There are many factors that affect the accuracy of overprinting. In addition to the film errors caused by plate-making factors, several main aspects of printing are described as follows:
1. The effect of printing plate on the accuracy of overprinting
Lithographic printing plates are mostly thin and light aluminum. In the printing process, due to its shape or volume changes will affect the accuracy of overprinting.
The deformation of the printing plate is roughly caused by two aspects, the first is stretching deformation, and the second is baking deformation. Stretching and deformation refers to the fact that during the proofreading process, the operator imposes improper force when installing and pulling the plate, which artificially causes the metal plate to extend (elongate) in the circumferential direction of the drum, resulting in inaccurate overprinting caused by the change of the layout. Baking deformation refers to the fact that the linear size of the printing plate changes due to improper baking when baking is performed to improve the durability of the printed PS plate. As we all know, most metals have the characteristics of thermal expansion. When the printing plate is heated to 250 degrees Celsius in the oven, the plate base will be baked and affect the stability of the plate size. The temperature in the oven is uneven or over-baking is especially serious, and it will cause a waste version in severe cases.
2. The effect of roller lining on overprint accuracy
It can ensure that the linear speed of the printing plate, rubber, and impression cylinder are the same, so that the transfer can be performed effectively under a certain pressure. If the plate cylinder liner is too thick, the cylinder radius will increase accordingly. In the case of the same angular velocity, the linear velocity of the drum surface with an increased radius will increase. The resulting speed difference will not only cause the wear of the printing plate and text, but also cause the size of the printing plate to "compress" in the front and back directions. If the drum liner is too thin and the radius decreases accordingly, the linear speed of the drum surface will slow down, and the dots will be elongated during the transfer process, which will cause the image area to "enlarge", resulting in inaccurate overprinting. On two-color machines, this phenomenon is particularly apparent.
3. The effect of paper on the accuracy of overprinting
In lithography, the paper that has the greatest impact on overprinting is paper, especially offset paper. This type of overprinting caused by paper is difficult to control.
The composition of paper is mainly fiber. The characteristics of the fiber are water swelling and dehydration shrinkage. In the offset printing process, when the first color is printed and then the second color is printed, the overprinting caused by the expansion and contraction of the paper is very obvious. The paper has high structural 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 expansion and contraction of the paper is affected by the temperature and humidity of the surrounding environment, and it is also pressed by the roller during the printing process. Squeezing makes the paper extend from the grip area to the fan surface, and the more it reaches the sides of the trailing tip, the greater the overprint error. This phenomenon is often referred to as "swing angle".
4. The impact of machinery on overprinting
The printed paper must be effectively controlled by the mechanical components from the beginning to the end of the machine, and the relative position must be stable during the transmission and handover process. Only in this way can the requirements of overprint accuracy be met, otherwise the overprint accuracy will be greatly affected. The main impacts are mainly in the following situations: one is the paper positioning, that is, the problem of mechanical regulations causes the overprint to be inaccurate; the second is the paper transfer and handover process, due to the unstable handover, resulting in changes in the relative position of the paper Overprint is not allowed.
In addition, when the paper is peeled from the drum, it is subjected to adhesion and peeling tension. If the gripping force of the gripper cannot overcome the peeling force, the paper will twitch and a small displacement occurs. This phenomenon is manifested as inaccurate overprinting on a single machine and ghosting (ghosting) on ​​a multi-color machine.
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