GB2067944A

GB2067944A - Extrusion process

Info

Publication number: GB2067944A
Application number: GB8002686A
Authority: GB
Original assignee: CENTRY ALUMINIUM CO Ltd
Current assignee: CENTRY ALUMINIUM CO Ltd
Priority date: 1980-01-26
Filing date: 1980-01-26
Publication date: 1981-08-05
Also published as: GB2067944B

Abstract

A first billet 7 is extended through a die 6 with a ram 2 which has at its free end 3 a dummy block 4 abutting the terminal end 9 of the billet 7. Extrusion is discontinued and the ram 2 withdrawn with said dummy block 4. A second billet 10 is introduced behind the terminal end 9 of the first billet 7 and extrusion is resumed. The dummy block 4 comprises two parts, viz. a central plunger 12 and a resiliently deformable outer ring 13 which has a peripheral sliding contact surface 14 with an outside diameter slightly less than the inside diameter of the billet container 6. The ring and plunger members 13, 12 have co- operating frusto-conical engagement surfaces 16 so that as pressure is applied to the leading end face 11 of the dummy block the plunger 12 is displaced relative to the ring 13 and the ring 13 is resiliently expanded. <IMAGE>

Description

SPECIFICATION Extrusion method and apparatus for use therein The present invention relates to a method of extrusion wherein successive billets of material are extruded through a die and to apparatus suitable for use in the method.

In known methods of extrusion of aluminium billets the flow of metal from the billet through the die is such that discontinuities such as blisters are formed as air is trapped in the extruded metal as the terminal portion of the billet is extruded. This terminal portion can constitute as much as 10% of the billet length which has to scrapped since extruded metal formed with such discontinuities is not normally acceptable. Naturally this results in significant additional production costs which are becoming ever greater with the continuing rise in aluminium prices.

In practice it is necessary to withdraw the die with the terminal billet portion which then has to be sheared away from the die. The die then has to be re-mounted and a new billet introduced into the extrusion machine. This rather cumbersome procedure results in significant interruptions and machine down-time which in turn results in still further increased operating costs.

It is an object of the present invention to minimise one or more of the above disadvantages.

The present invention provides a method of extruding billets of extrudable material comprising the steps of extruding a first billet by forcing a leading portion of said billet through a die with a ram which has at its free end a dummy block, as defined hereinbelow, abutting the terminal end of the billet, said leading portion having a length such that the extrudate formed therefrom is substantially free of discontinuities; disontinuing extrusion and withdrawing the ram with said dummy block; introducing a second billet behind the terminal end of the first billet; bringing the ram and said dummy block into engagemerit with the terminal end of said second billet; and resuming extrusion.

As used herein the expression dummy block is used to refer to the piston-like member disposed at the end of the ram and which slidably passes along the billet container upstream of the die to force the billet through the die.

Desirably the leading end face of the dummy block is substantially flat or concave so as to avoid the terminal end face of having any concavity formed therein which could result in air becoming trapped between the adjacent end faces of the first and second billets. Most desirably the leading end face of the dummy block is made concave so that the terminal end face of the billet becomes convex in the course of extrusion.

Preferably the dummy block is in two parts, comprising a central plunger member and a resiliently deformable outer ring member (which may or may not be circular depending on the cross-sectional shape of the billet container), said ring member having a peripheral sliding contact surface with a predetermined outside diameter slightly less than the inside diameter of the billet container, said ring and plunger members being provided with co-operating frusto-conical engagement surfaces diverging in the direction of the leading end of the dummy block so that, as pressure is applied to the leading end face of the dummy block in use thereof, the plunger member is displaced relative to the ring member and the ring member resiliently expanded whereby the outside diameter of the peripheral sliding contact surface is increased towards the inside diameter of the billet container so as to substantially correspond thereto.

By means of the present invention the formation of discontinuities in the extrudate during extrusion of the terminal portion of a billet is substantially avoided. An additional advantage is that the invention allows the production of continuous extrudate without any discontinuities arising at the ends of successive billets. This enables more efficient utilisation of extrudate since the latter can always be cut to the desired length and no waste end-prices need arise after cutting.

Still further advantages are the reduction in machine down-time, elimination of die disassembly, shearing operations and production of scrap billet. Yet another possible advantage is the scope for using flat face dies in place of the normally concave-faced ones.

Further preferred features and advantages of the invention will appear from the following description given by way of example of some preferred embodiments of the invention illustrated with reference to the accompanying drawings in which: Fig. 1 is a generally schematic partially sectioned side elevation showing the principal parts of an extrusion apparatus being operated in accordance with the invention; and Fig. 2 is a longitudinal section of a preferred form of dummy block.

Fig. 1 shows an extrusion apparatus 1 comprising a ram 2 having at its free end 3 a dummy block 4; a billet container 5; and a die 6. In the billet container 5 is shown the terminal portion of a first billet 7 which is being extruded into an extrudate 8. Immediately behind the terminal end face 9 of the first billet is a newly inserted second billet 10.

In use of the apparatus a leading end portion comprising approximately two thirds of the billet length of the first billet 7 is extruded through the die 6. Extrusion is then discontinued and the ram 2 and dummy block 4 withdrawn. The second billet 10 is then introduced behind the first billet 7 and the ram and dummy block brought back into engagement with the terminal end of the second billet (this stage is illustrated in Fig. 1) and extrusion then resumed. As the second billet 10 is forced against the first 7, any air between the adjacent ends is forced out, as shown by arrows X, escaping around the sides of the second billet 10 which initially has a diameter slightly smaller than the internal diameter of the billet container 5. As extrusion progresses the billet is deformed to fill the full internal diameter of the container 5 as shown in the terminal portion of the first billet.

Fig. 2 shows a preferred form of dummy block 11 particularly suited for use in the method of the invention and comprising a central plunger member 12 and a resiliently deformable outer ring member 13. The ring member 13 has a peripheral sliding contact surface 14 which is machined to a high tolerance and has an outside diameter slightly smaller than the inside diameter of the billet container 5.

The ring and plunger members 13, 12 are provided with complementary co-operating frusto-.

conical engagement surfaces 1 5 which diverge in the direction of the leading end 16 of the dummy block 4. When pressure is applied to the leading end 16 of the dummy block the plunger member 12 is forced back relative to the ring member 13 and the ring member 13 resiliently expanded as its frusto-conical surface 1 6 rides up over that of the plunger 12. The ring member 13 expands radially until it closely fits the interior of the billet container 5. This enables it to force the billet 7 along the container 5 to maximum effect. When however pressure is no longer applied through the ram 2, the ring member 1 3 contracts back to its original diameter and can then be easily withdrawn from the container 5.

As may be seen in Figure 2 the leading end face of the dummy block 11 is slightly concave having a shallow generally conical depression therein.

When this is forced against the terminal end of the face of a billet it results in the deformation thereof into a correspondingly convex shape as shown in greatly exaggerated form in Fig. 1. This helps to ensure that the possibility of air becoming trapped between the adjacent ends of the first and second billets 7, 10 as they are compressed together. In addition the members 12, 13 are shaped and dimensioned so that in the neutral condition of the block 11, the leading end of the plunger member 12 stands slightly proud of the ring member whereby displacement of the plunger member 12 is facilitated.

The dummy block 11 may be provided with any conventional means for use in the mounting thereof on the ram 2. Conveniently it is provided with a threaded socket 1 7. To facilitate screwing of the dummy block 11 into position the ring and plunger members 13, 12 are provided with radially extending bore and socket means 18, 1 7 respectively disposed in alignment with each other (in the neutral or upstressed condition of the block 11) for receiving a tommy bar or the like.

It will of course be appreciated that the maximum length of the leading portion in any given case will depend on a number of factors such as ratio of billet diameter to length and ratio of billet diameter to extrudate diameter. The optimum length may however be readily determined by simple trial and error.

Claims

1. A method of extruding billets of extrudable material comprising the steps of extruding a first billet by forcing a leading portion of said billet through a die with a ram which has at its free end a dummy block, as defined hereinbefore, abutting the terminal end of the billet, said leading portion having a length such that the extrudate formed therefrom is substantially free of discontinuities; discontinuing extrusion and withdrawing the ram with said dummy block; introducing a second billet behind the terminal end of the first billet; bringing the ram and said dummy block into engagement with the terminal end of said second billet; and resuming extrusion.

2. A method according to claim 1 wherein the leading portion has a length corresponding to approximately two thirds of the billet length.

3. A method according to claim 1 or claim 2 wherein is used a dummy block having a concave leading end face whereby the terminal end face of each billet is formed into a convex shape.

4. A method according to any one of claims 1 to 3 wherein is used a dummy block having a resiliently radially expandable outer-ring member having an outer diameter less than the internal diameter of the billet container and arranged so that during extrusion, the ring member expands radially to fit ciosely to the interior of the billet container for forcing the billet(s) through the container and when extrusion is discontinued contracts again whereby it can be readily withdrawn from the billet container.

5. A dummy block for use in a method according to any one of claims 1 to 4 wherein the billet container has a pre-determined internal diameter, which block comprises a central plunger member and a resiliently deformable outer ring member, said ring member having a peripheral sliding contact surface with a predetermined outside diameter slightly less than said predetermined inside diameter of the billet container, said ring and plunger members being provided with co-operating frusto-conical engagement surfaces diverging in the direction of the leading end of the dummy block so that, as pressure is applied to the leading end face of the dummy block in use thereof, the plunger member is displaced relative to the ring member and the ring member resiliently expanded whereby the outside diameter of the peripheral sliding contact surface is increased towards the inside diameter of the billet container so as to substantially correspond thereto.

6. A dummy block according to claim 5 wherein the leading end face of the dummy block is concave.

7. A dummy block according to claim 6 wherein the plunger member has a concave leading end face in the form of a substantially conical depression.

8. An extrusion apparatus for use in the extrusion of billets of extrudable material, said apparatus having a dummy block according to any one of claims 5-7 mounted on the free end of the ram.

9. A method of extruding billets of extrudable material according to claim 1 substantially as described hereinbefore with particular reference to Fig. 1 or Figs. 1 and 2 of the accompanying drawings.

10. A dummy block for use in a method according to claim 1 substantially as desribed hereinbefore with particular reference to Fig. 1 or Figs. 1 and 2 of the accompanying drawings.