With the right exposure light source, the next step is to find the best exposure time. The best exposure time is determined by observing the depth of the stencil cure. The optimum exposure time must be sufficient to cure the stencil. There are several techniques for determining the optimal exposure time. The most famous one is the exposure calculator (Figure 4). It is represented by a series of gradually increasing neutral gray filters, which can simulate 100%, 70%, 50%, 33%, 25% and other exposure results at one time.
The exposure calculator test is performed by first exposing and processing the test screen, and then evaluating the exposure template based on the color change, instead of the resolution. As shown in Figure 5, the unused diazonium salt shows a strong yellow bottom tone. In the correct exposure, the yellow bottomtone on the leftmost column disappears completely. However, this does not fully explain that exposure is optimal and thorough. Thorough exposure also ensures that there is no color change in the middle part of the exposure calculator, which is the immediate right column. If there is a color change, double exposure times need to be tested again. No change in color until two consecutive steps indicates that the best exposure was obtained.
When you determine which step to complete the exposure, the exposure coefficient for that step is multiplied by the exposure time to get the best exposure time. In the case of underexposure, the 100% exposure area or coefficient 1 always seems to be correct because there is no other area in addition to itself. When this happens, you must double the exposure time to do another experiment.
This exposure calculator is the most suitable tester for diazonium salt templates. However, for dual nitrogen template, the two parts often change at the same time. Therefore, the judging method becomes such that once the diazo portion stops changing, it means that the correct exposure is achieved.
However, since the exposure calculator does not exhibit a color change in contrast to a polymerized emulsion template, although it can be used to determine the resolution at different exposures, it does not indicate the cure. An alternative method for sensitive emulsions of photosensitivity can be used - the sensitive gray scale (Figure 6). This tool has 21 gray scales, each with a difference of 0.15 in density. The longer the exposure time, the more grayscale becomes harder. If used correctly, the best exposure can be found just by testing once. In most cases, the 7th order is cured, indicating that the correct exposure has been achieved. If the first test only cured to the 5th order, the best exposure time would double. If cured to the sixth order, the best exposure time needs to increase by 40%. If it cures to the 8th level, it means that there is overexposure and some details may be lost. Just like the “pinhole†caused by scratches or dust, the exposure time will need to be reduced to 70%. If it is cured to the 9th order, it means that the exposure time has reached twice the best time. The greatest benefit of using this method is that it can be used to control the curing of any type of stencil, such as diazonium salts, dual nitrogen salts, or photopolymer emulsions.
The final recommended test exposure method does not use the test film at all. The method uses a digital irradiometer to determine the amount of photosensitizer used in the coating at a certain point. Its working principle is: 365nm color filter is installed in the phototube of the optical integrator, and then the photocell is placed in the vacuum box behind the coated screen and finally exposed. At the beginning, due to the large amount of photosensitizer absorbed, no light reaches the photocell, so at this time the radiometer reads 0. During the exposure, as the photosensitizer is consumed, the emulsion coating begins to “transparentâ€, more light A large area of ​​emulsion coating can be achieved. The increase in the amount of light can be measured by a radiometer and finally the light gradually becomes stable.
The information collected by the optical integrator is shown in Figure 7. The best exposure is represented by a line that is stabilized. The measured light intensity indicates that the photosensitizer has been exhausted completely, and no photosensitizer has prevented the light from reaching the photocell, which means that the stencil has been fully exposed.
From Figure 7 it can be seen that the diazonium salt emulsion curve finally reached the level, while the dual nitrogen salt emulsion curve showed a continued slow rise. This is due to the additional photochemical effect, which is also reflected in the color change test method.
This method has its advantages, but it cannot be used for exposure testing of polymeric emulsion products. This is because the residual photosensitizer still has great light absorption even after exposure. This will be further explained later when the post-exposure effect is discussed.
Going to the next section, this section needs to explain why the effectiveness of the light source and the quality of the bulb are very important for the production of stencils with better printing performance. In the real world, it is impossible in most cases to produce a very perfect template that is very consistent from top to bottom and from side to side. This is because the intensity of the light received by the entire screen during the exposure is inconsistent. For example, a typical point light source is the best light source for copying details. However, only the light source is far away from the screen to ensure that the “hot spot†is not in the middle, so as to ensure that the edge of the template is not underexposed.
Insufficient stencil exposure may occur in both cases. The first case is an inappropriate light source, which means that the light source is not a sensitive light source for the stencil chemistry. In this case, the underexposure portion is mainly concentrated on the place where the emulsion and the screen are bonded, so when the emulsion is underexposed, the entire coating easily falls off. The second case is similar to the case of underexposure in the first case, but the exposure stencil has good adhesion in use and is not easy to fall off. The exposure light source in this case is correct and appropriate, and the underexposure is due to other reasons such as exposure time. Therefore, since the entire emulsion layer is hardened at the same time, it is possible to avoid the "apex hardening", that is, the closest and the furthest emulsions that are in contact with the screen are hardened. Although the emulsion layer is still underexposed, the resulting stencil will have better durability than the first case.
It is very necessary to use a good light source during the exposure. It can make the stencil with a wider exposure range and can avoid pinholes or emulsion shedding during development.
The fluorescent exposure system allows for a more consistent hardening of the stencil but may result in loss of detail, especially fine halftones. For a positive film, it is difficult to ensure that light from different angles is very sharp on the emulsion layer.
When exposing larger screen screens, multiple exposure lights are needed to provide a wider light coverage, but it must be extra careful. The pattern details of the overlapping areas of the light may be lost, usually in a direction perpendicular to the light arrangement. Figure 8 shows an enlarged view of the fine detail loss of 305 lines/inch