For offset printing, ghosting, overprinting, and bar are called the three major faults of offset printing. These failures will cause the entire picture of the printed product to be blurred and the levels to be distorted. Therefore, in the production of offset printing, we must take measures to avoid the occurrence of failures. In order to solve the faults that occur, the type of fault must be clarified. However, for ghosting and overprinting, there is a certain similarity between the two. If the fault is judged incorrectly, the problem will be difficult to solve. Therefore, the correct distinction between ghosting and overprinting is of great significance for controlling the quality of printing.
1. The concept and characteristics of ghosting
Ghosting refers to light lines that appear next to printed lines or side shadows that appear next to printed dots. The side shadows are lighter than the main lines or main dots. Double ghosts are also called double shadows (when printing graphics and text, double shadows or ghost images appear in the text. In the picture, there is a ghost image next to the dot, a light-colored line next to the printed line, or two or more same marks appear on the printed product, and their positions are very close, this phenomenon is also called ghosting). The appearance of ghosting makes the dot increase more severe, and reduces the sharpness of the edges, affects the tone and color reproduction of the image, and makes the entire print's graphics and text blurry, and the sharpness is significantly reduced.
Types of ghosting: According to the direction of ghosting, ghosting is divided into three categories: vertical ghosting, horizontal ghosting, and A and B ghosting.
â‘ Vertical ghosting, that is, up and down ghosting. The silhouette of the dot is fixed in the direction perpendicular to the axis of the drum at the upper and lower ends of the original dot, but the silhouette is light and heavy.
â‘¡ Horizontal ghosting, that is, ghosting. The dot silhouettes are on the left and right sides of the original dots. Some dot silhouettes are flat with the roller axis, and some dots have a smaller angle with the roller axis. The position of ghosting is fixed, but there are some light and heavy,
â‘¢AB ghosting only occurs on two-color machines or multi-color machines. The biggest feature of ghosting is that the dot silhouette on the two front and back sheets of paper occurs in the opposite direction of the normal dot, that is, the dot silhouette on the first sheet of paper is on the left of the normal dot. Side or upper side, the dot silhouette on the second sheet is in turn on the normal right or lower side,
In lithographic offset printing, if ghosting occurs when printing fine text, the entire layout of the product is blurred and cannot be consulted, resulting in waste products. For color prints, severe ghosting affects the image tone. Reproduction, color reproduction, clarity, uniformity, etc., ghosting will affect the high-key, middle-tone and low-key of the picture to varying degrees, especially the middle-tone of the picture is the most obvious, and there are often irregularities in printing Less ghosting will make it impossible to accurately control the layers that need to be emphasized, and it is difficult to achieve the desired effect
2. The concept and characteristics of inaccurate overprint
Overprinting is actually the process by which each color image is transferred to the specified image position of the substrate again and again in printing. It is a process in which one color is overprinted with another color. According to the quality requirements of the printed matter, the contour overprint of each color image is superimposed on the same image position according to the standard. We call it overprint accurate. Conversely, when each color image cannot be overlapped on the same image position in each transfer, that is, there is an unequal amount of circumferential or axial deviation in the image position, which is not completely overlapped, we call it product overprinting inaccuracy.
The inaccuracy of overprinting in multi-color machine printing is reflected in two aspects. One is the inaccuracy of partial overprinting on a printed sheet (generally in the shape of a fan in the trailing tip). The other is that each printed sheet itself is accurately overprinted, but the specification positions (axial and circumferential) of the front and rear printed sheets are different.
3. Phenomenon and causes of overprinting
The causes of inaccurate overprinting are generally as follows:
â‘ Mechanical aspects: The parts processing and assembly of the whole machine do not meet the requirements before the machine leaves the factory, and the parts are worn prematurely after being put into normal production, resulting in inaccurate product overprinting. Improper debugging of the whole machine makes the machine or parts overloaded during high-speed operation, resulting in mechanical wear and inaccurate overprinting.
â‘¡ Improper adjustment of the machine: the paper positioning time is improper, the paper advances or does not go into place when it reaches the front gauge. Improper handover time between front and side rules. Improper handover time between the side rule and the delivery tooth. Improper handover time between the delivery tooth and the impression cylinder, and the synchronization time is too short, resulting in unstable paper handover. The height of the front gauge and the tension of the side gauge are improper. Passing the paper to bite the teeth and the teeth are uneven or the pads are uneven. The impression rollers have different bite levels, or uneven tooth pads. The clenching force of the impression cylinder is less than that of the paper feeding.
â‘¢In terms of process operation: the precision of pre-press imposition is not high. The printing set is not exposed properly, or the exhaust of the printing set is insufficient, causing the film to move. The printing plate is installed improperly, causing the image to be partially elongated, or the bolts are not tightened after the printing plate is installed. The water and ink are unbalanced during printing, and the water content of the layout is too large, causing the paper to stretch. The deviation of upper and lower blades of white material cutting exceeds the standard, the edge of the paper is not vertical, or the edge of the bite is moon-shaped.
â‘£Materials: The moisture content of the paper itself is not uniform, causing the paper to deform. The flatness of the paper is poor.
4. The connection and difference between ghost and overprint
For ghosting and overprint inaccuracy, although the two mechanisms and formation methods are different, there are many things in common: overprint inaccuracy and ghosting are both directional (circumferential and axial). From the perspective of distribution, there are global and local phenomena. The causes are also mechanical and technological operations. The same fault cause will cause several quality defects. For example, mechanical streaks appear on the image. If you look closely with a magnifying glass, there is a ghost phenomenon on the edge of the image dot; similarly, if the plate cylinder or blanket cylinder has axial movement, axial overprinting will also occur. And axial ghosting. Inaccurate overprinting and ghosting will directly affect the printing effect of the product, resulting in poor image tone and gradation, blurry text, and scrapping in severe cases, which will directly cause economic losses.
Although there are many similarities between the two, there are also great differences in many aspects.
â‘ Print form:
Ghost image can be directly reflected in one (monochrome) printing, and does not need to be reflected after two or more printings. The ghosting problem emphasizes the word “heavyâ€, and two shadows of the same color, one dark and one light, appear repeatedly on a dot or image.
When the position of the printing specifications is not clear, it can not see the problem of inaccurate registration when printing on a white paper at a time; it can only distinguish the printed image or the position of the text by comparing the front and back printed sheets. Therefore, the problem of overprinting is to focus on the word "set". Even when printing a single color, it is necessary to pass two or more times of overprinting to clearly reflect the overprinting problem.
â‘¡ The direct causes of quality problems are as follows:
The direct cause of ghosting is that the dots on the blanket during printing cannot overlap with the dots of the subsequent printing, which means that the blanket or blanket cylinder cannot be restored to its original position in time after the previous printing. Caused by overlap.
The problem of inaccurate overprinting is mainly due to the failure of the overprinting positioning system to varying degrees, so that the color and color of the printed image position in the overprint cannot overlap at the same position.
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