Dimensional stability of paper The dimensional stability of the paper refers to the performance of the paper's dimensional change when the paper is stressed or affected by the environment (temperature, humidity) and other factors. The dimensional stability of the paper is related to the elastoplasticity and hygroscopicity of the paper. (1) Elastoplasticity of paper Elasticity refers to the ability of an object to recover after being deformed by force. It is a reversible deformation. When the paper is subjected to external forces, in addition to instantaneous deformation, there is also deformation caused by the extension of time. Therefore, the elasticity of paper consists of two parts, the sensitive elasticity that recovers instantaneously after the removal of the external force and the hysteresis elasticity that gradually recovers with time. Plasticity refers to the property of permanent deformation that cannot be recovered when the applied external force is greater than the elastic limit of the paper after the force is applied to the object. It is also called reproducibility or reproducibility. Paper is mainly composed of cellulose and hemicellulose, both of which are high molecular substances and have a certain elasticity, which determines the elasticity of the paper. The fibers inside the paper interweave and the presence of air determines the paper's plasticity. At the same time, the filler in the additive will increase the plasticity of the paper, and the rubber compound will increase the elasticity of the paper. When the curved single fiber is straightened, the paper will show elastic deformation and plastic deformation; the function of the fiber interweaving point restores the paper deformation, and the distance between the single fiber and the molecule changes. Sensitive elastic deformation occurs; and the polymer hook property of the fiber changes the inter-molecular distance between the fiber surfaces, resulting in hysteresis elastic deformation: at the same time, the molecules between the individual fibers are displaced, and the surface binding force is destroyed, resulting in plastic deformation. Therefore, the Wangyi standard is maintained during papermaking, so that the paper produces uniform and stable elastoplastic deformation in both tension and compression, which can ensure stable printing tension and accurate overprinting. The size of die-cut finished products is consistent. The paper is subjected to a combination of pressure and tension during the printing process. The paper will produce elastoplastic deformation after being subjected to printing pressure. This deformation has little effect on overprinting, and can still maintain the consistency, balance and stability of all imprinting unit pressures and printing overprinting. At the same time, the paper is also subjected to a pulling force during printing, which is elongated in the circumferential direction and produces a corresponding deformation in the axial direction. The paper has been under tension during printing. In order to make the two-color overprinting accurate, the method of enlarging the plate diameter should be used on the plate making. The diameter of the cylinder of the latter color plate should be larger than that of the previous color, so as to ensure the stability of the printing tension . However, the elastoplastic deformation of the paper is in a non-linear relationship with the tension. The initial deformation is large, and the deformation gradually decreases to a certain degree. Therefore, the method of uniformly increasing the diameter of the plate cylinder cannot completely solve the effect of elastoplastic deformation of paper on overprinting. . In addition, this non-linear relationship will also affect the stability of the printing tension. After the tension is released, due to the shrinkage of the paper, the size of the finished product die-cut has a certain deviation from the standard size, which causes problems such as inaccurate overprint and fluctuation of overprint printed to the die-cut. In summary, the elastoplastic deformation of paper will have a certain effect on the accuracy of overprinting. Adjusting the printing tension and other processes can only reduce the error of overprinting relatively, and cannot fundamentally solve the problem of overprint fluctuations. (2) Hygroscopicity of paper Paper is hygroscopic and is a factor that cannot be ignored due to inaccurate overprinting. The paper has hygroscopicity mainly for the following two reasons: one is that the paper has capillary action, because the paper is made of interlaced fibers, there are pores between the fibers, and the fibers themselves have submicroscopic pores; second, the plant fibers are hydrophilic Both cellulose and bovine cellulose contain hydroxide (-OH), and the free -OH has strong water absorption, and hydrogen bonds can also be formed between adjacent cellulose molecules. In the case of a certain temperature, the relationship between the moisture content of the paper and the relative humidity of the air. In the high and low positions of relative humidity, the paper moisture content caused by the relative humidity changes greatly, so the paper storage environment and use environment should be as consistent as possible, and kept in the middle humidity range, so that the impact of humidity on the paper moisture content changes To a minimum. In the case of a certain humidity, the temperature change and the water content change have a linear relationship. The temperature increases and the water content decreases. Therefore, it is also very important to maintain the consistency of the ambient temperature. The process of paper moisture absorption and dehumidification will cause the paper size to change, causing overprint errors. In the gravure printing process, the oven is used for drying, and the paper size after dehumidification will be relatively reduced, resulting in inaccurate two-color overprint in the circumferential direction and the axial direction. The absorption and dehydration of single-molecule fibers are mainly manifested in the change of fiber width. The axial overprint is not allowed to be larger than the circumferential overprint. This is also the main reason for the film's accuracy and the deviation of printed products, especially the wider paper width. Products with higher overprint accuracy or more color numbers are more likely to appear. By maintaining the consistency of the paper roll with the storage and production environment, ensuring a certain storage time, choosing a medium humidity operating environment, reasonably arranging the color sequence, taking pre-shrinkage and other measures, fully considering the effect of size reduction on color registration after dehumidification, you can change Overprint error is minimized. 4. Internal structure of paper Changes in the papermaking process, such as changes in the pulp-web speed ratio (pulp-web speed ratio refers to the ratio of pulp flow speed to net-flow speed), changes in pulp types, and changes in the proportion of fillers, all affect the internal structure of the paper. Overprint fluctuations caused by these changes are difficult problems for all printing companies to solve. Standardizing and standardizing papermaking operations can minimize the impact. 5. Paper roll shape During the transportation process, the paper roll is easily deformed and the concentricity is poor due to the impact of the transportation tool, which causes unstable printing tension during unwinding and intensified overprint fluctuations. Paper rolls with severe deformation are difficult to print products with high overprint accuracy. Therefore, the concentricity of the paper rolls should be kept as much as possible during transportation. In addition, the paper core is the basis for the overall shape of the roll of paper, which plays an important role in the quality of printing overprint. First of all, the size of the paper core must meet the requirements, otherwise it will cause slippage during rewinding or unwinding; secondly, the moisture of the paper core should not be too high, otherwise the outer diameter of the paper core will shrink after dehumidification, and the unwinding will cause looseness, which will also cause tension Instability and intensified volatility of overprinting. Process factors Unreasonable process will also lead to fluctuations in overprinting. In fact, each process in the gravure platemaking process has an impact on the stability and accuracy of overprinting. Only some of the main factors will be discussed below. The plate blank is the basic material for making the printing plate, and its mechanical properties determine the mechanical properties of the finished printing plate. Therefore, it is required that the surface of the plate blank is flat, the concentricity is good, the dynamic and static balance is good, and the plate shaft meets the machining accuracy. The poor concentricity of the printing plate cylinder and uneven surface will cause the surface linear velocity of a certain part of the printing plate to be different from other normal areas, which will cause the overprint to change and produce the problem of overprint fluctuation. If there is a problem with the dynamic and static balance, the impulse change of the rotating plate cylinder will occur, which will aggravate the wear of the mechanical parts. The worn mechanical parts will lead to more serious overprint fluctuation problems. If the plate axis does not meet the machining accuracy, it will also cause overprint fluctuations and other printing failures. If the plate axis is too large, it will bring difficulties to the plate installation work and damage the components; if the plate axis is too small, the plate cylinder will vibrate under the action of printing pressure. (2) Turning Car milling is to process the plate blank coated with copper layer according to the requirements of the plate diameter. The following two points are required: ①The axial diameter of the printing plate after turning is kept the same, otherwise it will cause the inconsistent linear speed of the surface of the finished printing plate, resulting in the fluctuation of overprinting. ②The copper surface after grinding needs to be smooth and flat, the purpose is to keep the surface linear velocity consistent and facilitate the next process such as electric carving and chrome plating. Lathe mills have high requirements for processing precision. According to the different tensions used in paper and printing, different printing manufacturers use a color-by-color incremental plate diameter (20μm ～ 50μm) method to ensure the accuracy of overprinting. (3) Electric carving (corrosion), chrome plating, grinding For electric engraving (corrosion) to make gravure, you should choose a suitable engraving knife, engraving angle and screen line number, etc., and handle the edges of the pattern (especially at the color scale block). Chrome plating and grinding must also meet the printing requirements, to avoid printing faults such as virtual edges, ink drawing, and uneven edges at the overprint color block during printing. These faults will cause errors in the scanning signal and eventually lead to overprint fluctuations. 2. The effect of printing process on overprint Printing process design mainly includes color sequence arrangement, ink viscosity control, printing speed control, color density value control and some product quality control. In the process design, we must combine the actual situation of production, comprehensively consider the impact of quality control standards on overprinting, especially the impact of printing pressure control on overprinting. Because if the printing plate or the printing material does not meet the requirements, excessive printing pressure will be used, which will increase the fluctuation of overprinting. Therefore, reasonable and stable printing process conditions are an important factor to avoid overprint fluctuations. Operating factors The operations that affect overprinting can be divided into two parts: operations in preparation work and operations in printing production. 1. Operations in preparation The operations in the preparatory work mainly include the work of installing the plate, installing the squeegee, and setting the machine parameters. For the problem of overprint fluctuations, the following matters should be paid attention to in these tasks: (1) When installing the plate, pay attention to check whether the plate shaft meets the size requirements, and take corresponding measures to deal with the size of the plate shaft. (2) Pay attention to check the dynamic and static balance of the printing plate. (3) Pay attention to check and keep the transmission parts clean, lubricated, and damaged parts should be repaired in time. replace. (4) Pay attention to keep the positioning points and surfaces clean and in place. (5) All components are installed in place. (6) Adjust the horizontal position of the ink cart. (7) The squeegee is installed flat and straight. According to the conditions of the printing plate and ink, select the appropriate hard and soft blades, sharpen the blade, and avoid the faults such as ink drawing and virtual edges caused by the bad knife and affecting the overprint signal. (8) Set accurate and appropriate parameters. 2. Operations in print production In the process of printing production, we should pay attention to the following matters: (1) Select the appropriate tension and appropriate controllable parameters according to different papers. Implement standardized and standardized operations to minimize overprint fluctuations. At the same time, the operator is required to make accurate judgments and take timely measures to solve the problem after the fault occurs. The degree of resolution of overprint fluctuations can reflect the overall management and technical level of a company. Standardization, standardization, and dataization are the development direction of the printing industry and the basis for solving the problem of overprint fluctuations. Glass Dab Rigs,Big Eyes Glass Dab Rigs,Hand Painted Bong,Hand Painted Glass Dab Rigs ShanXi Hippies Glass Tech. Co., Ltd. , https://www.hippiesglass.com
1. The Influence of Plate-making Process on Overprint
(1) Quality of plate blank
(2) Keep proper during production. Stable printing speed.
(3) Control the printing pressure.
(4) Control the viscosity and performance of the ink to reduce the occurrence of printing problems.