. The inner tube 33 opens at both ends thereof, i.e., it passes through the closed end 34 in the portion denoted as 33a. The gap between the outer and inner tubes 32 and 33 is closed at one end thereof by the closed end 34.

The one-end-closed double pipe 31 is made of a single piece of metallic material 35 having a good plastic deformability or forging formability such as aluminum alloy grade A3004. For example, the metallic material 35 has a shape of a short hollow cylinder with a thick wall and a center throughhole 35a with a diameter slightly greater than the inner diameter of the inner tube 33 of the pipe 31.

Figs. 4A and 4B schematically illustrate an arrangement 36 for producing the one-end-closed double pipe 31 from the single piece of metallic material 35.

An upper die 37 and a lower die 38 are assembled to compose a forging die having a molding cavity 39 for accommodating the short, thick wall, hollow cylinder 35.

The molding cavity 39 has a height greater than that of the metallic material 35.

A center pin 40 is fixed to the lower die 38 in an upward position so as to protrude upward from the center of the molding cavity 39. The protruded portion of the center pin 40 has the same diameter as the inner diameter of the inner tube 33. The throughhole 35a of the metallic material 35 has a diameter slightly greater than the diameter of the protruded portion of the center pin 40 so that the latter may be engaged in the former.

The protruded portion of the center pin 40 has a height greater than that of the metallic material 35. Knock-out pins 41 are placed through the lower die 38 to push out a forged product from the molding cavity 39.

A tubular punch 42 is held by a not-shown press machine movably in the vertical direction and coaxially with the center pin 40. The front portion (lower side in Figs. 4A and 4B) of the tubular punch 42 has ring-shaped inner and outer lands 43 and 44 radially protruding at selected distances from the inner and outer side wall surfaces thereof, respectively. The pressing front face of the tubular punch 42 has tapered surfaces 46 and 47 with an apex at a selected position thereof. A first space between the outer land 44 and the molding cavity 39 is equal to the wall thickness of the outer tube 32 of the pipe 31; and a second space between the inner land 43 and the center pin 40 is equal to the wall thickness of the inner tube 33 of the pipe 31.During cold forging, the metallic material 35 placed in the molding cavity 39 is pressed by the tubular punch 42, thereby forcing the metallic material to plastically flow separately into the first space between the mold cavity 39 and the outer land 44 of the tubular punch 42 and into the second space between the inner land 43 of the tubular punch 42 and the center pin 40, in such a manner as shown by the broad arrows of Fig. 5, to form the outer and inner tubes 32 and 33.

During cold forging for producing the one-end-closed double pipe 31 of this example, a flow dividing ridge 48 is always generated in the closed end 34 between the inner and outer tubes 33 and 32. The flow dividing ridge 48 is a branching point at which the plastic flow of the metallic material 35 is separated into flows toward the outer and inner tubes 32 and 33 as shown by the broad arrows in Fig. 5. The flow speed or displacement is zero in the radial direction at the flow dividing ridge 48.

The length ratio Hin/Hout (see Fig. 2) of the inner and outer tubes 33 and 32 varies with the position of the flow dividing ridge 48. Specifically, an inward deviation of the flow dividing ridge increases the relative material supply for the outer tube 32, and thereby increases the height of the outer tube 32, (i.e., smaller Hin/HoU value). To the contrary, an outward deviation of the flow dividing ridge increases the relative material supply for the inner tube 33, and thereby increases the height of the inner tube 32 (i.e., greater Hin/Hout value).

The length ratio of the outer and inner tubes 33 and 32 also varies with the ratios Ain/Aall and A0ut/Aaii of the cross sections Ain and Acut of the inner and outer tubes 33 and 32 to the cross section AaLl of the metallic material 35 or with the wall thickness ratio of the inner and outer tubes 33 and 32. However, it is usually very difficult to so select such cross sectional ratios or wall thickness ratio that provide a desired length ratio of the inner and outer tubes 33 and 32. Moreover, variation in any of these ratios would not only vary the mechanical properties of the forged product such as strength and heat conduction but also cause undesired increase in the weight of the forged product because of an excessive wall thickness.

According to this embodiment, the length ratio of the inner and outer tubes 33 and 32 is controlled by adjusting the position of the flow dividing ridge 48, not by such cross sectional or wall thickness ratios. The desired positioning of the flow dividing ridge 48 may be effected, on the one hand, by selecting the apex location and/or taper gradient of the tapered surfaces 46 and 47, and on the other hand by selecting the length ratio (Lin/Loue) of the inner and outer lands 43 and 44. In this example, these two modes are combined.

Namely, the location of the apex 45 and/or the taper gradient of the tapered surfaces 46 and 47 and the length ratio of the inner and outer lands 43 and 44 are both determined by a preliminary experiment and calculation so as to control the position of the flow dividing ridge 48 for providing a desired value of the length ratio Hin /Hout of the inner and outer tubes 33 and 32. The used experimental data will be hereinlater described in detail.

For example, to provide a common height of the inner and outer tubes 33 and 32, the flow dividing ridge 48 must be positioned on a radius rl as defined by: Ain/Arl = Aout/A2ri where art represents the cross sectional area of the closed end 34 in the region within the radius rl and 2-rl represents the cross sectional area of the closed end 34 in the region outside the radius rl.

To provide a greater height of the inner tube 33 than the outer tube 32, the flow dividing ridge 48 should be positioned outside the region defined by the radius rl, and to the contrary, to provide a greater height of the outer tube 32 than the inner tube 33, the flow dividing ridge should be positioned inside the region defined by the radius rl.

On the other hand, a smaller value of the ratio Ain/AoUc of the cross sectional areas Ain and Aouc of the inner and outer tubes 33 and 32 causes the flow dividing ridge 48 to move inward, and to the contrary a greater Ain/A value causes the ridge 48 to move outward.

out When the length ratio (Lin/Lout) of the inner and outer lands 43 and 44 is varied, the metallic material 35 plastically flowing separately toward the outer and inner tubes 32 and 33 is subjected to varied friction force because of the varied contact area between the flowing material and the lands 43 and 44, thereby varying the resisting force against the plastic flow to vary the position of the flow dividing ridge 48. In this case, a smaller Lin/L value increases the plastic flow toward out the inner tube 33 and thereby causes the flow dividing ridge to move outward, and to the contrary a greater Ljn/L0ut value increases the plastic flow toward the outer tube 32 and thereby causes the flow dividing ridge 48 to move inward.

This relationship is demonstrated by the experimental data plotted in Fig. 6 showing a variation of the length ratio Hin/Hou of the inner and outer tubes 33 and 32 when the length Lin of the inner land 43 is varied, with the length Lout of the outer land 44 kept constant at 1.7 mm. The tubular punch 42 used in this experiment did not have a tapered surface, as shown in Fig. 7. The punch 42 had inner and outer diameters of 9.5 mm and 56.8 mm, respectively. A center pin 40 having an outer diameter of 7.5 mm and a molding cavity, defined by dies 37 and 38, having an inner diameter of 60 mm were used.

Although the position of the flow dividing ridge 48 is preferably the same as or close to that of the apex 45 at which the tapered surfaces 46 and 47 intersect, it can be purposely shifted by adjusting the length ratio of the lands 43 and 44 and/or the taper gradient of the tapered surfaces 46 and 47, as described above. For example, a greater gradient of the inner tapered surface 46 increases the plastic flow toward the inner tube 33 and thereby causes the flow dividing ridge 48 to move outward, and to the contrary a greater gradient of the outer tapered surface 47 increases the plastic flow toward the outer tube 32 and thereby causes the flow dividing ridge 48 to move inward.

This relationship is demonstrated by the experimental data plotted in Fig. 8 showing the variation of the length ratio Hin/Hou of the inner and outer tubes 33 and 32 when the taper gradient of the outer tapered surface 47 is varied. Figure 9 shows the tubular punch 42 used in this experiment, that did not have an inner tapered surface, but had an inner land 43 with a length Lin of 20 mm, an outer land 44 with a length Lout of 1.7 mm, an apex 45 of the outer tapered surface 47, the apex being distant from the axis of the punch 42 by a radius of 8.8 mm. The metallic materials 35 used had two levels of height of 39 mm and 35 mm and a common diameter of 59 mm. It can be seen from Fig. 8 that the length ratio H. /Hout of the inner and outer tubes 33 and 32 is also varied by the gradient of the tapered surface 47 or the height of the metallic material 35.

A one-end-closed double pipe 31 is produced by cold forging in the forging apparatus 36 in the following sequence.

A single piece of metallic material 35 is placed in the mold cavity 39, with the center pin 40 engaged in the throughhole 35a of the metallic material 35, as shown in Fig. 4A. A press machine (not-shown) is then operated to lower the tubular punch 42 so that the pressing front face (tapered surfaces 46 and 47) of the punch 42 is pressed against the metallic material 35 to compress and crush the metallic material 35 within the mold cavity 39, as shown in Fig. 4B. This causes the metallic material 35 to plastically flow within the cavity 39, forming oppositely directed separate flows divided by the ridge 48 as denoted by the broad arrows in Fig. 5.The metallic material 35 is thus pressed up both through a first space between the mold cavity 39 and the outer land 44 of the tubular punch 42 and through a second space between the inner land 43 of the punch 42 and the center pin 40 to form the outer and inner tubes 32 and 33 which are continuous through the closed end 34 thereby composing a monolithic one-end-closed double pipe 31.

It should be noted in this sequence that the metallic material 35 flows up with the flow front moving upward free from mechanical constraint during forming of the outer and inner tubes 32 and 33, as can be seen from Figs. 4B and 5. This free movement without constraint of the flow front advantageously prevents the outer and inner tubes 32 and 33 from buckling, i.e., a wavy deformation along the longitudinal or flow direction, during forming thereof.

Generally, the tendency to buckle depends on the wall thickness of the tubes 32 and 33. An experimental study by the present inventors showed that, in the case of a one-end-closed double pipe having a relatively thin wall as shown in Figs. 4A and 4B (for example, as thin as 1 to 2 mm), the buckling easily occurs when the ratio HoU/D is 2.5 or more, where Hout and D are the height and outer diameter of the outer tube 32.

The arrangement of Figs. 4B and 5 prevents such buckling and is therefore particularly advantageously applied for the production of one-end-closed double pipes having the ratio H0ut/D of 2.5 or more. For example, Figs. 10 and 11 show a receiver with the main body 49 composed of a one-end-closed double pipe 31, usually having a Hout/D of 2.5 or more.

After the forging is completed, the tubular punch 42 is raised off and the forged product or a one-end-closed double pipe 31 is then pushed out of the mold cavity 39 by the knock-out pins 41. If the forged product is also raised off the cavity 39 together with the punch 42, the product is then removed from the punch 42 by means of a suitable remover (not shown).

When the cold-forged, one-end-closed pipe 31 is used as the main body of a receiver 49 composing part of a refrigerating cycle, the inner tube 33 should have a length Hin equal to or up to about 20 mm shorter than the length Hout of the outer tube 32 so that the Hin/Hout value fall within the range of from 0.9 to 1.0.

For example, when a forged product should have a ratio /Aout of 0.091 between the cross sectional areas of the inner and outer tubes 33 and 32, a tubular punch not having a tapered front surface (the length ratio of the outer and inner lands equals to 1.0) would yield but an Hin/Hout value of 1.4 to 1.6, not falling within the above-recited preferred range.

A tubular punch 50 shown in Fig. 9 according to a preferred embodiment of the present invention has an apex 45 of a tapered surface 47 located 8.8 mm distant from the axis of the punch 50 or on a radius of 8.8 mm, the tapered surface 47 with a gradient angle of 70, an inner land length Lin of 20 mm and an outer land length L of 1.7 mm. Forging by using this punch 50 yielded a out product with dimensions falling within the preferred Hin/Hout range of 0.9 to 1.0 with Hin of 235 mm. When Hi, is 195 mm, a tapered surface 47 with a gradient angle of 130 is successfully used.

Although the tubular punch 50 of Fig. 9 had a pressing front face in which the inner region with respect to the apex 45 is not tapered but flat, the inner region with respect to the apex 45 may also be tapered such as the tapered surface 46 of the punch 42 shown in Fig. 5 in producing a forged product with desired dimensions, if the position of the apex 45 and the gradient angle are selected based on a preliminary experiment and calculation.

It can be naturally understood that the outer region with respect to the apex 45, instead, may not be tapered but flat, such as the tubular punch 51 shown in Fig. 12.

Further, the pressing front may be entirely flat, such as the punches 52 and 53 shown in Figs. 7 and 13.

In this case, a desired length ratio Hin /H0ut of the inner and outer tubes 33 and 32 can be achieved if the position of the flow dividing ridge 48 is controlled by selecting the length ratio Lin/Lou of the inner and outer lands 43 and 44 based on a preliminary experiment and calculation.

The tubular punch 52 of Fig. 7 has an inner land 43 longer than an outer land 44 and the punch 53 of Fig. 13 has an outer land 44 longer than an inner land 43.

A one-end-closed double pipe forged by using any one of the tubular punches 42, 50, 51, 52 and 53 is used as the main body 49 of a receiver (a component of a refrigeration cycle) as shown in Figs. 10 and 11, in vertical sectional and top views, respectively. The receiver main body 49 has an open end squeezed to reduce the diameter and welded to a closure dome 54 spaced from the tip of the inner tube 33 to provide a path for coolant flow. The receiver main body 49 contains a drying agent or desiccant 55 packed between felt sheets (56, 57) and porous plates (58, 59). The porous plates 58 and 59 are staked by the inner and outer tubes 33 and 32. As shown in Fig. 11, the closed end 34 of the receiver main body 49 has a coolant inlet 60, screw holes 61 and 62, and positioning holes 63 and 64, which are machined afterwards.

Although the forging machine 36 has the center pin 40 fixed to the lower die 38, a center pin 66 may be fixed inside the tubular punch 42 such as a forging machine 65 shown in Fig. 14. In this case, the center pin 66 must have a length sufficiently greater than the height of the metallic material 35 so that the protruded end portion thereof is engaged in a hole 38a open at the center of the lower die 38 during forging.

Although the center pins 40 and 66 extend through the throughhole 35a of the metallic material 35 in the above-described examples, a center pin 68 does not extend through the metallic material 35 as used in a forging machine 67 shown in Fig. 15. In this case, the center pin 68 is fixed inside the tubular punch 42, as in the case shown in Fig. 14. The tip of the center pin 68 is positioned on the same level of the pressing front face of the tubular punch 42, so that these both press and crush a metallic material 69 during forging. The metallic material to be placed in the mold cavity 39 of this forging machine 67 is in the form of a short solid cylinder but need not have a throughhole, as shown in Fig. 16.

A one-end-closed double pipe 70 produced by the forging machine 67 has the lower end of the inner tube 33 that is also closed by a closed end 71. When the oneend-closed double pipe 70 is used as a receiver main body 49 shown in Figs. 10 and 11, the closed end 71 must be drilled to open a throughhole communicated with the lower end of the inner tube 33.

The metallic material 35 is not limited to an aluminum alloy but may be copper, low carbon steel or other metals or metal alloys having good plastic deformability or forging capability.

When a tubular punch (42, 50, 51) has a tapered surface (46, 47) on the pressing front face, the length ratio Lin/Lou of the lands (43, 44) may be 1.0, because the position of the flow dividing ridge 48 is controlled by the apex position and the taper gradient of the tapered surfaces 46 and 47.

It can be reasonably understood by a person skilled in the art that the forged product of one-end-closed double pipe (31, 70) of the present invention may be used in various applications other than a receiver 49.

To summarize, the described embodiments of the present invention have the remarkable advantages that a one-end-closed double pipe is efficiently produced by forging a single piece of metallic material by using a tubular forging punch having a pressing front face defined by tapered surfaces with an apex at a selected position and a selected taper gradient and/or having inner and outer lands with a selected length on the inner and outer side walls; and that the length ratio of the inner and outer tubes of the pipe can have a desired value by controlling the flow dividing ridge by selecting the apex position and/or taper gradient of the tapered surfaces and/or the length ratio of the inner and outer lands.

Claims

1. A process for producing a one-end-closed double pipe comprising an outer tube and an inner tube, the inner tube being disposed inside the outer tube with a gap therebetween which is closed at one end of the pipe, by forging a piece of metallic material, the process comprising the steps of: providing a die having a molding cavity, a tubular punch, and a centre pin disposed along the centerline of the punch; placing the piece of metallic material in the molding cavity of the die; and pressing the metallic material with the tubular punch to force the metallic material to plastically flow separately into both a first space between the die and the tubular punch and a second space between the tubular punch and the centre pin to form the outer and inner tubes of the pipe, with the relative quantities of the separate flows of the metallic material into the first and second spaces being determined by the shape of the front portion of the tubular punch, thereby producing a desired ratio of the lengths of the outer and inner tubes.

2. A process according to claim 1, wherein the front face of the tubular punch has a tapered surface with a selected gradient and with an apex located at a selected position, the gradient and the position being so selected as to provide the required relative quantities of the separate flows of the metallic material.

3. A process according to claim 1, wherein the front portion of the tubular punch has outer and inner lands radially protruding outwards and inwards from the outer and inner side wall surfaces thereof, respectively, the ratio of the lengths of the lands being so selected as to provide the required relative quantities of the separate flows of the metallic material.

4. A process according to claim 1, wherein: the front face of the tubular punch has a tapered surface with a selected gradient and with an apex located at a selected position; the front portion of the tubular punch has outer and inner lands radially protruding outwards and inwards from the outer and inner side wall surfaces thereof, respectively; and the gradient and the position of the apex of the tapered surface of the front face, and the ratio of the lengths of the lands, are so selected as to provided the required relative quantities of the separate flows of the metallic material.

5. A process according to any one of claims 1 to 4, wherein the metallic material flows through the first and second spaces with a flow front free from mechanical constraint.

6. A process according to claim 5, wherein the outer tube of the pipe has a ratio HoUt/D of not less than 2.5, Hout and D being the height and diameter of the outer tube, respectively.

7. A process for producing a one-end-closed double pipe, substantially as herein described with reference to, or with reference to and as illustrated in, Figures 2 to 16 of the accompanying drawings.

8. All novel features and combinations thereof.