The effect of printing speed on printing quality in offset printing (one)
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In addition to the process and materials, the printing speed also plays a certain role. Familiarity with the relationship between them is a must for the press operator, especially in modern offset presses. Today's high-speed development is especially necessary.
1. The effect of printing speed on printing pressure
To make the ink on the plate can be clearly transferred to the surface of the substrate. There must be enough printing pressure. However, the applied printing pressure should not be too large. Excessive printing pressure will cause the substrate to continue to deform, the print becomes thicker, the dots are graded, and the paste is affected, which affects the sharpness of the graphic and at the same time increases the wear of the printing plate. The printing plate's printing durability decreased. When the printing pressure is insufficient, the printing plate and the blanket are not sufficiently reliably contacted, and the blotting becomes shallow, which affects the sharpness 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 decreased, but it was not. Since the embossing time is reduced as the printing speed is increased, the pressing time of the printed surface is reduced, and the amount of ink transferred to the surface of the substrate is also reduced, and the density is reduced. This causes the ink of the print to become lighter. In actual production, in order to compensate for such defects, a method of increasing the printing pressure is often employed.
Furthermore, in the case of the same print, the printing speed is also directly related to the size of the printing press cylinder. In order to obtain a print of the same print quality, the printing speed of a large press cylinder must be slower than the printing speed of a press smaller than the impression cylinder.
2. The effect of printing speed on paper
As a kind of substrate, paper will produce certain deformation under pressure, mainly elastic deformation and plastic deformation. In modern multi-color high-speed printing presses, the embossing interval between the two color groups is very short, only a few tenths of a second. In such a short period of time, the wet-pressed overprinted ink cannot be completely dried at all. In this process, the paper has to be wetted and embossed several times, and its surface strength will decrease. Therefore, the water absorption of the printing paper is not too strong. Otherwise, the overprint will not be caused by excessive water absorption of the paper. It is accurate, and it is also prone to paper pulling.
Practice has proved that in the case of a certain viscosity of the ink, the printing speed is the main influencing factor of whether the paper will be pulled during the printing process. In general, the faster the printing speed, the more pronounced the paper pulling phenomenon.
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 effect of changes in printing speed on the ink is enormous.
The ink is output from the ink fountain, and is transferred by the ink fountain roller, which is quickly beaten after numerous extrusions, shearing and tearing between the ink rollers, and generates a large amount of heat, which causes the ink temperature to rise and stick. Decline and increase mobility. The faster the printing speed, the more heat is generated and 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, which is the blurring, unclearness of the dots, and even the occurrence of paste, flying ink, etc., which seriously affects the quality of the printed matter.
4. The effect of printing speed on the printing plate
In one printing cycle, the plates are in contact with and roll against the water roller, the inking roller and the blanket cylinder. Obviously, the increase in printing speed will have a great impact on the printing plate, thereby accelerating the wear of the printing plate, resulting in partial loss of the dot.
In short, a good print must be equipment. A comprehensive reflection of many aspects such as technology and technology. In actual work, it is necessary to avoid frequent acceleration and deceleration. This is not only beneficial to ensure the quality of printed matter, but also an important guarantee for the good operation of the equipment.
The problem of printed corners and the "one line" view (on)
The "one line" view is the author's many years of mechanical maintenance work, after a lot of practical and theoretical research. The "one line" view emphasizes that the paper should always be in a line state at each transmission and transfer link, without deformation, to ensure the accurate printing of the printed products to the utmost extent.
Corner concept
The horns are also called the paper tail shape, which generally shows the following two phenomena.
(1) The printed paper tail sleeve is not allowed, the first color is large in the paper feeding direction, and the latter colors are gradually reduced. This overprinting problem starts from the position of the paper opening and becomes more serious as it goes to the end of the paper.
(2) Observed with the naked eye, the rule line is printed accurately, but the printed matter is discolored, the color of a batch of printed products is inconsistent, the color change of the paper tail is serious, the layer of the wire is not layered, the imaginary, the color of the single print is light and heavy, the front The text on the tail of several colors (especially the first color) is not clear. When inspected with a 20-fold magnifying glass, it can be found that the rule line on the paper tail has obvious jump and the dot is deformed.
The corner problem is a typical form of printing overprint fault. Most overprint faults are caused by the corners or accompanied by the corners. There are both mechanical and operational reasons as well as paper problems. Strictly speaking, all multi-color printing equipment has corner problems. High-quality new presses have a lighter corner problem, and lighter corners can only occur when printing thinner or ruffled paper. Some corner problems are inherent to certain models. In the 1970s and 1980s, the Heidelberg multi-color offset press, due to limitations in equipment design and manufacturing, had problems with the corners at the factory, but it was not serious. After using for a period of time, due to various factors, the problem of the corner of the machine gradually makes the operator feel a headache. Through maintenance, the problem of corners can be minimized to the extent that it is not visible to the naked eye. The domestic multi-color offset printing machine generally has a serious problem of cornering at the factory, and the provisions on the overprinting accuracy in the national standard are too loose, and are not suitable for printing high-quality printed products.
One of the reasons for the corner: the front edge of the paper is wavy when it is transferred to the first group of impression cylinders. When printing, under the common rolling of the impression cylinder and the blanket cylinder, the wave is flattened, but formed on the paper. Micro wrinkles, which are partially or completely flattened during the subsequent imprinting of individual units. Therefore, in the paper feed direction, the first color print of the print is the largest, and the print of the rear unit is gradually reduced. The change in the rule line starts from the mouth of the paper, and the change from the end to the end of the paper. This is the most typical corner problem.
The second reason is that the paper is deformed under the peeling force of the blanket during the printing process, so that the front edge of the paper is not in an ideal straight line state. Under the printing pressure of the rear unit, the wave deformation is flattened and A slight wrinkle is formed on the paper, causing a horn.
The third reason: the vertical section of the paper can not maintain the "one line" state during printing, or the surface of the impression cylinder and the longitudinal surface of the blanket surface in the pressing area cannot be kept "one line" after the blanket cylinder and the plate cylinder are pressed together. "The state, the image on the plate can not be accurately transmitted to the paper, causing the printed matter to be inaccurate and corner.
The fourth reason is: the paper is finished in the first color or a certain color after the printing process, due to the unreasonable handover time, etc., the paper is deformed at the front edge when the paper is handed over to the back impression cylinder. Being flattened and forming micro wrinkles on the paper, causing corners