Offset printing is one of the most popular printing methods we can find. One of the characteristics of this printing system is that it works in an indirect way -similar to a lithography-, that is to say, the ink is transferred by pressure thanks to the different rollers it has. In addition, this type of printing stands out because it allows a large number of prints to be made in just a few minutes, reducing the cost of printing the larger the print run.
If you want to know exactly what offset printing is and how it works, we recommend you to read this article. Below, we answer these and other common questions, such as what are the offset printing formats and how this method differs from digital printing.
What is offset printing
The offset printing system is an indirect printing system. This means that the image is not printed directly on the substrate used, but through rubber rollers. But what exactly happens? How does the offset printing system work?
Offset printing works as follows. First, ink is applied on rubber rollers to the metallic plates -there is a minimum of 4 – to create the image we want. As we said, the image does not go directly to the paper -which is why it is an indirect printing and is called offset-, but it goes to a rubber cylinder and then to the final support -paper or plastic-.
The roller printing system is as follows: First, some rollers with ink, others are wetters that bathe the areas of the illustration plate in which the image does not use ink -hydrophobic areas-. A solution that repels the ink is used for this purpose. The inking rollers then apply an oily ink to the areas of the image with hydrophobic compound and the shape of the motif to be printed – which has been previously etched on the illustration plate.
The plate is a flat metal alloy support. It has a photosensitive and lipophilic film on one side. In addition, it is monochromatic, which means that it can only transfer one color, so it is necessary to decompose the image for each of them. It would also take 4 plates to make a four-color process according to the CMYK color gamut, and even 6 plates to make a hexachrome process.
When the plates have been broken down into the different colors, they are placed on the printing units of the press, specifically on the rollers. These, in turn, are in contact with the dampeners and the inking units. The plate receives water on one side and ink on the other. However, thanks to the lipophilic characteristic of the plate emulsion, only the area of the image that is of interest receives the color and the area that does not repel it.
Once it receives the ink, it is transferred to the rubber and immediately to the paper, which passes to another roller and continues with the printing process receiving the ink of the missing color – until it passes through the four plates and at the end of the machine there would be a heater for drying the ink, obtaining the final printed image.
Characteristics of offset printing
We have just seen what offset printing is. Now, why is it so widely used? Offset printing provides a lot of advantages, the main one being that it offers an almost exact reproduction of the image on paper. In addition, it can be used on all types of paper. This is achieved thanks to its characteristics.
- Printing machines are divided into printing units. In other words, the product coming out of the printer passes through several stages – or printing units – until the desired print is obtained. All printing units are the same, but what changes is the ink that is applied to them. In other words, each printing unit is only capable of printing one of the colors. The printing plates are located in the printing units.
- The printing is of better quality than printing directly on paper. This would result in a much faster wear of the printing plates.
- The scale used for four-color printing is CMYK (Cyan, Magenta, Yellow & Black), although other colors can also be used, such as special Pantone colors like gold or metallic. In addition, lacquers and varnishes – for this, the printer needs to have additional units installed.
- The order in which the colors are printed is as follows. The first color is black, then cyan, then magenta and finally yellow. Yellow is the most transparent, which is why it also acts as a paper protector.
Offset printing formats
Offset printing formats are very varied. This system admits a great number of sizes and types of paper. In general, the most used sheets in offset printing formats are 52 x 70 cm; and 70 x 100 cm.
Difference between digital and offset printing
The difference between digital and offset printing is that digital printing does not use plates to print, but the information goes directly to the machine. However, we can find many types of digital printing, such as the one used in home printing machines, up to machines that exceed one million euros -at industrial level-. The difference with respect to offset printing is that this printer performs immediate printing, however the size of the sheet, as well as the range of papers that can be used, is more limited.