AU2001239446A1 - Extrusion die - Google Patents

Description

"Extrusion Die"

This invention relates to an extrusion die for use in the extrusion of metallic

materials, and in particular to a die suitable for use in the extrusion of aluminium. The

invention also relates to a method of manufacturing a die of this type.

When extruding aluminium it is important to ensure that the speed of movement

of the aluminium through the extrusion die is uniform across the die. This has been

achieved in the past using a die having a die cavity of finite bearing length, and by

varying the bearing length across the die. It has been found, however, that the extruded

product sometimes contains surface imperfections resulting from the engagement

between the bearing surface and the aluminium being extruded. Rather than provide the

bearing surface downstream of the entrance to the die cavity, it is known to use a die

having a so-called zero bearing, and to provide a chamber upstream of the die cavity of

varying bearing length to control the extrusion speed over the die. 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.

Another problem which has been faced when extruding aluminium is that, where

the extrusion is, for example, of channel section, the sides of the channel tend to deflect

thus, if the die is shaped to include a die cavity in which the parts thereof which form the

sides of the channel are parallel to one another, the sides of a member extruded using the

die may be splayed, rather than parallel to one another. In order to correct such

splaying, it is known to provide a pre-chamber located upstream of the die cavity, the

pre-chamber being of greater width than the part of the die cavity immediately adjacent thereto, the pre-chamber being offset laterally relative to the die cavity. Such a

technique results in a side loading being applied to the metal being extruded. Although

off-setting the pre-chamber laterally from the die cavity can correct splaying in some

circumstances, it is of limited application and may not be able to apply a sufficient force

to correct splaying in, for example, extrusions of relatively low wall thickness. Similar

problems are experienced when hollow members are extruded using a die comprising a

male part and a female part.

According to the present invention there is provided an extrusion die comprising

a die body having a die cavity formed therein, the die body defining a male portion which

projects into a female portion, and wherein the leading edge of the part of the die cavity

defined by the male portion and the leading edge of the part of the die cavity defined by

the female portion are not co-planar, when the die is not in use.

It has been found that, in prior arrangements, the magnitude of the load applied

to the metal being extruded, and hence to the die, is sufficient to cause the male portion

to deflect relative to the female portion. If the die is a zero bearing die, such deflection

results in the leading edges of the die cavity being spaced apart in the extrusion direction.

Such spacing results in side loadings being experienced by the metal being extruded and

can result in splaying as described hereinbefore. By designing the die such that, at rest,

the leading edges are not co-planar, this effect can be reduced as the die can be arranged

such that the leading edges become co-planar or substantially co-planar when deflection

occurs in use. Although it is convenient for the leading edges to become co-planar, the

advantages of the invention also arise if, throughout the die, the leading edge of a part on one side of the cavity aligns with the part thereof on the other side of the cavity.

It should be noted that, in general, both of the male and the female portions

deflect, and that it is the relative deflection between these portions for which the

invention is intended to compensate.

Although the description herein refers to deflection, it will be appreciated that

some compression of the material of the die may also occur as a result of the application

of loads thereto, in use, and that the invention can also be used to overcome

disadvantages associated with misalignment of bearings caused, in use, by such

compression.

The die cavity is preferably shaped such that its width increases from a minimum

adjacent the leading edges thereof. Such an arrangement is referred to hereinafter as a

zero bearing die.

The invention is particularly advantageous with zero bearing dies as a relatively

small amount of deflection causes total misalignment of the bearings of such dies. In

dies of the non-zero type, even when deflection has occurred, it is likely that a part of

each bearing surface will remain aligned with a part of the opposing bearing surface.

The die cavity may be shaped to define the male and female portions, the male

portion taking the form of a tongue portion. In such an arrangement, the die is used to

form an extruded member including at least one elongate channel of any cross-sectional

shape.

The die cavity may be shaped to define at least one further tongue portion. The

die body may define at least one further die cavity. The die body may have a substantially planar front face, a groove being provided

in the front face, the leading edges of the die cavity being defined at the intersection

between the die cavity and the groove, the groove being of non-uniform depth.

Alternatively, the die body may have a non-planar front face, the front face

having been machined to define the leading edges of the die cavity.

A preform chamber may be located upstream of the die cavity, the preform

chamber being shaped to achieve a uniform extrusion speed across the die. The preform

chamber may be of non-uniform depth, and hence of non-uniform bearing length, and/or

may be of non-uniform width.

In an alternative arrangement the die may be designed to produce an extruded

member of hollow form, the male portion projecting into an opening formed in the

female portion.

According to another aspect of the invention there is provided a method of

manufacturing an extrusion die comprising calculating the likely deflection of a male

portion of the die, in use, and forming a recess around at least part of a die cavity of the

die, the depth of the recess being non-uniform and shaped such that, in use, the leading

edge of apart of the die cavity to one side of the cavity aligns with the leading edge on

the opposing side of the cavity.

The recess is conveniently formed using a grinding operation, but it will be

appreciated that other techniques could be used.

The step of calculating the likely deflection is conveniently achieved using a finite

element analysis technique. The invention will further be described, by way of example, with reference to the

accompanying drawings, in which:

Figure 1 is a sectional view of an extruded aluminium member;

Figure 2 is part of a view of a die used in the extrusion of the member of Figure

1;

Figure 3 is an enlargement of part of Figure 2;

Figure 4 is a diagrammatic sectional view along the line 4-4 of Figure 3;

Figure 5 is a view similar to Figure 4 illustrating an alternative embodiment;

Figure 6 is a view similar to Figure 2 illustrating an alternative die;

Figure 7 is a diagrammatic view illustrating one of the die cavities of the die of

Figure 6;

Figure 8 is a diagrammatic sectional view along the Line 8-8 of Figure 7;

Figure 9 is a view similar to Figure 6 illustrating a further alternative die;

Figure 10 is a diagrammatic sectional view illustrating an alternative die; and

Figure 11 is a plan view illustrating the die of Figure 10; and

Figure 12 is a diagrammatic view illustrating part of a method for use in

manufacturing the dies of Figures 1 to 11.

Referring to Figures 1 to 4, Figure 1 illustrates an extruded aluminium member

10 of relatively complex shape. The member 10 includes several regions which can be

regarded as channel-shaped regions 12 including limbs 14 which are generally parallel

to one another. The pairs of limbs 14 forming several of the channel-shaped regions 12

illustrated in Figure 1 are denoted by the references 14a, 14b and 14c in the drawing. Figure 2 illustrates part of the die body 16 of an extrusion die for use in the

production of the member 10. The die body 16 is provided with openings defining a pair

of die cavities 18, each die cavity 18 being designed for use in the production of the

member 10. Figure 3 illustrates one of the die cavities 18 in greater detail.

As illustrated in Figure 3, each die cavity 18 comprises an opening shaped to

conform, generally, with the cross-sectional shape of the member 10 to be extruded.

The width of the cavity 18 increases from a minimum adjacent the leading edges 18a,

18b of the cavity 18 (see Figure 4), thus the die is of the zero bearing type.

The leading surface of the die body 16 is of generally planar form, and is

provided with a recess 20 aligned with and of the same general shape as the cavity 18,

but of greater width. An additional recess 22 (see Figure 4) is formed in the base of the

recess 20, the recess 22 again being of the same general shape as the die cavity 18 but

of greater width. As illustrated in Figure 4, the leading edges 18a, 18b of the die cavity

18 are defined at the intersection between the die cavity 18 and the recess 22.

As best shown in Figure 3, the part of the die body 16 located between the parts

of the die cavity 18 which, in use, form each pair of limbs 14 takes the form of a tongue

portion 24 received within a correspondingly shaped female portion 26 of the die body

16. In use, the application of a load to the material to be extruded tends to cause the

tongue portions 24 to deflect relative to the female portions 26. In order to reduce the

risk of such deflection causing the limbs 14 of the member 10 becoming splayed, or to

reduce the degree by which they become splayed, the die is manufactured in such a

manner that, when the die is not in use, the leading edges 18a of the die cavity 18 defined by parts of the tongue portions 24 are not co-planar with those defined by parts

of the female portions 26 but rather are positioned such that the deflection of the tongue

portions 24 (to the position shown in broken lines in Figure 4) brings the leading edges

18a associated therewith closer to the plane containing the leading edges 18b associated

with the female portions 26, and preferably into the same plane. By ensuring that

leading edges 18a, 18b are substantially co-planar, in use, the application of side loadings

on the material being extruded, and hence splaying of the limbs 14 can be reduced.

In the embodiment of Figures 1 to 4, the recess 22 is not of uniform depth but

rather, as illustrated in Figure 4, contains regions 22a on one side of the die cavity 18 of

relatively small depth and regions 22b on the other side of the die cavity 18 of greater

depth. The regions 22a are provided on the tongue portions 24, and the depths of the

regions 22a, 22b are chosen to ensure that when the tongue portions 24 occupy their

deflected positions, in use, the leading edges 18a, 18b are substantially co-planar.

If desired, the recess 22 may be off-set laterally from the opening of the die

cavity 18 in some parts of the die, such lateral off-setting also resulting in side loadings

being applied to the metal being extruded to correct for splaying of the limbs 14 in the

conventional manner. This technique may be used, for example, where the amount of

splaying of the limbs 14 is relatively small and may be corrected relatively easily using

this technique or where the use of providing parts of the recess 22 of different depth is

not practical or it is not practical to fully correct splaying using this technique.

Although as described hereinbefore, in use, the leading edges of the die cavity

become co-planar or substantially co-planar, this need not be the case. In order to achieve the benefit of the invention, all that is required is that, in use, the leading edges

on opposing sides of the die cavity align or substantially align with one another. The

plane in which the leading edges of one part of the die align need not be the same as that

in which the leading edges of other parts of the die cavity align.

Figure 5 illustrates an alternative to the arrangement of Figures 1 to 4. In the

arrangement of Figure 5, instead of using the recess 22 to cause the leading edges 18a,

18b to be non-co-planar, the leading face of the die body 16 is not of planar form but

rather is shaped so that, for example, the leading face of the parts of the die body 16

defining the tongue portions 24 are raised relative to the parts defining the female

portions 26.

It will be appreciated that in both of the arrangements described hereinbefore,

the spacing of the leading edges 18a, 18b in the direction of extrusion, varies smoothly

and continuously over the face of the die, for example from a maximum at the tips of the

tongue portions 24 to a minimum at the ends remote therefrom.

Although in the arrangements described hereinbefore the recess 22 is of flat

bottomed form, it could, if desired, be of angled form. A flow control pre-chamber of

varying bearing length or shape may be provided upstream of the die cavity, if desired,

to ensure that the extrusion speed across the die is substantially uniform. Alternatively,

a bearing surface of variable bearing length may be provided downstream of the leading

edges 18a, 18b of the die cavity to achieve this effect. Further, although in the

description hereinbefore the tongue portions 24 are of parallel sided form, it will be

appreciated that this need not be the case and that the invention, is applicable to dies having tongue portions of any shape, for example of curved form or of V-shaped

section. Several N-shaped tongue portions are illustrated in Figure 3 and denoted by

reference numeral 28.

The distances through which the tongue portions deflect, and hence the distances

through which the leading edges of the die cavity should be spaced when at rest are very

small. Figure 6 illustrates a die having four die cavities 18 formed therein, each including

several tongue portions 24 and corresponding female portions 26. Figure 7 is a view,

to an enlarged scale, of one of the cavities 18 shown in Figure 6. In Figure 7, the shaded

area is the die cavity 18. A recess 22 is formed around part of the die cavity 18 with the

result that the leading edges 18a of the parts of the cavity defined by the tongue portions

24 lie in one plane and the leading edges 18b defined by the female portions 26 lie out

of that plane. The recess 22 is only formed on the female portions 26 and is not of

uniform depth. The depth of the recess 22 in various places is marked on the drawing.

Further, the recess 22 is not of flat bottomed form, but rather is of angled form as

illustrated in Figure 8.

Figure 9 illustrates a die for producing a member of an alternative cross-section,

the die including two die cavities. The load experienced by the die body is not uniform

but rather varies depending upon the distance from the edge of the die body. As a result,

a tongue portion located near the centre of the die will deflect by a different amount

from a similar tongue portion located near the edge of the die body. The spacing of the

leading edges of the die cavities, at rest should be modified accordingly, and Figure 9

indicates the spacing of the leading edges 18a, 18b at various points around the two die cavities 18, at rest.

Comparing Figure 9 with Figure 2, it will be appreciated that in Figure 9 the two

cavities are identical to one another, those of Figure 2 being inirror-images of one

another. It is advantageous to produce identical extrusions as any treatment processes

carried out immediately after extrusion can be simplified. The technique of the present

invention allows dies containing several cavities arranged to produce identical extrusions

to be manufactured relatively easily. In Figure 9, the chambers 22 are dimensioned to

ensure that the correct proportions of metal to be extruded are supplied to the two die

cavities 18.

The arrangement illustrated in Figures 10 and 11 differs from those described and

illustrated hereinbefore in that it is intended for use in the extrusion of hollow members.

The die comprises a female part 30 defining a plurality of openings. Each opening

receives part of a male part 32. The male and female parts 30, 32 define therebetween

die cavities 18. Each die cavity 18 is of zero bearing form and includes leading edges

18a defined by the male and female parts 30, 32. The male part 32 will deflect relative

to the female part 30, in use, as described hereinbefore. Such deflection would, in a

typical arrangement, result in the leading edges 18a , 18b becoming misaligned. In

accordance with the invention the die is designed such that, at rest, the leading edges 18a

defined by the male part 32 are spaced from those defined by the female part 30 in the

extrusion direction and such that, in use, the deflection of the male part 32 results in the

spacing of the leading edges 18a, 18b reducing, thus reducing, for example, the tendency

of a circular cross-section extrusion to become elliptical. Preferably the spacing of the leading edges 18a, 18b is reduced to zero in the extrusion direction, in use, but reducing

the spacing to a very small amount may be acceptable in some circumstances.

In order to manufacture an extrusion die in accordance with the invention it is

necessary to determine how much each part of the die will deflect, in use. Once the

deflection has been determined, the die can be designed to ensure that, in its deflected,

in use condition, the leading edge at one side of the die cavity aligns or substantially

aligns with that at the opposite side of the cavity for all parts of the die.

The determination of how much deflection will occur can be achieved using a

range of techniques. For example, a skilled technician may be able to determine, from

his own knowledge and to a reasonable degree of accuracy, how much deflection is

likely. In an alternative technique, a computer model may be used to determine the

forces likely to be experienced by parts of a die, and hence the likely deflection of those

parts. The model conveniently uses a finite element analysis approach. In another

technique, a die having a cavity and other characteristics similar to the die to be

manufactured may have a load applied thereto and the deflection of parts thereof

measured.

Once the likely deflection has been determined, recesses are formed around the

die cavity, the recesses being shaped, and in particular the depth of the recesses being

controlled, to ensure that, in its deflected state, proper alignment of the leading edges

occurs. The formation of the recesses is conveniently achieved using a grinding process.

Figure 12 illustrates a grinding wheel 40 having a grinding surface 42 of frusto-conical

shape. The grinding wheel 40 is mounted for rotation about an axis angled to the intended extrusion direction to form a recess, the base of which is angled at 10° to the

front face of the die. The wheel 40 is conveniently of diameter approximately 15mm.

In order to improve the wear resistance of the die, a nitriding process is also

preferably undertaken.

Although as described above a grinding technique may be used to form the

recess, other techniques could be used, if desired.

Claims (10)

1. An extrusion die comprising a die body having a die cavity formed therein, the

die body defining a male portion which projects into a female portion, wherein a leading

edge of a part of the die cavity defined by the male portion and a leading edge of an

opposing part of the die cavity defined by the female portion are out of alignment with

one another when the die is not in use.

2. A die as claimed in Claim 1 wherein the said leading edges are spaced apart from

one another by a distance such that, in use, deflection of the male portion brings the

leading edges substantially into alignment.

3. A die as claimed in Claim 1 or Claim 2, wherein a recess of non-uniform depth

is provided on the die, at least part of at least one of the leading edges being defined at

the intersection between the die cavity and the recess.

4. A die as claimed in any one of the preceding claims, wherein the die cavity is of

zero bearing form.

5. A die as claimed in any one of the preceding claims, wherein the male and female

portions co-operate to produce, in use, an extrusion having at least one channel shaped

region.

6. A die as claimed in any one of the preceding claims wherein the male and female

portions co-operate to produce, in use, an extrusion including at least one hollow region.

7. A method of manufacturing an extrusion die comprising determining how much

at least a part of the die will deflect, in use, and forming a recess around at least part of

a die cavity of the die, the recess being of non-uniform depth, the depth of the recess being chosen to ensure that, in use, the leading edge of a part of one side of the die

cavity substantially aligns with that of an opposing part of the die cavity.

8. A method as claimed in Claim 7, wherein the recess is formed using a grinding

technique.

9. A method as claimed in Claim 7, wherein the deflection is determined using a

finite element analysis technique.

10. An extrusion die substantially as hereinbefore described with reference to the

accompanying drawings.