Unstruck planchets - Production
As a rule, the planchets are no longer produced within the Mint. This operation is usually contracted out to external suppliers. In any case, the planchet errors generated in this phase of the process are the same regardless of the place where they are actually produced.
Melting of metal
During this operation, various metals are mixed to create the alloy of which the planchet will be formed. The metal usually arrives at the external firm in the form of rods or ingots of standard weight. The ingots are loaded into large containers according to a well-defined proportion and then are transferred through conveyor belts inside the furnace, where the metal is melted and mixed to obtain the correct alloy. In these cases, if for any reason the proportions are not respected or the metal is not mixed in a uniform way, the planchet will be composed of an alloy different from the one established. The liquid metal mixture is then poured into water-cooled molds (or with other systems) in order to generate very large solid blocks of metal weighing a few tons. These metal blocks are cut with a special machine to obtain metal bars which are subsequently inserted into another furnace called the annealing furnace, where they will remain exposed for about 15 minutes at a temperature of 700 °C for aluminum alloys and 800 °C -900 °C for other metal alloys. At this temperature the metal does not melt, but becomes softer and takes on a very bright yellow-white color.
Most of the coins are minted in alloys formed by two or more elements of which at least one is a metal, and whose resulting material has metallic properties different from those of the relative components obtained with a fusion process. However, production methods have been developed relatively recently that involve the use of pulverized mixtures. In these processes, alloys are prepared by mixing powders of various materials compressed at high pressures and then heated to temperatures just below the melting point (sintering). In this way, compact and homogeneous materials are obtained. Other techniques for the preparation of alloys consist in ion implantation or by refining.
Lamination
This operation takes place inside a machine called a laminating machine, equipped with presses made up of at least 4 compressor cylinders (2 pushing and 2 working) placed one above the other and with containment walls placed on the sides. During this phase, the metal is exposed to a temperature lower than that of the annealing furnace and therefore a very fast heat loss occurs. The pressure exerted by the compressor cylinders and the cooling of the metal modify its molecular structure giving it greater hardness. Long metal plates are formed due to the containment effect of the side walls of the laminator. The metal thinning operations are designed in such a way that they can be guided both by a computer and by manual intervention by the operator.
A new method has been introduced, where the metal is poured directly in the liquid state in the form of a plate and then solidified between two cooled cylinders from which the metal plate emerges already with the final thickness. Once the desired thickness has been reached, the metal plates are conveyed into the quenching chamber, that is to say a room equipped with cold water sprays which have the function of rapidly bringing the metal to room temperature. The metal plates are inserted into a milling and planing machine which removes and cuts any imperfections and oxidations on the surface of the plates.
With a special machine, the final part of the sheets is sheared to obtain a regular shape and 3 or 4 sheets are welded together (currently a single sheet is used). In rare cases it may happen that one of the welded plates has a lower width due to an incorrect shearing and that some of the planchets produced from this plate will have a straight clip.
The welded plates are rolled to form metal coils that are easier to handle. These coils are unrolled and inserted into another laminating machine that further thins and elongates the metal, until it reaches the predetermined thickness. In some cases, it may happen that the thickness reached does not conform to the standard one and that the blocks produced from this slab will be of irregular thickness. The plates are rolled up again and stored in the form of coils. For the Italian Mint, the external supplier takes care of the entire process of preparing the planchets and then deliver it ready for minting to the Mint.
Cut - Clipping
This operation consists in converting the metal plates into rough planchets, also known as the 1st type. This is done by an eccentric mechanical press called a punching press or shearing press. Older generation presses require the metal plate to have a reduced width and therefore it must first be cut to have the necessary width. The new generation presses are able to accommodate the metal plates as prepared by the last laminating machine mentioned above.
The punching presses are equipped with special feed rollers on which the metal coils are mounted which are then unrolled and inserted into the press. Inside the press, the plates are hit with a series of metal cylinders called clipping punches which, thanks to the high pressure exerted by the press, perforate the metal plate and make the planchets fall under the base (on which the plates slide) equipped with circular holes. In some cases it may happen that the guides or feed rollers do not work properly and advance the metal plates at a slower speed than the predetermined one. In this case, curved, irregular or elliptical clips can be created. The perforated plate that comes out of the punching press is reduced into smaller pieces that are sent to the furnace ready to be recycled.
The planchets are subjected to a deburring operation which consists in the removal of a thin burr produced by the shearing of the metal plate. This operation is carried out by tumbling piece by piece, that is, by rolling and hitting the rounds inside a container similar to a barrel (Tumbler) and possibly with the addition of abrasive material.
Hemming
This operation consists in perfecting the edge and the contour of the 1st type planchets and is carried out for several reasons:
- Sometimes the 1st type planchets have a slightly larger diameter than the collar. With the hemming operation, the diameter of these planchets is brought back to normal. The minting operation is facilitated if the edge of the coin is already partially formed.
- 2nd type rounds move along the various stages of the minting process more efficiently and quickly than 1st type planchets and this is because the outer part of the round (edge) is in contact with the surfaces on which it is rests.
- The raised edge protects the inner part of the round from scratches and abrasions.
- The 2nd type planchets tend less to stick to each other.
To obtain the border, the planchet is inserted inside an edging machine in a cavity placed between a rotating wheel and the semicircle-shaped walls (the walls are not smooth but hollowed in the shape of a "V").
The block is pushed by the rotating movement of the wheel and as it advances, the space between the wheel and the walls narrows. This causes the planchet to be flattened forming a sort of raised edge. With the same operation, incuse inscriptions and various decorations can be obtained on the edge of the planchet by engraving the letters and embossed decorations on the guides. These planchets with the raised edge are called 2nd type planchets.
The planchet is subjected to an annealing treatment which consists in inserting them in an oven which has the function of heating them to a temperature that stretches the molecular structure of the metal giving it the necessary malleability.
Polishing
The planchets are placed inside tanks (Tumblers) with detergent solutions, steel granules and ceramic material which have the function of eliminating any impurities, smoothing and then polishing the metal.
After polishing, the planchets are washed with running water and dried with hot air jets.
The planchets are checked in order to discard any defective planchets (rejects).
With the traditional method, the inspection can be done manually with finder machines. With the new method, the inspection takes place automatically with computers equipped with cameras.
Finally, the weight is checked by means of automatic weighers which compare the weight of the planchet with that of a sample and start those with exact weight, light ones and heavy ones in different containers.
Unstruck planchets are the final product of the production process that we have just analyzed.
These are therefore metal discs that will be delivered to the mint and intended exclusively for the minting of new coins in compliance with the issue decree.
Collected as a curiosity, these planchets are useful for the study of counterfeits, standard coins and the sector of minting errors.
Regularly produced planchet do not officially belong to any category of minting errors. Even if for some experts they fall within striking errors, it is impossible to establish with certainty whether they put them into circulation before arriving to the mint or whether the planchet feeding arm installed on the monetary press pushed it directly into the basket of the minted coins, thus skipping the minting process.
Furthermore, it is not possible to define it as a striking error because in reality dies did not in any way touch the planchet at the time of minting. In some cases, unstruck planchets were found inside the rolls of coins intended for circulation.
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