Factors Affecting Overprinting Accuracy of Color Newspapers

The publishing of newspapers in our country has made great progress in recent years, and the proportion of color newspapers has greatly increased every year. At present, most color newspaper printing uses imported rotary presses, greatly improving the printing quality of color newspapers. However, relatively speaking, in the rotary offset printing, there are many factors that affect the accuracy of color printing overlays and are complex and changeable, including mechanical aspects, Process aspects and materials. This article only discusses the effect of commonly used paper, printing plates, blankets, etc. on the overlay accuracy of color newspapers in printed materials.

First, printing plate
In the lithography, after processing of the image and character of the original, the graphic information of the original is transferred to the PS plate, and batch copying of the original is completed by printing. PS version is generally based on industrial pure aluminum as the base, the thickness of 0.3mm, its technical indicators directly affect the accuracy of overprinting.

1. Plane geometry of PS plate
The PS plate has to be perforated prior to drying, so its overall dimensions are particularly important. The PS plate should be cut into rectangles to ensure that the two diagonal lines are of equal length, so that they can maintain precise positioning of the perforation, bending plate and machine before the drying. If the printing plate is not cut properly, it will cause deviation in the machine-mounted version, which will inevitably result in overprinting. It is important to emphasize here that the positioning components of the plate press, bending machine, and plate cylinder must be supported, and the accuracy of the matching between them must be ensured to ensure accurate positioning and to eliminate the improper cutting due to the plate material. Impact.

2. Thickness of PS plate
The cross-sectional geometry of the PS plate is usually rectangular. When it is wrapped on the roller, its cross-section becomes a fan shape from a rectangle. At this time, the front and the plate of the plate correspond to the same wrap angle, but the plate has a certain degree of Thickness, resulting in a change in the length of the front and back of the plate, the length of the front of the plate is greater than the length of the back, indicating that the front of the plate has a certain degree of elongation, this phenomenon is called metal cracking. When the printing plate is mounted on the roller, due to the non-uniform tension and internal stress, the size of the plate surface is elongated due to the cracking of the surface of the plate, as shown in Figure 1, the elongation of the plate (â–³L ) can be calculated by:

This kind of elongation is slight, but for products with high requirements for overlay accuracy, it must be taken into account and corrected accordingly.

The thickness of the base is different, and the overlay error is also different. Because the thick plate base is wrapped on the roller, the friction force on the back surface of the plate after being attached to the roller surface is relatively large. The pull plate force is only concentrated at the two ends of the plate mouth. The whole plate is not completely attached to the roller, and the stress distribution of the various particles Inhomogeneous, besides minor natural cracking, the tensile deformation is small, it shows a rigid body in the printing process, and there is almost no extrusion deformation, so its surface size is basically stable; for the thin plate base, the printing of the liner paper is added. After the plate is mounted on the roller, the friction force is reduced due to the isolation of the liner. Under the action of the tension, the stress distribution of each particle is even and the printing plate is attached to the surface of the roller. Therefore, the tensile deformation is large. Overprint errors are large.

Based on the above analysis, plates of the same thickness should be used for the same print. If plates of different thicknesses are used, they should be reduced in color sequence to reduce overprint errors.

Second, paper
In the printing of color newspapers, the substrates are mainly newsprint. The printability of the substrates directly affects the accuracy of overprinting.

1. Dimensional stability
The surface of newsprint is porous and hydrophilic and hygroscopic. It contains a certain amount of moisture. When the temperature and humidity of the environment changes, the moisture content of the paper also changes: as the relative humidity increases, it increases with temperature. As it rises and falls, the result is a change in the size of the paper. The temperature of the paper store is usually 18 to 25°C, and the relative humidity is controlled at 60% to 65%. In general, every 10% change in relative humidity changes the water content by 1%; under a certain relative humidity, the temperature of paper changes by ± 5% per change ± 5%, and the paper's moisture content changes by ± 0.15%. Changes in water content exceeding ±0.1% will affect overprint accuracy. In newsprint printing, newsprint is exposed to water several times, resulting in changes in water content, which in turn leads to changes in dimensional stability, thus affecting the accuracy of overprinting. Therefore, during the production, transportation and storage of newsprint, it is necessary to control as much as possible the factors affecting the change in the moisture content of the paper, maintain a stable moisture content, and avoid unnecessary printing failures.

2. The effect of paper stretching on overlay accuracy
Newsprint formed two silk directions in the manufacturing process, namely horizontal and vertical. After one fiber of newsprint is completely wet, its diameter can be increased by 30%, its longitudinal direction only increases by 1% to 2%, and the horizontal expansion and contraction amount is 2 to 3 times of the longitudinal expansion and contraction volume, which has a great influence on the overprint accuracy of color newspapers. About 0.15%, the maximum overprint error can reach more than 0.3mm, resulting in the paper can only register one side. It can be seen that the newsprint water expansion has a great influence on the registration accuracy of color newspapers, especially in the cross-web direction of the paper, and the accuracy of registration is affected more by the water absorption and expansion.

In order to ensure that the paper has sufficient tension during the printing process, the newspaper must be oriented in the longitudinal direction. The dimensional stability in the cross-web direction is more difficult to control. The fan-shaped expansion of the paper suction section and the fan-shaped shrinkage after drying can be controlled by the expansion of the fan-shaped expansion roller and the borrowing line during the imposition; the direction of the longitudinal thread can be adjusted by the unit The registration roller is eliminated (for satellite-type printing presses, the paper path between the color sets is very short, and the expansion and contraction after the paper absorbs water is small, and there is generally no effect on the registration), but it is used in the printing process of the color newspaper. Which kind of equipment controls the amount of water is very important.

At present, there are many types of newsprint used by printing companies, and their hygroscopicity and dimensional stability are different. At the same time, the design of color newspaper layouts is increasingly diversified, and most of the rotary machines have large structures and long paper paths. Therefore, higher requirements are imposed on the accuracy of overprinting. Therefore, different papers should not be used for the same prints, and papers should be used. The stretching rate is an important parameter that determines the degree of deformation of the paper during printing. Before printing, the appropriate paper is selected according to the printing requirements of the printing product, and the temperature and humidity of the printing shop must be kept constant.

Third, the blanket
In lithographic offset printing, blankets are used as intermediate media for graphic and text transfer, and their properties such as elongation and tightness after being coated on rollers will directly affect the overlay accuracy of color newspapers.

1. Elongation
Elongation is the performance index that characterizes the elongation of the blanket under a certain tensile force. It mainly depends on the number and strength of the fabrics at the bottom. It requires that the fabric at the bottom is fine and uniform, smooth and firm, with low elasticity, and the adhesive layer is Adhesive bonding performance is good, not easy to crack, the vertical and horizontal direction is absolutely vertical, the direction of the wire to follow the radial direction. The elongation of the blanket should be as small as possible to ensure accurate overprint, complete dot, and clear graphics. If the blanket is easily stretched, the adhesive layer is thinned and the elasticity is reduced, which will certainly cause defects such as increased distortion of the dots and inaccuracy.

2. Tightness
Differences in the tightness of the blanket can also cause overprinting. After the blanket is tightened on the roller, the degree of tension, the relative friction generated on the surface of the roller, the pushing deformation of the blanket, and the elongation caused by the dimensional change of the graphic and the registration error are closely related. The deformation of the blanket depends on the direction and size of the friction and the elongation of the blanket.