A MULTI-METALLIC FOIL TECHNOLOGY FOR MINTING MEDALS AND COINS WITH TWO OR THREE COLORS

STATE-OF-THE-ART

So far, the technology used for producing bi-metallic (or bi-colored) coins, hereafter named bi-metallic ring technology, has consisted on the utilization of an inner disk (center) and an outside ring of different materials, generally with different colors.

The coins are produced in two stages: firstly the center is placed inside the outer ring with a little clearance, and secondly the two parts are assembled together with the impartion of the surface details by the minting dies (Figure 1). In the last years, several technical solutions for ensuring the mechanical joint between the inner disk and outside ring, have been developed by the producers of disk blanks, manufacturers of presses as well as by the mint houses.

More recently, a new type of bi-metallic collection coin has been presented, in which the bi-colored effect is achieved by mounting a small foil over a limited zone of the surface of a disk blank.

DETAILED DESCRIPTION OF THE INVENTION

The invention, hereafter named as multi-metallic foil technology, is based on a concept entirely different to that of bi-metallic ring technology. Multi-metallic foil technology employs two or three disk blanks of different materials, having at least two different colors or shades. One (or two) of these blanks is very thin (hereafter named as foil), while the other is much thicker (hereafter named as disk) and has a larger diameter (Figure 2).

The coins and medals are obtained by assembling the foil with the disk, by mechanical means, during the coining operation. Whenever two foils are to be utilized, these must be assembled in opposite sides of the disk.

The multi-metallic foil technology is based on a sequence of four different operations; three cold metal forming stages (preforming, rimming and coining) and one intermediate annealing treatment.

The metal forming sequence, starts with the preforming operation. This operation utilizes one flat die in conjunction with a special purpose die in order to produce an intermediate disk geometry having, in one of the sides, a considerably high distance from the table (lower floor surface of a coin with uniform level from which the volumes of the relieves grow) to the surface of the rim (Figure 3). In other words, the preforming operation transforms the blank into an intermediate non-symmetrical disk having a near-flat surface in one of the sides and a high rim on the opposite side.

The second metal forming stage is the rimming operation in which the preformed disk is bent along its diameter in order to generate a suitable profile for subsequent assembly with the foil (Figure 4). Close dimensional tolerances are required as the final diameter reduction does not exceed approximately 1 %. Sharp fillets at the rim to table corner are preferable to round corners with concave curvatures. The rimming operation is crucial for the overall success of the coining process.

The main design parameters utilized for a blank having an initial diameter equal to 33.65 mm are listed bellow.

X=0.8 mm Z=0.3 mm E=2.0 mm

General guidelines for designing and manufacturing two- or three-colored coins are as follows (Figure 5):

Z<X

Z«E

45°<θ<90°

The third metal forming stage is the coining operation in which the two parts are assembled by locking the foil (1) into the rimmed edge of the disk (2), during the imprint of the surface details (Figure 6). This operation requires the foil to be previously positioned on the surface of the disk. The clearance between the foil and the rimmed edge of the disk must be equal to Y=0.3 mm if a blank having an initial diameter equal to 33.65 mm is to be chosen. As a general rule, the clearance between the foil and the rimmed edge of the disk must be comprised within the range 1 %-3% of the initial diameter of the blank.

Due to the level of strain accumulated during the preforming and rimming stages and due to the fact that the rims after being bent are preferential stress raiser zones, there might be a necessity of annealing the disk. The annealing softens the material, and therefore the initial ductility of the blank is completely recovered prior to the final coining stage.

The multi-metallic foil technology can be applied to all the metals and metal alloys currently utilized in the production of coins and medals. Proper selection of the metals to be used require the combination of technical and aesthetical criteria.

The multi-metallic foil technology is an alternative to conventional bi-metallic ring technology utilized worldwide, as well as to galvanized surface treatments that can also induce bi-color effects on a monometallic disk.

Multi-metallic foil technology allows the coining of gold-silver specimens with larger diameters and lower costs than those that would arise from the utilization of monometallic gold blanks. Therefore, it presents a unique opportunity for minting gold with large diameters without increasing the final cost of the product. As a consequent this technology opens new market opportunities for brilliant uncirculated coins and proof coins as well as for medals.

Finally, it must be emphasized that variations to this technology by employing three disks, two being very thin and one thicker placed in-between, are also possible.