2.2 Computer Direct Plate Making
Computer-to-plate, called CTP (CompmerToPlate), is a high-tech platemaking method that is used more and more widely.
2.2.1 Process Flow
Original manuscripts, graphic mixing, proofing, laser platemaking mechanism, development, lithography.
2.2.2 Features
(1) Graphical information is stored digitally in various memories, which are convenient for storage and modification;
(2) Eliminating the film production process;
(3) Simplification of the plate making process;
(4) high plate making precision;
(5) Printing plate has high printing resistance;
(6) The cost of plate is higher;
(7) Higher plate-making machine investment.
2.2.3 CTP Plate Making Machine
The CTP platesetter has an inner drum type, an outer drum type, and a platform type, and the light source is a laser.
2.2.4 Media
The CTP plate has a metal plate and a polyester plate.
("Photosensitive (photosensitive) CTP plate: The plate has a photosensitive coating on its surface. It uses laser imaging to operate in a dark room and the plate making speed is fast.
(2) Thermal-type (thermal-sensitive) CTP plate: The surface of the plate has a heat-sensitive coating. Using thermal imaging technology, it can be operated in a bright room, and the quality of the plate is good.
2.3 Wetting
At present, China's offset printing industry is still dominated by water offset printing, and wetting is an indispensable process.
2.3.1 Principle of Wetting
Surface free energy of blank part of printing plate> Surface tension of wetting fluid> Surface free energy of printing plate graphic part> Surface tension of ink, ink is similar to printing plate graphic and miscible, low surface tension liquid can be wetted on high energy surface, otherwise Can't. The wetting fluid can only wet in the blank part of the printing plate and cannot be wetted in the graphic part. After the printing plate is wetted, the ink and the wetting liquid are incompatible with each other and cannot adhere to the blank portion, and can only be attached to the graphic portion.
2.3.2 The role of wetting fluid
(1) wetting the printing plate to prevent the dirty printing plate, the wetting liquid forms a water repellent film on the blank part of the printing plate, resisting the expansion of the ink to the blank part;
(2) The hydrophilic layer is added, the printing plate is worn by friction, the hydrophilic layer is destroyed, the electrolyte in the wetting liquid reacts with the plate, and a new hydrophilic layer is formed;
(3) Decrease the surface temperature of the printing plate, the ink roller runs at a high speed, the temperature rises, the viscosity of the ink drops, the dot is enlarged, and the wetting fluid can reduce the plate surface temperature.
2.3.3 Types of wetting fluid
2.3.3.1 Ordinary wetting fluid
Common wetting fluids are mainly composed of phosphoric acid, ammonium nitrate, hydrophilic colloids, surfactants, etc. The surfactants can reduce the surface tension of the wetting fluid and improve wetting.
The general wetting fluid has good wetting effect and low cost. The general amount of surfactant is 0.1%. Too much will make the ink seriously emulsified.
2.3.3.2 Alcohol Wetting Fluid
Alcohol wetting liquid main raw material is ethanol or isopropyl alcohol, the volume in the dampening fluid is 8% -25%.
Alcohol wetting solution can reduce the surface tension of the wetting fluid, improve the film formation condition, and has a good wetting effect. With less fluid, the ink emulsification is less; the printing quality is good. Alcohol wetting fluid is volatile, unstable in concentration; high in price; flammable and explosive; special wetting devices are required and fluid compensation devices are provided.
2.3.4 pH of the wetting fluid
The pH of the wetting fluid is preferably 5.0-6.0. If the pH is too small and the acidity is large, the plate will corrode, and the ink will dry slowly. If the pH is too large, it will be alkaline, destroy the lipophilic layer, and the ink emulsification will be serious. (to be continued)
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