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As the chart above graphically indicates, the same holds true for another double-cantilevered beam, the mesh filament. The left and right edges of the chart represent the area of the mesh closest to its attachment point on the frame, while the center corresponds to the mesh nearest the screen's center—where the diving boards meet. Plotted on the graph is the minimum squeegee pressure necessary to bring a 28"-wide (I.D.) screen into contact with the substrate for each of three actual screens, measured at one-inch intervals: The top curve represents the results for a 7-Newton screen at 3/8" off-contact, the second, a screen at 14 Newtons with 1/4" off-contact and at bottom, a 40-Newton screen with 3/32" off-contact and an 85-Newton screen at 1/32"
The graph confirms what was suggested by the student's touch test and our diving-board illustration: screen printing is inherently a non-uniform-pressure ink-transfer method of printing. In other words, the pressure the ink feels near the edge is greater than the pressure it feels near the middle of the screen, resulting in a curve that rises sharply near the frame edge. Visualized in three-dimensional terms, the high-to-low pressure pattern resembles a series of concentric rings (see diagram, page 76). At 7 to 25 N/cm, as the squeegee passes over the image area, interface pressure and shear rate on the ink all along the length of the squeegee are constantly changing—another well-disguised variable.
Cutting the curve
As we examine the graph further, however, we discover a hopeful trend in the differences between the plotted curves. As the tension gets higher and off-contact gets lower, the curve tends to flatten out, becoming more uniform across a wider area of the screen. Indeed, at 85 N/cm, that uniform area has spread within 2-1/2" of the 24" frame members, keeping the severe rise of the curve outside the bounds of the typical print image area. Though the curve never becomes completely flat, we can see that as tension goes higher and off-contact lower, we approach a point where (as was true, you'll recall, with mesh elongation) for all practical purposes, the ink feels the same interface pressure across the entire width of the squeegee. And again, a complex variable is made into a constant. This now produces uniform registration, ink-film thickness, color, and halftone dot size from the middle to the edge of the print. On textiles, uniform ink penetration and cure will also result.
Higher tensions, then, not only sharpen our cutting tool but also make our ink-transfer machine cut more consistently across our image area. And as our graph indicates, if our goal is to turn variables to constants, the only response we can make to those who ask How high is high enough? is to pose another question: How well would you like to print?
Now that would be a passably good tagline to the end of our high-tension discussion ... if we were at the end. But my mission up to now has only secondarily been to recap high-tension-related quality-improvement concepts. Primarily, I've tried to lay the groundwork for discussion of the key role high tension plays in— as the title has suggested all along—elevated production efficiency. While quality is by no means unimportant to pursue, efficiency has much more to do with speed. No screen printer (at least no one I've ever met) would turn down an opportunity to turn out print jobs faster, as long as quality didn't suffer in the process. So I'm assuming that those same printers would be delighted if someone were to demonstrate that dramatically elevating mesh tension not only allows them to increase printing speed, but that as print speed goes up, quality—far from suffering or even staying the same—actually continues to improve.
I propose to do that from this point forward, answering the "How high is high enough?" contingent by rephrasing my response, asking How FAST would you like to print?
The faster, the better
Since the advent of frame and mesh technologies which facilitate printing at tension levels approaching 100 N/cm, there has been a mounting body of evidence to support my contention that as screen tension goes up, reject rates and press downtime drop dramatically while press speeds increase. The result is enormous impact on that all-important subject that motivated the writing of this series: The number of saleable prints generated per hour, per day, per week and per month—in other words, our yield—quite possibly the most important statistic in a for-profit manufacturing operation. And thus, our high-tension investigation is about to come up to speed
Next time: Newman begins a demonstration of how image quality and production speed go hand-in-hand in four key functions of our ink-transfer machine.
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n our last session, I suggested that we think of our ink-transfer machine, the screen, from an unusual but practical perspective, as a cheese-grater-like cutting tool. From that point of view, we considered the concept of interface pressure as the key to keeping our cutting tool sharp, but along the way, we confronted a serious problem unique to screen printing: as ink is transferred from screen to substrate, it is subject to not one but two points of interface pressure. The first, and desirable, point is where the squeegee meets the mesh. The second and, as we discovered, undesirable point, occurs where the mesh/stencil meets the substrate. High tension emerged as the means to apply a nearly equal and opposing upward force to the downward force of the squeegee, successfully preventing the substrate from becoming a significant aspect in the pressure equation. Thus, to recap, the result was more control and an end to ink abuse: ink could be sheared from the mesh cleanly and deposited onto a garment surface rather than mashed into and through the fabric.
