The basic principle of offset printing

Offset is a category of lithography, with water (wetting) offset printing and waterless offset printing. Printing is indirect printing.

The basic principle of water offset printing

What people usually call water offset printing is the use of oil (offset ink) water (wetting liquid) polarity principle of non-affinity. From the perspective of polarity theory, substances are composed of molecules, and molecules are divided into polar molecules, non-polar molecules, and amphiphilic molecules. The molecular structure (stereo-geometry) of the polar molecule is asymmetrical, the center of gravity of the molecule is not coincident, the electron cloud molecules are not uniform, the dipole distance is not zero, and the larger the dipole distance is, the stronger the polarity property is, for example, The wetting fluid in water offset printing is a typical polar-based substance.

Non-polar molecular characteristics and polar molecules are completely the opposite, and the ink in water offset printing is a typical non-polar nature-based material.

Amphiphilic molecules are composed of polar genes and non-polar genes, for example, various surfactants that play a role of wetting in low surface tension wetting fluids and disperse in inks, and are typical amphiphilic molecules.

In the case of water offset printing, these polar-based substances and nonpolar-based substances and amphiphilic molecules co-exist in the printing process. They must follow the similarity of the material structure, and they are similar to each other, and they are not similar. Insoluble, incompatible with the objective law.

Most of the water offsets use the PS version, and the dried part of the printed version is about 3 microns above the blank. This type of plate is also known as a plano-plate (Figure 1-1). From this, it can be concluded that the basic principle of water offset printing is that the non-inked blanks (substances with polar structure) and the inked parts (substances made of non-polar molecules) on the plate are in one plane. For water (wetting) offset printing, the blank part is hydrophilic and oleophobic and the graphic part is hydrophobic and hydrophobic. Utilizing the principle of mutual exclusion of oil and water, the water is first applied to the printing plate to form a water-repellent water-repellent film on the blank part, and then the printing plate is inked, so that the graphic part is accompanied by ink. Under the influence of printing pressure, the graphic on the printing plate is transferred to the surface of the printing object by a blanket cylinder.

Waterless Offset Features

The waterless offset printing plate is a plano-gravure structure (Figure 1-2). The blank part of the printing plate is convex and is a silicone rubber (low energy surface) that does not absorb ink, and the graphic part can absorb the ink well. Obviously, offset printing follows the laws of interfacial chemical wetting, adsorption and selective adsorption.

Figure 1-2 Gravure structure

Compared with waterless offset printing, waterless offset printing has the following main advantages:
1, high product quality. Due to the low dot gain rate and good effect of layered copying, it is particularly noticeable in dark shades. In addition, without the participation of water, the density of the ink layer is quite stable, and there is no problem of lowering the density after drying. Ensure the consistency of color, at the same time, high overlay accuracy, no traces of water and so on.
2, improve production efficiency. Since there is no wetting device, the time for registering and color adjustment is reduced, waste of paper is reduced, and production efficiency is improved.
3, is conducive to environmental protection. Because there is no need to use chemicals such as alcohol, the working environment can be improved.
The main disadvantages of waterless offset printing are: First, the cost of the printing plate is too high; second, it is suitable for low-speed small-format printing; and third, it is low against the printing force. Because of this, the offset printing used by printing companies is mainly offset printing with water participation. This is the focus of this book.

Source: Ke Yin Network