Screen printing with moiré

One, know Moire fringe

Moire fringe is an optical phenomenon first discovered by French researcher Morr in the 18th century. From a technical point of view, the moiré fringe is the visual result of the interference between two lines or two objects at a constant angle and frequency. When the human eye cannot distinguish the two lines or two objects, only the visible The pattern of interference, this optical phenomenon is moire.

Moire fringes is a potential problem for halftone screen printing. The so-called halftone printing is a method of expressing a hierarchy by decomposing a continuous-tone document into photographs or other methods into dots of different sizes. Calls are made by printing large dots, and bright dots are printed by printing dots with smaller dots. Moire fringes occur between dots of the same color, especially multi-color printing or four-color printing.

The formation of moiré between the dots is a common problem at all levels of screen printing. Dots and screens can also form another form of moiré. The distribution of moiré on the screen can produce indistinguishable patterns that are clearly different from the original.

Moiré can be produced in three ways:
1. Interference between two or more color dots;
2. Interference between colored dots and screen mesh;

3. As an additional factor, interference occurs due to the characteristics of the substrate itself. The purpose of using the Moire fringe protection system is to predict the moire fringe based on the number of screen meshes you have selected, the number of screens, the number of colors printed, and the screen angle.

Second, to avoid the moire fringe

It is possible to use mathematics to predict and analyze the moiré fringe, and the calculation result is only the theoretical moiré fringe. The actual moire fringe that affects the screen printing is the visual moiré fringe that is harmful to the printing result. The Moire stripe protection system provides silkscreen workers with a simple visual control tool. Using this tool will avoid moiré fringing at any step of the replication process.

Serilor?log contains two tools for predicting moiré:

1. Analog screen mesh instrument, which is a set of analog mesh screen number of the film, the number of simulated meshes from the order you ordered the test film --- Advanced test and Basic test film. Both sets of test pieces included four mesh numbers: MG-S-B: 100T/cm, 110T/cm, 120T/cm, and 130T/cm, (255T/inch, 280T/inch, 305T/inch). 330T / inch). The Advanced Test Piece also contains the breakdown series for MG-S-A: 140T/cm, 150T/cm, 165T/cm, and 180T/cm (330T/inch, 380T/inch, 420T/inch, 460T/inch). For coarse mesh screen printing or specifically for textile screen printing, MG -S -C: 77T/cm, 81T/cm, 85T/cm, and 90T/cm; MG -S -D: 48T/cm, 54T/cm, 61T/cm and 68T/cm.
Test sheets for special purpose and high tension screens need to be specially ordered.

2. Moiré fringe recognizer/proportioner, which overlays the indexer and film to detect the number of film screens and the separation angle.

The two sets of tools can be used interchangeably to comprehensively analyze the number of screen meshes/adding lines/net point angles and the Moiré function. You need to install a light-stable glass table in the darkroom to provide a strong and stable light source. The weak light makes it difficult to find moiré.

Moire Strip's two sets of protective tools can help you to choose the correct screen, but also help you select color separation parameters (screen number and angle) and printing parameters - whether it is from the original to the print control, but also important The parameter control.

3. The following example can help you understand the purpose of the Mohr protection tool

1 The color separation film provided by the customer shall select the correct screen mesh number, first put the screen mesh simulation sheet on the glass worktable, and then place the color separation film in the area of ​​the selected mesh number in order. The color separation film must be completely aligned with the corresponding position of the analog film. When using multicolor or four-color printing, each color separation sheet must be placed on a mesh simulator for inspection.

In some special cases, the moire pattern appears very clearly in one of the four dichroic films. Experience has shown that the use of screens of different meshes for this separation can solve these problems. The number of meshes used can be selected through the Garnandstandard test piece to select the mesh that is closest but does not generate moire. Note: This is your last resort to solve this problem. In some cases, it will not work, and it will change the amount of ink deposits and cause additional trouble: drying speed, screen definition, color deviation issues.

2 The number of meshes can not be changed but the color separation film can be changed. Due to special technical factors, you may not be able to change the number of meshes (printing parameters and ink deposition amount have been set). At this time, you need to modify the color separation parameters. To avoid the formation of moire.

Use the mesh simulation piece that matches the number of screen meshes selected, and use the Moire fringe indexer to cover the mesh simulation piece according to the screening angle of the dichroic film. The screening line used by your color separation film can be determined. number. When you turn the indexer, different overlays will produce moire. If the Moiré pattern appears only in the highlight area (below the 10% dot percentage), the printed image is hardly affected, but when the moire pattern occurs on all grayscale steps or occurs at 50% of the dots, you must change the pattern. Web angle. When visual moiré appears, replace it with a screen or dichroic patch until no moiré appears when all dichroic patches overlap the screen.

Before checking the screening angle, check the scanner's recommended value. These screening angles vary for different printing processes (offset, flexo) and country-specific. For example: North American offset printing screen angle and European standard DIN16547 (1) are nearly unified, but belong to different color versions. As shown (not shown):

European flexo screen angle North American offset printing screen angle European DIN16547 standard (1)
European DIN16547 standard (2)

The diagram also shows that it is very important to keep the three deepest primary colors different from each other by 30°. However, the specific screening angle is not static. Many printers have worked with color separators to improve the screen angle and use them in screen printing. The greatest help that can be obtained with the Moire fringe protection system is to determine the matching relationship between the number of meshes, the number of screen lines, and the screening angle by visual inspection.

3 The number of meshes and the color separation film cannot be changed. In some cases, the screen printing worker can neither change the number of meshes nor replace the color separation film. There are two ways to check the moiré pattern. One is to change the screen. Tension———(adjust the number of meshes), the other is to change the stretch angle, the Moire protection system will guide you to change the stretch angle to prevent the moiré. Place the indexer on the mesh simulation plate and rotate the indexer until the moire pattern is generated. The angle of inclination on the indexer is to avoid the angle of the Moiré pattern.

4. In other aspects, there are several reasons for the generation of moire, which is related to the printing process, although the moiré protection system is not designed to detect this moiré. It can still help you identify these moire fringes.

1 screen tension. If the screen tension is too low or the screen tension of the printing process is not stable, moiré fringes will be generated. You will see that the moire pattern will appear in a fixed position in the pattern, and dot gain and dot loss will occur. In order to do the adjustment printing, the stretching tension must reach 20 Newtons/cm.

2 scraper selection. Moire pattern can also be caused by improper angle or hardness of the scraper. Too much scraper pressure or too sharp edge contour of the scraper can cause printing distortion. To avoid these problems, it is recommended to use the following three-layer composite scraper for hardness. print.

Source: Bai Songfang, Guangdong Henghui Industrial Company