Color Screen Printing Relevance Model (3)

(The Dependency Model for Process-color Screen Printing) Step 19: Screen Tension The appropriate screen tension is required to obtain a stable and consistent off-plan gap and to promote accurate ink transfer. Tension should be the same in warp and weft directions. Without balanced tension, moire and overprinting will occur. The tension gauge alone can not be used to obtain information on the balance tension of the screen. The precise tension needs to measure the percentage of elongation of the warp and weft cables, and the tension gauge is used to evaluate the geometric performance of the screen (unity, size, and number of screen lines). Using the obtained balance tension data, the probability of occurrence of moiré will be reduced. If the elastic limit of the screen is exceeded, tension disappears and eventually collapses. This is why it is important to observe static and dynamic tension when printing. The dynamic tension at both ends of the squeegee may exceed the elastic limit first. Especially when the angle of the screen shows absolute tension and the ink pool is too small. Note that the non-deformable screen should not be used for color overprinting. Once the correct screen geometry is obtained under the required tension, the deformation of the screen will destroy the geometric performance and cause moire. In general, the minimum static tension level must ensure that the squeegee has similar dynamic tension at both ends and in the middle. The maximum static tension level must ensure that the dynamic tension is below the elastic limit of the screen, and it cannot produce deformation that leads to a decrease in geometric accuracy. . Step 20: Select the template After the screen is stretched and the related work is ready, the choice of template is the next stop in the screen-making journey. For color overprinting, you should produce as thin a stencil as possible to control the emulsion thickness on both sides of the screen and the flatness of the print side. The flat template can ensure that the stencil and the substrate in the printing are relatively parallel. The EOM (the amount of emulsion deposited on the screen) should be as small as possible on the side of the stencil printing, but it also benefits the flatness of the stencil. The lower the Rz value (surface roughness measurement), the flatter the template, and the easier it is to obtain the resolution and exposure you need. The squeegee side emulsion may have a higher Rz value (unevenness), but it cannot exceed the highest point of the screen fiber. In short, the thinner and flat the template, the better the printing edge, and the more the ink transfer on the screen, the less ink will accumulate in the UV ink. The default parameters for direct exposure emulsion stencils should include high cure, dual cure, high resolution, fast drying, and a wide exposure range. For overprinting, you want to make the dried coating very transparent. Step 21: Color separation test image The test image (color balance scale, gray scale, overprint scale, density scale, scale, etc.) should be included in your separation film. By default, the elliptical positive dot is generated with the minimum gray component substitution. The maximum density of the image area is 3.8-4.2. The film should be clearly imaged and the small dots should be complete. Step 22: Adjust the Angle of the Color Separator and Screen After selecting the number of lines (step 8) and the number of screen lines (step 19), you must adjust the angle of the color separator to suit the angle of the screen. Film and screen There are three factors that must be balanced: 1) the best screen reference angle; 2) the symmetrical balance of the film at this angle; 3) the priority of the angle and frequency at a given line number. In balancing these factors, your goal is to make the dots and the mesh opening with minimal interference, that is, to minimize the occurrence of moiré. Once you have the correct tension and good mesh, there is an optimal angle (α), the geometric pattern will be the best, and the print will be cleaner. The symmetric angles arrange the halftone dots on the screen at an angle that does not generate a moire. When the halftone arrangement is ideally suited to a screen, the alpha and symmetry angles are the same. Most conventional color separations are frequency-first (for example, they have a fixed number of lines and varying color-to-color angles to allow the ink to be fully transferred and to avoid moiré). The direction priority is to maintain the same angle of the screen and the film and change the number of lines to adapt to the multi-color angle and expand the color space printing. Step 23: Output the test image Once the color separation sheets are suitable for the screen, you can output a set of test images for color overprinting. At the same time to ensure that the density of the positive film between 3.8-4.2. Only 1% change at 50%. Step 24: Screen Printing Images are screened with a test image when the stencil has dried and the humidity is less than 4%. The film should be mounted directly on the prepared screen for exposure. The exposure must allow sufficient cross-linking of the stencil material to prevent corrosion of the stencil on the squeegee side of the screen plate. Exposure criteria are applied to each screen to ensure consistent exposure. Ideally, the distance between the light source and the image should be 1.5 times the diagonal of the image. If the distance causes overexposure, you should choose a smaller distance. However, it should be noted that the exposure intensity gradually decreases from the center of the screen toward the periphery. The stencil resolution should be detected with an exposure detector. The application of this technique allows you to obtain the minimum dot size for high-profile and dark-sounding (dark dots may be smaller). Step 25: Estimate dynamic tension The dynamic tension should be consistent along the edge of the doctor blade. Its value is related to the position of the blade edge, the off-gauge gap, and the initial or static tension of the screen. Because the dynamic tension is almost inconsistent, it should be measured and calculated where the cutting edge approaches the end of the frame, that is, where the elastic limit of the tension is reached first. The highest dynamic tension shows the tension across the squeegee, and the minimum is the tension in the middle. The tension difference between the two ends of the doctor blade and the middle position becomes noticeable when the ink pool is small, and it is the task of the doctor blade to compensate for this uneven tension. At this point, good quality images can already be printed. Continuously setting standards for standardization work is a method of making your color overprint a “screen printing experience”. Screen printing, like many other forms of production, also has a series of variables. By setting these variables, you Get consistent quality prints.