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First, the spectral characteristics and intensity of the exposure light source are the decisive factors affecting the exposure. Screen emulsions are sensitive to UV light in the 365-420 nm wavelength range. The emulsion must be completely cured and requires a certain amount of energy to cause a chemical reaction.
In general, incandescent lamps and floodlights emit light with a wavelength of 365 nm to 420 nm, so it is not the best choice for exposure light sources. Metal halide lamps, mercury lamps, and some fluorescent lamps produce photochemical light sources, but they emit different amounts of UV light and blue-violet light. Except for special circumstances, all the screen emulsions can use the above three light sources as the exposure light source. Since they emit slightly different ratios of UV light and blue-violet light, the exposure time is slightly different, and the images on the finished screen are slightly different.
The intensity of the light source refers to the energy of the light source reaching the screen emulsion. It does not refer to the brightness of the light source seen by the human eye. This is because the UV light is invisible light. The length of the exposure light source, the power of the exposure device, and the distance from the light source to the screen are the three most important factors affecting the intensity of the light source. In the work, the intensity of the light source should be checked regularly, because the lamp tube will be attenuated during use, resulting in a weakened light source.
Second, the distance from the light source to the screen In the plate making process, once the distance from the light source to the screen changes, the best exposure time must be recalculated, it is a squared proportional relationship. As an example, if the distance is increased by twice, the current exposure time is 4 times the original exposure time. Conversely, if the distance is reduced by half, the exposure time is reduced by 1/4.
Exposure effects of the moisture of the emulsion coating layer:
The imaging speed and resolution of the emulsion are also factors that affect the exposure time. With the advent of new sensitizing emulsions, the exposure speed has been significantly accelerated, and several types of sensation emulsions are now used. Let's use an analogy. The relationship between the thickness of the screen emulsion and the exposure time is like the relationship between the ink layer thickness and the ink drying time.
If the emulsion coating layer has not yet dried, it is exposed, so that it is almost impossible to achieve the optimum exposure. So, after the emulsion layer is completely dry, it is exposed again. Otherwise, the residual humidity in the emulsion coating will greatly inhibit the thermal cross-linking reaction during exposure. Even if it does not help to prolong the exposure time, it will only end up facing the embarrassing situation of underexposure.
The effect of screen mesh number and color on exposure time:
The number of screens and the thickness of the fibers of the screen fabric determine the open area of ​​the screen and the thickness of the screen, which in turn affects how much emulsion can stay on the screen. Obviously, if the area of ​​the opening is large and the mesh line is thick, then the resident dose of the emulsion is more and the exposure time required is longer. On the contrary, the exposure time is short! In general, the exposure time of the white screen is short, but the white screen is more prone to light diffusion, thus affecting the resolution of the image, that is, reducing the resolution. Dyeing screen material effectively inhibits light diffusion. Therefore, a better resolution can be obtained. On the other hand, no matter which type of screen material is used, inevitably there will be a certain degree of light diffusion. This is a natural feature of screen printing.
The density of positive film affects the exposure time:
If the density Dmin (i.e., background density) of the non-image area of ​​the positive film used for plate making is too high, or the density Dmax of the image area is too low, it will have a bad influence on the exposure. In screen printing, the positive film needed for platemaking should have a clean, transparent background and a high-density (high density) graphic area. Let us suppose that if the background and the graphic area of ​​the positive film are translucent, there is no doubt that light must pass through the graphic area to reach the emulsion.
In short, the distance between the exposure light source and the light source to the screen will have an impact on the exposure, so we must grasp the degree of this.
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Influence of the light source on exposure time: During the exposure of the screen, the photo-curing reaction of the emulsion is similar to the curing reaction of the UV ink or other ink on the substrate. The screen emulsion is sensitive to the wavelength of the mixed spectrum of UV light and blue-violet light. Because the scraping knife repetitively scrapes back and forth on the screen during printing, the exposed emulsion completely covers the cable after curing. Specifically, there are two factors that affect exposure: