The pointers presented here address the relationship between image and screen size, as well as how image position on screens can affect the quality of finished graphics.
Screen printers who push the envelope in production volume often find themselves limited by the size and configuration of their presses. Even using the maximum screen size the press will accommodate allows the use of only a portion of the screen for image area. Exceeding the maximum image area causes image distortion, which can lead to registration problems and make accuracy in diecutting and other finishing processes impossible to achieve.
Image-to-frame ratio is critical, regardless of whether you’re printing one large image or multiple smaller images simultaneously. The term refers to the size of the image are relative to the total screen area, which is usually represented by the frame’s inside dimensions.
In general, screens on a smaller press provide less free mesh around the perimeter of the image area. Even with minimal off-contact, the smaller screens require greater squeegee pressure to bring the image into contact with the substrate. As a result, the mesh may deflect more than desired and cause image distortion (Figure 1A).
Larger screens, used on larger presses, provide more free space around the image areas. The availability of more free mesh area reduces deflection and image distortion during the squeegee stroke. The result is a printed image that is more likely to fall within desired tolerances (Figure 1B).
Maximum image size
The eight-and-six rule is a popular way to figure out maximum image size. The rule maintains that a fixed measurement should always be used for image placement on the screen. Specifically, the image should be positioned at least 8 in. from the frame edge, where the squeegee stroke begins, and a minimum of 6 in. from either of the frame’s sides. This rule applies in many cases, but some applications require even more free mesh area at the edges to ensure accurate, distortion-free printing.
Squeegee length also factors into the relationship between image size and total mesh area. In general, using a squeegee that greatly exceeds the width of the image cancels out the positive effects of free space around the image area. The squeegee still approaches the edges of the frame, leading to mesh deflection and image distortion.
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