This application represents the U.S. national stage entry of International Application No. PCT/EP2017/082544 filed Dec. 13, 2017, which claims priority to German Patent Application No. 10 2016 125 406.1 filed Dec. 22, 2016, the disclosure of which is incorporated herein by reference in its entirety and for all purposes.

The invention relates to a die for a press, particularly for a powder press for manufacturing green compacts. In particular, the press is used to manufacture sinterable green compacts—i.e., green compacts that can be sintered after the pressing process. In particular, metallic and/or ceramic powders can be pressed into green compacts in the die.

Known dies of this type include a so-called shrink ring, it being possible for a core (particularly made of hard metal) to be arranged within the shrink ring which then forms the inner peripheral surface of the die. On the one hand, the inner peripheral surface of the die forms the receptacle for the powder or the green compact to be produced. In particular, at least one upper punch of the press can travel into the die along an axial direction via an upwardly open first end face of the die. The at least one upper punch slides along the inner peripheral surface of the die and increasingly compresses the powder. In particular, at least one lower punch can be additionally provided which moves into the die via a downwardly open second end face of the die along the axial direction, or moves in the die between an upper position and a lower position. The powder is thus pressed into a green compact between the at least one upper punch and the at least one lower punch, the inner peripheral surface of the die particularly defining a lateral contour of the green compact.

In particular, the die has a collar on an outer peripheral surface via which the die can be received and clamped in the press. The collar extends in a radial direction over the outer peripheral surface, so that the die can be placed onto and/or supported on a support of the press. Moreover, such dies are substantially cylindrical, the cylindrically shaped outer peripheral surface usually being received via a radial clearance in the press, thus enabling a centering of punch(es) and die relative to one another—i.e., a coaxial arrangement of punch(es) and die.

A die can have punch guide zones on its respective end faces, in which case a pressing zone is present at a distance from the end faces and adjacent to the punch guide zones. The pressing zone is the area in which the powder is compressed with the greatest pressing force. The pressing zone is clearly defined in the die and bounded along the axial direction. Furthermore, a demolding zone—i.e., a zone of the die through which the green compact is pushed out of the die (demolded) and provided for removal from the press—can be provided at least on one end face. During the pressing of the powder, an equally strong bonding pressure is applied to the inner peripheral surface of the die. In the process, the inner peripheral surface of the die is elastically expanded in the radial direction (or in the direction of a normal vector that is respectively present on the inner peripheral surface, which is thus arranged so as to be perpendicular to the respective surface of the inner peripheral surface). This expansion in the pressing zone now results in strong frictional forces during demolding. These frictional forces can extend into the demolding zone, because the die is usually cylindrical and therefore has a substantially constant stiffness (i.e., a substantially constant resistance to an elastic expansion in the radial direction or in the direction of a normal vector that is respectively present on the inner peripheral surface and is thus arranged so as to be perpendicular to the respective surface of the inner peripheral surface) along the axial direction.

This expansion only in the pressing zone of the die also has the effect that the green compact cannot be produced with dimensional accuracy. In particular, a conicity of the green compact can occur during the demolding of the green compact. In that case, the die rebounds in the pressing zone as demolding progresses, so that the green compact is increasingly constricted at its lower end and consequently takes on an overall conical shape.

In order to reduce these frictional forces, it has been proposed to provide drafts on the inner peripheral surface of the die, so that, as the green compact moves along the axial direction from the pressing zone and through the demolding zone to the end face, a relaxation of the green compact occurs.

Known designs of such dies are associated with high costs, for example due to the material used for the die and/or also for auxiliary devices that are required for handling the die or for assembly and disassembly in the press.

Proceeding from this background, it is an object of the present invention to at least partially solve the problems described with reference to the prior art. In particular, unwanted but previously process-related conicities on the green compact are to be avoided. Non-rotationally symmetrical components are also to be manufacturable with high accuracy. In particular, a die for a press is to be provided which has a lower weight than conventional dies without impairing the dimensional accuracy of the green compacts to be produced. Furthermore, it should be preferably possible to reduce the frictional forces that occur during demolding of the green compact without the need for drafts.

A die according to the features of claim1is proposed in order to achieve this object. Advantageous developments are the subject of the dependent claims. The features listed individually in the claims can be combined in a technologically meaningful manner and supplemented by explanatory facts from the description and details of the figures, with additional design variants of the invention being indicated.

For this purpose, a die contributes to the arrangement in a press, the die extending along an axial direction between a first end face and an (opposite) second end face and forming an inner peripheral surface between the end faces. The die extends from the inner peripheral surface along a radial direction toward an outer peripheral surface and toward at least one centering surface that is disposed in the radial direction on a first diameter. The die has a pressing zone that is spaced apart from the end faces. In the vicinity of the pressing zone, the die has a greater maximum first stiffness (i.e., the greatest first stiffness present there), at least relative to zones of the die that are arranged on the end faces, compared to a pressing force acting on the inner peripheral surface (or the bonding pressure acting there) in a direction of a normal vector (i.e., of a normal vector that is present on the respective portion of the inner peripheral surface, which normal vector is thus arranged so as to be perpendicular to the respective surface of the portion of the inner peripheral surface). The maximum first stiffness is at least 10%, particularly at least 15%, preferably at least 20%, especially preferably at least 40% greater than a minimum second stiffness that is present in at least one of the zones arranged on one of the end faces (i.e., the least second stiffness present there). Very especially preferably, this applies to both zones that are arranged on the end faces.

The maximum first stiffness is preferably greater, particularly by at least 10%, preferably by at least 15%, especially preferably by at least 20%, or even by at least 40%, than a maximum second stiffness in at least one zone that is arranged on one of the end faces (i.e., the greatest second stiffness present there). Very especially preferably, this applies to both zones that are arranged on the end faces.

The stiffnesses denote particularly the resistance of the inner peripheral surface to a deformation in the radial direction (or in the direction of a normal vector that is respectively present on the inner peripheral surface, which is thus arranged so as to be perpendicular to the respective surface of the inner peripheral surface). The unit of stiffness is: N/m [Newton/meter].

As an example, the stiffness can be determined as follows: By means of an FEM analysis, in which the deformation, particularly the elastic deformation, of the die at a certain pressing force [N] acting particularly perpendicularly on the inner peripheral surface of the die is determined (i.e., the displacement of the material of the die in the direction of the normal vector of the inner peripheral surface of the die, which can be specified in [m]). The ratio of these quantities (pressing force [N]/material displacement [m]) represents the stiffness of the die.

The lower the stiffness of the die, the greater the elastic deformation of the die. The die should therefore be as stiff as possible in the pressing zone in order to ensure the dimensional stability of the green compact. In particular, the die should have the least stiffness in the vicinity of the lower and/or upper end face in order to have greater elasticity particularly in the demolding zone, so that the frictional forces in this zone are minimized and, if necessary, the surface of the green compact is not impaired or only to a slight extent.

The die is intended particularly for a powder press for manufacturing green compacts. In particular, the press is used to manufacture sinterable green compacts—i.e., green compacts that can be sintered after the pressing process. In particular, metallic or also ceramic powders can be pressed into green compacts in the die.

In particular, the die comprises a so-called shrink ring, it being possible for a core (particularly made of hard metal) which then forms the inner peripheral surface of the die. On the one hand, the inner peripheral surface of the die forms the receptacle for the powder or the green compact to be produced. In particular, at least one upper punch of the press can travel into the die along an axial direction via an upwardly open first end face of the die. The at least one upper punch slides along the inner peripheral surface of the die and increasingly compresses the powder. In particular, at least one lower punch can be additionally provided which moves into the die via a downwardly open second end face of the die along the axial direction. The powder is thus pressed into a green compact between the at least one upper punch and the at least one lower punch, the inner peripheral surface of the die particularly defining a lateral contour of the green compact. The pressing force is introduced into the powder by means of the punches. The pressing force is maintained over the punches and the die. At the same time, the pressing force acts on the die in the direction of the normal vector.

In particular, the die has a respective punch guide zone on the end faces (optionally immediately adjacent thereto) as zones, with a pressing zone being present at a distance from the end faces and (optionally immediately) adjacent to the punch guide zone. It is in the pressing zone that the powder is compressed with the greatest pressing force. In particular, the pressing zone is defined by the region along the axial direction in which the powder is arranged during the application of the greatest pressing force.

Furthermore, a demolding zone is present at least on one end face—i.e., an area of the die through which the green compact is pushed out of the die (demolded) and provided for removal from the press.

In particular, the die is aligned with the punches in the press by means of the at least one (outer) centering surface. However, it is also possible in particular for the die to be centered relative to the punches by means of other surfaces, e.g., parts of the outer peripheral surface. In particular, the smallest radial clearance between die and press in the radial direction (with the exception of connections for cooling lines, etc.) lies between the centering surface (or the respective surface used for centering) and the press. In particular, the at least one centering surface lies on the largest first diameter of the die—that is, the die extends only within the first diameter.

In particular, the centering surfaces, or the top and bottom side of the die, in the immediate vicinity of the centering surfaces are used as collars for clamping the die in a receptacle (an adapter) of the press. However, other surfaces are also suitable here for use as a collar for clamping the die by receiving the presses.

The die that is proposed herein is designed in particular such that a maximum or the greatest possible first stiffness is present (only) in the vicinity of the pressing zone. By virtue of this high first stiffness, dimensionally accurate production of the green compact by the press and the pressing process can be ensured. On the other hand, a second stiffness in the vicinity of the end faces of the die is designed to be substantially smaller, because a substantially lesser load acts on the die in these regions (which are bounded in the axial direction) as a result of the pressing force component acting in the direction of the normal vector.

In particular, a majority of the material that is usually present in cylindrically shaped dies can be saved due to the lesser second stiffness. Weight savings of at least 25%, preferably of at least 50%, and especially preferably of at least 75% can thus be achieved compared to cylindrically designed dies.

In particular, the die has integrated cooling lines and/or heating lines that are required for temperature-controlling the die during the pressing operations.

The design and layout of the die is created particularly through calculation and simulation of the loads and deformations of the die that occur (e.g., by means of FEM calculations: finite element method). Programs for topology optimization can also be used here.

The lesser second stiffness particularly has the effect that the frictional forces can be reduced during demolding of the green compact from the die. In particular, drafts on the green compact and/or on the inner peripheral surface of the die are no longer absolutely necessary, so that very dimensionally stable and cylindrical outer peripheral surfaces of the green compact can be produced. What is more, the stress on the die due to the friction during demolding is reduced, so that the wearing of the die can be reduced. In addition, the restorative forces of the die during the demolding of the green compact via one end face are reduced, so that the green compact is less constricted and therefore has a very little or even no (unwanted) conicity.

The savings of the material of the die now results particularly in substantial weight savings. However, the handling of the die, particularly during assembly in the press or disassembly from the press, can be facilitated in this way. Exclusively manual handling of the die, meaning movement of the die without mechanical aids (e.g., crane, hoist, or the like), is now even possible in some circumstances. However, the aids used can be designed for less weight in any case, meaning that a substantial cost reduction can be achieved in this regard as well. Currently, only presses with a pressing force of up to 1500 kN [kiloNewton] are able to be set up manually. As a result of the weight reduction proposed herein, presses with a pressing force of up to 4000 kN can be set up manually in the future. In particular, it is also not necessary to change adapters for the die in the case of manual setup. This eliminates the need for a second adapter and an adapter station. The risk of damaging the die or the punch as a result of the contact of the die with the sharp-edged punches of the press can also be reduced.

The first stiffness along a peripheral direction of the die can be different or vary in the peripheral direction. In particular, the die is therefore not designed to be rotationally symmetrical about an axis parallel to the axial direction (or, in particular, only in the case of a rotation of 180 degrees along the peripheral direction). Such a design of the die is advantageous, for example, when non-rotationally symmetrical, e.g., cuboid, green compacts are produced.

In particular, the at least one centering surface is arranged (at least partially, or exclusively) in the pressing zone along the axial direction. According to another embodiment, the at least one centering surface can also be arranged at least partially in one of the zones that adjoin the end faces and, in particular, completely outside of the pressing zone. In this case, however, it is crucial that the first stiffness in the pressing zone be elevated in comparison to the second stiffnesses in the other zones.

In particular, the at least one centering surface has a first height along the axial direction, the first height corresponding to no more than 80% of a shortest distance between the end faces. The shortest distance is preferably determined in the region of the transition from the end faces to the inner peripheral surface.

According to a preferred embodiment, the die, along the radial direction between the inner peripheral surface and the first diameter, has at least

one cross section that is reduced at least in the axial direction or

connecting regions that are arranged at a distance from one another in the peripheral direction.

The cross section that is reduced in the axial direction describes the shape of the die at the end faces in the region between the inner peripheral surface and the first diameter. A sort of constriction of the shape of the die can be provided here, meaning that the die has a shorter distance between the end faces in this region than in the vicinity of the inner peripheral surface.

The connecting regions describe the shape of the die along the peripheral direction. Free spaces (i.e., spaces without material of the die) can be present here between the inner peripheral surface and the first diameter. In that case, spokes can be formed by the connecting regions that connect the inner peripheral surface to a centering surface that is arranged on the first diameter.

In particular, the connecting regions are arranged so as to be additionally spaced apart in the axial direction. In particular, spokes can thus be formed that are arranged in at least partially identical positions in the peripheral direction but in different positions in the axial direction.

In particular, a second diameter is arranged between the inner peripheral surface and the first diameter, with a cross-sectional area of the die present on a second diameter corresponding to no more than 80%, particularly no more than 60%, preferably no more than 40% of the inner peripheral surface. As explained above, areas without material—i.e., free spaces—are thus provided on this second diameter. In particular, an additional cross-sectional area is provided between the second diameter and the first diameter that is greater than the cross-sectional area present on the second diameter.

In particular, a plurality of centering surfaces are arranged on the first diameter, with the centering surfaces being arranged so as to be spaced apart from one another along the peripheral direction. In particular, at least three centering surfaces are provided which are arranged so as to be spaced apart from one another along the peripheral direction.

The at least one centering surface can be embodied so as to extend circumferentially in the peripheral direction. This means, for example, that this centering surface is continuous over the periphery.

The die can have at least one retaining portion that is arranged at a distance from the at least one centering surface in the axial direction. The retaining portion is provided particularly for the purpose of facilitating the handling of the die. In particular, the retaining portion serves as a handle for manual handling of the die. Preferably, the retaining portion is embodied as a single piece with—i.e., is integrally connected to—the die. Alternatively, the retaining portion can also be secured to the die by means of screws, for example.

In particular, the retaining portion is arranged in the radial direction between the inner peripheral surface and the first diameter.

The retaining portion preferably extends in the manner of a ring.

As will readily be understood, the special shape of the dies that is proposed here can be produced using the known manufacturing methods such as turning, milling, sawing, drilling and grinding, wire cutting, die sinking, and hard milling, etc. It is especially advantageous, however, to manufacture the die or at least the shrink rings by means of so-called additive methods, e.g., laser sintering (3D printing process for producing spatial structures from powdery starting material through sintering, with the workpiece being produced in layers). This enables a truly free design of the die to be achieved in which the weight of the die can be reduced to the greatest possible extent.

A method for manufacturing at least one green compact with a press is also proposed in which the press has at least one die as described above and at least one punch that can travel along the axial direction via an end face of the die into a receptacle for the green compact that is formed by the inner peripheral surface, the method comprising at least the following steps:

placing a powder in the receptacle;

moving the at least one punch in the die along the axial direction and compressing the powder into a green compact in the pressing zone;

demolding the green compact from the die via an end face of the die;

wherein, at a distance from the end faces, the die has the pressing zone and, in the vicinity of the pressing zone, a greater maximum first stiffness, at least relative to zones that are arranged on the end faces, compared to a pressing force acting on the inner peripheral surface in a direction of a normal vector at least in step b), the maximum first stiffness being at least 10% greater than a minimum second stiffness that is present in at least one zone that is arranged on one of the end faces.

In particular, it is proposed that the green compact be removed from the die in step c) via a first zone that is arranged on the first end face, the maximum first stiffness being at least 10% greater than at least the minimum second stiffness that is present in the first zone.

The remarks regarding the die apply equally to the method, and vice versa.

By way of precaution, it should be noted that the number words used here (“first,” “second,” . . . ) serve primarily (only) to distinguish a plurality of similar objects or quantities; that is, they do not prescribe any dependency and/or order of these objects or quantities relative to one another. Should a dependency and/or order be required, this is explicitly stated herein or it obviously follows for a person skilled in the art when studying the embodiment specifically described.

