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The color difference of printed matter is roughly divided into two types, one is the same color of the same batch of printed matter, but there is color difference with the color of the sample; the other is that the color of the same batch of printed matter has color difference, and some are consistent with the color of the sample, but some Does not match the color of the sample. In this paper, the author introduces the causes of the chromatic aberration of offset printing and how to control it.
Offset color difference
1. The color of the same batch of prints is the same, but there is color difference with the color of the sample
(1) Human factors
Regardless of the skill level of the captain, it is related to the captain's sense of responsibility. Since the color of the same batch of prints can be consistent, the captain's skill level is good, but why is it inconsistent with the color of the sample draft, but only the captain? There is a problem with the sense of responsibility.
(2) Dry back density value
After the offset printing product is printed, the ink is still in a wet state. At this time, the measured density value is different from the density value measured after drying. It should be controlled by the same wet density test method, that is, the first print is wet. The wet density value and the Lab value measured in the state, and then the product is color-controlled in the wet state according to the first measured density value and Lab value.
(3) Paper
First, different papers will have different effects on the color of the printed ink layer, and the color effect will appear as the whiteness of the paper is different, resulting in chromatic aberration.
Second, the gloss of the print depends on the gloss and smoothness of the paper. The same amount of ink is measured by a densitometer, and the paper having a high smoothness and gloss has a high density value, and the density value is low.
Third, the printed matter will have different degrees of hue change after surface treatment such as printing and glazing. Some of these changes are physical changes, and some are chemical changes. The physical change is mainly to add specular reflection to the surface of the product, which has a certain influence on the color density. For example, the color density of the printed matter after the UV varnish is increased, and the color density of the printed matter after the upper varnish is lowered. The chemical changes mainly come from varnish, UV varnish, etc. These materials contain a variety of solvents, which cause a chemical reaction of the color of the printed ink layer, which in turn causes color changes.
(4) Printing pressure
If the printing pressure is insufficient or uneven, the ink layer is prone to unevenness, so the process requires “three flats”, that is, the printing plate, blanket and substrate and the surface of the lining are required to be relatively flat to be thinner. The ink layer acts to achieve a more uniform ink color through a balanced printing pressure.
2. There is color difference in the color of the same batch of printed matter, and some are inconsistent with the color of the sample.
(1) Human factors
It is believed that the first few prints are in the same color. When the ink is not balanced, the printing is normal. During the printing process, the amount of ink is continuously adjusted, and the side is opened, which will inevitably cause color difference in the batch color.
(2) fountain solution
The main principle of offset printing is that oil and water are not mutually soluble. The water here refers to the fountain solution. The amount of dampening solution directly affects the color of the ink layer. The more the amount, the more severe the emulsification. The emulsified ink is dull and the color is light. Therefore, it is necessary to strictly control the amount of dampening solution in the operation, and it is required that the less the dirt, the less the better.
(3) Ink roller
The slipperiness, viscosity, center of gravity, hardness and surface finish of the ink roller largely determine the quality of the ink color of the printed matter. The ink roller should be cleaned during normal work, so that the paper dust and impurities retained on the ink roller can be removed. The ink is evenly transferred, and the pressure between the ink rollers and between the ink roller and the printing plate should be adjusted at the same time, otherwise the ink transfer will be uneven and a serious chromatic aberration will occur.
(4) Equipment maintenance
If the equipment is unstable, such as paper feeding is unstable, frequent shutdown, empty sheet, arbor, and multiple sheets in printing will affect the stability of the equipment operation, which will inevitably cause chromatic aberration. Therefore, it is necessary to maintain the equipment and pay attention to the equipment. The lubrication of the parts prevents the wear from getting worse and the equipment is in a stable state.
Offset color difference control
1. Standardized operation
Although there are many factors that affect the color difference, the human factor is undoubtedly the most important, so in the daily work, the standardization operation is extremely important. The first is to correct the "three flats", that is, the blanket is flat, the roller is flat, and the ink roller is flat; secondly, the "two small" is adjusted, that is, the water volume is small and the pressure is small; in addition, the "three-times" is to be checked, that is, the sample is checked frequently. Stir the ink fountain and check the moisture on the plate.
2. Must have a strong sense of responsibility and establish a global concept
The problem of chromatic aberration is not solved by a certain individual or a certain part of the person. A team must be fully involved in this shift, and all staff members are responsible. The leader is responsible for all the work within the shift, but other people in the shift also have to take responsibility. For example, the paper loader is not finished with paper, and there is also an unshirkable responsibility for quality and color difference. After the paper loading work is completed, the equipment is inspected for a limited period of time, and the ink storage amount of the ink fountain and the mixing ink fountain are checked.
3. Focus on self-learning and promotion
Knowledge accumulation is the basis for solving problems. It is necessary to establish a sense of self-learning. For some key basic problems, we must grasp the problems before they arise. In the usual work, sum up experience, master the necessary professional knowledge, study hard, be humble and serious. In particular, the leader, not only must learn to master professional business knowledge, but also have the responsibility to pass on the help of other members of their class, and teach their assistants more work experience, so that they can better assist themselves in managing product quality.
As the substitute teacher of the offset printing workshop, the author has the following two experiences in controlling the chromatic aberration problem.
(1) Strengthen management, pay attention to process control, and focus on key processes of key products, and carry out key control during key periods. For example, the yellow process of “Jinmantang” products and the local gold process of “Aishang” products are difficult to find chromatic aberration due to the strong wavelength of the color. It is necessary to make full use of the combination of color difference meter measurement and visual inspection, especially in the case of just changing or At the beginning of the shift, it is necessary to carry out important control over the color difference, ensure that the color difference data is correct under the premise of ink fountain balance, and guide normal production under the condition of visual inspection. Put an end to the adjustment when opening the class.
(2) Be good at summarizing and analyzing. After the end of each batch of production, summarize the advantages and disadvantages of the products, carefully summarize the problems, analyze the reasons, and prevent the micro-duplication, and repeat the control in the future production process.