Offset printing color failure analysis and elimination
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Color registration is not allowed to be a special case of inaccurate overprinting. First analyze their relationship and characteristics.
First, the definition of color inaccuracy
Imprinting is not allowed to mean that the different colors of the printed matter are not overprinted according to the original requirements. According to the different reasons for formation, it can be divided into two types: inaccurate rules and inaccurate color registration. The misalignment means that the position of the graphic printing on the paper is inconsistent, and the rule line between the sheet and the sheet is not straight. Monochrome machine overprinting multi-color products will not produce the phenomenon of overprinting if the rules are not allowed. The rule is not the main reason for the inaccurate overprinting of the monochrome offset printing machine. Inaccurate color registration refers to the phenomenon that the multi-color graphic can not be registered regardless of how the school version is applied. That is, the school plate cannot register the color and color, which is a special case of overprinting. Multi-color offset printing machines often have inaccurate color registration, and color registration is not one of the printing failures that often occur in multi-color printing.
Second, the color classification is not allowed
The color registration is not allowed. According to the actual production situation, there are mainly the following forms of expression.
1. The upper and lower sets are not allowed
The upper and lower color registration is not allowed to register the tip of the bite, or the method of registering the tip can not register the bite. That is to say, only one end of the up and down direction can be registered.
2. Going to the color is not allowed
Insufficient color registration means that you can't register the outside when you are in the right position, or you can't register the body when you register outside, that is, you can only register one end of the body or the outside.
3. Single corner color is not allowed
The inconsistency of single-angle coloring means that there is always a corner or two corners in the four corners of the paper. No matter how the school board is, there will always be some corner sleeves, which is also called the corner.
4. Local color registration is not allowed
Partial color registration is not allowed to refer to a certain part of the paper, but all the gauge lines can be registered.
Third, the reason for the formation of color misregistration
Inaccurate color registration may be caused by inaccurate printing or deformation of the plate. These are caused by human factors. Improper imposition is generally not a single figure and other pictures are not problematic. The deformation of the plate generally occurs in a certain part of the direction of the direction to form the upper and lower color inaccuracy. With the popularity of CTP processes and automatic plate changing, such reasons are no longer the focus. The following analysis of other reasons:
1. Paper stretching deformation
Paper has the characteristics of water absorption elongation and dehydration shortening. When the paper absorbs (or peels out) the water after printing the first color, it will elongate (or shorten), and when the second color is printed, the color registration will be inaccurate. If the paper is unevenly stretched and irregular, the type of misregistration that is produced is not fixed. Multi-color machine overprinting multi-color color and color between the printing time interval is long, the amount of paper stretching between the various colors may be large, so the resulting color registration may be obvious. The multi-color machine has a short printing time interval between color and color, and the paper generally absorbs water and stretches, and the elongation is not too large and gradually decreases in color sequence, so the color registration is generally not too large. Paper stretching and deformation is inevitable, we can only minimize the deformation of the paper and remediate the problem of color misregistration caused by paper deformation.
2. The thickness of the drum liner is inconsistent
If the thickness of the plate cylinder liner is inconsistent, the color of the upper and lower sleeves will be inaccurate. Similarly, the thickness of the impression cylinder will be inconsistent. The thickness of the impression cylinder is inconsistent. The main reason is that the surface of the impression cylinder is stuck with paper or ink layer, the thickness of the printing paper is inconsistent, and the impression cylinder is worn. In addition, local unevenness of the lining may also cause local color misregistration.
3. The drum pressure is too large
If the pressure of the drum is too large, the blanket will be greatly deformed. If the paper has a large elasticity, the paper is liable to be deformed by extrusion, that is, the paper is rushed longer and deformed toward both sides, resulting in inaccurate color registration.
4. The drum has a bite bite due to insufficient bite
Insufficient bite force can cause the paper to slip in the teeth, causing the color to be inaccurate.
Fourth, preventive measures and solutions
According to the above analysis, it is generally difficult to deal with the occurrence of color registration failure, so prevention is particularly important.
1. Preventive measures
The main precautions for monochrome machine printing are as follows: 1 The semi-finished products that need to continue to be colored should be sealed and stored with plastic film after printing to avoid deformation caused by dehumidification or moisture absorption after printing. 2 pre-pressing paper treatment or over-water air pressure treatment, so that the paper is fully hygroscopic to reduce the amount of water absorption deformation during printing. 3 Small amount of water during printing to reduce the amount of paper deformation during printing. 4 Multi-color overprinting interval should not exceed 1 day. The longer the interval is, the more difficult it is to control the deformation of the paper. Generally, the multi-color overprinting product should be overprinted once, and not printed overnight. Long-single products should also be printed in batches or cross-printed with two monochromatic machines (premise the registration). 5 Generally, do not adjust the teeth and the lining during the printing process to prevent the color registration from being inaccurate due to parameter changes. 6 plate and school version method is correct, tight plate and pull plate can not be used too much artificially caused the plate to stretch deformation. 7 Try to use horizontal silk paper for paper printing. The paper feeding direction of the horizontal paper is perpendicular to the paper yarn direction, and the paper deformation is small in the coming direction and relatively large in the up and down direction. Because the upper and lower direction of the color is not allowed to adjust the correction, the direction is not easy to adjust the correction, so the horizontal silk paper printing is better.
The main precautions for multicolor machine printing are as follows: 1 prepress drying or air pressure treatment to reduce the amount of paper deformation during printing. 2 Small amount of water during printing to reduce the amount of paper water deformation. 3 Reasonably lining the rollers to compensate for the elongation of the paper. The plate cylinder lining is arranged in descending order, and the general decreasing amount of the four-color machine lining is 0.1 mm, 0.05 mm, and 0.03 mm. 4 Correctly install and proof the plate to prevent the plate from stretching and deforming. 5 Control the roller pressure to prevent the paper from being deformed. The hardness of the rigid liner is not more than 0.25mm. 6 Use rigid lining as much as possible to reduce the amount of compression deformation of the lining during printing. 7 Make sure the bite force meets the standard to prevent the paper from slipping.
2. Solution
After the color registration fault occurs, the following methods can also be used to remedy and deal with the following solutions.
2.1 The upper and lower sets are not allowed. Adjust the plate package value to change the length of the text to achieve, the formula is as follows.
ΔL/ΔR=Lp/Rp
ΔL is the amount of change in the circumferential length of the printed image, and ΔR is the amount of change in the radius of the printing plate cylinder, that is, ΔR=R before -R, Lp is the length of the graphic on the printing plate, and Rp is the free radius of the plate cylinder.
When △L is negative, the length of the graphic is reduced. When ΔL is positive, the length of the graphic is increased. That is to say, when the plate cylinder lining is increased or the radius of the impression cylinder is increased, the length of the printed graphic will be reduced. Otherwise, in contrast.
Whether it is a monochrome machine or a multi-color machine, any method that occurs when the upper and lower sets are not allowed to be faulty can be calibrated by this method. When the upper and lower color registration is small, it can be corrected by the drawing method, or can be adjusted by changing the printing pressure. The specific relationship is as follows: only increase the pressure between the plate cylinder and the blanket cylinder, and the length of the graphic is shortened; Increase the pressure between the blanket cylinder and the impression cylinder, and the length of the graphic increases.
2.2 to go to the color is not allowed. When the monochrome machine prints the second color, it is found that the color registration is not allowed to be printed twice or overprinted. Multi-color machine printing appears to change the color of the paper to the printing, to test whether the paper is not caused by the wrong direction, because the printing paper will cause the paper to expand in the direction of the wrong direction. If it is possible to change the wire after printing, it is best to change the paper to the printing. If not, adjust the roller pressure of the color in which the color is not correct. In addition, the top plate method can be used to adjust the version of the version.
2.3 single-angle color is not allowed. The top plate and the pull plate method are used to make the plate for local adjustment of the deformation.
2.4 partial color is not allowed. Check the condition of the roller in the corresponding part, find out the reason, and the flat color is not allowed. Check the bite force of the bite in the wrong color and calibrate the bite force.
For products with high color registration accuracy or multi-color text overprinting (especially for hollowing out text and dot text overprinting), it is recommended to use multi-color machine printing, do not use monochrome machine overprinting, and try to concentrate them in the imposition. Near the bite, don't put it on the tip, which will help improve the overprint quality of such products. Color misregistration may occur in both monochrome and multicolor machines. The color difference between the monochrome machine and the multi-color machine is mainly different. When the multi-color machine prints, the paper transfer time between the groups is very short, the deformation of the paper is generally enlarged, and the monochrome machine is very different. The deformation of the paper between different prints is complex, may expand or shrink, and the amount of change may vary. Therefore, the overprinting precision of the monochrome printing multi-color product is difficult to control, so the products with higher printing precision requirements are printed by multi-color machine.