GB2143454A - Apparatus and methods for extruding tubes with off-centre bores - Google Patents

Abstract

Apparatus and methods for extruding tubes having a longitudinal off-center passage therein, from billets having off-center bores comprises a die 30 for forming the outer shape of the tube and a mandrel 71 for forming its passage both being located off-center, so that flow of the material through the die will not push the mandrel to one side and cause the passage in the tube to be incorrectly located. <IMAGE>

Description

SPECIFICATION Apparatus and methods for extruding tubes with off-center bores This invention relates to metal working and more particularly to apparatus and methods for extruding metal to form tubular members.

Metal tubes have been extruded for many years. For the most part, these have been tubes of regular cross section, such as conventional tubes with concentric inside and outside surfaces. Extruding tubes of this type does not present a great problem.

There has developed a need to extrude tubes in which the bore is off center with respect to the tube's outer surface. The outer wall of such tube might be round, non-circular, substantially oval, or other desired shape, for instance, with the bore being positioned so far off center that the thickest portion of the wall might be three or more times as thick as the thinnest portion of the wall.

It is desirable that the bore of the extrusion be formed to finish size during the extrusion process so that machining of the bore later will be unnecessary.

In extruding tubes, a die having an aperture of suitable size and shape forms the outer wall of the tube while a mandrel of suitable size and shape and having a portion thereof extending into or through the die aperture forms the bore or passage of the tube.

Typically, the mandrel extends through the die aperture and forms therewith an annular opening through which material is extruded to produce the tube. Tubes of uniform section, such as a hollow cylinder, are readily extruded and present no problem since a round billet with a round bore is extruded through a round annular opening formed between a round die aperture and a round mandrel. When the billet, the die aperture and the mandrel are concentric, the extrudible material, as it is being extruded, flows into and through the die aperture about the mandrel uniformly and does not tend to push the mandrel to one side.Ideally, the flow of the extruding material is as nearly axial or linear as possible with uniform velocity distribution in the flow stream and, at the same time, creating uniform pressure forces acting about that portion of the mandrel which is disposed near and within the die aperture.

It is readily seen that unless these pressure forces about the mandrel are substantially equal, the mandrel may be forced off center, and the extruded tube will have a wall which is too thick on one side of the bore and too thin on the opposite side. This problem becomes more real when consideration is given the fact that the mandrel is usually long and slender, hence more readily deflected from true position. It must be sufficiently long to extend through both the billet bore and the die aperture. In the cases discussed hereinbelow, the billet was about 36 inches long by about 11 inches in diameter and the mandrel about 2 inches in diameter by about 42 inches long.

The problem of displacing or deflecting the mandrel to one side in the extrusion process is greatly multiplied as the extrudible material becomes stronger, harder, and/or more resistant to flow. Such materials require much greater forces to extrude them through a die, other things being equal.

U. S. Patent 4,333,527 which issued on June 8, 1 982 to Robert S. Higgins, et al. for SIDE POCKET MANDREL AND METHOD OF CONSTRUCTION shows a tubular structure, a portion of which could be extruded as a tube having an off-center longitudinal passage.

U. S. Patent Application, Serial No.

06/350,283, filed February 19, 1982 by David T. Merritt, et al. and erititled SIDE POCKET MANDREL, now U. S. Patent No.

4,416,330, shows a similar structure, a portion of which could be extruded as an irregular tube.

U. S. Patent Application, Serial No.

06/503,728, filed june 13, 1983 by Olen R.

Long for APPARATUS AND METHODS FOR EXTRUDING IRREGULAR TUBES, now U. S.

Patent No. - - - - - - - teaches the extrusion of irregular tubes including an irregular tube suitable for use in constructing side pocket mandrels such as that illustrated and described in the just-mentioned application, Serial No. 06/350,283, of David T.

Merritt, et al., of which Olen R. Long is a coinventor. In this sole application of Olen R.

Long, the irregular tube has a wall that varies in thickness about the irregular bore. However, the variation in wall thickness about the bore, while presenting problems, was extrudible using a centered die aperture and centered mandrel, even though the extrusions were of Type 4140 Steel which is hard and strong and is forced through the die only with great force.

The present invention makes it possible to extrude tubes of various sections having an off-center passage therein for use in making the receptacle portion of side pocket mandrels such as those taught in U. S. Patent 4,333,527 mentioned above, or for other uses. Such extrusions may be of high strength material, such as Type 4140 Steel, for instance.

None of the prior art of which applicants are aware discloses methods and apparatus for extruding tubes having off-center longitudinal passages therein wherein the wall at its thickest region is much thicker than the wall at its thinnest region. U. S. Patent 4,333,527 and U. S. Applications, Serial No.

06/503,728 and 06/350,283, all three of which are mentioned hereinabove, are incorporated herein by reference thereto for all purposes.

The present invention is directed to apparatus and methods of extruding a tube having an off-center longitudinal passage therein using an extrusion press having a containing vessel for containing a billet of extrudible material; a die closing one end of the containing vessel and having an aperture of suitable shape through which the billet material may be extruded; a ram movable into the opposite end of the containing vessel for forcing the extrudible material through the die aperture; and an off-center mandrel associated with the ram and having a portion extending through the die aperture and forming therewith an annular opening through which the extrudible material is extruded to produce a tube of desired cross section.

It is therefore one object of this invention to provide extrusion apparatus for extruding tubes having an off-center longitudinal passage therein, the mandrel of such apparatus being off center relative to the ram of the apparatus.

Another object is to provide such apparatus wherein the mandrel is slidably carried by the ram and is extensible and retractable relative thereto.

Another object is to provide such apparatus in which both the mandrel and the die aperture are off center relative to the ram of the apparatus.

Another object is to provide a method of extruding tubes having an off-center longitudinal passage therein using an extrusion apparatus having an off-center mandrel for forming the off-center passage in the tube.

A further object is to provide a method of extruding a tube of the character described using an extrusion apparatus in which both the mandrel and the die aperture are off center.

Another object is to provide such a method using extrusion apparatus in which the mandrel is carried by the ram, is extensible and retractable relative thereto and wherein the mandrel is round, or non-circular, or elliptical, or substantially oval, or irregular, or other desired shape in cross section.

Another object is to provide methods for extruding such tubes using extrusion apparatus of the character described and using tubular billets each having an off-center bore.

Other objects and advantages will become apparent from reading the description which follows and from studying the accompanying drawings wherein: Figure 1 is a fragmentary schematical view of an extrusion press in the act of extruding a tube in accordance with the present invention; Figure 2 is a cross-sectional view of the extruded tube taken along line 2--2 of Figure 1; Figure 3 is an isometrical view of a tubular billet of extrudible material; Figure 4 is a longitudinal view of a side pocket mandrel having a portion thereof formed from a short length of tube extruded in accordance with the present invention; Figure 5 is a cross-sectional view taken along line 5--5 of Figure 4; Figure 6 is a view similar to Figure 2 but showing a cross section of a round tube having an off-center bore;; Figure 7 is a diagrammatical view showing pressure forces acting inwardly about the outer surface of the mandrel of the extrusion press during the extrusion operation; Figure 8 is a cross-sectional view of a conventional prior art die for extruding tubes having an off-center bore; Figure 9 is a diagrammatical view showing the relationship of mandrel, die, and billet, with the mandrel centrally located; Figure 10 is a diagrammatical view similar to Figure 9 but showing the die aperture centered and the mandrel positioned off center; Figure 11 is a cross-sectional view of a tube which may be like the tube of Figure 2 but showing a line on a plane running through the center of the tube bore and perpendicular to the plane of symmetry; and Figure 1 2 is a cross-sectional view taken through a tubular billet such as the billet of Figure 3 and showing the off-center bore thereof.

Referring now to Figures 1 and 2, it will be seen that an extrusion press or apparatus is schematically shown and is indicated generally by the reference numeral 20. The press 20 has a body 22 having a cylinder portion 24 and a fixture support portion 26, as shown.

Extrusion die means 30 is held by fixture 32 mounted on the support portion 26 of the press. The die means 30 has an extrusion aperture 38 of the desired cross section, and its entrance end is flared to provide a suitable guide which may be in the form of rounded corners as at 40. In some instances, it may be desirable to form the guide with a taper, chamfer, or other desired shape.

Guide means 44 is mounted adjacent the exit end of the die. The guide means 44 includes a flange 46 adjacent the die means and having a tubular guide 48 with a bore 49 attached to its rearward side as shown and extending in a direction away from the die means.

A containing vessel 50 having a bore 56 is supported upon the support portion 26 of the press, and the rearward end 52 of the vessel 50 is preferably held against the face side 54 of the die means. Thus, the die means 30 closes the rearward end of the bore 56 of the vessel 50.

Ram means 60 is provided which includes a ram 61 which is telescopable into the containing vessel, as shown, and its forward end 62 is movable thereinto by main piston 66 to which the ram 60 is attached as by retaining ring 68 and bolts 69. The main piston 66 is slidable in bore 24a of cylinder portion 24 of the press. The forward end of ram 61 carries a pusher plate 70 having an aperture 76, and various such pusher plates may be had and interchanged as needed depending upon the sizes and shapes of the extrusions to be made.

A mandrel such as mandrel 71 is used to form or shape the bore or passage 72 through the extrusion while the die aperture 38 forms or shapes the exterior surface 73, seen in Figure 2. The mandrel 71 must extend through the die aperture 38 to form an annulus 74 between the die and the mandrel, and it is through this annulus that hot metal is extruded to form the tubular extrusion 75.

The mandrel 71 could, for small tubes, be attached to a pusher plate having no aperture.

In such case, it is difficult to insert billets into the containing vessel unless they are quite small. For larger tubes, it is preferably carried by the ram means as seen in Figure 1 so that it projects through the aperture 76 of the pusher plate 70 at all times. The aperture 76 in the pusher plate 70 provides lateral support for the mandrel 71 to help maintain it in position in the aperture 38 of the die 30.

Even so, uneven flow of the extruding metal through the die might force the mandrel to one side, causing the passage through the extrusion to be improperly positioned. This is especially true if high strength material such as Type 4140 Steel is extruded.

The mandrel 71 has its inner end attached to base member 80 (shown in dotted lines), and the base member 80 is attached to auxiliary piston 81 slidably mounted in auxiliary bore 82 of main piston 66 as shown.

Since the auxiliary piston 81 is slidable in bore 82 of the main piston 66, the mandrel is extensible and retractable as the auxiliary piston moves forward and back relative to the main piston. When the mandrel 71 is retracted, it extends outward of the pusher plate 70 a few inches. By moving the auxiliary piston forward, the mandrel is extended. The limit of extension for the mandrel is reached when the forward side of base member 80 engages the inner side of the pusher plate 70.

If desired, a bushing or plug (not shown) having an opening therethrough is placed temporarily in the die aperture to support the mandrel 71 in proper position until the extruding material pushes it out of the way.

When both mandrel 71 and the main piston 66 are retracted, a hot tubular billet such as billet 86 of suitable extrudible material is placed before the vessel 50 and oriented to align its bore 87 with mandrel 71 and die aperture 38. It is then ready to be moved into vessel bore 56. This billet is of a size to fit freely in the vessel 50, and its bore 87 freely accommodates mandrel 71. Hydraulic pressure from a suitable source is applied to the main and auxiliary pistons 66 and 81. The auxiliary piston, being much less massive than the main piston, moves first and extends the mandrel 71. The mandrel extends through the bore 87 of the billet 86 and also through the die aperture 38. If the bushing, previously mentioned, is used in the die aperture, the mandrel, of course, will extend through the aperture in the bushing and will be held in proper position thereby.Hydraulic pressure is built up in cylinder 24a, and the ram begins to move toward the die means and the pusher plate 70 engages the billet, pushing the billet ahead of it until its forward end engages the die means. The ram is moved yet farther into the vessel. When sufficient ram force is applied to the billet in the containing vessel, the billet first begins to upset and fill the available space within the vessel between the pusher plate and the die means. Then it commences flowing through the annulus 74 between the exterior of the mandrel 71 and the inner wall of the die aperture 38. This flow of the extruding material through the die aperture will push the bushing from position in the die aperture as before explained.The ram advances slowly toward the die, and the extrudible material of the billet is extruded rather rapidly, the extrusion being much smaller in section than the billet. If the velocity of the extrusion becomes excessive, the heat in the area of the die becomes excessive and metal begins to wipe off at the die and the extrusion is diminished in cross section and poorly shaped, usually with unwanted longitudinal grooves. It, therefore, is poor in quality and very likely unfit for its intended purpose.

Figure 2 shows a cross section of the extrusion 75. This cross section may be exactly like that disclosed in previously mentioned Application Serial No. 350,283 (now U. S. Patent 4,416,330) incorporated herein by reference.

It should be clear that the outer shape 73 of the extrusion 75 is determined by the shape of the die aperture 38, while the opening or inner shape 72 of the extrusion is determined by the shape of the mandrel 71.

Understandably, the aperture 76 in the pusher plate, like the shape of mandrel 71, need not be round, but is shaped to conform to the shape of the mandrel and is sufficiently large to allow free movement of the mandrel therethrough. The clearance about the mandrel should be minimal, perhaps on the order of 0.008 to 0.010 inch (0.020 to 0.25 millimeter), and preferably this clearance increases a little toward the forward end of the mandrel to prevent excessive friction between the mandrel and the extrusion.

The guide tube 48 prevents bowing of the extrusion. It is formed with its longitudinal passage 49 therethrough having a cross section corresponding to the exterior shape of the extrusion and with only slight clearance to permit unhindered movement of the extrusion therethrough. The clearance between the guide tube and the extrusion, is preferably on the order of 0.005 to 0.025 inch (0.12 to 0.62 millimeter), depending of course upon the size of the extrusion and the straightness required in the extrusion. In forming extrusions such as that illustrated herein and having a major axis in its cross section of about 4 to 5 inches (102 to 127 millimeters), a clearance of about 0.010 inch (0.25 millimeter) about the extrusion was provided to control bowing in the extrusion within satisfactory limits.

At the end of the ram stroke, the pusher plate stops before it strikes the die in order to avoid damage to either the die or the pusher plate. Upon retraction of main piston 66, the ram and the mandrel must be retracted from the extrusion. In so doing, the extrusion is dragged rearwardly through the die for a short distance before the mandrel pulls free thereof.

The containing vessel is then withdrawn from around the extrusion and gotten out of the way. At this time, it will be seen that the rearward end of the extrusion has a large malformed head about one inch (25.4 millimeters) thick and as large as vessel bore 56 and now spaced several inches from the die.

This head is cut off and discarded as scrap.

The extrusion is then removed from the die and guide tube and is laid aside to cool. The extrusion is preferably allowed to lie straight as it is allowed to cool down below the transition temperature (the temperature below which the metal will not flow or become distorted because of its own weight).

The extrusion 75 may be cut into lengths as desired which are suitable for use in constructing side pocket mandrels such as the side pocket mandrel illustrated and described in Application Serial No. 350,283, supra, or that indicated by the reference numeral 90 in Figures 4 and 5. It is seen that the side pocket mandrel 90 is provided with lower body section 92 having a cross section as seen in Figure 5. The cross section seen in Figure 5 corresponds to that seen in Figure 2 and discussed earlier. A valve receptacle 94 and flow ports 95 have been formed in this body member 92 before welding it into the side pocket mandrel as at 97 and 98 as shown.

It is readily understood that tubes with offcenter passages therein are not limited to the substantially oval shape seen in Figure 2 but may be formed in a variety of shapes. For instance, their outer shape could be elliptical, polygonal with almost any number of sides, rectangular, trapezoidal, and the like. The bore could be of any such shape also.

In the manufacture of side pocket mandrels, such as the side pocket mandrel 90 illustrated in Figure 4, it is sometimes desirable to make them with circular exterior walls particularly in the larger sizes for use in high pressure wells since the circular side pocket mandrels have superior collapse strength, other things being equal. A circular tube 100 having an offcenter bore 101 is seen in Figure 6. Tube 100 would be suitable for use in the construction of round side pocket mandrels similar to oval mandrel 90 of Figure 4.

As was stated earlier, extruding such tubes having off-center bores, such as the tubes 75 and 100, presents a problem, especially if such tubes are to be formed of high strength material such as Type 4140 Steel which is a very suitable material for high strength side pocket mandrels.

The problem, as mentioned earlier, relates to the need to maintain a balance of forces acting inwardly about the mandrel. The diagram of Figure 7 shows a tube 11 4 being extruded. A billet 11 5 is represented by a dotted line circle. The billet material is being crowded through the annular opening between the exterior of the mandrel 11 6 and the die aperture 11 7. The inward flow of the material applies inward forces to the mandrel.

These forces are represented by the inwardly directed arrows. Ideally, these forces should be equal. If these forces are unbalanced, lateral displacement of the mandrel relative to the die aperture is likely to result, and this would cause the passage in the extruded tube to be laterally displaced. Such tubes would be unfit for use. This would be costly in time, materials, and money.

Actual attempts to produce tubes such as tube 75, using prior art knowledge and experience, the accepted procedure called for use of a die having a cross section like that seen in Figure 8.

In Figure 8, the die 110 has a substantially oval aperture 111 through which a mandrel (not shown) would extend and form with aperture 111 as annular opening through which a substantially oval tube, such as tube 75 (Figure 2), could be extruded. In order to substantially equalize the forces tending to displace the mandrel laterally and distort the annular opening just mentioned, and at the same time keep the mandrel in its conventional central location, it was necessary to provide a second aperture in the die 110.

This second aperture is indicated by reference number 11 2. As a tube is extruded through aperture 111, material simultaneously extrudes through aperture 112 also. The second extrusion thus formed is called a sacrificial extrusion. It is considered a necessary evil. It is expensive. Type 4140 Steel is expensive, although it is one of the more economical grades of premium high strength materials suitable for mandrels such as mandrel 90.

This sacrificial extrusion wastes, in this case, about 30 percent of the material.

Experimental extrusions were made using a die such as that shown in Figure 8. In a very short time the web between the two die openings began to deteriorate or melt, beginning at the entrance end of the die. This experiment was made using a small die and producing a tube much too small for use in side pocket mandrels. In larger sizes, the problem would have been-foreseeably worse, so this prior art die design was abandoned and experiments then pursued using arrangements in accordance with the illustrations of Figures 9 and 10.

Figure 9 shows diagrammatically the relationship between the mandrel, the die aperture, and the billet if the sacrificial strip is not resorted to. The mandrel 1 20 is centered with respect to the billet 1 22 and is properly positioned with respect to the die aperture.

The annular opening 1 26 between the aperture and the mandrel is the opening through which the material of billet 1 22 would be extruded to form a tube of the shape of opening 1 26. The flow of the extrudible material will not be balanced about the mandrel because the thick material in the region A of the billet will only be partially used to form the thin part of the tube. The excess material from region A must flow around the mandrel 1 20 to help form the thick portion of the tube. Extruding Type 4140 Steel in this case is considered impossible because of the extreme pressure required to extrude the material and because of the extreme imbalance of side loads on the mandrel as the material flows from region A toward the opposite side of the mandrel.

Figure 10 presents a similar case. It shows aperture 1 24 centered with respect to billet 1 22 while mandrel 1 20 is properly positioned with respect to aperture 1 24 to provide an annular die opening 1 26 of proper shape. The mandrel in this case is off center while the aperture is centered. This case is better than the case presented in Figure 9 but is still considered a very difficult case for extrusions in sizes approximating that of extrusion 75, and especially so if high strength material is to be extruded.

In accordance with the present invention, the relationship between the billet and the aperture/mandrel can be ideally established in a manner which will now be described.

In Figure 11, there is shown a cross section of a tube to be extruded. This tube happens to have the same shape as does the tube 75 illustrated in Figure 2 and is therefore indicated by reference numeral 75. A horizontal line 76 is drawn through the center of bore 72 of the tube, dividing the cross section into two portions which are indicated by the letters A and B. This horizontal line 76 is perpendicular to the plane of symmetry, indicated by the vertical line 77 which also passes through the center of bore 72.

The areas of the portions A and B are calculated and compared to determine their ratio. For the sake of illustration, assume that portion A has three times the area of portion B, that is, the ratio of A to B is 3 to 1. This same ratio must be established in the tubular billet to be used.

Referring now to Figure 12, it will be seen that the billet 86 is bored as at 87 and has a horizontal line 88 drawn through the center of the bore, dividing the cross section into two portions which are indicated by the letters C and D.

The bore 87 is ideally located in the billet 86 such that the ratio of the area of portion C to the area of portion D will be equal to the ratio of A to B as before determined. In the case at hand, CD must equal A:B or 3:1.

Since these ratios are 3 to 1, it is readily seen that the upper portion, C, of the billet will go to make the upper portion, A, of the tube and that the lower portion, D, of the billet will go to make the lower portion, B, of the tube.

In calculating the ratio of A to B, the area within the periphery of the tube is calculated and then the area of the bore 72 is subtracted from that area to obtain the area of the metal in the tube cross section. Similarly, in calculating the ratio of C to D, the area within the periphery of billet 86 is calculated, then this are is reduced by the area of bore 87.

Thus, since the ratio in each case is 3 to 1, the area is divided by four, that is, three plus one, to obtain the area of the lower portion D of the billet. The remainder of the area, of course, constitutes the upper portion C of the billet.

Having A:B::C:D, the lines of flow of the extruding material through the die should be as straight and uniform as possible.

It is readily understood that the ratio of A to B of the tube is determined first. Next, the bore of the billet is located to cause the ratio of C to D to equal A to B. Then the mandrel and the die aperture are located in accordance therewith.

In actual experiments, tubes shaped virtually like that represented in Figure 2 were extruded from Type 4140 Steel in accordance with this invention without using excessive pressures in the main cylinder of the extrusion press. The extrusions produced after only a few practice pushes were of excellent quality except for the bore being slightly out of position. As anticipated, the bores of the tubes were formed to finished size by the mandrel as the tubes were extruded, and it was clear that further work on the bore would not be required.

It is possible that the location of the bore in the billet may need to be positioned closer to or farther from the center of the billet to compensate for such things as excessive looseness of fit of the billet in the containing vessel, uneven heat transfer from the billet to the containing vessel, and the physical charac teristics of the material to be extruded.

Thus, it has been shown that apparatus and methods have been provided for extruding tubes having off-center bores; that such tubes may be extruded of high strength materials such as Type 4140 Steel; that the off-center bore of billets used in forming such tubes can be precisely located, and that the die aperture and the mandrel of the press also can be precisely located off center to cause the lines of flow of the extruding material to be as straight as possible, thus avoiding unbalanced side loads on the mandrel at the aperture area; that when the billet bore, the die aperture, and the mandrel are located substantially as taught hereinabove, excessive pressures in the apparatus cylinder will be avoided; that tubes of many exterior shapes (circular, noncircular, oval, rectangular, polygonal with any desired number of sides, or other shapes) may be extruded, each with an off-center bore.

While the extrusion of high strength materials has been discussed hereinabove, and particularly Type 4140 Steel, the apparatus and methods of this invention could be ap plied to the extrusion of other materials also.

While tubes suitable for use in constructing side pocket mandrels have been illustrated and described hereinabove, tubes of the type disclosed herein may find utility in many places, even in other oil tools.

The foregoing description and drawings of the invention are explanatory and illustrative thereof, and various changes in sizes, shapes, materials, and arrangements of parts, as well as certain details of the illustrated construction, may be made within the scope of the appended claims without departing from the true spirit of the invention.

Claims (14)

1. Apparatus for extruding a tube having an off-center longitudinal passage therein, comprising: a containing vessel for containing a billet of extrudible material; die means closing one end of said containing vessel and having an aperture therethrough of desired shape; ram means axially aligned with said containing vessel and movable thereinto for forcing said extrudible material through said aperture of said die; and an elongate mandrel associated with and movable by said ram means, said mandrel being off center relative to said ram and having a portion extending through the aperture of said die and forming therewith an annular opening through which said extrudible material is extrudible.

2. The apparatus of Claim 1 wherein said mandrel is slidably carried by said ram means and is extendable and retractable relative thereto.

3. Apparatus for extruding a tube having an off-center longitudinal passage therein, comprising: a containing vessel for containing a billet of extrudible material; die means closing one end of said containing vessel and having an aperture therethrough of desired shape, said aperture being off center relative to said containing vessel; ram means axially aligned with said containing vessel and movable thereinto for forcing said extrudible material through said die means; and mandrel means carried by said ram means, said mandrel means being off center relative to said ram means and having a portion extending through said die aperture and forming therewith an annular opening through which said extrudible material is extrudible.

4. The apparatus of claim 3 wherein said mandrel means is slidably carried by said ram means and is extendable and retractable relative thereto.

5. The method of extruding a tube having an off-center longitudinal passage therein us ing an extruding press having a containing vessel for containing a billet of extrudible material, a die closing one end of said containing vessel, a ram movable into the other end of said containing vessel to force said extrudible material of said billet through said die to produce an extrusion, said die having an aperture of desired shape for controlling the outer shape of the extruded tube, and an off-center mandrel movable by said ram, said mandrel being of desired section and having a portion thereof projecting through said aperture of said die for controlling the inner shape of the extruded tube, said method comprising the steps of: placing a tubular billet of extrudible material in said containing vessel; moving said ram into said containing vessel and extending said mandrel through said tubular billet and said die; and moving said ram further into said containing vessel to force said extrudible material through said die to produce an extruded tube having an off-center longitudinal passage therein.

6. The method of claim 5 wherein said aperture of said die is located off center with respect to said containing vessel and said ram.

7. The method of claim 6 wherein said mandrel is carried by said ram and is slidable relative thereto between extended and retracted positions.

8. The method of claim 7 wherein said die aperture is circular.

9. The method of claim 7 wherein said die aperture is non-circular.

10. The method of claim 9 wherein said die aperture is substantially oval.

11. The method of claim 5, 6, 7, 8, 9, or 10 wherein the center of the bore of said tubular billet is offset from the longitudinal axis thereof.

1 2. The method of claim 11 wherein said mandrel is of circular cross section.

1 3. The method of claim 11 wherein said mandrel is of non-circular cross section.

14. Apparatus for extruding a tube substantially as hereinbefore described with reference to and as shown in the accompanying drawings.

1 5. A method for extruding tubes substantially as hereinbefore described.