Today I would like to talk about what happens before the printing ink comes into contact with the paper. In this process the image is not placed directly on the printing material in one single step, instead it is put together from several component parts. This process is made possible thanks to 4 colour printing. You can now read about how this is defined, how the process works and what is understood by multi-coloured printing and spot colours.
Printers, which produce large runs of print projects, often use offset printing as digital printing is more suitable for the production of smaller print runs. Four-colour printing is the technique applied in all modern print processes for colour reproductions.
The basis for this consists of the four colours: cyan, magenta, yellow and key (black) – CMYK for short. Theoretically all colours can be mixed from the three subtractive primary colours cyan, magenta and yellow. So yellow and magenta make the colour red, while cyan, magenta and yellow make black. However, the colour pigments are insufficient, which is why just a limited colour space can be shown. Furthermore, the overprinting of cyan, magenta and yellow does not result in dark black, but rather a dark brown tone. For this reason, jet-black has been added as a fourth primary colour. In order to be able to print different shades of colour, different rasterisation is used, this means that many tonal values are shown through raster points of different sizes.
Process of 4 colour printing in offset printing
In order to print an image via an offset process, it must first of all be separated with the help of special computer software into the four primary colours (colour channels) of the subtractive colour model.
In the prepress, the artwork for each colour is individually rasterized and exposed on a separate printing plate. Therefore only the cyan, magenta, yellow and key (black) sections of the image can be seen on each plate. Up until a few years ago this exposure often took place indirectly via the conventional “Computer to Film” (CTF) process. However, this process has been replaced by the higher quality and time-saving “Computer to Plate” technique (CTP). The imaging in the computer-to-plate recorder takes place directly on the printing plate. In future this procedure could be replaced by the “Computer to Press” process.
For the flat printing process known as offset printing the exposed printing plates are fitted in the respective printing unit of the machine. The offset process uses the conflicting behaviour of different substances. This means that there are ink-repellent and ink absorbing areas on the printing plate. The ink absorbing (lipophilic, meaning water-repellent) image areas are differentiated from the image-free (hydrophile, meaning water absorbing) areas, which are ink-repellent through minimal moistening with water. Therefore the printing ink is only transferred from the ink absorbing areas on a rubber blanket, from where it is then pressed onto the object (paper) which is to be printed. The printing substrate is then layered with the colours cyan, magenta, yellow and key (black) one after the other, so that the complete printed image is created.
The colour, which is created from CMYK during the print process, is known as the process colour. There is also the spot colour. These spot colours are defined by a number or a similar indication.
Special colours such as HKS or pantone are required, if a shade of colour needs to be 100% reproducible, so that the CI (Corporate Identity) of a company with logo, slogan or similar design elements look identical at any time, all over the world and can be recognised by the observer. This is extremely important for companies in order to position themselves on the market and to attract regular customers. For example, the shade of colour of a chocolate manufacturer can often be recognised from afar when shopping.
In order to always be able to adjust a certain colour, it requires a fixed special colour. Many companies of a certain market size bring their own colour guides with company colours they have created themselves for printing approval. The creation of this type of colour through the mixture of CMYK with other chromatic colours is possible, but often does not lead to the desired result, as each result deviates slightly from the previous result. The printing of spot colours is therefore carried out via multi-colour printing. As opposed to four-colour printing in this case 5 or more colour channels are set up depending on the number of special colours.
Depending on whether business paper, business cards or flyers for a one-off event or whether private greetings cards are to be given to a printer shop for completion, you should decide in advance which layout elements are to be produced and in which colours.