WO2009122384A1

WO2009122384A1 - Extrusion die

Info

Publication number: WO2009122384A1
Application number: PCT/IB2009/051442
Authority: WO
Inventors: Edward George Feldcamp
Original assignee: Preform Dies Limited
Priority date: 2008-04-04
Filing date: 2009-04-06
Publication date: 2009-10-08
Also published as: GB0806078D0

Abstract

Extrusion die (1) comprising a die body (10) having bearing walls, wherein the bearing walls comprise leading edges (17) surrounding a die cavity (12) having a die cavity width. The extrusion die comprises a feed control chamber (18) formed directly upstream of the die cavity (12), wherein the feed control chamber (18) has a base comprising the leading edges (17), and side walls (23) extending with a height (H1) of less than 7 mm from the base in an upstream direction of the die towards an inlet of the feed control chamber (18), wherein the side walls (23) of at least part of the feed control chamber (18) diverge away from one another in an extrusion direction of the die, and wherein a ratio between a width of the inlet of the feed control chamber and the die cavity width is chosen as to define a volume flow of material to be extruded.

Description

Extrusion die

The invention relates to an extrusion die, for example for use in the extrusion of aluminium or other materials. It is well known that, during the extrusion of aluminium and other materials, it is common for the section of the final extruded product to be different from that of the die cavity of the die used to extrude the material. For example, when extruding sections of channel-shaped form, e.g. U-shaped form, it is common for the 'legs' of the extruded product to be splayed or tow in relative to one another compared to the shape of the corresponding parts of the die cavity. When extruding hollow sections of, for example generally circular section, it is common for the extruded product to be of, for example, generally elliptical rather than circular form. A number of other deformations are also known.

Research has shown that there are several causes for such deformations. For example, one cause of deformation is the passage of material through the die cavity at a non-uniform speed. Where the material passes through some parts of the die cavity at a speed higher than that at which material passes through other parts of the die cavity, deformations tend to occur in the extruded product.

In order to control the speed at which material passes through the die cavity, it is known to locate a so-called pre-chamber or sink- in upstream of the die cavity. The pre- chamber will usually be of varying widths around the die. The pre-chamber usually has a fixed depth (i.e. height) around the die. The local width of the pre-chamber is usually dependent on the die cavity width, and the location on the die body. The die cavity width is defined as the distance between leading edges of the bearing, as seen in a cross-sectional view. The friction caused by the sidewall of the pre-chamber, is used to control the metal flow. The widths and depths are being chosen so as to ensure that the material passes through all parts of the die cavity at substantially the same speed. The pre-chamber can also have a non-uniform depth around the die cavity, wherein the depth of the various parts of the pre-chamber are chosen so as to ensure that the material passes through all parts of the die cavity at substantially the same speed. Dies having such a pre-chamber are known from EP 0 837 745. The complete disclosure of this document is included by reference. Although the pre-chamber may be designed to successfully achieve a substantially uniform material speed throughout the die cavity, the regions of the pre- chamber of greatest sidewall length apply relatively large frictional loadings to the material being extruded, thus relatively high extrusion pressures are required, and this is undesirable. This can result in high extrusion temperatures, giving the extruded product a bad surface finish and excessive die bearing wear. Lower pressures are beneficial for extending the 'life' of the die.

Another cause of deformation in the extruded product, arises from deflection, in use, of leading edges of the die cavity, especially near tongues of the die. The deflection may result in misalignment in the x, y and/or z plane of the bearing of opposing parts of the die cavity. It is known to correct for this effect by adjusting the x, y and/or z to make parallel or realign the opposing bearings.

In order to correct for the occurrence of deformations, it is known for die correctors to modify the shape of the die cavity to achieve extruded products of the desired shape. In other words, the form of the die cavity is not equal to the form of the product desired. For example, if a die is intended to produce a product of channel shaped form, the die cavity of the die may have the parts thereof which form the legs of the channel inclined towards one another. The splaying which occurs, in use, brings the legs to the desired positions. Such die correction is typically achieved through trial and error, and use of the die corrector's knowledge gained over years of experience.

Although such techniques can work satisfactorily, they are inefficient, and also it is preferred for the die cavity and the extruded product formed using the die to be of the same shape.

Another technique for correcting for splaying is known from EP 1 268 096, by the same inventor, the disclosure being included by reference in this application. This technique is known as micro bearing adjustment (MBA). Here, a recess is provided on or near one of the leading edges of the bearing. The die preferably is a zero-bearing die, in that the bearing walls diverge away from one another in the extrusion direction. The angle by which the walls diverge may be relatively small. The recess may have a non-uniform depth around the die cavity, the depth being chosen such that, in use, deflection of the parts of the leading edge brings the opposite leading edges in alignment with one another. Another known technique involves displacing the pre-chamber laterally relative to the die cavity, resulting in the application of sideways acting forces in the material being extruded. These forces themselves cause splaying/tow in/deflection of parts of the extruded product. It will be appreciated that by appropriate location of the pre- chamber, the forces so applied to the material can counter and compensate for the forces that cause the deformation which would otherwise occur.

It is an object of the invention to provide a die wherein relatively low extrusion pressures may be used. It is a further object to provide a die of simple and convenient form which permits improved control over the extrusion process, thereby allowing the occurrence of deformations to be reduced or avoided. The invention is also intended to enable faster extrusion speed, therefore increased productivity. A further object may also be lowering the extrusion temperature.

According to the present invention at least one of these goals is achieved by providing an extrusion die comprising a die body having bearing walls. The bearing walls comprise leading edges surrounding a die cavity. The die cavity has a die cavity width that can vary along the die cavity in accordance to the desired extrusion product. A die can have multiple die cavities, allowing simultaneous extrusion of products in a single run.

The extrusion die may further comprise a feed control chamber formed directly upstream of the die cavity. The feed control chamber may have a base that may comprise the leading edges of the die cavity. The side walls of the feed control chamber extend a height from the base in an upstream direction of the die towards an inlet of the feed control chamber. The inlet of the feed control chamber may be defined as that part of the feed control chamber where the walls are closest together. The width of the inlet is being defined as the shortest distance between the side walls, as seen in a cross- sectional view of the die cavity. The height may be relatively small compared to conventional, known chambers. The height of the feed control chamber may be less than 7 mm, preferably less than 6 mm and more preferably less than 5 mm.

In an embodiment the side walls of at least part of the feed control chamber diverge away from one another in an extrusion direction of the die. This means that the distance between the side walls in a first part of the feed control chamber is smaller than the distance between the side walls in a part of the feed control chamber located downstream, i.e. in the extrusion direction, of the first part. The first part may be the inlet.

The relieved form of the chamber, i.e. the diverging of the side walls in a downstream direction of the die, results in relatively less resistance to flow of the material supplied passing through the feed control chamber. During extrusion, metal may accumulate in the diverged section of the feed control chamber. Material supplied later will then slide over a layer of material accumulated in the feed control chamber. This way, friction is reduced. Such a relieved feed control chamber directly upstream from the bearing can be called a soft bearing or soft zero bearing. By minimizing the distance (height) between the inlet of the relieved chamber and the die cavity, a feed control chamber is obtained that effecting extrusion parameters only locally. The inventor has realised that using a relieved and small chamber directly in front of the bearing results in a local control of the extrusion parameters, in particular extrusion speed/volume that is more accurate. The relieved feed chamber generally functions as a pre-bearing lowering extrusion pressure at the actual bearing.

By reducing pressure on the actual bearing, the deflection according to EP 1 268 096 is reduced and the limited relieved feed control chamber can actually replace the MBA. However a combination of a relieved chamber according to the invention with misaligned leading edges of the bearing, preferably a zero bearing, during non-use, said leading edges aligning during use, said misalignment being arranged such as to correct for deflection of the one leading edge with respect to the other leading edge as a result of the extrusion pressure, preferably having a misalignment that is non-uniform along the die cavity as a result of different deflection of the leading edges, is possible and desirable for at least tongue portions of the die/die cavity. In an embodiment the edges of relieved chamber according to the invention are during non-use misaligned, said edges aligning during use, said misalignment being arranged such as to correct for deflection of the one leading edge with respect to the other leading edge as a result of the extrusion pressure, preferably having a misalignment that is non-uniform along the die cavity as a result of different deflection of the leading edges

The extrusion die may be a zero-bearing die. With this, a further reduction in friction may be obtained. Although the term zero bearing suggests that the die cavity is of zero bearing length, in practise the die cavity is likely to have a finite but very small bearing length.

The die according to the invention having a zero-bearing has a extrusion ratio (total surface area profile cross section divided into the diameter (6", 7" or 8") of the aluminium billet) of less than 50/1. The relieved chamber according to the invention will increase the ratio preventing too fast extrusion.

The die cavity width is preferably less than 3mm and more preferably less than 2.5 mm or advantageously less than 2 mm . In the combination of a relieved chamber according to the invention, preferably combined with a zero bearing, and manufacturing small wall thickness extrusion profiles, sufficient control without a large increase in friction is obtained. The relieved chamber is less effective for larger wall ticknesses.

In an embodiment the relieved feed control chamber is laterally offset with respect to the die cavity. The relieved feed control chamber forms a restriction of the surface area through which the extruded material can move. The surface formed by the leading edges is generally perpendicular to the extrusion direction. The surface of the leading edges generally extends in a direction along the die cavity and a lateral direction.

The numerical difference of the width of the inlet of the relieved feed control chamber and the die cavity width may be substantially constant throughout the die. The lateral offset of the relieved feed control chamber, e.g. the lateral distance between the edge of the die cavity and the edge of the inlet, may be constant throughout the die. This provides a starting point with a relatively constant ratio, and a relatively constant volume flow throughout the die. With this, the two chambers may be effectively used for their intended function, relatively independent from each other. Having a generally constant depth for the relieved chamber reduces the amount of variables complicating the design and manufacture, without limiting the amount of control and correction power for overcoming splaying/towing.

In an embodiment, the numerical difference is less than 10 mm. The offset in this case may be less than 5 mm. This provides for a relatively small side loadings control chamber.

In an embodiment a ratio between a width of the inlet of the feed control chamber and the die cavity width may be chosen as to define a volume flow of material to be extruded. Although the feed control chamber is of limited depth, the chamber may still be used to control the flow of the material supplied. Instead of using friction of the sidewalls to control the amount of material supplied, as is the case in conventional dies, it is possible to use the ratio between the width of the inlet of the feed control chamber and the width of the die cavity to accurately regulate the amount of material being supplied to the die cavity. The ratio may be increased or decreased, in order to increase or decrease the volume flow through the die cavity, respectively. The reduced friction and reduced pressure results in a relatively local control of flow, i.e. changes in properties of the feed control chamber, such as height or ratio, only affect a relatively small region on the die. Therefore, the relieved form may allow for an accurate and relatively local control of the amount of material supplied, and at the same time may allow reduced pressures to be used and an increase in extrusion speed.

In an embodiment, the side walls of the feed control chamber diverge at an angle of approximately 0.1° to 10°, and preferably at an angle of approximately 0.8° to 6°. The slope of the walls may be used to control the extrusion speed. A larger angle may increase extrusion speed, and vice versa. By providing more diverging wall, the friction in the relieved feed chamber will be lowered.

In an embodiment of the invention a die is provided having a relieved feed control chamber of limited height directly upstream of the bearing having varying relieve angles along the die cavity. This will allow a local control of the extrusion speed at the bearing directly downstream from the feed control chamber.

In an embodiment the relieved feed control chamber comprises at least one area along the die cavity having a non-relieved side walls, said area being an area of the die or die cavity having the lowest friction. This area could be an area having a die cavity of large width or e.g. the most central part of the die body where extrusion pressure is highest.

In an embodiment the angle of relieve is non-uniform along at least a part of the die channel. This allows to locally control extrusion parameters. The angle of relieve of the relieved feed control chamber according to the invention is directly dependent on local extrusion parameters, such as die cavity width or the relative position on the die, with respect to local extrusion pressure.

The feed control chamber may have a substantially uniform height around the die cavity. The height can be less than 4.5 mm. The substantial uniform height allows for relatively easy manufacturing of the die and control of the variables for calculating possible designs of the die. A lower height will lower the friction and improve the extrusion parameters.

The feed control chamber can have a non-uniform height around the die cavity. In an embodiment a height of the feed control chamber may be larger than a lateral offset between the leading edge of the die cavity and the inlet of the relieved feed control chamber. The lateral offset is equal to the lateral distance, as measured perpendicular to the extrusion direction, between the side wall at the inlet of the feed control chamber and the leading edge of the die cavity. In other words, it may be the projected distance, as measured parallel to the in the plane of the die, between the edge of the inlet and the edge of the die cavity. The angle between the surface of the leading edges and a fictional line between a tip of the leading edge and a tip of the inlet of the relieved feed control chamber is 45 degrees or more. This reduction angle of the relieved chamber can vary along the die cavity. By varying the angle the inlet width of the relieved chamber according to the invention is increased or decreased, therefore regulating flow. The larger the reduction angle, the higher the friction/control. By varying the reduction angle the friction in the relieved chamber is varied. The larger the reduction angle, the lower the friction, reducing the local flow control of the relieved chamber. In an embodiment the reduction angle is maximum, preferably more than 75 degrees, most preferably in the order of 85-90 degrees, at that point along the die cavity needing most friction.

In an embodiment the die body has at least a part of a die cavity arranged such that the relieved chamber and further feed control chamber are locally one single chamber. The relieved chamber has no reduction in width (reduction angle 90), while width of the relieved chamber and further feed control chamber are equal. This will allow local maximum friction. In an embodiment, the feed control chamber is arranged to control side loading during extrusion. At least a part of the relieved feed control chamber is arranged as side loadings control chamber. A midpoint of the inlet of the side loadings control chamber is laterally shifted with respect to a midpoint of the die cavity. The lateral shift or mid point is immediately available from a cross sectional view of the die along the die cavity. Such cross sectional views are shown in the Figures. A midpoint of the inlet and the die cavity can be defined as the centre-point between the side walls and leading edges of the side loadings control chamber and die cavity respectively. The inlet of side loadings control chamber is laterally shifted. The shift of the inlet of the side loadings control chamber is chosen as to apply a side loading on the material being extruded. This may be used to correct for the occurrence of splaying or other deformations, for example of the type outlined hereinbefore. The shift of the inlet of the side loadings control chamber may be used locally on the die, for instance to correct the flow of material being extruded in a region of the die where splaying occurs. Prior art arrangements of relatively complex extrusion profiles experience problems, in that occurrence of deformations in one area may not be corrected without affecting the material supplied in adjacent areas where no correction is needed. The relieved form allows for an accurate and relatively local control of the material supplied. The side loadings control chamber, in combination with the shift of the inlet, allows for side loadings to be applied precisely where needed, without affecting regions of the die where correction is not desired. The control of material supplied is thus accurate and may be exerted on a relatively small part of the die.

In an embodiment in some parts of the die the inlet of the side loadings control chamber is not shifted, but rather in line with the midpoint of the die cavity, for example in regions not showing splaying.

The shift may be relatively small compared to the die cavity width. Preferably, the shift is in the order of the die cavity width, or smaller. The shift may be in between 0.1-30% of the die cavity width, preferably 0.1%- 10%.

In an embodiment the die comprises a relieved side loading control chamber having different shifts along the die cavity, in order to correct splaying at different parts of the die. This may be necessary, since certain parts of the die cavity may possibly show more splaying than others. It will be understood that some parts of the die cavity do not show splaying, and hence no shift is needed there. In an embodiment the lateral shift is non-uniform along the die cavity. This allows a similar correction for splaying as described in the MBA-disclosure.

In an embodiment the shift varies smoothly along the die cavity, allowing local corrections for splaying. The varying shift is arranged dependent on the amount of local expected deflection of the leading edges/bearing as a result of extrusion pressure. In an embodiment the varying shift of the relieved chamber along the die cavity is calculated in a calculation method for designing a die and die cavity. In an embodiment the varying shift is dependent of the die design, e.g. more shift near tongues in the die body, and of the die cavity position in the die, e.g. close to the edge, resulting in less pressure and therefore less deflection, resulting in a lower amount of shift.

In an embodiment, the extrusion die further comprises a further feed control chamber upstream from the relieved feed control chamber. The further feed control chamber allows controlling the flow of extrusion material before it enters the relieved feed control chamber and eventually the die cavity.

In an embodiment the further feed control chamber is a volume control chamber formed directly upstream of the side loadings control chamber. The volume control chamber has a base comprising the inlet of the side loadings control chamber. The volume control chamber can be formed in line with a preform chamber as known from EP 0 837 745.

In a preferred embodiment the height of the relieved chamber, when used in combination with a upstream further feed control chamber, is less than 3 mm. If the height is less than 3 mm a more local control of extrusion parameters is obtained, while more global parameters are controlled with the upstream chamber(s).

A midpoint of the inlet of the volume control chamber may be positioned in line with respect to the midpoint of the inlet of the side loadings control chamber. A ratio between a width of the inlet of the volume control chamber and the die cavity width may be chosen as to define a volume flow of material to be extruded. The width and/or other dimensions of the inlet of the volume control chamber may be chosen as to define the volume and/or speed of material supplied to the die cavity. This way, an additional chamber, next to the side loadings control chamber, may be used to further increase the accuracy with which the material to be extruded may be controlled. The side loadings control chamber, situated between the die-cavity and the volume control chamber, may be used to apply side loadings to the material being extruded, in order to correct for the occurrence of splaying or other deformations. The volume control chamber may be adapted in order to ensure that the correct quantity of material is supplied to each part of the die cavity. This ensures that the material flows through the die cavity at a substantially uniform speed. By achieving these effects, using two separate chambers that follow each other in a downstream direction, it will be appreciated that the two effects can be controlled substantially independently of one another. This means that if corrections are made to one chamber, for example by changing the width of the inlet of the volume control chamber, these corrections need not alter the shape of the other chamber.

In an embodiment the side walls of the volume control chamber diverge in the extrusion direction of the die. As described before, the relieved form of the chamber, i.e. the diverging of the side walls in a downstream direction of the die, results in relatively less resistance to flow of the material supplied passing through the chamber, but still allows for an accurate control of the flow.

The side walls of the volume control chamber may diverge at an angle of approximately 0.5° to 10°, and preferably at an angle of approximately 1° to 6°. The angle may be used to control the extrusion speed.

Preferably, the volume control chamber has a substantially uniform height around the die cavity. The volume control chamber has a varying width (or lateral offset) around the die cavity in order to control the extrusion speed. The height may be in the range of 4 mm to 20 mm. Preferably, the height is equal to approximately 8-12 mm, but it will be appreciated that the invention is not restricted to this height, and that other heights, either greater or less than 8-12 mm, may be used.

In an embodiment, the width of the inlet of the volume control chamber is nonuniform around the die cavity. The width dimension may be chosen to supply a correct quantity of material. The width dimensions may depend on various die factors, e.g. applied pressure, die dimensions, relative position on the die, and size of the die cavity. Preferably the width is dependent on the relative position on the die. A position closer to the edge of the die will preferably have a width larger than a similar chamber closer to the middle of the die.

According to a further aspect a die is provided comprising a die body in which a die cavity is formed, a first feed control pocket being formed in the die body upstream of the die cavity, and a second flow control pocket being formed in the die body between the first pocket and the die cavity, wherein at least part of the first pocket has a width dimension controlled in order to achieve the supply of material, in use, at a substantially uniform speed throughout the die cavity, and the second pocket includes at least a region which is laterally offset relative to the adjacent part of the die cavity. The die body can comprise any of the above mentioned features of above described embodiments. The invention will further be described, by way of example, with reference to the accompanying drawings, in which:

Fig. 1 illustrates part of a die, illustrating a die cavity and associated side loadings control chamber and volume control chamber;

Fig. Ia is a sectional view along the line Ia-Ia of Figure 1;

Fig. 2 is a view similar to Figure 1 illustrating an alternative embodiment;

Fig. 2a and 2b are sectional views along the lines Ha-IIa and Hb-IIb of Figure 2;

Fig. 3 is a view similar to Figure 1 illustrating another alternative section; Fig. 3a, 3b and 3c are sectional views along the lines IHa-IIIa, IHb-IIIb and IIIc-

IHc of Fig. 3;

Fig. 4 illustrates a die intended for use in the extrusion of a hollow section; and

Fig. 4a is a view along the line IVa-IVa of Figure 4;

Fig. 5 illustrates a further embodiment; Fig. 5a is a view along the line Va-Va of Figure 5;

Fig. 6 is a view of another embodiment of a die;

Fig. 6a and 6b are sectional views along Via- Via and VIb-VIb of Figure 6 respectively.

In this application 'height' will be an indication for a dimension of a chamber in an upstream direction. 'Depth' of a chamber is a similar dimension in the downstream direction. The upstream or downstream direction of a die can be indicated with the 'z- direction'. The die cavity extends generally in the x-y plane. The 'lengthwise direction' of the die cavity is in general the direction in which the die cavity runs in the x-y plane. 'Width' is an indication for a dimension generally in the x-y plane, extending perpendicular to the lengthwise direction of the die cavity. When describing different parts of the die, it may be convenient to use a local coordinate system x'-y'-z'. The z'- direction is the local upstream or downstream direction. The y'-direction is the local direction in which the die cavity extends in the x-y plane. The y'-direction thus indicates the local lengthwise direction. The x '-direction is the direction in the x-y plane that is perpendicular to the y'-direction. The x'-direction is thus equal to the local width direction of the die. The surface of the leading edges of a bearing extend in the x'-y' plane. The extrusion direction is the z-direction.

Figure 1 shows a x-y-z coordinate system. Figure Ia shows a cross section of a die. In figure 1 the local x'-z' coordinate system is indicated. Referring firstly to Figures 1 and Ia, there is illustrated a flat faced die 1 comprising a die body 10 in which is formed an opening 11 defining a die cavity 12. The die cavity 12 has a width w. The die cavity 12 is of so-called zero bearing form in that the side walls 13 thereof diverge away from one another in the extrusion direction z,z' by a small angle β. The die cavity 12 in this arrangement is at least partly of zero bearing form, and it will be appreciated that the invention is not restricted to such arrangements, and that arrangements having parts where finite bearing lengths are used also fall within the scope of the invention.

Although preferably of uniform, zero bearing length around the entire die cavity 12, the invention may be applied to arrangements in which the die cavity is of non- uniform bearing length, preferably limited to bearings having a length of less than 2.5mm, more preferably less than 1.5mm. In an embodiment the length of the finite bearing depends on a width w of the die cavity 12.

The width w of the die cavity 12, at the leading edges 17 thereof, is substantially uniform throughout the entire die cavity, in this embodiment, and is intended for use in the formation of an extruded product of uniform wall weight/thickness.

Directly upstream of the die cavity 12 a feed control chamber 18 is formed. The chamber 18 has a base 26 comprising the leading edges 17 of the die cavity 12, the die cavity 12 having a width w. The base 26 extends on both sides of the die cavity 12, and has a width Wl '. From the base 26 sidewalls 23 extend with a height Hl towards an inlet having a width Wl of the relieved feed control chamber 18. The inlet is formed between edges 27 of the feed control chamber 18.

The inlet has a width Wl, defined as the distance between the two edges 27. The feed control chamber 18 may be of substantially uniform depth, i.e. the height Hl of the chamber is constant throughout the die around the die cavity 12. The height Hl may for example be approximately 2 mm, but other depths are also possible within the scope of the invention. The chamber 18 is of relieved form having a relief angle γ. The side walls 23 diverge away from one another in the extrusion direction (z '-direction) of the die 1. The sidewalls 23 may immediately diverge from the edges 27, such that a sharp edge is formed in the feed control chamber 18. It is also possible that the sidewalls are positioned relatively parallel from the edges in a downstream direction, before diverging away from one another. This way, a desired amount of friction may be re- introduced in the volume control chamber, when necessary. In the embodiment shown in Figure 1, relieved control chamber 18 is generally similar to a zero bearing.

The purpose of the relieved feed control chamber 18 is to locally control the flow of the material through the die cavity 12. The feed control chamber 18 may be used to locally control the speed of the material being extruded. The relieved feed control chamber 18 is of generally the same shape as the die cavity 12, but of increased width relative thereto. The relieved feed control chamber is laterally offset with respect to the die cavity in the x' -direction, as clearly shown in Figure 1.

Preferably, the relieved feed control chamber 18 has a sharp edges 27 and diverging sidewalls 23. The relieved form, angle γ with respect to z, allows control over extrusion speed to be achieved by controlling the width Wl rather than the length of the parallel side walls of the side loadings control chamber. It will be appreciated that a reduction in the required applied extrusion pressure may be achieved. The relief angle γ of the sidewalls 23 may be approximately 2°-6°. Other angles, ranging from 0.5° to 10° are also possible. The larger the relieve angle, the less friction the feed control chamber will enforce on the flow of extrusion material.

A back flow of extruded material results in the relieved feed control chamber 18 from base 26 along side walls 23 back to the inlet. Extrusion material more internally will flow over this back flow and will experience relatively low friction, but distinctive control. The relieved feed control chamber 18 can be referred to as a soft bearing.

Where it is anticipated that deflections or other deformations are likely in the extruded product, the relieved feed control chamber 18 may be arranged to include regions, where required, which are shifted laterally relative to the die cavity 12, i.e. in the local x '-direction, rather than being disposed substantially symmetrically relative thereto. The relieved feed control chamber 18 will become a relieved side loading control chamber.

The lateral offset distance in the in local x' -direction is very small in relation to the die cavity geometry (width). The shift is preferably less than 25%, but more preferably less than 15% of the die cavity width. For example, it is typically less than 0.3 mm.

The purpose of such offsetting is to apply sideways acting loadings to the material being extruded which counter the forces that give rise to the generation of deformations in the extruded product. For example, where splaying of part of the extruded product is anticipated, the side loadings control chamber 18 is offset to apply forces to the material which oppose those causing splaying with the result that the extruded product is of substantially the same shape as the die cavity 12.

The degree of offsetting of the side loadings control chamber 18 is typically non- uniform around the die cavity 12. For example, there will typically be parts of the die cavity 12 for which no correction in this manner is required (and so the side loadings control chamber 18 will be disposed symmetrically), parts where a significant offset is required, and other parts where less offsetting is needed. The direction of offsetting needs not be uniform. The direction of offsetting may be in the positive x '-direction, as well as in the negative x'-direction. The direction may depend upon the shape of the product to be extruded and the anticipated deformations to be corrected by the side loadings control chamber 18.

In an embodiment the lateral offset increases closer to tongue-parts of the die. The tongue parts of a die will encounter most deflection in a Z-direction during extrusion resulting in more local splaying. The splaying is corrected by a relative large lateral offset near tongue parts formed in the die. An example tongue part is formed by the tongue in between the legs of a U-shaped die cavity.

Directly upstream of the side loadings control chamber 18, a further feed control chamber 16 may be formed. Here the feed control chamber 16 is a volume control chamber 16 having a base 36 comprising the leading edges 27 of the side loadings control chamber 18. The base 36 extends on both sides of the leading edges 27 of the side loadings control chamber 18, and has a width W2'. From the base 36, sidewalls 33 extend in an upstream direction towards an inlet of the volume control chamber 16. The inlet is formed by edges 37. The inlet of volume control chamber 16 is of width W2. The width W2 may be significantly greater than the width w of the adjacent, associated part of the die cavity 12.

The volume control chamber 16 may be disposed substantially symmetrically about the die cavity 12, i.e. a midpoint line 39 between the leading edges 37 of the volume control chamber 16 is aligned, in the z' direction, with the midpoint between the leading edges 17 of the die cavity 12. Figure Ia shows a cross section not showing the lateral shift. The chambers 18 and 16 in this embodiment are however laterally shifted and a side loading is applied as the chamber are asymmetrically disposed around line 39.

The local width W2 of the volume control chamber 16 is chosen to ensure that substantially the desired quantity of material is supplied to each part of the die cavity 12, thereby achieving a substantially uniform speed of material flow through the die cavity 12, in use. Hence, the width W2 may vary on the die, as can be seen in Fig. 1. Near end 40, the width W2 is larger.

Several factors are taken into account in determining how wide each part of the volume control chamber 16 should be. One important factor to take into account is the width w of the associated part of the die cavity 12. However, other factors including, for example, the proximity of that part of the die cavity 12 to the edge of the die body 10, may also be taken into account as friction in the extruder will result in the effective applied extrusion pressure being lower close to the edges of the die body 10 than towards the centre thereof. In the arrangement of Figure 1, which is intended for use in the production of an extruded product of substantially constant wall weight/thickness, it can be seen that the width W2 of the volume control chamber 16 is substantially uniform adjacent those parts of the die cavity 12 which are approximately the same distance from the edge of the die body 10, the width W2 increasing in regions where the die cavity 12 is closer to the edge of the die body 10, thereby compensating for the reducing applied extrusion pressure in those regions.

The volume control chamber 16 may be of uniform depth and depends on the section size and geometry. A possible depth is for example a depth of approximately 5 mm, but it will be appreciated that the invention is not restricted to this depth, and that other volume control chamber depths fall within the scope of the invention.

As best shown in Figure Ia, a cross section along Ia-Ia, the volume control chamber 16 has sidewalls that diverge in the downstream direction, such that a width W2' at its base 36 is greater than the width W2 at the inlet. The sidewalls 33 may immediately diverge from the edges 37, such that a sharp edge is formed in the volume control chamber. It is also possible that the sidewalls are positioned relatively parallel from the edges downstream, before diverging away from one another. This way, a small amount of friction may be re- introduced in the volume control chamber, when necessary.

By using a volume control chamber 16 of relieved form, it will be appreciated that the resistance to flow of the material passing through the volume control chamber 16 is minimised, as described earlier. The relief angle of the volume control chamber 16 is, in this embodiment, approximately 2°, but it will be appreciated that the invention is not restricted to this angle, and that other relief angles may be used. In Figure Ia, the relief angles are exaggerated for clarity.

The volume control chamber 16 preferably has a sharp edge 37 and the diverging sidewalls 33. The sharp edge allows control over extrusion speed to be achieved by controlling the width W2 rather than the length of the parallel side walls of the side loadings control chamber. It will be appreciated that a reduction in the required applied extrusion pressure may be achieved.

In other embodiments the depth/height of the volume control chamber 16 is used to further control extrusion parameters.

In the embodiment shown in Fig. 1, it will be appreciated that the speed at which the material passes or is fed through the die is controlled by the volume control chamber 16 to ensure that a substantially uniform extrusion speed is achieved across the entire die cavity. The side loadings control chamber 18 is used to control the flow of material through the die cavity 12, increasing feeds to potentially slower details and applying side loadings to the material being extruded to counter or correct for the occurrence of deformations in the extruded product. By achieving these effects, using two separate chambers that follow each other in a downstream direction, it will be appreciated that the two effects can be controlled substantially independently of one another.

Figures 2, 2a and 2b illustrate an arrangement of a die 110 similar to that of Figures 1 and Ia, but in which the extruded product includes a small region 120 of increased wall thickness. The die cavity 112 thus includes a region 120 of increased width. It will be appreciated by the man skilled in the art that the extrusion material will tend to flow too fast near region 120. Figure 2 is a schematic example. In an actual die of this shape the prechambers near the ends would have been more ballooned and local shifting of the relieved chamber would correct for deflection. A cross-sectional view of this region along Hb-IIb is shown in Fig. 2b. As the region 120 forms less of a restriction to the flow of material through the die cavity, to ensure that the extrusion speed is substantially uniform about the entire die cavity 112 the adjacent parts of the volume control chamber 116 are of reduced width W4'with respect to width W4, the width of the volume control chamber 116 along cross section Ha-IIa.

The dimensions of the corresponding part of the side loadings control chamber 118 are substantially the same as elsewhere on the die cavity 112. The side loadings control chamber 118 has a width W3 that is generally uniformly laterally offset around the die cavity. In this example the edge 128 is about 2 mm offset with respect to leading edge 117. The lateral offset of 1 mm is constant or uniform around the complete die cavity.

Width W3' is larger than width W3, however the width is only larger to the extend that leading edges 117 are spaced further apart. The lateral offset is still 1 mm. A reduction in the width W3' of the volume control chamber 116 to achieve full control over the extrusion speed is not possible, in this embodiment, without impacting upon the dimensions of the side loadings control chamber 118. Thus, in this arrangement, the part of the volume control chamber 116 in the region 120 is not of relieved form, but rather is of parallel walled form (or the walls may even slightly converge), and serves as a factional part slowing the passage of material to the adjacent part of the die cavity 112. It will be appreciated that, in this arrangement, feed control is achieved primarily by appropriate selection of the width W4' of the volume control chamber 116, but that in the region 120, feed control is achieved by a combination of control over the width of the volume control chamber 116 and control over the parallel side wall length thereof.

Figures 3, 3a, 3b and 3c illustrate another die cavity 312, this time intended for use in the formation of a product of tapering wall weight or thickness. Again, the width W7 of the volume control chamber 316 is chosen to control the speed of material flow through the corresponding parts of the die cavity 312, the width W7 being dependent upon the width w6 of the adjacent part of the die cavity 312 and the distance from the edge of the die body 310. As with the arrangement of Figure 2, feed control simply by controlling the width W7 of the volume control chamber 316 is thought to be insufficient and so the volume control chamber 316 includes part (see Figure 3a) which is of fully relieved form and so the width of the volume control chamber 316 alone achieves the feed control, part (see Figure 3 c) which is of parallel walled form, and an intermediate part (see Figure 3b) in which the walls of the volume control chamber 316 include a parallel section 322 and a relieved section 324, providing an intermediate level of control to the speed of the material being extruded. The parts of the volume control chamber 316 which are not fully relieved achieve feed control by a combination of the width of the volume control chamber 16 and the parallel side wall length thereof.

As the widths w6, W6 and W7 vary along the die cavity 310, they are indicated with respective widths w6\ w6", W6\ W6", W7' and W7". The arrangements described hereinbefore are all flat face dies intended for use in the formation of extruded products of solid form. However, the invention is also applicable to extrusion dies intended for use in the formation of extruded products of hollow form. Figure 4 illustrates such a die, the die being intended for use in the production of box section components, Figure 4a indicating that in such an arrangement, if desired, the volume control chamber 416 may be of relieved form on just one side of the die cavity 412. Still however, both side walls 433 diverge away from each other in the downstream direction. In the arrangement illustrated, the part of the volume control chamber 416 on the both male (or mandrel) 426 of the die and the part provided on the female die body 428 are of relieved form. In the embodiments described hereinbefore, the volume control chamber controls the speed at which the material passes or is fed through the die. The side loadings control chamber 18 may be used to counter or correct for the occurrence of deformations in the extruded product, by applying side loadings to the material being extruded. The side loadings may be applied by laterally shifting the side loadings control chamber by a small amount. The control of the flow of the material being extruded can be exerted by adjusting separate chambers, and thus precise control of the material being extruded is possible.

Figures 5 and 5 a illustrate another section, but exaggerating the distance by which parts of the second pocket 518 are offset relative to the die cavity 512, for clarity.

In each of the arrangement described hereinbefore it will be appreciated that the speed at which the material passes or is fed through the die is controlled by the first, feed control pocket 516 to ensure that a substantially uniform extrusion speed is achieved across the entire die cavity. The second, flow control pocket 518 is used to control the flow of material through the die cavity 512, increasing feeds to potentially slower details and applying side loadings to the material being extruded to counter or correct for the occurrence of deformations in the extruded product. Line 539 is a line through the middle of die cavity 512. Both midpoints 540 and

541 are however laterally shifted with respect to this middle point line. Double shifting, shifting of both the relieved chamber according to the invention and the further feed control chamber increase the effect of side loading and allow more correction for splaying. Figures 6, 6a, and 6b illustrate another die 610 with a die cavity 612. The bearing of the die cavity is preferably a zero bearing. In this embodiment, a relieved feed control chamber 618 formed by a single pocket is arranged upstream from the bearing. The relieved feed control chamber 618 is used to control the flow of material to be extruded. The pocket 618 has a height H7. In this embodiment the height H7 of the pocket is uniform along the die cavity.

The side walls 617 of the relieved chamber 618 diverge in the extrusion direction of the die. The width W9, W9' of the inlet is substantially constant throughout the die 610, as can be seen in the cross sectional views of Fig. 6a and Fig. 6b.

To control the speed of material flow through the corresponding parts of the die cavity 612, the side walls 617 of the feed control chamber 618 may be adapted. For example, the die may contain regions, such as shown in Fig. 6b, where parallel side walls 617' with height H6 are used to induce friction, intended to slow down the material.

A skilled person will recognize the region according to Fig 6b as a relieved chamber directly upstream from the die cavity and a further upstream feed control chamber. Clearly the relieved chamber (part of the pocket 618) according to the invention has a varying height along the die cavity.

The parts of the double feed control chamber 618, according to Fig 6b, which are not fully relieved achieve feed control by a combination of the relieve of the side walls and the parallel side wall length thereof. Other regions, such as the one shown in Fig. 6a, contain a relieved chamber 618 with continuously diverging side walls 617. The chamber here is fully relieved. Here, no parallel side walls 617' are present. The relief angle of the relieved side walls 617 may be equal in the different regions. Although the width of the first region W9' is equal to the width W9 of the latter region, control of speed of material flow is possible by changing the amount of relief of the side walls. Alternatively, the width W9,W9' of the inlet 641 of the single relieved feed control chamber 618 may be used to control the speed of material flow through the corresponding parts of the die cavity 612.

The single pocket may also be used to correct for deformations, such as splaying. To this end, the inlet W9 may be shifted, locally, with respect to the centerline 639 to induce local side loadings on the material to be extruded. With this, a single relieved pocket may be used to accurately control the material flow.

If desired, a die corrector can modify the die, after manufacture, to fine tune the corrections achieved thereby. For example, the bearing lengths of parts of the further feed control chambers 16, 116, 316, 416 and 516 may be altered by changing the lengths thereof that are relieved. If desired, a die corrector can modify the die, after manufacture, to fine tune the corrections achieved thereby. For example, the parallel side wall lengths of parts of the feed control chamber may be altered by changing the lengths thereof that are relieved. It will be appreciated that a wide range of modifications and alterations may be made to the arrangements described hereinbefore without departing from the scope of the invention.

Claims

1. Extrusion die comprising a die body having bearing walls, wherein the bearing walls comprise leading edges surrounding a die cavity having a die cavity width, and wherein the extrusion die further comprises a feed control chamber formed directly upstream of the die cavity, wherein the feed control chamber has a base comprising the leading edges, and side walls extending with a height of less than 7 mm from the base in an upstream direction of the die towards an inlet of the feed control chamber, wherein the side walls of at least part of the feed control chamber diverge away from one another in an extrusion direction of the die, and wherein a ratio between a width of the inlet of the feed control chamber and the die cavity width is chosen as to define a volume flow of material to be extruded.

2. Extrusion die according to claim 1, wherein the extrusion die is a zero-bearing die.

3. Extrusion die according to claim 1 or 2, wherein the side walls of the feed control chamber diverge at an angle of approximately 0.5° to 10°, and preferably at an angle of approximately 2° to 6°.

4. Extrusion die according to any one of the preceding claims, wherein a height of the control chamber is larger than a lateral offset between the leading edge of the die cavity and the inlet.

5. Extrusion die according to any one of the preceding claims, wherein the feed control chamber has a substantially uniform height around the die cavity.

6. Extrusion die according to any one of the preceding claims, wherein in at least a part of the die, the feed control chamber is a side loadings control chamber, wherein a midpoint of the inlet of the side loadings control chamber is laterally shifted with respect to a midpoint of the die cavity, wherein the shift is chosen as to apply a side loading to the material to be extruded.

7. Extrusion die according to claim 6, wherein the shift is in between 0.01% to 30% of the die cavity width.

8. Extrusion die according to claim 6 or 7, wherein the extrusion die further comprises a volume control chamber formed directly upstream of the side loadings control chamber, the volume control chamber having a base comprising the inlet of the side loadings control chamber, and side walls extending with a height of less than 10 mm from the base in the upstream direction of the die towards an inlet of the volume control chamber, wherein a midpoint of the inlet of the volume control chamber is positioned in line with respect to the midpoint of the inlet of the side loadings control chamber, and wherein a ratio between a width of the inlet of the volume control chamber and the die cavity width is chosen as to define a volume flow of material to be extruded.

9. Extrusion die according to claim 8, wherein the numerical difference of the width of the inlet of the side loadings control chamber and the die cavity width is substantially constant throughout the die.

10. Extrusion die according to claim 9, wherein the numerical difference is less than 10 mm.

11. Extrusion die according any one of the claims 8-10, wherein the side walls of at least part of the volume control chamber diverge away from one another in an extrusion direction of the die.

12. Extrusion die according to claim 11, wherein the side walls of the volume control chamber diverge at an angle of approximately 0.5° to 10°, and preferably at an angle of approximately 0.8° to 6°.

13. Extrusion die according to any one of the claims 8-10, wherein the volume control chamber has a substantially uniform height around the die cavity.

14. Extrusion die according to any one of the claims 8 to 12, wherein the width of the inlet of the feed control chamber is non-uniform around the die cavity.