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Historically, of course, screens have been anything but rigid. As a result, print speeds have been generally slow and inconsistent. For example, what happens when we attempt to print, say, a few lines of black type on a white shirt at low tension — less than 25 Newtons, or under 650 lbs. total screen force for a typical 18" X 20" I.D. printing frame? Not much, of note. The screen doesn't have much difficulty popping back up off the substrate as the squeegee passes.
But what if we want a few lines of white type on a black background ... on that same white shirt (a large reverse print); or a large area of opaque white ink on a black shirt? Suddenly, we've got a huge area of tacky ink to lay down, and you know what happens. Because the screen force is weak, the ink acts as an adhesive, substantially delaying snap speed or even — if tension is low enough — preventing peel entirely.
Complicating things (as we learned in our discussion of interface pressure), the squeegee, like a sort of large rolling stone, is smashing and abusing everything — from ink to stencil to substrate — in its path.
Shooting the moon
In low-tension situations, particularly as screens get larger, the squeegee may easily run ahead of the mesh peel, mashing and smearing the mesh into the ink and creating a halfmoon-shaped release line that follows the squeegee at a distance. And it's no coincidence that this half-moon pattern resembles the curve on our "non-uniform- ink-transfer" chart (see above).
Meanwhile, of course, the print head may not be lifted until the halfmoon completes its slow course - unless the printer is willing to live with an even more severely smeared and blurred print. For the manual printer, this means either adjusting each squeegee stroke speed to be synchronous with the peel rate of each individual screen or, for sake of printing rhythm, going with the slowest stroke for all the screens. For the automated printer, there is no choice: the machine cannot cycle until the slowest print head lifts. As a result, printing one large area of color - a large white underbase on a black shirt, a large, solid, opaque color, or a tacky fluorescent, puff or metallic - can have a drastic impact on productivity. The entire machine's cycle time can be substantially slowed down, typically 20-50 percent, sometimes more.
To avoid such slow-downs, printers often significantly increase the off-contact distance as a means of assisting the mesh in its efforts to pull itself out of the muck left in the rolling stone's path. But this only partially improves snap, and does so mostly on the edges with little improvement near the center of the screen. It also severely erodes quality by further enlarging and distorting the image and (as illustrated in our diving-board discussion, Printwear, Feb. '94) introduces more inconsistency into the interface-pressure equation along the length of the squeegee, causing non-uniform ink deposit, substrate penetration and more rejects. Either way, the number of saleable prints - our all-important yield - suffers. Additionally, raising the off-contact distance fatigues the manual printer and prematurely wears out the stencil, squeegee and mesh due to massive, excessive squeegee pressure.
Small wonder then that most screen printers have traditionally assumed print speed to be dependent on the ink's adhesive properties and percentage of substrate coverage. At this industry's typically low and medium screen tensions, the assumption is warranted, and visually summed up by the "Simpsons" prints.
Our "before" sample was printed at 7 Newtons (or 250 lbs. of total screen force on a 25" X 36" frame) and required one flash. At a rate of 350 pieces per hour, the job was plagued by mis-registration and poor opacity, plus noticeable smearing and bleeding. Screens had to be wiped after every 200 shirts. Dreadful, right?
These squeegee-stroke problems actually begin with the flood stroke. At lower tensions, the same inconsistency we observed last month in our discussion of squeegee and substrate interface pressure also comes into play during the flood stroke. As the floodbar moves across the mesh surface, it displaces the ink. The ink has to move somewhere.
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Rock 'n' roll: What durometer did you say that squeegee was? Seriously, though, compensating for low-tension with increased off-contact and squeegee pressure can be like printing with a stone.
