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Screen Printing Application Technique - The Value of Using High(er) Screen Tension
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 today’s 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 today’s marketplace.
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
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