EP0326602A1

EP0326602A1 - Continuous extrusion apparatus

Info

Publication number: EP0326602A1
Application number: EP88906966A
Authority: EP
Inventors: John East; Ian Maxwell
Original assignee: CMB Packaging UK Ltd; MB Group PLC
Current assignee: Crown Packaging UK Ltd
Priority date: 1987-08-18
Filing date: 1988-08-17
Publication date: 1989-08-09
Also published as: FI891780A; FI891780A0; WO1989001369A1; ZA886074B; GB8719518D0; GB8819561D0; JPH02501720A; GB2208618A

Abstract

A continuous extrusion apparatus has a rotatable wheel (2) with a peripheral groove (5). A shoe (4) encompassing part of the wheel (2) covers an arc of the groove (5) and terminates at an abutment (17) which penetrates and closes the groove (5). The shoe (4) and wheel (2) define a passageway for receiving a metal feedstock rod (1A) and propelling it to an extrusion zone (30) adjacent the abutment (17), from which zone (30) the feedstock (1A) is extruded through a die (9). The wheel groove (the "main groove") (5) has an adherent metal lining (12) for gripping and propelling the feedstock (1A). The lining (12) incorporates at the bottom a subsidiary groove (13) of width smaller than the main groove (5), into which subsi diary groove (13) a narrow metal feedstock rod (1A) of width less than that of the main groove (5) is urged and so pro pelled to the extrusion zone (30). The abutment (17) maintains the shape of the main and subsidiary grooves (5, 13) as the wheel (2) rotates. A ramp (16) projects into the main groove (5) at a position upstream of the extrusion zone (30) and urges the narrow feedstock rod (1A) into greater frictional engagement with the subsidiary groove (13) as the rod (1A) is pro pelled into the extrusion zone (30).

Description

CONTINUOUS EXTRUSION APPARATUS

Technical Field

This invention relates to a continuous extrusion apparatus for the extrusion of a rod-like metal feedstock, which apparatus comprises a rotatable wheel having a groove in its peripheral surface, a shoe member enclosing an arc of the groove to define an arcuate working passageway, an abutment protruding from the shoe into the groove to block the passageway and form upstream thereof an extrusion zone, and an extrusion die carried in the shoe member adjacent the extrusion zone and defining an orifice through which the feedstock metal is extruded from the extrusion zone and so exits from the working passageway and shoe.

Background Art

Such an apparatus is known in the art as a "CONFORM" machine. One such machine has been described in the British patent specification No. 1,506,845 (United Kingdom Atomic Energy Authority, London).

It will be understood from that specification that as the wheel rotates to carry the rod-like feedstock with it, the progress of the rod is hindered by its sliding against the internal surface of the stationary shoe, since the groove and shoe are dimensioned so as to fit closely against the rod-like feedstock which is being fed to the entrance to the passageway. That specification also teaches (a) that to increase the frictional engagement of the feedstock with the wheel, it is desirable in some cases, for instance when extruding a copper feedstock, to line the wheel groove with a coating of copper; and (b) that the undesirable frictional resistance of the stationary shoe to movement of the feedstock may be reduced by providing rollers on the shoe within the entrance to the working passageway. British patent specification No. GB 2,069,389 (B.I.C.C.) and European patent specification No. EP 0 071 490 (BABCOCK WIRE EQUIPMENT LTD) also teach the use of an abutment which is smaller that the wheel groove so that a tyre, or lining, of feedstock material is carried with the wheel groove past the abutment and remains adherent to the wheel groove surfaces and assists in driving particulate feedstock materials, in particular, to the extrusion die.

This latter prior art teaches that the passageway must be filled as soon as possible; and as a consequence considerable energy is needed to overcome the frictional resistance between the feedstock and the shoe.

We have discovered that if a tyre of a metal which will grip the feedstock lines the groove and is profiled so as to present an annular cleft to the feedstock so that the feedstock becomes wedged in the cleft as it progresses along the passageway, a feedstock smaller in cross section than the passageway can be used, so that the frictional engagement of the feedstock with the stationary shoe is reduced, and energy is saved. In order to maintain the shape of the cleft, an abutment is provided which conforms in shape to that of the cleft, so that the cleft profile is maintained by the abutment as the wheel rotates in contact with it.

Disclosure of the Invention

Thus, according to the present invention there is provided a continuous extrusion apparatus of the kind having a wheel, and a shoe encompassing part of the periphery of the wheel, the wheel having a peripheral groove therein, the shoe covering an arc of the groove and terminating at an abutment which penetrates and closes the wheel groove, so that the shoe and wheel together define a working passageway for receiving a metal feedstock rod and propelling it towards an extrusion zone adjacent the abutment, from which zone the feedstock rod is extruded through a die, and the wheel groove (hereinafter referred to as the 'main groove') having an adherent metal lining for gripping and propelling the feedstock rod, is characterised in that - the groove lining incorporates at the bottom thereof a subsidiary groove of width smaller than that of the main groove, into which subsidiary groove a narrow metal feedstock rod of width less than that of the main groove may be urged and so propelled thereby to the extrusion zone; and that the abutment is shaped so as to perpetuate the shape of the main and subsidiary grooves as the wheel rotates.

According to one preferred feature of the present invention, a deflector ramp carried by the shoe projects into the main groove at a position upstream of and adjacent to the extrusion zone, and is arranged to urge a said narrow feedstock rod into greater frictional engagement with the surface defining the subsidiary groove as the feedstock rod is propelled into the extrusion zone.

Preferably, the transverse cross-sectional shape of the subsidiary groove closely matches that of a selected narrow feedstock rod that is to be fed into and be propelled by the subsidiary groove.

In one preferred arrangement, the narrow feedstock rod is of circular cross section, and the subsidiary groove is at least in part semi-circular in transverse cross section.

The subsidiary groove may have opposed side wall parts which are at least in part parallel and which receive a selected narrow feedstock rod'with an interference fit. Alternatively, the subsidiary groove may have opposed side wall parts which are at least in part convergent in a radially inwards direction, whereby a selected narrow feedstock rod may become wedged In the subsidiary groove as it is propelled towards the extrusion zone.

A selected narrow feedstock rod may have a radial dimension that is greater than its transverse dimension.

As compared with conventional apparatus In which the conventional tooling (i.e. the abutment and the feedstock- contacting parts of the shoe member) ensures that the wheel groove and the shoe member closely fit and contact the feedstock rod, the benefits accruing from the use of the present invention include the following:

1. a substantial reduction in the frictional resistance of the shoe member to movement of the feedstock rod, and hence in the energy otherwise lost in overcoming friction; 2. a variety of feedstock cross sections can be used with but a single shoe and main groove profile, using merely a different abutment to accommodate in the groove lining a different feedstock rod. This avoids the need for, and expence of having, several different sets of shoe and associated main groove defining parts (or wheels) each dedicated to one particular form of feedstock rod;

3. the extrusion loads and temperatures arising in operation can be controlled by varying the input volumetric feed rate without varying the speed of rotation of the wheel, that is by feeding a feedstock rod of a different cross section;

4. it is possible to extrude various high strength alloy feedstock rods which hitherto have been too difficult to extrude; and 5. the force per unit area on the abutment is reduced, so that the reliability and service life of the tooling are improved.

Other features of the present invention will appear from a reading of the description that follows hereafter and of the claims appended at the end of that description.

One embodiment of the present invention will now be described by way of example and with reference to the accompanying diagrammatic drawings.

Brief Description of the Drawings

Figure 1 is an end view of a known CONFORM machine for extruding feedstock rod;

Figure 2 is a sectional view taken on the vertical plane

II-II of Figure 1; Figure 3 is a sectional view taken on the horizontal plane

III-III of figure 2;

Figure 4 is a sectional view similar to that of Figure 2, showing a CONFORM machine modified in accordance with the present invention, and having a narrow feedstock rod in the course of entering the working passageway of the machine;

Figure 5 is an enlarged sectional view taken on the inclined plane V-V of Figure 4;

Figure 6 is a sectional view similar to that of the Figure

4, but showing instead the feedstock rod being extruded; Figure 7 is an enlarged sectional view taken on the inclined plane VII-VII of Figure 6;

Figure 8 is an enlarged sectional view taken on the horizontal plane VIII-VIII of Figure 6;

Figure 9 is a fragmentary sectional view similar to those of Figures 4 and 6 showing one practical form of the machine shown in the Figures 4 and 6; and

Figure 10 is a perspective view of an abutment incorporated in the machine of Figure 9.

Modes of carrying out the Invention

Referring now to the drawings, as shown in the Figures 1-3, a conventional CONFORM machine for extruding a feedstock in the from of a solid metal rod 1 comprises a wheel 2 supported for rotation on a shaft 3 which is carried in bearings (not shown). A shoe 4 is held stationary by fixings (not shown), and closely encompasses an arc of the periphery of the wheel 2. An annular groove 5 in the periphery of the wheel and an internal surface 6 of the shoe define a working passageway in which the transverse section of the feedstock rod 1 fits closely. An abutment portion 7 of the shoe supports an abutment 8 which protrudes from the shoe into the groove 5 so blocking

(closing) the downstream end of the passageway, as shown in the Figures 2 and 3. A die Insert 9 is located in the inner surface of the shoe upstream of and close to the abutment 8, and feedstock metal (extrudate) 11 emerging from the die orifice exits from the shoe 4 through an exit passage 10 in a substantially radial direction.

In Figure 2, it can be seen that the feedstock rod 1 is a tight fit in this prior art passageway defined by the wheel groove 5 and the shoe inner surface 6. That shoe surface 6 is designed to push the rod 1 firmly into the wheel groove 5, so that it becomes firmly engaged by friction with the wheel groove 5, and is therefore driven by the wheel to the abutment 8 with sufficient force to generate the extrusion pressure necessary to force the feedstock metal through the extrusion orifice of the die 9. In this arrangement, there is considerable sliding frictional resistance between the shoe inner surface 6 and the feedstock rod 1. Furthermore, the apparatus of Figures 1-3 is dedicated to the processing of only one particular cross section of feedstock rod.

Figure 4 shows an apparatus similar to that of the Figures 1-3, so that like parts in Figure 4 are denoted by the references assigned to corresponding parts in the earlier Figures: namely, wheel 2; shaft 3; shoe 4; abutment portion 7; die 9; and exit passage 10.

However, in the Figures 4 and 5 it will be seen that the wheel has been modified in that its groove (the main groove) is provided with an adherent annular metal lining or tyre 12 (of the same metal as the feedstock rod to be used), in which tyre is formed an annular cleft or subsidiary groove 13 of cross sectional shape matching that of the narrow feedstock rod 1A which is to be received in that subsidiary groove.

Thus, the working passageway of the machine is formed by the radially-opposing surfaces of the shoe 4 and the groove lining 12 respectively and the transversely-opposing annular side walls of the wheel groove.

Since the feedstock rod is substantially narrower than the transverse distance between the opposing side walls of the wheel groove, it does not fill the entire passageway defined by the shoe inner surface 14 and the groove profile 15. Thus, when lying in the subsidiary groove 13, the feedstock rod does not make any contact with the side walls of the wheel groove, nor with the inner surface of the shoe.

In addition, a ramp 16 has been added to the shoe inner surface 14, and projects radially Inwards towards the subsidiary groove 13.

Figure 4 shows the feedstock rod 1A at an early stage during its introduction into the working passageway. At that stage the rod is roughly tangential to the lining 12, with the leading end striking and rubbing against the inner surface 14 of the shoe and a trailing portion wiping against an entry sill 4A of the shoe. Friction at the point of contact between the rod 1A and the groove lining 12 drives the rod along the passageway, and so causes the rod to bend and conform to the shape of the passageway and so settle in the cleft 13 in the lining 12. The frictional forces exerted on the rod 1A are thereby enhanced, and propel the rod to the extrusion zone 30 upstream of the abutment. It should be noted that in this preliminary condition, the frictional resistance of the shoe to the movement of the rod is much less than in the corresponding case of the prior art embodiment of Figure 2. Figure 6 shows the apparatus of Figure 4 at a time when the feedstock rod 1A has advanced past the ramp 16 into the extrusion zone 30 adjacent the abutment 17, and is being extruded through the orifice in the die 9. The ramp is intended to force the rod 1A firmly Into greater frictional engagement with the cleft 13, whereby to cause the wheel to exert on the rod 1A a driving force sufficient to generate in the ext rus i on zone 3 0 an ext rus i on pre s sure hi gh enough to extrude the metal of the rod through the extrusion orifice of the die 9.

Whereas the round rod 1A is shown in the Figure 7 to have been flattened by the ramp 16 and only partially pushed into the cleft 13, as may happen if the rod is of a soft metal such as annealed copper, harder metals such as brass, alloyed aluminium or steel may retain the original round cross section and simply be driven deeper into the cleft.

Whereas only one ramp 16 has been shown, other ramps may be incorporated on the shoe at other upstream positions if desired; or if desired, the ramp may be omitted in those cases where its presence is not found to be necessary.

After passing the ramp 16, the rod strikes the abutment 17 and becomes progressively upset in thickness until it fills the extrusion cavity 30 adjacent the die. The continuing entry of further feedstock rod into that cavity produces the extrusion pressure necessary to extrude the feedstock metal 1A through the die orifice.

Referring now to the Figure 8, it will be seen that the extrusion cavity defined by the abutment 17, groove lining 12 and shoe surface 14/die 9 is completely filled with feedstock metal 1A. The continous line shown separating the feedstock metal 1A from the lining metal 12 at the boundary of the cleft 13 is purely notional, since those metals will have become united at this part of the extrusion zone. However, that line serves to indicate the shape of the abutment 17, which shape is shown more clearly in the Figure 10.

The shape of the abutment is important because the abutment serves not only to block the passageway and so cause the extrusion pressure to build up in front of it, but also because it regenerates the shape of the cleft in the lining ready for receiving further feedstock rod at the entrance to the working passageway.

The abutment 17 is made as a separate block for fitting into the abutment portion 7 of the shoe, so that it may be replaced when worn or when a feedstock rod of a considerably different cross section (e.g. a square cross section) is to be used.

The die block 9 is also replaceable so that various cross sections of extrusion product can be readily produced (e.g. larger round sections, or square sections). A change of die orifice relative to the feedstock cross section may be used to vary the extrusion ratio, if desired. Alternatively, the diameter of feedstock may be varied without changing the apparatus of Figures 4-6 to achieve a change of extrusion ratio.

Referring to Figures 8-10, it will be seen that the arcuate annular portion 18 of the abutment has a radius 'r' such that it forms a cleft 13 in the lining 12 having a cross sectional shape close to (preferably less than) that of the feedstock rod to be extruded, so that the cleft 13 is able to provide the frictional engagement necessary for slip- free transport of the rod to the extrusion cavity 30 adjacent the die 9. The arcuate annular portion 18 of the abutment connects with divergent side surfaces 19, which in turn connect with parallel side surfaces 20 to define the abutment.

The 'abutment penetration' (defined as the area of the abutment face protruding beyond the inner surface of the die 9 as shown in Figure 8) is set preferably to a value close to that of the cross section of the feedstock rod 1A, in order to ensure that the rod will fill the pressurised cavity upstream of and adjacent the die 9.

If no ramp 16 is used, the distance between the face of the die 9 and the 'apex' of the cleft (denoted 'X' in Figure 8) is preferably set to a value sufficiently close to the feedstock rod diameter as to ensure that the rod is pressed into the subsidiary groove constituted by the cleft 13 before the pressurised cavity is reached and that adequate grip is thus achieved.

It is desirable to provide a specific abutment cross section for each type of cross section of feedstock rod to be used, which is of course much cheaper than providing a whole shoe and an associated wheel with a different groove width, as would otherwise have been required with the prior art apparatus.

Figure 9 shows an actual apparatus in which the wheel 2, lining 12, shoe 4, internal surface 14, and ramp 16 work as already described to drive a rod to the abutment 17. However, in this apparatus the shoe includes a recess to accommodate a thrust block 21, and a spacer 22 which adjoins a die support block 23, which not only supports a die 9 but also defines the ramp 16. The die thrust block 21 has a passage 25 for cooling water which is sprayed on to the extrusion product emerging from the die through the passage 10.

Also accommodated in the recess is an abutment thrust block 24 which permits adjustment of the penetration 'X' of the abutment 17 into the lining 12.

A further passage 26 directs cooling water tangentially towards the tyre 12 across the faces 18, 19 of the abutment .

The apparatus of Figure 9 is in fact a modification of an apparatus which is suitable for extruding particulate feedstocks, such feedstocks being fed through a hopper entry 28. There is no need to alter this arrangement if it is desired to carry out this present invention. However, if desired, an adjustable wear plate 27 may be provided to act as an entry sill and so protect the shoe and hopper from abrasive wear as the feedstock rod enters the working passageway. Movement of the plate 27 towards the wheel may be used to increase the radial force exerted on the feedstock at the tangent point shown in the Figure 4, and thereby develop an increased arc of contact of the rod with the wheel lining 12, for improving the driving force applied to the rod.

By way of non-limiting example, the apparatus of Figure 9 has been used to extrude copper wire of 2 mm diameter from a feedstock rod in the range 6-8 mm diameter. Typically, a rod of 7 mm diameter was fed to the passageway, using an abutment having an arcuate surface of 3 mm radius and a penetration value 'X' of 4.75 mm. A wire of 2 mm diameter was extruded. As will be readily understood from the foregoing description, the apparatus may be provided with alternative sizes of abutment to extrude other sections from feedstocks of various cross sections.

For reasons of metallurgical cleanliness, it is desirable that the tyre be made of the same metal as the feedstock which is to be extruded. One method of preparing the tyre 12 is to wind a tube of the metal into the wheel groove, and to compress it by means of progressive rolling so as to expand and fill the re-entrant circular cross section of the groove as depicted in Figures 5, 7 and 8. However, alternative methods of preparing the tyre include wire winding, metal sputtering, or the use of a split wheel and pre-machined tyre. Preferably, the transverse cross-sectional shape of the subsidiary groove closely matches that of a selected narrow feedstock rod that is to be fed into and be propelled by the subsidiary groove, and preferably is such as to provide a good grip on the feedstock rod entering it.

Where the narrow feedstock rod is of circular cross section as in the above embodiments, the subsidiary groove is preferably at least in part semi-circular in transverse cross section.

The subsidiary groove may have opposed side wall parts which are spaced apart to receive a selected narrow feedstock rod with an interference fit. Such side wall parts may be at least in part parallel; or they may be at least in part convergent in a radially inwards direction, so that a selected narrow feedstock rod may become wedged in the subsidiary groove as it is propelled towards the extrusion zone.

If desired, the selected narrow feedstock rod may have a radial dimension that is greater than its transverse dimension.

The bottom and sides of the subsidiary groove may have any other profile which is directed at providing a good frictional drive between the wheel and the feedstock rod entering the subsidiary groove. For example, the base of the subsidiary groove may be corrugated, serrated or stepped across the transverse width thereof.

Whereas in the embodiments described above, the groove lining 12 extends but a short way up the respective parallel side walls of the wheel groove, in other embodiments the lining extends to a greater distance up those side walls, being let into recessed annular parts of those side walls (as shown in the Figure 5 by the dotted lines 32) if desired.

Claims

1. A continuous extrusion apparatus of the kind having a wheel (Fig.2: 2), and a shoe (4) encompassing part of the periphery of the wheel (2), said wheel having a peripheral groove (5) therein, and said shoe (4) covering an arc of said groove (5) and terminating at an abutment (8) which penetrates and closes the wheel groove (5), so that the shoe (4) and wheel (2) together define a working passageway for receiving a metal feedstock rod (1) and propelling it towards an extrusion zone adjacent the abutment (8), from which zone the feedstock rod (1) is extruded through a die (9), the wheel groove (hereinafter referred to as the 'main groove') (5) having an adherent metal lining for gripping and propelling the feedstock rod (1), characterised in that - the groove lining (12) incorporates at the bottom thereof a subsidiary groove (or cleft) (13) of width smaller than that of the main groove (5), into which subsidiary groove (13) a narrow metal feedstock rod (1A) of width less than that of the main groove (5) may be urged and so propelled thereby to the extrusion zone (30); and that the abutment (17) is shaped so as to perpetuate the shape of the main and subsidiary grooves (5, 13) as the wheel rotates.

2. An apparatus according to claim 1, wherein a deflector ramp (16) carried by the shoe (4) projects into the main groove (5) at a position upstream of and adjacent to said extrusion zone (30), and is arranged to urge a said narrow feedstock rod (1A) into greater frictional engagement with the surface defining the subsidiary groove (13) as the feedstock rod (1A) is propelled into the extrusion zone (30).

3. An apparatus according to claim 1 or claim 2, wherein the transverse cross-sectional shape of the subsidiary groove (13) closely matches that of a selected narrow feedstock rod (1A) that is to be fed into and be propelled by the subsidiary groove (13).

4. An apparatus according to claim 1, 2 or 3, wherein the narrow feedstock rod (1A) is of circular cross section, and the subsidiary groove (13) is at least In part semicircular in transverse cross section.

5. An apparatus according to claim 3 or 4, wherein the subsidiary groove (13) has opposed side wall parts which are at least in part parallel and which receive a selected narrow feedstock rod (1A) with an interference fit.

6. An apparatus according to claim 1 or 2, wherein the subsidiary groove (13) has opposed side wall parts which are at least in part convergent in a radially inwards direction, whereby a selected narrow feedstock rod (1A) may become wedged In the subsidiary groove (1A) as it is propelled towards the extrusion zone (30).

7. An apparatus according to any preceding claim, wherein the selected narrow feedstock rod (1A) has a radial dimension that is greater than its transverse dimension.

8. An apparatus according to claim 4, wherein the abutment (17) has an arcuate surface (18) corresponding in shape to that of the transverse cross section of the narrow feedstock rod (1A), which surface has adjacent divergent side wall parts (19), which in turn connect with respective parallel side wall parts (20), whereby to define a subsidiary groove (13) having a corresponding, arcuate surface which is flanked by radially outward divergent surfaces.

9. An apparatus according to any preceding claim, wherein (a) the wheel groove (5) includes a pair of annular seal surfaces (34) spaces apart and an annular groove surface (36) of width greater than the gap between the annular seal surfaces (34) and depth greater that the length of penetration of the abutment (17) into the groove (5), (b) the abutment (17) has a pair of annular seal surfaces (20) spaced apart to cooperate with the seal surfaces (34) of the wheel groove (5), and comprises a convex protrusion (18) of width less that the width apart of the seal surfaces (20) of the abutment (17), said convex protrusion (18) extending into the annular groove surface (15), and flank portions (19) extending to each side of the convex protrusion (18) to join the protrusion (18) to each seal surface (20) at a level within the annular seal surfaces (34) of the groove (5) or within the groove surface (36), said flank portions (19) thus being deeper into the groove (5) than the entry surface of the die (9).

10. An apparatus according to claim 9, wherein the abutment (17) comprises a convex protrusion (18) having an arcuate surface (18) supported by divergent side walls (19) which connect with parallel side seal surfaces (20) to define a replaceable abutment (17) which when in use defines a cleft (13) having a concave surface flanked by divergent surfaces.

11. An apparatus according to any preceding claim, wherein the abutment (17) is adjustable towards or away from the wheel groove (5).

12. An apparatus according to any preceding claim, wherein a feedstock guide plate (Fig. 9: 27) is mounted on the shoe (4) at the entrance to the working passageway for guiding a feedstock rod (1A) into that passageway, said guide plate

(27) being adjustable towards or away from the wheel groove (5).

13. An apparatus according to any preceding claim, wherein the groove lining (12) comprises a hoop of metal tube or a winding of metal wire, which hoop or winding has been radially crushed so as to fill the wheel groove (5), and has thereafter been machined to form the subsidiary groove ( 1 3 ) .

14. A method of operating a continuous extrusion apparatus according to any preceding claim, which method comprises the steps of:

(a) lining the wheel groove (5) with a tyre (12) of the metal of a feedstock rod (1A) which is to be extruded;

(b) forming in the periphery of the tyre (12) an annular cleft (13) (constituting said subsidiary groove) comprising a concave annulus flanked on either side by an annular flank portion, said flank portions being deeper into the wheel groove than the entry surface to the die (9); and

(c) entering into the working passageway a feedstock rod (1A) of cross section smaller than that of the passageway so that the feedstock rod (1A) becomes wedged in the cleft (13) and is driven thereby to the abutment (17) and is there extruded through the die (9).

15. A method according to claim 14, including the step of pressing the feedstock rod (1A) firmly into the cleft (13) at a position adjacent the extrusion zone (30) by means of a ramp (16) which projects into the wheel groove (5).

16. A method according to claim 14 or 15, wherein the wheel groove (5) Is lined by winding a metal tube or wire around the annular bottom surface (36) of the groove (5), and thereafter radially compressing the winding of metal tube or wire so as to crush and solidify it.

17. A method according to claim 16, wherein the crushed metal filling the wheel groove (5) is machined to form the desired cleft shape (13).

18. An apparatus comprising any novel and inventive combination of features disclosed in the present specification, which combination has not been specifically claimed in any preceding claim.

19. A method comprising any novel and inventive combination of steps disclosed in the present specification, which combination has not been specifically claimed in any preceding claim.

20. An apparatus susbstantially as hereinbefore described with reference to and as illustrated by the Figures 4 to 10 of the accompanying diagrammatic drawings.

21. A method susbstantially as hereinbefore described with reference to and as illustrated by the Figures 4 to 10 of the accompanying diagrammatic drawings.

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