In order to obtain an ideal screen-printed product, exposure (printing) is an extremely important process in the screen-making process of the photosensitive method, and how to effectively control the exposure during the printing 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 .
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