Influence of Screen Distance in Screen Printing and Adjustment (I)

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.

(to be continued)