Printing Quality Control--Color Control (1)

Color control

The printing industry has been making efforts to control color for many years. Now many color control systems for printing machines are operating around the world. The purpose of these systems is to optimize the printing conditions and control the stability of color reproduction by controlling printing characteristics.

The test items for most color control systems are the same, and the various items are combined in different forms for different reasons. Test items that may be included are field, overprinted, overprinted dot patches, tricolor gray balance dot patches, dot gain, ghosting, slick, or plate exposures.

For many years, the industry has attempted to control the color by controlling the density of solid ink, but the overall impact of all printing parameters is more important than the density of the field. In order to detect the printing characteristics, some control strips and test plates were industrially manufactured to monitor the time of the problem and the time when one of the printing conditions was out of control.

As mentioned earlier, people expect a consistent and consistent hue of prints. Hue is generated by overprinting dots, and most of them are parallel dots. The change of density in the field will cause the dot coverage to change. When the dot coverage changes, the most obvious result is the hue change.

It is hoped that there will be some dot gains, and some dot gains will always appear, but as long as the dot gain is stable and under control, it is a normal increase, and this increase has already been compensated for during color separation.

The color control is best able to show the printing hue and the dot gain of each printing ink, and then maintain a stable hue and saturation by changing the density of the solid ink. This kind of control is like controlling the speed of the car. It cannot measure the position of the accelerator pedal, but it can measure the speed of the car. The accelerator is just like the dot size, and the speed is like the density on the ground.

In color reproduction, it is important to see the density or hue, rather than a single dot size, the eye can not see every dot, the eyes see the entire appearance produced by the dots. The eye can not distinguish the dots, as long as the color appearance looks correct, then the coverage of the printing dot is correct, regardless of the dot increase in the ink layer.

Second, the basic principle of quality control

For printed matter, size, negative number, price, quota per day, etc. can all be expressed in numbers. However, for actual printing quality, there is a long-term lack of quantitative evaluation data and there is no objective quality control standard. In addition to registration. In addition to the commonly used criteria such as dirtiness, the printing quality is always evaluated according to the dot, the fidelity of color reproduction, and the quality stability of a batch of products.

The most common quality defects are color deviations between printed images and originals and color changes during printing. The color chart attached to the end of the book shows the color change of a cover during the printing process. The blue-violet color is obtained by overprinting cyan and magenta. The hue change may be due to the change of the ink layer thickness or the ink layer thickness and dot Incorrect adjustment of value, ink print.

The color image 2 shows that when judging the middle green color, the result of the evaluation has a subjective color due to the difference in the surrounding colors and the difference in the visual perception between the lighting and the printing personnel.

In order to visually judge the fidelity of print reproduction, many auxiliary measures were adopted to reduce the difficulty of visual evaluation. Such as the use of D65 standard light source lighting, add print control elements on the register, set the signal bar to determine the dot change and determine the dot distortion, ghosting, and ink overprint the control block.

For a truly objective assessment, objectively measured variables and appropriate measurement methods are absolutely necessary. The quality of printed images is affected by many process parameters. Many of these process parameters are often not independent variables but affect each other. For example, when the thickness of the ink layer is increased, the adjustment value of the dot network will always increase, and the trapping percentage will also be affected. This situation determines the complexity of the print quality control. The key to an objective and orderly quality control method is to determine the objectively measured variables and adopt appropriate measurement methods.

Maintaining print colors throughout the printing process is primarily dependent on three parameters: ink thickness, dot coverage, and trap rate. Each of these three parameters has its own physical quantity that can be measured objectively. Figure 5-1 shows the vertical and horizontal relationships between the three quality parameters and related physical quantities. This figure describes the basic principles of quality control for printed images.

1. Ink thickness. The visual color of a printed image depends to some extent on the thickness of the ink layer. For coated paper, in order to make the cyan, magenta, yellow, red, green, and blue-violet printing colors coincide with its correct position on the chromaticity diagram, the thickness of the ink layer should be 0.7 to 1.1 μm. Depending on the type of ink, the offset ink layer can reach 2.5 μm.

When the ink layer thickness is appropriate, the printed matter can obtain a larger copy color gamut.

For clear inks, the change in the thickness of the printing ink layer is noticeable from the change in color saturation, and the brightness is also affected, resulting in the deviation of the process color from the specified color location, limiting the reproducible color gamut.

As already mentioned above, the visual inspection of the ink thickness is not sufficient, and the density of the ink layer is controlled by measuring the solid density in a press shop using a densitometer. This is because the thickness of the ink layer is closely related to the density of the field. The absorption characteristics of the ink layer depend on the hue of the ink, the thickness of the ink layer, the characteristics and concentration of the pigment in the ink. However, because the hue of the three primary color inks is standard, the concentration of the pigment is also specified within a certain range, so only the thickness of the ink layer can be adjusted by a worker as a variable. Of course, increasing the thickness of the ink layer to a certain degree will reach his and the field density, and it will not exceed him and the density when measured on a container full of ink. Therefore, the density of infinities does not exist.

Figure 5-2 shows an example of ink layer thickness adjustment on a four-color offset press. The two dashed lines indicate that the thickness of the ink layer in offset printing is usually 0.7-1.1 μm. From the figure, it can also be seen that the curve begins to flatten when it reaches a very thick ink layer, but that is for offset printing. It is no longer applicable.

(to be continued)