In addition to the process and materials, the printing speed also plays a role in affecting the printing quality. Familiarity with the relationship between them is a must for printing press operators, especially in modern offset presses. High-speed development is particularly necessary today.
1. The effect of printing speed on printing pressure
To make the graphics coated with ink on the printing plate can be clearly transferred to the surface of the substrate. There must be sufficient printing pressure. However, the applied printing pressure should not be too large. Excessive printing pressure will cause the substrate to continue to deform, the prints will become thicker, the dots will be graded, and the paste version will affect the clarity of the graphic. At the same time, it will increase the wear of the printing plate and make The printing plate's durability is reduced. When the printing pressure is insufficient, there is insufficient and reliable contact between the printing plate and the blanket, the imprint will become lighter, affecting the clarity of the graphic.
As the printing speed increases, the ink color of the print becomes lighter. At first, it was thought that the printing speed increased, and the printing pressure was reduced, but it was not. Because when the printing speed increases, the imprint time decreases, the pressure on the surface of the printed product decreases, the amount of ink transferred to the surface of the substrate will also decrease, and the density will decrease. As a result, the ink color of the print becomes lighter. In actual production, in order to make up for such defects, methods of increasing printing pressure are often adopted.
In addition, in the case of the same print, the printing speed is also directly related to the size of the impression cylinder of the printing press. In order to obtain the same print quality imprint, the printing speed of a printing machine with a large impression cylinder must be slower than that of a printing machine with a small impression cylinder.
2. Effect of printing speed on paper
As a kind of printing material, paper will produce certain deformation under pressure, mainly elastic deformation and plastic deformation. In modern multi-color high-speed printing presses, the stamping interval between two color groups is very short, only a few tenths of a second. In such a short period of time, the wet press wet overprinting ink cannot be completely dried at all. In this process, the paper undergoes several wetting and embossing, and its surface strength will decrease. Therefore, the water absorption of the printing paper should not be too strong, otherwise, not only will the overprinting be caused by the paper absorbing too much water It is accurate, and paper fuzzing is also very easy.
Practice has proved that under the condition of a certain ink viscosity, the printing speed is the main influencing factor of the paper during the printing process. Under normal circumstances, the faster the printing speed, the more obvious the paper is rough.
3. The effect of printing speed on ink transfer
Fundamentally speaking, the printing process is the process of ink transfer, the ink is mainly from the ink fountain → ink fountain roller → ink roller → printing plate → blanket → substrate surface. In this process, the impact of changes in printing speed on the ink is huge.
The ink is output from the ink fountain, passed through the ink fountain roller, after being squeezed, sheared and torn between the ink rollers, it is quickly smoothed, and at the same time generates a lot of heat, which heats up the ink temperature and viscosity Decline and enhanced liquidity. The faster the printing speed, the more heat generated, the higher the ink temperature. If the generated heat cannot be taken away in time, then the ink at too high temperature is reflected on the printed matter because the dots are blurry and unclear, and even scumming and flying ink occur, which seriously affects the print quality.
4. The effect of printing speed on the printing plate
In a printing cycle, the printing plate must be in contact with the water roller, the inking roller and the rubber cylinder, and the roller. Obviously, the increase in printing speed will have a great impact on the printing plate, thereby accelerating the wear of the printing plate and causing the dots to be partially lost.
In short, a good print must be equipment. Comprehensive manifestation of many aspects such as technology and technology. In actual work, it is necessary to avoid frequent acceleration and deceleration as much as possible. This not only helps to ensure the quality of the printed matter, but also an important guarantee for the good operation of the equipment.
The problem of dithering of printed matter and "one line" view (Part 1)
2006 at 9:12 on June 29 Source: Printing Machinery Co., Ltd. Qingdao developed Author: Ning Ronghua
The "one line" viewpoint was summarized by the author after years of practical and theoretical research in mechanical maintenance work for many years. The "one line" view emphasizes that the front edge of the paper should always be in a line state at every transmission and handover link, without distortion, to ensure the accuracy of printed products to the greatest extent.
Concept of flick angle
The flick angle is also called the tail of the paper, and generally shows the following two phenomena.
(1) The end cover of the printed paper is inaccurate, the first color is larger in the paper feeding direction, and the following colors gradually decrease. This type of overprinting problem starts from the position of the paper gripper, and the situation becomes more serious as it reaches the end of the paper.
(2) Observation with the naked eye, the ruled line overprint is accurate, but the printed products are discolored, the color of a batch of printed products is inconsistent, the tail of the paper is more discolored, there is no layering in the network line, the hair is false, and the color of the single printed product is lighter before the front Several colors (especially the first color) are not clear on the end of the paper. When inspected with a 20x magnifying glass, it can be found that the rule line on the tail of the paper jumps obviously, and the dots are deformed.
The flicker problem is a typical manifestation of printing overprint failure. Most overprint failures are caused by or accompanied by flicker angle. There are both mechanical and operational reasons and paper problems. Strictly speaking, all multi-color printing equipment has the problem of flick angle. The high-quality new printing presses have less problem with flicking angles, and the lighter flicking phenomenon only occurs when printing thinner or ruffled paper. Some swing angle problems are inherent in certain models. Heidelberg multi-color offset presses in the 1970s and 1980s, due to limitations in equipment design and manufacturing, had the problem of flicking at the factory, but it was not serious. After being used for a period of time, due to the influence of various factors, the machine's angle swing problem gradually makes the operator feel headache. The maintenance can minimize the angle swing problem to the extent that it is not visible to the naked eye. The domestic multi-color offset printing machine generally has a more severe angle swing problem, and the national standards on overprint accuracy are too loose and are not suitable for printing high-quality prints.
One of the reasons for the flick angle: the front edge of the paper is wavy when it is transferred to the first set of impression cylinders. During printing, the wave shape is rolled flat under the joint rolling of the impression cylinder and the rubber cylinder, but it is formed on the paper Micro-wrinkle, this micro-wrinkle is partially or completely crushed in the subsequent imprinting process of each unit. Therefore, in the paper feeding direction, the first color print of the printed product is the largest, and the print of the subsequent units gradually decreases. The change of the rule line starts from the mouth of the paper, and the greater the change to the end of the paper. This is the most typical angle swing problem.
The second reason for the flick angle: during the printing process, the paper is deformed by the peeling force of the blanket, so that the front edge of the paper is not an ideal straight line state. Under the printing pressure of the rear unit, this wave deformation is flattened and Micro-wrinkles are formed on the paper, causing flicking.
The third reason for the flick angle: during printing, the longitudinal section of the paper cannot maintain the "one line" state, or the longitudinal section of the surface of the impression cylinder and the blanket surface in the combined pressure area cannot be maintained "one line" after the rubber cylinder and the plate cylinder are pressed together "In the state, the image on the printing plate cannot be accurately transferred to the paper, resulting in inaccurate overprinting and flicking of the printed product.
The fourth reason for the flick angle: during the transmission of the first color or a certain color of the paper, due to the unreasonable transfer time, the front edge of the paper is deformed when it is transferred to the rear impression cylinder. This wavy deformation Is flattened and forms micro-wrinkles on the paper, causing a flick angle
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