Traditional halftone images (generally formed by amplitude modulation dots relative to the FM network) are regularly arranged by dots with different sizes and equal distances (spacing between dot centers) to form a continuous sense visually and achieve the purpose of reproduction . Among them, the regularly arranged dots are all spread out from the center, so the number of net lines arranged by the dots determines the size of the single dot area. For example, when the number of network lines is 100 lines / inch, the area of ​​a single dot is: 1/100 × 1/100 square inch. On a digital phototypesetting machine, the area of ​​a single halftone dot is generally composed of a 16 × 16 pixel grid structure. These pixels are arranged in a specific order to form a dot representing a certain percentage of tones. Therefore, based on this structure, the maximum tone level that can be obtained is 162-1 = 255 levels, which is equivalent to an improvement of the tone value between each level by 0.35%. The shape of the dot can also be determined according to the pixel size. At present, the most widely used dots are round dots, square dots, diamond dots and oval dots. In screen printing, it is more meaningful to have diamond dots and oval dots. 1. Intermediate tone mutation and analysis of various dot shapes The dot has the most obvious effect on the midtone area of ​​the image. This is because of the middle tone. That is, near the range of 50% dots, the adjacent dots expand and the borders touch each other. As a result, the dots are not clear during printing. In severe cases, it will cause paste and even scrapped versions. We call the expansion of the midtone region the midtone mutation. In screen printing, due to the high viscosity of screen printing ink, the geometric shape where the dots overlap each other determines the size of the midtone mutation. The round dots maintain a uniform surrounding feature throughout the tonal range. Around the midtones, the angle formed by the dots is usually an acute angle (less than 90 °). Due to the relatively dense angles, the areas between the dots are easy to ooze out and fill with ink. The mid-tone dots of the screen printing are enlarged or the mid-tone mutation is up to 15%, so for the screen printing, the round dots are the worst dot shape. There are two reasons for choosing round dots in printing: first, the uniform dot shape of the round dots is helpful to reduce the influence of ellipses and other dot-shaped natural lines, and can well avoid the formation of moire; second, relative to printing production That is to say, on high-speed rotary presses and sheet-fed offset presses, large dot deformation will occur when the paper passes through the blanket and impression cylinder, so the round dots will become elliptical, which is helpful to reduce the midtone mutation (this will be Introduced later). However, in screen printing, due to the small force, the round dots will generally not be deformed. In order to improve the defects of round dots, square dots are introduced. The following dot shape adopted in the design is: the dots start to be round, and the dot shape gradually changes from circular to square in the mid-tone range, and completely becomes square dots at 50% dots, and then gradually changes back to circular. The four corners of the square dots touch at the same time, the angle is greater than 90 °, but there may still be a sudden change in midtones. In an ideal situation, the square dots expand to 10-12% at the mid-tone. If the printing conditions are poor, the mutation will be greater. And because the straight network line destroys the boundary of the dot, this style change is more obvious to the human eye. In order to find a solution that is more suitable in the entire tonal range, two other dot shapes were designed. For diamonds and ellipses, the geometry of the midtone regions and the change in the tone value at the edge of the dot will be discussed below. The dots here also start from a round dot, transition to a rhombus or ellipse, reach a standard shape at the midtone, and then change back to a circle. By changing the shape of the halftone dots, the halftone dots start to contact at a lower tone range, generally at 40%, and the halftone dots maintain growth up to 60%. In addition to superimposing the dots at different tone values, this type of dot can also make the mesh angle of the contact range higher than 90 °, which is helpful to prevent the ink from oozing and ensure that the imprint is clear and legible. However, with the transition of the dots to diamonds or ellipses, the chances of the dots becoming linear increase, which leads to the appearance of moire. The thin and narrow dots are also easy to produce regular halftone stripe patterns visually. This phenomenon is especially noticeable in areas with a large range of the same tone (such as an image depicting a large area of ​​blue sky) or areas where the tone changes slowly (such as a flesh tone image). 2. Resolution and number of network cables The shape of the dots is determined by the resolution of the output device. In order to use 255-level tones as much as possible, the minimum output resolution of the imagesetter should be 16 times the number of design screens. For a halftone image of 50 lines / inch, the minimum output resolution required is 50 * 16 = 800dpi. If you feel that the fixed resolution of the imagesetter is not ideal, you can also derive the discretized tone number from the following formula: Tone level = {rounded (output resolution / network line number)} 2-1 For example, if the resolution of the imagesetter is 400dpi and the resulting halftone image is 50 lines / inch, the tone level obtained is: (400/50) 2-1 = 64-1 = 63 levels Although the calculated tone level is only 1/4 of the ideal range (255 levels), the difference is not obvious to the observer. The minimum tonal value acceptable in screen printing is determined by the ink, substrate and observation environment. The higher the smoothness of the substrate, the closer the observation distance, the higher the tone level required. Lower tones are sufficient, but images with a lower resolution will form deformed or other irregular dots when describing complex screen angles and converting dot shapes within the tonal range, as shown in Figure 1, when zoomed in The dot shape looks rough, and it is easy to cause mesh clogging, resulting in extremely irregular or even unpredictable moire pattern. The only solution is to use a suitable network cable aperture to reduce the effect of interference fringes. The design of halftone dots requires a high technical level and solid mathematical skills. The design of specific dot patterns or the transition sequence of dot patterns involves the theory of image processing and image replication. At present, a large number of screen printing uses diamond-shaped or elliptical-shaped dots to reduce the abrupt change of color tone and achieve good results. We believe that with the improvement of technological level, in the near future, ideal dot patterns suitable for screen printing will definitely appear. True & Bright Metal Packaging Co.,Ltd.KaiPing , https://www.tnbcan.com
Correct selection of halftone dot shape in screen printing
In the reproduction of halftone images, the shape of the dots has a great influence on the result of printing reproduction. What kind of dots are used to help reduce dot expansion and avoid the occurrence of moiré has been the focus of debate. For a long time, in the field of screen printing, people usually ignore the influence of dot shape on halftone printing. Many screen printing manufacturers have made extremely incorrect assumptions about the choice of dot shape. This is also the main reason why the printing quality of halftone images in screen printing is still very backward.