WO2016020234A1

WO2016020234A1 - Method for producing a stamping blank for coins and medals, stamping blank, and use of the stamping blank

Info

Publication number: WO2016020234A1
Application number: PCT/EP2015/067356
Authority: WO
Inventors: Gerd Wagner
Original assignee: Reischauer Gmbh
Priority date: 2014-08-07
Filing date: 2015-07-29
Publication date: 2016-02-11
Also published as: DE102014111245A1

Abstract

The invention relates to a method for producing a stamping blank for coins and medals, wherein a metal powder is provided, and the metal powder is pressed into a plate-shaped or disk-shaped green compact having two end faces located opposite each other, and wherein the green compact is sintered using a mold corresponding to the green compact, thus forming the stamping blank. At least one of the end faces of the coin blank is provided with a profile, at least sectionally.

Description

Method for producing a coin blank for coins and medals, blank blank and use of the blank blank

The invention relates to a method for producing a coin embossing coin and medals and the blank obtainable by the method and a use of the embossing blank.

For the production of coins and medals, which may consist of base metals or also of precious metals, first flat metal disks are punched out with preferably circular or regular polygonal cross-section of rolled sheets. Prior to further processing into coins, these blanks can be polished and optionally electroplated. The coins or medals are stamped in an embossing tool engraved with a profile for the coin relief. During the embossing process, the material of the blank flows to the formation of the relief and possibly also to the formation of an annular edge bar. DE 199 59 500 A1 discloses a method for producing coins and medals made of metal, in which coin blanks are provided by cold forming with a raised core zone and a raised edge zone on at least one side of the blank. However, this method is feasible only on a laboratory scale and not suitable for the mass production of coin blanks, since for the cold forming of the blank by extrusion to high pressures over a long period of time must be expended.

A method for the production of metallic coins using powder metallurgy produced coin blanks is known from DE 26 33 323. According to this method, the powder metallurgical process steps are carried out with the aim to achieve a special material identification of the coin for the purpose of identification against counterfeiting. For this purpose, among other things, it is proposed to produce an inhomogeneous distribution of the materials within the coin blank or to add a non-alloyable foreign substance. Powder metallurgical process for the production of Coins and medals, however, have so far remained insignificant in the industry.

There continues to be a need for a method of making medals or coins, particularly precious metals, that can be inexpensively performed with less material and lower energy costs.

This object is achieved by a method according to claim 1.

Advantageous embodiments of the invention are specified in the subclaims, which can optionally also be combined with one another.

According to the invention, a method is proposed for producing a coin and medallion embossing blank, which comprises the following steps:

Providing a metal powder;

Pressing the metal powder into a plate-shaped or disk-shaped green body with two opposite end faces; and

Sintering of the green compact to form the stamping blank with a shape corresponding to the green compact, wherein at least one of the end faces of the stamping blank is at least partially provided with a profile.

With the method according to the invention blanking blanks are obtained which have a compared to conventionally produced blanks significantly improved ductility and can therefore be minted with much less time and energy to coins and medals of any kind. In addition, valuable material can be saved for the production of the embossing blanks, since no punching waste accumulates and the powder metallurgical production proposed according to the invention can take place with exactly the metal weight required for the embossing blank.

Surprisingly, not only the profiling of the embossing blank leads to improved flow behavior during the embossing process. Rather, the powder metallurgy production of the blank contributes to a significant higher ductility, which is particularly evident in a comparison of flow curves in the stress-strain diagram. Without intending to be bound by theory, it is believed that dislocations and dislocations in the grain structure of the embossing blank produced by powder metallurgy processes can be achieved with significantly lower forces than in the embossing blank conventionally produced by stamping from semi-finished products.

In addition, the inventive method allows a high freedom of choice with respect to the introduced during the formation of the green compact in the blanking profile. Thus, the embossing blank obtained after sintering of the green compact can be advantageously used for embossing coins and medals with a high relief which either can not be produced with satisfactory quality in the conventional way or requires repeated embossing operations at high pressures. All of the metals and metal alloys commonly used for circulation coins, collector coins, bullion coins or medals, and also in the form of a powder mixture, can be used for producing the coin and medal coin blanks according to the invention. Preferably, the metal powder is a powder of a noble metal, copper, nickel, aluminum or iron and mixtures and alloys thereof. Particularly preferred are powders of steel, copper, gold, silver, platinum, palladium, rhodium, iridium, ruthenium and / or osmium, and mixtures and alloys thereof.

Examples of such metals and metal alloys are Au999, Au917, Au900, Ag999, Ag925Cu, Ag900Cu, Ag625Cu, Ag333Cu, Cu99, Cu75Ni25. The slab-shaped or disk-shaped green compact and thus also the slug blank obtained after sintering of the green compact can, as seen in plan view on one of the end faces, have any arbitrary, even irregular base surface. Preference is given to a round, in particular elliptical or circular, base area or a base area in the form of a regular polygon. Furthermore, with the method according to the invention also green compacts and embossing blanks in bar form with a substantially rectangular base area can be displayed, as they are also used for investment products. The oppositely disposed end faces of the embossing blank are bounded by a peripheral circumferential surface with, taking into account manufacturing tolerances, substantially constant height. The lateral surface may be substantially perpendicular to the plane formed by the end faces, but also be rounded. The edges formed at the meeting of the end faces and the lateral surface can be sharp, round or chamfered.

The dimensions of the stamping blank obtained after sintering from the green compact lie in the range of conventional coins and medals. Preferably, the width of the end faces of the embossing blank is from 10 to 100 mm. The height of the end faces bordering peripherally bounding lateral surface is preferably in the range of 0.2 to 5 mm, more preferably in the range of 1 to 3 mm. The dimensions of the green body are up to about 20% greater than those of the blanking blank due to the volume shrinkage occurring during sintering. The volume shrinkage during sintering is calculated in relation to the dimensions of the green compact. Remaining, slight deviations of the embossing blank from the nominal dimension of the coin or medal can still be compensated during the embossing process because of the good ductility of the embossing blank. According to a preferred embodiment, the at least sectionally profiled end face of the green body and of the embossing blank obtained therefrom by sintering have a profile with at least one bulge in the axial direction. The axial direction is the direction perpendicular to the plane formed by the end faces, which also points in the direction of the height of the lateral surface. Due to the bulge in the axial direction additional material for the expression of the coin or medal is provided on the front side, which flows due to the better ductility of the invention produced milled blanks even at lower embossing pressures and allows the formation of a coin relief with high quality. The profile may extend over the entire end face or be arranged in a predetermined section of the end face. Preferably, the profile comprises an axial bulge of at least one edge zone surrounding the end face. This allows the simple expression of a so-called edge bar, without the edge of the blanking blank must be compressed as in the semi-finished stamped blanks.

But at least one end face may also have a plurality of profiled sections, which are each arranged laterally at a distance from each other. This makes it possible to create coins and medals with complex images at a low embossing pressure and / or in only one embossing pass.

The plurality of profiled sections may have different base areas and / or a different width of the respective base areas of the profile.

The profile height is preferably in a range of 0.01 to 2.5 mm, depending on the dimension and material of the embossing blank. The profile height is considered to be the maximum height of the profile above a profile-free flat surface of the front side which adjoins the profiled section and corresponds to the coin base in the pronounced coin or medal. If the end face of the embossing blank has no profile-free flat surface, the axial height between the deepest indentation and the highest elevation of the embossing blank is understood as the profile height.

For the embossing of circulating coins with edge bar stamping blanks can be used with a peripheral circumferential bulge and one or more bulges in a central portion of the front page. The profile height of the bulge in the middle section is preferably not higher than the profile height of the edge-surrounding bulge. This ensures that the stamped blanks are stackable and the coins stamped out from the stamping blank are automatic.

The ratio of the profile height to the thickness of the embossing blank is preferably up to about 0.5. The thickness of the embossing blank is considered to be the axial distance between the lowest-lying profile-free planar surfaces on the mutually opposite end faces of the embossing blank. For embossed blanks without peripheral bulge the thickness of the embossing blank corresponds to the height of the lateral surface. The ratio of profile height to the thickness of the embossing blank is preferably in the range from 0.05 to 0.5, preferably from 0.1 to 0.5 and particularly preferably in the range from 0.1 to 0.25, in particular from 0.1 to 0 ; 2. Stamped blanks with a ratio of the profile height to the thickness of the stamping blank of 0.1 to 0.25 are particularly suitable for embossing circulation coins or conventional collector coins by simple ring embossing.

According to a preferred embodiment, the thickness of the embossing blank is in a range from 0.2 to 5 mm, preferably from 0.5 to 3 mm and particularly preferably from 1 mm to 3 mm, and the ratio of the profile height to the thickness of the embossing blank is in the range from 0.1 to 0.25, preferably from 0.1 to 0.2. The pressing force required for the complete expression of a design is generally the higher the thinner the embossing blank. In contrast, blanked blanks according to the invention with the dimensions mentioned can be embossed with lower pressing force and preferably in only one pass to coins and medals with fully formed motifs. In particular, at low thicknesses of the blanks of up to 3 mm can therefore be advantageously used with a significantly lower pressing force than stamped from semi-finished stamping blanks.

For the production of coins and medals with high relief motifs, the ratio of profile height to thickness of the embossing blank may be in a range of more than 0.25 to 0.5. In this case, fewer passes are required at the same pressing force to obtain fully defined motifs.

For fine metals such as Ag999 and Au999, coin and medal coinages can be considered as high relief, which has a ratio of relief height above the coin base to coin thickness greater than 0.25. For less ductile coins, the ratio of relief height above the coin base to the thickness of the coin may be lower.

Furthermore, there is the desire for the production of coins or medals with increasingly complex images, which have both high-lying, ie lying above the coin base, as well as low-lying, ie lying below the coin base relief sections. The invention takes this request into account by the fact that the end face has at least sections a profile with one or more indentations and bulges in the axial direction. This embodiment has the advantage that less material has to be displaced to form deep-set relief sections during embossing.

The stamping blank may be profiled at least in sections on one or both end faces. The profiles on the two end faces can be the same or different. Preference is given to blanking blanks with profiled sections on both end faces, which differ from one another.

The base of the formed on the front side of the embossing blank profile may be regular or irregular. For example, a relief to be imprinted in the later embossing step may already be predetermined in outlines and / or in the profile cross section.

If the face has a profile with a regular base, known geometric shapes can be used. In particular, the base can be formed ellipsoidal, circular or polygonal.

The profile may be in the form of a pyramid, a truncated pyramid, a cone, a truncated cone, a dome, a spherical segment, a prism or a cylinder. Preferably, the profile is formed in the form of a cone or truncated cone. It can be shown that this profile shape in combination with the good ductility of the embossing blank has particularly favorable flow properties. A trained as a recess on the front side profile can be designed as a negative mold of the profile shapes described above.

The metal powder used for producing the sintered embossing blank preferably has a mean particle size of from 20 μm to 150 μm.

The pressing of the green compact from the metal powder is preferably carried out at a pressure of 200 MPa to 800 MPa, preferably 200 MPa to 600 MPa. By pressing the green compact metal powder is compressed so far that handling of the green compact is guaranteed without damage.

The sintering of the green compact to form the stamping blank is preferably carried out at atmospheric pressure under a protective gas atmosphere, preferably nitrogen. The sintering temperature is preferably about 60-95% of the solidus temperature of the metal or metal alloy used. The sintered density of the sintered blank blank thus obtained is preferably at most 98% of the theoretical density.

In order to produce a coin or medal provided with a relief, the embossing blank is placed in a die with two stamping dies, i. H. an upper punch and a lower punch, and provided a stamping tool provided and then converted by applying the die on the blank blank under pressure to a coin or medal. The intended in the dies as negative reliefs motifs are transferred to form positive impressions of the reliefs on the embossing blank. The embossing blank is preferably arranged in the tool so that the profile of the embossing blank at least partially engages in the indentations and / or bulges of the negative relief in the embossing die. This ensures that the required for the forming process material flow to form the relief is minimized and can be worked with lower press forces. The high ductility of the sintered embossing blank contributes to a further improvement of the flow behavior compared to blanks punched out of semi-finished products.

The invention further relates to an embossing blank which is obtainable by the method according to the invention. The sintered obtained by the process according to the invention

Embossing blank can be advantageously used for embossing of reliefed coins and medals. Due to the improved ductility of the sintered embossing blank in combination with the at least sectional profiling on one or both faces of the embossing blank, the relief of the coin or coin can be achieved with significantly lower pressing force or fewer embossing passes with simultaneously improved relief structure.

The relief on the coin or medal is preferably a high relief. Such embossments previously required the use of high embossing pressures associated with several repetitions of the embossing process and possibly a heat treatment. The use of the embossing blank according to the invention can complex motifs in Hochreliefausführung with a smaller number of Embossing passes or without intermediate heat treatment are designed.

Further features and advantages of the invention will become apparent from the following description of preferred embodiments with reference to the accompanying drawings, which should not be construed as limiting. In the drawings show:

- Figure 1 is a schematic representation of Prägerohlingen invention with different profiles, shown as Figures 1 a to 1 e;

FIG. 2 shows a flow curve recorded with sintered embossing blanks in comparison to embossing blanks conventionally produced from semifinished products; and

FIG. 3 shows the graphic representation of the embossing force necessary for the expression of a coat of arms motif with a given relief height.

FIGS. 1 a to 1 e each show a sintered blank blank 10 with end faces 12, 14 opposite one another and a lateral surface 16 enveloping the end faces. At least one of the end faces 12, 14 is profiled in sections.

1 a shows the sintered blank blank 10 with a profile 18 in plan view and in cross section, the profile 18 having a conical shape and extending over a central portion of the end face 12. The profile height h is indicated as the maximum height of the cone above the edge of the stamping blank or above the flat surface of the end face 12 adjoining the cone. With D-ι the diameter of the cone is designated at its base, and with D _0, the diameter of the stamping blank. The proportions shown are not to scale. Preferably, the diameter of the embossing blank D _{0 is} in the range of 10 mm to 100 mm, and the diameter of the cone on the base D-1 is preferably from 30 to 80% of D ₀ . The thickness of the embossing blank corresponds in the embodiment shown here, the height of the lateral surface (not shown) and is preferably from 0.2 to 5 mm. The ratio of the profile height h to the height of the lateral surface is preferably in the range of 0.05 to 0.5. In Fig. 1 b a formed on a portion of the end face 12 truncated cone profile 18 is shown in cross section. The preferred dimensions of the embossing blank 10 correspond to those of Fig. 1 a.

Fig. 1 c shows a sintered blank blank 10 in plan view, with two spaced at a distance to each other on the front side 12 arranged cone profiles 18 whose diameter is denoted at the base with D-ι and D ₂ , wherein D _! > D ₂ is.

Fig. 1 d shows a sintered blank blank 10 with a over the entire end face 12 extending profile 18 in the form of cones. The preferred dimensions of the embossing blank correspond to those of FIG. 1 a

Fig. 1 e shows a sintered blank blank 10 in plan view, which has a profile 18 with an irregular base. The profile traces the outlines of a portrait motif.

According to the invention, the sintered embossing blank 10 is produced by a powder metallurgy process in which a metal powder is provided and pressed into a plate-shaped or disc-shaped green body with two opposite end faces, and wherein the green body forms a blank corresponding to the green body to form the blank blank 10 Form is sintered, wherein at least one of the end faces 12, 14 of the embossing blank 10 is profiled at least in sections.

Preparation example

To produce a sintered embossing blank, 12 g of a powder of Cu99 having a particle size of <100 μm were weighed and pressed at a pressing pressure of 400 MPa into a disk-shaped green compact in the form of a round blank. In one part of the green compacts, a central conical profile was introduced in the pressing tool on one of the end faces. Another part of the green compacts was pressed for comparison with flat faces.

The green compacts thus obtained were sintered at about 950 ° C for one hour under a nitrogen atmosphere. Sintered embossing blanks having a sintering density of at most about 98% of the theoretical density and a diameter of 30 mm were obtained. The central cone profile had a diameter the base of 15 mm and a profile height of 0.3 mm. The thickness of the embossing blank was 2.1 mm.

Determination of the flow curve

The flow curves according to FIG. 2 were determined for sintered embossing blankets and for conventional embossing blankets from semi-finished products, each with a series of compression tests. The 30 mm diameter and 12 gram weighted billets were upset with various forces in the range of 200 kN to 2000 kN. The changes in diameter and thickness of the blanks were measured. From these measurements and the applied force, the true stress and true strain were determined and the flow curve calculated.

An elongation of 0.04 results in a 67% lower stress for the sintered blank blank compared to the coin blank conventionally made of semi-finished product. This means that the force to be applied for a 4% elongation in the sintered blank blank is only about 33% of the force that must be expended on the blanket coin blank at the same elongation.

At a stress of 1300 MPa results for the sintered blank blank about a three times higher elongation compared to the conventional semi-finished coin blank. With the same force, the plastic deformability of the sintered blank blank is therefore 3 times higher than the deformability of the conventionally produced coin blank.

stamping tests

Coin impressions with different pressing forces (500 to 1000 kN) were carried out on the previously described sintered embossing blank rings with and without cone profile. The embossing was carried out as a simple ring embossing with a single stroke. On the front of the embossed coin a portrait motif was shown. The reverse of the coin was designed as a coat of arms motif. The relief height of the pronounced coin was determined as a function of the pressing force at a fixed position of the coat of arms motif and measured with reference to the coin base.

For comparison purposes, a conventionally made from semi-finished by punching blank coin blank was used with the same dimensions for comparison purposes.

The results of the stamping tests are shown in FIG. 3 and partly in the following table.

Table: Results of embossing tests

The relief heights of the coins embossed from the inventively produced sintered stamped blanks with cone profile are in the range of the pressing forces of 500 to 1000 kN in the range of 270 to 280 μm. The coat of arms motif was fully formed already with respect to the relief height and the relief shape already at a pressing force of 800 kN.

The sintered blank blank without profile reached the required relief height at a pressing force of 800 kN and exhibited a complete shaping of the coat of arms motif at about 1000 kN.

For the coin blank produced from semifinished products in a conventional manner, however, the required relief height of the coat of arms motif can only be expected from a minimum force of approx. 1200 kN. The pressing force required for the complete development of an ordinary relief on a coin can be reduced by at least 30% in accordance with the exemplary embodiment shown by using the sintered cone blank with tapered profile produced according to the invention.

Claims

claims

1 . Method for producing a coin and medal coin blank comprising the following steps:

Providing a metal powder;

Sintering of the green compact to form the stamping blank having a shape corresponding to the green compact,

wherein at least one of the end faces of the stamping blank is provided at least in sections with a profile.

2. The method according to claim 1, characterized in that the metal powder is a powder of a noble metal, copper, nickel, aluminum or iron, their mixtures and / or alloys, preferably a powder of steel, copper, gold, silver, platinum, palladium , Rhodium, iridium, ruthenium and / or osmium and their mixtures and / or alloys.

3. The method according to claim 1 or 2, characterized in that the profile comprises at least one bulge in the axial direction.

4. The method according to claim 3, characterized in that the bulge is formed at least in one of the end face peripheral edge zone.

5. The method according to any one of claims 1 to 4, characterized in that at least one end face has at least two sections each provided with a profile, which are arranged laterally at a distance from each other.

6. The method according to any one of claims 1 to 5, characterized in that the profile has a profile height in the range of 0.01 to 2.5 mm.

7. The method according to any one of claims 1 to 6, characterized in that the profile has a ratio of profile height to the thickness of the embossing blank of up to 0.5.

8. The method according to claim 7, characterized in that the ratio of profile height to the thickness of the embossing blank is in the range of 0.05 to 0.5, preferably in the range of 0.1 to 0.25.

9. The method according to claim 7 or 8, characterized in that the thickness of the embossing blank is in a range of 0.2 to 5 mm, preferably from

1 to 3 mm, and that the ratio of profile height to the thickness of the die blank is in the range of 0.1 to 0.2.

10. The method according to any one of claims 1 to 9, characterized in that the profile has a portion with a recess in the axial direction.

1 1. Method according to one of claims 1 to 10, characterized in that the profile has a regular base, in particular an ellipsoidal, circular or polygonal base

12. The method according to any one of claims 1 to 1 1, characterized in that the profile in the form of a pyramid, a truncated pyramid of a cone, a truncated cone, a dome, a spherical segment, a prism or a cylinder is formed.

13. The method according to any one of claims 1 to 12, characterized in that the profile extends over the entire end face.

14. The method according to any one of the preceding claims, characterized in that the embossing blank is formed in a tool to a coin or medal with a relief, wherein the tool comprises a stamping ring and an upper punch and a lower punch.

15. The method according to claim 14, characterized in that the upper punch and / or the lower punch has a negative relief with indentations and / or bulges and the embossing blank is arranged in the tool so that the profile of the embossing blank at least partially in the indentations and / or bulges engages the negative relief.

16. Sintered stamping blank, obtainable by the method according to one of claims 1 to 13, with two opposite to each other End faces, wherein at least one of the end sides of the embossing blank is provided at least in sections with a profile.

17. Use of the stamping blank according to claim 14 for embossing a coin or medal.

18. Use according to claim 15, characterized in that the

Coin or medal has at least one high relief.