As we all know, the quality of screen printing is affected by many variables, of which screen pitch is one of the most important factors. In most cases, the printing quality and registration problems are mostly blamed on inappropriate mesh pitch or some adjustments made to compensate for the uncomfortable mesh pitch.
The screen pitch is the distance between the printing surface of the screen printing plate and the surface of the substrate (see Figure 1). In screen printing, in order to prevent the screen from sticking to the substrate, a certain distance must be maintained between the screen and the substrate. Once the screen is stuck to the substrate, it will seriously affect the printing quality. The role of the screen distance is to effectively overcome this problem, so that the screen only contacts the substrate when the scraper scrapes, once the scraper scrapes, the screen rebounds , The screen is separated from the substrate instantaneously, avoiding ink dragging and smudging.
In addition, we must correctly understand the concept of "rebound distance", which is closely related to the screen pitch in screen printing, but is different from the screen pitch.
"Rebound distance" refers to the distance that the screen plate rebounds slowly after the scraper scrapes the ink. It occurs at the beginning of printing and only appears during the scraping of the scraper. It ensures that the screen plate can rebound quickly and leave the substrate .
The general screen pitch in screen printing is 9.5-12.7mm, but it is more reasonable and more commonly used is 1.5-3.0mm. Excessive screen pitch brings many problems to printing quality and registration, so the screen pitch should be as small as possible. When printing some products with higher precision, the allowable amount of screen deformation directly determines the size of the screen pitch. In the printing of electronic products, the screen pitch of stainless steel screens is often smaller, about 0.1-0.2mm.
Therefore, whether it is printed graphics or textiles, the screen pitch is a parameter that deserves special attention. Once the screen pitch is inappropriate or inconsistent, it will cause unnecessary losses to the printing and needs to be carefully controlled.
1. The relationship with positioning registration is significant
In multi-color and four-color screen printing, the problem of color registration must be handled well, and the screen pitch plays a crucial role in registration. Strictly speaking, it is impossible to make the printed matter and the original completely consistent with the existence of the screen pitch, which is caused by the expansion and contraction of the screen in the printing process. The larger the screen pitch, the greater the pressure of the doctor blade and the greater the stretch of the screen, resulting in a shift in the pattern on the substrate. The small network distance can improve this situation to a certain extent. Therefore, in order to reduce the image shift, it is required to select the smallest possible pitch. For some prints that require high registration accuracy, the recommended screen pitch is 1.5mm, and the maximum should not exceed 3mm.
As the network distance increases, many problems will arise. First, in order to ensure that the screen is in contact with the substrate and realize ink transfer, the pressure of the doctor blade will inevitably increase, and the force exerted on both ends of the doctor blade is bound to be greater. Compared with the screen frame, the longer the scraper, the more prominent this phenomenon. In this way, not only the degree of deformation of the pattern is increased, but also the printing quality is seriously affected.
In addition, sometimes the printed pattern is slanted and elongated, part of the reason for this is the mesh distance problem, but it is also related to the position of the pattern, the length of the blade and the direction of the blade.
It is very important to maintain a uniform screen pitch in screen printing. If the screen distances on the front, back, left, and right sides of the screen are inconsistent, the image at the corresponding position will shift. In this case, no matter how hard the operator works, the second color will never register with the first color. Obviously, if various colors cannot be registered during printing, the entire job will be scrapped.
It is worth mentioning that in electronic circuit printing, some pattern distortions can be pre-compensated by calculation when the pattern is not counted. In addition, some manufacturers have introduced devices that have the capability to overcome these problems.
Second, it is important for image quality
The screen pitch setting is not suitable. During the printing process, the pressure of the blade should be increased or decreased, which will affect the quality of the printing and will cause poor copy accuracy, blur, bubbles, ghosting, dot loss, poor edge definition, and moire. , Incomplete patterns and other issues.
If the pitch is not too large, it will cause other problems. For example, the squeegee needs to increase the pressure of the ink scraping so that the screen can contact the substrate, but before the screen and the substrate are in contact, the ink has been squeezed onto the printing surface of the screen. If this situation is serious, it is bound to be Will smear the substrate or the printed pattern on the substrate. Excessive squeegee pressure will also damage the printed image, causing dot deformation or dot expansion.
The greater the pressure of the blade, the faster the rebound speed of the screen after printing, and the ink cannot be completely transferred to the substrate, which will cause moire, incomplete patterns, brushing, bubbles, poor edge definition, etc. Printing defects.
Another reason for the poor printing quality is the low screen tension and the screen pitch is too small. In this case, the screen will wrinkle during the ink scraping process of the doctor blade, forming ripples on the printed matter, which will eventually cause severe image distortion and ghosting. [next]
Third, determine the thickness and color of ink adhesion
The uneven thickness of the ink may be caused by the following factors: (l) the screen is not parallel to the substrate; (2) the pitch, squeegee, and stencil tension change indefinitely; (3) the squeegee is deformed due to the excessive pitch. Any one of these factors may cause uneven adhesion of the ink, and the final print will have different shades of color.
The small mesh distance can ensure the uniformity of ink adhesion. If the mesh distance is too large, it is easy to cause uneven ink adhesion, especially at the two ends of the scraper.
Fourth, affect the durability of screen and scraper
The screens, screens, and moon scrapers used in the printing process are all elastic. In the process of repeated use, if the force is too large, their elasticity will decay and disappear. If the screen distance is large, the scraping pressure will inevitably increase. Under such conditions, the durability of the screen and the scraper will be greatly weakened.
5. Related to production speed and labor intensity
If the mesh distance is too large, it will reduce the production speed. This is because during the printing process, the squeegee squeezes the screen to make contact with the surface of the substrate before squeezing the ink to force it through the mesh to reach the substrate. In this process, the pressure of the tongue can be consumed and transferred, which greatly reduces the production speed. In manual printing, the printer must use more force to print, which will be particularly tiring and sometimes easy to be injured.
Six, how to reduce the network distance
To reduce the screen distance, Shouguang needs to increase the screen tension. The high screen tension can reduce the screen pitch and increase the printing speed. However, the degree of increase in screen tension is limited by other factors, but the minimum needs to meet the needs of printing.
It should be remembered that the larger the screen version, the less likely it is that the screen version will reach high tension. Therefore, when considering the screen pitch of the screen version, both the size of the screen version and the tension of the screen version need to be considered. Generally speaking, the tension should be in the range of 18-25N / cm, so it is not advisable to blindly increase the screen tension. In addition, the tension of each screen in each set of screen should be consistent. In fact, the consistency of the screen tension is more important than the tension itself. This not only ensures the accurate registration between colors, but also ensures the consistency of the doctor pressure.
In addition to the tension of the screen, another noteworthy is the ink. The viscosity of the ink is related to the rheology of the ink, the composition of the ink, the size of the ink particles, the surface tension and the storage time. If the viscosity of the ink is large, you can add some additives to the ink or switch to other inks. Under the premise of unchanged screen tension, reducing the viscosity of the ink can make the screen spacing smaller.
The technical specifications of the screen are also an important factor affecting ink transfer. The smaller the mesh area of ​​the screen (such as a mesh with a diameter of 27-31 μm), the smaller the force required to peel off the surface of the ink and substrate. The smaller the required force, the smaller the net distance can be.
The surface of the substrate is also a factor that affects ink transfer and release. The surface of the screen and the substrate with strong ink absorption is relatively easy to detach. The absorption capacity of the textile surface is strong, so the screen is easily separated from the substrate, which is why the screen pitch of textile printing is smaller than that of other graphic printing. The tension of the surface of the substrate will affect the surface energy of the ink, so it also has a certain effect on the post-press release.
Other factors that determine the screen pitch include screen size, blade length (relative to screen), printed pattern size (relative to screen), and type of pattern (solid pattern and non-solid pattern that requires a larger mesh distance).
Some of these factors can be changed, while others cannot be changed. Each printing has its own specific printing conditions. We must understand that it is important to adjust the screen pitch under different printing conditions. By standardizing the operating procedures and testing the newly purchased materials, it is ensured that the edition and commissioning can be completed successfully as quickly as possible.
7. Measuring method of net distance
There are two ways to regulate and determine the pitch. Many fully automatic printing machines and some manual screen printing machines have corresponding screen pitch adjustment functions. The problem is that most can only be adjusted roughly. Only a few high-end equipments are equipped with a readable dial that can display the screen pitch digitally. Therefore, in most cases, it has to be adjusted by the operator himself.
In practice, the operator has figured out some methods and techniques for measuring and controlling the screen pitch. The easiest way is to use the gasket to place the gasket on the four corners of the printing table, and then lay the screen flat on the On each gasket, different gaskets are used for different mesh distances. To this end, some shims with different thicknesses can be made to meet the needs of different mesh pitches and ensure the accuracy of mesh pitch setting. Another simple method is to use a ruler or wedge to measure the distance from the bottom of the screen to the worktable or vacuum table (see Figure 2).
If you have a printing machine with a readable dial, you can directly read the value of the pitch, of course, this is the most convenient. However, as mentioned earlier, in most cases, manual adjustment is required. If the pitch is not adjusted properly, the trouble is greater. The final option for screen printing operators is to use a mechanical screen gauge. Place the measuring instrument on the surface of the screen, set the meter to zero, and then slowly press down firmly until the screen touches the surface of the substrate. Subtract the thickness value of the screen from the value shown in the measuring instrument, and the difference is The net distance there. Measured with a pitch gauge, as long as the method is correct, the results are more accurate. The use of this measuring instrument requires that it must be reset before each measurement, and at the same time, it has a limited range of movement on the screen. Therefore, in this sense, it is not very convenient to use this measuring instrument on some printing machines. However, so far, the screen printing industry has only this kind of measurement tool.
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