Effect of Adjustment of Web Distance in Screen Printing on Printing Quality

In screen printing, many printing variables determine the final print quality, where the web spacing is an important variable. Therefore, it is necessary to accurately adjust the pitch in printing to ensure good print quality. In order to control the network distance, we must first fully understand and be familiar with the influence of variables such as the network distance on the print quality, so that we can print better quality products better, faster, and more steadily.

The role of net distance

Network distance, also called screen spacing, is the vertical distance between the bottom surface of the screen plate and the surface of the substrate. It is the main five variables in screen printing (scraper hardness, mesh distance, printing speed, pressure, ink One of the suitability). In screen printing, the space between the plate and the substrate is the key to ensuring that the screen plate does not stick to the substrate during the printing process.

If the screen is stuck to the substrate, it will cause a lot of problems. The network distance solves this problem. During the printing stroke, it can make the screen contact the printing material only at the edge of the squeegee, and the squeegee passes through. In this area, there is a stripping area where the screen and the printing material are separated, leaving a clear print. Peeling refers to the upward separation movement that the frame gradually generates during the printing stroke. It occurs at the beginning of the scraping stroke and the squeegee is in the moving state during the printing cycle. The moving screen stays in contact with each pad. Different shims can be used to adjust different mesh distances. The calibration wedge can also be used to adjust the pitch, which is better in accuracy and versatility.

Ruler adjustment. This method is to close the ruler against the bottom of the frame, and observe the distance from the ruler to the surface of the printing platform.

Use a mechanical web meter. Suspend the grid gauge above the screen and make the instrument read 0 on the surface of the screen. Press down until the screen contacts the substrate. Read the value from the instrument and subtract it. The thickness of the screens and stencils is the net distance at this point. As long as it is used properly, the data measured with a web meter is very accurate and repeatable. Its disadvantage is that each reading must be re-adjusted. The instrument is limited to moving around the screen, which is inconvenient to use and difficult to use on some printers. At present, the Screen Printing Technology Foundation (SPTF) has introduced a new type of instrument that measures the distance between the screens. It is a more effective and reliable electronic testing instrument that can quickly and accurately determine the network at any position on the screen. Distance, easy to operate. The operator of the prepress process can use this instrument to know the size of the mesh at any time.

Variables affecting the grid spacing include the frame size, screen tension, doctor blade length, image area relative to the size of the screen frame, and the type of image (eg, large area solids and reverse images, due to the large surface area in contact with the substrate. It requires a larger network distance. Some of these variables can be changed and some cannot be changed. The printing substrate operator should make corresponding adjustments according to requirements, and the network distance should be as small as possible within the acceptable range. The tension of the screen is the influencing factor of the distance adjustment, and it is also an important factor that affects the printing quality. This is something that should be considered when making a plate, but the effect of tension on the life of the screen will only be demonstrated when printing. If the tension of the screen is too low, the screen can easily be debonded in a continuous stretch. In addition, in order to allow the screen to quickly rebound, had to increase the distance between the network, and therefore require the pressure of the squeegee to increase accordingly, but it is no doubt more likely to damage the screen, but also will deform the pattern. Generally, the adjustment of the net distance is based on the stretching tension, and the tension large distance is small, and vice versa; according to the size of the net frame, the large net distance of the net frame is also large. However, it should be noted that when adjusting the grid distance, it is necessary to ensure that the net points on the screen are the same.

Network distance influence on printing quality

Inappropriate grid spacing can adversely affect the quality of printing, and may cause problems such as poor copying accuracy, smudging, blistering, ghosting, ink bars, missing layers, blurred image edges, web marks, incomplete images, and overprinting. .

Excessive net distance affects print quality

Regardless of the size of the tension, an excessively large mesh distance will cause the image to expand, causing changes in image accuracy and size. Under greater tension, the separation of the screen from the substrate will be faster than the speed of the blade, which will cause the blade to deform. Oversized screens can also be very damaging to the screen, causing premature damage to the stencil. Stickies may also fall off due to excessive tension.

Under normal tension, if the net distance is too large, a large amount of squeegee pressure is required to make the screen printing plate contact with the printing material. This may damage the image level due to the excessive ink amount. Excessive pressure on the squeegee can also damage the image, causing smudging and dot gain. In addition, the net distance is too large, the screen will bounce back too quickly, so that the ink can not be fully transferred to the substrate, the print will appear net marks, incomplete images, ink, bubbles, blurred edges and other issues. Large squeegee pressure will also cause the squeegee blade to deform, resulting in deformation of the printing area edge, a large amount of ink, these will cause the ink layer uneven, showing that the printing crystal is the image color variation, become a defective product. Excessive mesh distance will also slow down the printing speed, because the force exerted by the squeegee is not directly used to transfer ink, but must first overcome the tension of the screen to make it in contact with the substrate, which spreads the squeegee pressure Reduced printing efficiency. In manual printing, the operator must apply more force to print, which is very easy to fatigue.

In addition, it should be noted that in screen printing, because the increase of the net pitch has the same effect as the increase of screen tension, some people want to increase the squeegee pressure by increasing the net pitch, so that the screen gets a higher tension, but This method has a lot of negative effects (especially the dot increase), so the screen tension should be increased in the printing instead of increasing the pitch.

The effect of small net distance on printing quality

In most cases, the net distance should be as small as possible, but too small grid distance will cause the separation of the screen and the substrate to be unsatisfactory, so that the screen cannot be ejected from the workpiece immediately, and the separation speed between the screen and the substrate is relatively low. The slow speed of the squeegee blade will leave a net mark and cause the pattern to become dirty. In addition, if the mesh distance is too small, the ink will adhere unevenly, and the squeegee will ripple on the screen when printing, which will affect the registration and cause image ghosting. The correct net distance should be under the condition that the screen plate has a certain tension, so as to ensure that the scraping pressure of the squeegee during the screen printing process is minimized, and the screen can be ejected from the workpiece immediately. The choice of net spacing is related to the screen tension. High tension screens can choose a relatively small screen pitch, and low tension screens should choose a relatively large screen pitch. The adjustment range of the general network distance is 0.1-12.7mm, but 1.5-3.Omm is more common and more feasible. When the quality of the printed product is required to be high, the size of the net distance will be limited by the range of deformation that the screen can allow. In the printing of electronic products, stainless steel nets are often used, and the net pitch is relatively small, 0.1-0.2 mm.

Registration issues

In multi-color printing, the accuracy of overprinting is an important factor in measuring the quality of the printing, and the net distance plays a decisive role in the stable printing and registration. The screen is deformed due to its elasticity. The larger the distance, the more the squeegee forces the screen to extend, causing the image to be deformed on the printing material, and the peeling will make the problem worse. In order to reduce this distortion, the network spacing should be as small as possible. For products with higher printing accuracy, the net distance should be less than 3.Omm. Practice has proved that using 1.5mm is more ideal. In addition, many problems will arise. For example, in order to make the screen and the substrate contact with each other, the good transfer of the ink is achieved, the pressure to increase the squeegee must be increased, and excessive force is exerted on the screen plate in the squeegee. The cutting edge will multiply, and the longer size of the squeegee will bring more problems. The result is not only an increase in image edge distortion but also a significant reduction in print quality. The increase in grid distance also tends to skew and lengthen the entire image. This effect will vary depending on the position of the image, the length of the squeegee relative to the frame, and the direction of travel of the squeegee. The uniformity of the mesh distance is also an important factor affecting the registration. If the screen pitches of the screen before and after printing are inconsistent, partial overprinting of the printed matter will occur. In this case, the next color is printed, and it is difficult to register with the first color in any case. In addition, the impact of registration and shrinkage and expansion of the substrate. When multi-color printing, the net distance used must be the same, otherwise it will cause deviation in printing accuracy.

How to adjust the optimal network distance

Since the determination of the network distance is related to a variety of factors, it is necessary to consider different materials and printing equipment to adjust the optimal network distance. To use the smallest possible network distance, consider the following aspects:

(1) Improve screen tension. The screen tension is high, so that the network distance can be reduced and the printing speed can be increased. Therefore, according to the actual level of equipment, materials, and stretch netting, try to obtain a screen with higher tension. However, it should be noted that it is difficult to stretch the large-size screen to a higher tension. Therefore, when determining the screen pitch, the size and tension of the screen must be taken into consideration. In general, screen tension can choose 18~25N/cm. Multi-color printing also needs to be consistent with the tension of each screen, so as to ensure that the screen pitch of each screen is the same. In this way, not only can the print registration between color and color be guaranteed, but the relative printing pressure can also remain the same.
(2) Reduce the ink viscosity. The viscosity of the ink is determined by factors such as thixotropy, ink composition, size and amount of pigment particles, surface tension, storage time, and the like. To reduce the viscosity of the ink, you can improve the ink, or find another good printing ink. The lower viscosity of the ink will also reduce the requirements for web spacing and stretch tension.

(3) Select the appropriate screen. The geometry of the screen is also an element that affects the ability of the ink to transfer. Using a screen with a small surface area (eg, a wire with a wire diameter of 27 to 31 mm), the force required to separate the ink from the printing material will be smaller, the required force is small, and the mesh distance may be small.

(4) The characteristics of the substrate. Substrate also has a great influence on ink transfer and peeling. Absorbent materials are more easily peeled than non-porous materials. The tension on the surface of the substrate is related to the surface energy of the ink. Therefore, it also affects the peeling of the ink. Taking a textile substrate as an example, textiles have water absorption, and there is no problem of peeling from the screen. This is the reason why the net distance of textile printing is smaller than that of other graphics. Smaller net distances for printed textiles can minimize the deposition of ink underneath the screen. Because textiles are generally printed with multiple table tops and print heads, ensuring consistent web spacing at each print station is the key to achieving good overprinting and ink uniformity. Textile printing sometimes requires several garments to print the same pattern. The thickness of these garments is different, and the net pitch must be adjusted accordingly.

In a word, as one of the five major variables in screen printing, the screen pitch has a great influence on the quality of screen printing. Therefore, the best screen distance should be debugged according to various factors such as equipment, materials, and substrates. In this way, you can print beautiful products.

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