Overprint failure: "swing angle" concept and formation reasons

The concept of rejection angle

The throwing angle is also called the paper-end scattered shape and generally shows the following two phenomena.

(1) The tail of the printed paper is inaccurate, the first color is large in the paper-feeding direction, and the back color gradually decreases. This type of overprint problem started from the position of the paper, and the more serious the situation was at the end of the paper.

(2) Observed with the naked eye, the registration of the rules is accurate, but the printed products appear discolored, the color of a batch of printed products is inconsistent, the color of the tail of the paper is severe, and there is no sense of layering in the line of the cable, and the hair is empty. The front of the sheet is light and heavy, and the front The text on the end of a few colors (especially the first color) is not clear. When examined with a 20x magnifier, it can be seen that there is a significant jump in the rule line on the tail of the paper, and the dots are deformed.

The problem of rejection angle is a typical manifestation of overprinting faults in printing products. Most overprinting faults are caused by the rejection angle or accompanied by the rejection of corners. There are both mechanical and operational reasons as well as paper problems. Strictly speaking, all multicolor printing equipment has the issue of throwing angles. High-quality new printers have a lighter rollover problem, and only lighter rollovers occur when printing thin or flounced paper. Some of the rollover problems are inherent to some models. Heidelberg multicolor offset printing press in the 1970s and 1980s, due to the limitations in the design and manufacture of equipment, there was a problem of rejection at the factory, but it was not serious. After a certain period of use, due to the influence of various factors, the machine's rollover problem gradually gives the operator a headache. Through maintenance, the problem of rejection angle can be minimized to a degree that cannot be observed with the naked eye. The domestic multi-color offset printing press is generally more serious when it comes to the rejection angle, and the national standard on the accuracy of overprinting is too loose, not suitable for printing high-quality prints.

One of the reasons for rejection

When the paper is conveyed to the first group of impression cylinders, the front mouth is wavy. During printing, under the common rolling of the impression cylinder and the blanket cylinder, the waves are flattened, but a slight wrinkle is formed on the paper. The wrinkles are partially or completely flattened during the subsequent imprinting of each unit. Therefore, in the paper-feeding direction, the first color print of the print is the largest, and the print of the rear unit gradually decreases. Changes in the rules line from the beginning of the paper, the more the change to the end of the paper. This is the most typical rejection problem.

The second reason for rejection

In the printing process, the paper is deformed by the peeling force of the blanket, so that the front edge of the paper is not in an ideal straight line state. Under the action of the printing pressure of the rear unit, this wave deformation is flattened and a slight wrinkle is formed on the paper. Causes rejection.

Three reasons for rejection

During printing, the longitudinal section of the paper cannot maintain the "one line" state, or the longitudinal section of the surface of the impression cylinder and the surface of the blanket after the blanket cylinder and plate cylinder are pressed together cannot maintain the "one line" state. The image cannot be accurately transferred to the paper, resulting in misprints and rejections.

Four reasons for rejection

After the paper is printed in the first color or in a certain color, the front edge of the paper is deformed due to an unreasonable delivery time, etc., and the wavy deformation is flattened and formed on the paper when the paper is transferred to the following impression cylinder. The formation of micro-wrinkles, resulting in rejection angle.

The premise of ensuring the accuracy of overprinting of printed products is to minimize the deformation of the paper during each transfer and imprinting process, especially the deformation of the front edge of the paper. Although there are compensators in Heidelberg multicolor offset presses, this compensation does not have any practical significance for the larger swing angles and the deformation of the dot ends of the paper tails. There are many reasons for the phenomenon of print rejection, and most failures are difficult for the compensator to compensate.