Network Discomfort and Adjustment in Screen Printing (I)

As we all know, the quality of screen printing is affected by many variable factors, among which the net distance is one of the most important factors. In most cases, the print quality and registration problems are mostly attributed to improper network coverage or some adjustments to compensate for network discomfort.

Network distance, the distance between the printing surface of the printing plate and the surface of the substrate (see Figure 1). In screen printing, in order to avoid the screen sticking to the substrate, a certain distance must be maintained between the screen printing plate and the substrate. Once the screen sticks to the substrate, it will seriously affect the printing quality. The role of the screen distance is to effectively overcome this problem, so that the screen will only touch the substrate when the squeegee is scratched. Once the screen is scraped, the screen will rebound. , Screen and the substrate immediately disengaged, to avoid the occurrence of drag ink and dirt and other phenomena.

Figure 1 Platform printing net pitch

In addition, the concept of “rebound distance” must also be correctly understood. In screen printing, it is closely related to the grid distance, but it is different from the grid distance.

“Rebound distance” refers to the distance that the screen slowly rebounds upwards after the squeegee blows ink. It occurs at the beginning of printing, and only appears during the squeegee process. It ensures that the screen can rebound quickly and move away from the substrate. .

The net distance in screen printing is generally 9.5 - 12.7mm, but it is more reasonable and more commonly used is 1.5-3.0mm. The network distance has brought many problems to the printing quality and registration. Therefore, the network distance should be as small as possible. When printing some high-precision products, the allowable amount of screen deformation directly determines the size of the screen pitch. In the printing of electronic products, the net distance of the stainless steel screen is often smaller, about 0.1-0.2mm.

Therefore, whether it is a printed graphic or a textile, the pitch is a parameter that deserves special attention. Once the network is not suitable or inconsistent, it will bring unnecessary losses to the printing and need to be carefully controlled.

I. Significant relationship with positioning and registration

In multi-color and four-color screen printing, the problem of color registration must be handled well, and the net distance plays a crucial role in registering. Strictly speaking, there is a distance between the web and the manuscript. This is due to the telescopic deformation of the screen during the printing process. The larger the net distance, the greater the pressure of the squeegee, and the larger the screen is stretched, causing a shift in the pattern on the substrate. The small net distance can improve this situation to a certain extent. Therefore, in order to reduce the image shift, it is required to select the smallest possible net distance. For some printed products with high registration accuracy, it is recommended that the net distance be 1.5mm, and the maximum should not exceed 3mm.

As the network distance increases, many problems arise. First of all, in order to ensure that the screen is in contact with the substrate and the ink is transferred, the pressure of the blade must increase, and the force applied to both ends of the blade must be greater. Relative to the net frame, the longer the scraper, the more prominent this phenomenon is. This not only increases the degree of deformation of the pattern, but also seriously affects the printing quality.

In addition, sometimes the printed pattern is stretched and elongated, which is caused in part by the problem of the grid distance, but is also related to the position of the pattern, the length of the blade, and the direction in which the blade runs.

It is very important to maintain the uniformity of the net distance in screen printing. If the screen pitches around the screen are different, the image at the corresponding position will shift. In this case, no matter how hard the operator is working, the second color will never register with the first color. Obviously, if the colors in the printing cannot be aligned, the entire job will be scrapped.

It is worth mentioning that in electronic circuit printing, some pattern distortions can be pre-compensated by calculation when the pattern is not counted. In addition, some manufacturers have introduced devices that have the ability to overcome these problems.

Second, it is essential for image quality

The network spacing is not suitable. During the printing process, the squeegee pressure must be increased or decreased. This will affect the printing quality. It will cause poor copying accuracy, blurring, bubbles, ghosting, missing dots, poor edge definition, and moiré. , patterns are not complete and other issues.

The network distance is not too large and will cause other problems. For example, the squeegee needs to increase the squeegee pressure so that the screen plate can contact the substrate. However, before the screen plate comes into contact with the substrate, the ink is squeezed onto the screen printing surface. If this is serious, it is bound to The printed material on the substrate or the substrate will be smeared. Excessive squeegee pressure can also damage the printed image, resulting in dot distortion or dot gain.

The greater the pressure of the squeegee, the faster the rebound speed of the screen after printing, and the ink cannot be completely transferred to the substrate. As a result, moire, incomplete patterns, drawing, bubbles, and poor edge definition are caused. Print rickets.

Another reason for the poor print quality is the low screen tension and the small net pitch. In this case, the screen will wrinkle during the squeegee squeegee process to form ripples on the print, which will eventually cause serious image distortion and ghosting.

Third, determine the ink thickness and color
Inconsistent thickness of the ink may be caused by the following factors: (1) the screen is not parallel to the substrate; (2) the pitch, doctor blade, and screen tension are indefinite; (3) the doctor blade is deformed due to excessive mesh distance. Any one of these factors may cause the ink to adhere unevenly, and the final print may have different shades of color.

The small net distance can ensure uniform ink adhesion, and the net distance is too large, which can easily cause the ink to adhere unevenly, especially at the two ends of the scraper, the ink adhesion is more uneven.

(to be continued)

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