by Scott R. Sabreen and Mike Young, The Sabreen Group, Inc.
One of the most prevalent problems within the industrial screen printing industry is the habitual use of “under tension” screens. This may be rather difficult to appreciate at first, since, unlike other graphic-art reproduction processes, it is possible to produce a fairly good looking result using screen process as the creative medium, even with vital parameters improperly set. Why is that so? One of the reasons screen printing is such a unique process is that it is flexible enough to accommodate a variety of application requirements with seemingly unlimited versatility. The very reason screening is extensively used for industrial applications is that it can be adapted with relative ease, in almost any imaginable configuration, to conform to specific needs. It is a forgiving imaging and selective coating process, i.e., wide tolerance parameters. However, its versatile nature also can create a critical problem. While the process is easy to apply, it is equally easy to render it poorly.
The tension levels originally adopted for a specific fabric type in any printing operation are usually known internally as the house standard. Unfortunately over the course of time, those levels tend to become etched in stone and, in many instances, no longer serve a useful purpose for todays critical printing demands. Contrary to belief, the ultimate screen tension level does not quantifiably exist (at least by definition), nor could any one be characterized as being the most desirable for a given application.
The demands placed on screen printing operations are continuously being increased in an attempt to reach higher levels of quality and performance. The caliber of work printed today is more complex than ever before, with demands for six sigma manufacturing. This means a requirement to handle larger multiple images per sheet, rendering truer likeness to all computer-generated originals or phototools, and a consistent perfect color match. While these higher standards for excellence become crucial as a basic requirement, reducing production cost also remains critical.
The quality of printing is steadily improving due to the continuous development and greater use of superior consumable materials, such as inks, meshes, photographic emulsions, and substrates, etc. The use of higher screen tension, perhaps, has the greatest single impact on overall print quality while contributing greatly to production performance. Using higher tension prolongs screen integrity, causes less wear and tear on the squeegee blade and stencil, yields more exacting image reproduction, promotes superior control of coating uniformity, and in many instances, permits faster production speed.
Gaining the maximum from high-tensioned screens requires obtaining and using higher tension levels than those currently employed in existing printing operations. For example, using 20 N/cm² (Newtons per square centimeter, as measured by a tension meter) instead of making do with a weaker 16 N/cm² or moving up to 25 N/ cm² rather than 22 N/ cm². The purpose is not to achieve super tension (anything above 50 Newtons) rather, the goal is to reach a higher plateau or level of performance, to better satisfy the ever increasing demands of todays marketplace.
In order to learn how tension improvement best works in favor of the screen printing process, a comparison was made between the two extremeslow 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 operators 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 substrates 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 substrates 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 airlines system to a devicenot 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