In order to obtain the ideal screen printing product, exposure (stencil printing) is an extremely important process in the photosensitive screen manufacturing process. How to effectively control the exposure during the printing process is the most critical step. Whether the exposure control is correct or not is an important factor in deciding whether or not to obtain high quality prints. So how to determine the appropriate exposure time, to avoid exposure screen failure it? The following will specifically analyze the key issues that affect the quality of screen exposure, and provide reference for determining the correct exposure time.
Effect of exposure of light source on exposure
The choice of printing plate light source is the first problem to be considered in the screen printing plate making process. The correct selection and use of light sources is of great importance for improving the quality of screen plates, simplifying operation, saving energy, maintaining health, and reducing costs.
In order to improve plate-making quality, it is necessary to fully study the matching relationship between the photosensitive material and the light source, the output power of the light source, the distance between the light source and the screen, and the wavelength of the light source, and grasp the exposure time well.
The photosensitive material is selective to the light source. Table 1 shows the characteristics and radiation wavelengths of several common light sources. It can be seen that all the emulsions can be selected from metal halide lamps, high-pressure mercury lamps and some fluorescent lamps (cold lamps) as the printing plate light source, but Due to the difference in the light emission range, the exposure time is slightly different from the image quality.
Light source intensity is also a factor that affects exposure time. The intensity of the light source refers to the amount of energy that the light source acts on the photosensitive film. It is related to the distance between the light source used, the light source and the screen (lamp distance). If the lamp distance changes, the optimal exposure time must be recalculated. The exposure time is squared with the lamp distance. Experience shows that if the lamp distance increases by 1 time, the exposure time needs to be increased by 3 times. On the contrary, if the lamp distance is reduced by 1 time, the exposure time should be reduced to a quarter.
Overexposure and underexposure effects on screen
Correct exposure is the precondition for ensuring print quality. The properly exposed screen can faithfully reproduce the original, avoiding unclear lines and overprinting. Overexposure can cause "diffraction" and affect the edge definition and resolution of screen images. Underexposure is a frequent occurrence. If it is not discovered in time, after printing on the machine, not only the printing quality is poor, but also raw materials are wasted, and the screen is difficult to recycle. . There are several manifestations of underexposure.
1) Breakage and pinhole occurred during development
Insufficiently exposed screens, due to the effect of water pressure during development, may cause irregularities in the edges of the image. The film may have pinholes, and the photosensitive adhesive may flow along the adhesive surface of the screen, or may squat down from the screen. After discovering this problem, it is necessary to determine the exposure time through experiments and reprint. In addition, underexposure will affect the service life of the screen on the printing press. If there is no problem with the short version, if the printing amount is several thousand or tens of thousands of impressions, there may be pinholes in the screen image due to insufficient curing of the photoresist. Or it is damaged, or the plate is detached due to corrosion of strongly corrosive solvents and ink. Therefore, underexposed screens cannot be printed on the machine.
2) The printed image is incomplete
Underexposure can easily cause the uncured photosensitive adhesive to flow into the through-holes of the graphic. After drying, a transparent film is easily formed on the screen to block the openings, resulting in incomplete print images and affecting the product quality.
Some people think that the sharpness of the pattern can be improved by reducing the exposure time. Actually, this method is wrong because the underexposure does not increase the sharpness. The sharpness is determined by the resolution of the line after the negative contrast is achieved. The accuracy of the dot shows the quality of the printed edge. Increased clarity cannot be improved by reducing the exposure time, especially when the print volume is large.
3) The screen is difficult to recycle
The consequences of underexposure can also occur during the printing process. Crosslinking occurs due to the continuous immersion of the ink and solvent into the underexposure film and combined with the screen, making it difficult to remove the screen when the screen is reclaimed, resulting in screen discard. .
Effect of screen mesh and color on exposure
At present, there are many kinds of screens, and the products are uneven. However, it must be clear that the mesh number and wire diameter of the screen determine the aperture ratio and the thickness of the screen, and at the same time, it affects the online presence of the emulsion. Retention, apparently, the aperture ratio is large, silk thick silk screen can be coated emulsion is thick, so the exposure time should be increased accordingly, otherwise the exposure time is shortened.
In general, white screens are prone to light scattering, reflecting ultraviolet rays to the edges of the images that should not be exposed, causing the edges of the images to be unclear and jagged. Although the exposure time is short, the screen resolution and lines are affected. Sharpness. The yellow screen can absorb ultraviolet rays at a wavelength of 350 to 420 nm, effectively reducing the damage of the scattered light to the screen. Experiments have shown that with the warm yellow screen plate, ultraviolet rays shine on the yellow screen, and the reflected light does not contain harmful wavelengths of 350 to 420 nm. Since the elimination of ultraviolet light scattering can provide clear graphics with sharp edges, when making a fine screen, yellow screen printing should be selected as much as possible, and the screen printing resistance should be improved by selecting the best exposure time. Help screen recovery.
Influence of temperature and humidity on exposure
Temperature changes have a greater influence on the emulsion layer. Under normal circumstances, the temperature is below 4Â°C, the sensitivity of the film is very slow; and when the temperature rises by 1.8Â°C, the sensitivity of the photosensitive layer increases by 3 times. The photocuring speed of the adhesive layer is also increased accordingly, and the exposure time is reduced. However, when the temperature is too high (above 40Â°C), the unexposed photosensitizer is prone to dark reaction, resulting in self-crosslinking of the photosensitive resin. As a result, the dots of the high-light part are lost and the screen resolution is significantly reduced. Therefore, the operating environment should be controlled. About 25 Â°C.
Humidity changes will also affect the screen exposure quality. When the screen is not completely dry or the environment is exposed to humidity, the screen printing resistance will be reduced, and the relative humidity of the operating environment will be controlled at 50% to 70%. The best exposure.
Correct choice of exposure time
There are several ways to determine the best exposure time for a screen, depending on the emulsion used. In general, the manufacturer of the emulsion will provide the basic parameters for the exposure of your product. You can select the appropriate one for your own reference. The best exposure time, or take the following method to determine the exposure time.
1. Grading exposure test
The grading exposure test method is a commonly used exposure time test method, which uses the calculated approximate exposure time as the medium exposure time, and then carries out different degrees of exposure on the same screen part to obtain the best exposure time. . The specific steps are: first, expose the screen to 50% of the approximate exposure time, then cover 1/5 of the screen, and then press 25% of the approximate exposure time, that is, cover 1/5 more times and press Exposing 50% of the previous exposure time, you can get 5 areas with different exposure levels. After the screen is washed and dried, you can find the best exposure time after proofing.
2. Exposure test strip (sheet) test method
The exposure test strip is a necessary tool for screen printing. It can accurately test the correct exposure time of different photosensitive materials and different film thicknesses for different light sources, and plays an important auxiliary role in improving the screen quality. Commonly used EC test strips and wedge test strips, the latter control effect is better, you can increase or decrease the exposure time as you like.
Since there are many factors affecting the quality of screen exposure, the correct choice of exposure time with exposure control tools is the most effective means of ensuring screen quality.
Source "Printing Technology"