USE OF TENSILE STRESS FOR DEFORMING A METAL OBJECT IN THE FORM OF A CIRCUMFERENTIAL SURFACE

Abstract: Process for deforming a metal object which comprises a side wall in the form of substantially a continuous circumferential surface, the object being moved, in a relative movement in the longitudinal direction of the circumferential surface, past a forming tool, in such a manner that the forming tool acts on the side wall and, in the process, deforms the side wall, the side wall coming into contact with a forming tool only on the inside or the outside, characterized in that the object is at least pulled past the forming tool. Preferably, a combination of pulling and pushing the object past the forming tool is used. In the immediate vicinity of where the side wall comes into contact with the forming tool, it is preferable, in addition to the pressure which is exerted by the forming tool, for an extra pressure to be exerted on the side wall, at right angles to the side wall and directed from the side wall towards the forming tool. The invention also provides a number of devices which are suitable for carrying out the process.

Description: USE OF TENSILE STRESS FOR DEFORMING A METAL OBJECT IN THE FORM OF A CIRCUMFERENTIAL SURFACE The invention relates firstly to a process for deforming a metal object which comprises a side wall in the form of substantially a continuous circumferential surface, the object being moved, in a relative movement in the longitudinal direction of the circumferential surface, past a forming tool, in such a manner that the forming tool acts on the side wall and, in the process, deforms the side wall, the side wall coming into contact with a forming tool only on the inside or the outside. For the purpose of the present application, a contact zone where the side wall and the forming tool come into contact with one another is defined on the side wall. That part of the side wall which, as seen in the direction of movement of the body, is situated beyond the contact zone is referred to as "the processed side", and that part of the side wall which is situated before the contact zone is referred to as "the unprocessed side". A process as outlined in the preamble is described in EP 0,864,385. In the known process, a metal side wall is moved onto and into a profiled die as a result of being pushed in the processing direction. A profiled die is to be used as the forming tool. In the known method, on the unprocessed side the side wall is subjected to an axial compressive load which may lead to instability of the side wall and increases the risk of undesirable deformation, and even collapsing, of the side wall on the unprocessed side. The process referred to relates to a "body-necking" operation. Body-necking is understood to mean the reduction of the circumference of a part of an object which is in the form of a circumferential surface over at least part of the height of the said part. The body-necking operation referred to takes place without the use of an internal support body during the body-necking, such as a mandrel or ram. In the known process, this results in poor efficiency in the transfer of the shape of the profiled forming tool to the side wall. This is a consequence of the elasticity of the side wall itself: in the event of a single-sided axial load and in the absence of a support body, the side wall is pushed away from the forming tool on the processed side. The deformation of the side wall then differs from the maximum achievable deformation for a given tool profile. The use of a support body as a second forming tool to promote the shape transfer from a first forming tool is preferably avoided, since using a support body increases the risk of damage to the inner or outer surface of the side wall. In the known process, use is also made of excess internal pressure, the body functioning as part of a pressure vessel. Internal excess pressure offers the side wall support against excessive axial loads but does not present a solution to the drawback referred to above. The above drawbacks are eliminated or reduced considerably by the fact that, according to the invention, the object is at least pulled past the forming tool. This ensures that, in a section of the side wall which, as seen in the direction of movement of the object, is located beyond a zone of contact with the forming tool, a tensile stress is generated during the deforming, with the result that the efficiency of the transfer of the tool profile to the side wall is increased in that section of the side wall which is situated beyond the contact zone. Also, there is a greater degree of freedom in the selection of, inter alia, the material from which the side wall is made, the extent to which the circumference changes, and the shape of the contour of the forming tool. Preferably, the process according to the invention is combined with the known process. If the object is both pushed and pulled past the forming tool, a more efficient transfer of shape from the tool to the side wall both in front of and beyond the contact zone is achieved. When the process according to the invention is used, at least the same stress level is reached in the side wall in the vicinity of the contact zone as in the known process, at a lower axial load. Exerting a transverse pressure, at right angles to the circumferential surface and directed from the side wall towards the forming tool, to add to the pressure which is exerted on the side wall by the forming tool, makes a further contribution to controlling the forces which are active in the deforming operation. To exert this extra pressure, in practice use is often made of the side wall itself as part of a pressure vessel, but the process according to the invention is not limited to this variant. It should be noted that the process according to the invention differs from the process which, within the specialist metal-processing world, is known by the name of "deep-drawing", in that a deep-drawing operation uses both an internal and an external forming tool. Furthermore, for deep-drawing relatively soft material with a low yield stress, for example lower than 300 MPa, and an associated high elongation at break, is selected. Moreover, during a deep-drawing operation, it is impossible, or at least very uncommon, to combine the generation of a tensile stress in the side wall with the generation of axial compressive stress, i.e. for an object which is to be deep-drawn to be both pushed and pulled past the forming tool, as is the case in a preferred embodiment of the invention. By contrast, in an embodiment of the invention the process is preferably applied to a side wall consisting of hard material with a high yield stress of > 300 MPa, more preferably > 700 MPa. The yield stress is a threshold stress above which plastic deformation occurs. Preferably, the invention is applied in an embodiment wherein the object has a wall thickness of less than 0J4 mm. and more preferably less than OJO mm. The invention can be applied particularly after a wall ironing operation, whereas deep-drawing or re-drawing operations are commonly applied prior to wall ironing. Wall-ironed steel is a fully hard material with a yield stress of > 700 MPa, as is known, for example, from EP 0,733,415. In addition, an ironed wall is very thin (thinner than 0J4mm in case of aluminium and steel, or even thinner than 0J0 mm in case of steel), while the tangential loadability reduces with a third power of the wall thickness. Consequently, wall-ironed material, in particular steel is difficult to deform plastically. To enable sufficient stress to be generated in this material, it is important that the side wall should follow the shape of the die as efficiently as possible. This is accomplished by proceeding according to the process of the invention. Wall-ironed steel is widely used as material for packaging. The process according to the invention is suitable in particular for reducing the circumference of a side wall over at least part of the height during the deformation, the process known as "body-necking". During body-necking, there is a high risk of undesirable wrinkling. Undesirable wrinkling is caused by compressive stress in the tangential direction (in the transverse direction tangentially to the side wall). The tangential stress must therefore be kept as low as possible. In practice, this is achieved, for example, by limiting the reduction in circumference to low levels. According to the Von Mises criterion, the stress in, for example, the axial direction has to be increased in order to move back above the yield stress when the tangential stress is reduced. By using the process according to the invention, it is possible to increase the stress in the axial direction, since, according to this process, the material is forced to follow the contour of the forming tool. In this way, at least the Non Mises criterion is satisfied in the zone of contact without the axial load on the unprocessed side being increased. A relatively great reduction in circumference is achieved by a sequential succession of steps of relatively small reductions in circumference. The invention also provides a number of devices which are suitable for carrying out the process. A device for deforming a metal object which comprises a side wall in the form of substantially a continuous circumferential surface, comprising a forming tool, which interacts with the side wall, for deforming at least part of the side wall during relative movement of the object past the forming tool, in such a manner that the side wall comes into contact with a forming tool only on the inside or the outside, on which tool an entry side and an exit side are defined, and a pusher element which can move with respect to the forming tool, in the direction from the entry side towards the exit side, to bring about the relative movement of the object past the forming tool, is characterized, according to the invention, in that the pusher element, on the exit side of the forming tool, can be coupled to the object and extends to at least the entry side of the forming tool. This has the advantageous result that very simple means, for example a push-rod, can be used to pull the object past the forming tool, generating a tensile stress in the side wall beyond the contact zone. In a preferred embodiment, the forming tool comprises, on the entry side, a substantially closed housing which, during deforming, interacts with the object to form a pressure vessel, with the object of bringing about the relative movement of the object past the forming tool. This ensures that a pushing force is exerted on the object on the unprocessed side, while a pulling force is exerted on the process side. One advantage of this device is that an axial compressive stress can be generated in a body with any desired neck shape (in cross section) and neck profile (in longitudinal section) without technical adjustments. The device described above has the considerable drawback that it is difficult or impossible to fit into an existing body-necking device which is described, for example, in EP 0,864,385. A device which is easy to fit into the known device, comprising a forming tool for deforming a metal object which comprises a side wall in the form of substantially a continuous circumferential surface, comprising a forming tool, which interacts with the side wall, for deforming at least part of the side wall during relative movement of the object past the forming tool, in such a manner that the object comes into contact with a forming tool only on the inside or the outside, on which tool an entry side and an exit side are defined, and a centring means situated on the exit side of the forming tool for the purpose of monitoring the position of the body with respect to the forming tool during deforming, is characterized in that there is an enclosing means, which interacts with the centring means and to which the object can be coupled in or close to an open end on the exit side of the forming tool interface in a clamping manner. This ensures that a pulling force can be exerted in the axial direction at the open end. Another significant advantage of this device is that it is also easy to use in operations in which the circumference of the side wall is at least increased over at least part of the height. The invention will now be explained in more detail with reference to drawings, in which: Fig. 1 diagrammatically depicts forces and moments of force for (A) a known process and (B,C) embodiments of the process according to the invention; Fig. 2 diagrammatically depicts an embodiment of the process according to the invention and a device for carrying out the process; and Fig. 3 (parts A,B,C) diagrammatically depicts another embodiment of the process according to the invention and a device for carrying out the process. Figure 1 diagrammatically depicts a side view of forces (indicated by means of straight arrows) and moments of force (indicated by means of curved arrows) which occur during the deformation of a side wall of an object. Cross sections through a forming tool (1) and cross sections through part of a rim-necked side wall (2) in the vicinity of an open end are shown. Part A of Fig. 1 relates to the known process. In the known process, compressive force d is exerted on the side wall (2) of the object in the direction of movement of the object with respect to the forming tool (1). This compressive force d must at least cancel out all the oppositely directed forces (comprising the processing force and the frictional force), so that the side wall (2) of the object moves past the forming tool (1). The force d also causes a couple (K j ), which contributes to the side wall (2) following the contour of the forming tool on the entry side. The magnitude of K, increases as the axial load used rises. However, elasticity of the body itself results in a couple (K 2 ) which is directed away from the forming tool being generated on the exit side, so that the side wall does not successfully follow the contour on the exit side of the forming tool. The principal ideal of the process on which the invention is based is that the direction of K 2 can be reversed if a tensile stress is generated on that section of the side wall which, as seen in the direction of movement of the object, is situated beyond the contact zone, as a result of the object being pulled past the forming tool by force t. This is indicated in part B of Fig. 1. If only a pulling force is exerted, couple K, will be directed away from the forming tool on the entry side, owing to the elasticity of the side wall itself, leading to a reduction in the transfer of shape at the entry side. Tests have shown that the transfer of shape is most efficient if a combination of tensile force t and compressive force d is used, as illustrated in Fig. 1, part C. The two couples K, and K 2 can then be directed in such a manner that the side wall is forced to follow the contour of the forming tool. The magnitudes of the couples can be adjusted on the basis of the values of the external forces t and d, and the application of a transverse pressure directed from the side wall towards the die. In practice, for the latter pressure it is usual to employ an excess pressure inside the object. According to the invention, the value of t can be selected to be greater than, equal to or less than the value of d. The values which are set will depend, inter alia, on the shape of the object, its materials properties and the desired change in shape. Figures 2 and 3 relate to embodiments of the invention in which the diameter of a substantially cylindrical, continuous side wall provided with a base is reduced. This operation is referred to by the term "body-necking". This body-necking operation is preceded by a rim-necking operation, in which the open end of the side wall, on the side remote from the base side, is narrowed as described, for example, in EP 0,750,953. Prior to the rim-necking, a can is obtained, for example, by deep-drawing followed by wall-ironing, known by the acronym DWI. Necked cans obtained in this way are used, for example, as a packaging can for foodstuffs or personal care products. In Fig. 2, the forming tool (1), which in this figure is shown as an annular, profiled die, forms part of a housing (11) which is substantially closed on the entry side and which will be referred to in the context of the present application by the term "necking housing". A moveable pusher element (14), in this case a push-rod, is introduced on the unprocessed side through the open neck of the body. As a result of interaction between the pusher element (14) and the base of the object, tensile force t is exerted on that section of the side wall which, as seen in the direction of movement of the object, is situated beyond the contact zone. In practice, it is sometimes desirable for a base support (12) to be used in order to support the base, since in deep-drawn and wall-ironed cans the base material has a relatively low yield stress compared to the (wall-ironed) side wall. It should be noted that the push-rod shown does not come into contact with a section of the side wall which is situated in the contact zone, in contrast to a device which is known, for example from EP 0,851,974. Therefore, there is only contact between the side wall and a forming tool on the outside of the side wall. A major advantage of the current device above the known device is that the object is easy to remove from the pusher element after it has moved completely past the forming tool. The removal operation is facilitated further by the fact that the object, after a complete body-necking operation during which the entire object is moved through the die, is situated entirely on the exit side with respect to the die, with an open neck directed towards the die. Furthermore, the necking housing (11) in the device shown in Fig. 2 has an opening (9) which is suitable for connection to a pressure source. The necking housing (11) interacts with the necked rim of the side wall to create a pressure vessel. In this way, excess pressure (compared to the pressure outside the object beyond the contact zone) exerts an axial compressive force d on that section of the side wall which, as seen in the direction of movement of the object, is situated in front of the zone of contact with the die. Preferably, the circumference of the necking housing (11), measured in cross section on the inside of the necking housing, is the same as the external circumference of the object (2) which is to be deformed. This ensures that the pressure is transferred to the object only in the axial direction. The excess pressure is vented through an opening (10) in the immediate vicinity of the zone of contact between the forming tool (1) and the side wall (2). This makes it possible to maintain in the object an internal excess pressure, which is independent of the level of the pressure at opening (9), with respect to the external ambient pressure in the immediate vicinity of the contact zone, with the aid of a passage (6) in the push-rod. It should be noted that pressure differences can also be used to generate tensile stress in the unprocessed side of the side wall. In this context, in one embodiment pusher element (14) would not have to act directly on a base of the object. A vent opening similar to (10) would then have to be arranged on the exit side of the forming tool. Preferably, a circumference seen in cross section through the pusher element is complementary to the circumference of the neck shape, so that the pusher element and the object fit together reasonably tightly, for example in an almost gastight way. Figure 3 shows various views of cross sections through an alternative device which is suitable for carrying out the inventive process. Part A shows a can which has previously been rim-necked and is provided with a base and side wall (2). The rim neck is in contact with a centring means (4). The external diameter of the centring means at the location where it is in contact with the object corresponds to the internal diameter of the neck. The centring means is provided with a gas passage (6). A closure sleeve (7), which can be displaced parallel to the longitudinal axis, is located concentrically around the centring means. Between the closure sleeve (7) and the centring means (4) there are segmented enclosing means (3) which enclose the neck of the body (2) through relative axial movement of the closure sleeve (7) around the centring means. The enclosing means are released with the aid of springs (8) when the closure sleeve (7) is retracted. In the securely clamped state, the centring means is suitable for transmitting force t to the side wall. Part B of Fig. 3 shows the rim neck of the body (2) in the state in which it is clamped securely between the centring means (4) and an enclosing-means segment (3). A cross section through the release springs (8) is shown in part C of Fig. 3. In this embodiment, four release springs are located in recesses in the centring means (4), and the enclosing means (3) are divided into four segments. A base support (5) is used to generate axial compressive load on the side wall on the unprocessed side. The drawings shown do not constitute any limitation. The embodiments may be adapted to a desired situation. In some cases, it is also possible to carry out deforming operations in accordance with the process in which the circumference of the side wall increases. The devices shown can be used in this case too if the essential components are replaced by complementary equivalents. For example, in Fig. 3 the passage (6) would be used not as a passage for a pressurized medium, but rather as a passage for an internal forming tool (not shown). The diameter of the forming tool, where it is suitable for contact with the side wall, would be substantially equal to the diameter of the centring means 4. In the context of the present application, deforming a side wall is understood to mean both congruent and non-congruent deformation.

Claims: CLAIMS 1. Process for deforming a metal object which comprises a side wall in the form of substantially a continuous circumferential surface, the object being moved, in a relative movement in the longitudinal direction of the circumferential surface, past a forming tool, in such a manner that the forming tool acts on the side wall and, in the process, deforms the side wall, the side wall coming into contact with a forming tool only on the inside or the outside, characterized in that the object is at least pulled past the forming tool. 2. Process according to Claim 1, characterized in that the object is pushed past the forming tool. 3. Process according to Claim 1 or 2, in which a contact zone where the side wall comes into contact with the forming tool is defined on the object, which forming tool exerts a pressure on the side wall of the object, characterized in that, at least in the immediate vicinity of the contact zone, an additional pressure is exerted on the side wall at right angles to the circumferential surface and directed from the side wall towards the forming tool. 4. Process according to one of the preceding claims, characterized in that material with a yield stress of 300 MPa or higher is selected for the material from which the side wall is made. 5. Process according to one of the preceding claims, characterized in that material with a yield stress of 700 MPa or higher is selected for the material from which the side wall is made. 6. Process according to one of the preceding claims, characterized in that an object with a wall thickness of less than 0J 4 mm is selected. 7. Process according to one of the preceding claims, characterized in that an object with a wall thickness of less than 0.10 mm is selected. 8. Process according to one of the preceding claims, characterized in that a circumference of the side wall becomes smaller during the deforming. 9. Device for deforming a metal object which comprises a side wall in the form of substantially a continuous circumferential surface, comprising a forming tool, which interacts with the side wall, for deforming at least part of the side wall during relative movement of the object past the forming tool, in such a manner that the side wall comes into contact with a forming tool only on the inside or the outside, on which tool an entry side and an exit side are defined, and a pusher element which can move with respect to the forming tool, in the direction from the entry side towards the exit side, to bring about the relative movement of the object past the forming tool, characterized in that the pusher element (14), on the exit side of the forming tool, can be coupled to the object (2) and extends to at least the entry side of the forming tool. 10. Device according to Claim 9, characterized in that the forming tool comprises, on the entry side, a substantially closed housing which, during deforming, interacts with the object to form a pressure vessel, with the object of bringing about the relative movement of the object past the forming tool. 11. Device for deforming a metal object which comprises a side wall in the form of substantially a continuous circumferential surface, comprising a forming tool, which interacts with the side wall, for deforming at least part of the side wall during relative movement of the object past the forming tool, in such a manner that the object comes into contact with a forming tool only on the inside or the outside, on which tool an entry side and an exit side are defined, and a centring means situated on the exit side of the forming tool for the purpose of monitoring the position of the body with respect to the forming tool during deforming, characterized in that there is an enclosing means (3), which interacts with the centring means (4) and to which the object can be coupled in or close to an open end on the exit side of the forming tool interface in a clamping manner.

Inventors: SCHAAPER HANS NICOLAAS, VERMEIJ JOHANNES, SCHAAPER, HANS, NICOLAAS, VERMEIJ, JOHANNES
Applicants: CORUS STAAL BV, SCHAAPER HANS NICOLAAS, VERMEIJ JOHANNES, SCHAAPER, HANS, NICOLAAS, VERMEIJ, JOHANNES
Application year: 2000
Main classification: B21D51
IPC codes: B21D22, B21D51