The key technology of capillary photosensitive film making screen

In screen printing, when producing high-definition graphics, such as halftone dot images or fine text, in order to obtain high-quality printing products, screen plates with high-accuracy capillary photosensitive films are often used for making screens. This article will systematically introduce the key technologies and precautions that should be mastered in the process of making a photosensitive photosensitive film.

1. Pretreatment

The use of capillary photosensitive films for making screens requires a high degree of cleanliness of the screen surface. As the screen is woven, made, and stretched, the surface will be stained with oil stains, and the filaments are extruded from the mold cavity at a certain temperature. The surface of the mesh is smooth, especially the heat setting using PET weaving. The screen, which is a weak polar material, has a small surface tension, and has low adhesion strength with adhesives, photosensitive adhesives, and the like. In order to solve this contradiction, it is necessary to process the screen before attaching the film to improve the adhesion of the screen and the photosensitive layer. Pre-treatment includes screen surface roughening and degreasing.

1) Coarse. Coarsening is to make the roughness of the mesh smooth, thereby increasing the bonding area between the photosensitive film and the screen, and improving the adhesion of the photosensitive film on the screen.

2) Degreasing. Degreasing removes oil from the surface of the screen, improves the hydrophilicity of the screen, makes the film more intimate between the film and the screen, and can more securely “grab” the screen that has been degreasing. Print force, an effective method for attaching thin lines and small dots in a graphic to a screen.

2. Attach the diaphragm

1) The effect of film on screen quality

Attaching a capillary photoconductive film to the screen is a very important procedure, and the quality of the film will affect many aspects.

1 affect the combination of the capillary speed sensitive film and screen fastness, screen printing resistance and screen life and recycling times;

2 affect the degree of thinning of the photosensitive membrane and the uniformity of the photosensitive layer on the screen;

3 whether there will be pin holes (sand holes) and saw teeth (dog teeth) on the screen;

4 Influence the sharpness of the edge of the line or dot and the size of the screen Rz value (surface roughness).

2) How is the operating environment different?

Changes in temperature affect the swelling rate of the photosensitive layer on the capillary photosensitive film. During the summer or when the temperature is high, the photosensitive layer swells rapidly. When the temperature is low, the film swells slightly. Therefore, if the winter film is still used in summer filming speed, there will be the following shortcomings:

A. The photosensitive layer has not yet swelled, it has already scraped off the water, resulting in the photosensitive film and the screen can not be completely secure;

b. After the dry photosensitive film is not fully swollen, whitening or white spots and agglomeration may occur on the screen printing surface;

c. If you ignore the above operation, the screen plate will be dropped when exposed. Even if you do not drop the film, it will reduce the resistance of the screen.

To solve the above problems, the following methods can be used.

a. Use dry sticking method as much as possible, and the mist water temperature on screen and capillary membrane should be maintained at 20～30°C.

b. When the capillary photosensitive film is affixed to the screen, the length of water residence time should be determined according to the temperature and humidity of the operating environment, so as to ensure that the photosensitive layer of the capillary film has a sufficient wetting process, and then scrape off the printing. Superfluous moisture on the surface of the capillary membrane. Such as the use of 18 # capillary film in the winter than the summer should extend the residence time of 10 to 15 seconds, and then scrape off excess water.

c. The length of stay can also be judged by whether the scraped water is sticky. If it is not sticky, it means that there is not enough time to stay before the wiper, and the mist can be evenly sprayed. If the residence time is too long, the photosensitive adhesive layer will be dissolved and thinned excessively, resulting in an increase in pinholes and irregularities in the pattern edges.

d. When using a dry-film capillary film, lay it on a flat, damp paper with a certain thickness, and then place the screen tightly against the photosensitive surface of the capillary membrane and spray it after compaction. Otherwise, the bottom of the capillary film scrapes excess moisture to cause local photosensitive film to stick unevenly, resulting in a pattern or spot after the capillary film is dry. With the wet method, the filming speed should be fast, and it should be close to the screen plate with uniform water film under the same pressure, and the excess moisture on both surfaces should be scraped and dried.

3. Film drying

1) Strict control of drying temperature

Drying temperature should be controlled 35 ~ 40 °C, can not be higher than 40 °C, this temperature is the actual temperature on the screen, rather than the indicated temperature on the device. When the actual temperature on the screen is greater than 40°C, the dark reaction of the photosensitive layer will be accelerated. The longer the drying time is, the darker the degree of dark reaction will be, resulting in difficulty in development after exposure, or even showing no pattern, even causing Difficult to recycle screens, etc.

2) Complete removal of water

If the photosensitive film is not fully dried, it will be exposed even if the exposure time is just right, but due to residual moisture in the photosensitive layer, it will also affect the penetration of ultraviolet light, making the photosensitive film underexposure, affecting the screen printing resistance, causing A burr occurs on the edge of the pattern, and the photosensitive layer is thinned. In severe cases, the film can even be washed out during development. Therefore, it is essential that the dryness completely controls the thickness of the photosensitive layer to ensure the quality of the screen and the resistance to printing.

It is worth noting that after drying the photosensitive film to remove the protective film (PET sheet), it must be dried again at the same temperature, so that the moisture in the film between the transparent PET film and the photosensitive film can be further removed before exposure. . For screens that have been pre-patched and stored without peeling off the PET film, they must be dried before exposure so that the moisture absorbed by the photosensitive layer on the screen can be removed together with the air. quality.

3) How to determine if the drying is complete?

1 After the capillary membrane is dried, it is very easy to peel off the transparent film base (PET sheet), and there is no harsh sound. At the same time, the surface of the photosensitive film is bright, which means that the photosensitive film is completely dry, and vice versa.

2 After exposure, from the vacuum state of the printing machine for a while, the printing plate can be separated from the photosensitive film on the screen or separated automatically. This indicates that the photosensitive film has dried out, otherwise it has not dried.

4 exposure control

When the exposure is insufficient, if the film is not completely hardened, the pattern on the screen may be easily deformed during development, the image clarity is poor, the dots are lost, the resistance force is reduced, and pinholes and blisters are easily generated on the screen; after the screen is recovered There are shadows of the original text on the screen. When the exposure is excessive, the screen is difficult to develop, the resolution of the image is degraded, or no image can be produced at all. Therefore, the screen exposure must be just right, to ensure that the original as much as possible to achieve 100% reduction. Therefore, the exposure time should be determined according to the operating conditions such as ambient temperature, humidity, working voltage, performance of the UV lamp of the exposure machine, quality of the emitted UV light wavelength and its use time, quality of the capillary photosensitive film and batch, etc. In addition, the exposure ruler is used to detect the exposure before exposure, and the correct exposure time can be easily determined.

5. The choice of capillary photosensitive film

The proper combination of the thickness of the capillary photosensitive film and the screen mesh number is also a key technology to ensure the screen quality. The thickness of the capillary photosensitive film and the mesh mesh number can be determined with reference to Table 1. Photographic film on the screen is too thin, easy to produce dot distortion, surface roughness is too large, it is recommended that the minimum thickness of the photosensitive film should be controlled at about 8μm; when using light-curing ink, the thickness of the photosensitive film should be reduced accordingly, 4μm can.

The above points are the key technology for making screens with capillary photosensitive films. In addition, after screen exposure, punching and drying, if it is left for a certain period of time, that is, after exposure and development, it will stay for 24 hours before being used again. Improve the quality of the screen during use.

Author: Sanwa International Printing Equipment Co., Ltd. Chen Shan
Source: "Printing Technology"