Choosing a wire mesh is the next important decisive step. A high-quality wire mesh, with a parameter index from the manufacturer, should have similar and uniform numbers of latitude and longitude lines, and also have comparable or identical pressure curves in both directions.
An inconsistent warp and weft line number or inconsistent warp and weft pressure curve is called an unbalanced silk screen, which can cause moire and overprint inaccuracy. A good screen must have enough area to support the necessary gray levels and clean ink transfer.
Your screen parameter index should include the data of the change range of the screen diameter, the number and pressure curve of the warp and weft screen, and the fabric thickness. Most manufacturers provide a data range in the description. If not, they should be asked why not. The ideal wire mesh is to have as thin a wire diameter as possible and a high enough wire count to meet the tension requirements (to check the wire mesh count, you can use a wire mesh counter). The screen opening should be large enough to cover the entire dot pattern in a wide range of tones. The silk screen should also be woven flat and dyed to reduce the gloss of the exposure.
The wire count meter allows you to count the number of wires in the warp and weft directions. With this tool, you can view the mesh to be used and ensure that it provides the features you need. More importantly, you can use it to determine the final properties of the warp and weft directions of the machine-printed screen.
Step 1: Determine the image size
The physical size of the image determines all other printing problems on the machine, such as screen frame size, squeegee length, stroke length, and ink pool size. Of course, this parameter is given by the customer.
Step 2: Set standards for stroke length
The first printing setting is the length of the back and forth stroke of the doctor blade. It should be set to the smallest area that just covers the entire image area. The shorter stroke length increases the number of ink pools, and allows for printing of narrow-width images. It also reduces the discontinuous phenomenon of ink, and can print clean dots. As the stroke length decreases, the time that the ink remains on the screen will decrease, so the viscosity of the ink can also be maintained at a higher level.
Step 3: Choose the blade length
The squeegee length should choose the shortest length that can exactly reproduce the image. You should leave enough length to be able to bypass the end of the blade to ensure that no images are missed.
Step 4: Maximize the ink pool
The ink pool is the maximum area of ​​the screen minus the printing area. The printing area is the area covered by the doctor blade, which is slightly larger than the image area. The distance between the screen frame and the moving squeegee and the top of the squeegee is an important factor in setting the off-plate gap. If the obtained ink pool is too small, the dynamic tension of the screen at the two ends of the doctor blade and the middle are too different, resulting in inconsistent ink transfer between the two ends of the image and the middle. As the ink pool decreases, the off-plate gap decreases, and the ink transfer becomes less secure.
Step 5: Select the frame
The last parameter related to the image size is the size selection of the screen frame. The ideal screen frame is the largest screen frame suitable for printing. However, if the screen frame is more than twice the size of the image, there is no advantage. On the contrary, for a large image, the image / screen frame ratio is not allowed to be 1: 2.
In addition, do not let the printing size be the sole determinant of the screen frame size. The screen frame must be suitable for printing, but the screen frame must also provide an ink pool of available size. If the size of the screen frame is too large for you, then the image is too large for your workshop.
Mesh frame manufacturing is also an aspect to be considered. The mesh frame should be as hard as possible and resistant to bending and twisting.
Step 6: Determine the off-page gap
The purpose of the separation gap is to facilitate the separation of the screen and the substrate in printing. Unless the separation gap is set correctly, the size and accuracy of the image may change.
Regardless of the tension, an excessively large release gap will cause the image to expand. Under higher tension, it will cause negative screen lag, which means that the screen and the substrate are separated at a faster speed than the scraper, which will produce blade sound and excessive blade deformation.
Insufficient off-plate gap will cause the screen / substrate to be difficult to separate. This extreme causes a positive screen lag, which means that the separation of the screen and the substrate is slower than the doctor blade, and it will leave a screen mark. The correct release gap will show zero lag, or just a little positive lag, that is, if the doctor blade passes a certain point, then that point is immediately separated from the substrate.
Step 7: Calibrate the printing press
Parallel is your first consideration when performing color overprinting. The blade edge, aisle, screen and printing platform must be parallel to each other at all points. This ensures that the gap between releases must always be consistent, and consistency is very important for every part of multicolor printing.
Initially, the automatic release function of the printing press should not be used. If so, the release gap must be adjusted accordingly. The use of the automatic release function will compensate for the deficiencies caused by inappropriate geometry and excessive ink, but it is carried out under conditions that damage the stability of the screen and the accuracy of the image.
Step 8: Wire mesh tension
Appropriate screen tension is required to obtain stable and consistent release gaps and promote accurate ink transfer. The tension should be the same in the warp and weft directions. Without balanced tension, moire and overprinting will occur.
The use of a tensiometer alone cannot obtain information on the equilibrium tension of the wire mesh. Accurate tension requires measuring the extension percentage of the warp and weft wires, and using the tensiometer to evaluate the geometric properties of the wire mesh (unification of size, shape, and number of wires) Using the balance tension data obtained, the chance of moire will decrease.
If the elastic limit of the wire mesh is exceeded, the tension disappears and eventually collapses. This is why it is important to observe static and dynamic tension during printing. The dynamic tension at both ends of the doctor blade may exceed the elastic limit first. Especially when the corner of the screen shows absolute tension and the ink pool is too small.
Note that the non-deformed screen is best not used for color overprinting. Once the correct geometric properties of the screen are obtained under the required tension, if the screen is deformed, it will destroy the geometric properties and cause moire.
In general, the minimum static tension level must be able to ensure that the two ends and the middle of the doctor blade have similar dynamic tension, and the maximum static tension level must ensure that the dynamic tension is below the elastic limit of the wire mesh, and cannot cause deformation that causes a decrease in geometric accuracy .
Step 9: Choose a 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 template as possible, controlling the thickness of the emulsion on both sides of the screen and the flatness of the printing side. The flat template can ensure that the template and the substrate are relatively parallel during printing.
The EOM (the amount of milk deposited on the silk screen) should be as small as possible on the stencil printing side, but it should also contribute to the flatness of the stencil. The lower the Rz value (measured value of surface roughness), the flatter the template is, and the easier it is to obtain the resolution and exposure you need. The emulsion on the stencil squeegee side can have a higher Rz value (unevenness), but it cannot exceed the highest point of the screen fiber.
In short, the thinner and flatter the stencil is, the better the printing edge is, and the more the ink transfer on the screen is full. For UV printing, the accumulation of ink will be reduced. The default parameters of the direct exposure emulsion template should include high curing, dual curing, high resolution, fast drying, and wide exposure range. For overprinting, you want to make the dried coating very transparent.
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