Color printing overprint accuracy
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Color plate making is a color decomposition process, and color printing is a color combination process. This color combination process is called overprinting. The minimum quality requirement for color printing is that the overprinting is accurate, that is, each monochrome image is superimposed in the same position at the same position, and finally the batch of printed matter becomes a faithful copy of the original. However, in the production process, due to various reasons, it is easy to cause the overprinting error of the product to exceed the standard, so that a large number of products become defective products or waste products, which causes great losses to the enterprise. Therefore, the majority of printing workers have made unremitting efforts to this end and have summed up some valuable experiences. In order to better communicate and learn, the issue of printing overprinting accuracy is discussed here.
First, the factors affecting the accuracy of overprinting
There are many factors affecting the accuracy of overprinting. In addition to the film error caused by the plate making factors, several main aspects of printing are described as follows:
1. Influence of printing plate on overprinting accuracy
Lithographic printing plates are now mostly thin and light aluminum. During the printing process, the accuracy of the overprint is affected by changes in its shape or volume.
The deformation of the printing plate is roughly caused by two aspects, the first is the stretching deformation, and the second is the baking deformation. Stretching deformation refers to the inaccurate overprinting caused by the change of the layout of the metal plate caused by the operator's improper application during the plate-making process. The baking deformation refers to a change in the linear size of the printing plate due to improper baking in order to improve the printing durability of the dried PS plate. As we all know, most of the metals have the property of being thermally expanded. When the plate is heated to 250 degrees Celsius in the oven, the plate base will be baked soft, which will affect the stability of the plate size. The temperature in the oven is uneven or excessively baked. In severe cases, it will cause waste.
2. The influence of the roller lining on the overprinting accuracy can ensure that the linear speeds of the printing plate, the rubber and the embossing 3 roller are consistent, and the transfer can be effectively performed under a certain pressure. If the plate cylinder liner is too thick, the drum radius is increased accordingly. At the same angular velocity, the line speed of the roller surface with an increased radius is accelerated. The resulting speed difference will not only cause the printing plate to wear out, but also cause the layout image size to be "compressed" in the front and rear direction. If the drum liner is too thin and the radius is correspondingly reduced, the linear speed of the roller surface becomes slow, and the dot will be elongated during the transfer process, which will cause the image area to be "magnified", resulting in inaccurate overprinting. On a two-color machine, this phenomenon is particularly noticeable.
3, the impact of paper on overprint accuracy
In lithography, the most important influence on overprinting is paper, especially offset paper. This overprinting caused by paper is difficult to control.
The composition of the paper is mainly fiber. The characteristics of the fiber are water swelling and syneresis. In the offset printing process, when the first color is printed and the second color is set, the overprinting caused by the expansion and contraction of the paper is not obvious. The paper has high structural strength, good smoothness, thick coating and small water absorption. It is especially serious for some papers with loose structure, light sizing and strong water absorption.
The expansion and contraction of the paper is affected by the temperature and humidity changes of the surrounding environment, and is also squeezed by the drum during the printing process. The squeezing causes the paper to extend outward from the mouth of the mouth, and the overprinting error is greater on both sides of the tip. This phenomenon is often referred to as the "corner".
4, the impact of mechanical aspects on overprinting
Printed paper must be effectively controlled by the mechanical components from start to finish, and the relative position must be stable during transmission and transfer. Only in this way can the requirements of overprinting accuracy be met, otherwise the overprinting accuracy will be greatly affected. The main influences are as follows: First, the paper positioning, that is, the problem of mechanical rule parts caused by overprinting; second, the paper is in the process of transfer and transfer, due to the unstable transfer, causing the relative position of the paper to change. Overprinting is not allowed.
In addition, when the paper is peeled off from the drum, it is subjected to adhesion and peeling tension. If the gripper force does not overcome the peeling force, paper twitching occurs and a small displacement occurs. This phenomenon is manifested as inaccurate overprinting on a single machine and as a ghost image on a multi-color machine.
Second, the control method of overprint precision
For the factors affecting the accuracy of overprinting, the corresponding control measures are adopted to achieve the purpose of accurate overprinting.
1. For the baking process of the printing plate, the temperature in the oven must be...
The school version must be standardized, the position of the plate is centered, the plate is positive, and the plate screws are fastened. When pulling the plate, tighten the screws. As long as the force is moderate, it will generally avoid the deformation of the plate.
2. The center distance of the roller is strictly measured according to the standard. It is necessary to make the axis of the roller shaft horizontal and the lining calculation is accurate. After the good lining, do a stress test. Generally, the methods of “playing the screen”, “drawing the map” and “pressing the bar” are adopted to ensure the “pressure flat” to meet the printing precision requirements.
3. Solving the influence of paper on overprinting accuracy is difficult 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 as relatively constant as possible. At the same time, according to the "lag effect" of the paper, the pre-press processing of the paper is achieved. The usual method is to hang and hang the paper, so that the water content of the paper is relatively balanced with the air humidity, which can reduce the sensitivity of the paper to the environment.
For printings with higher precision, you can also use the method of first printing the paper on the paper to make the paper fully absorb and press the paper before the overprint. This is more effective for the "corner" and the expansion effect, but the cost is higher.
4, the printing process requirements, in the case of ensuring no dry version, the use of the smallest water.
5. The overprinting caused by the machine is not accurate, mainly the fangs of the regular parts. The requirements of the pre-regulation are: according to the thickness of the paper, the height position is correctly adjusted, and the connection of the front gauge paper point is parallel to the drum axis; the requirement for the far-sighted view is: the paper pull is reliable. Each paper and paper conveying component must ensure that the paper runs in a straight line. The force of each paper tooth should be uniform and uniform, and the opening of the paper should be accurate. When the two teeth are handed over, the paper should have enough time to ensure effective control of the paper. In addition, the rollers and the dental shaft should ensure no turbulence.
To meet the above requirements, under normal circumstances, the overprinting accuracy can be improved to achieve control purposes.