Comparative analysis of ghosting and overprinting

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For offset printing, ghosting, overprinting, and the bar together are called the three major failures of offset printing. These faults will cause the entire picture of the print to be blurred and levelly distorted. Therefore, in offset printing, measures should be taken to avoid failures. In order to solve the fault, it is necessary to clarify the type of fault. However, for ghosting and overprinting, there is a certain similarity between the two. If the fault is judged incorrectly, the problem is difficult to solve. Therefore, correct distinction between ghosting and overprinting is of great significance for controlling the quality of printed matter.

1. The concept and characteristics of ghosting

Ghosting refers to the shallow lines appearing beside the printed lines or the side shadows appearing next to the printed dots. The silhouette is lighter than the main line or the main net point. The ghost image is also called double shadow. (When printing graphics, the text appears double shadow or virtual shadow, printed When the picture appears, there is a ghost shadow next to the dot, a light line appearing next to the printed line, or two or more identical prints appear on the print, and its position is very close. This phenomenon is also called ghosting). The appearance of ghosting makes the dot increase more serious, and reduces the sharpness of the edge, affecting the tone and color reproduction of the image, making the image of the entire print blurred and the clarity is obviously reduced.

The type of ghosting: according to the direction of the ghost, the ghosting is divided into three categories: vertical ghosting, horizontal ghosting, and A, B ghosting.

1.1 Vertical ghosting, that is, ghosting up and down. The silhouette of the dot is fixed in the direction perpendicular to the axial direction of the drum at the upper and lower ends of the original dot, but the silhouette is light and heavy, as shown in Fig. 1(a).

1.2 horizontal ghosting, that is, to go to the ghost. The silhouette of the dot is on the left and right sides of the original dot, and some of the dot silhouette is parallel to the roller axis, and some dot silhouette has a smaller angle with the roller axis. The position of the ghost is fixed, but it is light and heavy, as shown in Figure 1(b).

The 3AB ghost image is only produced on a two-color machine or a multi-color machine. The biggest feature of the ghost image is that the dot silhouette on the two sheets of paper is generated in the opposite direction of the normal dot, that is, the dot silhouette on the first sheet is on the left of the normal dot. On the side or upper side, the dot silhouette on the second sheet of paper is reversed on the normal right or lower side, see Figure 1 (c).

(a) Longitudinal ghosting (axial ghosting) (b) Lateral ghosting (circumferential ghosting)

Figure 1 Types of ghosting

In lithographic offset printing, if a fine image is printed, if the ghost image is produced, the entire layout of the product is blurred and cannot be consulted, resulting in waste. For color prints, severe ghosting affects the image tone. Reproduction, color reproduction, sharpness, uniformity, etc., the ghosting will have different degrees of influence on the high-profile, middle-tone and low-key of the graphic, especially the effect of the middle-tone of the graphic is most obvious, and the irregular time when printing Less ghosting will make the level that needs to be emphasized not accurately controlled, and it is difficult to achieve the desired result.

Figure 2 Ghost example

2. The concept and characteristics of overprinting inaccurate

Overprinting is actually the process in which we refer to the image of each color being transferred to the position of the specified image of the substrate one at a time in printing, which is the process of overprinting one color with another. According to the quality requirements of the printed matter, the outline overprinting of each color image is superimposed on the same image position according to the standard, which we call accurate overprinting. On the other hand, when the images of the respective colors cannot overlap at the same image position in each transfer, that is, the circumferential or axial deviation of the image positions is unequal, and the overlap is not completely, which is called product overprinting.

From the perspective of the inaccurate overprinting, there are: the circumferential overprint is not allowed, as shown in Figure 3. Axial overprinting is not allowed, as shown in Figure 4.

Figure 3 Vertical overprinting is not allowed

Figure 4: Horizontal overprinting is not allowed

Overprinting in multi-color printing is not allowed to be reflected in two aspects. First, partial overprinting on a sheet of paper is inaccurate (generally common in the shape of a fan tip). The other is that each sheet is printed accurately for each color, but the specification positions (axial and circumferential) of the front and back sheets are different.

3. Overprinting is not allowed and causes

The reasons for the inaccuracy of overprinting are generally as follows:

3.1 Mechanical aspects: The parts processing and machine assembly before the machine leaves the factory do not meet the requirements, and the parts are worn out prematurely after being put into normal production, resulting in inaccurate product overprinting. The machine is not properly debugged, and the horizontal accuracy is poor (horizontal and vertical horizontal errors should be ≤0.03/1000mm), so that the machine or parts are overloaded during high-speed operation, causing mechanical wear and misregistration.

3.2 improper adjustment of the machine: the paper positioning time is not correct, the paper advances before the arrival of the paper or does not go. The time for the handover of the front and side rules is improper. The timing of the transfer of the side gauge and the transfer paper is improper. The transfer time between the transfer paper and the impression cylinder is improper, and the synchronization time is too short, resulting in unstable paper transfer. The height of the front gauge and the tension of the side gauge are not appropriate. The paper is squeezing the teeth and the teeth are not the same or the pads are not flat. The impression cylinder has a light or heavy tooth or an uneven pad. The impression cylinder bite force is less than the transfer bite force.

3.3 Process operation: Prepress imposition accuracy is not high. The drying sleeve is not properly exposed, or the drying plate is insufficiently pumped, causing the film to move. Improper loading of the plate causes the image to be partially elongated, or the bolt is not tightened after the plate is installed. The ink is unbalanced during printing, and the paper surface is too large to cause the paper to expand and contract. The size of the upper and lower knives of the white material is not exceeded, the edge of the paper is not vertical, or the edge of the paper is moon-shaped.

3.4 Material: The paper itself has uneven water content, which causes paper deformation. The flatness of the paper is poor.

4. Contact and difference between ghosting and overprinting

For the sake of ghosting and overprinting, although the mechanism and formation of the two are different, there are many things in common: overprinting and ghosting are directional (circumferential and axial). From the perspective of distribution, there are both overall and local phenomena. The reasons for this are also due to mechanical and process operations. The same cause of failure can cause several quality ills. For example, mechanical streaks appear on the image. For example, if you use a magnifying glass to look closely at the edge of the image dot, there is a phenomenon of ghosting. Similarly, if the plate cylinder or blanket cylinder has axial sway, axial overprinting may occur. And axial ghosting. Inaccurate overprinting and ghosting will directly affect the printing effect of the product, making the image tone and level unsatisfactory, the text is ambiguous, and in serious cases it will be scrapped, directly causing economic losses.

Although there are many similarities between the two, there are also many differences in many aspects.

4.1 printed form:

Ghosting can be directly reflected in one (monochrome) printing and does not need to be reflected after two or more printings. The ghosting problem emphasizes a "heavy" word in which two shadows of one deep and one light of the same color are repeated on one dot or image.

When there is no clear print specification position, it does not see the problem of overprinting when printing on a piece of white paper; it can only distinguish whether the printed image or the position of the text is consistent by comparing the front and back sheets. Therefore, the overprinting problem is to focus on a "set" of words, even when printing a single color, it is necessary to pass the second or more overprinting to clearly reflect the overprinting problem.

4.2 The direct cause of the quality problem:

The direct cause of ghosting is that the dots on the blanket in printing cannot overlap with the dots printed in the next printing, that is, the blanket or blanket cylinder cannot be restored to the original position and the subsequent printing phase after the previous printing. Caused by overlap.

The problem of inaccurate overprinting is mainly due to the different degrees of failure of the overprint positioning system, so that the positions of the printed images in the overprint cannot be overlapped at the same position.

In the offset printing process, offset printing uses the principle of ink repellent to print, involving physical changes and chemical reactions, plus offset printing machine to high speed, multi-color, automatic development, complex structure, high technical requirements, heavy The faults of shadow and overprinting are endless. Therefore, for offset press operators, it is necessary to have a certain level of culture and professional technical knowledge, but also to pay attention to accumulating certain operational experience and ability to eliminate faults, in order to adapt to the growing development of offset printing. Need.