Overprinting problem in color printing

Overprinting problem in color printing

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In the production of color newspaper printing, the problem of overprinting is almost the most important criterion for evaluating the quality of newspapers. There are many factors that can affect the accuracy of color registration registration in actual production. It can be said that almost all the links in the production of color newspapers can affect the overprinting of newspapers, such as: photo, printing, paper, fountain solution , printing process, printing equipment, etc. In this article we will discuss some representative issues that affect the quality of overprinting.

Prepress process:

(1) Photographic process leads to:

    When the color image is subjected to pre-press processing and imposition process, it is finally exposed on the film by the laser imagesetter. This exposure is after the color separation and is exposed on the four-color film by the imagesetter. However, due to the influence of mechanical transmission and other factors, the four films cannot achieve a complete correspondence (in the sense of registration). This kind of error always exists and cannot disappear because of the increased level of operation.

(2) film imposition error

    In many printing manufacturers, since there is no direct out-of-the-line imagesetter, only four or fewer films can be used for collage and then printed. However, since this process is a manual process, the operator's proficiency and meticulousness are highly demanded. The same imposition process, for different levels of operators, the results will never be the same. But even the best imposition and printing operators will never achieve the elimination of such overprinting errors. This registration error always exists objectively.

(3) Plate punching and bending error

    The dried printing plate must be punched and bent before being put on the machine, but since this operation still depends largely on the operation of the person, no matter how fine the special equipment is used, the error cannot be avoided. .

The above three points are all parts of the pre-press process that are easy to affect the printing overprint. Although we find that the overprinting error of each link is not very large, and the error of any single link is not enough to produce overprinting under normal conditions. It is a serious influence, but we also mentioned that since these errors can only be reduced and cannot be avoided, the influence of the error superimposition on the printing overprint cannot be ignored. It can be seen that improving the quality and responsibility of the operators in the prepress stage is very important for the overprinting of the color newspaper.

Printing process:

Factors affecting the stability of the tension of the paper tape (such as the failure of the tension control system to cause the overprinting error caused by the tension of the paper tape) and the factors affecting the accurate transfer of the graphic (such as the wear of the roller bearing of the printing press, running the eccentricity, etc.) The pressure is not normal, the axis of the drum is not parallel, the positioning pin of the plate is offset, etc., so that the newspaper printing plate can not be accurately transferred to produce overprinting error).

(1) Factors affecting the stability of the tape tension

Different types of high-speed wheel transfer machines have different methods and methods for controlling the tension of the paper strips, and the paper feeding paths between the printing units of the printing press are also different. From the current mainstream equipment used in domestic color newspaper printing, the following printing components will affect the color registration:

During the printing process, the tension of the tape is affected by the above components, and any component problems will affect the stability of the tape tension. In turn, it affects the registration accuracy of newspapers. The following is a detailed analysis of the impact of each component on the color registration.

1 paper holder.

The brake pads of the press paper roll tension brake system are worn. The gap between the brake pad and the brake disc changes, and the brake seal airbag leaks, etc., which will have a great influence on the stability of the tape tension.

2 floating rollers.

The floating roller is an important part to ensure the stability of the printing paper belt tension during the printing process. Tension control generally refers to a tension control system consisting of components such as pneumatic control elements, cylinders, and floating rollers that maintain tension during the printing process. The mechanical factors that affect the color registration register mainly include the steel wire connecting the sliders at both ends of the floating roller, the sliding guide and the sliding bearing on the slider. In actual production, when the paper has a change, such as changing paper, loading a new paper roll, etc., if the wire of the part is fastened. The sliding bearing and the guide rail have good working performance, and the floating roller is sensitive to the change of the paper tape, so the tension stability of the paper tape is good when printing; otherwise, if the steel wire is slack, the sliding bearing is dry or lacking oil or the paper ash is accumulated on the guide rail, The floating roller is slow to respond to changes in the tape, and the tension stability of the tape is poor.

3 pre-tension unit, paper pulling roller for each part.

In order to achieve the best printing effect, different parts of the entire printing line have different requirements on the tension of the paper strip. The paper-feeding rolls with independent driving ability are installed in different parts of the machine, and the paper path is controlled by tension to meet each Part of the tension requirement. The function of the pre-tensioning unit is similar to that of the paper-feeding roller. The function is to further control the tension of the paper tape entering each printing unit during the printing process, to compensate for the influence of the paper path change on the tension of the paper tape, and to stabilize the paper tape tension of the printing unit. For better printing results. However, the premise of normal work is that each paper-feeding roller, especially the paper-feeding roller before and after each printing unit, must be well driven, and the speed overshoot value is set correctly, otherwise the tension of the paper strip of each printing unit will be affected, resulting in inaccurate overprinting. .

4 sets of printing and cutting paper roll.

For a shaft-driven printing press, the registration between the upper and lower units is mainly achieved by a paper feed roller. The overprinting and overcutting paper roll is composed of a shaft-fixed paper feed roller and a cantilever roller. Under the control of the registration motor, the cantilever roller can be horizontally rotated around the fixed roller axis. Therefore, the levelness of the cantilever roller and its stability during printing will directly affect the color registration between the upper and lower units. At the same time, the stability of the tape tension is also affected.

5 various parts of the paper feed roller.

The paper take-up roller is the guide roller of the printing paper path. The reasonable setting of the paper-feeding roller of each part of the printing machine can optimize the paper-feeding mode and also facilitate the control of the paper tape tension. Therefore, it is necessary to ensure that the level of each paper roll is the same, especially the level of the paper feed roller of the upper and lower printing units is consistent, which is an important factor affecting the registration between the upper and lower units.

6 In addition, it is necessary to carry out regular inspections on the rotating bearings of the above-mentioned various types of rollers. If necessary, new bearings should be replaced when necessary to prevent bearing wear, oil shortage and jamming, and the bearing should be in the inner and outer ring to ensure the bearing is in good working condition. Avoid color registration overprint due to bearing problems.

(2) Factors affecting the accurate transfer of graphics

The printing unit is a direct printing processing component that realizes the precise transfer of graphics from the printing plate to the paper. Therefore, any mechanical deviation of the printing unit will affect the accurate transfer of the newspaper graphic. According to experience, the mechanical factors affecting the registration of color newspapers in the printing unit are as follows:

1 version of the mouth. The version of the pin is a positioning component that allows the plate to be accurately positioned on the plate cylinder. If the plate pin of a certain color plate is worn, loose, detached or improperly installed, the corresponding color image of the plate is inaccurately overprinted.

2 roller bearings. The roller bearing is the support of the printing cylinder, and is also the main mechanism for providing printing pressure to each printing cylinder and parallelizing the cylinder axis. If the roller bearing wears and the bearing eccentric position is deviated, the non-parallel printing pressure of the drum will change, which makes the color printing overprint and inaccurate.

3 clutch pressure link. The clutch pressure link is a control operation mechanism for making the printing cylinder clutch pressure, stabilizing the bearing eccentricity, avoiding damping vibration, and ensuring that the axes of the printing cylinders are parallel. The precision of the gap between the connecting rod support and the copper sleeve and the bearing is very high. If the connecting rod support, the copper sleeve is seriously worn or the bearing is loose, the registration accuracy of the newspaper will be affected.

4 sets of standard adjustment mechanism. The registration mechanism is an adjustment mechanism for accurately engraving the various colors and images. During the printing process, the registration mechanism realizes the axial and circumferential registration by axially pulling the printing plate or the blanket cylinder, and keeps the axial position of the corresponding roller unchanged. of. The bearing of the register mechanism is rotated at high speed together with the drum, which is prone to oil shortage and outer ring failure, resulting in wear or gap formation at the contact between the bearing end face and the roller. When the roller is pulled circumferentially, there is a gap or the axial position of the drum is uncertain. , resulting in inaccurate color printing.

5 version of the mouth. The occurrence of paper breaks, roll blankets, high-speed emergency stop and other events often cause the position of a plate cylinder to be deviated during the printing process. For the shaft drive high-speed wheel transfer machine, its axial and circumferential registration The adjustment range is limited, so when the position of the plate is deviated, the circumferential overprint of the corresponding layout will be affected, and even the front and back overprinting will be inaccurate.

6 The main motor is also one of the mechanical factors affecting the registration of color newspapers. Because there is a gap between the printing roller gears, when the motor driving voltage fluctuates or fails, if the speed of the motor is unstable and the main motor speed is unstable, the drum gear will have a pre-shooting and a back-flushing phenomenon, causing the color printing to be overprinted. Precise, even ghosting, flowering.

printing material:

The printing materials mentioned here mainly refer to newsprint, dampening solution, etc.

(1) Newsprint is affected by the temperature and humidity of the environment.

In the printing workshop, the paper moisture content change requirements can not exceed ± 0.1%, otherwise it will affect the accuracy of overprinting. When the humidity in the printing workshop is too large, the newsprint is easy to absorb moisture and expand and contract, which is manifested as “flank” on both sides of the paper strip. When the humidity in the workshop is lower than the newsprint, the edge of the paper roll will appear tight. Such a paper certainly cannot guarantee normal overprinting when it is printed on the machine, and other problems such as breakage of the paper tape or wrinkles are likely to occur.

Both the shop temperature and the plant humidity are closely related to the moisture content of the paper. Experiments have shown that when the relative humidity of the workshop changes by 10%, the water content of the paper will change by 1%. If the relative humidity of the workshop exceeds 80%, the moisture content of the paper will change correspondingly when the humidity changes by 10%. 1.5~2%; when the temperature changes, the humidity of the workshop will change accordingly. When the temperature of the workshop changes by 5 °C, the water content of the paper changes by 0.15%.

The water content of the paper is closely related to the surface stability of the paper. Under the premise that the shape stability of the paper cannot be guaranteed, the stability of the overprint cannot be guaranteed. Therefore, it is very important to ensure the stability and standardization of the temperature and humidity of the workshop for the overprinting accuracy of the printed color report. It is generally recommended that the temperature of the printing workshop should be maintained at 18 to 24 ° C, the temperature change should not exceed ± 3 ° C, and the relative humidity should be controlled within the range of 55 to 65%.

(2) The effect of dampening solution on overprinting.

Similar to the ambient temperature and humidity mentioned above, the effect of color registration is similar. The dampening solution also affects the surface area of the paper by changing the humidity inside the paper, which has a certain influence on the overprint during the printing process. We can't avoid this kind of influence. In practice, we always ask to use the minimum amount of water while ensuring the balance of ink and water. Minimize the resulting overprint changes.

In summary, in the printing of color newspapers, the standardization operation of the prepress is guaranteed; the stability of the machine work is maintained; and the printability of the printed materials is fully understood; these factors can help us further ensure the overprint quality of the color printing.