Method of extrusion, and extrusion press

Abstract: In an extrusion press (1) the volume between a pressure pad (4), a billet container (5), an extrusion die (6) and the billet (12) to be extruded is evacuated by suction through the pressure pad and a hollow extrusion stem (3) before the commencement of extrusion. Preferably the extrusion ram (2) is hollow and contains a vacuum reservoir (7) connected to this volume by a vacuum line (8) and valve (9). The vacuum line (8) may have a compressed air line (10) and valve (11) connected into it.

Description: This invention relates to a method of extrusion, and to an improved extrusion press, to make the extrusion process more efficient and less wasteful of the material being extruded. Although the present invention will be particularly described with reference to the extrusion process as applied to aluminium alloys, the invention can be applied to other materials, which are pressure-extrudable. An extrusion press is a machine that is designed to apply a known and controlled directional force, of known magnitude, against a known orifice via a quantity of aluminium alloy physically contained. The aluminium alloy is contained during the application of the applied force by a component forming part of the machine and known as the container. The purpose of the container is to allow the force applied to the aluminium alloy contained within the internal dimensions or bore of the container to be converted into linear movement in the direction of the applied force through a component containing one or more precision orifices or dies constituting a die plate. During the time that the aluminium alloy is subjected to linear movement and pressure the container is physically clamped to the die plate. An extrusion die plate is a means of providing physical resistance to the linear movement of the contained aluminium alloy in order to allow the aluminium alloy to be shaped by the detailed geometry of the orifice. The extrusion die plate is manufactured with one or more precision orifices designed to allow the aluminium alloy that passes through the orifice or orifices to assume the required product shape. The temperature of the aluminium alloy is raised to a known value prior to the application of the applied force. The action of applying sufficient force to an aluminium alloy under these conditions results in the aluminium alloy being physically forced through the orifice, to produce a product of cross-section conforming to the shape of the orifice. The method of generating the directional force, known as the extrusion pressure, is in principle that of a hydraulic ram and cylinder. The extrusion pressure generated by the principal or main ram of the extrusion press machine is transmitted to the aluminium alloy contained in the container in a linear manner by means of a steel rod attached to the main ram via the ram crosshead and known as the extrusion stem. A pressure pad is employed between the extrusion stem and the contained aluminium alloy. The pressure pad is designed to transmit the extrusion pressure from the extrusion stem to the contained aluminium alloy while permitting an aluminium alloy film of approximately 0.4mm thick to be left in the container bore after the pressure pad has passed through the container under extrusion pressure. The consolidated aluminium remaining in the container after extrusion is known as the "discard". The process of preparing an extrusion press to extrude a quantity of aluminium alloy often results in entrapping unwanted air in the cavities within the container prior to the aluminium being subjected to the extrusion pressures. If the entrapped air is pressurised by the extrusion process the results can be seen as severe damage to the finish of the extruded product, in the form of blistering, often accompanied by audible sound of the gases upon release to the atmosphere after passing through the die orifice. The sequence of events relating to the physical movements of an extrusion press to perform the function of extruding hot aluminium alloy through an extrusion die can be broken down into specific stages known as press cycles. A standard press cycle for the extrusion process in order to eliminate the problem of entrapped air is a cycle called the de-gas or "burp" cycle. The de-gas cycle requires the aluminium alloy to be consolidated in the container at approximately half of the required extrusion pressure. The de-gas cycle then decompresses the components that are normally under extrusion or clamping pressure as in the case of the main ram, container and die plate. The components are physically moved to break the seal around the area of the extrusion die to permit pressurised gases to escape. The de-gas cycle is then completed by re-initiating the container "close and seal" cycle as well as returning the main ram to its previous extrusion condition. It has already been proposed, in UK patent specification no. GB-A-1,462,163, to remove air from the void volume within a billet container by applying a source of vacuum to an opening in the sidewall of the container. However, this method of evacuation will tend to draw air past the ram and will not work in evacuating the void space until the ram has consolidated the billet, at which point the opening will have been closed off by the ram. Attention is also drawn to the disclosure of US-A-4117703, in particular the embodiment described with reference to figure 7 thereof. The present invention accordingly aims to overcome the problems described above and to provide an improved and more efficient extrusion press and extrusion process. In one aspect the present invention provides a method of extrusion comprising evacuating the void volume between a pressure pad, a billet to be extruded, and the extrusion face of a die, within the billet container, by suction through the pressure pad and a hollow extrusion stem; characterized in that the pressure pad is of variable cross-section, and the cross-section of said pressure pad is expanded to near fill the cross-section of the said container; and extrusion of a billet in said container is subsequently commenced in a hermetically-sealed pressure vessel formed by said pressure pad, said container bore and said sealed extrusion die. Preferably this volume is substantially evacuated by connection to a source of vacuum via a vacuum reservoir capable of being evacuated to the minimum operational pressure within the time of a press-cycle; which reservoir is preferably contained within the main ram of the extrusion press and communicates via a hollow extrusion stem and the pressure pad with the said void volume. The reservoir may be suitably connected to said volume via an on/off vacuum valve and a vacuum line. Preferably an air pressure line and valve is connected into said vacuum line in order to purge the said volume with compressed air during or after extrusion. The hollow vacuum reservoir within the extrusion ram may be suitably directly connected to a vacuum pump, as a source of vacuum. In a second aspect the invention provides an extrusion press having a ram, a hollow extrusion stem, a pressure pad, a billet container having a bore, and an extrusion die sealable to said container, and a vacuum reservoir connected via a first vacuum line to a vacuum pump and via a second vacuum line and the hollow stem and the pressure pad to a volume defined between the pressure pad, the container, a billet being extruded and the extrusion face of the die, characterized in that said void volume may be evacuated by suction through said pressure pad and said hollow extrusion stem before extrusion is commenced; in that said pressure pad is of variable cross-section and is expandable to near fill the cross-section of said container before extrusion is commenced; and in that said pressure pad, said container bore and said sealed extrusion die form a hermetically-sealed pressure vessel, which is subject to evacuation before extrusion is commenced. The reservoir is preferably contained within the hollow ram. Preferably, in this aspect, the vacuum line is connected, via an on/off valve, to a source of compressed air which may be used to purge the void volume defined above. The pressure pad is of variable cross-section and expands to near fill the cross-section of the container. The pressure pad as a whole is usually generally bell-shaped. The advantages of extrusion press operation according to this invention can be summarized as follows: 1. The gases can be partially removed prior to the aluminium being consolidated in the container without interrupting the normal loading extrusion cycle and so reduce the amount of time between production cycles of the extrusion press. 2. The action of removing the gases is specifically directed at the volume between the pressure pad and the extrusion die face, within the confines of the container bore (that volume not occupied by the mass of the aluminium alloy to be extruded) and known as the normal atmospheric air gap. Due to the fact that the pressure pad as well as container bore and sealed extrusion die form a hermetically sealed pressure vessel, the action of removing the gases is possible due to the fact that the aluminium alloy has not been consolidated and the unoccupied volume around the aluminium alloy billet will be influenced by a change in the pressure as determined by the vacuum system. The conventional gas removal cycle provides a means of decompressing the gases, but only at the extrusion die face as the aluminium alloy has been consolidated within the container bore, thus effectively sealing the gases trapped in the container between the consolidated aluminium alloy and the pressure pad. The conventional gas removal cycle may leave gases under pressure in the area of the pressure pad with the possibility of damage to the pressure pad outside diameter forming the seal with the container as the pressurised gases are forced between the outside diameter of the pressure pad and the container bore under extrusion pressure. 3. A fixed pressure pad is designed to expand to a given diameter forming a seal with defined clearance in relation to the container wall or bore during the period it is subjected to extrusion pressure. It thus follows that during the conventional de-gas cycle the pressure pad will expand twice, once when consolidating the aluminium alloy and then once again after retracting to expel the pressurised gases. The vacuum process reduces the number of times the pressure pad expands by 50% thus reducing the possibility of mechanical failure of the pressure pad through fatigue. 4. The elimination of the conventional de-gas cycle also reduces the number of operations involving de-compression and reversal of the motions of the main ram and container components as well as the number of operations of the control gear during the extrusion cycle with subsequent reduction in component fatigue and wear. A summary of the principle of operation, according to the invention, is described as follows: The press cycle is standard for a conventional fixed pad and is initiated in automatic mode. The container closes and seals against the die face. The hot aluminium alloy billet to be extruded is brought up to the press centre line by the press loaders and is pushed forward into the container by raised pressure points on the pressure pad, allowing the pressure pad bell to remain open. The pressure pad and billet advance until the main ram position device signals that the pad main outside diameter is within the bore of the extrusion container. At this point the billet has displaced the majority of the air at atmospheric pressure and the vacuum valve is initiated to equalise the pressure difference between that in the remaining container cavity around the billet and the vacuum stored in the reservoir contained in the extrusion press ram. It should be noted that the vacuum can draw air from the front as well as the back of the billet during its vacuum cycle prior to billet compression. The vacuum cycle cannot be used on the first billet of every die as air will be drawn through the die. This has the effect of unnecessarily lowering the vacuum potential as the die cavities are clear of aluminium and permit free air flow. The pad continues to advance with full vacuum applied until the resistance of the billet against the die face closes the pad bell and the press indicates that the billet has consolidated. At this point the vacuum is disconnected from the pad by the operation of the appropriate valve. Normal extrusion takes place until a second main ram position device signals that the pressure pad is within a short distance of its final extrusion position determined by the commissioning engineer. At this point the compressed air valve is initiated to pressurise the interior of the pad and place an opening pressure on the pad bell. The press continues to extrude until the limit of extrusion position is reached. During the conventional "strip or return" part of the cycle the main ram decompresses and starts to retract the pad away from the compressed discard aluminium on the die face. The air pressure, applying force to the bell, holds the bell against the discard face as the pad retracts and so opens the bell positively and in a controlled manner. The application of air pressure to the pad interior prior to the pad being withdrawn from the discard results in a positive force being applied to the pressure bell, such as to initiate the separation of the bell from the body of the pad. The pad continues to retract with a positive airflow venting through the gap between bell and pad body via the pad drillings, acting as a purge, until it reaches its fully back position. The invention will further be described, by way of example only, with reference to the accompanying drawings, which represent a sequence of operation according to the invention: Figure 1 shows an extrusion press with a container and die disconnected; Figure 2 shows the press with the container sealed to the die; Figure 3 shows a billet inserted between a pressure pad and container; Figure 4 shows the billet inserted within the container and vacuum applied to the volume between billet, pressure pad, die face and container; Figure 5 shows the vacuum disconnected and a ram advancing to extrude the billet; Figure 6 shows the situation at the end of the extrusion phase, with compressed air connected to the pressure pad; Figure 7 shows the ram retracting, the extrusion die disconnected from the container and compressed air purging the container and pressure pad; Figure 8 shows the ram retracted with compressed air still purging the pressure pad; and Figure 9 shows a situation similar to that of figure 1 with the ram fully retracted and the vacuum and compressed air sources isolated from the pressure pad. In the drawings, a dotted area represents those parts where a vacuum exists and a solid area indicates those parts where compressed air is applied. The drawings show an extrusion press 1 having a ram 2, a hollow extrusion stem 3, a generally bell-shaped pressure pad 4, a billet container 5 and an extrusion die 6 sealable to the container 5. A vacuum reservoir 7 is contained within the hollow ram 2, and the reservoir 7 is connected via a first vacuum line (not shown) to a vacuum pump (also not shown) and via a second vacuum line 8 and the hollow stem 3 and the pressure pad 4 to a volume defined between the pressure pad, the container 5, a billet being extruded and the extrusion face of the die 6. The vacuum line 8 incorporates an on/off valve 9. The vacuum line 8 is further connected to a compressed air line 10 via an on/off valve 11, which source of compressed air may be used to purge the void volume thus defined. The pressure pad 4 may be of variable cross-section and expands to near fill the cross-section of the container. As will be apparent from the drawings, figure 1 shows the state of the extrusion press wherein the ram 2 is fully retracted and wherein the container 5 and the die 6 are disconnected, i.e. unsealed. In figure 2 the container is sealed to the die. Figure 3 shows a billet 12 inserted between the pressure pad 4 and the container 5, with the ram 2 to be advanced forward to a position to load the billet into the container. The billet is brought up to the press centre line by press loaders (not shown). As shown in each of figures 1, 2 and 3, the valves 9 and 11 remain closed during these operations. Figure 4 shows the billet 12 inserted within the container 5 and vacuum applied to the volume between the billet, the pressure pad 4, the face of the die 6 and the container 5, by opening the valve 9 and allowing the vacuum to be established through the line 8, whereby the vacuum reservoir 7 is effectively connected to the container 5. The ram 2 has advanced further forward to the position shown at which the vacuum is applied. Figure 5 shows the state where the vacuum reservoir has been disconnected by closing the valve 9 and wherein the ram 2 is advancing to extrude the billet, the ram being forward to a position to achieve normal extrusion. Figure 6 shows the situation at the end of the extrusion phase, wherein the ram has advanced further forward and is approaching the limit of extrusion, and wherein the valve 11 has been opened to connect compressed air to the pressure pad 4. Figure 7 shows the ram 2 retracting, the extrusion die 6 disconnected from the container 5, and compressed air purging the container 5 and pressure pad 4. Figure 8 shows the ram returned almost to its fully retracted position, with compressed air still purging the pressure pad 4. Figure 9 shows a situation similar to figure 1, with the ram 2 fully retracted, and the valve 11 closed so that both the vacuum source and the compressed air source are isolated from the pressure pad 4.

Claims: A method of extrusion comprising evacuating the void volume between a pressure pad (4), a billet (12) to be extruded, and the extrusion face of a die (6), within the billet container (5), by suction through the pressure pad and a hollow extrusion stem (3); characterized in that the pressure pad (4) is of variable cross-section, and the cross-section of said pressure pad is expanded to near fill the cross-section of the said container; and extrusion of a billet in said container is subsequently commenced in a hermetically-sealed pressure vessel formed by said pressure pad, said container bore and said sealed extrusion die. A method as claimed in claim 1, characterized in that the said void volume is connected to a source of vacuum via a vacuum reservoir (7). A method as claimed in claim 2, characterized in that the vacuum reservoir is contained within the hollow extrusion ram (2) of the extrusion press (1). A method as claimed in claim 2 or 3, characterized in that the vacuum reservoir is connected to the said void volume via a vacuum line (8) which incorporates an on/off valve (9). A method as claimed in claim 4, characterized in that a compressed air line (10) and valve (11) is connected into the vacuum line. An extrusion press (1) having a ram (2), a hollow extrusion stem (3), a pressure pad (4), a billet container (5) having a bore, and an extrusion die (6) sealable to said container, and a vacuum reservoir (7) connected via a first vacuum line to a vacuum pump and via a second vacuum line (8) and the hollow stem (3) and the pressure pad (4) to a volume defined between the pressure pad, the container (5), a billet (12) being extruded and the extrusion face of the die (6), characterized in that said void volume may be evacuated by suction through said pressure pad and said hollow extrusion stem before extrusion is commenced; in that said pressure pad is of variable cross-section and is expandable to near fill the cross-section of said container before extrusion is commenced; and in that said pressure pad, said container bore and said sealed extrusion die form a hermetically-sealed pressure vessel, which is subject to evacuation before extrusion is commenced. A press as claimed in claim 6, characterized in that the vacuum reservoir (7) is within the ram (2). A press as claimed in claim 6 or 7, characterized in that the second vacuum line (8) has a source of compressed air (10) connected into it.

Inventors: NEWMAN MICHAEL JAMES, NEWMAN, MICHAEL JAMES
Applicants: INDALEX LTD, INDALEX LIMITED
Application year: 1990
Main classification: B21C27
IPC codes: B21C27, B21C23