Basic principles of offset printing
Offset printing belongs to the category of lithographic printing, which is divided into water (wetting fluid) offset printing and waterless offset printing. The printing method is indirect printing.
1. The basic principle of water offset printing
Generally, what people call water offset printing is the use of oil (offset printing ink) and water (wetting fluid) incompatible polar principle. From the perspective of polarity theory: matter consists of molecules, and molecules are divided into: polar molecules, non-polar molecules and amphiphilic molecules. The molecular (stereogeometric) configuration of the polar molecule is asymmetric, the positive and negative charge centers of the molecules do not coincide, the electron cloud molecules are not uniform, and the dipole distance is not zero. The wetting fluid in water offset printing is a typical material with mainly polar properties.
The characteristics of non-polar molecules are completely opposite to those of polar molecules. Ink in water offset printing is a typical non-polar substance.
Amphiphilic molecules are composed of polar genes and nonpolar genes. For example, various surfactants that act as wetting agents in low surface tension wetting fluids and as dispersion agents in inks are typical amphiphilic molecules.
When printing with water offset printing, these polar-based materials, non-polar-based materials and amphiphilic molecules coexist in the printing process, they must follow the similar structure of the material, compatible, compatible, not similar The objective law of incompatibility and incompatibility.
Most of the water-based offset printing uses the PS version, and the printed image and text part of the printed plate is about 3 microns higher than the blank part. This type of printing plate is also called planographic printing plate (Figure 1-1). From this, it can be concluded that the basic principle of water-based offset printing is that the blank part (substance with polar structure) on the printing plate and the ink part (substance composed of non-polar molecules) are on the same plane. For water (wetting fluid) offset printing, the blank part is hydrophilic and oleophobic, and the graphic part is oleophilic and hydrophobic. Using the principle of mutual repulsion of oil and water, the plate is first filled with water to make the blank part form a water-repellent water film; then the plate is inked to make the graphic part adhere to the ink. Under the action of printing pressure, the pictures on the printing plate are transferred to the surface of the substrate through the blanket cylinder.
Two characteristics of waterless offset printing
The waterless offset printing plate has a flat intaglio structure (Figure 1-2). The blank part of the printing plate is raised and is a silicone rubber (low-energy surface) that does not absorb ink, while the graphic part can absorb ink well. Obviously, offset printing follows the laws of interfacial chemical wetting, adsorption and selective adsorption.
Figure 1-2 Schematic diagram of flat intaglio structure
Compared with waterless offset printing, the main advantages of waterless offset printing are:
1. High product quality. Due to the low dot expansion rate and good level reproduction effect, especially in the dark part, the density of the ink layer is quite stable without the participation of water, and there is no problem of density reduction after drying. To ensure the consistency of the color, at the same time, high accuracy of overprinting, no water streaks, etc.
2. Improve production efficiency. Since there is no wetting device, the registration and color adjustment time is reduced, paper waste is reduced, and production efficiency is improved.
3. 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; Because of this, the offset printing currently adopted by printing companies is mainly offset printing with water participation. This is the focus of the discussion in this book.
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