Screen Printing - December 2012/January 2013 - (Page 20)
SOLUTION SOURCE BOOK FINE TUNING FOR FINE DETAILS
asic alignment of major press components is a consideration that’s often overlooked. That’s unfortunate, because it can have a major impact on your ability print graphics with halftones, fine lines, and small text. Use these pointers to start making the adjustments needed to fine tune your graphics press for highquality, fine-detail work. Press alignment Press-alignment problems cause squeegee pressure to vary along the print stroke. To gauge the severity of your alignment problem, just consider how much adjustment is normally required to go from not printing to a full print. The greater the amount of necessary adjustment, the more severe the problem, and the greater the benefits of correct it. The issue stems from the press table—the bed or platen—and the mechanism that carries the squeegee assembly being out of parallel in the direct of squeegee travel. In the example shown in Figure 1, the space between the squeegee carriage rails and the press bed is greater at the front than at the back. In this case, the image will begin printing at the rear of the press where the space is the least. As pressure increases, the printed area increases toward the front until the entire image appears. Let’s accept that in the ideal printing situation, you use the minimum amount of squeegee pressure that will produce a print. Consider the setup adjustment for this situation. That ideal, minimum pressure is now occurring at the beginning of the stroke. The pressure increases as the stroke progresses and, to some degree, the squeegee is effectively overpressured at the end of the stroke (Figure 2). Excessive pressure interferes with fine-detail imaging by inducing smear underneath the stencil, causing fine reverse text
or high-percentage dot areas to fill in and blur. The variance in pressure can yield a smear across the print, making it impossible to get equal control of print quality for the entire image. Why is this problem so common? One is mechanical; the other is perceptual, on the part of the printer. Mechanically speaking, with time and use, machinery moves and settles. It’s sometimes repaired for various reasons, and alignment is not checked after the repairs are made. Sometimes, wear simply takes its toll on the equipment. Perceptually, many printers are stuck on the notion that it’s more important to have the screen parallel to the press table than to have the squeegeecarriage mechanism parallel to the table. Of the three—the table, screen, or squeegee—the screen is the least critical for absolute alignment. In Figure 3, the carriage rails and table are parallel, but the screen is not. The parallelism of the rails and table will maintain even pressure, and the elastic nature of the screen will allow the stencil to compensate for its alignment—within reason. It would be ideal to have all three parallel to have predictability within the screen-printing operation. On most presses, the screen is constantly the easiest of the three elements to adjust, and the most difficult to keep adjusted. It is usually designed to be adjusted a lot to change off-contact distances, but it’s the most difficult to keep adjusted because people often tweak the off-contact in ways that aren’t calibrated to very exact standards. In addition, you must also consider peel adjustments, which actively and deliberately move the screen out of parallel to the table and squeegee as part of their function. Consult “A Look at Screen Lift-Off” on p. 14 for information about the effect of peel adjustments.
distance at front of stroke
distance at back of stroke carriage rails table
FIGURE 1 In this example, the distance from the squeegeecarriage rails to the table at the front of the print stroke is greater than the distance at the back of the print stroke. Therefore, the back of the image prints first during setup, but the front of the image does not.
FIGURE 2 Continuing the example shown in Figure 1, after initial press setup, the ideal minimum squeegee pressure occurs at the back of the stroke, but the front of the image is not printing at all. As a result, you increase squeegee pressure to make the front of the image print correctly. Unfortunately, as the squeegee travels, it now becomes overpressured at the back of the stroke. Such overpressure can result in smearing and a loss of fine image detail at the back of the print stroke.
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
Screen Printing - December 2012/January 2013