The screen pitch is also called the screen interval, which is the vertical distance between the bottom of the screen and the surface of the substrate. It is the main 5 variables in screen printing (squeegee hardness, screen pitch, printing speed, scraping pressure, The suitability of ink) One. In screen printing, the space between the printing plate and the substrate is the key to ensuring that the screen printing plate does not stick to the substrate during the printing process.
If the screen and the substrate are stuck, it will cause many problems. The screen distance solves this problem. In the printing stroke, it can make the screen contact the printing material only at the blade edge of the squeegee. In this area, there is a peeling area where the screen and the printing material are separated, and then a clear print is left. Peeling refers to the upward separation movement that the screen frame gradually generates during the printing stroke. It occurs at the starting point of the ink wiping stroke and the ink scraper is in motion during the printing cycle. The screen is moved until it contacts each pad. Different shims can be used to adjust different mesh distances, and calibration wedge can also be used to adjust the mesh distance, which has better accuracy and versatility.
Ruler adjustment. This method is to put the ruler close to the bottom of the screen frame, and directly observe the distance from the ruler to the surface of the printing platform.
Use a mechanical pitch gauge. Suspend the screen gauge above the screen, and make the instrument read 0 on the screen surface, and then press down until the screen is in contact with the substrate. Read the value at this point from the instrument and then subtract The thickness of the screen and the template is the pitch of this point. As long as it is used properly, the data measured by the net distance meter is very accurate and repeatable. Its disadvantage is that each reading must be readjusted. The instrument is limited to moving around the screen, which is inconvenient to use and difficult to use on some printing machines. Currently, the Screen Printing Technology Foundation
(SPTF) has launched a new type of network distance measuring instrument, which is a more effective and reliable electronic testing instrument. It can quickly and accurately determine the network distance at any position of the screen version, and it is easy to operate. The operator of the prepress process can use this instrument to know the size of the screen at any time.
The variables affecting the screen pitch are also the screen frame size, screen tension, doctor blade length, image area relative to the screen frame size, and the type of image (such as large-area field and reverse image, 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. Operators of printing substrates should make corresponding adjustments according to the requirements, and the network distance should be as small as possible within the acceptable range. The tension of the screen is an influential factor in the adjustment of the screen pitch and an important factor influencing the printing quality. This should be considered when making a plate, but the effect of tension on the life of the screen will only show up during printing. If the tension of the screen is too low, the screen will be easily degummed under continuous expansion and contraction. In addition, in order for the screen to pop up quickly, the screen distance has to be increased, so the pressure required to scrape the glue is also increased accordingly, but this is undoubtedly more likely to damage the screen and deform the pattern. Generally, the adjustment of the mesh pitch should be based on the tension of the stretched net, and the mesh pitch is small when the tension is large, and vice versa; according to the size of the mesh frame, the mesh pitch of the mesh frame is also large. However, it should be noted that when adjusting the screen spacing, it is necessary to ensure that the screen spacing of each point on the screen is the same.
The effect of screen pitch on printing quality
Inappropriate screen pitch will adversely affect the printing quality, and there may be problems such as poor copy accuracy, smudging, blistering, ghosting, ink sticks, missing levels, blurred image edges, screen marks, incomplete images, inaccurate overprinting, etc. .
The effect of excessive pitch on printing quality
Regardless of the amount of tension, an excessively large mesh pitch will cause the image to expand, causing changes in image accuracy and size. Under greater tension, the separation speed of the screen and the substrate will be faster than the speed of the blade, which will cause the blade to deform. Excessive mesh pitch will also cause serious damage to the wire mesh, which will cause premature template damage. Sticky web pulp may also come off due to excessive tension.
Under normal tension, if the screen distance is too large, a large scraping pressure is required to bring the screen plate into contact with the printing material, which will destroy the image layer due to the excessive amount of ink. Excessive pressure of the squeegee will also damage the image, causing smudging and dot expansion. In addition, if the screen distance is too large, the screen will bounce back too quickly, so that the ink cannot be transferred to the substrate sufficiently, and the printed product will have screen marks, incomplete images, ink bars, bubbles, and blurred image edges. High scraping pressure will also deform the scraping blade, causing deformation of the edge of the printing area and large amount of ink adhesion. These will cause uneven ink layer, and the color variation of the image on the printed crystal will be large and become a defective product. Excessive screen pitch will also slow down the printing speed, because the force applied by the squeegee is not directly used to transfer the ink, but must first overcome the tension of the screen to make it contact the substrate, thus dispersing the pressure of the squeegee Reduced printing efficiency. In manual printing, the operator must apply more force to print, which is easy to fatigue.
It should also be noted that in screen printing, because increasing the screen pitch has the same effect as increasing the screen tension, some people want to increase the scraper pressure by increasing the screen pitch to obtain a higher tension on the screen, but This method has many negative effects (especially the increase of the dot), so the screen tension should be increased during printing, and the screen distance should not be increased.
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