Parameter Analysis Affecting Offset Quality
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The advantages and disadvantages of offset printing products, in addition to the quality of printing equipment and printing raw materials, also depend on the technical level of the printing operator. The level of technology is mainly reflected in the understanding of the relationship between process technology parameters and printing quality, and The selection and mastery of process technical parameters, the main technical parameters of offset printing, printing pressure, overprinting accuracy, ink supply, water supply impact on printing quality, depends on whether the process parameters are ideal. The best process parameters can be used to obtain high-quality prints; for unsuitable process parameters, only general prints or even unqualified prints can be obtained.
The offset process parameters that affect the quality of the offset include the offset working conditions and the offset process parameters.
First, the working conditions of the offset press
The type, parameters and performance of the offset press will have a large impact on the quality of the offset. Whether the offset printing machine runs at high speed, high efficiency, accurate overprinting and uniform ink color are important factors affecting the quality of offset printing products. In the printing process, the position and time of the paper in the transmission, delivery and printing are strictly required, so the transmission and assembly precision between the various devices of the printing machine have high requirements.
All offset presses have five basic modules: paper feed system, dampening system, inking system, imprinting system, and delivery unit. The working conditions of the printing unit such as the construction of the printing unit, the paper feeding route, the transfer system, the inking system, the wetting system, etc., have an impact on the quality of the offset printing. Among them, the working conditions of the offset printing machine which have the greatest influence on the quality of the offset printing are the paper feeding device and The performance of the imprinting device.
1. Paper feeding device
The paper is positioned to ensure that the graphic is in a certain position on the upper part of the print each time it is printed. This is required for both color printing and monochrome printing. For the color printing discussed herein, the color of the print generally consists of four colors of cyan, magenta, yellow, black, or more. For each of the color printing, the corresponding ink needs to be transferred to a strictly defined position on the surface of the substrate by the printing device, thereby ensuring that the inks of the respective colors are accurately overlapped together to achieve accurate overprinting, so as to realistically reproduce the rich color of the original. Therefore, the role of the paper positioning device on the printing press is very important. Generally, the longitudinal positioning mechanism is called the front rule (referred to as the front gauge), and the lateral positioning mechanism is called the side rule (referred to as the side gauge). The performance, accuracy value and adjustment of the positioning device not only have a decisive influence on the quality of the printed product, but also have a close relationship with the level of labor productivity.
In order to facilitate the inspection of the overprinting accuracy of the printing, two cross lines for overprinting are generally printed at the same position around the middle of the left and right sides of the printed sheet. When printing, the overlapping and adjusting of the crosshair offset and the yaw angle are checked and adjusted. In the case, the registration of printed graphics can be guaranteed.
2. Imprinting device and printing pressure
The main body of the offset printing press is a printing device, also known as an imprinting device. Its structural performance and manufacturing precision are directly related to the quality of the printed matter and the efficiency of production. At the same time, it is the benchmark for the adjustment of all relevant components. Regardless of the offset press, the stamping unit follows the three-roller principle: It mainly consists of a plate cylinder, a blanket cylinder and an impression cylinder. The main functions of the plate cylinder are:
1 Load the plate and adjust the position of the plate.
2 The length of the print can be changed.
3 Change the contact pressure between the plate cylinder and the blanket cylinder. The blanket cylinder is wrapped with a thick, elastic blanket on the outer surface of the cylinder to transfer the print from the plate. The role of the blanket cylinder is:
1 transfer ink;
2 to make up for the comprehensive error of the machine;
3 buffer absorption.
The impression cylinder is the most important one in the offset printing machine. Its surface has high processing precision, acid and alkali resistance and corrosion resistance. Its simplified body also has high rigidity and strength. It can resist shock and vibration. It is equipped with a caries mechanism. The main function of the impression cylinder is:
1 Support the paper and finish printing;
2 transfer paper;
3 In modern offset presses, the impression cylinder is the basis for adjustment of the maneuvering relationship of other parts.
It should be specially pointed out that the printing pressure is obtained by the elastic force generated by the compression deformation of the blanket on the blanket cylinder. Plate cylinders and impression cylinders are hard surfaces that are not easily deformed after application of pressure. Therefore, during the printing process, the blanket cylinder and the other two rollers are not in a straight line, but a curved surface with a constant width. The maximum compression deformation value of the blanket is usually called the printing pressure in production. In fact, the maximum compression (deformation value) is the sum of the radii of the two-phase cylinders plus the difference between the thickness of the paper, the liner, and the actual center distance of the two rollers.
At the moment when the ink contacts the paper, under the certain pressure between the rollers, the binder molecules are forcibly pressed into the fiber gap of the paper surface, and then the gap capillary phenomenon in the paper layer is slowly and freely permeated. Therefore, the pressure is an important condition for the binder to be dry and permeable on the paper surface. When the printing pressure exceeds the certain value, the printing ink pressure begins to spread to the blank portion due to the excessive printing pressure, and the ink transfer rate begins to decrease. The printed dot prints are severely enlarged, and the image is obviously distorted. Therefore, printing pressures exceeding a certain value are not suitable for printing.
Second, the process parameters of the offset printing process
The printing process parameters commonly used in offset printing are: ink hue, ink temperature, ink drying speed, paper thickness, paper smoothness, paper surface efficiency, paper relative moisture content, paper surface strength, paper p H value, fountain solution p H value, blanket hardness, blanket thickness, ambient temperature, ambient relative humidity, dryer temperature, drying device stop time, chill roll temperature, printing speed, surface hardness of the ink roller and water roller, thickness of the liner, pressure between the rollers , plate wear and so on.
In the actual offset printing production, it is not necessary or impossible to adjust and control all the above parameters. The parameters such as dampening solution, blanket, ink roller, water roller and lining are generally not adjusted and changed during the production process, and can be used as a system. Pre-set conditions. From the perspective of offset quality control, the parameters of the control adjustment required for the offset printing process mainly include ink control, printing parameters, proofing, proofs, plates, ink grades and ink types, ink sample files, layouts, substrates, and paper feeding methods. Wait.
Ink Control: Actually called Color Control (Colorant C ontrol), because the use of colorants instead of ink is more versatile, controlling how to process PS pages with colorants, it contains attributes or units that describe the embedded in the PS page. The color information is converted to the details of the device colorant page, such as device color space type, print color sequence, and the like.
Printing parameters: Define the units or attributes required for the traditional printing process, including whether direct proofing, ink drying methods, printing color sequence, fountain solution properties, paper position, number of available color groups (printing units), and color group drying Method, color group order list, double-sided printing color group use order, printer type, cornice position, maximum printing speed of the printing machine, paper turning method, glazing type, and the like.
Proof of proof: The proofs provided by the prepress department are the basis for the control of the printing process. There must be a signal strip on the proof. Samples: Samples from previous printing processes are used as the basis for the current printing process.
Plate: Most of the plates used in modern offset printing are P S versions. The P S version is based on the polymer photosensitive layer as a graphic text, and the printed plate image is slightly higher than the blank portion. The hydrophilic portion is a film of aluminum oxide. Waterless offset printing uses a unique pre-coated photographic offset printing plate. The blank portion is a silica gel that repels ink. The image portion is a layer of slightly concave polymer, and the surface of the plate is resistant to oxidation. The quality of the P S version directly affects the quality of the print.
Ink: The color characteristics of the three primary colors of ink directly affect the degree of vivid color of the printed matter. Different types of three primary color inks have different color shift, gray scale and efficiency, so the color of the ink is different. Therefore, when printing fine products , should use the same type of colorful inks of the same manufacturer. In addition, during the printing process, try not to add too much auxiliary agent to the original ink to make the ink emulsification or saturation decrease. When changing colors, if the previous color ink is not washed, the color shift of the latter color will increase, and the gray level will increase, thus affecting the color reproduction of the printed matter. Ink zone sample information: Defines the ink zone setting parameters for the printing device, regardless of the device details, including the ink zone width of the printing machine in the paper feed direction, and the ink amount required in the horizontal direction of the ink zone (specifying the amount of ink required for each ink zone) , the value is between 0 and 1, and 1 is 1 0 0 %).
Layout: Represents the layout structure for layout printing, including layout name, whether to automatically execute the layout result, the maximum number of documents referenced during execution, the maximum number of documents consumed, whether to insert additional sheets, how to form signatures and layout units Attributes, etc. Among them, the layout unit includes the cutting method, the page object details, the bleeding attribute, whether the page has been trapped, and the like.
Printing materials: Printing materials are the basis for the bright colors of printed materials. If the whiteness of the substrate is high, the brightness of the color can be improved. Conversely, the gray substrate is like adding a small amount of black to each color of ink, showing a dull color. In addition, the amount of substrate absorbency will affect the saturation of the color. For a highly absorbent substrate, the imprint does not form a sufficiently full ink layer on the surface, and the binder excessively penetrates into the substrate to dry the blot. A paper with a large capillary hole causes some pigment to penetrate into the paper together with the binder, thereby lowering the color saturation and affecting the vividness of the color.