The process of planning and producing New Zealand coins generally incorporates 6 stages:
- Striking the coin
The coin minting process begins with an approved coin design from the Bank.
Mint engravers have the challenging task of transforming the two-dimensional work of the coin designer into a three-dimensional sculpture.This can be achieved using two different methods:
- Plaster model – From the chosen design, a large-scale plaster model is prepared with the details cut into the plaster by hand. The plaster is placed on a scanning machine where a digital camera photographs the details from all angels. As a digital image, changes can now be made on screen.When complete, the file is turned into a cutting programme, instructing a computer-controlled engraving machine to cut the design into a master punch.
- Direct engraving model – Digitally sculpting the design on a computer tablet and transferring the end result to an automated milling machine which engraves it to a master punch.
Once the master punch is created, it is pressed or ‘hobbed’ against another blank die to produce working dies, these are used to make coins. The master punch always has a positive image so that a negative of the design can be transferred to the working die. This allows the coin to be struck with a positive, recognizable image. Each working die can on average make about 500,000 coins.
A coin’s metal composition can include several metals such as copper, nickel and zinc. The different metals for each coin are melted together in the furnace, where temperatures can reach up to 1450 degrees Celsius. A sample of this molten metal is then analysed to confirm the composition is correct. It is then poured into a holding furnace where it is cooled before being cast through this device into a continuous strip, which is wound into coils. These coils can weigh up to three tonnes.
Blank discs of metal of the correct size are punched from the strip in a blanking press, much like a biscuit or cookie cutter. Next, the blanks are placed in a rimming machine. This gives them a slightly raised rim around the edge that prevents the coins from sticking together, makes them easier to stack and protects the surface of the coin.
The current $1 and $2 produced by The Royal Mint are solid alloy and plated steel, which have a steel centre encased in a layer of a different metal such as nickel, brass or copper. After rimming the blanks are electroplated. Electroplating is where a layer of copper, nickel or brass is electroplated directly onto the surface of the blanks.
The current 10 cent, 20 cent, and 50 cent coins manufactured by the Royal Canadian Mint, using its own innovative multi-ply-plated steel (MPPS) technology. Instead of plating a steel core with a single, thick nickel layer, the core of our silver-coloured circulation coins are plated with alternating layers of nickel, copper and a final outer layer of nickel.
The blanks are rinsed after each plating stage. A special process is then used to harden and dry them, so they can are ready for striking.
Striking the coin
Striking is when the design is added to both sides of the blank — using tools known as dies. Stillage, or crates, that hold around 850kg of clean and shining blanks are tipped into a container, known as a hopper, above a coin press. Each blank is fed into perfectly-sized recesses in a circular plate. As the plate rotates,the blank is held by a collar as it sits between a pair of dies. With one strike of up to 100 to 150 tonnes of pressure from above and below, the obverse and reverse (or heads and tails) are added, often together with the milled edge from the collar, and the blank disc becomes a coin. Coining presses can strike up to 850 coins per minute.
An operator regularly inspects coins for quality, as they are ejected from the press and routinely changes the dies to ensure uniform quality. Once they meet the operator’s approval, the finished coins are packaged, boxed and shipped to the Reserve Bank vaults.
In 2015 the Bank introduced a recycling system for the old or damaged coins. Old or damaged coins are destroyed by smelting them and make them into metal ingots.