circulating coins
Minting is the process of transferring a design from a die onto a blank piece of metal.
Dies
The manufacture of the dies commences with a sketch of the proposed design. A three-dimensional model of the design, several times larger than the finished coin, is then created in ‘Plaster of Paris’. Silicone rubber is then used to produce a ‘negative’ moulding of the plaster, which in turn is used to cast a ‘positive’ model of the design in epoxy resin.
The epoxy resin model is mounted on a Reducing Machine that traces the three-dimensional design and cuts a much smaller steel replica to the actual size required. This tool is normally called the Reduction Punch. A heat treatment process then follows to harden this tool.
A ‘master’ die is produced from the Reduction Punch using a cold forging process called hobbing. This is achieved by pressing the design into another piece of soft steel using very high forces in a hydraulic press. This master die is then hardened and used in the same way to produce a ‘positive’ tool called a hob or hub.
It is from this tool that the working dies are hobbed, machined to suit the coining press on a computer controlled lathe and hardened. Finally, an electro-plating process is used to deposit hard chromium on the surface of the die to reduce wear in the coining process and to extend the life of the dies. On average each working die produces 250 000 coins but some dies produce over one million coins.
The Mint also uses a state of the art process called Filtered Arc Deposition to deposit extremely hard and smooth coatings such as titanium nitride onto the surface of some dies.
Blanks
Blanks are cut from strips of a metal alloy, which usually consists of a mixture of base metals, the most common of which is copper. The composition of these alloys is carefully controlled. Blanks are usually round in shape but an exception is the blank for Australia’s 50 cent coin, which is dodecagonal (12 sided).
Rimming
Blanks for most circulating coins have their edge rimmed. This is a process where the edge of the blank has been raised by rolling the blank through a specially shaped groove. The raised metal assists in the coining operation by partially forming the rim so that the dies do not have to displace as much metal.
The rimming machine can also apply edge lettering or security marking to the edge of the coin blanks.
Annealing
During the preparation of the blanks the action of the rolling and rimming operations creates a change in the condition of the metal. This effect is called Work Hardening. The blanks need to be softened slightly before they can be struck with the die. This softening process is carried out in a furnace by heating the blanks to around 850 degrees Celsius and then cooling them again. After annealing, the blanks are then transferred to a burnishing unit.
Burnishing
Burnishing is used to make the surface of the blanks brighter, remove any discolouration and in some cases apply a minute amount of lubricant to assist in coining. This is carried out in a Burnishing Machine that tumbles the blanks amongst a mixture of small steel balls and ceramic media combined with special chemicals, which etch and polish the surface. After burnishing, the blanks pass through a hot air drier. The annealed, burnished and dried blanks are stored ready for coining.
Coining
The prepared blanks are fed into a hopper at the coining press. The Mint operates several different types of coining presses for general circulating coins. These presses have a coining capacity of 100, 150 and 200 tonnes force. The choice of press depends on the size and alloy (metal) of the struck coin.
Blanks are fed automatically into tungsten carbide collars. The collar locates the blank prior to striking and controls the finished size and shape of the edge of the coin. The coining action is such that the blank is struck simultaneously with two dies to produce the obverse and reverse designs. The presses can strike up to 650 blanks per minute. The larger the coin and harder the metal, the more pressure required, which usually means a slower strike rate. Damaged or wrongly sized blanks are automatically rejected without slowing down the production process.
The finished coins collect in a small catch box that is regularly checked for quality by the press operator. A representative sample of the coin batch is also checked by Quality Control. If the sample fails, the press is shut down for inspection of the die and the entire batch of coins is quarantined. If the quality of the coin is below the accepted standard, the batch is then destroyed.
Bagging
Once approved by Quality Control, the coins are bulk packed into plastic bags. The Royal Australian Mint can package up to two million circulating coins per day using one machine and one operator – a second person is required occasionally to take away the completed packed coins and refill the feed bins.
Coins are first counted electronically, and after bagging, are weighed as an additional check. The plastic from which the bags are made comes flat in rolls, and in one operation is shaped, filled and sealed.
Storage
Coins for local consumption are stored in a vault until dispatch is required. Overseas orders are further packaged for protection during transport.