Detailed analysis of exposure quality before the h

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Detailed analysis of exposure quality before printing

printing is one of the most important processes in printing, and how to effectively control the exposure is the most critical step in the printing process. Whether the exposure control is correct or not is an important factor to determine whether high-quality prints can be obtained finally. So how to determine the appropriate exposure time to avoid version failure in exposure? Next, we will analyze several key problems that affect the exposure quality of the printing plate:

the influence of the printing light source on the exposure

the selection of the printing light source is the first thing to consider in the printing plate process. The correct selection and use of the light source is of great significance for improving the quality of the printing plate, simplifying the operation, saving energy, maintaining health and reducing the cost

in order to improve the quality of plate making, we must fully study the matching relationship between photosensitive materials and light sources, the output power of light sources, the distance between light sources and plates, and the wavelength of light sources, and master the exposure time

photosensitive materials have selectivity to light sources. Table 1 shows the characteristics and radiation wavelengths of several common light sources. It can be seen that all photosensitive emulsions can choose metal halogen lamps, high-pressure mercury lamps and some fluorescent lamps (cold light lamps) as their light sources. They still have small dielectric constant and dielectric consumption in the high-frequency range. However, due to the difference in the luminous range, the exposure time and image quality are slightly different

light source intensity is also a factor affecting exposure time. Light source intensity refers to the energy of the light source acting on the photosensitive film. It is related to the light source used and the distance between the light source and the plate (lamp distance). If the lamp distance changes, the best exposure time must be recalculated. The exposure time is squared with the lamp distance. Experience has proved that if the lamp distance is doubled, the exposure time needs to be increased by three times; On the contrary, if the lamp distance is reduced by one time, the exposure time should be reduced to one quarter of the original

the influence of overexposure and underexposure on the plate

correct exposure is a prerequisite to ensure the printing quality. The correctly exposed plate can faithfully reproduce the original and avoid pattern lines. 2. The status quo and development trend of the fixture are not clear, and color drift occurs in dot overprint. Overexposure will cause light diffraction, affecting the edge definition and resolution of the plate image; Underexposure often occurs. If it is not found in time, after printing on the machine, not only the printing quality is poor, raw materials are wasted, but also the version is difficult to recycle. There are several manifestations of underexposure

1. In the development process, there are damages and pinholes

underexposed versions. Due to the effect of water pressure, the edges of the image will be irregular, the adhesive film has pinholes, and the photosensitive glue flows along the gluing surface of the wire, or from the hole downward. After finding this problem, we should determine the exposure time through experiments and print again. In addition, insufficient exposure will affect the service life of the plate on the printing machine. It is no problem for the short version. If the printing volume is thousands of prints or tens of thousands of prints, the plate image may appear pinholes or damage due to the insufficient curing of the photosensitive adhesive, or the plate film may fall off due to the erosion of strong corrosive solvents and inks. Therefore, the underexposed version cannot be printed on the machine

2. The printed image is incomplete

the exposure is insufficient, which is easy to cause the uncured photosensitive glue to flow into the graphic through hole. After drying, it is easy to form a transparent film on the plate to block the hole, resulting in the incomplete printed image and affecting the product quality

some people believe that the sharpness of the pattern can be improved by reducing the exposure time. In fact, this practice is wrong, because insufficient exposure will not increase the sharpness. Sharpness is the quality of the printing edge shown by the sharpness of lines or the accuracy of points after the negative contrast meets the requirements. Improving sharpness cannot be improved by reducing exposure time, especially when printing volume is large

3. It is difficult to recover the version

the consequences of insufficient exposure will also occur in the printing process. Due to the continuous immersion of ink and solvent in the underexposed photosensitive film, cross-linking will occur, which will be combined with silk, making it difficult to remove the film during the recovery of the version, resulting in the scrapping of the version

the influence of silk mesh and color on exposure

at present, there are many kinds of silk, and the products are uneven, but it must be clear that the mesh number and diameter of silk determine the opening rate and cloth thickness of silk, and at the same time, it also affects the upper retention of photosensitive emulsion. Obviously, the coated photosensitive emulsion of silk with large opening rate and thick silk thread is thick, so the exposure time should also be correspondingly lengthened, otherwise the exposure time will be shortened

generally speaking, white silk is prone to light scattering, reflecting ultraviolet light to the edge of the image that should not be exposed, resulting in unclear image edges and sawtooth. Although the exposure time is short, it affects the resolution of the version and the clarity of lines. The yellow silk can absorb ultraviolet light with a wavelength of 350 ~ 420nm, effectively reducing the damage of scattered light to the plate. Experiments have proved that when the plate is made of warm yellow silk and ultraviolet rays are irradiated on the yellow silk, the reflected light does not contain harmful wavelengths of 350 ~ 420nm. Due to the elimination of UV scattering, we can get pictures and texts with clear edges. Therefore, when making a fine version, we should choose the yellow silk version as much as possible, and improve the printing resistance of the version by selecting the best exposure time, which is also conducive to the recovery of the version

influence of temperature and humidity on exposure

temperature change has a great influence on the photosensitive emulsion layer. Under normal circumstances, the photosensitive sensitivity of the film is very slow when the temperature is below 4 ℃; When the temperature increases by 1.8 ℃, the photosensitive sensitivity of the photosensitive adhesive layer increases by 3 times, the light curing speed of the photosensitive adhesive layer also increases correspondingly, and the exposure time decreases. However, when the temperature is too high (above 40 ℃), the unexposed photosensitizer is prone to dark reaction, which causes the photosensitive adhesive to produce self crosslinking. As a result, the highlights are lost and the resolution of the plate is significantly reduced. Therefore, the operating environment should be controlled at about 25 ℃

humidity changes will also affect the exposure quality of the version. When the version is not completely dry, because the service life of the product depends on the quality of a product, the printing resistance of the version will be reduced when the exposure is dry or the environmental humidity is high. The relative humidity of the operating environment is controlled at 50% - 70%, and the exposure effect is the best

correct selection of exposure time

there are several methods to determine the best exposure time of the version, which depends on the photosensitive emulsion used. Generally speaking, the manufacturer of photosensitive emulsion will provide basic exposure parameters for your products. You can select the best exposure time for yourself by referring to these parameters, or adopt the following methods to determine the exposure time

1. Graded exposure test method

graded exposure test method is a commonly used exposure time test method for printing. It takes the calculated approximate exposure time as the medium exposure time, and then exposes each part of the same plate to different degrees, so as to obtain the best exposure time. The specific steps are as follows: first expose the plate according to 50% of the approximate exposure time, then cover 1/5 of the plate, and then expose according to 25% of the approximate exposure time, that is, cover more than 1/5 each time, and expose according to 50% of the previous exposure time. Five areas with different exposure degrees can be obtained. The plate is washed, dried, and hit 4. Power supply: after exchanging 220V 10%2a samples, the best exposure time can be found

2. Exposure test strip (film) test method

exposure test strip is a necessary tool for silk printing. It can accurately test the correct exposure time of plates with different photosensitive adhesives and film thicknesses for different light sources, and plays an important auxiliary role in improving the quality of plates. Commonly used are EC test strip and wedge test strip. The latter has better control effect and can increase or decrease the exposure time at will

since there are many factors that affect the exposure quality of the version, the most effective and direct way to ensure the quality of the version is to correctly select the exposure time with the help of the exposure control tool. I won't list other factors here

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