Analysis of the problem of quality before screen printing
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Printing is an extremely important process, and how to effectively control the exposure during the printing process is the most critical step. The correctness of exposure control is an important factor in determining whether or not to obtain high-quality prints. So how to determine the appropriate exposure time to avoid screen failure in the exposure? The following will analyze several key issues affecting the quality of the screen exposure, and provide a reference for determining the correct exposure time.
The effect of the light source on the exposure
The choice of the light source is the first problem to be considered in the screen printing process. The correct selection and use of the light source is of great significance for improving the quality of the screen, simplifying operation, saving energy, maintaining health and reducing costs.
In order to improve the quality of plate making, 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 master the exposure time.
The photosensitive material is selective to the light source. Table 1 shows the characteristics of several common light sources and the radiation wavelength. It can be seen that all the emulsions can be selected from metal halide lamps, high pressure mercury lamps and some fluorescent lamps (cold lights) as the light source. The exposure time is slightly different from the image quality due to the difference in the range of illumination.
The intensity of the light source is also a factor influencing the exposure time. The intensity of the light source refers to the amount of energy applied to the photosensitive film by the light source. It is related to the distance between the light source used and the light source and the screen (light distance). If the light distance changes, the optimal exposure time must be recalculated. The exposure time is squared with the lamp distance. Experience has shown that if the lamp distance is increased by 1 times, the exposure time needs to be increased by 3 times; conversely, if the lamp distance is reduced by 1 times, the exposure time should be reduced to the original quarter.
Overexposure and underexposure on the screen
Correct exposure is a prerequisite for ensuring print quality. The correctly exposed screen can faithfully reproduce the original, avoiding unclear pattern lines and color drift in dot overprinting. Excessive exposure will cause "light diffraction" phenomenon, affecting the edge definition and resolution of the screen image; underexposure is a common occurrence. If it is not found in time, after printing on the machine, not only the printing quality is poor, the raw materials are wasted, and the screen is difficult to recycle. . There are several manifestations of underexposure.
1) Breakage and pinholes occur during development
Underexposed screen, due to the effect of water pressure during development, the edges of the image will be irregular, the film has pinholes, the photoresist will flow along the glue surface of the screen, or it will smash downward from the mesh. After discovering this problem, it is necessary to test the exposure time and re-print the plate. In addition, underexposure will affect the service life of the screen on the printing press. For short-running, if the printing volume is several thousand or tens of thousands of marks, pinholes may appear in the screen image due to insufficient curing of the photoresist. Or the damaged film, or the erosion of the solvent and the ink, causing the film to fall off, so the underexposed screen can not be printed on the machine.
2) The printed image is incomplete
Insufficient exposure, easy to cause uncured photosensitive adhesive to flow into the through hole of the graphic. After drying, a transparent film is formed on the screen to block the mesh, resulting in incomplete printing images and affecting product quality.
Some people think that it is wrong to increase the sharpness of the pattern by reducing the exposure time. In fact, this method is wrong because the lack of exposure does not increase the definition. The sharpness is after the contrast of the film reaches the requirement, by means of the sharpness of the line or The accuracy of the dots shows the quality of the printed edges. Improving clarity does not depend on reducing exposure time to improve, especially when printing is large.
3) The screen is difficult to recycle
The consequences of underexposure will also occur in the printing process. As the ink and solvent are continuously immersed in the underexposed photosensitive film to produce cross-linking, combined with the screen, it is difficult to remove the film during the screen recycling process, resulting in the screen rejection. .
Influence of mesh size and color on exposure
At present, there are many kinds of wire mesh, and the products are uneven. However, it must be clear that the mesh number and wire diameter of the wire mesh determine the opening ratio of the wire mesh and the thickness of the mesh cloth, and at the same time affect the online station of the emulsion. Retention, obviously, the screen emulsion that can be coated with a large opening ratio and a thick thread is thick, so the exposure time should be lengthened accordingly, and the exposure time is shortened.
In general, white screens are prone to light scattering, reflecting ultraviolet light to the edges of the image that should not be exposed, resulting in unclear edges and jagged edges. Although the exposure time is short, it affects the resolution and lines of the screen. Sharpness. The yellow screen absorbs ultraviolet light with a wavelength of 350-420 nm, which effectively reduces the damage of the scattered light to the screen. Experiments have shown that the warm yellow screen is used to make the ultraviolet light on the yellow screen, and the reflected light does not contain harmful wavelengths of 350 to 420 nm. By eliminating the UV scattering, you can get sharp edges and texts. Therefore, when making a fine screen, you should choose a yellow screen plate as much as possible, and improve the screen printing durability by selecting the best exposure time. Conducive to the recycling of the screen.
Effect of temperature and humidity on exposure
The temperature change has a great influence on the emulsion layer. Under normal conditions, the temperature sensitivity is below 4 °C, and the sensitivity of the film is very slow. When the temperature is increased by 1.8 °C, the sensitivity of the photoresist layer is increased 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 40 ° C), the unexposed photosensitizer is prone to dark reaction, which causes the photosensitive adhesive to self-crosslink. As a result, the high-light part of the dot is lost, and the screen resolution is significantly reduced. Therefore, the operating environment should be controlled. At around 25 °C.
Humidity changes will also affect the exposure quality of the screen. If the screen is not completely dried or the ambient humidity is high, the screen printing durability will decrease, and the relative humidity of the operating environment will be controlled at 50% to 70%. The exposure is best.
Correct exposure time
There are several ways to determine the optimal exposure time of the screen. Depending on the emulsion used, in general, the manufacturer of the emulsion will provide the basic parameters of the exposure for your product. You can choose these parameters to suit your needs. Optimum 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 a medium exposure time, and then exposes portions of the same screen to different degrees to obtain an optimum exposure time. . The specific steps are: first expose the screen by 50% of the approximate exposure time, then cover 1/5 of the screen, and then expose it by 25% of the approximate exposure time, that is, cover 1 /5 each time, and press 50% of the previous exposure time is exposed, and 5 areas with different exposure levels can be obtained. The screen can be washed and dried, and the best exposure time can be found 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 quality of the screen. Commonly used are EC test strips and wedge test strips, which have better control effects and can increase or decrease the exposure time as desired.
Since there are many factors affecting the quality of the screen exposure, the correct selection of the exposure time by means of the exposure control tool is the most effective means to ensure the quality of the screen.