Correct Selection of Halftone Dot Shapes in Screen Printing

In halftone image reproduction, the dot shape has a great influence on the print reproduction results. It is always the focus of people’s debates as to which shape of dot is adopted to help reduce dot gains and avoid moiré. For a long time, in the field of screen printing, people usually neglect the effect of dot shapes on halftone printing. Many screen printers make extremely incorrect assumptions about the choice of dot shapes. This is also the main reason behind the poor quality of halftone printing in screen printing.

The traditional halftone images (usually formed by the amplitude modulation dots relative to the FM network) are arranged by the dots of different sizes and the same distance (the center point of the dot points) and form a continuous feeling visually to achieve the purpose of copying. . Among them, the regularly arranged outlets are spread outward from the center, so the number of network lines formed by outlets determines the size of a single outlet. For example, when the number of screen lines is 100 lines/inch, the area of ​​a single network point is: 1/100×1/100 inch2.

In a digital photosetter, the halftone single dot area is generally composed of a 16 x 16 pixel grid structure, and the pixels are arranged in a particular order to form a dot representing a certain percentage of hue. Therefore, based on this configuration, the maximum tone level that can be obtained is 162-1 = 255 steps, which corresponds to a 0.35% increase in the tone value between each level. The shape of the dot can also be determined depending on the size of the pixel. At present, the most widely used dot shapes are circle dots, square dots, diamond dots and ellipse dots. In screen printing, it is more meaningful to use diamond dots and elliptical dots.

1. Midtone mutations and various dot shape analysis

The effect of halftones on the midtone areas of the image is most pronounced. This is because of the midtones. That is, near the 50% network point range, neighboring network points are expanded and the boundaries are in contact with each other. As a result, dots are not clearly printed during printing. In severe cases, it may cause stencils or even discarded copies. We call the enlargement of the midtone region a midtone mutation. In screen printing, due to the high viscosity of the screen printing ink, the geometry of the overlap of the screen dots determines the size of the midtone mutation.

The circle dot maintains a uniform surrounding feature throughout the entire tonal range. Near the midtones, the angle formed by the dots is usually an acute angle (less than 90°). Due to the dense angles, the areas between the dots are easily oozing out of each other and filled with ink. The middle dots of the screen printing are enlarged or the midpoint mutation is as high as 15%. Therefore, for screen printing, the dot dot is the worst dot shape.

There are two reasons for choosing circular dots in printing: Firstly, the uniform dot shape of the circle dots helps to reduce the effect of oval and other dot-shaped natural lines, and it can well avoid the formation of moire; secondly, relative to printing production. In high-speed web offset presses and sheet-fed offset presses, the paper passes through blankets and impression cylinders and causes large dot distortions, so the dots will become ovals, which will help reduce the midtone mutations (this will After the introduction). However, in screen printing, due to the small force, the circular dot will not be deformed.

In order to improve the defects of circular dots, square dots are introduced. The following point shapes used in the design are: the dots start to be round, the dots in the midtone range gradually change from round to square, and they become square dots at 50% dots, and then gradually change back to circular. The four corners of the square dot are in contact at the same time, and the angle is greater than 90°, but there is still the possibility of a midtone mutation. In an ideal situation, the square dots expand to 10-12% at the midtones. If the printing conditions are poor, the mutation will be greater. And because the linear network line breaks the boundary of the dot, this change of style is more apparent to the human eye.

In order to find a solution that is more suitable for the entire tonal range, two other dot shapes are designed. Diamonds and ellipses. The following will discuss the geometry of the midtone region superimposition and the change of the tone at the edge of the dot. The dots here also start from the circle dot, transition to a diamond or ellipse, reach the standard shape at the midtones, and then change back to a circle. By changing the shape of the dot, the dots start to contact in the lower tonal range, which is generally 40%, and the dot maintains growth of up to 60%. In addition to the ability to overlay dots at different tonal values, this type of dot also allows the screen angle of the contact range to be higher than 90°, which helps prevent the ink from oozing out and keeps the prints legible.

However, with the transition of the dots to diamonds or ellipses, the chances of the dots becoming linear increase, resulting in the appearance of moire, and the fine and narrow dots are also very likely to produce a regular halftone stripe pattern visually. This phenomenon is particularly noticeable especially in areas with a large range of the same hue (for example, an image depicting a large area of ​​blue sky) or areas where the hue changes slowly (for example, meat-tone images).

2, resolution and network lines

The dot shape is determined by the resolution of the output device. To use 255 shades as much as possible, the minimum output resolution of the imagesetter should be 16 times the number of design lines. For a 50 line/inch halftone image, the minimum required output resolution is 50*16=800 dpi. If you think that the fixed resolution of the imagesetter is not ideal, you can also derive the number of discretized tones by the following formula:

Hue level = {round (output resolution / number of screen lines)} 2-1

For example, if the imagesetter has a resolution of 400 dpi and the resulting halftone image is 50 lines/inch, the obtained tone levels are: (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 that can be accepted in screen printing is determined by the ink, the substrate, and the viewing environment. The higher the substrate smoothness, the closer the viewing distance, the higher the required tone level.

Lower shades are desirable, but imagesetters with lower resolutions describe complex mesh angles and can cause malformations or other irregularities when converting mesh shapes within the tonal range, as shown in Figure 1, when zoomed in Dots appear to be rough, which can cause the mesh to clog, resulting in a very irregular or even unpredictable moire pattern. The only solution is to use a suitable mesh aperture to reduce the effects of interference fringes.

Halftone dot design requires a high level of technology and a solid mathematical foundation. The design of specific point or dot transition sequences involves theories of image processing and image copying. At present, a large number of diamond-shaped or elliptical dots are used in screen printing to reduce tone changes and achieve good results. We believe that with the improvement of the level of science and technology, in the near future, the ideal form for screen printing will certainly emerge.