Print chromatic aberration is roughly divided into two kinds, one is the same batch of printed products with the same color, but with the sample color chromatic aberration; the other is the same batch of printed products with color chromatic aberration, some with the sample color, but some with the sample color inconsistency. In this paper, the author introduces the causes of offset printing color difference and how to control it.
Offset printing color difference generated
A. The same batch of prints with the same color, but there are color differences with the sample color
(1) Human factors
Since the color of the same batch of prints can be consistent, it means that the skill level of the captain is good, but why the color is not consistent with the sample and continue to print, it only means that the captain's responsibility is problematic.
(2) dry back density value
Offset printing products just after printing, the ink is still in a wet state, the density value measured at this time is different from the density value measured after drying, the same wet density test method should be taken to control, that is, the first print is the wet density value and Lab value measured in the wet state, and later the product in the wet state according to the first measured density value and Lab value for color control.
(3) paper
One, different paper on the printing ink layer color will have a different impact on the color effect will be revealed with the different whiteness of the paper, resulting in color differences.
Second, the gloss of the print depends on the gloss and smoothness of the paper. The same amount of ink with a densitometer to measure the density value, smoothness, glossy paper with high density value, and vice versa low density value.
Third, printed products through printing, varnishing and other surface treatment will have different degrees of color changes, some of these changes are physical changes, some are chemical changes. Physical changes are mainly for the product surface to increase the mirror reflection, which has a certain impact on the color density. For example, the color density of the printed product will increase after the UV varnish, and the color density of the printed product will decrease after the matte varnish. Chemical changes are mainly from varnish, UV varnish, etc. These materials contain a variety of solvents, which will cause a chemical reaction in the color of the printing ink layer, which in turn causes color changes.
(4) printing pressure
If the printing pressure is not enough or uneven, the ink layer is prone to the phenomenon of uneven thickness, so the process requires "three flat", that is, the requirements of the printing plate, blanket and substrate and lining surface are relatively flat, in order to thin the ink layer through the role of balanced printing pressure, so that the print to obtain a more uniform ink color.
B. The same batch of prints with color differences, some inconsistent with the color of the sample
(1) human factors
That the first few prints color phase accurate, in the case of water and ink has not been balanced, the normal printing, in the printing process constantly adjust the amount of water and ink, while adjusting the side of the open, is bound to cause the existence of color differences in the batch color.
(2) Fountain solution
The main principle of offset printing is that oil and water are not mutually soluble, where the water refers to the fountain solution. The amount of fountain solution directly affects the color of the ink layer, the more the amount, the more serious emulsification. Emulsified ink lusterless, color is light, so in the operation to strictly control the amount of fountain solution, required in the case of not on the dirty, the less the better.
(3) Ink rollers
The ink roller's slipperiness, stickiness, roundness, hardness and surface finish largely determine the quality of the printed ink, usually work to clean the ink roller, so that the paper dust and impurities left on the ink roller can be removed to ensure uniform ink transfer, and at the same time to adjust the pressure between the ink roller and the ink roller and the printing plate, otherwise it will cause uneven ink transfer and produce serious color differences.
(4) Equipment maintenance
If the equipment is not stable, such as unstable paper delivery, frequent stops in printing, empty sheets, crooked sheets, multiple sheets, etc., will affect the stability of the operation of the equipment, is bound to cause color differences, so to maintain the equipment, pay attention to the lubrication of the critical parts of the equipment to prevent the intensification of wear and tear, so that the equipment in a stable state.
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Offset printing color difference control
A. Standardization of operation
Although there are a variety of factors affecting the color difference, but the human factor is undoubtedly the most important, so, in the daily work, standardized operation is extremely important. The first is to correct the "three flat", that is, the blanket flat, roller flat, ink roller flat; secondly, to adjust the "two small", that is, small water, small pressure; and then to do "three diligent", that is, diligent inspection sample, diligently stirring the ink hopper, diligently check the layout moisture.
B. Be extremely responsible and establish a global concept
The problem of color difference cannot be solved by a certain person or a certain part of the people who are responsible for it, a team should be fully involved and responsible for the shift. The leader should be responsible for all the work in the shift, but other people in the shift should also be responsible, for example, the paper loader is not done after loading the paper, but also has an unshirkable responsibility for the quality and color difference, and its correct practice is to inspect the equipment within a limited time after the paper loading work is completed, check the ink storage capacity of the ink hopper, stir the ink hopper, etc.
C. Focus on self-learning and improvement
Knowledge accumulation is the basis of problem solving, establish the consciousness of self-learning, and for some key basic problems, make sure to master before problems arise. In the usual work to sum up more experience, master the necessary professional knowledge, diligent learning, humble and serious. Especially the leading machine, not only itself to learn to master professional business knowledge, to other members of their own shift also have the responsibility to pass on the helper, more to teach the assistant work experience, so that they can better assist themselves to manage the quality of products.
