Effect of light source on exposure time in screen printing plate

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:

1. Exposure light source The spectral characteristics and intensity of the light source are decisive factors in 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. Therefore, they are 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 screen emulsions can use the above three light sources as exposure light sources. 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 mean the brightness of the light source seen by the human eye, 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.

2. Distance from the light source to the screen During the plate making process, once the distance from the light source to the screen is changed, the optimal exposure time must be recalculated. This 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.