How to improve overlay accuracy

Overprint accuracy is a common quality requirement for all prints, and it is also a prerequisite for adjusting the ink color. Overprinting is accurate, and it is easy to adjust the ink color. The overprinting effect of the dots is obvious. The images are more layered and difficult to color. In the actual printing process, there are many factors that affect overprinting, and are intricate and complicated. For example, whether the imposition is correct, whether the printing plate is accurate, whether the bending plate and the printing plate are reasonable, the tension setting, the water resistance of the paper, etc. We should solve each one by one.

1. Guarantee the precision of prepress printing

Starting from the prepress process, ensure that the manual imposition, printing, and positioning holes or grooves are accurate. If CTP is used directly to print plates, automatic punching and automatic bending will reduce some human error.

2. The upper version should be accurate

The bending and loading of the printing shop operators are also crucial. For the same PS version, the bending version should be on the same bending machine, which can reduce the bending errors caused by machine errors. For the adjustable positioning pins on the plate cylinder, apply the same standard to the uniform standard. When loading the plate, use the positioning pin as a reference to ensure horizontal alignment of the two or four plates on each plate cylinder. As for the non-adjustable positioning pins, the positioning pins should be used as the reference to ensure that the front and rear and left and right positions of two or four plates of the same roller are aligned, which provides a favorable guarantee for printing overprinting.

3. Paper performance closely related to overprint accuracy

In the actual printing process, we must face different businesses, different brands, different specifications of the paper, some paper positive and negative surface smoothness difference is quite large, some paper ink absorption, water resistance, water absorption is different, its own water content is different, Various deformations will occur under different pressures and tensions. These will affect the accuracy of overprint in different degrees and in different aspects. Therefore, in the actual printing process, it is necessary to understand the characteristics of various papers, and to control tension according to different papers. The amount of water supply, maintaining the proper temperature and humidity in the workshop, minimizes the deformation of the paper during the printing process, which is an important prerequisite for ensuring accurate overprinting. In fact, the paper's deformation is related to the amount of moisture, the size of the external force and the external environment. Practice has shown that the paper's moisture content can not be changed too much, otherwise it will have a great impact on overprinting, so to control the moisture content of paper, first, to control the temperature and humidity of the workshop, under normal circumstances, the temperature is controlled at 18-24 °C The relative humidity is controlled at 60% to 65%; the second is to control the amount of water supply on the layout.

In order to reduce the deformation of the tape, the tension control should be appropriate, so that the paper does not wrinkle, does not run and the paper edge is stable. To minimize the paper's compression and deformation, when the paper is printed in magenta and cyan printing and then into yellow and black, the lateral deformation is increased or decreased, which is not conducive to horizontal overprint of yellow, black, magenta, and cyan. The phenomenon of layout registration and another layout inaccuracy, how can this situation be solved?

(1) Check whether the sun is accurate and whether the loading is correct.

(2) Overprinting due to poor roundness of the roll is not allowed, and the roll can be replaced.

(3) Mechanical overprinting is not allowed, that is, overprinting due to mechanism, gear wear, or roller walking is not allowed.
(4) some characteristics of the paper itself, such as the strength of the tape, anti-deformation capacity. If the lateral deformation of the paper increases, some tension can be increased appropriately; otherwise, the tension is reduced.

(5) Overprinting caused by uneven moisture content of the paper is not allowed. If the paper's moisture content and water absorption are different, under the influence of printing pressure, there will be different degrees of pressure deformation. Under the effect of tension, the deformation of the two sides of the paper strip is different, which also has a great influence on the overprinting. Therefore, the amount of water supply for each layout is controlled reasonably, so that the paper of each layout is less and evenly absorbed. The New Zealand water supply unit of the Group's Indian Printing Factory is relatively reasonable. Each split-board layout has four control areas and the size can be controlled independently.

(6) Check whether the pressure between the two blanket cylinders is proper and whether the pressure on both sides is the same.

4. Printing pressure

The influence of the tape tension on overprinting The change in the printing pressure and the tape tension of the web rotary press directly or indirectly affects the deformation of the tape, thereby affecting multicolor overprinting. The general printing pressure can reach the printing requirement, the key is to control the tension of the tape reasonably to reduce the deformation of the tape, so as to achieve accurate overlay. If there is no smooth tension to make the tape in tension, it will not be possible to make the overprinting accurate and ensure the folding quality. However, the existence of pressure and tension will inevitably cause the paper tape to undergo compression deformation and tensile deformation. The amount of extension increases as the pull force increases. If the tension is too small, it will lead to lateral wrinkles, misprints, and other ills in the front of the tape. If the tension is too large, the tape may be stretched and pleated in a longitudinal direction, exceeding the paper's tensile limit and easily breaking the paper, which may also cause overprinting. Not allowed; unstable tension or non-circular paper core, will cause the tape to jump so that there is ghosting, overprinting and other phenomena. Therefore, in the printing, the compressive deformation and the tensile deformation should be as small as possible to facilitate the overprinting accuracy.

From the actual production, it was found that for medium-sized machine UNISET, when using Japanese paper (45g/m2) printing color newspaper, the tension is controlled within 10N, the effect of printing is better, the overprint is accurate, the paper is smooth and not pleated. On the contrary, the tension of some home-made papers (48g/m2 or 48.8g/m2) should be controlled at 15N or more, otherwise pleating, running of paper edges, etc. may occur.

Keywords: Overprint