Color Screen Printing Relevance Model (2)

(The Dependency Model for Process-color Screen Printing) Step 8: Determine the number of lines You must print the image according to the customer's resolution requirements. In general, the visual distance should be the most basic criterion for determining the number of lines. However, the number of lines required by the customer is often limited by the printable range, because if the number of lines is too high, it is impossible to distinguish within your visual distance, resulting in a large amount of information being redundant. The number of lines will influence your ability to maintain "point information." With the increase in the number of lines, the size of the dots continues to decrease until the dots cannot be printed. The determination of the number of lines should also take into account the tonal range and spatial frequency, and the short or compressed tone range can easily print large highlights and dark points at any line number. Unfortunately, over-compression of the tonal range causes a reduction in contrast, and in the case of high spatial frequencies, too small a contrast cannot be printed with a high number of lines. Step 9: Determine image latitude Determine the latitude of an image is the next issue to consider. The latitude here refers to the range of allowable variations in color overprinting, within which full stacking and juxtaposition can still be produced, resulting in good image contrast, no moire, and optimal resolution. The latitude dependency is the size of the basic halftone rose pattern printed under the number of selected lines. Step 10: Determine the dot resolution The film is obtained by the photo method. The dot resolution is not a problem because in practice, the photo dot has a “hard” and high-definition edge. However, with the film obtained by the photo-composing machine, a good dot is difficult to obtain because the photo-setter dot is a collection of a large number of small dots based on pixels. On digital devices, not only the number of lines and tonal ranges but also the resolution of the dots must be determined. If the digital output resolution (dpi) is set too low, a banding phenomenon will occur. If the resolution is too high (the number of lines does not change), the number of gray scales will increase to exceed the printing capacity of screen printing. The relationship between dpi and gray scale, line number can be expressed by the following formula: gray scale = (dpi/number of lines) 2-1, with 150 gray scales as an example, this formula shows that the number of lines is 30-100 lines The /inch range requires an output resolution of 370-1230 dpi. Step 11: Select the wire mesh Selecting the wire mesh is the next important decisive step. A high-quality screen, with a parameter indicator from the manufacturer, should have a similar uniform number of latitude and longitude lines, and also have comparable or identical pressure curves in both directions. The warp and weft pressure curves with inconsistent number of latitude and longitude lines or inconsistent lines are called unbalanced screens and can cause moire and overprinting. A good screen must have enough area to support the necessary grayscale and clean ink delivery. Your screen parameters should include data on the range of screen diameter changes, number of warp and weft screen lines and pressure curves, and fabric thickness. Most manufacturers provide a range of data in the description. If not, then you should ask him why he did not. The ideal screen is to have as thin a wire diameter as possible and a sufficiently high number of wires to meet the need of pulling force (if you want to check the screen wire number, you can use a screen wire counter). The mesh opening should be large enough to cover the entire dot pattern in a wide range of tones. The screen should also be relatively flat and dyed to reduce exposure to light. The screen line meter allows you to screen the number of lines in the warp and weft directions. Using this tool, you can view the mesh you are using and make sure it provides the features you need. What's more, you can use it to determine the final warp and weft properties of the machine screen. Step 12: Determine Image Size The physical size of the image determines all other on-machine printing issues such as the frame size, doctor blade length, stroke length, and ink pool size. Of course, this parameter is given by the customer. Step 13: The first printing setup for the stroke length setting criteria is the length of the squeegee stroke. It should be set to the minimum range that can just cover the entire image area. The shorter stroke length increases the number of ink fountains and allows for the printing of narrow latitude images. It also reduces discontinuous ink instabilities and prints clean dots. As the stroke length decreases, the time for the ink to remain on the screen is reduced, so the viscosity of the ink can be maintained at a higher level. Step 14: Select the length of the doctor blade. The length of the doctor blade is to choose the shortest length that just reproduces the image. You should leave enough length to bypass the end of the blade to ensure that no image is missing. Step 15: Maximize the ink pool The ink pool is the largest area of ​​the screen minus the print area. The print area is the area covered by the squeegee, which is slightly larger than the image area. The distance between the frame and the moving squeegee and the top of the squeegee is an important factor in setting the clearance. If the ink pool is too small, the dynamic tension of the screen will be too different at both ends and in the middle of the squeegee, resulting in inconsistent ink transfer at both ends of the image. As the ink pool decreases, the off-plan gap decreases and the ink transfer becomes less safe. Step 16: Selecting the last parameter related to the frame size is the size of the frame. The ideal frame is the largest frame that is suitable for printing. However, if the frame is larger than twice the size of the image, there is no advantage. On the contrary, a large image does not allow the ratio of the image/frame to be 1:2. Also, don't let the print size be the only deciding factor in determining the frame size. The frame must be suitable for printing, but the frame must also provide a usable size ink pool. If the size of the frame is too large for you, the image is too large for your workshop. The manufacture of the frame is also an aspect to be considered. The frame should be as hard as possible and have good resistance to bending and twisting. Step 17: Determining the off-grid gap The purpose of the off-grid gap is to facilitate separation of the screen from the substrate during printing. Unless the off-set gap is set correctly, the size and accuracy of the image may change. No matter what the tension is, excessive off-gauge gap will cause the image to expand. At higher tensions, it can cause negative screen lag, meaning that the screen and the substrate are separated at a faster speed than the doctor blade. This will result in blade noise and excessive blade distortion. Insufficient off-grid gaps will cause the screen/substrate to be difficult to separate. This extreme leads to a positive screen lag, which means that the separation of the screen from the substrate is slower than the squeegee and it leaves a net mark. The correct off-gaps will show zero lag, or just a little lag, that is, if the squeegee passes a certain point, the point is immediately separated from the substrate. Step 18: Correcting the printer parallel is the first thing you should consider when doing color overprint. The squeegee blade edge, aisle, mesh and printing platform must be parallel to each other at all points. This ensures that the off-gaps gap is always consistent, and consistency is very important for every part of multi-color printing. The automatic detachment function of the initial printing press should not be used. If this is the case, the off-page gap must be adjusted accordingly. The use of the automatic disengagement function compensates for deficiencies caused by improper geometry and excess ink, but it is performed under conditions that impair screen stability and image accuracy.