About Color Control in Offset (4)

Fourth: Neutral gray balance in color reproduction

First, the concept of neutral gray balance

The reproduction work of the printed matter is color restoration, tone reduction, texture restoration, and resolution restoration. The so-called tonality is the color classification by lightness, the color is three variables, the neutral gray is a three-dimensional color space, the neutral gray is composed of Y, M, C three colors, the three variables of the color (hue, saturation, lightness ) Hv /c means that neutral ash has no hue and saturation, and there is only one-dimensional variable brightness.

We know that the three primary colors R, G, and B of the shade transition to the white field, and the three primary colors Y, M, and C of the pigment transition to the black field. In addition, four-color printing is subject to two conditions: one is that only four colors of Y, M, C, and K can be used; and the other is that the ink layer on each sheet is evenly distributed.

Neutral ash refers to a series of colors such as Y, M, and C. They absorb R, G, and B light in equal amounts. Only lightness changes without changes in hue and chroma.

When the three primary colors Y, M, and C of the pigments are overprinted in the proper proportions on the spot or halftone, the neutral gray scale can be completely reproduced. Then, the three primary colors of Y, M, and C are said to have reached a neutral gray balance. The monochromatic density of the three primary colors that achieves gray balance is called the equivalent neutral gray density.

Second, emphasize the significance of neutral gray balance

1. Correctly copy the original gray.

2. At the time of plate making and printing, neutral gray balance is used as a measure of the appropriateness of the ratio of the three primary color inks used in color separation and printing.

Third, the background removal and neutral gray balance

1. The concept of background color removal, the so-called under color removal (UCR) refers to the removal of a portion of the three primary colors of Y, M, and C to form a neutral gray background, that is, the approximately black bottom of the color materials Y, M, and C. Color, and replaced with black ink.

For example: suppose a certain color is overprinted by Y, M, C three-color, Y dot area is 80%, M dot area is 80%, C dot area is 100% (field), if according to neutral gray balance, 80% Y 80% of M can overlap with 90% of C to form black ink equivalent to 90% of dot area. Then:

C =80%Y +80%M+100%C =80%Y +80%M+90%C +10%C =90%K +10%C

2. The need to remove the ground color, 1 improves the printing applicability: in general under the dark tone, in the dark tone area if you use the three primary colors Y, M, C printing, the dot area is often very high, in high-speed multi-color printing, It is prone to process problems. If the background color is removed, the amount of ink is greatly reduced and the printability is improved. 2 is conducive to control the neutral gray balance when printing. 3 is conducive to ink drying, reducing the Y, M, C trichromatic color inks overprinting. 4 As the amount of Y, M, C color inks is reduced, the amount of black ink is increased, thereby reducing the cost, and black ink is relatively inexpensive. 5 The use of high-speed multi-color printing presses, the biggest problem affecting the speed and quality of high-speed printing presses is the ink drying problem, which means that the ink drying speed will directly affect the speed and product quality of high-speed multi-color printing presses to solve this There are many ways to solve the problem, such as changing the composition and structure of the ink and improving the drying method of the printing machine. The other is to improve the process, improve the black ink concentration and gloss, and reduce the Y, M, C trichromatic ink in the achromatic color. The amount of ink at the site, which effectively thins the ink layer to speed up the drying process, is the fundamental reason for using UCR in color reproduction.

3. Background removal should pay attention to several points

1 Undercolor removal generally works in midtones, shadows neutral gray and near-neutral gray areas.

2 Correctly control the removal amount.

3 master the control of the removal point, which is to remove the scope of control.

Fourth, the relationship between the dark lines and the amount of background removal

The depth of the black plate and the amount of background removal are closely related to each other. The amount of background removal is large, and the black version should be deep. On the contrary, the amount of background removal is small, and the black version should be shallow. At the same time, the length of the black tone should be determined by the length of the dark tone density range of the original. Dark tone density range is long, black version should be long, dark tone density range is short, black version should be short. Neutral gray balance is an important condition for copying and restoring a manuscript. It is also a technique difficult to master in plate making and printing because Y, M, and C inks have a certain degree of difficulty in reproducing the neutral gray balance.

V. Neutral gray balance control The main factors affecting the neutral gray balance:

1 ink color characteristics.

2 The proportion of three primary colors.

3 color order of printing.

4 paper properties.

5 process operations.

1. The color characteristics and the law of change of ink, the control of the neutral gray balance, and the factors affecting the neutral gray balance must first be clear about the law of color change. 1 Three primary colors of ink can be mixed in black in a certain proportion. 2 After the tricolor inks are mixed to obtain a similar black color, plus different proportions of diluting agents, neutral ash of different colors is obtained. 3 The three primary colors can be mixed in different proportions to obtain the inter-color or complex color of different hue, and the hue of the three primary colors is proportional to the large-scale hue. 4 The two primary colors of ink are mixed in equal amounts to obtain the standard inter-color.

2. Neutral gray balance on the requirements of the ink, a high transparency: When the ink is coated into a uniform film, the degree of coloration of the background color of the material under the ink film, the clearer the background color, the higher the transparency of the ink. 2 Fine pigment particles: Fine pigment particles should be printed on high-grade and delicate products. 3 Strong coloring: The coloring power of ink is a quantity that expresses the color intensity or concentration of the ink. For the same color ink, the coloring power depends on the dispersion of the pigment on the binder, and the higher the dispersion, the stronger the coloring power; It also depends on the content of the pigment, the higher the content, the stronger the coloring power. 4 Light resistance: Under the effect of light, the color of the pigment will change and fade in different degrees, which is the light-resistant characteristic. High efficiency of 5 colors: Color efficiency is a comprehensive index of ink color difference and gray scale. 6 good chemical stability: the color of the ink will change under the effect of some chemicals, therefore, the ink is required to have good chemical stability, water resistance, acid and alkali resistance, ethanol resistance and other properties.

3. The relationship between the performance of paper and neutral gray balance, the quality of the printing company's products is affected by the raw paper, the ideal paper whiteness is 100% reflectance, total reflection of 380 ~ 700nm light. In the process of plate making and printing, it is necessary to consider "different from paper to paper", and adopt appropriate technological contents to adapt to the used paper, especially in the thickness of the dot, the number of colors, the size of the layout, etc.

Exquisite high-grade offset printing products include domestic coated paper, offset paper, painted newspapers, imported coated paper, egg-printed paper, textured paper, etc., due to the different degrees of tightness, whiteness, paper absorption, smoothness, and elasticity of the paper. , and the curve that reproduces the reproduction of the tone is also different. The use of coated copy paper printing tone level reflects very rich, and use offset printing printing tone level is relatively flat, the same 157g / m 2 coated paper due to different origins, domestic or imported paper also have a certain gap.

Paper with low absorption, high whiteness, and high smoothness has a better reproduction curve. The amount of ink used is thin and the colors are still bright.

4. The relationship between printing color sequence and neutral gray, color offset printing press uses Y, M, C, K color sequence, first consider the lowest transparency of the ink on the first color printing, and then print high transparency ink, Color arrays are arranged according to the size of the ink's transparency. At the same time also taking into account the viscosity of the ink, due to the small viscosity of the Y ink, you can avoid the phenomenon of dirty printed back; light color printing on the front, cleaning the ink roller is very easy; also facilitate four-color overlay accuracy. The multicolor offset printing press uses the inverted color sequence K, C, M, Y. Since the multicolor printer is wet and wet, the printing ink is dried without the oxide conjunctiva, and the other color ink is overprinted. The viscosity of the four-color inks is K>C>M>Y, which satisfies the requirements of the four-color machine. The color sequence of the multicolor machine is gradually lowered from high to low according to the ink viscosity, and the viscosity is high. India, the overprint fastness is better.

In addition, the multi-color machine ordering is taken into consideration, so that the lowest transparency of the first printing, black ink, the lowest transparency, that is, the darkest, as the outline of the screen with a darker color, it should be placed in front of printing, so that high transparency of the M, C two colors on the back of India, can make the whole picture colorful.

The different color sequence arrangement has a great influence on the neutral gray balance. Different color sequences will directly affect the color tone of the printed product quality and color. In the actual production, a comprehensive analysis should be carried out. Considering the change factor a little bit more, the color sequence must be arranged according to the effect of different types of manuscripts and customer's requirements in order to achieve the perfect reproduction of the printed matter.

Under normal circumstances also have to consider several kinds of ink after the overprint color cast problem: M, C, Y is easy to green after overprinting, C, M, Y red after overprinting, C, Y, M after overprinting is easy to reddish, M, Y, C overprinted after brownish green, Y, C, M superimposed after the superficial purple, Y, M, C superimposed after the blue-green.

Source: PACK.CN