In order to learn how tension improvement best works in favor of the screen printing process, a comparison was made between the two extremes—low and high tension. Fig. 1 illustrates precisely what happens when printing with a low tension screen, as opposed to one of a higher level shown in Fig. 2. For the purpose of clarity, actual tension in these comparisons is not relevant since the important factor is to highlight graphically the differences between the effects of low tension to that of an elevated one.

As shown in Fig. 1, the results of using a lower tension inevitably mean that a higher off-contact distance becomes necessary in order to compensate for the deficiency found in the less-than-ideal mechanical aptitude of the weaker fabric. The amount of off-contact used should be determined solely by screen tension alone. Screening is an “off-contact” printing process, although not necessarily with textiles. The screen needs to be kept just slightly separated from the substrate except at the point of squeegee contact in order to obtain crisp sharp reproduction, clean image (appearance), and uniform coating (deposit integrity). The only other possible factor that requires adjustment to off-contact is if screen separation is poor and the printing press is not fitted with a “peel-off” device (a subject matter for later inclusion). Higher tension automatically improves screen “snap” regardless of the functional use of peel-off.
 

Most screen printing technicians concur that high off-contact leads to gross misuse of squeegee pressure and always will remain a press operator’s nemesis. Excessive off-contact, due to weakly tensioned screens, promotes image distortion, unsightly streaks, registration problems, non-uniformity of coating deposits, and squashes the integrity of tonal dots (see Fig. 3). This latter residual effect is largely due to the brute force that the squeegee blade applies in an effort to complete the ink transfer process. In such a scenario, the greatest mechanical downward pressure point of the squeegee is exerted directly onto the substrate’s surface, a condition that is far from favorable. This is precisely why tonal dots somehow appear “crushed” and consequently, the finished print could take on a bland, linear flat appearance, as well as tone gain or loss. What actually happens is that the squeegee blade effectively is left to take on virtually 100 percent of the manipulative workload. Thus, the squeegee overwhelmingly becomes an uncontrollable but dominating partner in the transferring process. The weaker screen will create more fabric-roll (a phenomenon that lifts the fabric in front of the squeegee tip unseen by the naked eye), which causes a messy appearance with non-opaque colors and banding with graduating tone dot (sometimes called blends or vignettes). For greater control and superior results of the entire process, the workload of the ink transferring burden must be shifted through to the screen itself. The screen (fabric and tension) should always be the influential partner here than with the less controllable components such as the squeegee blade.

Taking the same screen but with higher tension and consequently lower off-contact (Fig. 2), the mechanics of the process will tend to transfer much of the built-up pressure away from the substrate’s surface. It will then redistribute it along the fibers of the screen fabric. Here, the squeegee needs only enough force to deflect very slightly the stronger stretched fabric before it just encounters the substrate.

The screen mesh takes most of the pressure from the squeegee to complete ink transfer before the blade actually makes contact with the print surface. Removing much of this direct negative force automatically allows the squeegee blade to function at the lightest optimal pressure possible. Technically, the mechanical activity of ink being transferred from the screen is actually by means of a hydraulic action. This is precisely what happens when the ideal conditions are set correctly and properly engineered into the process. In fact, the squeegee does not need to make physical contact with the surface. In 1995, the Virginia-based SPTF (Screen Printing Technical Foundation) irrefutably proved after exhaustive tests that a half-pound contact pressure (between blade and substrate) per linear inch of squeegee (200 g/cm) is all that is required to successfully provide a distortion-free printed image and ink deposit uniformity. Any more is wasteful, unnecessary, and can be detrimental to many high-end applications. Do not be confused between the distinction of physical contact pressure and air pressure applied to the squeegee itself. The pressure gauge of a press shows air pressure released from the airline’s system to a device—not actual contact pressure on the substrate. They are entirely different.

The endeavor of screen printing is to bring together many functions into one entity. Using the least amount of squeegee pressure and off-contact, through higher screen tension, is a better way to achieve superior quality, greater productivity, and a healthier bottom-line.

There are a few applications that require weaker screen tension than described above, particularly when conforming to soft, difficult, or irregular surfaces. Higher tension alone probably can benefit 97 percent of the screen-printable market. This can mean the difference between ordinary and superior printing results at a lower cost.

Scott R. Sabreen is the founder and president of The Sabreen Group, Inc. (TSG). TSG is a global engineering company specializing in secondary plastics manufacturing processes – surface pretreatments, bonding, decorating and finishing, laser marking, and product security. For more information call toll-free (888) SABREEN or visit www.sabreen.com and www.plasticslasermarking.com