Various types of inks are used to produce each banknote, such as optically variable ink that changes colour as the note is tilted, as well as ink that reacts in near-infrared light. Colour control and rheology (the flow of ink) become critically important when printing this type of security document; therefore the process can be incredibly complex. The US cotton notes are soil-resistant, chemical-resistant and durable.
The Bureau of Engraving and Printing’s paper has been sourced from the same company (Crane and Company) since 1879 and it is 75% cotton and 25% linen blend. There are fluorescent security threads, watermarks and red and blue fibres embedded in the paper at the time of manufacturing. Consistent colour and feel of the paper is critical to the success of the notes over multiple runs. The fibres of the paper have been super calendered (flattened) to increase smoothness.
There are three main printing processes used to produce US banknotes: offset lithography, intaglio and letterpress.
The first pass of the notes are printed on an offset lithographic KBA Simultan press, where the face and back of the note are printed simultaneously. This sheetfed press is capable of printing 8 colours on each side. It is 52 feet long, 14 feet wide, 12 feet tall and holds up to 20,000 sheets. The notes are printed 32 per sheet and this press is capable of speeds of up to 10,000 sheets per hour.
The sheets are then left to dry in storage for 72 hours before moving to the intaglio process. The back of the notes are printed first using engraved plates to achieve tactility. Only in the last 12 years has the engraving process become digital and previously all plates were hand-engraved. The new process uses a plastic mould and assembly process for making a full step and repeat master die through heat and pressure. This plastic master is treated with a light coating (conductive agent) and placed into a nickel bath for 14-16 hours where the nickel “grows” or adheres to the plastic master. The Bureau of Engraving and Printing has some of the last siderographers (SID-er-og-ra-phers) in the world, and these individuals bring together each individual intaglio engraving onto a master plate and correct for imperfections. The Bureau has two intaglio presses, including an Intaglio 10 Press, which uses a direct inking system and can produce 8,000 sheets per hour. The second intaglio press is called the Super Orloff, which has an indirect inking system of plate to blanket to engraving.
The next step in the process is to print the face of the notes using an intaglio press, whereby finely engraved line work acts as a security feature by introducing a three-dimensional tactile element called a “rumble strip”.
Lastly, the notes are trimmed for the Currency Overprinting Processing Equipment (COPE), which is a letterpress that applies serial numbers and seals. Only after this step are the notes considered legal currency. The sheets are then trimmed down into single notes and packaged in various groupings:
Bundles (10 straps of 100 notes each – 1,000 notes)
Bricks (4 bundles – 4,000 notes)
Cash packs (4 bricks – 16,000 notes)
Palletized on skids (40 cash packs – 640,000 notes!)
The packaging process includes various levels of security wrap and barcodes, with five layers of shrink wrap around the pallet alone. One pallet of $20 notes has a total value of $12,800,000! These pallets are locked in the federal reserve vaults and are ready to be shipped to federal banks in armoured trucks.