Resolution and Solution of Exposure Problems in Screen Printing Plates

Analysis of Exposure Problems in Screen Printing Platemaking

With the increasing number of outdoor large-format advertisements and the emergence of some special printing materials, commercial printing companies are now seriously treating screen printing. The printed matter obtained by screen printing has a thick ink layer and a bright color. After printing, the printed matter can withstand the influence of outdoor natural conditions. Therefore, screen printing has become the preferred printing method for some prints.

To get a good screen print, it's easy and easy to say. It's hard to say. The key is how to effectively control the exposure when using screen printing. How to make a good online version? In the actual work, the printers are often overwhelmed by the exposure time. Whether the exposure process is correct or not determines whether high-quality printing can be obtained in the end. How to avoid exposure errors? In the first step, we may listen to the recommendations of the emulsion suppliers. They have a direct understanding of their own emulsion products, so their recommendations are constructive. Secondly, test and test the specific conditions of the company or the printing shop to determine the optimal exposure time. Because the screen printing process, many changes 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! Here are some key issues related to screen exposure!

The consequences of improper exposure

In the process of screen printing, when the screen is overexposed, it cannot be started printing, of course, there is no choice but to re-plate. In actual production, people are more likely to make the mistake of underexposure. This kind of mistake will bring very negative influence, and it will affect the entire screen printing process. If such a screen is used for printing, the screen will have small pinholes, serrated 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 outlets will change, eventually affecting. To halftone dot reproduction and tone reproduction. In addition, underexposure can also have another negative effect, that is, when the screen is regenerated, it is very difficult to remove the emulsion on the screen.

The relationship between screen exposure and resolution

In the process of screen printing, people hope that the exposed screen will have high-quality images and are resistant to printing. Of course, it is also hoped that the screen will be easily reprocessed. To achieve these requirements, the material must be selected correctly. 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 during the plate making process are minimized. In addition, the spectral characteristics of the exposure light source are also an important factor affecting the quality of the screen.

In the exposure, we are trying to achieve the best exposure to reproduce the level of detail in the original. Some screen printers often have this experience: they want to reproduce the level of detail in the manuscript on the screen, but the final print is not satisfactory! In fact, underexposure can present more levels of detail on the screen, but the overall effect of printed images is often not ideal!

Now, under good exposure conditions, the exposure light source, a wide variety of screen fibrous materials and light-sensitive materials can reproduce very subtle image details. However, even if you use emulsions with large exposure latitude, it is best to take the exposure seriously, and you won’t be lucky enough to do so. All in all, it is best to be successful at the first exposure and try to be accurate.

Influence of light source on exposure time

During the exposure of the screen, the photo-curing reaction of the emulsion is similar to the curing reaction of the UV ink or other ink on the substrate. The screen emulsion is sensitive to the wavelength of the mixed spectrum of UV light and blue-violet light. Because the scraping knife repetitively scrapes back and forth on the screen during printing, the exposed emulsion completely covers the cable after curing. Specifically, there are two factors that affect exposure:

1, exposure light

The spectral characteristics and intensity of the light source are decisive factors in the exposure. Screen emulsions are sensitive to UV light in the 365-420 nm wavelength range. The emulsion must be completely cured and requires a certain amount of energy to cause a chemical reaction.

In general, incandescent lamps and floodlights emit light with a wavelength of 365 nm to 420 nm. Therefore, they are not the best choice for exposure light sources. Metal halide lamps, mercury lamps, and some fluorescent lamps produce photochemical light sources, but they emit different amounts of UV light and blue-violet light. Except for special circumstances, all screen emulsions can use the above three light sources as exposure light sources. Since they emit slightly different ratios of UV light and blue-violet light, the exposure time is slightly different, and the images on the finished screen are slightly different.

The intensity of the light source refers to the energy of the light source reaching the screen emulsion. It does not mean the brightness of the light source seen by the human eye, because the UV light is invisible light. The length of the exposure light source, the power of the exposure device, and the distance from the light source to the screen are the three most important factors affecting the intensity of the light source. In the work, the intensity of the light source should be checked regularly, because the lamp tube will be attenuated during use, resulting in a weakened light source.

2, the distance from the light source to the screen

In the platemaking process, once the distance from the light source to the screen has changed, the optimal exposure time must be recalculated. It is a squared proportional relationship. As an example, if the distance is increased by twice, the current exposure time is 4 times the original exposure time. Conversely, if the distance is reduced by half, the exposure time is reduced by 1/4.

Exposure effects of moisture in the emulsion coating layer

The imaging speed and resolution of the emulsion are also factors that affect the exposure time. With the advent of new sensitizing emulsions, the exposure speed has been significantly accelerated, and several types of sensational emulsions are now used. Let's use an analogy. The relationship between the thickness of the screen emulsion and the exposure time is like the relationship between the ink layer thickness and the ink drying time.

If the emulsion coating is not yet dry, it is exposed, and it is almost impossible to achieve optimum exposure. So, after the emulsion layer is completely dry, it is exposed again. Otherwise, the residual humidity in the emulsion coating will greatly inhibit the thermal cross-linking reaction during exposure. Even if it does not help to prolong the exposure time, it will only end up facing the embarrassing situation of underexposure.

The effect of screen mesh number and color on exposure time

The number of screens and the thickness of the fibers of the screen fabric determine the open area of ​​the screen and the thickness of the screen, which in turn affect how much emulsion can stay on the screen. Obviously, if the area of ​​the opening is large and the mesh line is thick, then the resident dose of the emulsion is more and the exposure time required is longer. On the contrary, the exposure time is shorter! In general, white screens have a short exposure time, but white screens are more prone to light diffusion, thus affecting the resolution of the image, ie, reducing the resolution. Dyeing screen material effectively inhibits light diffusion. Therefore, a better resolution can be obtained. On the other hand, no matter which type of screen material is used, inevitably there will be a certain degree of light diffusion. This is a natural feature of screen printing.

The density of positive film affects the exposure time

If the density Dmin (i.e., background density) of the non-image area of ​​the positive film used for plate making is too high, or the density Dmax of the image area is too low, it will have a bad influence on the exposure. In screen printing, the positive film needed for platemaking should have a clean, transparent background and a high-density (high density) graphic area. Let us suppose that if the background and the graphic area of ​​the positive film are translucent, there is no doubt that light must pass through the graphic area to reach the emulsion.

Method for controlling exposure in plate making

In platemaking, there are several ways to determine the optimal exposure time for a screen, which is mainly determined by the emulsion. If you use diazo photosensitive material, you can determine the degree of hardening of the emulsion material according to the color change; if you use photopolymer material to make plates, we can use a subjective color method to determine whether the best exposure time is reached.

1, the best exposure time test method

Before the actual screen exposure, the exposure time must be determined by the exposure test. In the test, we will use some test strip tools.

For diazo photosensitive materials, we can use Exposure Caculator to determine the color change of diazo photosensitivity. EC offers a range of acceptable exposure times. The EC consists of a series of density-increasing neutral gray image filter blocks. Each image is the same. Neutral gray images of different density levels "filter out" different percentages of the light source range. It can produce a total of 5-6 simulation exposure effects. .

In the exposure test, underexposed and overexposed areas can be identified after testing with EC on a screen plate. In general, the exposure time in the test is very long, generally 2-3 times the actual exposure time. On the CE, the area where the color does not change is the optimal exposure area. The disadvantage of this EC test strip is that only 5 or 6 exposure indication results can be produced. If you want to perform some flexible exposure tests, we recommend using a wedge test strip, because we can increase the exposure time as much as we want, not as limited as the EC.

For example, if you perform 10 exposure tests at 5 second intervals, the wedge test strip is more convenient than the EC. However, there is also a problem with the use of a wedge-shaped test strip. The ease of repetitive exposure to manual operations depends on the vacuum structure of the exposure device. If the vacuum device does not have to be turned on each time the test strip is moved during exposure, the test strip is more convenient to use. However, the construction of some exposure devices makes the use of wedge test strips very cumbersome.

For single-component photosensitive polymer emulsions and stencils, the above test methods are completely inapplicable because they do not have a change in color. Experienced screen printers use the resolution to determine how exposed the screen is. Of course, tests can also be conducted until the emulsion hardness and resolution are considered to be optimal.

2, the actual exposure process control method

After 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 light source is uniform. Alternatively, we can use light control positive film (ECP) to control the exposure. The ECP consists of a series of neutral gray density filter blocks. If the screen is exposed at the optimum exposure time, a specific density block can be achieved after washing with water. If you did not make exposure compensation for the exposure device's aging decay at the time of exposure, or if the exposure time required for the plate-making material was not set correctly, the control image area on the ECP will give you an indication that you are not properly exposed at this time. Therefore, ECP is a useful tool bar for actual exposure.

Conclusion

To the point, the exposure technology in screen printing plates is a key link. There are many factors that affect it, and the factors are dynamic. Faced with a complex exposure process is not always in tune. The most effective means is to use some exposure control tools and profoundly grasp these factors that affect exposure. It is reasonable to obtain a beautiful silk screen print!