A Look at Static and Dynamic Screen Tension

Discover how dynamic tension can affect the quality of printed graphics

Achieving proper and consistent mesh tension is a primary concern when attempting to create a controllable printing environment. Exactly which tensions are considered optimum for any given mesh count, thread diameter, screen size, ink system, or press is often a subject of debate. Virtually all of the recommended tensions come from the manufacturers of the mesh, stretching system, or frames (in the case of retensionables). They all focus on the single, easily defined static, or resting, tension of the mesh. Within these varying recommendations is another unseen and seldom discussed element of the tension equation: dynamic tension. Dynamic tension defines the continuously changing mesh tension at the substrate surface throughout the printing cycle. Static tension is useful only in the sense that it is either consistent or inconsistent within the single frame and from frame to frame in a multicolor job. But it is the dynamic tension that directly affects the deposition of the ink film. Dynamic tension's effect on image quality The goal of producing and maintaining a consistently high static tension is to overcome the variation at the dynamic level and consequently deliver a uniform and controllable ink deposit. Static and dynamic tensions are related. As the static tension increases, the variation in dynamic tension decreases. Put another way, increasing the static tension decreases the negative influence of changes in dynamic tension at the print surface. That's why there is such a preoccupation with high static tensions. Printers know this from direct observation. They often refer to the "sweet spot" of the screen. This is the center of the screen that prints perfectly. The ink deposit is sharp, clean, in register, and generally flawless. As the image moves out from the center, the ink deposit becomes progressively more difficult to control. This is especially the case with halftone images. Dot gain is highest in the areas where the image is closest to the frame edges. The distance from the inside edge of the frame to the image is referred to as the free mesh area of the screen. The smaller the free mesh area, the more difficult the image will be to control toward its outer edges. Since the mesh is fixed to the frame, there is no elastic stretch beyond the point of attachment. The closer the squeegee comes to the edge of the frame, the faster and greater the change in tension. As the tension changes during the print stroke, so does the fluid transfer rate of the ink. The point at which the ink begins to move through the mesh opening depends on the tackiness, viscosity, and rheology (thixotropy) of the ink. In order for the ink to transfer--or pass--through the mesh opening, the appropriate amount of energy is needed to overcome the shear resistance of the ink fluid. This energy comes in the form of stored kinetic energy in the stretched mesh. The higher the mesh tension, the greater the stored kinetic energy. The ink will not transfer until the kinetic energy is greater than the shear force. The point at which the two are equal is called the yield point. Another way of considering this is to think of higher mesh tension as resistance. As squeegee pressure is applied, the mesh either stretches or resists. If the mesh is stretching, the applied squeegee pressure is really transferring energy to the mesh, thereby increasing the stored kinetic energy in the mesh filaments. When the resistance (stored kinetic energy) reaches the yield point of the ink, the ink passes and a print results. To overcome the yield point, printers traditionally increase off-contact and squeegee pressure. This is the universal solution to improving print quality. In many cases it seems to work because the level of image detail or resolution is low enough that excessive application of force is not noticed. All the printer sees is that applying more squeegee pressure and increasing off-contact makes the print better. Hidden within the experience is the exact point where the forces are overcome. Instead of trying to find this point by making small pressure and off-contact adjustments, printers simply crank down until the squeegee blade is doubled over. For simple line work or broad fields or color, finding the yield point is less critical. When screen printers take this approach, what they are really doing is increasing the mesh tension. They are using the squeegee as a stretching device to force the mesh to the print surface. The higher the off-contact, the more force required. Printers may get acceptable results by using this approach, but it is the wrong way to go. The problem lies in the uniformity of the applied force. The precise problem is that the force is not applied evenly. The lower the static tension of the screen, the greater the off-contact distance and squeegee pressure needed to reach the yield point of the ink. More importantly, there's an even greater change in dynamic tension across the screen. As an example, imagine a screen measuring 30 x 40 in. with a static mesh tension of 16 N/cm (static tension A). The free mesh area from the edge of the frame, where it is attached, to the image is 4 in. As the tension values in Table 1 indicate, the 32-in. image is too large for the 40-in. screen. If the off-contact is 0.25 in. in the center of the screen (the center is labeled as 0 in the chart), the actual dynamic tension at the print surface is 25 N/cm. Assuming that you allow a tension tolerance of

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