Choosing the right frame and mesh is critical to achieving quality prints. This overview looks at different frame types, mesh parameters and performance criteria, screen-preparation tips, and recommendations for ensuring long-lasting screens.
• Some meshes (high tension, low elongation) have been developed to counteract the first problem and allow higher tensioned screens, but it is always best to follow the manufacturer’s or suppliers’ recommendations for proper mesh tension level.
Proper frame size is determined by the printed image. The squeegee should have a run-on and run-off area of a minimum 4 in. to the inside of the frame from the image, and side clearance of at least 3 in. from either side of the squeegee to the inside of the frame. An image 11 x 15 in. would require a minimum frame size of 17 x 23 in. for a problem-free print. Usually, the more space around the print, the easier it is to get a good printing result. Tightly jammed images on screens that are too small end up causing production problems, including blurred edges, missed edges on the print, and distortion.
When printing halftone images, a rule of thumb for determining correct mesh count to use is 3.5 x halftone line count to get the minimum mesh count. For example, a 65-line halftone image would require a 230-thread/in. mesh or higher. The higher the mesh count, the better the reproduction of the image on the stencil and the print and the less chance of moiré patterns. Avoid any directly divisible mesh/halftone combinations (e.g., 75 line halftone screen on a 300-thread/in. mesh) as this will also cause interference patterns on the stencil and the print. Also keep in mind that yellow mesh is often recommended for high resolution printing with halftones. The reason why is discussed in the section on color mesh that follows.
Sawtoothing and sharp edges
Coarse meshes will cause sawtoothing on the printed image. This is a result of the stencil being unable to bridge the gap between threads. Capillary and Indirect stencils, because their edge is actually below the mesh, print sharper lines. Photopolymer-based emulsions (one-pot, SBQ type) are sharper than diazo-based or regular emulsions because they form their molecular shapes in straighter lines.
Other mesh considerations include the following:
• Monofilament meshes clean and reclaim better than multifilament
• Meshes with the designation 6XX, 10XX, etc., are using an old silk measuring language,which does not readi- ly translate into threads per inch. Under no circumstances use silk.
• Mesh becomes more expensive the higher the mesh count.
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