There are many factors that affect the accuracy of overprinting. 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 plate on overprint accuracy
Lithographic printing plates are now mostly thin and light aluminum. During the printing process, the overprint accuracy is affected by changes in its shape or volume.
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 refers to the change in the linear dimension of the printing plate due to improper baking when the baking is performed in order to improve the printing resistance of the dried PS plate. As we all know, most of the metals have the characteristic of thermal expansion. When the printing plate is heated to 250 degrees Celsius in the oven, the plate base will be baked softly, affecting 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, roller lining on the accuracy of overprinting
It can ensure that the linear speeds of the printing plate, the rubber and the impression cylinder are the same 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. In the two-color machine, this phenomenon is particularly apparent.
3, the impact of paper on overlay accuracy
In lithography, paper, especially offset paper, has a major influence on overprinting. 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.
The paper's stretching deformation is affected by the temperature and humidity of the surrounding environment. In the printing process, it is also squeezed by the roller. Squeeze causes the paper to extend outward from the mouth of the mouth and extend to the edges of the tip. The overprint error is greater. This phenomenon is often referred to as "sweep angle."
4, the impact of mechanical overprint
The printing paper has to 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 type of image appears to be misprinted on a stand-alone machine, and appears as a ghost image on a multi-color machine.
1. Effect of printing plate on overprint accuracy
Lithographic printing plates are now mostly thin and light aluminum. During the printing process, the overprint accuracy is affected by changes in its shape or volume.
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 refers to the change in the linear dimension of the printing plate due to improper baking when the baking is performed in order to improve the printing resistance of the dried PS plate. As we all know, most of the metals have the characteristic of thermal expansion. When the printing plate is heated to 250 degrees Celsius in the oven, the plate base will be baked softly, affecting 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, roller lining on the accuracy of overprinting
It can ensure that the linear speeds of the printing plate, the rubber and the impression cylinder are the same 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. In the two-color machine, this phenomenon is particularly apparent.
3, the impact of paper on overlay accuracy
In lithography, paper, especially offset paper, has a major influence on overprinting. 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.
The paper's stretching deformation is affected by the temperature and humidity of the surrounding environment. In the printing process, it is also squeezed by the roller. Squeeze causes the paper to extend outward from the mouth of the mouth and extend to the edges of the tip. The overprint error is greater. This phenomenon is often referred to as "sweep angle."
4, the impact of mechanical overprint
The printing paper has to 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 type of image appears to be misprinted on a stand-alone machine, and appears as a ghost image on a multi-color machine.
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