Color platemaking is a process of color decomposition, and color printing is a process of combination of colors. This color combination process is called overprinting. One of the most important quality requirements for color printing is that the overprinting is accurate, that is, each monochrome graphic is superimposed on the corresponding identical position in order, and finally the batch of printed material becomes a faithful copy of the original. However, in the production process, due to various reasons, it is easy to cause product overprinting errors to exceed the standard, so that a large number of products therefore become inferior or waste products, so that enterprises suffer a lot of losses. Therefore, the vast number of printing workers make unremitting efforts to this end, summed up some valuable experience. In order to better communicate and learn, the issue of printing overlay accuracy is discussed here.
First, the factors that affect the accuracy of overprinting There are many factors that affect overprinting accuracy. In addition to the film errors caused by the factors of platemaking, several major aspects of printing are described as follows:
1. Effect of Printing Plates on Overlay Accuracy Lithographic printing plates are now mostly thin and light aluminum. In the printing process, overprinting accuracy may be affected by changes in the shape or volume of the warp.
The deformation of the printing plate is generally caused by two aspects. The first is stretching and deformation, and the second is baking deformation. Stretch deformation means that during the school printing process, due to the operator's improper use of force during plate loading and printing, artificially causing the metal plate to extend (stretch) in the circumferential direction of the platen, resulting in overprinting of the plate graphic and text. Baking deformation means that the linear dimension of the printing plate changes due to improper baking in order to improve the printing durability of the PS plate that has been dried. As we all know, most of the metals have the property of thermal expansion. When the printing plate is heated to 250°C in the oven, the plate base will be baked softly, which will affect the stability of the layout size. The uneven temperature in the oven or overbaking is particularly serious, and it can cause scrappage in severe cases.
2. The impact of the roller lining on the overprint accuracy can only ensure that the linear speeds of the printing plate, the eraser and the embossing roller are the same when the radius of the lining is equal, so that the transfer can be effectively performed under a certain pressure. If the plate cylinder liner is too thick, the radius of the cylinder increases accordingly. With the same angular velocity, the linear speed of the drum surface with an increased radius will increase. The resulting speed difference will not only cause the printing plate to wear, but also cause the size of the layout image to be “compressed†in the front-rear direction. The drum liner is too thin and the radius is reduced accordingly. The linear speed of the drum surface will be slow, and the dots will be stretched during the transfer process, which will cause the graphic area to “magnifyâ€, resulting in inaccurate overprinting. On the two-color machine, this phenomenon is particularly evident.
3. Effect of paper on overprint accuracy In offset printing, paper that affects printing more is paper, especially offset paper. This type of overprint due to paper is not easy to control.
The composition of paper is mainly fiber. The characteristics of fibers are water swelling and syneresis. In the offset printing process, when the first color is printed and then the second color is set, the overprint due to the retraction of the paper is inaccurate. Paper structure has high strength, good smoothness, thick coating, and low water absorption. It is particularly serious for some papers with loose structure, light sizing and strong water absorption.
In addition to the paper itself being affected by changes in temperature and humidity in the surrounding environment, the paper's stretching deformation is also squeezed by the rollers during the printing process. Extruding makes the paper expand outward from the mouth of the mouth, and the more the two sides of the tip, the greater the overlay error. This phenomenon is often referred to as "sweep angle."
4. Mechanical influence on overprinting The printed paper must be effectively controlled by the mechanical components throughout the machine, and the relative position must be stable during the transfer and transfer process. Only in this way can the requirements for overprinting accuracy be met, otherwise overprinting accuracy will be greatly affected. The most influential factors are the following: First, the paper positioning, that is, the problem of the mechanical parts of the rules caused by overprinting is not allowed; Second, the paper is in the transfer and transfer process, due to the transfer of instability, resulting in changes in the relative position of the paper caused Imprinting is not allowed.
In addition, when the paper is peeled off from the drum, it is subjected to adhesion force and peeling tension. If the gripping force cannot overcome the peeling force, paper whip occurs and a small displacement occurs. This phenomenon appears to be misprinted on a monochrome machine and appears as a ghost image on a multicolor machine.
Second, the overprint accuracy control method For the impact of overprinting accuracy factors, take the appropriate control methods to achieve the purpose of overprinting accuracy.
1. For the roasting treatment of the printing plate, the temperature in the oven must be even, so that the plate is heated evenly, the temperature should not be too high, and the time should not be too long. Short version or product with high precision is best not to bake.
It is necessary to standardize the operation of the school edition. The layout of the plate is centered, the plate is mounted on the plate, and the plate screws are fastened. When you pull it, tighten the screws. As long as the force is moderate, deformation of the plate is generally avoided.
2. The center distance of the rollers is strictly measured according to the standard, and it must be done that the roller axis lines are horizontal and the lining calculation is accurate. After a good lining, do a stress test. Generally use "play screen", "draw map", "pressure bar", etc., to ensure that "pressure flat" to meet the printing accuracy requirements.
3. It is difficult to solve the effect of paper on overlay accuracy, and there are many related factors. In order to improve the printability of the paper and enhance its stability, the temperature and humidity of the printing environment should be kept relatively constant. At the same time, according to the "hysteresis effect" of the paper, prepress processing of the paper is done. The commonly used method is hanging and the paper is hung by hanging, so that its moisture content is relatively balanced with the air humidity, which can reduce the sensitivity of the paper to the environment.
For high-precision prints, it is also possible to use a method in which water is printed on the paper first, so that the paper can absorb water and be pressed sufficiently to deform the paper before overprinting. This is better for "slewing angle" and expansion, but at a higher cost.
4. The printing process requires the use of minimal water in the case of guaranteed non-dry version.
5. Mechanical overprinting is inaccurate, mainly rule components and tooth decay. The requirements of the pre-gauge are: According to the thickness of the paper, adjust the height position correctly, and the connection line of the front regulation paper point is parallel to the axis of the drum; the requirements of the pull gauge are: pull the paper reliably, moderate tension, and block the axis of the paperboard and the drum Vertically. Each paper and paper feeder must ensure that the paper runs straight. The grippers must have uniform gripping force and accurate closing time. When the two teeth are handed over to each other, they must have enough time for mutual annihilation to ensure effective control of the paper. In addition, the rollers and the shaft must be free from turbulence.
Satisfy the above requirements, under normal circumstances can improve the overlay accuracy, achieve control purposes. (Shandong Xinhua Printing Factory Linwei Plant Miaowei Xi)
First, the factors that affect the accuracy of overprinting There are many factors that affect overprinting accuracy. In addition to the film errors caused by the factors of platemaking, several major aspects of printing are described as follows:
1. Effect of Printing Plates on Overlay Accuracy Lithographic printing plates are now mostly thin and light aluminum. In the printing process, overprinting accuracy may be affected by changes in the shape or volume of the warp.
The deformation of the printing plate is generally caused by two aspects. The first is stretching and deformation, and the second is baking deformation. Stretch deformation means that during the school printing process, due to the operator's improper use of force during plate loading and printing, artificially causing the metal plate to extend (stretch) in the circumferential direction of the platen, resulting in overprinting of the plate graphic and text. Baking deformation means that the linear dimension of the printing plate changes due to improper baking in order to improve the printing durability of the PS plate that has been dried. As we all know, most of the metals have the property of thermal expansion. When the printing plate is heated to 250°C in the oven, the plate base will be baked softly, which will affect the stability of the layout size. The uneven temperature in the oven or overbaking is particularly serious, and it can cause scrappage in severe cases.
2. The impact of the roller lining on the overprint accuracy can only ensure that the linear speeds of the printing plate, the eraser and the embossing roller are the same when the radius of the lining is equal, so that the transfer can be effectively performed under a certain pressure. If the plate cylinder liner is too thick, the radius of the cylinder increases accordingly. With the same angular velocity, the linear speed of the drum surface with an increased radius will increase. The resulting speed difference will not only cause the printing plate to wear, but also cause the size of the layout image to be “compressed†in the front-rear direction. The drum liner is too thin and the radius is reduced accordingly. The linear speed of the drum surface will be slow, and the dots will be stretched during the transfer process, which will cause the graphic area to “magnifyâ€, resulting in inaccurate overprinting. On the two-color machine, this phenomenon is particularly evident.
3. Effect of paper on overprint accuracy In offset printing, paper that affects printing more is paper, especially offset paper. This type of overprint due to paper is not easy to control.
The composition of paper is mainly fiber. The characteristics of fibers are water swelling and syneresis. In the offset printing process, when the first color is printed and then the second color is set, the overprint due to the retraction of the paper is inaccurate. Paper structure has high strength, good smoothness, thick coating, and low water absorption. It is particularly serious for some papers with loose structure, light sizing and strong water absorption.
In addition to the paper itself being affected by changes in temperature and humidity in the surrounding environment, the paper's stretching deformation is also squeezed by the rollers during the printing process. Extruding makes the paper expand outward from the mouth of the mouth, and the more the two sides of the tip, the greater the overlay error. This phenomenon is often referred to as "sweep angle."
4. Mechanical influence on overprinting The printed paper must be effectively controlled by the mechanical components throughout the machine, and the relative position must be stable during the transfer and transfer process. Only in this way can the requirements for overprinting accuracy be met, otherwise overprinting accuracy will be greatly affected. The most influential factors are the following: First, the paper positioning, that is, the problem of the mechanical parts of the rules caused by overprinting is not allowed; Second, the paper is in the transfer and transfer process, due to the transfer of instability, resulting in changes in the relative position of the paper caused Imprinting is not allowed.
In addition, when the paper is peeled off from the drum, it is subjected to adhesion force and peeling tension. If the gripping force cannot overcome the peeling force, paper whip occurs and a small displacement occurs. This phenomenon appears to be misprinted on a monochrome machine and appears as a ghost image on a multicolor machine.
Second, the overprint accuracy control method For the impact of overprinting accuracy factors, take the appropriate control methods to achieve the purpose of overprinting accuracy.
1. For the roasting treatment of the printing plate, the temperature in the oven must be even, so that the plate is heated evenly, the temperature should not be too high, and the time should not be too long. Short version or product with high precision is best not to bake.
It is necessary to standardize the operation of the school edition. The layout of the plate is centered, the plate is mounted on the plate, and the plate screws are fastened. When you pull it, tighten the screws. As long as the force is moderate, deformation of the plate is generally avoided.
2. The center distance of the rollers is strictly measured according to the standard, and it must be done that the roller axis lines are horizontal and the lining calculation is accurate. After a good lining, do a stress test. Generally use "play screen", "draw map", "pressure bar", etc., to ensure that "pressure flat" to meet the printing accuracy requirements.
3. It is difficult to solve the effect of paper on overlay accuracy, and there are many related factors. In order to improve the printability of the paper and enhance its stability, the temperature and humidity of the printing environment should be kept relatively constant. At the same time, according to the "hysteresis effect" of the paper, prepress processing of the paper is done. The commonly used method is hanging and the paper is hung by hanging, so that its moisture content is relatively balanced with the air humidity, which can reduce the sensitivity of the paper to the environment.
For high-precision prints, it is also possible to use a method in which water is printed on the paper first, so that the paper can absorb water and be pressed sufficiently to deform the paper before overprinting. This is better for "slewing angle" and expansion, but at a higher cost.
4. The printing process requires the use of minimal water in the case of guaranteed non-dry version.
5. Mechanical overprinting is inaccurate, mainly rule components and tooth decay. The requirements of the pre-gauge are: According to the thickness of the paper, adjust the height position correctly, and the connection line of the front regulation paper point is parallel to the axis of the drum; the requirements of the pull gauge are: pull the paper reliably, moderate tension, and block the axis of the paperboard and the drum Vertically. Each paper and paper feeder must ensure that the paper runs straight. The grippers must have uniform gripping force and accurate closing time. When the two teeth are handed over to each other, they must have enough time for mutual annihilation to ensure effective control of the paper. In addition, the rollers and the shaft must be free from turbulence.
Satisfy the above requirements, under normal circumstances can improve the overlay accuracy, achieve control purposes. (Shandong Xinhua Printing Factory Linwei Plant Miaowei Xi)
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