Exposure is an extremely important process, and how to effectively control the exposure during the exposure process is the most critical step. The correctness of the exposure control is an important factor in determining whether high-quality prints can be obtained in the end. So how to determine the appropriate exposure time and avoid screen failure during exposure? The following will specifically analyze several key issues that affect the quality of screen exposure and provide a reference for determining the correct exposure time.
The influence of printing light source on exposure
The choice of printing light source is the first issue to be considered in the screen printing plate making process. The correct selection and use of light sources are of great significance for improving the quality of screen printing, simplifying operations, 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 and radiation wavelengths of several commonly used 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 the light source for printing. Due to the difference in luminous range, the exposure time and image quality are slightly different.
Light source intensity is also a factor that affects the exposure time. Light source intensity refers to the amount of energy that the light source acts on the photosensitive film. It is related to the light source used, the distance between the light source and the screen (light distance), and if the light distance changes, the optimal exposure time must be recalculated. The exposure time has a square relationship with the lamp distance. Experience has shown that if the lamp distance is doubled, the exposure time needs to be increased by a factor of 3. On the contrary, if the lamp distance is reduced by a factor of 1, the exposure time should be reduced to a quarter.
The effect of overexposure and underexposure on the screen
Correct exposure is a prerequisite for ensuring printing quality. The correctly exposed screen can faithfully reproduce the original, to avoid unclear pattern lines and color drift in dot printing. Overexposure will cause "light diffraction" phenomenon, which affects the edge definition and resolution of screen images; underexposure often occurs. If it is not found in time, after printing on the machine, not only the printing quality is poor, the raw materials are wasted, but the screen is difficult to recycle . There are several manifestations of underexposure.
1) Damage and pinholes occurred during the development process
Underexposed screens, due to the effect of water pressure during development, will result in irregular image edges, pinholes in the film, photosensitive glue flowing along the glued surface of the screen, or down from the mesh. 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 are thousands of prints or tens of thousands of prints, the pinholes may appear in the screen image due to insufficient curing of the photoresist. Or damaged, or due to strong corrosion solvents and ink erosion caused the plate film to fall off, therefore, underexposed screen can not be printed on the machine.
2) The printed image is incomplete
Insufficient exposure can easily cause uncured photoresist to flow into the graphic holes. After drying, a transparent film is easily formed on the screen to block the mesh, resulting in incomplete printed images and affecting product quality.
Some people think that the clarity of the pattern can be improved by reducing the exposure time. In fact, this approach is wrong, because underexposure does not increase the clarity. The clarity is after the contrast of the negative film meets the requirements. The accuracy of the dots shows the quality of the printed edges. Improving sharpness cannot be improved by reducing the exposure time, especially when the printing volume is large.
3) The online version is difficult to recycle
The consequences of underexposure will also occur in the printing process, as ink and solvents continue to immerse in the underexposed photosensitive film to produce crosslinking, which is combined with the silk screen, making it difficult to remove the film during screen recycling, resulting in screen scrapping .
The Influence of Screen Mesh Number and Color on Exposure
At present, there are many kinds of wire mesh, and the products are uneven, but it must be clear that the mesh number and wire diameter of the wire mesh determine the opening rate of the wire mesh and the thickness of the mesh cloth, and at the same time, affect the online presence of the photosensitive emulsion. Remaining amount, obviously, the photosensitive emulsion that can be coated on the wire mesh with large opening rate and thick wire is thick, so the exposure time should be correspondingly longer, otherwise the exposure time is shortened.
In general, white screens are prone to light scattering, reflecting ultraviolet rays to the edges of the image that should not be exposed, resulting in unclear image edges and aliasing. Although the exposure time is short, it affects the resolution and lines of the screen. Clarity. The yellow silk screen can absorb ultraviolet rays with a wavelength of 350-420nm, effectively reducing the damage of scattered light to the screen. The experiment proves that the warm yellow silk screen is used for plate making, and the ultraviolet light is irradiated on the yellow silk screen, and the reflected light no longer contains harmful wavelengths of 350-420nm. Due to the elimination of ultraviolet scattering, you can get clear edges and graphics. Therefore, when making fine screens, you should choose yellow screen making as much as possible, and improve the screen durability by selecting the best exposure time. Conducive to online recycling.
Effect of temperature and humidity on exposure
The temperature change has a greater impact on the photosensitive emulsion layer. Under normal circumstances, the temperature is below 4 ℃, the photosensitive sensitivity of the film is very dull; and when the temperature is increased by 1.8 ℃, the photosensitive sensitivity of the photosensitive adhesive 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 40 ℃), unexposed photosensitizers are prone to dark reaction, which causes self-crosslinking of the photosensitive resin. At about 25 ℃.
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 resistance of the screen will be reduced. The relative humidity of the operating environment is controlled at 50% to 70%. The best exposure.
Correct choice of exposure time
There are several ways to determine the optimal exposure time of the screen, depending on the emulsion used. Generally speaking, the manufacturer of the emulsion will provide the basic parameters of exposure for your product. You can refer to these parameters to choose your own The best exposure time, or take the following methods to determine the exposure time.
1. Hierarchical exposure test method
The grading exposure test method is a commonly used exposure time test method for printing. It uses the calculated approximate exposure time as the medium exposure time, and then exposes the parts of the same screen to different degrees to obtain the best exposure time. . The specific steps are: first expose the screen at 50% of the approximate exposure time, then cover 1/5 of the screen, and then expose at 25% of the approximate exposure time, that is, cover 1/5 more each time, and press Expose 50% of the previous exposure time to get 5 areas with different exposure levels. After the screen is washed and dried, the best exposure time can be found after proofing.
2. Exposure test strip (film) test method
The exposure test strip is a necessary tool for screen printing. It can accurately test the correct exposure time of different photosensitive resins 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, the latter has a better control effect, and can increase or decrease the exposure time as desired.
Since there are many factors that affect the quality of the exposure of the screen, it is the most effective means to ensure the quality of the screen by correctly selecting the exposure time with the help of the exposure control tool.
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