Silk screen CTS system (2)
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Analysis of Exposure Problems in Screen Printing Plates
With the increasing number of outdoor large-format advertisements and the emergence of some special printing materials, commercial printing companies are now taking the printing method of screen printing seriously. The printing ink obtained by screen printing has a thick ink layer and a bright color, and after glazing printing, the printed matter has been affected by outdoor natural conditions. Therefore, screen printing has become the preferred printing method for some prints.
To get a good screen print, it is easy and easy to say. It is difficult and difficult to say. The key is how to effectively control the exposure when screen printing? How to make a good online version? In actual work, printers are often overwhelmed by exposure time. The correctness of the exposure process determines whether or not high quality printing can be achieved. How to avoid exposure errors? The first step is to listen to the advice of the emulsion supplier who has a direct understanding of their emulsion products, so the recommendations are constructive. Secondly, test and test the specific conditions of the company or the printing workshop to determine the optimal exposure time. Because the screen printing process, many variables determine the exposure time recommended by others, just a reference standard, to achieve the best exposure, it is necessary to test, practice the truth is this truth! Below, focus on a few key issues related to the exposure of the screen!
Consequences of improper exposure
In the process of screen printing, when the screen is overexposed, you can't rush to start printing. Of course, there is no choice but to re-plate. In actual production, the mistake that people are more likely to make is underexposure. This kind of error will have a very negative impact, which will affect the entire screen printing process. If such a screen is taken for printing, the screen will have small pinholes, jagged edges, and the pressure of the printing press will cause the emulsion to fall off, the jagged dots will appear on the printed matter, and the percentage of the dots will change, eventually affecting Copying to halftone dots and reproduction of tones. In addition, underexposure will have another negative effect, that is, it is very difficult to remove the emulsion on the screen when the screen is regenerated.
The relationship between screen exposure and resolution
In the process of screen printing, people hope that the screen after exposure has high-quality images and is resistant to printing. Of course, it is also hoped that the screen will be easily reprocessed. To meet these requirements, the material selection must be correct first. The quality of the screen depends on whether the plate-making equipment has good performance, whether the selection of the photosensitive material and the screen material is correct, and whether the dynamic change factors in the plate-making process are minimized. In addition, the spectral characteristics of the exposure source are also an important factor affecting the quality of the screen.
In the exposure, the goal we want to achieve is to achieve the best exposure to reproduce the level of detail of the original. Some silkscreen workers often have the experience that they want to reproduce the details of the original on the screen, but the final printed prints are not satisfactory! In fact, underexposure can present more levels of detail on the screen, but the overall effect of the printed image is often not ideal!
Now, under good exposure conditions, exposure light sources, a wide variety of mesh fiber materials and photographic materials can replicate very fine image detail. However, even if you use an emulsion with a large exposure latitude, it is best to take the exposure seriously, and you will not be able to do so. All in all, it's best to be successful once in the exposure, try to be accurate.
The effect of light source on exposure time
During the exposure of the screen, the photocuring reaction of the emulsion is similar to the curing reaction of UV ink or other ink on the substrate. The screen emulsion is sensitive to the mixed spectral wavelengths of UV light and blue-violet light. Since the doctor blade in the printing repeatedly scrapes back and forth on the screen, the exposure emulsion hardens to completely cover the screen fiber. Specifically, there are two factors that affect exposure:
1. Exposure light source
The spectral properties and intensity of the source are the decisive factors influencing the exposure. Screen emulsions are sensitive to UV light in the 365 nm to 420 nm wavelength range. The emulsion is fully cured and requires a source of energy to cause a chemical reaction.
In general, incandescent and floodlights emit light at wavelengths from 365 nm to 420 nm, so they are not the best choice for exposure sources. Metal halide lamps, mercury lamps, and some fluorescent lamps emit actinic light sources, but they emit different ratios of UV light and blue-violet light. Except for special cases, all of the screen emulsions can use the above three light sources as the exposure light source. Since the ratio of UV light and blue-violet light emitted by them is slightly different, the exposure time is slightly different, and the images on the final screen are also different.
The intensity of the light source refers to the energy of the light source reaching the screen emulsion. It does not refer to the brightness of the light source seen by the human eye because the UV light is invisible light. The length of use of the exposure source, the power of the exposure device, and the distance from the source to the screen are the three most important factors affecting the intensity of the source. In the work, the intensity of the light source should be checked regularly, because the lamp will attenuate during use, resulting in weakened light source intensity.
2. The distance from the light source to the screen
During the plate making process, once the distance from the source to the screen changes, the optimal exposure time must be recalculated at this time, which is a square proportional relationship. As an example, if the distance is increased by two times, the current exposure time is four times the original exposure time. Conversely, if the distance is reduced by half, the exposure time is reduced by a factor of 1/4.
Effect of exposure on the humidity of the emulsion coating layer
The imaging speed and resolution of the emulsion are also factors that affect the exposure time. With the advent of some new emulsions, the exposure speed has been significantly accelerated, and several types of sensory emulsions are now in use. Let us say, for example, the relationship between the thickness of the screen emulsion and the exposure time is like the relationship between the thickness of the ink layer and the drying time of the ink.
If the emulsion coating layer has not been dried, the exposure is taken, so that it is almost impossible to achieve optimal exposure. Therefore, after the emulsion layer is completely dried, it is exposed again. Otherwise, the residual humidity in the emulsion coating layer will greatly inhibit the thermal crosslinking reaction during exposure. Even if you want to extend the exposure time, it will not help, and in the end, you can only face the embarrassing situation of underexposure.
The effect of mesh line number and color on exposure time
The number of mesh lines and fiber thickness of the wire mesh determines the open area of the screen and the thickness of the wire, which in turn affect how much emulsion can reside on the wire. Obviously, the opening area is large and the wire is thicker, so the residual milk dose is more, and the exposure time required is longer. On the contrary, the exposure time is shorter! In general, the white screen has a shorter exposure time, but the white screen is more prone to light diffusion, thus affecting the resolution of the image, that is, the resolution is reduced. The dyed wire mesh material effectively suppresses light diffusion. Therefore, a better resolution can be obtained. Having said that, no matter which kind of screen material is used, there will inevitably be a certain degree of light diffusion, which is a characteristic of screen printing.
The density of Zhengyang tablets affects the exposure time
If the density Dmin (i.e., background density) of the non-image area of the positive-working sheet used in the plate making is too high, or the density Dmax of the image area is too low, the exposure may be adversely affected. In screen printing, the positive film required for plate making should maintain a clean, transparent background and a high-density (high density) image area. We might as well assume that if the background and the graphic area of the positive film are translucent, there is no doubt that the light must reach the emulsion through the graphic area.
Method of controlling exposure in plate making
In the plate making process, there are several ways to determine the optimum exposure time of the screen, which is mainly determined by the emulsion. If you use a diazo sensitizing material, you can determine the degree of hardening of the emulsion material based on the color change; if you choose a photopolymer material, we can use a subjective color method to determine whether the optimal exposure time is achieved.
1. Test method for optimal exposure time
The optimum exposure time must be determined by exposure testing before the actual screen exposure. In the test, we will use some test strip tools.
For diazo sensitizing materials, we can use the exposure time calculation strip EC (Exposure Caculator) to determine the color change of diazo sensitivity. The EC offers a range of acceptable exposure time options. The EC consists of a series of neutral gray image filter blocks with increasing density. Each image is the same. Neutral gray images of different density levels "filter" out different percentages of the light source range, which can produce 5-6 kinds of simulated exposure effects. .
In the exposure test, an area of underexposure and overexposure can be identified by using an EC test on a screen. Usually, the exposure time in the test is very long, usually 2-3 times the actual exposure time. On the CE, the area where the color does not change is the optimal exposure area. The shortcoming of this EC test strip is that only 5 or 6 exposure indication results can be produced. If you want to do some flexible exposure testing, we recommend using a wedge test strip because we can increase the exposure time as we like, unlike the EC.
For example, if a 10 exposure test with an interval of 5 seconds is performed, the wedge test strip is more convenient than the EC. However, there is also a problem with the use of wedge test strips, and the ease of reproducible exposure for manual operations depends on the vacuum structure of the exposure apparatus. If the vacuum device is not required to be opened each time the test strip is moved during the exposure process, the test strip is more convenient to use. However, the construction of some exposure devices makes the use of wedge test strips cumbersome to develop.
For single-component photosensitive polymer emulsions and dies, the above test methods are completely unsuitable because they do not reflect color changes. Experienced screen printing technicians use resolution to determine how exposed the screen is. Of course, it is also possible to test until the emulsion hardness and resolution are considered to be optimal.
2. Control method during actual exposure
When we determined the optimal exposure time through testing, the next problem was solved. We can use some control tools for exposure control. For example, we can use the Exposure Control Tool (ECT) to check if the exposure source is uniform. Alternatively, we can use the Light Control Positive (ECP) to control the exposure. The ECP consists of a series of neutral gray density filter blocks. If the screen is exposed to the optimum exposure time, after washing with water, a specific density block can be reached. If you do not make exposure compensation for the aging decay of the exposure device during exposure, or if the exposure time required for the plate material is not set correctly, the control image area on the ECP will give you an indication that the exposure is not correct at this time. Therefore, ECP is a useful toolbar for actual exposure.
Conclusion
To sum up, exposure technology in screen printing is a key link. There are many factors that affect it, and the factors are dynamic. Faced with a complex exposure process is not the only thing that can't be done. The most effective means is to use some exposure control tools and to grasp these factors that affect the exposure. It is reasonable to get a beautiful silk screen print!