Mesh Movement and Its Impact on Screen Tension, Part Two

Continuing his exploration of mesh tension loss, Dr. Anderson focuses on optimizing stretching procedures to consistently produce stable, high-tension screens. Learn how to minimize thread deflection and achieve predictable mesh-tension levels, whether you use a pneumatic or mechanical stretching system with rigid frames or rely on retensionable frames for both stretching and printing.

Continuing his exploration of mesh tension loss, Dr. Anderson focuses on optimizing stretching procedures to consistently produce stable, high-tension screens. Learn how to minimize thread deflection and achieve predictable mesh-tension levels, whether you use a pneumatic or mechanical stretching system with rigid frames or rely on retensionable frames for both stretching and printing.

Part of this series revealed new findings about the way in which mesh threads interact during and after screen stretching. The article introduced the phenomena of thread deflection and realignment and explained how these conditions lead to unpredictable drops in mesh tension that continue long after initial stretching. Now, the discussion moves from the theoretical to the practical by reviewing common tensioning tools and techniques and assessing their effectiveness in overcoming thread deflection. On the following pages, you’ll discover how to adjust your own tensioning methods to get stable, high-tension screens time after time.

What Is the Optimum Mesh-Stretching Process?
There is no single component you can add or action you can take to completely eliminate mesh deflection (Figure 1) from the screen-stretching process. However, you can cost effectively produce screens with stable, consistent, and repeatable tension levels by modifying your stretching procedures with various functions and options.

[Figure 1: Mesh Deflection
When tensioning screen mesh, friction between intersecting mesh threads causes bowing of the threads, particularly those located toward the center of each screen edge. When other forces are introduced to the mesh, these friction bonds may be broken, allowed the deflected threads to straighten in a process called fiber realignment. This realignment of threads leads to tension loss.]

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An optimum mesh-stretching process would apply even tension through controlled extension of each and every thread that makes up a screen. It would produce no sideways deflection of threads and maintain the mesh count of the screen fabric. And it would produce stable, high-tension screens very quickly and inexpensively, with minimal environmental risk and no image-distortion or registration issues. Unfortunately, this is not the perfect world in which such a stretching system could exist. So you must make compromises in your existing mesh-stretching methods, adding new steps to the procedure and, if your situation warrants it, upgrading to more effective stretching technologies. Before you can enhance your stretching methods, however, you should understand the inherent strengths and weaknesses of all the techniques commonly used today.

The Pros and Cons of Modern Stretching Methods
Screen printers take a variety of approaches to mesh tensioning. Some methods are driven by fast-turnaround demands, others by application requirements. This discussion considers the most used methods for producing screens on both rigid and retensionable frames and highlights the benefits and drawbacks of each. It also suggests how you can combine the best attributes of several stretching techniques to minimize the possibility of tension loss on your printing screens.

Rigid-Frame Systems
Rigid-frame screens (also known as fixed-frame screens) are the most common worldwide. Producing these screens involves stretching the mesh, then attaching it to a rigid frame. With this screen type, any tension loss the mesh experiences after it is attached to the frame is permanent for the life of the screen.

Types of stretching devices Tensioning mesh for rigid frames requires a tensioning system that is independent of the frame. Your choice include mechanical or pneumatic tensioning units that rely on some form of clamp system to grip each side of the mesh. On mechanical systems, mesh tension is typically increased manually with cranks or levers that stretch the screen along two axes. Pneumatic systems also tension mesh along two axes simultaneously. But these systems use pneumatic pistons, rather than operator force, to stretch the screen. The pulling action of the pneumatic clamps may be triggered and controlled by a human operator or completely automated with the use of a computer-based control system.

Mechanical systems tend to be simpler, with fewer moving parts, than pneumatic systems. As a result, they cost less than pneumatic stretchers, but are also less controllable and more dependent on operator skills. Pneumatic systems rely on very accurate pressure controls that allow for consistent and repeatable tension levels. But these features lead to substantially higher price tags for pneumatic units. If your budget allows it, the pneumatic systems are the preferable option.

Number of clamps How tension forces are applied is not the only aspect of stretching equipment to consider. You also have to think about the number of clamps the system uses to attach to the edges of the mesh. The ideal stretching system would pull each mesh thread individually to ensure correct tension. Obviously, physical limitations make this impractical, if not impossible, to achieve with a stretching device. So how many clamps should be used?

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A single clamp securely holds the ends of all mesh threads along one edge of the screen and pulls them evenly. This approach would be perfect if screen tensioning occurred only in one direction. But as Part 1 of this series pointed out, stretching is a two-dimensional process, and the forces and mesh alignment that result in the corners of the mesh will be different than those at the middle of each edge. Because a single-clamp system locks onto an entire edge of the screen, you have no way to compensate for the uneven tensioning forces on the screen. Consequently, single-clamps stretchers maximized fiber distortion.

Multiple-clamp tensioning systems (Figure 2), on the other hand, allow you to vary the pulling forces at different locations along each mesh edge, providing more balanced tension across the screen and limiting thread deflection. The result is slightly lower screen distortion than you would experience with a single-clamp system. The downside to multiple-clamp systems is that small areas of unclamped thread ends may be trapped between adjacent clamps, creating narrow bands of mesh with improper tension levels. To minimize or eliminate this problem, adjacent clamp areas must overlap in some way. Overall, the advantage of multiple clamps over single clamps is small, unless you add another capability to the clamps – lateral movement.

[Figure 2: Multiclamp Tensioning
Multiclamp tensioning systems allow you to vary pulling forces at each clamp to provide more balanced tension over the entire mesh. Some models feature clamps that are free to move laterally, which allows them to compensate for thread deflection and reduces the potential for tension loss after the stretching process.]

Lateral clamp movement The vast majority of stretching systems allow little or no sideways motion of the individual clamps during the tensioning process. But during the last decade, a growing number of pneumatic-clamp units have been introduced that mount the clamps on rails with bearings to facilitate smooth, easy lateral movement during stretching. The later movement of the clamps helps reduce the likelihood of tension loss due to thread deflection because the clamps move the thread ends away from the center of the mesh edge, especially near the corners. The net effect is reduced thread bowing, which means that less fiber realignment will occur and less tension will be lost after stretching.

On the downside, moving the clamps, sideways can introduce several new problems. First, moving the clamps laterally has the effect of reducing the mesh count by spreading the mesh threads over a greater distance. If the threads ended up evenly distributed, this would not be an issue. However, lateral movement creates uneven thread distributions. The main concern is that mesh secured by the clamps is held at a fixed thread count, but the mesh between the clamps is not fixed, so, as the clamps move apart, the unfixed mesh experiences a drop in tread count. Because the bowing of the fibers is greatest at the edges of the mesh, this is where you find the most lateral movement of the clamps and variance in thread counts.

However, the mesh at the edges of the frame is not normally used to carry the printing image – the image areas of the stencil are usually centered on the screen. So any thread count changes that come from stretching with laterally moving clamps tend to have minimal effects on the printed image. The only cases in which this condition might impact the image is when large, continuous-tone elements are placed near the screen edges. In these situations, you’ll see narrow bands that represent changes in ink coverage caused by the larger mesh openings or possibly by loss of parts of the stencil.

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To assist in further diminishing this effect, you can use a greater number of smaller clamps, which would allow better distribution of tension forces through many smaller lateral motions, instead of just a few large ones. More clamps mean that each clamp will be lighter, move sideways more easily, distribute the sideways movement more easily, and cause less variations in the mesh counts between the clamped and unclamped mesh threads. Smaller clamps also allow greater flexibility and interchangeability for different frame sizes. So, although single-clamps systems fix the mesh count across the width of the frame, they also produce greater tension-loss properties due to fiber realignment. Using multiple smaller clamps that allow lateral motion provides a superior solution.

Single-stage versus multistage stretching The greater the tension applied to mesh in a single stretching step, the greater the thread deflection and the higher the tension loss the screen will experience due to fiber realignment. Stretching screens to the desired tension level in one action is possible, but produces highly unstable and inconsistent mesh tensions.

As tension forces on mesh threads increase these forces trigger fiber realignment. The realignment of threads continues until the friction forces at thread intersections are greater than the tension forces acting upon the threads. Any time you introduce higher tension forces, this rapid thread realignment and tension loss recurs. However, when tension increases are applied incrementally, with brief rest periods between stretching stages, more of the thread realignment and tension loss occurs during the stretching process than after the process is complete. In other words, you’ll minimize the overall degree of thread deflection by using a greater number of small stretching and rest cycles as opposed to a single stretching stage followed by a long rest period.

In multistage stretching, the time for each cycle should be short so that the overall stretching time remains the same as a standard single-stage tensioning cycle (Figure 3). Even with this approach, you can expect additional tension loss after the final stretching cycle. But the loss will be much smaller and easier to predict and control than if you had stretched the screen all at once.

[Figure 3: Multistage Tensioning
This graph depicts tension level over time when a multistage stretching procedure is employed. Note that total stretching time with the multistage process is identical to the time required for stretching in a single step. However, the multistage process results in a higher and more stable tension level. The shaded area at the top of the multistage graph represents the small degree of variability in final tension that results from using different mesh counts, frame types, etc.]

A rapid-tensioning study by the Screen Printing Technical Foundation, Fairfax, Virginia, revealed the advantages of multistage stretching with rest time between stages. The addition of the “fiber realignment” concept is further testament to the benefits of short rest periods between small, rapid tensioning cycles.

Vibration during stretching As a mesh is stretched, the motion between threads reduces friction forces at thread intersections. This allows some fiber realignment to occur automatically during the tensioning process. To break these friction bonds more rapidly, it could be useful to add vibration to the mesh, either applying it through the tensioning-system clamps or some other method. However, because mesh stretching is a dynamic process, adding vibration with current stretching techniques may have limited benefits while creating a more complex stretching process.

Vibration between stretching stages Friction between threads is the dominant force that resists fiber realignment during the rest periods in multistage stretching. However, applying vibration during these periods may assist in increasing the local tension forces and breaking the contact between the mesh threads. Once this contact is broken, the fibers will be free to move, encouraging and accelerating tension loss as thread positions stabilize.

As simple as this concept sounds, putting it into practice remains difficult. One problem is that fiber realignment is greatest at the edges of the mesh. This means that the forces acting upon the mesh are not equal at all locations. Consequently, no single vibration frequency can be used to break all the friction bonds that keep the threads deflected. Additionally, screen mesh is excellent at absorbing and dissipating vibration energy, so using vibration forces alone to break these bonds is unlikely to give you the desired results.

I would recommend an alternative procedure: During the rest period between tensioning stages, apply a continuous vibration/deflection cycle at multiple points around the mesh, staying with 5 to 15 percent (relative to mesh width) of the screen’s edges. The vibration and deflection should be random in amplitude, frequency, and direction, and be applied for 90 percent of the rest time between each and every stretching cycle. This would provide enough variance in additional applied forces to trigger thread realignment regardless of the location of deflected threads. The rest periods that result with the procedure would be shorter than with conventional multistage tensioning practices and lead to more consistent and stable tension levels.

Rigid-frame summary The ideal stretching system to employ for screens affixed to rigid frames can be summed up as follows:
• The system would rely on small, pneumatic stretching clamps;
• It would allow sideways (lateral) motion of the clamps during tensioning;
• It would stretch the mesh through a series of short tensioning cycles with brief rest periods between each stretching stage;
• It would apply random deflection and vibration forces at multiple points around the mesh during rest periods;
• And it would target a narrow range of stable tension values for the final mesh.

Automating as much as of this process as possible by using computer-controlled stretching equipment could further increase the consistency and repeatability of your stretching results. But if this is the route you plan to take, make sure that automation doesn’t become a substitute for skilled screen-stretching personnel.

With the multistage stretching procedure recommended here, screenroom employees must have a great understanding of the stretching process in order to measure the progress and effectiveness of the system. These employees should be able to carry out all of the stages of the process manually and recognized and understand problems before taking steps to solve them. With such skilled operators, production of high-quality screens can continue, even if equipment breaks down. When tasks such as screen stretching are deskilled through automation, you’ll eventually see the results in reduced print quality and, possibly, lost customers.

Retensionable Frames
Many of the tension issues that arise with screens on rigid frames also occur with retensionable frames (Figure 4). This article doesn’t distinguish between the various types of retensionable frames available today, but focuses on the thread deflection and realignment concerns that apply to all frames in this category.

[Figure 4: Retensionable Frames
Retensionable frames hold each edge of the screen mesh secure during stretching, which prevents a screen’s thread count from shifting but creates unequal distribution of forces and maximizes thread deflection. Fiber realignment can be accelerated on retensionable frames by running a hard-rubber roller over the mesh immediately after stretching.]

On retensionable frames, loose mesh is fixed to the frames sides, which are then rotated or expanded to tension the mesh. The effect is identical to a stretching system for rigid frames that uses only one clamp per side. This approach maintains the mesh count at a fixed value because all the threads are locked into the frame sides. But it also results in the maximum mesh distortion and the greatest potential for tension loss due to fiber realignment. The primary advantage of retensionable frames is that they can reapply tension to the mesh if the tension level drops. This capability also makes them heavier and more expensive than rigid frames.

Mesh tensioning on retensionable frames is normally conducted as a multistage process, and mesh stabilization through fiber realignment is assisted with successive stretching and rest cycles. Once the mesh is stretched to the target tension level, it is ready for us. However, action from printing, cleaning, and reclaiming triggers further fiber realignment, which causes additional tension loss. So the mesh is retensioned on the frame prior to its next use.

As the cycle of mesh use, reclaiming, and retensioning continues, the tension loss experienced after each reclaiming stage is reduced, and the mesh requires less retensioning to reach the desired tension level. Eventually, the mesh attains a stable state when no more tension loss occurs due to fiber realignment. At this point, the mesh is completely broken in and ideal for use on jobs that require a high degree of accuracy and repeatability.

This scenario sounds perfect, but the use of retensionable frames creates other difficulties. One problem is that until mesh deflection has been completely eliminated through multiple usage cycles, the screen’s tension is unstable and can vary unpredictably. Furthermore, relying on multiple printing, cleaning, and retensioning cycles to provide complete fiber realignment increases the risk of mesh damage and is slow and takes expertise. Nevertheless, retensionable frames allow you to get more use out of screens than rigid systems, which can lead to cost savings in mesh.

The good news is that you can accelerate fiber realignment on retensionable frames. If your goal is to produce stable screens prior to using them for printing, the best solution is to mount the screens on press and then run a hard-rubber roller (sometimes called a “roller squeegee”) over the mesh. This would also eliminate the time and labor of coating, exposing, and reclaiming the screen multiple times. The roller tool is recommended because it can apply deflection forces and dynamic vibrations that induce fiber realignment without creating friction that might damage the mesh, as a standard squeegee would.

Retensionable-frame summary The ideal stetching system to employ for screens affixed to retensionable frames can be summed up as follows:
• The retensionable frame is basically a single-clamp system for each side of the screen;
• It maintains a stable mesh count but maximizes mesh distortion;
• Use in printing, followed by cleaning or reclaiming, causes fiber realignment and tension loss;
• Retensioning the mesh through multiple stages produces stable and consistent tension properties;
• This frame type allows in-house control of mesh stretching, while ensuring consistent final tension values for complete sets of frames;
• And, mesh stabilization can be accelerated by using a low-friction roller to deflect and vibrate the screen so that fibers realign more rapidly.

No More Stress from Tensioned Mesh
Rigid and retensionable frames both bring advantages and disadvantages to the tensioning process, and neither type merits a better rating than the other in terms of tension stability. In the end, your frame-type selection should be based on the economic production merits it has based on your applications and the quality levels you demand.

Regardless of the frame type you sue, understanding the concepts of thread deflection and fiber realignment is critical for producing high-quality screens. By applying the suggestions presented here to your own stretching procedures, you’ll create stable printing screens with predictable tension levels, reduce waste, and maximize profits.

Author’s note: The original research for this article was carried out at the Welsh Centre for the Printing & Coating, University of Wales, Swansea, UK. This was part of an industry and government funded research project between 1993 and 1996. The research was supervised by Dr. Tim Claypole and Prof. David Gethin, with support from Dr. Eifion Jewell. The research has continued, with the development of a greater understanding of the screen-printing process, diagnostic tools, and high-speed screen printing.