US3924429A

US3924429A - Method and apparatus for reducing extrusion start-up pressure

Info

Publication number: US3924429A
Application number: US523403A
Authority: US
Inventors: Jr Francis Joseph Fuchs
Original assignee: Western Electric Co Inc
Current assignee: AT&T Corp
Priority date: 1974-11-13
Filing date: 1974-11-13
Publication date: 1975-12-09
Anticipated expiration: 1992-12-09
Also published as: CA1041946A; DE2550216C2; SE404661B; FR2290965B1; JPS5430908B2; GB1514750A; DE2550216A1; CH603266A5; JPS5171258A; BE835365A; ES442594A1; FR2290965A1; SE7512646L

Abstract

Extrusion start-up or peak pressure is reduced by initially commencing extrusion of the billet through a multiple aperture die and then closing off all but one of the said apertures in the die.

Description

United States Patent Fuchs, Jr. Dec. 9, 1975 METHOD AND APPARATUS FOR [56] References Cited REDUCING EXTRUSION START-UP UNITED STATES PATENTS PRESS 2,723,028 11/1955 Carter 72/261 [75] Inventor: Francis Joseph Fuchs, Jr., Princeton 3,063,560 11/1962 Edgecombe 72/261 Junction, NJ. 3,740,985 6/1973 Fuchs, Jr. 72/60 3,767,368 10/1973 Fuchs, Jr. 72/60 [73] Ass1gnee: Western Electric Company, Inc.,

New York Primary ExaminerCarl E. Hall [22] Filed: Nov. 13, 1974 Attorney, Agent, or FirmD. P. Kelley; A. S. Rosen [21] Appl. No.: 523,403

[57] ABSTRACT [52] U.S. Cl. 72/60; 72/261; 72/271; Extrusion start-up or peak pressure is reduced by ini- 72/284; 72/468 tially commencing extrusion of the billet through a [51] Int. Cl. B21D 22/10 l ipl p r re di n hen c ing ff l but ne [58] Field of Search 72/60, 61, 62, 63, 261, f he i aperture in the di 12 Claims, 5 Drawing Figures atent Dec. 9 1975 Sheet 1 of 3 3,924,429

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. .w OhIm ZOGQEPXM TIME US. Pitfillt Dec.91975 Sheet 2 of3 3,924,429

METHOD AND APPARATUS FOR REDUCING EXTRUSION START-UP PRESSURE BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates, broadly speaking, to improved method and apparatus for reducing extrusion start-up or peak pressure. More specifically, this invention relates to method of and apparatus for eliminating or substantially reducing start-up or peak pressure, which may be encountered in commencing hydrostatic extru- SlOn.

2. Description of the Prior Art Various methods and means for extruding billet material through a die are not well known. See, for example, U.S. Pat. No. 3,667,267 (1973) to Fuchs, which teaches method and apparatus for continuously hydrostatically extruding an elongated billet of indefinite length to produce wire of indefinite length.

In commencing hydrostatic extrusion of certain materials to obtain certain ratios of reduction, it has been noted that, when the nose of the billet enters and initially contacts the zone of deformation of the die, the pressure of the extrusion fluid rises to a peak value until extrusion commences. After extrusion commences, the pressure of the extrusion fluid falls to a lower, substantially constant, level known as the run-out pressure. Thus, in the hydrostatic extrusion of copper at reduction ratios greater than 20, there is always a peak pressure at the commencement of extrusion which is greater than the run-out pressure.

Although the peak pressure occurs only at the commencement of an extrusion operation, the pressure vessel for containing the extrusion operation, the die, die stem and related components must be designed to withstand this peak pressure which may, in some instances, be as much as 25 percent or more above run-out pressure, even though virtually the entire extrusion operation is performed at the lower run-out pressure. Thus, for virtually the entire extrusion operation, the pressure vessel, die, die stem and related components are overdesigned by as much as 25 percent or more. From another point of view, elimination or substantial reduction of peak pressure permits much higher (e.g., by an order of magnitude or so) ratios of reduction in equipment of a given design and pressure capability. Indeed, for certain materials, ratios of reduction are now attainable which heretofore were not attainable because of design limits on maximum pressure of equipment.

An example of the relationship between peak pressure and run-out pressure can be seen in the hydrostatic extrusion of a 0.300 inch diameter copper billet to produce 0.013 inch diameter wire, respresenting a reduction ratio of 500, wherein the peak pressure in the extrusion fluid is 370,000 psi and the run-out pressure in the extrusion fluid is 280,000 psi.

More, as peak pressure is reached, extrusion commences with a burst of speed, which may be undesirable.

In non-hydrostatic extrusion operations, it has been noted that, for certain materials and for certain ratios of reduction, the initial effort required to commence the extrusion operation is substantially in excess of the effort required to maintain the extrusion operation and, further, the extrusion operation may commence with an undesirable burst of speed.

Efforts have been made in the past to facilitate the extrusion of billets. In U.S. Pat. No. 2,630,220 (1953) to Sejournet, extrusion ofa hot billet or ingot of steel is commenced by interposing between the front end of the hot billet and the die a packet of glass fibers and glass plate, the glass melting under the heat of the billet and lubricating the die. In U.S. Pat. No. 3,345,842 (1967) to Richards, a hot billet is coated with a plurality of layers of glass of different viscosity-temperature characteristics (e.g., by sequentially dipping the entire hot billet into tanks of the various molten glasses) and is then extruded through a die. In the Sejournet and Richards patents, the molten glass acts as a lubricant.

More recently, eminently satisfactory solutions to the problem of reducing extrusion start-up or peak pressure have been taught in U.S. Pat. No. 3,767,368 (1973) to Fuchs, U.S. Pat. No. 3,766,766 (1973) to Fuchs and Venkatesan, U.S. Pat. No. 3,766,768 (1973) to Fuchs and Venkatesan, and U.S. Pat. No. 3,766,769 (1973) to Fuchs and Venkatesan. These solutions involve billet preparation, viz., the leading end of the bi]- let is provided with a nose of materials having a hardness gradient increasing from a relatively low value (i.e., high softness) at the front end of the nose to a value rearwardly thereof which may be equal to the hardness of the billet material.

The present invention represents a different approach to the problem of elimination or substantial reduction of peak extrusion pressure, not involving preparation of the billet itself in the manner of the patents mentioned in the preceding paragraph.

SUMMARY OF THE INVENTION One of the objects of this invention is to provide improved method and apparatus for eliminating or substantially reducing peak effort ordinarily required to initiate an extrusion operation.

A specific object of this invention is to provide improved method and apparatus for eliminating or substantially reducing peak pressure when commencing a hydrostatic extrusion operation.

A further object of this invention is to provide improved method and apparatus for smoothly commencing hydrostatic extrusion.

Still other and further objects of this invention will become apparent during the course of the following description and by reference to the accompanying drawings and the appended claims.

Briefly, the billet is initially extruded through a multiple aperture die and after extrusion has commenced, all but one of the apertures is closed, extrusion being continued through the remaining open aperture.

BRIEF DESCRIPTION OF THE DRAWINGS Referring now to the drawings in which like numerals represent like parts in the several views:

FIG. 1 represents an idealized plot of extrusion effort (e.g., hydrostatic extrusion fluid pressure) vs. time for two conditions, viz., conventional extrusion and extrusion according to the present invention;

FIG. 2 represents a view in elevation of the entrance end of a multiple aperture die used with the present invention;

FIG. 3 represents a medial longitudinal section of the multiple aperture die taken along the line 3-3 of FIG.

3 FIG. 4 represents a fragmentary view in elevation, partially in medial section, of extrusion apparatus employing the valve plate of the present invention; and

FIG. represents a view in elevation of the valve plate from the line 33 of FIG. 2.

DESCRIPTION OF THE PREFERRED EMBODIMENT In commencing the extrusion of a billet through a die for certain materials and certain ratios of reduction, when the nose of the billet enters and initially contacts the zone of deformation of the die, and until extrusion of the billet through the die begins, the effort required to force the billet through the die peaks, and when the billet starts to extrude through the die, the effort required to continue the extrusion falls off to a lower or run-out level. In terms of hydrostatic extrusion, when the nose of the billet enters and initially contacts the zone of deformation of the die, and until extrusion of the billet through the die begins, the pressure of the extrusion fluid rises generally along curve 1 of FIG. 1 and reaches a maximum value 2. When the billet begins to extrude through the die, the pressure of the extrusion fluid decreases, generally along curve 3, to a lower substantially constant level 4 known as the run-out pressure.

The difference between the maximum or peak pressure 2 and the run-out pressure 4 requires that the pressure vessel for containing the extrusion operation, the die, die stem and related components by overdesigned for this high transitory condition.

The present invention eliminates or substantially reduces the peak or maximum pressure 2, and permits the extrusion apparatus to be designed to operate substantially under the conditions of run-out pressure.

The multiple aperture extrusion die 5 with which the present invention is employed will, advantageously, be of the type disclosed in a pending US. Pat. application filed concurrently herewith in the name of Francis Joseph Fuchs, Jr. and entitled Multiple Aperture Die.

' Such a die 5 is shown in FIGS. 2 and 3 as comprising a conical mouth 6 at the apex of which is provided a first die aperture 7 extending through the said die 5 and communicating with passageway 8 at the rear end of the die 5. A plurality of second die apertures 9 is formed through the conical mouth 6 adjacent the entrance end 10 of the die 5, the said second die apertures 9 being radially equispaced around the die 5 as shown particularly in FIG. 2. Each of the second die apertures 9 is provided with a bevelled entry 11 arranged asymmetrically relative to the longitudinal axis of the said second die aperture 9 and extending toward the first die aperture 7. More specifically, a line joining the longitudinal axes of the first die aperture 7 and a second die aperture 9 will bisect the bevelled entry 11 of the said second die aperture 9. Advantageously, the angle between the bevelled entry 11 and the conical mouth 6 should lie in the approximate range of 4550. Each second die aperture 9 communicates with a passageway 12 at the rear end of the die 5.

Die 5 has been shown with six second die apertures 9 spaced 60 apart. It will be understood that die 5 may be provided with fewer or with more second die apertures 9 than shown.

The present invention may use die 5 to particular advantage in hydrostatic extrusion apparatus of the type disclosed in US. Pat. No. 3,740,985 (1973), although the invention is capable of use with other types of ex- 4 trusion apparatus. The said hydrostatic extrusion apparatus will be briefly described hereinbelow to aid in understanding the present invention.

US. Pat. No. 3,740,985 discloses coating the elongated surface of billet material with a shear transmitting medium which may, for example, be beeswax or polyethylene wax, and exerting through the shear transmitting medium a frictional or viscous drag force along the elongated surface of the billet material thereby to advance the billet material through a die to produce extruded product.

The shear transmitting-medium desirably has high viscosity and shear strength, is capable of lubricating the die, provides good wetting action on the billet material, and has minimal viscosity variation with respect to pressure, temperature and shearing rate.

The means which exerts, through the shear transmitting medium, viscous drag force along the elongated surface of the billet material comprises trains of gripping element segments, each train being continuously propelled by pinion gears around a separate endless path, all of the trains of grippingelements meeting along one length of travel from a first station upstream of a die to a second station downstream of the die and cooperating along said common length of travel to form a continuously moving train of centrally apertured gripping elements moving toward the die. The inner peripheries of the said apertures engaging the outer surface of the coating of shear transmitting medium on the billet material generate shear forces in the said medium which shear forces produce a frictional or viscous drag force along the elongated surface of the billet material to build up axial stress in the billet material and advance the billet material through the die. A pressure cylinder surrounds the centrally apertured gripping elements upstream of the die and exerts a normal pressure gradient on'the said gripping elements increasing toward the die, whereby a normal stress gradient is built up in the billet material increasing toward the die. These axial and normal stresses stress the billet material far above its yield strength and increase its ductility, or capacity for deformation without fracture.

Specific details of the extrusion apparatus are not part of the present disclosure. Reference should be made to US. Pat. No. 3,740,985 if information on such details is desired.

In the practice of the present invention with the extrusion apparatus disclosed in US. Pat. No. 3,740,985, die 5 is supported, as shown in FIG. 4, on die stem 13, the said die stem 13 having apertures extending therethrough in registry with

apertures

7 and 9 of die 5. The rear of die stem 13 is supported on die stem support 14 which is positioned in a counterbore in support plate 15, the latter being mounted to suitable auxiliary supporting structure (not shown). Die stern support 14 and support plate 15 are each provided with a central first aperture 17 and with second apertures 18 surrounding the said first aperture 17, the first aperture 17 and the second apertures 18 registering with the apertures in the die stem 13 and hence with first die aperture 7 and second die apertures 9, respectively, in die 5.

Valve plate housing 19, having an inwardly directed peripheral flange 20, is securely mounted in a suitable manner to the rear of support plate 15. Valve plate 21, having a central aperture 22 registering with first aperture 17in support plate 15 and having second apertures 23 adapted to register with second apertures 18 in support plate 15, is positioned in valve plate housing 19.,

flange retaining said valve plate 21 against the rear of support plate 15 with the central aperture 22 aligned with the first aperture 17. Means are provided to rotate valve plate 21 in valve plate housing 19 from a position in which all second apertures 23 in the valve plate 21 register with all second apertures 18 in support plate 15 to a position in which all said second apertures 23 have been shifted completely to the sides of the said second apertures 18 and the said second apertures 18 are blocked by the valve plate 21. Advantageously, said means may comprise, as shown in FIG. 5, a collar 24 mounted on valve plate housing 19, a threaded aperture 25 formed in said collar 24 and receiving a threaded shaft 26, a worm gear 27 at the end of the threaded shaft 26, and a geared sector 28 on the periphery of the valve plate 21 meshing with the worm gear 27. It will be apparent that, by rotating the threaded shaft 26, valve plate 21 is rotated in valve plate housing 19 so as to open or close second apertures 18 in the support plate 15, the first aperture 17 in the support plate 15 always being open. Valve plate housing 19, and particularly flange 20, should be strongly constructed so as to resist extrusion forces tending to push valve plate 21 away from support plate 15.

The operation of the invention will now be described.

Threaded'shaft 26 is operated so as to rotate valve plate 21 to a position in which all second apertures 23 register with all second apertures 18, i.e., all second apertures 18 are open, in addition to central aperture 17 being open. Billet 29, coated with shear transmitting medium such as beeswax or polyethylene wax, is advanced to and extruded through

apertures

7 and 9 of die 5 by means of gripping element segments 30 propelled toward and past the die 5 by pinion gears (not shown) engaging teeth 31 on the exterior surfaces of the said gripping element segments 30, in the manner taught in US. Pat. No. 3,740,985. The gripping element segments 30 cooperating as a train of gripping elements constitute a pressure chamber surrounding the billet 29 and exert, through the coating of shear transmitting medium, a frictional or viscous drag force along the surface of the billet 29 thereby advancing the said billet 29 against die 5 and through

die apertures

7 and 9 therein.

It will be understood that, in the manner taught in US. Pat. No. 3,740,985, the gripping elements are surrounded upstream of die 5 by a pressure cylinder (not shown) whereby a normal stress gradient increasing toward die 5 and indicated diagrammatically by arrows of increasing length in FIG. 4, is applied to the billet 29.

In the foregoing manner, axial and normal stress gradients, increasing toward die 5 to a value above the yield strength of billet 29, are built up in the billet 29 as the said billet is advanced, by frictional or viscous drag force along its surface, toward and into die 5 and through

die apertures

7 and 9 to produce simultaneously a plurality of extruded wires, wire 32 from aperture 7 and wires 33 from apertures 9. Up to this point, the pressure in the shear transmitting medium which may otherwise be denominated hydrostatic extrusion fluid pressure, and which is a specific manifestation of what may generically be termed extrusion effort, rises along curve 34 in FIG. 1 to a level indicated diagrammatically by the numeral 35.

Thereafter, threaded shaft 26 is operated, while the gripping elements continue to drive the billet 29 into and through the die 5, so as to slowly and gradually rotate valve plate 21 to a position in which all of the second apertures 18 are blocked off or closed and first aperture 17 remains open. Further extrusion of wires 33 will cease, and only wire 32 will continue to be ex- 5 truded, and the pressure in the shear transmitting medium will rise from point 35 along curve 36 to the level of curve 4 which represents the run-out pressure for single wire extrusion. Wires 33 downstream of valve plate 21 have been shown in FIG. 4 in phantom to indicate their temporary or transitory nature in the production operation. When the production of wires 33 has ceased, there will remain in apertures 18 of die stem support and support plate 15, in the corresponding apertures in the die stem 13 and in second die apertures 9 of die 5 plugs of the material constituting the billet 29. These plugs, in closing off second die apertures 9, convert the multiple aperture die 5 to a single aperture die for the continued production of wire 32.

In the operation hereinabove described, pressure peak 2 has been eliminated, and the extrusion apparatus need be designed only to withstand pressure of the level indicated by curve 4.

.The operation of the apparatus has heretofore been described in connection with the production of a single wire after all but one of the die apertures have been closed. It will be understood that the apparatus is not to be considered as being so limited, and that, upon closing the several of the die apertures, the remaining die apertures may be kept open to produce several wires. Thus, if die 5 has seven die apertures as shown, the valve plate may be designed to close down only five of these die apertures after commencement of extrusion, leaving two die apertures open for the continued production of two wires. As in the previously described operation, the undesirable transitory pressure peak is eliminated.

What is claimed is:

1. Method of producing wire from a billet, said method comprising:

a. applying force to the billet to commence extrusion of the billet simultaneously through first die aperture means and second die aperture means;

b. after extrusion of the billet simultaneously through the first die aperture means and the second die aperture means has commenced, and while maintaining force on said billet, gradually closing the second die aperture means;

0. continuing the application of force to said billet to continue the extrusion of said billet through said first die aperture means to produce wire.

2. Method as in claim 1, wherein:

d. force is applied hydrostatically to said billet.

3. Method of producing wire from a billet, said 55 method comprising:

a. applying force to the billet to commence extrusion of the billet simultaneously through a plurality of die apertures;

b. after extrusion of the billet simultaneously through the plurality of die apertures has commenced, and while maintaining force on said billet, gradually closing all but one of said die apertures;

c. continuing the application of force on said billet thereby to continue the extrusion of said billet through the remaining open die aperture to produce wire.

4. Method as in claim 3, wherein:

(I. force is applied hydrostatically to said billet.

5. Apparatus for extruding a billet to produce wire,

said apparatus comprising:

a. a die having a first die aperture and a second die aperture and an inlet end adapted to receive a billet and communicating with said first and second die apertures,

b. means for advancing said billet into the inlet end of said die to extrude said billet through said die to produce wire, and

c. means for closing the second die aperture after extrusion has commenced through both the first and second die apertures such that extrusion continues only through said first die aperture.

6. Apparatus for extruding a billet to produce wire,

said apparatus comprising:

a. a die having a first die aperture and a plurality of second die apertures, an inlet end adapted to receive a billet and communicating with said first and second die apertures, and an exit end adapted to discharge wire;

b. means for advancing said billet into the inlet end of said die to extrude said billet through said die to produce wire; and

c. valve means for closing the second die apertures after extrusion has commenced through both the first and second die apertures such that extrusion continues only through said first die aperture.

7. Apparatus as in claim 6, wherein:

(1. said valve means comprises:

i. a valve plate operatively'associated with the exit end of said die, said valve plate having a plurality of valve plate apertures adapted to register with all of said second die apertures;

ii. operating means to place said valve plate in a first position in which all of said valve plate apertures register with all of said second die apertures or selectively in a second position in which all of said valve plate apertures are completely out of registry with all of said second die apertures.

8. Apparatus as in claim 6, wherein:

d. said second die apertures surround said first die aperture,

e. said valve means comprises:

i. a valve plate operatively associated with the exit end of said die, said valve plate having a first valve plate aperture registering with said first die aperture and a plurality of second valve plate apertures adapted to register with all of said second die apertures;

ii. operating means to rotate said valve plate to a first position in which all of said second valve plate apertures register with all of said second die apertures or selectively to a second position in which all of said second valve plate apertures are completely out of registry with all of said second die apertures.

9. Extrusion die means comprising:

a. a die having a first die aperture and a second die aperture and an inlet end adapted to receive a billet and communicating with said first and second die apertures, and

b. means for closing the second die aperture after extrusion has commenced through both the first and second die apertures such that extrusion continues only through said first die aperture.

10. Extrusion die means comprising:

a. a die having a first die aperture and at least one second die aperture and an inlet end adapted to receive a billet and communicating with said first and second die apertures, and an exit end adapted to discharge wire; and

b. valve means for closing the at least one second die aperture after extrusion has commenced through both the first and second die apertures such that extrusion continues only through said first die aperture.

ll. Extrusion die means as in claim 10, wherein:

c. said valve means comprises:

i. a valve plate operatively associated with the exit end of said die, said valve plate having a plurality of valve plate apertures adapted to register with all of said second die apertures;

12. Extrusion die means as in claim 10, wherein:

c. said second die apertures surround said first die aperture,

d. said valve means comprises:

i. a valve plate operatively associated with the exit end of said die, said valve plate having a first valve plate aperture registeringwith said first die aperture and a plurality of second valve plate apertures adapted to register with all of said second die apertures;

UNITED STATES PATENT OFFICE fiERTIFICATE OF CORREQTION PatcntNo. QTQELLJLEQ Dated December 9. 1q75 lnventor(s) FRANCIS JOSEPH FUCHS, JR. Case 111 It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

In the Specification, column 1, line 16, "not Should read --no'w-- Column 3, line 30, "by" Should read --be-- Signed and Scaled this 1 second Day of March 1976 [SEAL] Attest:

RUTH C. MASON C. MARSHALL DANN Attesn'ng Officer 4 Commissioner ufPatems and Trademarks

Claims

1. Method of producing wire from a billet, said method comprising: a. applying force to the billet to commence extrusion of the billet simultaneously through first die aperture means and second die aperture means; b. after extrusion of the billet simultaneously through the first die aperture means and the second die aperture means has commenced, and while maintaining force on said billet, gradually closing the second die aperture means; c. continuing the application of force to said billet to continue the extrusion of said billet through said first die aperture means to produce wire.

2. Method as in claim 1, wherein: d. force is applied hydrostatically to said billet.

3. Method of producing wire from a billet, said method comprising: a. applying force to the billet to commence extrusion of the billet simultaneously through a plurality of die apertures; b. after extrusion of the billet simultaneously through the plurality of die apertures has commenced, and while maintaining force on said billet, gradually closing all but one of said die apertures; c. continuing the application of force on said billet thereby to continue the extrusion of said billet through the remaining open die aperture to produce wire.

4. Method as in claim 3, wherein: d. force is applied hydrostatically to said billet.

5. Apparatus for extruding a billet to produce wire, said apparatus comprising: a. a die having a first die aperture and a second die aperture and an inlet end adapted to receive a billet and communicating with said first and second die apertures, b. means for advancing said billet into the inlet end of said die to extrude said billet through said die to produce wire, and c. means for closing the second die aperture after extrusion has commenced through both the first and second die apertures such that extrusion continues only through said first die aperture.

6. Apparatus for extruding a billet to produce wire, said apparatus comprising: a. a die having a first die aperture and a plurality of second die apertures, an inlet end adapted to receive a billet and communicating with said first and second die apertures, and an exit end adapted to discharge wire; b. means for advancing said billet into the inlet end of said die to extrude said billet through said die to produce wire; and c. valve means for closing the second die apertures after extrusion has commenced through both the first and second die apertures such that extrusion continues only through said first die aperture.

7. Apparatus as in claim 6, wherein: d. said valve means comprises: i. a valve plate operatively associated with the exit end of said die, said valve plate having a plurality of valve plate apertures adapted to register with all of said second die apertures; ii. operating means to place said valve plate in a first position in which all of said valve plate apertures register with all of said second die apertures or selectively in a second position in which all of said valve plate apertures are completely out of registry with all of said second die apertures.

8. Apparatus as in claim 6, wherein: d. said second die apertures surround said first die aperture, e. said valve means comprises: i. a valve plate operatively associated with the exit end of said die, said valve plate having a first valve plate aperture registering with said first die aperture and a plurality of second valve plate apertures adapted to register with all of said second die apertures; ii. operating means to rotate said valve plate to a first position in which all of said second valve plate apertures register with all of said second die apertures or selectively to a second position in which all of said second valve plate apertures are completely out of registry with all of said second die apertures.

9. Extrusion die means comprising: a. a die having a first die aperture and a second die aperture and an inleT end adapted to receive a billet and communicating with said first and second die apertures, and b. means for closing the second die aperture after extrusion has commenced through both the first and second die apertures such that extrusion continues only through said first die aperture.

10. Extrusion die means comprising: a. a die having a first die aperture and at least one second die aperture and an inlet end adapted to receive a billet and communicating with said first and second die apertures, and an exit end adapted to discharge wire; and b. valve means for closing the at least one second die aperture after extrusion has commenced through both the first and second die apertures such that extrusion continues only through said first die aperture.

11. Extrusion die means as in claim 10, wherein: c. said valve means comprises: i. a valve plate operatively associated with the exit end of said die, said valve plate having a plurality of valve plate apertures adapted to register with all of said second die apertures; ii. operating means to place said valve plate in a first position in which all of said valve plate apertures register with all of said second die apertures or selectively in a second position in which all of said valve plate apertures are completely out of registry with all of said second die apertures.

12. Extrusion die means as in claim 10, wherein: c. said second die apertures surround said first die aperture, d. said valve means comprises: i. a valve plate operatively associated with the exit end of said die, said valve plate having a first valve plate aperture registering with said first die aperture and a plurality of second valve plate apertures adapted to register with all of said second die apertures; ii. operating means to rotate said valve plate to a first position in which all of said second valve plate apertures register with all of said second die apertures or selectively to a second position in which all of said second valve plate apertures are completely out of registry with all of said second die apertures.