Elevated Production Efficiency, Part VI

Room for Improvement
     As tension goes up, we're turning the tables on the force relationships within our ink-transfer machine. The higher we go in tension, the more completely we accomplish that reversal. The mesh is increasingly able to counter the crushing power of our "rolling stone" — the squeegee — to control the consistency of interface pressure on the ink (between both squeegee and mesh and floodbar and mesh) and finally, to almost instantaneously release itself from the adhesive grip of the ink while applying only the most minute pressure to the top of the substrate and the ink now residing upon it. Consequently, the ink's influence on peel is increasingly diminished. As screen tension goes higher, the squeegee and  floodbar speeds can be increased, in nearly direct proportion. From the squeegee's point of view, the snap force of the screen works like a series of compression springs, similar to those in a mattress box spring. The higher the tension, the stronger the spring. The stronger spring simply snaps up much faster and with more force, thus permitting the squeegee and floodbar to travel at much, much greater speeds. As our manual print demonstrates, such benefits continue to accrue far beyond 50 Newtons. It's my contention, therefore, that printers currently experimenting with even higher tensions will find that the point of diminishing returns on speed occurs (as I've previously stated) well in excess of 100 Newtons.

      Speaking of diminishing returns, you're no doubt asking how long our manual printer maintained this more-than-doubled production speed: No one can keep that up for eight hours.

      Assuming the "Wyoming" printer was working as fast as conditions allowed at 25N/cm, it does seem unlikely. Yet there's no statistical trickery here. The 20-22 dozen-per-hour figure represents the piece-per-hour range maintained by three printers on three separate machines over three eight-hour shifts.

      But as I mentioned last time, a 100+ percent increase in production can't be explained simply in terms of faster squeegees and floodbars. As it happens, high mesh tension also creates conditions which allow another significant reduction in cycle time, involving the squeegee stroke length.

Shorter trips
     In a low-tension print, the squeegee must pass a good distance beyond the image to allow the mesh to snap cleanly all the way to the end of the screen's image area. If too little of a screen's real estate is allowed between the image edge and the end of the stroke, the mesh will still be laying in the ink, struggling to pull free, as the head lifts. The trailing edge of the print smears, of course, because the ink release isn't complete.  Therefore, screen printers have habitually had to begin and end the squeegee stroke far beyond the actual image, very near the frame edge. But the squeegee should only need to pass beyond the image edge far enough to allow for snap to be completed. Therefore, when we boosted screen force from 25 N/cm or 700 lbs. to 2400 lbs. at 85 Newtons (a 243 percent increase) on the "Wyoming" print, we achieved a screen snap force in opposition to the adhesion of the printed ink that was previously so overwhelming, snap was (again, for all intents and purposes) instantaneous. The screen now snaps up so quickly behind the squeegee as it passes that there is little or no delay to account for. Consequently, as screen tensions become extremely high, with near-instantaneous snap, total stroke length can be reduced to near image length. The shorter stroke requires less time, and the few seconds shaved per print cycle on a multi-color print job can count up dramatically over a typical month, amounting to several days less press time.

The fatigue factor
     Naturally, this benefit is most apparent in automated printing, where squeegee-stroke length can be set precisely and is easily repeatable. But far from being left out, the manual printer can not only shave time by shortening his stroke length, but also greatly reduce stress to arms and shoulders.

      At low tension, the manual printer must begin his stroke near the back of the screen and pull the squeegee to a point near the frame member closest to him (or vice-versa) to ensure mesh snap, as mentioned above. This requires more arm and shoulder extension than would be necessary if the stroke began and ended near the border of the image area. In addition, at low tension, much of the physical effort that goes into the print stroke is expended to achieve sufficient downward pressure to force the mesh down from it's high off-contact position to the substrate surface in order to print. More effort is required to force the screen down to the mesh at the beginning of the stroke and to keep it down at the stroke's end while the screen peels past the image area. When high-tension conditions allow the stroke to begin and end just beyond the image's edge, several inches closer to the center of the screen, the difference in effort required to print is enormous. Remember, our "non-uniform ink-transfer" chart demonstrates that at the highest tensions, off-contact can be drastically reduced to as low as l/32nd or l/64th of an inch. And, though the resistance of the screen at 85 Newtons has increased by nearly 2000 lbs. of total screen force, the maximum squeegee pressure necessary to print within the typical image area actually drops dramatically, in this case approximately 50 percent. As a result, at 85 Newtons, our manual printer applies less downward pressure within the image area, and (with the shorter stroke necessary to accomplish peel at this tension) no longer struggles to overcome the low-tensioned screen's exaggerated resistance to pressure at the beginning and end of the stroke. The result is drastically reduced friction and drag on the squeegee, making it physically easier, less fatiguing to print, even while printing at much faster rates.

Real-world results
     We've observed some ways that high tension's effects on peel makes greater squeegee and flood speed possible, and reduces stroke length. Further, we've demonstrated that ultra-high-tension printing offers these three significant benefits not just to the "big time" printer — of four-color process work or 14 colors on black, armed with sophisticated automated equipment — but to anyone, manual or automated, no matter how simple the print.

      Now, however, we need to turn our attention to something that (I hope) has been nagging at you during this entire session. Doesn't this all strike you as ... well, a bit suspicious? Most printers might expect to experience print degradation at higher speed or, at best, to possibly maintain the same quality. But in both of our examples, image quality shows what can only be described as significant improvement.

      Your suspicions are likely to turn to disbelief when I tell you that, in fact, the quality improved despite that in each case the printers eliminated or reduced remedial measures commonly employed to protect quality. At 50 Newtons, the "Simpsons" printer reduced off-contact to l/32nd inch, re-engineered his art from trap to more-difficult butt registration and further increased machine cycle speed by eliminating flashing. Yet registration was more accurate, smearing was eliminated, better and smoother large-area coverage on the rough (and tough-to-print) canvas substrate was achieved while laying down less ink, and the intervals between screen-wiping sessions grew from 200 to 900 pieces. On the "Wyoming" print, at low tension, a flood stroke and a heavy squeegee stroke were required to force the ink through the small orifices of the 255 mesh. But at 85 Newtons, the print required just one squeegee stroke, and no flood and no wiping for eight hours, yet — here's the part that's really difficult to swallow — "Wyoming" exhibited greater detail at high tension than its low-speed, low-tension counterpart (see comparison above), despite the fact that a mesh more than twice as coarse was used. In addition, both large-area coverage and the finest details were printed from a single screen. To achieve the same quality result without the typical need to stop frequently to wipe screens, the low-tension printer's alternative would be to print the image with two screens — one fine mesh for detailand one coarse mesh for solid coverage— getting a good image, but totally giving up on speed. To top it all off, our "Wyoming" printer reported that his screens needed no wiping during the entire three-shift run. (What kind of yield could you maintain if you hardly ever had to stop and wipe screens?)

      Finer detail with a coarser screen? The achievement of good large-area coverage and fine detail from the same screen, without sacrificing one for the other? How do we account for all that? We're not just eliminating ink abuse. Something uniquely positive is happening to a key component of our ink-transfer machine — the ink itself — that we've not yet addressed.

      Next time: Newman examines the positive effects of ultra-high mesh tension on a final, and crucial, ink-transfer element.

PRINTWARE MAGAZINE

Spotlight On: Don Newman - October 1993
Elevating Production Efficiency, Part I - Tuning Up Tension, November 1993
Elevating Production Efficiency, Part II - Spaghetti with Rubber Bands, December 1993
Elevating Production Efficiency, Part III - The Cheese Grater, January 1994
Elevating Production Efficiency, Part IV - The Double-Cantilevered, Feburary 1994
Elevating Production Efficiency, Part V - Like a Rolling Stone, April, 1994
Elevating Production Efficiency, Part VI - Controlling the Stone, June 1994
Elevating Production Efficiency, Part VII - Out of the Stone Age, August 1994

THE PRESS reports

Newman Squeegees - June, 1998
Stretch Devices, L.O.C. Gauge - July, 1998
Newman Floodbars - August, 1998
Newman Roller Master 7700 Screen Stretcher - November, 1998

Screen Printing

The Roller Frame Story - March 1982