What are the details of the process that must be mastered in offset printing?
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Due to the physical and chemical aspects of the offset printing process, there are high requirements for mastering the process and technology. Due to the problems of the process and technical level of the management personnel and operators, the offset failure is also more.
01. Offset characteristics
Offset printing is performed using the principle of oil and water repulsion, on the same plate. The graphic part is lipophilic and hydrophobic, and the blank part is hydrophilic and oleophobic. This is the most obvious difference between offset printing and other printing, and it is also the key to offset printing technology.
In the case where both ink and water are present on the printing plate, the ink roller that transports the ink is in contact with water. Due to the high-speed rotation of the ink roller, water is mixed into the ink under the action of mechanical force, causing ink emulsification and normal printing. A reasonable emulsification value is reached. Therefore, offset printing actually utilizes the principle of oil and water repelling, and utilizes the characteristics of oil and water to dissolve and emulsify.
According to foreign experimental data, when offset printing is generally normal printing, the reasonable emulsification value of ink, water accounts for about 20 to 25%. There is no instrument measurement in China, which is determined by the operator's technology and experience. This includes unstable factors. , which increases the possibility of failure.
The water used in offset printing is not pure water. The PVA version contains phosphorus and chromic acid in water, which is very corrosive to the layout. It not only corrodes the blank part of the blank part of the layout, but also continuously accumulates the salt layer of the machine while corroding the graphic part. The base paint reduces the printing durability of the printing plate. The wetting powder added to the PS version contains more than ten chemical components. It is a slightly acidic wetting agent with a cleaning effect and is extremely corrosive to the printing plate.
02. Printing pressure
Printing pressure is also a major foundation of process technology. Because the offset printing is indirect printing, the image is transferred from the plate cylinder to the blanket for the first imprinting. After the blanket cylinder receives the ink, the imprint is transferred to the substrate. This is the second imprinting and two pressing. The pressure required for printing is different. How to achieve the "ideal pressure" of the realm, we must make some efforts.
Generally, the pressure between the offset printing press and the plate cylinder and the blanket cylinder is about 0.10 mm; the pressure between the blanket cylinder and the impression cylinder is about 0.20 to 0.25 mm. With the use of a better flatness PS plate and imported air cushion blanket, the pressure between the plate cylinder and the blanket cylinder can be smaller. For example, the imported Heidelberg four-color machine can be printed at 0.05 mm.
The pressure used in offset printing is achieved by calculating the radius of the drum, measuring the center distance, measuring and increasing or decreasing the number of linings. The offset pressure must be standardized and controlled by data. The liner should not be added arbitrarily, the pressure will be increased blindly, and the pressure will suddenly change. The line speed of the roller surface will be different, the friction of the contact surface will increase, and even greasy, ghosting and ink will appear. Other faults such as bars.
Therefore, the correct use and adjustment of printing pressure is an important part of the process technology required to print products.
03. Imprinting is not allowed
Imprinting is not allowed to be classified into vertical overprinting and horizontal overprinting. There is a saying that "the paper is always firmly controlled by the organic parts during the transmission process, and the overprinting can be guaranteed." In fact, this sentence is not comprehensive.
For example, the embossed biting shaft or bushing wear is strong, but it can be firmly controlled when transferring paper, but it cannot be properly controlled. This is like a person holding a ball in both hands, the two arms can be stretched straight, and the two arms can be bent, but the balls held in two different positions are different from the distance and stability of the human body. . Therefore, it should be said that "the correct control is in place to ensure accurate overprinting."
There are many reasons for inaccurate overprinting. Common ones are: paper stretching, paper feeding, inaccurate positioning, insufficient biting force, wear of the biting shaft, unsatisfactory handover, and wear of the transmission teeth.
In addition, some special factors may also cause inaccurate set-ups. For example, if the rubber layout is deformed, the image may be partially overprinted; the transfer shaft and the impression cylinder shaft may be axially tilted, which may cause lateral overprinting. .
04. Color order arrangement
In the past, the color sequence of the monochrome machine was always yellow, red, blue, and ink. This color sequence was arranged. The larger difficulty was to sign the yellow and red prints, especially the first color. Edition, related to the color reproduction and reproduction of printed matter.
If the yellow print is too deep, the color of the printed person has no sense of vitality, and the house is as old as the old temple, which makes it difficult to accept; if the yellow print is too light, the copied picture is dry and the color is biased toward blue-violet. Also not good-looking.
There are two main reasons for the yellowing of the yellow version: one is the retinal color-resolving cells of the human eye, which has the lowest resolution to yellow; the other is that people do not understand the oil absorption of paper, lack of instrument detection, and the same size. It often causes too deep and too weak.
The color sequence arrangement of the two-color machine can put the yellow plate and the blue plate in the same printing process, and the blue plate just plays the role of blue glass color filter, which is convenient for identifying whether the thickness of the yellow plate ink layer is suitable. However, due to the wet-stack printing, the two-color machine often produces irregular A and B ghost images on the two corners of the trailer. This requires that the color sequence be appropriately changed during the process arrangement.
Multi-color offset printing machine with more than four colors, because the three colors are wet and wet printing, the color sequence is arranged according to the viscosity of each color ink from high to low, otherwise the overprinting failure will occur, and the dark color of the ink will appear. Sometimes the first few colors will also produce A and B ghosts, and the color sequence should be adjusted as necessary.