In color printing of half tone images, different shapes of interference stripes (hereinafter collectively referred to as moiré) are often generated on the image, especially screen printing. The existence of these moirés will destroy the gradation effect of image colors and must be eliminated or weakened in the platemaking process. To eliminate or reduce the influence of “rhizard†on the image, it is necessary to understand the causes of the moiré and the main factors that cause the print to produce moiré in the production process of the screen, so as to avoid and reduce the occurrence of moire. Printing principle <br> <br> dot image print dot image belongs halftone printing, halftone printing, color shade and depth of the image size determined by the network, i.e., a small portion of the high profile dots, the large dot of low-key portion; network There are two major types of printing, namely, AM printing and FM printing. Causes of moiré
1. Amplitude-Modified Dot Printing Amplitude-modulating dot-dot printing is characterized by the same center-to-center distance between adjacent dots of the same number of screen lines. The resolution and fineness of the graphic and text are determined by the center-to-center distance (number of lines) of dots. In screen printing, regular screen dots on a screen are attached by regular screens. The overlapping of two regular “graphics†(printed images and screens) interfere with each other, ie they form a turtle. Pattern.
2. The feature of printing dot-tone printing at the FM network is that the dot size is equal, and the center-to-center distance between two adjacent dots is randomly changed. The depth of the image color is determined by the degree of density of the dot. In the highlight part of the image, the dot is loose, and the color is light; the dots of the low profile are tight and the color is deep. Because the dots are random and change randomly, the irregular dots on the image will not interfere with the silk screen of the regular textile. Because the dots of the images are arranged randomly, the moiré caused by the angle difference of the dots No longer exists in FM screen printing.
3. Comparison of AM and FM network printing Compared with AM screening, FM screen printing can make the image sharper, the color reducibility better, and the sensitivity to overprinting errors small, so the impact on color is small. Because FM screen printing is not limited by the screen angle in color printing, besides four-color printing, R, G, B and other colors can be printed to achieve seven-color printing, making the color reproduction of images more realistic. The level is more distinct, and the middle tone level is also better, and it is not easy to produce dot density jumps.
However, due to the strict requirements of platemaking and screen printing conditions such as film density, screen mesh number, wire diameter, opening size, and thickness of the photosensitive layer, the cleanliness of the platemaking and printing environment will be due to the amplitude modulation screening. Affects the reproduction of subtle levels of the image, so it requires strict printing conditions to achieve. For the above reasons, the printing of halftone images is still mainly based on AM printing.
There are many factors that cause images to form moire in color screen printing, such as the angle of screening of the color separation film, the matching of the screen mesh number and the dot line number, and whether the printing parameters are reasonable.
1. The dot angle and quality of the separation film (1) The screen angles of yellow, magenta, cyan, and black are different when they are overprinted. Some of these disturbances are normal, even, and there are no irregularities. The pattern will not have any effect on the image, while others will not be normal. That is, when the angles of the various types of screen lines are poorly arranged, the screen will have regular accumulation or evacuation, and the color of the piled-up part will be deep and evacuated. Some parts are light in color, forming a regular moiré on the surface of the picture, which interferes with the color uniformity of the image. In the event of such interference, the angles of the various color printing dots and the angle difference between the color printing sheets should be reset. The principle of setting is that the angle difference between each printing sheet is greater than 15°, and the angle difference between the main color printing sheets is 30°.
(2) The quality of color separation film is caused by the uneven moire of each color separation film dot. This is a problem with the quality of the film output, which will affect the quality of the screen printing, resulting in different shades of moiré when printing. At this time, the separation film should be re-made.
2. Number of screen lines
The mismatch between the number of screens of the color separation film and the number of screen meshes can lead to moiré. In general, if the number of screens is high, the number of screens to be selected is also higher. The practical experience is that the number of screens to be screened: the number of screens = 1 (3 to 6.25). In practice, most of them are controlled at 1:5 or more, which can effectively prevent the occurrence of moiré.
3. Improper control of platemaking parameters
1 Stretching tension is not suitable or the tension is not uniform, so that the ink volume of different colors is different, and the ink is uneven;
2 four-color screen tension is not consistent;
3Before and after the stretching of the net, the parameters of the screen changed, changing the proportional relationship between the number of screens and the number of screens;
When 4 sun screens are used, the relative positions of the various color films and screens are not correct, forming moire;
If the photosensitive layer on the 5th screen is too thin or the RZ value (surface roughness) is too large, the dots will be deformed and the edges of the dots will be uneven. It is recommended that the minimum thickness of the photosensitive layer should be controlled at about 8μm; the thickness of the photosensitive layer for photocurable ink should be about 4μm.
Source: PACK.CN
How to reduce the moire on the screen