Planographic printing: the differences between dry and wet offset

In indirect printing methods, the non-printable areas of the plate can be moistened with water or covered with silicone

Offset is by far the most widely used technology in the printing market. Since it is a planographic system, that is, a system in which the printable areas of the image are kept at the same level as the non-printable ones, pre-press work is very easy. On the other hand, the indirect printing method, where the image transfer from the printing plate to the substrate is carried out by means of a rubber roller or blanket, allows high speed work in medium and long print runs, and with top quality.

As we explained in a previous article, offset printing is the outstanding disciple of lithography, the technology that led to direct planographic printing, thanks to the incompatibility between greasy ink and water. There are two offset printing processes: wet and dry or waterless. What makes them different?

The characteristics of wet and dry offset

Both systems are based on the same principle: repulsion between the oily component of the ink and the substance with which the areas of the plate that are not to be printed are coated. In contrast to flexography, in gravure or rotogravure, the plate with the original image lacks relief; the printable areas are traced with oily ink, while the rest of the surface is treated with a substance that repels the ink. Wet offset differs from dry offset in that in the former case the plate is wetted with a solution of water and isopropyl alcohol, while in the latter case the areas where the ink is not to stick are covered with a layer of silicone.

Wet offset

Wet offset is the classic and most common form of indirect planographic printing. The parts of the plate with the design to be printed are hydrophobic, while the unprinted parts are hydrophilic. As the oil-based ink does not mix with water, it does not stick to the wetted surface and therefore only the inked areas are transferred to the substrate.

There are two types of wet offset: coldset, where the ink dries by evaporation and the absorption process of the paper, and heatset, where the press incorporates a heat-drying system to accelerate the evaporation of the solvents from the ink and subsequent cooling that settles the pigments on the substrate.

Dry or waterless offset

As already stated, it is also possible to print in offset without using water. Instead of water, the parts where image transfer shall not take place are covered with a layer of silicone that is incompatible with greasy ink. This calls for special plates, usually made of polymeric material and completely coated with silicone. The image to be reproduced is laser-engraved on the plate. With this operation, the silicone layer vanishes from the printing areas, exposing the ink-receptive surface.

The main disadvantage of dry offset is that is prone to colour peeling to due to the generation of dust particles at the border points between the printable and non-printable areas. This leads to the appearance of small white spots on the printed substrate.

At ROTATEK we develop rotary and semi-rotary presses that provide the best results with wet offset printing. Ask us.