Application of AM Screening in Screen Printing

After the successively adjusted originals are color-separated, they become unequal single-tone images. Only the continuous adjustment of the image into a halftone image can be printed. The process of converting continuous tone images into halftone meshes is called screening. When the dots printed after the screening are small enough, the eyes of the human beings are not clearly identified within a certain distance, and the halftone images composed of the dots are regarded as continuous tone images. It can be seen that the importance of screening in the screen printing of screens.

At present, the screening methods for continuous tone images are mainly divided into two categories: FM screening and AM screening.

FM screening: also known as random outlets. Random dots are composed of fixed-size particles, and the adjustment of the level is achieved by changing the amount of particles appearing. In a relatively fixed range, the appearance of dots reflects the depth of the color of the location. FM screening does not include elements such as screen number, screen angle, and dot shape. Multi-color random network overprint, there will be no problem with the wrong network. Typical FM screening, although showing very good results in plain printing, is not suitable for screen printing due to the small size of the dots, and even less suitable for screen printing with low mesh. Therefore, frequency-adding screens are not considered in screen-printing screen printing.

AM screening: Also known as conventional screening or traditional screening. AM screening has a fixed amount of space and number of lines. The level of adjustment is achieved by changing the size of the outlets, where the outlets in deep colors are large, the outlets in the shallow locations are small, and the depth is adjusted by the size of the outlets. The mesh of the AM screening is based on a grid, the center position is fixed, and it is arranged in a certain shape with a regularity of 900 degrees. The distances between the centers of two adjacent points are constant, and the size of the outlets gradually changes. In its relatively fixed range, the size of outlets reflects the depth of the location. AM screening is suitable for screen printing and is a realistic choice for screen printing. The network of the amplitude modulation and screening is used as the basis for the screen printing of the screen mesh and plays the following roles:

1. The task of presenting hue, lightness and purity on the printing;
2. The basic elements of image representation in the printing process;
3. In color synthesis is the organizer of image color levels and outlines.

The basic elements or parameters of AM screening include dot shape, number of screen lines, screen angle, and screen tones. These elements are related to each other.

The dot shape refers to the geometry of the amplitude modulation dot. It does not involve the visual effects of different color levels, but also has certain influence on the adaptability in the printing process. Dot shapes are square, round, chain, elliptical, etc., and sometimes special lines such as lines and crosshairs are used to produce special visual effects. The printing effect of outlets, the expansion rate of outlets, and the printing rate of outlets are the basis for choosing the shape of outlets. Screen printing usually chooses circular and elliptical dots. Square and chain dots are easily deformed due to acute angles.

First, dot shape

Circular spot

When circular screens represent the tone of the picture, the dots of the high and middle tone are all isolated, and only the dots at the dark tone touch each other. Therefore, the value of the enlarged halftone dot in the middle tone is smaller, and the intermediate layer can be better preserved. In the dark tone, because the contact is arc-shaped, more than 70% of the adjustment layer is easily blurred, and the performance level is poor.

2. The elliptical outlets are olive-shaped, and only connected at both ends at 50%, which reduces the intermediate adjustment skipping caused by dot gain. Therefore, it is suitable for fine-grained printing, printing on characters and characters. Use more.

è´°, number of lines

1. The size of the format affects the distance viewed by visitors. It is generally appropriate to watch at a distance of about 150cm, because only at this distance, the entire content of the picture can be fully seen. When viewing the screen at a distance of 150cm, if the number of mesh lines of the screen mesh is 50 lines/inch, according to the eye angle of 0.02, the human eye can not distinguish the screen points of the screen, and thus the mesh of the screen will be added. Error correction is seen as a continuous visual effect. Therefore, it can be considered that indoor display works are not only different from certain handicrafts requiring high screen-up lines for close observation, but also are not required for outdoor advertisements requiring lower screen lines to meet long-distance observations. The increase in cable collection is based on the size of the frames. According to the distance to be viewed, the number of screen lines required for this viewing distance is generally in the range of 40 to 70 lines/inch.

2. The reason why screen printing uses a mesh tone production method is because it hopes to enhance the image with rich layers, clear details and colorful colors, and increase its appeal. However, screen printing itself is also a dot-printing system, with screen barriers, and the ability to express the tone is limited. Compared with plain printing, screen printing is a kind of printing with a lower number of screens. If it is required that most of the tones used in screen printing can be printed out, a lower number of screens should be used. Under the premise that the line ratio is more than 3 times, the screen printing is only based on the 45 lines/inch screening, and the range of 5% to 95% of the gradation can be obtained. 80 lines/inch or less can get a range of 15%~85%. Screens with more than 100 lines per inch are still relatively low in screen printing, and it is almost impossible to obtain a good tone in screen printing. In screen printing, in order to pursue the clarification of details, the number of screen lines is required to be as high as possible, that is, the dots are small; however, the performance of color and tone requires that the number of screen lines is as low as possible, that is, the dots are thick. Therefore, in order to select the appropriate number of screen lines and maintain the best balance between details and tone, we must look for the screen performance needs. The screen printing feature is the balance of color, tone and detail that can be achieved when the number of screen lines is 45 lines/inch.

Third, the angle

The screen angle refers to the angle between the arrangement line of the screen and the horizontal. The angle measured in the counterclockwise direction is generally the angle of the screen. Because the arrangement of dots is a cross-directional two-column direction that intersects 900, according to a trigonometric function, the dot angle can only be within the first quadrant. The chain point and the elliptic point are different in the longitudinal and lateral shapes, and only the two columns with the difference of 1800 are in the same direction.

From the point of view of the visual effect network, the 450 point of view looks the most comfortable, the most beautiful; 150,750, the second; 900 the worst aesthetic. Therefore, the single-tone screen angle is generally set at 450.
In AM screening, the distances between centers are equal and regularly arranged. When two or more screens match each other, periodic interference patterns appear, and the pattern changes with the difference in the dot angle. In the 450 corner, all patterns will appear and repeat in 900. The normal pattern is dense and dense, looks comfortable, and has a normal tone reproduction performance, which is usually not noticeable. If the outlets are arranged improperly, the outlets are piled up and evacuated according to law, and the pattern is thick and the colors are not uniform. It looks uncomfortable and destroys the performance of the reproduction of the colors of the outlets. It is a taboo for screen printing. Unusual patterns are called "false network". Misplaced webs can make the layout rough, incomprehensible, and affect the reproduction of layers of prints. The pattern created by AM screening can be weakened and cannot be eliminated. A screen in different angles, there will be different patterns. The angle between the general network points is better than 300, and the wrong network is not obvious and acceptable. Therefore, two-color, three-color mesh screen adjustment angle is usually selected 300 angle difference, that is, the different color versions are set at 300,600 and 900 respectively.

Four-color silk screen printing with magenta, yellow, cyan and black four-color printing plates requires screening. This creates a problem that the angle between the 900 magenta, yellow, cyan, and black four-color dots is less than 300, so that only strong colors or dominant colors can be used to suppress the influence of bad dots on the visual effect. The angles of blue, magenta, and black are 300, and the difference between yellow and other colors is 150, that is, 00 is yellow, and magenta, cyan, and black are 150, 450, and 750, respectively. In addition, when the four dot meshes are heavy, the angle difference of 22.50 can also be used without the occurrence of the wrong mesh, so the following angles can be used: 22.50, 450, 67.50, 900.

Four tone

Addition tones are also called grayscale, which refers to the area coverage of outlets, calculated as the percentage of area occupied by opaque outlets. For example, 70% of the adjustments are 70% for non-transparent networks and 30% for transparent ones. In addition to the elevation, the print is deep, the tone is low, and the print is shallow.

Because fine dots are blocked and weakened on the screen, and larger dots are pressed and expanded on the surface of the paper, there is a gap between the adjustment of the screen on the film and the actual tone of the screen. If a 200-mesh screen is used and the number of screens is 50 lines/inch, the tone range is 10% to 85%. The number of screens is too high. Due to the effect of screens, the tone level is not necessarily high. Over 80 lines of screens can hardly achieve the 5% to 95% tone of the manuscript. Therefore, screen printing rarely uses more than 80 lines of screens. Therefore, in the face of manuscript gradation, it is necessary to adjust the screening tone according to the number of screen lines during screening.

Reprinted from: Long Chen Printing Technology Network