Maximizing Image Size and Position
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.
Three-quarter-automatic and most semi-automatic presses feature a series of mechanical edge guides that raise and lower between print strokes to ensure accurate substrate placement. The typical press-bed layout on these machines positions a row of guides at the side of the bed, farthest from the loading position—or, for this discussion, the back of the bed—and single guides along the left and right sides of the bed (Figure 2). The guides along the back of the press bed serve as the point where mechanical takeoff devices can grab the sheet and move it to the next print or finishing station.
It’s common practice to position the substrate against the back and left guides when working with sheets that are smaller than the maximum sheet size defined by the position of the press’s register guides. It’s also common, when these guides are used, to offset the stencil image on the screen so that the printed image is aligned correctly with substrate.
Applications that involve single, large images without critical tolerance can be printed successfully with the image positioned on one side of a screen—but this isn’t often the case for tight-tolerance applications, especially when the screen features multiple images that will be die cut.
The position of the image on the screen influences the apparent image-to-frame ratio. Although the centered and offset images take up the same area relative to the total screen area, offsetting the images actually makes the press behave as if it’s printing a larger image (Figure 3).
Calculating equivalent image-to-frame ratio
To figure out equivalent image-to-frame ratio, use the offset image position to define the image-to-frame distance at the back edge and along the side closest to the substrate-register stops. Double these distances and subtract them from the frame’s inside dimensions to get the equivalent size of the image area.
Also consider the squeegee’s influence if you’re offsetting images on the screen. The force of the squeegee is distributed equally over the entire mesh area, resulting in some mesh-thread elongation. However, because the offset graphic is typically aligned by positioning one edge of the stencil image—usually the edge nearest the press’s side register guide—the screen deflection and subsequent printed image distortion are more pronounced along the opposite side of the image.
Depending on the specific design of your press and the size of your image, it may be impossible to avoid offsetting images on screens, particularly on three-quarter-automatic presses, where you must positions substrates against register stops to ensure that take-up mechanisms can grab the materials. If your press requires manual unloading, you’re even less restricted. Such presses allow you to establish your own substrate-positioning guides and always center your images on screens, thereby avoiding the distortion issues associated with offsetting images.