Background art When extruding metal profiles two methods are generally applicable: direct extrusion and indirect extrusion. WO 01/68283 describes these two methods in greater detail and also discusses the advantages and disadvantages of each method. Direct extrusion has conventionally been prevailing due to the lower cost of tooling ; however, in some cases the indirect extrusion method are adopted due to its merit of producing comparatively less friction loss in the course of an extrusion process.

The subject matter in WO 01/68283 relates to an apparatus and a method for extrusion of metal and describes various equipment applicable in the apparatus and the method. A special die assembly is described in which a recess means is provided adjacent to the die for collecting shell material

from a billet during extrusion. A die provided with resistance means for the purpose of influencing temperature and flow of the billet material before extrusion is also described as well as a dummy block arranged for preventing the dummy block from adhering to the rear surface of the billet.

As mentioned above the present invention is related to the dummy block and it may be seen as a further development of the dummy block described in WO 01/68283. The dummy block according to WO 01/68283 comprises a foremost disk member with a generally convex front surface and being configured to elastically deform during the extrusion process when it is forced towards the rear end of the billet. The object of the deformation is to provide a billet-engaging surface having a first non-deformed configuration in its relaxed state, and a second configuration when the extrusion pressure is applied to the billet, the billet-engaging surface returning to its relaxed configuration at the end of extrusion when the extrusion pressure is reduced to zero. In its relaxed state the disk has an external diameter slightly smaller than the inner diameter of the billet charging bore of the container of the extrusion press, and an external diameter substantially corresponding to the inner diameter in its deformed state. Depending on whether the dummy block is to be used in direct or indirect extrusion, the"deformed"external diameter has to be chosen correspondingly. More specifically, in direct extrusion the dummy block should merely slide on the internal surface of the container during extrusion, whereas in indirect extrusion the dummy block should preferably expand so as to lock to the container.

As the extrusion takes place at a very high pressure, a large amount of elastic deformation energy is stored in the deformed disk, and due to the inherent elasticity of the disk member a slip-function between the billet rear end and the disk is provided. To further enhance the slip-function, and to avoid the use of additional slip means, the billet-engaging disk surface may be treated to prevent the billet material from sticking, for example by treating

the disk with PVD or plasma CVD, by ion-implanting, plasma-nitration or any other suitable tripological treatment.

Object of the invention The configuration of the dummy block described in WO 01/68283 in many aspects fulfils the proper functioning thereof. However, since the rim of the disk member in its deformed state engages the inner surface of the container of the extrusion press, very high requirements as to resistance to wear and tear of the disk member must be fulfilled which substantially increases the manufacturing costs of the disk member. In particular, if the dummy block is used in direct extrusion the wear of the outer rim of the disk member can be considerable. Additionally, after the extrusion stroke the dummy block must be displaced"backwards"in the container and due to considerable friction between the inner surface of the container and the outer rim of the disk member during the extrusion stroke there is a risk that the disk member does not flex back to its non-deformed state when the pressing force is released whereby the disk member may be caused to flex"backwards"when it is retracted. Experiments have also shown that when the known dummy block is used in a direct extrusion process, the small amount of metal deposited on the inner surface of the container can become loose during the withdrawal of the dummy block within the container and fall into the space between the disk member and the main body of the dummy block. This is highly undesirable.

Accordingly, it is an object of the present invention to provide a dummy block of the type mentioned in the opening paragraph in which the above- described disadvantages of the known dummy block have been eliminated.

Disclosure of the invention The object of the invention is achieved by providing the dummy block mentioned in the opening paragraph with a ring member at an outer rim of the disk member, said ring member having a foremost end adapted to be deformed elastically outwards by the disk member when the disk member is elastically deformed.

Thereby is obtained that the disk member itself does not engage the inner surface of the container whereby tear and wear on the rim of the disk member are considerably reduced. The ring member has a relatively small wall thickness at its foremost end, which means that only this end, if any part at all, must be provided with a billet slipping surface. The outer surface of the ring element is adapted to engage the inner surface of the container of the extrusion press in a sliding relationship (direct extrusion) or in fixed relationship (indirect extrusion). By providing a separate ring element outside the disk member, the ring element can be treated particularly for the engagement with the container, whereas the disk member can be treated particularly for engagement with the rear end of the billet.

In order to ensure correct outwards deformation of the ring element, it is preferably provided with an annular portion with a narrowed wall thickness adapted to allow the ring member to be elastically deformed.

Preferably the ring member is provided with an inner annular recess at a foremost end, with which the outer rim of the disk member engages.

Provision of such an annular recess ensures the mutual relationship between the rim of the disk member and the foremost end of the ring member. Thereby it is possible in a simple manner to ensure that the disk member is always positioned in front of the ring member, thereby ensuring that the disk member engages the rear end of the billet during the extrusion stroke.

In one embodiment the dummy block comprises a main body with a forwardly arranged through-going opening, while the disk member is provided with a rearwardly protruding stem that is axially displaceably arranged in the through-going opening of the main body. Thereby the disk member is safely centred in the dummy block ensuring identical radial extension in all directions of the elastically deformed disk member.

The rearwardly protruding stem of the disk member is preferably provided with an annular recess provided with a retaining ring that abuts on a rear wall of the main body, when the disk member is in its non-deformed state.

This fixes the disk member in relation to the main body such that the disk member and the main body appear as a single unit.

In a second embodiment of the invention the disk member is provided as a simple convex disk having a generally uniform thickness, while the ring member is provided with an annular inwardly protruding lip that extends over the outer rim of the disk member. Thereby is achieved that the disk member need not have rearwards protruding stem, and the manufacturing costs of the disk member are reduced considerably.

In this embodiment, the ring member is preferably connected to the main body by means of a bayonet fitting. Thereby, the disk member can be exchanged easily by simply removing the ring member.

The dummy block is applicable in an extrusion press provided with a container and the ring member is adapted to be deformed to an outer diameter that corresponds to an inner diameter of a cylindrical container or a liner provided therein of an extrusion press. The extrusion press may be for direct or indirect extrusion.

Brief description of the drawings In the following an embodiment of the invention will be described, by way of example only, with reference to the appended drawings, in which: Fig. 1 shows an extrusion press for indirect extrusion according to the prior art ; Fig. 2 shows in an enlarged view a portion of the extrusion press shown in Fig. 1; Fig. 3 shows a disassembled dummy block according to a first embodiment of the invention with a portion of the disk member cut away for better illustration; Fig. 4 shows an assembled dummy block according to a first embodiment of the invention with a portion cut away for better illustration ; Fig. 5 shows a cross-section of a dummy block according to a first embodiment of the invention as used in an extrusion press; Fig. 6 shows in an enlarged view a portion of Fig. 5; and Fig. 7 shows a cross-section of a dummy block according to a second embodiment of the invention.

Description of preferred embodiments of the invention Before preferred embodiments of a dummy block according to the invention are described in detail, reference is made to Figs 1 and 2 showing an

extrusion press for indirect extrusion according to the prior art as disclosed in WO 01/68283.

The extrusion press comprises a die stem-carrying portion 20 onto which is attached a hollow stem 21 carrying a die 22 mounted onto the stem 21 by means of a die holder 23. Furthermore, the extrusion press comprises a container 24 with a through-going opening, the container 24 being axially displaceable on guiding means (not shown), and a dummy block 25 attached on a press ram 26 driven by a press 27 mounted on a press-carrying portion 28. The stem-carrying portion 20. and the press-carrying portion 28 are interconnected by longitudinal posts 29 (one of which is shown cut in Fig. 1 for illustrative purposes).

As shown in Figs 1 and 2 a billet 30 is arranged in the container 24 and an extruded profile 31 is situated inside the hollow stem. In Fig. 2 it can further be seen that the die holder 23 comprises a resistance means 32 arranged in front of the die, and that the container 24 is provided with a liner 33 defining the wall of the through-going opening in the container. The dummy block 25 comprises a front surface 34 adapted to abut on a rear end 35 of the billet 30 when extruding the billet 30 through the die 22.

The extruding process takes place by first displacing the container 24 with the billet towards the press ram 26 until the front surface 34 of the dummy block 25 abuts on the rear end 35 of the billet 30. Then the container 24 and the press ram 26 are advanced together towards the die stem-carrying portion 20 until the resistance means 32 abuts on the front end 36 of the billet 30 within container 24. Thereafter a much greater force is applied to the press ram 26 forcing the billet 30 via the dummy block 25 towards the die 22, the container 24 at the same time being advanced at the same speed as the press ram 26. By this action the billet 30 is gradually extruded through an opening in the die 22, thereby forming an extruded product 31.

As it appears from Fig. 2 the dummy block 25 comprises a convex disk member 37 provided with the front surface 34. The configuration of the disk member 37 allows the disk member 37 to deform elastically to a substantially planar shape during the extrusion process when it is forced towards the rear end 35 of the billet 30. The object of this deformation is to provide a billet- engaging front surface 34 to build up elastic deformation energy in the disk member 37 during the extrusion stroke and thereby establish a slip-function between the rear end 35 of the billet 30 and the disk member 37 when the pressing force is reduced to zero. Such a slip-function allows the dummy block 25 to disengage from the billet 30 leaving a clean residual, which can then be re-used in a subsequent extrusion stroke.

In its relaxed state the disk member 37 has an external diameter slightly smaller than the inner diameter of the billet charging bore of the container 24. In its deformed state the external diameter of the disk member 37 corresponds substantially to the inner diameter of the billet charging bore of the container 24.

Reference is now made to Figs 3 and 4 that show a dummy block 1 according to a first embodiment of the invention in a disassembled state (Fig. 3) and an assembled state (Fig. 4). The dummy block 1 comprises a main body comprising a generally cylindrical main body cylinder 2 provided with mounting means for mounting the dummy block 1 on a press ram, and a main body disk 3 adapted to be fixed to the cylindrical main body 2 by not shown bolts to form the main body.

The dummy block 1 further comprises a disk member 4 (with a part cut away for better illustration) and a ring member 5 that are adapted for mutual engagement between the outer rim 6 of the disk member 4 and an annular recess 7 provided at the foremost end (8) of the ring member 5.

The disk member 4 is provided with a generally convex front surface 9 and a rearwardly protruding centrally located stem 10 adapted to engage a central through-going opening 11 provided in the main body disk 3 of the main body.

In the assembled state a retaining ring 12 is arranged in an annular recess 13 provided at the stem 9.

The assembled state of the dummy block 1 according to the first embodiment of the invention is better seen in Fig. 5 that shows a cross-section of the dummy block 1 positioned adjacent a metal billet 30 within a liner 33 in a container as known from the prior art shown in Figs 1 and 2. In Fig. 5 it is clearly shown how the disk member 4 is provided with a generally convex front surface 9 and how it is secured in the main body comprising the main body cylinder 2 and the main body disk 3 by means of the rearwards protruding stem 10 that extends through the through-going opening 11 and is secured by a retaining ring 12 that abuts on the rear wall of the main body disk 3. Fig. 5 also clearly shows how the outer rim 6 of the disk member 4 is in engagement with the recess 7 provided at the foremost end 8 of the ring member 5.

As explained above the extrusion process starts by applying a force to the press ram that is connected to the dummy block 1. Thereby the convex front surface of the disk member 4 is caused to abut on the rear end 35 of the billet 30, and the continued application of force to the press ram causes the disk member 4 to deform elastically until it achieves a generally planar front surface 9 as illustrated with dotted lines in Fig. 6. Fig. 6 also shows how the main body disk 3 supports the disk member 4 over the entire area which allows the disk member 4 to be relatively thin.

The deformation of the disk member 4 from having a convex front surface 9 to a generally planar front surface 9 causes the outer rim 6 of the disk member 4 to expand radially outwards. Since the outer rim 6 is engaged with the annular recess 7 provided at the foremost end 8 of the ring member 5,

the foremost end 8 of the ring member 5 is forced to flex outwards as well.

An annular portion 14 of the ring member 5 is provided with a narrowed wall thickness which allows the ring member 5 to be elastically deformed at this annular portion 14 as illustrated with dotted lines in Fig. 6.

After the extrusion stroke the force on the press ram is reduced and the inherent elasticity of the disk member 4 and the ring member 5 causes these elements to return to their original shapes. To further enhance the slip- function, and to avoid the use of additional slip means, the front surface 9 of the disk member 4 may be surface treated with e. g. PVD or plasma CVD, by ion-implanting, plasma-nitration or any other suitable tripological treatment.

As shown in Fig. 6 the outer rim 6 of the disk member 4 does not engage the inner surface of the container or a liner 33 provided therein during the extrusion stroke. This means that the outer rim 6 of the disk member 4 is much less exposed to wear and the requirements to surface treatment of the outer rim 6 are much lower than it is the case with the outer rim of the dummy block according to the prior art.

The inner surface of the container or of a liner 33 provided therein is engaged by the ring member 5 during the extrusion stroke. The ring member 5 is a construction element that can be manufactured at must lower costs than the disk member 4 and it is much preferred to be able to exchange this element only when it has been worn after a plurality of extrusion strokes.

Furthermore, it is possible to configure the ring member 5 in such manner that it engages the container or the liner 33 therein over a must larger area than it is possible with the outer rim 6 of the disk member 4. Thereby wear and tear on the ring member 5 can be reduced.

If the dummy block 1 is used in a direct extrusion process, the risk of material entering the space behind the disk member 4 during retraction

thereof within the container has been eliminated since only the ring member 5 engages the container and it safely covers this space.

Fig. 7 shows a cross-section of a dummy block 101 according to a second embodiment of the invention. In this embodiment the dummy block 101 comprises three elements only : a main body 102, a disk member 104 and a ring member 105. The main difference between this dummy block 101 and the dummy block 1 according to the first embodiment of the invention is that the disk member 104 is not provided with a rearwardly protruding stem that connects the disk member with the main body.

In this second embodiment the disk member 104 is provided as a simple convex disk having a generally uniform thickness. The disk member 104 is retained by means of the ring member 105 that is, at its foremost end 108, provided with an annular, inwardly protruding lip 107 that extends over the outer rim 106 of the disk member 104. In the shown embodiment the ring member 105 is connected to the main body 102 by a bayonet fitting comprising pins 111 or otherwise shaped extensions provided on the outer surface of the main body 102 and notches 112 provided internally in the ring member 105. The connection between the ring member 105 and the main body 102 may be provided by other means such as a threaded connection comprising external threads provided on the main body 102 and internal threads provided on the ring member 105. An advantage by using a bayonet fitting is, however, that an annular portion 114 of the ring member 105 has a narrowed wall thickness which prompts the ring member 105 to elastically deform at this annular portion 114 when it is forced radially outwards by deformation of the ring member 104 during the extrusion stroke.

In the extrusion process the front surface 109 of the disk member 104 is brought into contact with the rear end of a billet as described above. When a force is applied to the dummy block 101 the disk member 104 deforms elastically from the shown convex shape to a generally planar shape (not

shown). This deformation causes the outer rim 106 of the disk member 104 to expand radially outwards which in turn forces the ring member 105 to deform elastically outwards in the same manner as described above with reference to the first embodiment of the dummy block 1 according to the invention.

The inner diameter of the ring member 105 corresponds essentially to the outer diameter of the convex disk member 104 which ensures that the disk member 104 is always centred in relation to the ring member 105 and thereby also in relation to the main body 102. Due to the annular inwardly protruding lip 107 that extends over the outer rim 106 of the disk member 104, the disk member 104 is safely retained at the foremost end of the dummy block 101.

The invention has been described with reference to two preferred embodiments thereof. However, many modifications may be made to the configuration of the dummy block without departing from the general idea of the invention. For example, the disk member and the ring member may be configured to any suitable shape, and the mounting means for mounting the dummy block on a press ram may be configured to suit any press ram.

The dummy block according to the invention is primarily intended for extruding aluminium billets into any desired extruded profile. Nothing prevents, however, the dummy block from being configured to be used in the extrusion of other materials as well.