Elevated Production Efficiency, Part VII
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Elevated Production Efficiency, Part VII |
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Is he stoned? No, I'm not asking you to deny what you've seen in your shop with your own eyes. What you've experienced, and the conclusion you've drawn are absolutely accurate. What I am telling you, however, is that your observation, correct though it may be, is based on your experience. And, for almost all printers, that experience is with low (7-29 N/cm) to medium (30-49 N/cm) tension. As we've been discovering all along in this series, at high tension—65 to 100 Newtons (for now)—the old assumptions and understandings simply don't apply. Something different is happening, mechanically and hydraulically, to the ink at 85 Newtons that makes possible the previously impossible. To begin understanding that "something," we need to look at ink behavior. Many screen-printing inks are engineered to exhibit varying degrees of thixotropic behavior. That is, when the ink is subjected to the interface pressure between the squeegee and mesh, the ink flows and, when it leaves the screen and is no longer under a force, it stops flowing abruptly. How abruptly the ink stops flowing is determined by its degree of thixotropy. The more thixotropic the ink is, the more abruptly it stops flowing. The time during which the ink will continue to flow more freely is determined by the ink manufacturer depending on the way the ink is engineered to perform. For instance, process inks are designed to flow for a very short time after being sheared by the squeegee. Ideally, they stop flowing or "body up" immediately after they hit the substrate. This allows them to maintain their shape, keeping dot gain or growth of line width to a minimum after the mesh has snapped up from the substrate's surface. On the other extreme, inks intended for large-area coverage are non-thixotropic, and may be "timed" to flow a little or even a lot longer, in order to ensure even distribution and "leveling" of the ink after it is deposited on the surface. But in this case as well, the ink must have enough internal cohesion to body up at some point, or it would continue to flow past the point where it was stopped by the stencil edge, with resulting loss of edge definition or image sharpness. With this ink-behavior information-or, rheology—in mind, let's get back to the mystery, and resolve it by comparing in detail what happens to these inks when printed at low and then at extremely high tensions. As we've established in previous installments, when printing at low tension, the upward resistant force of the mesh is no match for the stronger downward force of the squeegee. In this case, there's too little squeegee interface pressure on the ink, and too much interface pressure where the mesh and stencil meet the substrate. So when the ink tries to make its way around the mesh threads, the ink's flow is mechanically interfered with. The result is that the ink develops very little hydraulic pressure, and hence very little ink velocity at the moment that the ink comes into contact with the substrate.
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At ultra-high tension, the interrelated factors decribed above combine to defeat the mesh-filament interference typical in low-tension situations. |
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Yet at 85 Newtons (with approx. l/64th-inch off-contact), we can get a clean, crisp, solid "J," full, round circles in the breastplate, sharp detail and halftone dots and smooth, even, large-area coverage—even though the ink must still find a way around and under those relatively fat threads in the 115 mesh. Why? Because we have dramatically faster ink velocity, increased squeegee speed, faster screen snap, and, finally, the development of angular ink velocity—changes in the ink transfer process that combine to cause and create the beneficial factors we have been discussing. These elements work together in the following way: The extremely taut 85-Newton mesh has sufficient upward-resistance force (2500 lbs. total screen force) to counteract the squeegee's downward force. The resultant squeegee interface pressure is maximized where we want it, on the ink, and minimized on the substrate, where we don't want it. The squeegee stroke now touches the mesh down ever-so-lightly to the substrate, and, moving on quickly, the mesh comes into contact with the substrate for only milli-seconds at the moment of actual ink transfer. And the squeegee can do so with much greater speed because the mesh now snaps instantly from the substrate. Remember, in this "Wyoming" print, there is no flood stroke but only one very fast squeegee stroke. The high squeegee interface pressure exerts tremendous hydraulic pressure on the ink, beginning to make it flow before the ink comes into contact with the substrate. The ink, then, gains velocity due both to increased stroke speed and, being under much higher squeegee interface pressure, achieves far greater downward and angular velocity as the ink is forced through the open areas between the filaments. When the screen kisses the substrate, the ink is flowing down at the very instant the screen is also gracefully withdrawing upward. Ink, squeegee and screen are all in fluid motion, never at rest, during ink transfer. The ink, now moving with far greater speed than in the low-tension scenario, experiences a complex reaction best described as a "frictional attraction" to the filament surface, and actually follows the curvature of the thread around and underneath each side of the filament, allowing the ink on each side to re-connect. The friction generated between the ink and filaments creates an angular or rotational velocity. (See drawing above) This frictional attraction occurs at low tension as well, but only to a very small degree. The upward force of the mesh keeps the interface pressure between the mesh filaments/stencil and substrate very low. The mesh filament is not mashed into the substrate, nor does it dwell on the substrate. The powerful snap force in the screen instantaneously separates the mesh from the substrate without physically interfering with the ink's newly found angular velocity. In contrast, at low tension, the filaments and stencil are mashed into and dwelling on the substrate surface, thereby cutting off the ink's path underneath the filament and eliminating the possibility for the ink to develop any angular velocity. Simply put, at 85 Newtons, we've dramatically accelerated the ink's movement, while getting the mesh filament the hell out of the way so the ink can flow properly, as it was engineered to do! As we saw last time, many benefits were available even to the one-color manual printer and, in the following case, we can see even more advantages for printers of multi-color work as illustrated by the "Classic Auto" print at the beginning of this chapter. His production yield at low tension using 196 mesh averaged 120 to 168 pieces per hour. At low tension, the printer had three options, each a compromise. Option number-one would have been to print a solid red, then flash to prevent smearing and screen pickup when the black outline and halftone shading were overprinted. The obvious problem here is that the flash slows production and he's likely to have a thicker print with a heavy hand. Option number-two: print the black over the red wet-on-wet at relatively higher speed and live with the degradation as the print becomes increasingly muddy. This printer selected option number-three: printing a bit more slowly, wet-on-wet, he chose to wipe screens every 15 or 20 minutes in order to keep print degradation to a minimum. In addition to the frequent wiping, the printer's four-color rotary carousel had to be turned full circle with each print cycle in order to print the solid red first and trap it with black outline and overprint the red with black halftone dots in the shaded areas. When "Classic Auto" was produced at 70 Newtons, through 1 205 mesh, specially designed, like the 115, to handle ultra-high tension, the printer found that each color required just a single pass and no flood stroke. Further, he was able to drastically reduce ink cost by basing the colors back 65 percent. A rolling stone gathers no moss The result? A yield of 216 to 288 pieces per hour over an eight-hour shift, nearly double that of the low-tension version, producing excellent large-area coverage, sharp detail, crisp edges and superb, soft and. Out of the stone age While the benefits I've described these past seven sessions are being realized in shops everyday, I would be less than candid if I expressed these ideas about high-tension printing without giving you a serious look, as well, at some possibly not-so-pleasant implications for you and your shop if you decide high-Newton printing is for you. The freedom to print dramatically faster and better while consuming less ink comes with the desire to negotiate an equally dramatic learning curve. If it didn't require knowledge, skill, training and hard work, then everyone in the entire industry would already be doing it. However, there's no free lunch. Next time: Newman enumerates numerous challenging but necessary modifications those committed to ultra-high-tension printing must make in their screen-printing theory and practice to ensure success.
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Classic example: The author contends that ultra-high tension nearly doubles production, with no flash, no wipe and no mud. The shaded areas (see close-up) aren't overprinted - the black dots fall on virgin cotton through reversed-out openings in the red: registration only achievable at ultra-high tensions.
ast session, we observed something that seemed to contradict conventional screen-printing wisdom.
End of the ink blockade