IMPROVEMENTS IN OR RELATING TO THE MANUFACTURE OF EXTRUSION DIES

Abstract: An extension die comprises a zero bearing die cavity (12) having a shape corresponding to the cross-sectional shape of the required extrusion. In order to control flow of the extrusion material through the cavity, a preform chamber (17) is provided which communicates with the die cavity but is of greater area. The depth and/or width of the preform chamber varies in different regions thereof, the dimensions of each region being related to the dimensions and position of the corresponding region of the die cavity (12). In use, extrusion material passes through different regions of the preform chamber at different velocities, such that the material subsequently passes through all regions of the die cavity itself at substantially uniform velocity. The depth of the preform chamber is not greater than about 5 mm, and may be of any smaller depth, down to less than about 0.5 mm. Notwithstanding its very small depth, the preform chamber allows accurate control of the velocity of the extrusion material while at the same time allowing a high velocity of extrusion to be maintained.

Description: "Improvements in or relating to the manufacture of extrusion dies" The invention relates to extrusion dies used for producing elongate profiles in metal (such as aluminium), plastics etc. In an extrusion process it is desirable for all parts of the material being extruded to pass through the die at substantially the same velocity, since if this is not the case the extruded profile is likely to be deformed. As is well known, in an extrusion die the velocity of the extrusion material through the die, at any particular region of the die cavity, depends on the width of the die cavity in that region, its position relative to the centre of the die, and the bearing length of the die cavity (i.e. its length in the extrusion direction) in that region. Since the width and position of each region of the die cavity are essentially determined for any particular profile to be extruded, it is normally necessary to control the velocity by adjusting the bearing length of the die cavity in different regions thereof, so that the velocity of extrusion material is as uniform as possible through the whole area of the die cavity. Thus, a narrow part of the die cavity will require a shorter bearing length than a wider part of the cavity in order to achieve the same velocity. The required variation in bearing length is normally achieved by forming in the back face of the die, i.e. the face further from the billet of material to be extruded through the die, an exit cavity which corresponds to the general shape of the die cavity plus an all-round clearance. The depth of this exit cavity is then varied so as to adjust the effective bearing length of the die cavity itself. Various methods of this kind for manufacturing an extrusion die are described, for example, in British Patent Specifications Nos. 2143445 and 2184371. The required bearing lengths of the die cavity are usually achieved by trial-and- error methods based on the knowledge of an experienced die designer or by use of a computer programme which calculates required bearing lengths from the shape and position of the die cavity. However, these methods suffer from certain disadvantages and International Specification No. WO 97/02910 describes a form of extrusion die where such disadvantages may be reduced or overcome. As described in WO 97/02910, there is provided on the front side of the die cavity (i.e. the upstream side with respect to the direction of extrusion) a preform chamber which is of generally similar shape to the die cavity but of greater cross- sectional area, so that regions of the preform chamber communicate with corresponding regions respectively of the die cavity. The flow of extrusion material through the die is then controlled by controlling the bearing length of the preform chamber rather than the bearing length of the die cavity itself. That is to say, each region of the preform chamber is given a bearing length which is related to the dimensions and position of that region so that, in use, extrusion material passing through each region of the preform chamber is constrained to move at a velocity such that the material subsequently passes through all regions of the die cavity at substantially uniform velocity. Such arrangement allows all regions of the die cavity to be of substantially constant bearing length since all control of velocity is carried out by adjusting the bearing lengths of different regions of the preform chamber. In particular the described arrangement allows all regions of the die cavity to be of substantially zero bearing length. A die cavity of substantially zero bearing length may be formed by providing in the die plate a die aperture which is negatively tapered throughout its length, i.e. the walls of the die aperture diverge as they extend from the front surface to the back surface of the die plate. Zero bearing dies are described in EP 0186340 and, as mentioned in that specification, a negative taper angle of at least 0.8° is preferred so that any friction stress between the walls of the die and metal flowing through it is negligible. It is believed that a negative taper angle of about 1.5° is more reliable. It will be appreciated that it is in practice impossible to provide a die cavity which is literally of zero bearing length, since there will normally be a small radius at the junction between the negatively tapered die cavity and the front surface of the die plate. EP 0186340 relates to arrangements where this radius of curvature is not greater than 0.2 mm. However, for the purposes of the present specification a die cavity is regarded as having zero bearing length when the die cavity increases in width as it extends away from the front face of the die plate, regardless of the radius of curvature at the upstream end of the die cavity, which may be greater or less than 0.2mm As acknowledged in EP 0186340, the design of a conventional zero bearing length die is such that modification of the profile of the aperture to hasten or slow the passage of metal is not possible. If a conventional zero bearing length die does not produce an extrusion of the required profile, there is no way in which the die can be corrected. However, the methods of WO 97/02910 allow control of the velocity of the metal upstream of the die, and therefore allows the use of zero bearing length dies for virtually all types of section. Hitherto, in carrying out the teaching of WO 97/02910, preform chambers have been used having bearing lengths in the range of at least 6-12 mm, since it has been considered necessary to have a bearing length of at least this sort of dimension in order to effect useful control of the velocity of the extrusion material. However, the present invention is based on the surprising discovery that effective control of the velocity of the extrusion material can actually be enhanced by considerably reducing the bearing lengths of the preform chamber to distances which might reasonably have been expected to be too small to provide effective velocity control. In the present specification the relevant dimensions of the preform chamber will be defined in terms of the depth of the chamber, in the direction of extrusion, rather than in terms of the bearing length of the chamber. The reason for this is that, during extrusion, in many cases the material being extruded bears against only a portion of the depth of the preform chamber adjacent the entry to the chamber. Accordingly, it is believed that the effective bearing length of the preform chamber is actually less than the physical depth of the preform chamber. The actual effective bearing length is therefore difficult to measure, but in any case it is believed that it is the physical depth of the preform chamber which is critical in determining the velocity of the extrusion material. It is not at present known whether this is simply because the effective bearing length is related to the physical depth of the preform chamber, or whether the distance of the entrance to the preform chamber from the entrance to the die cavity itself is critical, regardless of the effective bearing length within the preform chamber. According to the present invention, therefore, there is provided an extrusion die comprising a die cavity having a shape corresponding to the cross-sectional shape of a required extrusion, and a preform chamber having regions which communicate with corresponding regions of the die cavity and are respectively of greater area than said regions of the die cavity, the dimensions of each region of the preform chamber being related to the dimensions and position of the corresponding region of the die cavity so that, in use, extrusion material passing through said regions of the preform chamber is constrained to move at a velocity such that the material subsequently passes through the corresponding regions of the die cavity at substantially uniform velocity, characterised in that at least some of said regions of the preform chamber each have a depth, in the direction of extrusion, which is not greater than about 5 mm. The depth of said regions of the preform chamber may be less than about 2.5 mm, and is preferably less than 1 mm. In specific embodiments of the invention the depth may be about 0.5 mm or less. At least some of said corresponding regions of the die cavity, and preferably all such regions, may be of substantially constant bearing length. The invention is particularly applicable to extrusion dies where some, or preferably all, of said regions of the die cavity are of substantially zero bearing length. Using the preform chamber of the present invention, the metal is, in effect, extruded twice in quick succession: once through the preform chamber and once through the die cavity itself. It is found that, surprisingly, preform chambers of such small depth are still sufficient to allow the velocity of the extrusion material to be controlled sufficiently to provide uniform velocity of flow through the die cavity. Indeed, it is found in many instances that the control provided by these very small depths of preform chamber is actually greater than the control provided by preform chambers of greater depth. Furthermore, the reduced depth of the preform chamber means that the overall velocity of flow of the extrusion material may be significantly increased while still exerting the required control over the velocity through different parts of the die. The production rate of extrusion dies according to the invention may thus be considerably increased, particularly when used with zero bearing die cavities, when compared with conventional dies without a preform chamber, or when compared with dies having preform chambers of a depth greater than that required by the present invention. Also, the die according to the invention, in view of the reduced bearing length of the preform chamber, may be used at a lower pressure than prior art dies, if required., and hence at a lower temperature. Generally speaking, the rate of physical deterioration of an extrusion die is dependent on the extrusion pressure at which it is operated and the heat generated as a result of that pressure. Consequently, dies according to the present invention may deteriorate more slowly, and last longer, than prior art dies. It is also found that the invention makes it possible to extrude faster, due to the lower temperature. With prior art dies the speed of extrusion may be limited by the temperature of the extrusion. It is also believed that the surface finish of the extrusion may be improved by extruding at a lower temperature. It is also found that, by using very small depths of preform chamber in accordance with the invention, the velocity of flow of the extrusion material through a die cavity is less sensitive than in prior art arrangements to the position of the region of the die cavity with respect to the centre of the die. This means that the location and orientation of a number of cavities in a die is less critical, and this enables a greater number of cavities to be packed into a single die. For example, in a prior art extrusion die having, say, four cavities for extruding four profiles, it may be necessary for elongate parts of each profile to extend transversely to a radius of the die to ensure that, as far as possible, all parts of the elongate profile are substantially the same distance from the centre of the die. This necessary orientation limits the number of profiles which can be located within the area of the die. With the present invention, however, where distance from the centre of the die may be less critical, it is possible for elongate parts of profiles to extend away from the centre of the die and this may allow more profiles to be packed within the area of the die. This again will increase the productivity of the die. Also, since any reasonable number of profiles may be included in a single die, a greater number of different types of profiles can be accommodated in a single diameter of die, so that a manufacturer may need only one size press to produce a range of profiles which hitherto may well have required different sizes of press to produce different profiles. It is also found that the very small preform chamber depths are less likely to result in the distortions of the extruded profile which can occur with conventional extrusion dies or those having preform chambers which are of greater depth than those proposed by the present invention. For example with prior art dies it is common for the side walls of an extruded channel section to be inclined away from one another and for the connecting web to be bowed. Also, it is found that the shorter preform chamber depths of the present invention allow greater tolerances in the extrusion velocities in different parts of the die cavity. Although the small preform chamber depths of the invention still allow the velocities in different regions of the die cavity to be controlled to render them uniform, such velocities do not have to be as uniform, in order to avoid distortion of the profile, as was the case with prior art arrangements. As is well known, variation in extrusion velocity in different parts of the die cavity is indicated by deformation of the leading surface, or "nose", of the extrusion material as it emerges from the die cavity. If velocity is uniform throughout the die cavity the leading face of the extrusion material will be flat. With the present invention it is found that deformations of the leading face which, in prior art arrangements, would be accompanied by distortions of the extruded profile are not accompanied by such distortions when the extrusion die has a very small depth preform chamber controlling velocity of flow in accordance with the present invention. In arrangements according to the invention the control of the velocity in a particular region of the preform chamber may be effected by fixing the width and length of each region of the preform chamber and then adjusting the depth of the chamber to give the required result, or by fixing the depth of the preform chamber and then varying the width and/or length of each region of the chamber to give the required result. In the latter case, all regions of the preform chamber may then be of the same depth. Alternatively, the velocity of flow through a particular region may be controlled by adjusting any combination of the width, length and/or depth of the region. An advantage of the present invention, particularly when used with a zero length bearing, is that the preform chamber may be readily formed to the desired dimensions by a precision machining process, as opposed to the hand processes necessary to modify the shape of the actual die cavity in prior art dies. This enables die configurations to be accurately repeated, if required, without the die correction hand process hitherto required to be carried out by a skilled and experienced operator before a die could produce the desired extrusion. For example, a zero bearing length die has no need of hand "squaring", which might otherwise be necessary to ensure that the sides of the die cavity are parallel and normal to the front face of the die. In the present invention also the preform chamber and front face of the die may be polished by a machine process. The extrusion die may include one or more supplementary preform chambers communicating with the first said preform chamber, the area of the preform chambers increasing with distance from the die cavity, said supplementary preform chamber or chambers also having a depth, in the direction of extrusion, which is not greater than about 5 mm. As with the first said preform chamber, the depth of said supplementary preform chamber or chambers may also be less than about 2.5 mm, and is preferably less than 1 mm. In specific embodiments of the invention the depth of the supplementary preform chamber or chambers may be about 0.5 mm or less. At least one region, or each region, of the preform chamber may be substantially symmetrically disposed with respect to the corresponding region of the die cavity. Alternatively, at least one region of the preform chamber may be non- symmetrically disposed with respect to the corresponding region of the die cavity. For example, the width of said non-symmetrical region of the preform chamber may be greater on one side of the die cavity than it is on the opposite side of the die cavity. Alternatively or additionally the depth of said region of the preform chamber may be greater on one side of the die cavity than it is on the opposite side of the die cavity. The width of said region of the preform chamber may vary along each side of the die cavity, the width of the preform chamber along one side of the die cavity varying inversely as the width along the opposite side of the die cavity. In this case the total width of said region of the preform chamber may be substantially constant or may vary according to the width of the die cavity. The depth of said region of the preform chamber may also vary along each side of the die cavity, the depth of the preform chamber along one side of the die cavity varying inversely as the depth along the opposite side of the die cavity. At least a part of the bottom wall of the preform chamber, in which the entrance to the die cavity itself is formed, may be ground and/or polished to enhance the flow of extrusion material across said bottom wall as the material passes from the preform chamber into the die cavity. The side walls of the preform chamber may also be ground and/or polished. The die cavity and preform chamber or chambers are preferably integrally formed in a single component. Such arrangement maintains the strength of the extrusion die as a whole. In prior art arrangements according to WO 97/02910 the preform chamber may be formed in a separate plate which is bolted or otherwise secured to a plate in which the die cavity itself is formed. However, the present invention would require the plate in which the preform chamber is formed to be very thin and it might therefore be liable to bend or buckle when bolted or otherwise secured to the die cavity plate. The following is a more detailed description of embodiments of the invention, by way of example, reference being made to the accompanying drawings in which: Figure 1 is a diagrammatic section through part of an extrusion die in accordance with the invention, Figures 2-5 are diagrammatic front face views of different forms of extrusion die in accordance with the invention, Figure 6 is a diagrammatic section through part of a die for extruding hollow sections, Figure 7 shows a portion of the die of Figure 6 on an enlarged scale, Figure 8 is a diagrammatic front face view of a further form of extrusion die in accordance with the invention, Figure 9 is a diagrammatic section on the Line 9-9 of Figure 8, Figure 10 is a diagrammatic section through a modified form of the die of Figures 8 and 9, and Figure 11 is a diagrammatic front face view of a still further form of extrusion die in accordance with the invention. Referring to Figures 1 and 2, the extrusion die comprises a circular main body 10 formed with a number of die cavities 11, shaped according to the profiles to be extruded. In this case three die cavities are provided but, as is well known, any reasonable number of cavities may be employed in a single die. Extrusion material, such as plastics or aluminium or other metal, is passed through the die in the direction indicated by the arrow 15. Referring to Figure 1, the die cavity itself is indicated at 12. In this instance it is a zero bearing die cavity. That is to say the side walls 13 of the cavity diverge as they extend away from the inlet edges 14 of the cavity. Shaping of the material, which passes through the die in the direction of the arrow 15, is thus effected solely by the edges 14, there being no frictional engagement between the extrusion material and the diverging sides 13. As previously explained, the edges 14 will inevitably have a slight radius of curvature but this radius of curvature will be extremely small being, for example, of the order of 0.2 mm radius, although a lesser or greater radius of curvature may also be employed. As is well known, increase in bearing length, for a given width of die cavity, will slow the extrusion material as it passes through the cavity, and use of a zero bearing therefore permits the maximum possible velocity of flow. Although the invention is particularly applicable to zero bearing length dies, the invention may also provide advantage in dies which have a positive bearing length. In the vicinity of the die cavity edges 14, the side walls 13 are inclined at a very small angle (e.g. 1.5°) in order to provide adequate strength and support for the edges 14 which form the extrusion material. Further downstream, however, the cavity opens up more widely, and at a greater angle, as indicated at 16, to permit the free passage of the extrusion. In accordance with the teaching of WO 97/02910, a pre-chamber 17 is formed on the front face of the die at the entrance to the cavity 12. The preform chamber 17 is of generally similar shape to the die cavity 12, as may be seen from Figure 2, but is of greater area. The chamber 17 extends symmetrically to each side of the cavity 12. The material being extruded through the zero bearing die cavity 12 must therefore first be forced through the preform chamber 17 and, in accordance with the teaching of WO 97/02910, the velocity of the extrusion material may be controlled by appropriate selection of the dimensions of the preform chamber 17 so that the extrusion material subsequently passes through all regions of the die cavity 12 at substantially uniform velocity. According to the present invention the depth P, of the preform chamber 17 is much smaller than has hitherto considered to be necessary, and examples of typical dimensions will be given. In the arrangement of Figure 2 the width of each region of the preform chamber 17 is a substantially fixed percentage greater than the width of the corresponding region of the die cavity 12 and the velocity of the extrusion material passing through the different regions of the preform chamber is controlled by varying the depth P, of the preform chamber in different regions thereof, so that the extrusion passes through all regions of the die cavity itself at substantially uniform velocity. However, the velocity of the extrusion material may also be varied by varying the area of the preform chamber 17. Increase in the depth P, of the preform chamber 17 slows the extrusion material, whereas increase in the width of the preform chamber increases the velocity. Although in some circumstances both the depth and width of the preform chamber may vary in different regions of the chamber it may be preferable, in order to facilitate the adjustment, to maintain one of the parameters fixed and to vary the other. Hitherto, it has been considered that the preform chamber 17 should be of substantial depth in order to allow sufficiently accurate control of the velocity of the extrusion material. Typically, in prior art arrangements, the depth P, of the preform chamber might be in the range of 6 mm - 15 mm. According to the present invention, however, it has now been realised that effective and accurate control of the velocity may be achieved with very much smaller depths of the preform chamber, i.e. depths which are no greater than 5 mm and which may be as small as tenths of a millimetre. It will be noted From Figure 2 that it has not been necessary to orientate the die cavities so that corresponding regions of the different cavities are at substantially the same distance from the centre of the die. The use of a very shallow preform chamber, according to the present invention, renders the velocity of flow less sensitive to the position of each region of a die cavity with respect to the centre of the die, as previously explained, this making such orientation unnecessary. In Figure 2 the letters A-D indicate the depth of the preform chamber 17 in different regions of each die cavity. The depth in each region varies in order to allow for the shape and dimensions of the die cavity at that region and for the position of that region with respect to the centre axis of the die. As is well known, the velocity of flow through an extrusion die tends to decrease with distance from the central axis. In the die of Figure 2 the depths P, of the preform chamber 17 at the regions indicated may typically be as follows: A = 0.7 mm B = 1.0 mm C = 1.3 mm D = 2.3 mm In the arrangement of Figure 2 the die profile consists essentially of two elongate arms at an angle to one another. Where the depth of the preform chamber 17 is different at the two ends of an arm, the depth will normally increase or decrease continuously from one end of the arm to the other, so that the side wall of the preform chamber is tapered. It is found that by use of the very small depth of preform chamber described, accurate control of the velocity of extrusion material can be achieved. At the same time, the small depth of the preform chamber effects only a minimal overall reduction in the velocity of the extrusion material so that a high throughput of material, and hence a high rate of production, can be maintained. Figures 3, 4 and 5 show other typical designs of profile in an extrusion die. In Figure 3 the letters E, F, G indicate regions of the preform chamber of different depths and, typically, these depths may be as follows: E = 0.6 mm F = 1.3 mm G = 1.5 mm In the die Figure 3, the width of the preform chamber is increased, as indicated at 17a, in those regions of each die cavity where the width of the cavity is particularly small. This increase of the width of the preform cavity increases the velocity of the extension material through the preform chamber, and thus compensates for the subsequent reduction of velocity which occurs through the particularly narrow region of the die cavity. In the extrusion die of Figure 4, which is a more complex profile, the depths of the indicated regions of the preform chamber may be as follows: H = 0 J = 0.5 mm K = 0.7mm L = 0.8mm M = 0.9mm N=l mm O = 1.2 mm P= 1.3 mm Q= 1.5 mm R= 1.7 mm In the extrusion die of Figure 5, which is of an even more complex profile, the depths of the indicated regions of the preform chamber may be as follows: S= 1.0 mm T= 1.3 mm U= 1.5 mm V = 2.0 mm W = 2.5mm In the dies of Figures 4 and 5 a supplementary preform chamber 18 is not shown, but may be provided if required. Figure 6 is a diagrammatic section through a die for extruding hollow profiles, for example profiles of rectangular or circular cross-section. In known manner, the die comprises a male part 19 surrounded by a female part 20 so as to provide a generally annular die cavity 21 between them. In known manner, the material to be extruded flows around the male part 19 and through the cavity 21 between it and the female part 20. In accordance with the present invention, a preform chamber is formed on the front faces of the male and female parts of the die. The preform chamber is of generally similar shape to the die cavity 21, but is of greater area. It may extend symmetrically to each side of the cavity 21 , although asymmetrical arrangements are also possible as described herein. The portions of the preform chamber on the male and female parts are preferably of equal depth. The male part 19 and the female part 20 may each be formed with a zero bearing edge. However, during use of the die there may be slight axial movement of the male part 19 relative to the female part 20. In order to compensate for this, therefore, the zero bearing edge on the male part 19 is preferably initially located slightly upstream of the zero bearing edge on the female part, so that axial deflection of the male part, during extrusion, brings the bearing edges into alignment. Although such an arrangement may be preferred, Figure 7 shows a possible alternative arrangement for dealing with axial deflection of the male part during extrusion. Figure 7 shows, in enlarged cross-section, this alternative form of the die cavity 21. On the female part 20 of the die the bearing length of the die cavity is again zero, i.e. the side wall 22 of the cavity is inclined outwardly as it extends away from the inlet edge 23 of the cavity. Shaping of the external surface of the hollow section, as it passes through the die cavity in the direction of the arrow 24, is thus effected solely by the edge 23, there being no frictional engagement between the extrusion material and the inclined surface 22. As in the previous arrangements, the edge 23 will have a very small radius of curvature. In this case, however, in order to allow for slight axial movement between the male part 19 and the female part 20, the male part 19 does not have a zero bearing edge but has a small axial bearing surface 25 which extends to either side of the zero bearing edge 23 on the female part. The surface 25 may, for example, have a bearing length of about 1.0 mm. Again, the side wall 26 of the cavity downstream of the aperture is inclined so that the inner and outer side walls 26, 22 of the cavity diverge as they extend downstream. As before, a preform chamber is formed on the front faces of the male and female parts of the die, the female portion being indicated at 27 and the male portion at 28. The preform chamber 27, 28 extends symmetrically to each side of the cavity 21, but the portion 28 of the cavity on the male member is of smaller depth than the portion 27 on the female member, in view of the presence of the bearing surface 25 on the male part. Typically, in accordance with the present invention, the depth of the preform chamber portion 27 may be 1.5 mm and the depth of the portion 28 1.0 mm. If the hollow profile to be extruded is of generally rectangular section, or other section having corners, the depth of the preform chamber at each corner will generally be less than its depth along the sides of the section, and typically may be about 5 mm. The invention provides a preform chamber having a depth which is not greater than 5mm. In some of the arrangements previously described the depth of the preform chamber is in the range of 0.5-2.5 mm. It has been found with some extrusions of harder aluminium alloys, such as are used for example in the manufacture of rungs for aluminium ladders, that preform chamber depths at the upper end of the range according to the invention are preferred. For example, where a preform chamber depth of 0.8- 1.8 mm might give good results with some extrusion aluminium alloys, with harder alloys a preform chamber depth of about double this range (i.e. 1.6-3.6 mm) enables the resulting extrusion better to withstand stresses to which it is subjected in use, such as the stress resulting from swaging a hollow extruded ladder rung at its ends during attachment to the side stringers of the ladder. In the arrangements described above in relation to Figures 1 -7, each region of the preform chamber (for example the preform chamber 17 in Figures 1 and 2) is symmetrically disposed with respect to the corresponding region of the die cavity. That is to say, the width and bearing depth of the preform chamber are substantially equal on opposite sides of the die cavity. However, according to another important aspect of the present invention, the manner in which the material being extruded flows through the die cavity may be further controlled by making regions of the preform chamber non- symmetrical with respect to corresponding regions of the die cavity, for example by arranging for the preform chamber to be of different widths and/or depths on opposite sides of the die cavity. The effect of this non-symmetry is to impose a lateral force or bias on to the extrusion material as it passes through the die cavity, thereby affecting the shape and/or orientation of the extrusion. Figure 8 shows one form of die where such an arrangement may be advantageous. Referring to Figure 8: the die cavity 30 comprises a central hollow box section 31 from opposite ends of wliich project spaced parallel limbs 32 having inwardly turned flange sections 33 at their extremities. It is a common problem when extruding profiles from this type of die for the limbs on the extruded profile, formed by the die cavity regions 32, to diverge instead of being parallel. In conventional dies it is difficult to correct such divergent distortion by adjusting the bearing length of the die cavity itself using the conventional die correcting procedures of this art. Consequently, it is often necessary to compensate for and remove such distortion by modifying the shape of the die cavity so that, in the die cavity, the regions 32 are slightly convergent, with the result that the tendency for the corresponding parts of the extrusion to diverge cancels out this convergence in the die cavity so that the two limbs of the finished extruded profile are parallel. In practice, however, it may be difficult to correlate the required convergence of the regions of the die cavity with the divergence which occurs during extrusion so that they cancel each other out exactly. Shaping of the die cavity is therefore largely a trial-and-error process, which is wasteful of both time and material, and in the end it may not be practically possible to obtain an extruded profile where the limbs are precisely parallel. It has been found, according to this aspect of the present invention, that the preform chamber may be arranged to apply a controlled lateral bias to the material as it is being extruded, in a manner to compensate for lateral distortion of the extrusion, such as the divergence of the limbs of the profile. This lateral bias is achieved by making the preform chamber non-symmetrical with respect to the die cavity. One such arrangement is shown in Figures 8 and 9 where the preform chamber 34 is of greater width on one side of the region 32 of the die cavity than it is on the other side, the two differing widths being indicated at 34A and 34B respectively. The effect of the greater width 34A of the preform chamber on one side of the die cavity 32 is to impart a lateral bias to the extrusion material as indicated diagrammatically by the arrow 35 in Figure 9. The effect of this is to "push" each limb of the extrusion towards the other as they pass through the regions 32 of the die cavity in a manner to compensate for the tendency of the limbs of the extrusion to diverge. A similar effect may be achieved by forming the preform chamber with different bearing depths on opposite sides of the die cavity, as shown in Figure 10. In this case, the depth of the side wall 36A of the preform chamber 36 is greater than the bearing depth of the opposite side wall 36B of the preform chamber. The greater bearing depth of the wall 36A again imposes a lateral bias on the material being extruded through the regions 37 of the die cavity so as to "push" the extrusion laterally, as indicated diagrammatically by the arrow 38, thereby compensating for the tendency of the limbs of the extrusion to diverge. In the arrangement of Figure 9 the bearing depths of the two sides of the preform chamber are the same and in the arrangement of Figure 10 the width of the preform chamber 36 is the same on each side of the bearing cavity. This facilitates modification of the preform chamber to achieve the desired effect, since in each case there is only one dimension which needs to be varied. However, the invention may also include arrangements where both the width and bearing depth of the preform chamber differ on opposite sides of the die cavity. Since compensation for the tendency of the extrusion do distort is controlled solely by the dimensions of the preform chamber, it is far easier to make slight adjustments to compensate for distortion than is the case with the prior art methods described above. Thus, if distortion of the extrusion is occurring with a symmetrically shaped preform chamber, it is simply required to increase the width of the preform chamber on one side of the die cavity, or reduce the bearing depth of the preform chamber on the opposite side of the die cavity, to a sufficient extent to compensate for the distortion. Although the flow control arrangements described in relation to Figures 8-10 are particularly applicable to extrusion dies according to the first aspect of the present invention, where the bearing depth of the preform chamber is significantly less than has been the case heretofore, it will be appreciated that control of flow of the extrusion material may also be similarly effected in extrusion dies where the bearing depth of the preform chamber is greater, as in the prior art as exemplified by the disclosures in WO 97/02910. In the arrangements of Figures 8-10, the effect of the preform chamber being non-symmetrical is to apply a net bias to the extrusion material in order to compensate for distortion of the material as it passes through the die cavity. However, in the modified arrangement shown in Figure 11 the preform chamber is made non-symmetrical in such a manner that the effects of the non-symmetrical shapes of different regions of the preform chamber cancel out so that there is no net resultant bias acting on the extrusion material. Thus, as shown in Figure 1 1, the width of the preform chamber 39 on one side of the die cavity 40 varies between regions 39A of small width and regions 39B of greater width. The width of the preform chamber along one side of the die cavity varies inversely with respect to the width along the opposite side, so that each region of smaller width is opposite a region of greater width. As a result, some regions of the preform chamber tend to bias the extrusion material laterally in one direction while other regions tend to bias the extrusion material in the opposite direction. The amount of bias in each direction is so designed that they cancel out so that there is no resultant net lateral bias acting on the extrusion material. However, it is found that the opposing lateral biasing forces tend to "grip" the extrusion material as it passes through the die cavity and that this tends to inhibit distortion of the extrusion material so that its profile, as it emerges from the die cavity 40, corresponds more exactly to the shape of the die cavity. In the arrangement of Figure 11 the opposing lateral bias forces are applied by adjusting the width of the preform chamber 39, but it will be appreciated that a similar effect may be achieved by having different bearing depths of the preform chamber in different regions thereof, as described in relation to Figure 10. As may be seen from Figure 1 1 , although the width of the preform chamber varies along each side of the die cavity, the overall width of the chamber remains substantially constant, although it may also vary according to the width of the die cavity 40. In any of the arrangements in accordance with the invention there may be provided upstream of the preform chamber one or more supplementary preform chambers of greater width as indicated for example at 18 in Figure 1. The further preform chamber or chambers may also affect the velocity of extrusion material passing through the die cavity so that where one or more supplementary preform chambers are provided, the depth V λ of the primary preform chamber 17 and the depth P 2 of the supplementary preform chamber may be reduced. In other arrangements, however, the supplementary chamber, such as the chamber 18, may be of such width that it does not significantly affect the extrusion of material through the preform chamber 17 and die cavity 12. In any of the arrangements according to the invention the extrusion material will flow inwardly over the bottom wall of each preform chamber as it approaches the edge of the die cavity. Preferably therefore, this bottom wall of the preform chamber is ground and polished to reduce the resistance to flow. The side walls of the preform chambers may also be ground and polished. Preferably the bottom wall is polished to a mirror finish, and the side walls and bottom wall of the preform chamber, after it has been shaped and polished, may be surface-finished, for example it may be nitrided in the manner commonly used for the surface finishing of the die cavity itself. The preform chamber, and supplementary preform chambers where such are provided, may conveniently be initially formed by a milling process, followed by grinding and emery polishing of the bottom wall of the preform chamber. It is important that the grinding and polishing of the bottom wall is carried out to such an extent as to entirely remove any milling marks remaining on the bottom wall. It is found that where such nulling marks extend to the edge of the die cavity, they may have the effect of forming, on a very small scale, a "toothed" formation around the edges of the die cavity, where the tiny grooves formed by the milling process extend transversely to the edge of the die cavity. Such "toothed" formation can cause undesirable marking of the surface of the extrusion, and polishing of the bottom wall of the preform chamber therefore has the effect of smoothing the edge of the die cavity itself.

Claims: CLAIMS 1. An extrusion die comprising a die cavity having a shape corresponding to the cross-sectional shape of a required extrusion, and a preform chamber having regions which communicate with corresponding regions of the die cavity and are respectively of greater area than said regions of the die cavity, the dimensions of each region of the preform chamber being related to the dimensions and position of the corresponding region of the die cavity so that, in use, extrusion material passing through said regions of the preform chamber is constrained to move at a velocity such that the material subsequently passes through the corresponding regions of the die cavity at substantially uniform velocity, characterised in that at least some of said regions of the preform chamber each have a depth, in the direction of extrusion, which is not greater than about 5 mm. 2. An extrusion die according to Claim 1 , wherein the depth of said regions of the preform chamber is less than about 2.5 mm. 3. An extrusion die according to Claim 1 , wherein the depth of said regions of the preform chamber is less than about 1 mm. 4. An extrusion die according to Claim 1, wherein the depth of said regions of the preform chamber is less than about 0.5 mm. 5. An extrusion die according to any of the preceding claims, wherein at least some of said corresponding regions of the die cavity are of substantially constant bearing length. 6. An extrusion die according to any of the preceding claims, wherein all of said corresponding regions of the die cavity are of substantially constant bearing length. 7. An extrusion die according to Claim 5 or Claim 6, wherein said corresponding regions of the die cavity are of substantially zero bearing length. 8. An extrusion die according to any of the preceding claims, and further including one or more supplementary preform chambers communicating with the first said preform chamber, the area of the preform chambers increasing with distance from the die cavity. 9. An extrusion die according to Claim 8, wherein said supplementary preform chamber or chambers also have a depth, in the direction of extrusion, which is not greater than about 5 mm. 10. An extrusion die according to Claim 8, wherein said supplementary preform chamber or chambers also have a depth, in the direction of extrusion, which is not greater than about 2.5 mm. 11. An extrusion die according to Claim 8, wherein said supplementary preform chamber or chambers also have a depth, in the direction of extrusion, which is not greater than about 0.5 mm. 12. An extrusion die according to any of the preceding claims, wherein at least one region of the preform chamber is substantially symmetrically disposed with respect to the corresponding region of the die cavity. 13. An extrusion die according to any of the preceding Claims 1 to 11, wherein at least one region of the preform chamber is non-symmetrically disposed with respect to the corresponding region of the die cavity. 14. An extrusion die according to Claim 13, wherein the width of said non- symmetrical region of the preform chamber is greater on one side of the die cavity than it is on the opposite side of the die cavity. 15. An extrusion die according to Claim 13 or Claim 14, wherein the depth of said region of the preform chamber is greater on one side of the die cavity than it is on the opposite side of the die cavity. 16. An extrusion die according to any of Claims 13-15, wherein the width of said region of the preform chamber varies along each side of the die cavity, the width of the preform chamber along one side of the die cavity varying inversely as the width along the opposite side of the die cavity. 17. An extrusion die according to Claim 16, wherein the total width of said region of the preform chamber is substantially constant. 18. An extrusion die according to Claim 16, wherein the total width of said region of the preform chamber varies according to the width of the die cavity. 19. An extrusion die according to any of Claims 13-18, wherein the depth of said region of the preform chamber varies along each side of the die cavity, the depth of the preform chamber along one side of the die cavity varying inversely as the depth along the opposite side of the die cavity. 20. An extrusion die according to any of the preceding claims, wherein at least a part of the bottom wall of the preform chamber, in which the entrance to the die cavity itself is formed, is ground and/or polished to enhance the flow of extrusion material across said bottom wall as the material passes from the preform chamber into the die cavity. 21. An extrusion die according to any of the preceding claims, wherein the side walls of the preform chamber are ground and/or polished. 22. An extrusion die according to any of the preceding claims, wherein the die cavity and preform chamber or chambers are integrally formed in a single component. 23. A method of manufacturing an extrusion die according to any of the preceding Claims 1-22, wherein control of the velocity in regions of the preform chamber is effected by fixing the width and length of each region of the preform chamber and then adjusting the depth of the chamber in different regions thereof. 24. A method of manufacturing an extrusion die according to any of the preceding Claims 1-22, wherein control of the velocity in regions of the preform chamber is effected by fixing the depth of the preform chamber and then varying the width of the chamber in different regions thereof 25. A method of manufacturing an extrusion die according to any of the preceding Claims 1-22, wherein control of the velocity in regions of the preform chamber is effected by adjusting any combination of the width, length and/or depth of the chamber in different regions thereof. 26. A method of manufacturing an extrusion die comprising a die cavity having a shape corresponding to the cross-sectional shape of a required extrusion, the method comprising providing a preform chamber having regions which communicate with corresponding regions of the die cavity and are respectively of greater area than said regions of the die cavity, the preform chamber being non-symmetrical with respect to the die cavity in a manner to counteract lateral distortion of extrusion material passing through the die cavity, in use, by applying to the extrusion material a net lateral biasing force as it passes through the preform chamber, as a result of said non-symmetrical disposition of the preform chamber. 27. A method according to Claim 26, wherein the preform chamber is rendered non-symmetrical with respect to the die cavity by forming the preform chamber so as to be of greater width on one side of the die cavity than it is on the opposite side of the die cavity. 28. A method according to Claim 26, wherein the preform chamber is rendered non-symmetrical with respect to the die cavity by forming the preform chamber so as to be greater depth on one side of the die cavity than it is on the opposite side of the die cavity. 29. A method according to any of Claims 26-28, including the step of passing extrusion material through the preform chamber and die cavity, noting lateral distortion of the extrusion material as it passes through the die cavity, and modifying the symmetricity of the preform chamber in a manner to apply to the extrusion material a net lateral biasing force to counteract said distortion.

Inventors: FELDCAMP EDWARD GEORGE, FELDCAMP, EDWARD, GEORGE
Applicants: AILSA INVESTMENTS LTD, FELDCAMP GEORGE EDWARD, AILSA INVESTMENTS LIMITED, FELDCAMP, EDWARD, GEORGE
Application year: 1999
Main classification: B21C25
IPC codes: B21C25