First use the same kind of photosensitive adhesive, respectively coated on several screens. The operating methods and conditions of these several screens must be strictly identical, and then use the same light source within the same distance, but expose with different exposure time. The exposure time starts from 15 seconds and is exposed separately by a doubling method, namely 30, 60, 120, 240, and 480 seconds. Through the development, it can be seen that the photosensitive layer on each screen has a different expansion, as shown in FIG. 4 . In this way, we can use the point of non-expansion as the exposure time. If exposure distance is needed in the future, we can calculate the new exposure time according to the following formula. The calculation method is as follows:
If you change the light source, the new exposure time can be calculated by the following formula:
There are many factors that affect exposure. In addition to the main factors such as light intensity and distance, the sensitivity of the photosensitive colloid, the hardening process of the colloid, the coating thickness of the adhesive layer, the spectral sensitivity of the photosensitive colloid, the spectral scattering rate of exposure, and the light due to light absorption should also be considered. The loss and so on.
In the actual production operation, in order to determine whether the exposure time is correct, it is also possible to observe the color change of the photosensitive layer on the screen plate. The color of the photosensitive adhesives of different manufacturers may be different. Most of the currently used diazo photosensitive adhesives are green in color. By exposure, the color of the photosensitive adhesives will gradually turn light blue, and the exposure can be stopped. Excessive exposure can affect development. Underexposure can cause difficulty in development.
4.2 The distance between the light and the screen is determined
The light distance affects the exposure time and image quality. A short distance between the lamp and the screen will shorten the exposure time but affect the resolution. The different distances between the lamp and the screen will create a "bright spot" in the center of the image and cause severe sidewall corrosion around it. Increasing the distance between the lamp and the screen will reduce this effect and increase the resolution, but the exposure time is greatly extended.
In order to estimate the minimum light distance corresponding to the best exposure time for producing good sharpness, the diagonal of the image area on the screen is measured, and an exposure device is placed so that the light source and the vacuum platen are separated by the measured distance. If you want to expose images of various sizes, try to standardize one or two distances, because changing the distance between the light and the screen will cause confusion. There is an empirical method recommended to everyone; if you double the standard distance mentioned above, you need to triple the exposure time. Conversely, if you reduce this distance by half, you should reduce the exposure time by a quarter.
If you expose several screens at the same time, you must remember to measure the diagonal of the entire image and determine the distance between the screen and the screen. Because one screen is exposed at a time, changing the exposure distance is faster and more suitable than increasing the exposure time.
5 humidity of photosensitive adhesive
The complete drying of the screen material is very important for effective exposure. The more thoroughly the screen is dried, the shorter the exposure time, and vice versa. Therefore, pay attention to the consistency of the drying degree of each batch of the same version during the printing. Residual moisture can interfere with crosslinking of the diazonium sensitizer and polyvinyl alcohol in the photoresist. In fact, moisture affects the exposure, even if the optical integrator has shown that the screen receives the right amount of light, the screen will also show signs of underexposure.
The screen drying oven that controls heat and removes moisture is the most effective drying equipment before and after screen exposure. Dehumidification using strong hot air is another method. Coated screens should be dried under a yellow safety light in the dark and should not be exposed to high temperatures for long periods of time.
6 Supplementary notes on post-exposure
After rinsing and developing, place the screen in the exposure equipment again. Using this method instead of the first correct exposure is not a good idea. Although some dual nitrogen templates improve after exposure, their water resistance is affected. If the exposure is clearly insufficient, it is susceptible to solvent attack.
The post-exposure method is almost completely lost for the diazo photosensitive stencil material because the residual diazonium sensitizer is washed away during the stamping process without the occurrence of an additional cross-linking reaction. The only benefit is the rapid drying of the stencil material.
Determining the correct exposure parameters is a must for each plate maker. It is advisable to conduct an exposure experiment each time before using a certain brand of photopolymer to expose the exposure latitude and related exposure parameters of the product. The screen exposure only requires a little experience and guarantees to control some variables. Correct screen exposure is often easier than taking short cuts and guesswork.
Source: China Ceramics Trade Network