In printing and copying, gray balance is used to judge whether the printing color is balanced or color cast. It runs through the entire process of printing and copying. In color flexographic printing, it is also a problem that deserves special attention.

1. The concept and function of gray balance

1. The basic concept of gray balance

According to the color rendering theory of the subtractive color method, the superposition of the maximum saturation of the three primary inks should appear black. However, the hue, lightness, and saturation of the inks used in actual production all have different degrees of deviation. The whiteness, smoothness, and absorbency of the paper will also cause the instability of the color density value. , Reflected in the paper can not get a neutral gray, but a slightly brownish gray.

Due to the ever-changing colors on printed materials, people have explored the method of checking gray balance to control the reduction of printed colors. Under certain printing suitability, the dot ladder of the yellow, magenta, and cyan printing inks is combined to overprint according to a certain dot ratio from light to deep to obtain a gray balance. At this time, the yellow, magenta, The percentage of blue dots is the gray balance data for that dot. Since gray has neither hue nor saturation in the three color attributes, it is a neutral color, so gray balance is sometimes called neutral gray.

The most common method for judging the gray balance is to print a gray ladder, which is to print yellow, magenta, cyan, and black ink color marks next to the printed matter, so as to control and adjust the tone and color of the color image. In order to achieve the perfect reproduction of colors, the dot area ratio of the three primary colors ink must be changed according to the actual characteristics of the ink during printing, so that the three primary colors ink will display the correct gray balance after overprinting.

2. The role of controlling gray balance

Gray balance is one of the most important contents in the theory of printing and copying. Controlling the gray balance is to indirectly control all the tones on the entire screen by controlling the gray part of the screen. It is a measure to judge whether color separation and color printing are correct. It is a method to judge whether the printing color is biased and whether to faithfully come to the original manuscript. It is also the key to the scientific and scientific management of the printing process. [next]

Second, the impact in flexographic printing

Factors of gray balance

A set of printing gray balance data is for a certain thickness of ink, paper and ink layer. There are many factors that affect the gray balance in printing, both in printing process and in materials and equipment. The main influencing factors are:

1. The effect of ink color quality

Any slight changes in the primary color inks during printing will cause color shift. Different ink C, M, Y, B density (or dot area ratio) is different, there are four parameters to characterize the characteristics of the ink, namely ink color intensity, hue error, grayscale and color efficiency.

The ink color intensity determines the saturation of the ink color, and also affects the accuracy of the overprint and complex hue of the overprint and whether the neutral gray can achieve balance and other issues. Hue error, also called color shift. The color of the ink is impure, resulting in poor absorption of the spectrum, resulting in undesirable density, and causing hue errors.

The gray scale of ink refers to the achromatic components contained in an ink. It has a great influence on the saturation of the ink. The smaller the gray percentage, the higher the saturation of the ink.
Ink color efficiency means that an ink should absorb one third of the color light and reflect two thirds of the color light. The ink has undesirable absorption or insufficient absorption, which makes the color efficiency of the ink drop.

Before selecting different inks for printing, it is necessary to measure the basic characteristics of the ink (hue, saturation and lightness), the physical and chemical characteristics of the ink (drying speed and gloss, etc.) to establish a gray balance relationship suitable for this ink In order to take these factors into consideration when formulating the printing process and plate making. In addition, the printability of the ink also affects the gray balance. For example, changes in the viscosity, rheology, and ink layer thickness of the ink will affect the color reproduction of the original, causing the gray balance to be destroyed.

It should be emphasized here that in the process of pre-press color separation and plate making, since more than 20 kinds of ink data provided in Photoshop are for offset printing, there are no unique ink parameters for flexographic printing. If they are still made with offset printing parameters The flexographic version will inevitably produce out of control of the gray balance, so it must be based on the ink used by the printing company, pass the sample test, according to the characteristics of the flexo, and expand the figure as shown in the figure, etc. The color balance of flexo printing is taken into account when printing.

2. The influence of the characteristics of the printing material on the gray balance

The gray balance data is affected by the materials used in printing. Taking paper as an example, different types of paper have a large difference in the color rendering ability of the same ink. Coated paper is better than offset paper. Coated paper reflects the rich tone level. Offset paper is relatively bland, even if it is also 157g / cm2 coated paper, due to the different origin, there is a certain difference in the impact on the gray balance. The whiteness, smoothness, absorbency, gloss, opacity and pH of the paper will also affect the gray balance.

Therefore, in the process of flexo plate making and printing, it is necessary to consider "different from paper". Different papers have different gray balance data. The fineness of the dots and the size of the tone contrast should be adjusted appropriately. [next]

3. The impact of the number of screens on the gray balance

The quality of the printed matter is not that the higher the number of screen lines, the better, because the higher the number of screen lines, the smaller the dots, the finer the level of performance, but the more serious the dot expansion, which affects the gray balance. The lower the number of screen lines, the lighter the dot expansion, the firmer the adhesion on the printing plate and the higher the printing resistance.

4. The effect of ink layer thickness on gray balance

In flexographic printing, the thickness of the ink layer on the substrate is mainly determined by the number of lines of the anilox roller and the volume of the anilox. The line number of the anilox roller is high, the cell capacity is small, the thickness of the ink layer is thin during printing, and the dot expansion rate is small, which can reflect a longer tone range and make the color reproduction more realistic. The anilox roller has a low line number, a large mesh capacity, and a thick ink layer. However, the dot expansion is serious. It is easy to cause dot adhesion in the dark tone part, which makes it difficult to reproduce the color and tone level.

5. The effect of printing pressure on gray balance

The printing pressure of flexographic printing plate is the lightest among several other printing methods, only 1 ～ 3kg / cm2. Therefore, the quality of flexographic printing is closely related to the adjustment of printing pressure during operation. The size of printing pressure has an extremely important influence on the expansion of printing dots and the tone and color reproduction of the entire copied picture.

The pressure between the ink fountain roller and the anilox roller is mainly used to control the ink consumption of printing and evenly transfer the ink. The function of the ink fountain roller is to squeeze the ink off 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 low; otherwise, the amount of ink consumed will be high. If the pressure of the two rollers is too large (that is, the gap is small), the amount of ink consumed on the anilox roller is relatively reduced. Although the dots can be printed clearly and the characters and lines can be printed clearly, they lack a certain amount of ink and gloss. In addition, the pressure between the two cylinders will be too high, which will cause the two ends of the anilox roller and the ink fountain cylinder to bend, the printer gear will jump, and the teeth will break. If the pressure between the two rollers is too small (that is, the gap is too large), the effect of the anilox roller to transfer the ink is lost, resulting in uneven ink consumption on the printed part of the printing plate, poor product clarity, illegible dots, line spreading, and false writing hair.

The function of the pressure between the anilox roller and the plate cylinder is to evenly transfer the ink on the metal anilox roller to the printing part of the printing plate. The correctness of the printing pressure here has a great influence on the clarity of the printed dots. If the pressure is too heavy, the printed product network will expand seriously, and the printing plate's printing durability will decrease, which will easily pile up ink and produce dirty spots. Conversely, if the pressure is too low, it will affect the transfer of ink, no ink on the printing plate, of course, can not print.

The pressure between the plate cylinder and the impression cylinder is to ensure that the ink obtained on the printing plate is transferred to the printing material without error. This is the last key to flexographic printing and it is directly related to the quality of the printed product. The printing pressure we usually refer to is the printing pressure here. When the pressure between the two rollers is large, the ink transferred from the printing plate to the substrate is large, the size of the graphic on the printing plate will become larger, and the dot expansion will also be larger. The printed dots will be spread out, the middle color will be light, and the surrounding The deep circle has a great influence on the image level (the text and line version are spread on both sides of the imprint). The dark tone level is easy to blur, the image has poor clarity, the tone level loss is serious, the tone restoration is not good, and the gray balance cannot be achieved. If the pressure is too light, the image cannot be printed on the printing material.
The flexographic printing plate is elastic, and when subjected to vertical pressure, the printing plate will expand and deform in the horizontal direction, resulting in the actual ink area on the substrate being larger than the ideal area and the graphics becoming longer. The ink on the top of the printing plate is squeezed by the printing plate and the substrate and will expand outward along the edge of the dot. The greater the pressure, the more serious the expansion. The dot on the printed sheet is larger than the dot on the original plate. This is also the reason why the reduction and compression gradations are adopted in plate making.

The printing pressure greatly affects the expansion of the dots, and has an extremely important influence on the realization of the gray balance and the entire reproduction tone and color reproduction. Therefore, the printing pressure during flexo printing must be controlled within an appropriate range. [next]

6. Influence of anilox roller parameters on gray balance

The role of the anilox roller is to quantitatively and evenly 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 cell and the volume of the cell.
The line number of the anilox roll refers to the number of grid holes per unit length along the axis of the anilox roll. The higher the line number of the anilox roller, the closer the ink layer on the anilox roller is to the "continuous" state, and the thinner the ink layer. Conversely, the thicker the ink layer. When the line number of the anilox roller increases, the opening area of ​​the ink injection hole decreases and the volume of the anilox becomes smaller, so the amount of ink transfer also becomes smaller. It can be seen that the line number of the anilox roller is not as high as possible. It is necessary to comprehensively consider the situation of the printing plate dots and the requirements of printing density.

The angle of the cell refers to the angle between the arrangement direction of the cell and the axis direction, generally 60 °, 45 ° and 30 °. Most of the metal anilox roller is 45 °, and the laser engraving ceramic anilox roller is 60 °. The intensity at 60 ° is the best and the ink transfer volume is the largest.

The cell volume refers to the amount of ink contained per unit volume of the anilox roller. It is proportional to the size and depth of the opening of the net hole. The depth, shape, and amount of ink stored in the mesh hole directly affect the quality of the printed matter. If the volume of the cell is large, the amount of ink transferred is large, the ink layer is thick, and the dot is easy to increase; the volume of the cell is small, which is beneficial to prevent the increase of the ink dot transferred to the substrate and the flying ink to ensure the printing quality. Therefore, the smaller the capacity of the cells, the better the ink transfer requirements are.

It can be seen from the above that the three parameters of the anilox roller determine the amount of ink transfer of the anilox roller, which also greatly affects the gray balance and gradation of the printed image and the reproduction of color.

7. Effect of network expansion on gray balance

Dot expansion is a phenomenon of dot size change in the process of plate making and printing. It refers to the enlargement of the dot area on the substrate than the corresponding part of the dot on the printing plate. There are two main reasons for the expansion of dots: one is the increase of mechanical dots (also known as physical dots), and the pressure causes the dots to expand; the other is the increase of optical dots, and the light scattering produces a considerable dot expansion visually. Expansion of the dots will cause pasting or ghosting, resulting in a reduction in image clarity, loss of gradation, loss of tone, difficulty in restoring color tones, and inability to balance the three primary colors.

In flexographic printing, because flexographic printing uses an elastic printing plate, the phenomenon of dot expansion is more prominent. The main factors that cause the expansion of flexographic dots are: printing plate, printing pressure, anilox roller and so on.

The flexible version has elasticity. When subjected to vertical pressure, the flexible plate will expand and deform in the horizontal direction, resulting in the actual ink area on the substrate being larger than the ideal area. The thicker the printing plate, the lower the hardness and the worse the flatness, the more serious the dot expansion.

The greater the printing pressure, the easier the dot expansion.

The anilox roller has a high mesh line number and a small mesh hole, forming a thinner and more uniform ink layer, which can reduce the expansion of the dot. The lower the line number of the anilox roller, the larger the cavity volume, and the thicker the ink layer, the easier the dots expand. Therefore, the choice of anilox roller is extremely important to control the expansion of the dot. The expansion of the control network has also become the key to the success of flexographic printing.

The expansion of dots in printing is inevitable, the important thing is to be stable. Therefore, the tone compression and shrinkage must be done first during plate making, and the proportion of the three primary inks must be adjusted during printing to ensure that the three primary inks achieve neutral gray balance after overprinting, so as to achieve perfect reproduction of flexo colors.

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