Screen Printing - December 2012/January 2013 - (Page 10)

SOLUTION SOURCE BOOK MAXIMIZING IMAGE SIZE AND POSITION creen 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. 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. Image-to-frame ratio 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 S 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. Image positioning 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 sinA gle guides along the left and right sides of the bed (Figure 2). The guides along the back of the press bed serve as the Mesh Squeegee Frame point where mechanical takeoff devices can grab the sheet and move it to the Substrate next print or finishing station. It’s common practice to position the substrate against the back and left Force guides when working with sheets that are smaller than the maximum sheet size B 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 imFigure 1 age positioned on one side of a screen— For images of the same size, a smaller screen (A) will deflect more and produce but this isn’t often the case for tightgreater image distortion than a larger screen (B). tolerance applications, especially when SCREENPRINTING 10

Table of Contents for the Digital Edition of Screen Printing - December 2012/January 2013

Screen Printing - December 2012/January 2013
Prepress for Special-Effect Garment Printing
Tips for Optimum Screen Exposure
Maximizing Image Size and Position
Understanding Screen-Tension Loss
A Look at Screen Lift-Off
Staging Garment-Printing Jobs
How to Control Dye Migration
Fine Tuning for Fine Details
Application-Specific Considerations for Graphics Printers
Upping the Ante at SGIA 2012
Statement of Ownership
U.S. & Canadian Directory
Opportunity Exchange
Advertising Index

Screen Printing - December 2012/January 2013