Factors Affecting Printing Overlay Accuracy

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 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 “magnify”, 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.