1. The basis for the tension of the stretched net In screen printing, the mesh size, wire diameter, screen weaving method, and selected materials directly affect the tension of the stretch screen. When stretching the screen, the tension of the screen is measured according to these parameters. When measuring the tension, the detected tension point should be 10 cm away from the inner edge of the frame, otherwise the measured tension is inaccurate. See the table below for the maximum Zhang Guo value that can be achieved by SEFEN PET 1000. The maximum tension value indicated in the table is the display of the specific wire mesh strength, that is, the maximum tensile force value that various wire diameters can withstand and rebound with different wire diameters. It will lose elasticity and become plastic deformation, which should be paid attention to in the stretched net. The unit of tension: Newton / cm (N / CM), which can be measured with Newton tension meter. The tensiometer can measure the tension in the warp and weft directions. The so-called warp tension of the screen is the tension in the winding direction of the entire screen, that is, the tension in the edge typing direction; the weft tension of the screen is the tension in the width direction of the screen. It can be seen from the tension table that the same material produces different wire diameters and different meshes, and the tension is different, that is, the same mesh mesh, the tension is different, that is, the same mesh For several meshes, the mesh diameter is different, and the tension is different, because the tensile strength is proportional to the mesh diameter. For example: A is twice the radius of B wire, then the tensile strength of A wire is 4 times that of B wire. The tension value in the table is effective for high-strength frame with side length less than 1 meter. When the frame length is 2 meters, the tension value should be reduced by 15% -20%. When left and right, when the net is stretched, the tension given in the table is reduced by 20-25%. In order to ensure that the screen will not be torn during processing and printing, it is necessary to stretch the net a little lower than the tension given in the table. 2. Tension requirements 1. The mesh template for different purposes requires different tension. For example: color dot printing, in order to ensure the accuracy of the tone value and good reproducibility, the tension of 20-30N / CM is required. Fine prints, dials, etc., tension is 12-18N / CM. For general graphic printing, the tension is 8-12N / CM. For manual printing, rough printed products, or those that do not require accuracy, size, etc., the tension is> 6N / CM. It is worth noting that when the color block is overprinted, in order to make the overprint position accurate, not only the screen tension should be greater than 10N / CM, Moreover, the several stencils that are overprinted must reach the same tension, which is more important for dot printing, otherwise it will cause deviations in moire and hue. 2. Why do dot printing require higher tension screen templates? This is because A. The screen template with higher tension can obtain a lower screen pitch. When the screen pitch is doubled, the deformation of the printed image will increase by three times. Therefore, when the screen template tension is low, the ink is not uniformly distributed and the dots Expand the deformation and affect the hue. B. With lower screen pitch, less pressure can be used in screen printing, and the reduction of screen stencil wear will prolong the service life of screen stencil. C. The smaller scraping pressure is helpful to avoid ink and deformation around the dots, improve the sharpness of the dot edges, and ensure the printing effect. 3. Factors affecting the tension of the stretched net 1. Stretching equipment A. Pneumatic stretch net machine is a direct stretch net method. Because when the net is stretched, the cylinder support has been against the edge of the net frame, and at the same time, the reaction force of the wire mesh is pulled, so that the edge of the net frame is bent toward the center of the frame to obtain pressure. After the tension is reached, the net is dried and then discharged. Next, the tension is more stable, and the tension is easier to obtain. Disadvantages: a. During the entire process of stretching the wire, the wire wind has been extending outward (around the periphery) of the mesh frame viscose. Therefore, the mesh frame sets a resistance to the smooth extension of the wire mesh, which is often unreachable under high tension; b. Pneumatic stretching The net is controlled by air pressure. Therefore, if the reverse air pressure is adjusted to be high during the initial tension, the net will be broken under the instantaneous and rapid tension; c. The tension is controlled by the capacity of the cylinder, otherwise high tension cannot be achieved. B. The mechanical or electromechanical wire-drawing equipment can be flexible and changeable according to the needs of tension. The mechanical wire-drawing machine is an indirect stretch method, so when the wire tension reaches the required tension, the net frame rises against the wire The mesh is not affected by the frictional resistance of the mesh frame when the mesh is stretched, so the tension is easy to achieve uniformity. After stretching the net, the gap between the chucks is allowed to be between 2mm and 3mm, otherwise when the distance is far, the part of the wire mesh that is not clamped by the chuck is not directly pulled, which makes the tension on all the wires on the mesh surface unequal , The size of the opening and the screen diameter are inconsistent, and defects will appear in the printing, that is, the full screen will have uneven tension with the screen printing, the ink volume is inconsistent, and the depth will be inaccurate. Under the scraping pressure of the scraper, the wire mesh is easily broken, so it must be paid attention to. 2. Frame A. The screen frame material, cross-sectional size, shape and flatness after processing, strength and aging treatment directly affect the tension of the screen and the stability of the tension in use. In most cases, the main reason is that the tension of the screen template decreases when the screen frame is not strong enough. Secondly, when the degumming and stretching of the wire mesh and the frame have exceeded the limit tension of the wire mesh, the tension of the screen template will also be reduced. Therefore, the screen frame must be deburred before the use of the sticky surface and the spring side After these treatments, then apply a layer of glue evenly with the glue of the screen. After the screen is glued, the screen will be firmly adhered to the frame of the glue, to ensure that the screen is in the screen printing process. Under a large scraping pressure, the mesh cloth will not loosen from the mesh frame. Note that the glue of the sticky mesh must be able to penetrate into the glued face of the wire mesh, so that the wire mesh is completely adhered to the mesh frame. If necessary, when the sticky mesh glue is not completely dry, manually scrape the pressure of the mesh frame with the glue. , It is helpful to help the screen fit the screen frame completely. B. It is worth noting that the screen frame must have a cross-sectional size (wall thickness) and elastic coefficient that can resist large external forces, otherwise only the screen tension is pursued, and the strength of the screen frame is ignored, which is the unstable screen tension And the important reason for the drop in tension. C. The mesh frame is divided into aluminum alloy profiles and steel mesh frame, and the elasticity coefficient of steel is 2.9 times that of aluminum. Screens that require high tension hope to use steel mesh frames. For example, the screen for screen printing on discs has a tension of about 22-26N / CM and requires a small cross-sectional area. Therefore, a steel frame of about 3mm is used to make the screen frame, so that in the process of four-color screen production and printing The set is consistent, the overlay is accurate, the hue is good, and it is not easy to produce moire. D. For the positional requirements and the screen frame for dot printing, the material, size, tension of the stretched screen and the cutting direction of the screen must be the same. E. The unsuitable ratio of screen frame size to printed image size will affect printing accuracy and screen tension stability. For this reason, the large screen frame generally requires that the four sides of the printed graphic be at least 15CM away from the edge of the screen frame, the small and medium size and the screen frame are set to 10CM, which requires higher accuracy. The length and width of the printed graphic should be the inner frame length of the screen frame. 1/3 of the wide size. [next] 3. The methods and techniques of stretching the net A. Standard stretch net method: The so-called standard stretch net method, that is, after the wire net is loaded into the chuck, it stretches to 70% -80% of the allowable tension of the wire net in about 5 minutes, and stays for 10 After about 10 minutes, pull to the allowable tension, stay for about 10 minutes, make up to the allowable tension, stay for 10 minutes, make up to the allowable tension, and stick the net. The entire stretch process takes 1.5-2.0 hours. B. Net stretch method in 30 minutes: This method means that after the net drawing process is completed within 30 minutes, the net will not be stretched again. The net stretch process is: (1) Firstly, the tension in the warp and weft direction reaches half of the requirement. (2) Pause the net for about 60 seconds of stability. (3) Increase the tension by 2-4N / CM, stop for 60 seconds, and increase to the required tension by this method. (4) Stabilize the tension for about 10-15 minutes, and then stretch again to the required tension. (5) After repeating the steps of (4) again, the net can be glued, and the whole process of stretching the net ends. C, fast Screen-stretching skills in screen printing in screen printing-net-stretching method: In this method, the tension of the wire mesh is pulled to more than 15% of the required tension within 5 minutes and the mesh is immediately adhered. D. Comparison of several stretch net methods: (1) Fast stretch net, the result of stretch net means that it will have a loss of 2.5N / CM tension in the screen within the first hour. At the same time, due to the sudden change of the mesh under rapid tension, the wire is intentionally silk Netease is broken, this method of stretching the net is not recommended. (2) The 30-minute stretch method provides higher screen tension than the fast stretch method. After 1 hour, the screen tension loses 1N / CM, and after 24 hours, the screen tension loses only 1.8N / CM. (3) The standard stretched net is stretched in about 2 hours. The increase of the stabilization time within 2 hours and the increased tension is not large. At the same time, the stretched net is also long and does not save time. It is generally considered that a 30-minute stretch net method , Recommended. 4. When stretching the net, relax the four corners, leaving an adjustment margin: When stretching the net, if the four corners are not relaxed, because the short distance between the corners belongs to the high tension zone, when the mesh stretch distance is very short, especially for high tension, low elongation mesh, the four corners It is necessary to leave a stretch allowance according to the size of the frame before the net is stretched, otherwise, the tension in the stretched net has not reached the design tension parameter of the screen, that is, it is not the real tension of the screen, broken The net will be torn in advance from the four corners of the net frame, which is a very important issue in stretched nets. 5. The correct stretching process: The correct stretch net is: (1) First tear off the irregular end of the weft wire mesh, clip into the chuck in equal amount, then clamp the other end of the wire mesh, and flatten the wire mesh. (2) Flatten the wire mesh and clamp the wire mesh on both sides of the weft. (3) Pull the warp wire to 1/2 of the required tension. (4) Stretching the weft direction, the tension is close to the full data of the required tension. At the same time, we adjust the weft and warp directions to achieve the required tension, and stop for 10 minutes. (5) Adjust the tension in the warp and weft directions to the required tension after 10 minutes, about 10 minutes. (6) Repeat (5) twice, stick the net. (7) Note: A, the four sides of the chucks are close to each other, the maximum allowable gap is within 3mm, so that the mesh force is consistent. B. The chucks on the four sides correspond to each other one by one, to ensure that the mesh is close to square when the wire mesh is stretched, and the tension is uniform everywhere. 6. After stretching the net, the screen can only be made after 24 hours. Because of the high-tensile wire mesh, after 24 hours, its tension drops by about 10% -20%. The tension loss of the high-strength wire mesh in the stretched net appeared within the first 4 hours, and then gradually stabilized. Therefore, the stability of the tension between the stretched nets is determined by the internal requirements of the wire mesh polymer material, not the stretched net operation. The tension loss and stability of the screen version also change with changes in the environment and temperature. To make a fine screen version, this factor must also be taken into account. 7. The temperature and humidity change greatly and the drying time after the sticky net stretches is insufficient, so that the screen tension is easy to be lost. Under the guarantee that the wire mesh and the frame are all closely adhered, the sticky mesh glue must be dry, not only to stabilize the tension, but also to ensure the stick The solvent of the net glue is helpful when cleaning the screen. 8. Exceed the maximum allowable tension value of the screen That is, although the screen does not break during the stretching process, the screen will naturally break when it is impacted by time, humidity, temperature during the placement process, or when the screen is pre-treated. This is because the synthetic fiber has exceeded its own extension. Length, reaching plastic deformation and fatigue strength, when the external environment conditions change, it will naturally break, which must be paid attention to. In the actual production process, the tension of the wire mesh is not expected to reach the maximum allowable tension. Under the premise of ensuring the accuracy of the screen printing, it is reasonable as long as the screen can always rebound and the tension is stable for a long time. Do not pursue the high tension required to break away from the required tension, otherwise the screen life will be shortened, and the printing accuracy of large graphics and small screen frames should not be solved by increasing the tension. 4. The angle of the stretched net: it can be divided into flat stretched net and oblique stretched net. Its functions are as follows 1. In order to make the edges of the printed lines straight and smooth without distortion and jaggies, when stretching the net, the warp and weft of the silk screen and the edge of the frame are at a certain angle. This method is called diagonal stretch net. The angles of the stretched net are various, and the general angles are 15 degrees, 22.5 degrees, 45 degrees, 7.5 degrees, and 30 degrees. In these types of lines, the lines and edges obtained at 22.5 degrees have better straightness and smoothness, but the wire mesh is wasted in the process of stretching the net, and the quality of the edges of the lines obtained at 90 degrees is the worst. To sum up, in order to produce a good screen, the first condition is: choose a good quality screen, and then use high-quality screen stretching equipment and skilled and correct screen stretching techniques, according to the allowable tension parameters of the screen Without stretching the specified elongation of the screen, the stretched screen will provide a reliable basis for making a high-level, long-life, low-consumption screen printing template.
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2. For dot printing, due to the difference in the number of dot lines, in order to feed and eliminate obvious moire, it is necessary to stretch the net according to the smallest angle of moire when the wind is stretched. Otherwise, it is necessary to set the printed pattern and the screen at a certain angle to feed the moire or eliminate the obvious moire.