How to eliminate offset "ghost"?
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In the offset printing process, the "phantom" failure that occurs on continuously printed prints is called "ghosting." In the actual production process, there are many factors that lead to the appearance of “ghosts”. The author will elaborate on this and give corresponding solutions for the reference of the industry.
"Ghost" type and formation conditions
1. Type of "ghost"
Under normal circumstances, "ghost" can be divided into mechanical "ghost" and craft "ghost". Mechanical "ghosting" is caused by mechanical action, which is mainly affected by factors such as printing pressure, ink roller, printing plate and the like. The process "ghosting" is due to chemical changes in the printing process of the ink, and such "ghosting" is greatly affected by the production environment.
According to the positive and negative images of printed images, "Ghosts" can be divided into "Ghosts" and negatives "Ghosts". The part of the printed product whose pattern color is darker is called a positive image, and the extension in the circumferential direction forms a "shadow" which is lighter than the positive image, that is, the so-called masculine "ghost", as shown in Fig. 1a; The lighter part of the product itself is called the negative image, and the extension in the circumferential direction is formed by the "shadow" which is darker than the negative image, so-called negative image "ghost", as shown in Fig. 1b.
2. Conditions for the formation of "ghosts"
Unreasonable layout design or improper adjustment of the printing press during the offset printing process is the main reason for the “ghosting” of the printed products, specifically:
(1) There is a pattern or text with a large amount of ink at the mouth of the printing plate;
(2) There is a pattern with a large amount of ink and a large area at the tip of the printing plate;
The distance between the pattern portion of the 3 plate opening direction and the solid portion is exactly a multiple of the diameter of the inking roller.
Measures to avoid "ghosting"
Offset "ghosting" is a kind of fault that can be avoided. It is important to prevent the operator from taking effective measures in the prepress design, printing machine adjustment (ink supply system, water supply system, etc.) to avoid such failures. .
Prepress design
(1) When designing the pre-press, try not to design a real map with a small area at the cornice, and design a large-format graphic later.
(2) When designing the layout, try to balance the ink volume of the entire layout, and reduce the ink volume difference between the mesh portion and the solid portion.
(3) When arranging the position of the layout image, it is necessary to avoid the relationship between the distance between the pattern portion and the solid portion in the direction of the plate opening and the diameter of the inking roller.
If you can't change the prepress design, you can start from the following aspects to avoid the appearance of "ghosting":
(1) Place the ink stick. When there is a large area of the solid area on the layout, the prepress designer can place a moving ink strip on the cut portion other than the graphic to transfer excess ink on the ink roller.
(2) Rotate the design layout by 90°. Taking Fig. 1a as an example, if the ink roller first inks the text and then injects a large area on the ground, it is prone to "ghosting". If the design layout is rotated by 90°, the ink roller is simultaneously given to the text and a large area on the ground. Ink, you can effectively avoid "ghosting."
(3) Reverse the tip and mouth. Taking Fig. 1a as an example, when the plate is printed, the mouth and the tip are reversed. When printing, the ink roller first supplies the ink to the larger area, and then supplies the ink to the smaller area to avoid the ghost.
2. Ink supply system
(1) Adjust the amount of ink supplied in the "ghost" area.
(2) Increase the amount of movement of the ink roller. If the inking roller in the ink supply system cannot guarantee the uniformity of the ink layer on the inking roller, "ghosting" may also occur. Therefore, it is possible to adopt a method of increasing the amount of movement of the ink roller to avoid the occurrence of "ghosting".
(3) Adjust the pressure. In order to avoid such "ghosting", the operator should adjust the pressure between the ink rollers in the ink supply system to an optimum state.
3. Water supply
In the offset printing process, if the water supply is too large, the ink will be heavily emulsified, which will affect the transfer of the ink, and the ink on the inking roller is difficult to be evenly mixed, thereby causing "ghosting". Therefore, in the case where the amount of ink transfer is the same, adjusting the amount of water supply can reduce the degree of ink emulsification and reduce the probability of occurrence of "ghosting" to some extent.
4. Other aspects
In the printing process, if the blanket is not tightened, the printing plate is not tightened, the roller gear is worn, etc., it is easy to cause "ghosting". In this regard, the operator should carefully check and replace the worn roller gear in time.