Network distance - the key to quality control of screen printing
We are a big printing company in Shenzhen China . We offer all book publications, hardcover book printing, papercover book printing, hardcover notebook, sprial book printing, saddle stiching book printing, booklet printing,packaging box, calendars, all kinds of PVC, product brochures, notes, Children's book, stickers, all kinds of special paper color printing products, game cardand so on.
For more information please visit
http://www.joyful-printing.com. ENG only
http://www.joyful-printing.net
http://www.joyful-printing.org
email: info@joyful-printing.net
As we all know, the quality of screen printing is affected by many variables, and the grid distance is one of the most important factors. In most cases, the quality and registration problems of printing are mostly attributed to inappropriate adjustment of the mesh distance or some adjustments to compensate for the discomfort of the mesh.
The web distance, which is the distance between the printing surface of the screen printing plate and the surface of the substrate. In screen printing, in order to prevent the screen from sticking to the substrate, a certain distance must be maintained between the screen printing plate and the substrate. Once the screen is glued to the substrate, it will seriously affect the printing quality. The effect of the mesh distance is to effectively overcome this problem, so that the screen can only touch the substrate when the scraper is scraped. Once the scraper is scraped, the screen rebounds. The screen and the substrate are instantly detached to avoid the phenomenon of dragging and smudging.
In addition, the concept of "rebound distance" must be correctly understood. In screen printing, it is closely related to the mesh distance, but different from the mesh distance.
“Rebound distance” refers to the distance that the screen moves back slowly after the scraper is scraped. It occurs at the starting point of printing and appears only during the scraping process of the scraper. It ensures that the screen can bounce back quickly and break away from the substrate. .
The general mesh distance in screen printing is 9.5-12.7mm, but it is more reasonable and more used is 1.5-3.0mm. The network distance has brought many problems to the printing quality and registration, so the mesh distance 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 mesh distance. In electronic product printing, the mesh spacing of stainless steel screens tends to be smaller, about 0.1-0.2 mm.
Therefore, whether it is printed graphics or textiles, the mesh distance is a parameter that deserves special attention. Once the mesh distance is inappropriate or inconsistent, it will bring unnecessary losses to the printing and needs to be carefully controlled.
Significant relationship with positioning
In multi-color and four-color screen printing, color registration must be handled well, and the grid spacing plays a crucial role in registering. Strictly speaking, it is impossible to make the printed matter and the original completely in the presence of the mesh distance, which is caused by the telescopic deformation of the screen during the printing process. The larger the mesh distance, the greater the blade pressure and the greater the stretch of the screen, resulting in a pattern shift on the substrate. The network distance is small, which can improve the situation to a certain extent. Therefore, in order to reduce the image shift, it is required to select the mesh pitch as small as possible. For some prints with higher registration accuracy, it is recommended that the mesh distance be 1.5mm and the maximum should not exceed 3mm.
As the mesh distance increases, many problems arise. First of all, in order to ensure that the screen is in contact with the substrate and the ink is transferred, the pressure of the blade is inevitably increased, and the force applied to both ends of the blade is necessarily greater. Relative to the frame, the longer the blade, 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 skewed and elongated, which is partly due to the mesh distance problem, but also to the position of the pattern, the length of the blade, and the direction in which the blade is running.
It is important to maintain a uniform mesh distance in screen printing. If the screen distances of the screen are inconsistent around the front, back, left, and right, the image at the corresponding position will be offset. In this case, the second color will never be registered with the first color, no matter how hard the operator works. Obviously, if the various colors in the printing cannot be registered, the entire job will be scrapped.
It is worth mentioning that in electronic circuit printing, some pattern distortion can be pre-compensated by calculation when the pattern is not counted. In addition, some manufacturers have introduced devices that have the ability to overcome these problems.
Critical to image quality
The mesh distance setting is not suitable. During the printing process, the blade pressure should be increased or decreased, which will affect the quality of printing, resulting in poor copying accuracy, blur, bubble, ghosting, dot loss, poor edge definition, and moiré. , the pattern is not complete and so on.
The network distance is not too large, and it will cause other problems. For example, the doctor blade needs to increase the squeegee pressure so that the screen can touch the substrate, but before the screen is in contact with the substrate, the ink is squeezed onto the printing surface of the screen. If this is serious, it is bound to be A printed pattern on a dirty substrate or substrate. Excessive squeegee pressure can also damage the printed image, causing dot deformation or dot enlargement.
The greater the blade pressure, the faster the rebound speed of the screen after printing, the ink can not be completely transferred to the substrate, which will lead to the appearance of moiré, incomplete pattern, drawing, bubbles, poor edge definition, etc. Printing rickets.
Another cause of poor print quality is the low tension of the screen and the small mesh distance. In this case, the screen will wrinkle during the doctor blade squeegee, forming ripples on the printed matter, eventually causing severe image distortion and ghosting.
Determine the thickness and color of the ink adhesion
The uneven thickness of the ink adhesion may be caused by the following factors: (1) the screen and the substrate are not parallel; (2) the mesh distance, the blade and the screen tension are variable; (3) the screen spacing is too large, causing the blade to deform. Any of these factors may cause uneven ink adhesion, and the final print may vary in color.
The small mesh distance can ensure uniform ink adhesion, and the mesh distance is too large, which tends to cause uneven ink adhesion, especially at the two ends of the scraper, the ink adhesion is more uneven.
Affect the durability of screens and scrapers
The screen, screen and scratching moon used in the printing process are all elastic. In the process of repeated use, if the force is too large, their elasticity will be attenuated or disappear. When the mesh distance is large, the ink squeezing pressure will inevitably increase. Under such conditions, the durability of the screen and the squeegee will be greatly weakened.
Related to production speed and labor intensity
If the mesh distance is too large, the production speed will be reduced. This is because during the printing process, the doctor blade squeezes the screen to contact the surface of the substrate before the ink is forced to pass through the mesh to 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 can be particularly tiring and sometimes vulnerable.
Textile printing
There are several points that need to be specifically stated in terms of the distance between textile printing. Textiles are an extremely absorbent substrate, so the mesh distance is usually smaller than that of other graphic prints.
The ideal mesh distance should be 0.8-1.6mm. However, the specific settings must be based on factors such as screen tension, screen size, and ink used. The mesh distance is small, and the amount of ink attached to the screen printing machine can also be reduced.
Since textile printing is performed using multiple print heads on multiple printing stations, the uniformity of the web spacing of each printing station is particularly important to ensure registration of individual colors and uniform color tone of the ink. In addition, if you want to print the same pattern on several garments, the mesh distance must be adjusted accordingly as the thickness of the garment changes.
How to reduce the mesh distance
To reduce the mesh distance, the first light needs to increase the tension of the screen. The screen tension is high, which can reduce the mesh distance and increase the printing speed. However, the degree of increase in screen tension is limited by other factors, but at the minimum, it must meet the needs of printing.
It should be remembered that the larger the screen, the less likely the screen is to reach high tension. Therefore, when considering the screen distance of the screen, it is necessary to consider both the size of the screen and the tension of the screen. In general, the tension should be in the range of 18-25 N/cm, so it is not preferable to simply increase the tension of the screen. In addition, the tension of each screen in each set of screens should be consistent. In fact, the consistency of the screen tension is more important than the tension itself, so that not only the overprinting between the color and the color is ensured, but also the consistency of the blade pressure is ensured.
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, it is possible to add some additives to the ink or use other inks. Under the premise of the same tension of the screen, reducing the viscosity of the ink can make the mesh distance smaller.
The technical specifications of the screen are also an important factor affecting ink transfer. The smaller the mesh area of the screen (e.g., a mesh having a diameter of 27-31 μm), the less force is required to peel off the surface of the ink and the substrate. The smaller the force required, the smaller the mesh distance.
The surface of the substrate is also a factor that affects ink transfer and off-print. The screen and the ink-absorbent substrate surface are relatively easy to separate. The surface of the textile has a strong absorption capacity, so the screen is easily separated from the substrate, which is why the distance of the textile printing is smaller than that of other pictures. The tension of the surface of the substrate affects the surface energy of the ink, so it has a certain influence on the post-printing.
Other factors that determine the mesh distance are screen size, blade length (relative to the screen), printed pattern size (relative to the screen), and the type of pattern (solid pattern and non-solid pattern requiring a larger screen spacing).
Some of these factors can be changed, while others cannot be changed. Every print has its own specific printing conditions, and we must understand that it is important to adjust the mesh distance under different printing conditions. By standardizing the operating procedures and detecting newly purchased materials, it is ensured that the previous version and commissioning can be successfully completed as quickly as possible.
Method for measuring the distance of the grid
There are two ways to standardize and determine the mesh distance. Many automatic printing machines and some manual screen printing machines have corresponding network spacing adjustment functions. The problem is that most of them can only be roughly adjusted. Only a few high-end equipments are equipped with a readable dial, which can display the grid distance digitally. Therefore, in most cases, it is up to the operator to adjust.
In practice, the operator has explored some methods and techniques for measuring and controlling the mesh distance. The easiest way is to place the washer on the four corners of the printing table with a washer and then lay the screen on the fourth. On the washers, different gaskets are used for different mesh spacings. To this end, it is also possible to make some thick and thin gaskets to meet the needs of various mesh distances, and to ensure the accuracy of the grid spacing setting. Another simpler method is to measure the distance from the bottom of the screen to the table or vacuum table using a ruler or wedge.
If you have a press with a readable dial, you can read the value of the net distance directly, which is of course the most convenient. However, as mentioned above, in most cases, manual adjustment is required. If the network spacing is improperly adjusted, the trouble is relatively large. The last option for screen printing operators is to use a mechanical mesh gauge. Place the measuring instrument on the surface of the screen, zero the table, and then press down slowly until the screen touches the surface of the substrate. The value shown in the meter is subtracted from the thickness of the screen. The difference is The distance of the network here. Measuring with a grid gauge, as long as the method is correct, the result is more accurate. With this type of meter, it is required to 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 such a measuring instrument on some printing presses. However, so far, the screen printing industry has only one such measurement tool.
It is reported that the US Screen Printing Technology Foundation (SYTF) has recently introduced a more effective network distance measuring tool, the electronic measuring instrument, which can detect the spacing of any place on the screen faster and more accurately. The measuring instrument is convenient to use and can display the grid distance value at any time, so that the operator can quickly complete the debugging work of the upper version.