First, the concept and role of gray balance
1. The basic concept of gray balance
According to the subtractive color theory, the superposition of the maximum saturation of three primary inks should be black. However, the hue, brightness, and saturation of the ink used in the actual production are subject to different degrees of deviation. The whiteness, smoothness, and absorption of the paper also cause the color density value to be disordered. The three primary inks are stacked in equal amounts. Coupling, reflected on the paper, does not give a neutral gray, but a slightly dark brown.
Because of the ever-changing colors of prints, people have explored the use of gray balance methods to control the reproducibility of printed colors. Under a certain printability, using the dot scales of yellow, magenta, and blue three-color printing inks, from shallow to deep, a certain ratio of dots and dots is overprinted to obtain the gray balance, and the yellow, magenta, and yellow scales at that time are measured. The percentage of blue dots is the gray balance data for this point. Because gray has neither hue nor saturation in the three attributes of color and is a neutral color, gray balance is sometimes called neutral gray.
The most commonly used method to determine gray balance is to print a gray ladder rule, which is to print yellow, blue, green, and black ink color marks next to the print, in order to control and adjust the tone and color of the color image. In order to realize the perfect reproduction of the color, according to the actual characteristics of the ink, the dot area ratio of the three primary color inks must be changed during printing, so that the three primary color inks exhibit an accurate gray balance after overprinting.
2. Control the role of gray balance
Gray balance is one of the most important contents in print copy theory. Controlling gray balance is to indirectly control all shades on the entire screen by controlling the gray section of the screen. It is a standard to measure the accuracy of color separation and color printing, and it is a means to determine whether the printing color is color cast and whether it is faithful to the original manuscript. It is also a pivotal point for the data processing and scientific governance in the printing process.
In printing and copying, gray balance is used to determine whether the printing color is balanced or color cast, and it runs through the entire process of printing and copying. In color flexo printing, it is also a subject that should be particularly addressed.
Second, the impact of flexographic printing
Grey balance factor
A set of printing gray balance data is for a certain thickness of ink, paper, and ink layers. There are many factors affecting the gray balance in printing, both in terms of printing technology, as well as materials and equipment. The main influencing factors are:
1. Effect of ink color quality
Any slight change in the original color ink in the printing will result in color shift. Different inks C, M, Y, B density (or dot area rate) are different, the characteristics of the ink has four parameters, namely ink color intensity, hue error, gray scale and color efficiency.
The ink color intensity determines the saturation of the ink color, but also affects the correctness of overprinting and compound color hue and neutral gray balance. Hue error, also known as color deviation. The color of the ink is not pure, resulting in poor absorption of the spectrum and an undue density resulting in hue error.
The gray level of ink refers to the achromatic component contained in an ink. It has a great influence on the saturation of the ink. The smaller the percentage of gray scale, the higher the saturation of the ink.
The ink color efficiency means that one kind of ink should absorb one third of the color light and reflect two thirds of the color light. Inadequate absorption or absorption of ink in the ink makes the ink's color efficiency drop.
Before choosing different inks for printing, it is necessary to measure the basic characteristics (hue, saturation, and lightness) of the ink, the physicochemical characteristics (drying speed and glossiness, etc.) of the ink, and establish a gray balance relationship suitable for the ink. In order to make the same considerations for these factors in the formulation of the printing process and plate making. In addition, the printability of the ink also affects the gray balance. For example, the viscosity, rheology, and ink layer thickness of the ink change all affect the color reproduction of the original, resulting in destruction of the gray balance.
To exaggerate one point here, in the process of prepress color separation, because more than 20 kinds of ink data provided in Photoshop are for offset printing, there is no specific ink parameter for flexographic printing, if it is still made with the parameters of offset printing. Flexographic version will inevitably lead to loss of control of gray balance. Therefore, it is necessary to use the ink used by printing companies in detail to pass the proof test. According to the characteristics of flexo printing, factors such as the point expansion are more, and a set of flexographic parameters is established. The color balance of flexo printing is taken into consideration when coloring.
2. The influence of the characteristics of printing materials on the ash balance
The gray balance data is affected by the materials used in the printing. Take paper as an example, different types of paper have a greater difference in the color rendering ability of the same ink. Coated paper is better than offset paper, and the coated paper reflects a rich level of tone. Offset paper is relatively light, even if the same is 157g/cm2 of coated paper, because of the different origin, the impact of the gray balance also has some differences. Whiteness, smoothness, absorbency, glossiness, opacity, and pH of the paper also affect the gray balance.
Therefore, in the process of flexo platemaking and printing, it is necessary to consider “different from paper to paperâ€. Different papers have different gray balance data, and fine adjustment of the dots and the contrast of gradation should be appropriately adjusted.
3. The effect of screen number on the gray balance
The quality of printed matter is not that the higher the number of screens, the better. The higher the number of screens, the smaller the number of outlets, and the finer the level of performance, but the more severe the network expansion, affecting the gray balance. The lower the number of screen lines, the smaller the degree of network expansion, the firm adhesion on the plates, and the high resistance to printing.
4. The effect of ink layer thickness on ash balance
In flexographic printing, the thickness of the ink layer on the substrate is mainly determined by the number of lines and the capacity of the cells of the anilox roller. The number of lines of the anilox roller is high, and the capacity of the cells is small. When printing, the thickness of the ink layer is thin, and dot dot enlargement rate is small, so that the range of tone adjustment can be reflected to make the reproduction of colors more realistic. The number of anilox roller lines is low, and the dot volume is large, and the ink layer printed is thick, but the dot network enlarges severely. In the dark tone sector, it is easy to cause the dot adhesion to cause the color and tone levels to be difficult to reproduce.
5. Effect of printing pressure on ash balance
Flexographic printing pressure is the lightest among other printing methods, only 1 ~ 3kg/cm2. Therefore, the quality of a flexographic printing product is closely related to the adjustment of the printing pressure during manipulation, and the size of the printing pressure has an extremely important influence on the expansion of the printing dot and the reproduction and color reproduction of the entire reproduction picture.
The pressure between the ink fountain roller and the anilox roller is mainly used to control the amount of ink to be printed, and the ink is transmitted on average. The function of the ink fountain roller is to squeeze out the ink on the surface of the anilox roller, and the pressure can be slightly larger. If the pressure between the two rollers is large, the amount of ink on the anilox roller will be small; otherwise, the amount of ink will be greater. If the pressure of the two rollers is too large (ie, the gap is small), the amount of ink consumed on the anilox roller is relatively reduced. Although the dots can be printed clearly, the characters and lines can be printed clearly, but the ink volume and gloss are lacking. In addition, due to the pressure between the two rollers is too large and cause the two ends of the anilox roller and ink fountain roller bending, printer gear beat, broken teeth and other faults. If the pressure between the two rollers is too small (ie, the gap is too large), the anilox roller loses the effect of ink transfer, resulting in uneven printing of the ink on the printing plate of the printing plate, poor product clarity, riddled dots, lines spread, and false writing marks. hair.
The function of the pressure between the anilox roller and the plate cylinder is to transfer the ink on the metal anilox roller evenly to the printing department of the printing plate. Whether the printing pressure is accurate or not depends on the degree of clarity of the printed dots. Excessive pressure, the printing of the product outlets to expand the network is severe, the printing plate printing rate decline, easy heaping ink, resulting in dirty spots. Conversely, if the pressure is too small, the transfer of ink is affected, the printing plate is not inked, and of course it cannot be printed.
The pressure between the plate cylinder and the impression cylinder is to ensure that the ink obtained on the printing plate is accurately transferred onto the printing material. This is the last hinge of the flexographic printing and it directly relates to the quality of the printed product. The printing pressure we usually refer to refers to the printing pressure here. If the pressure between the two rollers is large, the printing plate will transfer more ink to the substrate. The size of the plate on the printing plate will become larger and the dot enlargement will be large. The printed dots will be spread out and the middle color will be light. The deep circle has a great influence on the image level (text and line version are printed and printed on both sides), dark tone levels, poor image clarity, serious loss of tone levels, bad tone reduction, and can not achieve the gray balance. If the pressure is too light, no image can be printed on the substrate.
The flexographic printing version has elasticity. When subjected to vertical pressure, the printing plate expands and deforms in the horizontal direction, which results in the fact that the actual inked area on the substrate is larger than the ideal area and the pattern becomes longer. The ink at the top of the printing plate's Internet point is also squeezed by the printing plate and the substrate. It also expands along the edge of the dot. The greater the pressure, the more severe the expansion. The dots on the printing sheet are larger than the dots on the original plate. This is also the reason why the miniatures and compression gradations are taken into account when making plates.
The printing pressure affects the dot enlargement to a great extent, and has an extremely important influence on the realization of gray balance and the entire reproduction tone and color reproduction. Therefore, the printing pressure during flexo printing must be controlled within a suitable range.
6. The effect of the anilox roller parameters on the gray balance
The role of the anilox roller is to quantitatively and quantitatively transfer the required ink to the printing plate. This involves the three main parameters of the anilox roller, namely the number of lines of the anilox roller, the angle of the cells, and the volume of the cells.
The number of lines of the anilox roller refers to the number of Internet access holes along the axial length of the anilox roller. The higher the number of lines of the anilox roller, the closer the ink layer on the anilox roller is to the “continuous†state and the thinner the ink layer thickness. On the contrary, the ink layer is thicker. When the number of anilox roller lines is increased, the area of ​​the ink-receiving hole is reduced, and the volume of the network hole becomes smaller, so the amount of ink transferred is also reduced. It can be seen that the number of anilox roller lines is not as high as possible, and the requirements of printing plate dots and printing density should be comprehensively considered.
The cell angle refers to the angle between the alignment direction of the cell and the axis direction, and generally has 60°, 45° and 30°. Most of the metal anilox rollers use 45°, and the laser engraving ceramic anilox rollers use 60°. The best intensity of 60°, the largest amount of ink transfer.
The cell volume refers to the volume of ink contained in the unit volume of the anilox roller. It is proportional to the size and depth of the opening of the mesh. The depth, shape, and amount of ink stored in the cells directly affect the quality of printed products. If the volume of the cell site is large, the amount of ink to be transmitted is large, the ink layer is thick, and the dot network is easy to increase. The volume of the cell hole is small, which is advantageous to prevent ink dots transmitted to the substrate from increasing and flying ink, thereby ensuring the printing quality. Therefore, the smaller the better the capacity of the cyberspace is, the better it is to meet the requirements for ink transfer.
From the above, it can be seen that the three parameters of the anilox roller determine the amount of ink transferred by the anilox roller, and it also greatly influences the gray balance and tone level of the printed image and the reproduction of the color.
7. Influence of network expansion on ash balance
Dot enlargement is a phenomenon in which the dot size changes during the process of plate making and printing. It refers to the enlargement of dot area on the substrate compared with the dot area of ​​the corresponding department on the printing plate. There are two main reasons for network expansion: one is the increase of mechanical network points (also known as the increase of physical network points), and the pressure causes the expansion of network points; the other is the increase of optical network points, and the light scattering generates visually large dot gains. The proliferation of outlets will result in stencil or ghosting, which will result in a decrease in the clarity of the image, loss of gradation, loss of tone, and difficulty in restoring tones. The three primary colors cannot be balanced.
In flexographic printing, because the flexographic printing uses an elastic printing plate, dot gain is more prominent. The main factors that cause the flexo dot expansion are: printing plate, printing pressure, anilox roller and so on.
The flexible version has elasticity. When it is subjected to vertical pressure, the flexographic printing plate will expand and deform in the horizontal direction, resulting in the actual inked area on the substrate being larger than the ideal area. The thicker the plate, the lower the hardness, the poorer the flatness, the more severe the dot expansion.
The greater the printing pressure, the easier it is to create dot gains.
The anilox roller has a high number of screen lines and a small network of dots to form a thinner and more uniform ink layer, which can reduce the expansion of the dot. The lower the number of lines of the anilox roller is, the larger the volume of the cells is, and the thicker the ink layer is, the network dots can be easily expanded. Therefore, the selection of anilox rollers is extremely important for controlling dot gain. Control network expansion has also become a pivotal point for the success of flexographic printing.
The expansion of printing outlets is inevitable. It is important that we do not mess things up. Therefore, it is necessary to do gradation compression and reduction processing when making a plate. When printing, it is necessary to adjust the ratio of the three primary color inks to ensure that the three primary color inks achieve a neutral gray balance after overprinting so that the perfect reproduction of the flexographic color can be achieved.
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