Mike Young
President,
Liberty International
Technology Inc., USA.
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We will examine the repercussions of off-centering the image and
the proverbial length of the squeegee/ flood coater, as they play a direct, dramatic and intricate role in the quest for superior print performance. Last but not least, we’ll appraise the benefits of squeegee “snowplow” to counter annoying every-day problems.
To Center or Not to Center the Image
You’ll recall that Part I discussed image- toframe size relationship in depth, and several ratio samplings were given based on job requirements, tolerances and the degree of difficulty in meeting them (according to inhouse processing ability and skill levels employed). Each sampling was based on the image being positioned dead center (in ‘X’ and ‘Y’ direction) on the screen.
This effectively means a healthy image ratio could still potentially pose a problem if it were offset to one side, because part of that image will now be inside the danger zone - the area where the amount of distortion would no longer be acceptable. |
Therefore, when part of an image is positioned beyond the acceptable area, as determined by the degree of print integrity sought, potential problems are still likely even if the overall image size was healthy to begin with in terms of its ratio. In other words, it is not simply a case of image ratio to that of the frame but also how close to the side of the frame one can safely print. Image-to-frame ratio is just one guide; how close the image is to the frame is something entirely different.
No person can provide precise figures here, because there are too many other factors that enter the equation. In any event, actual in-house ratios ought to be pre-determined by each shop according to jobs requiring crucial or special attention. As such, ratios developed internally may well be different from those given as a guide in this series and will certainly be more practical because they would encompass one’s own operational skill levels and processing abilities.
For ease of developing your own suitable ratio guides, consider printing fine graph lines
on several sheets of clear film in both ‘X’ and ‘Y’ directions. Space the lines, say, 6 millimeters (.24”) apart over the whole image area using maximum &ame size, standard prepress/ screen making, setup and printing procedures. Have on hand several different
squeegee lengths (with good, sharp edges),corresponding to the largest image size of con- cern to the smallest. Use maximum length floodcoater for the test if checking registration, but correct length-to- squeegee if checking out deposit uniformity.
For those of you who think this could easily be avoided by using a full-length squeegee blade with an offset image - think again ! Distortion will still be problematic, albeit somewhat less, because fabric clearance between the frame and image on one side (the right side in Figure 1b) is still considerably greater than that of the opposing side. Consequently, the former would “give” more overall (stretch further), thus making the image shift to the shorter (left) side. As mentioned, many operational styles of presses
call for offset image positioning in order to facilitate automated take-off systems; nevertheless, there is nothing wrong with centering the image for crucial jobs because quality invariably becomes more important than productivity in such cases.
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Figure 1 : Affects a, b, c, e, f, g, h, k and n.
Keyword list is found on Page 52 As noted earlier in this series, letters from the keyword list accompany each illustration,
denoting what problems are most likely to be created by the scenario portrayed. And
again, as before, some illustrations are slightly exaggerated to more clearly illustrate the points they make. |
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Figure 2 : Affects a, b, c, f, g, h, j and k. |
Deposit Uniformity & Squeegee Length
Keeping the image-to-screen ratio in mind as a guide, the actual ratio defined in-house depends entirely on how the operation’s idiosyncrasies (e.g., skill level, processing techniques, procedures) perform in regards to the requirements of frequently demandingjob specifications. That said, we need to recognise that screen printing effectively produces a three-dimensional entity rather than a twodimensional image. That is, an image that has been screened has length, width and - most importantly for many operations - depth (i.e., deposit thickness). Uniform ink coating and deposit thickness, by control in one form or another, are often prerequisites to obtaining good, solid coverage or opacity, as well as conductivity, protection barrier or a precise layer for many high-end/industrial requirements.
Consider this:
More and more commonly, today’s high- end applications require registration to be within stated tolerance of their 2-dimensional aspects. That’s just one objective. When a controlled, uniform deposit (i.e., a 3-dimensional aspect of the same print) becomes essential, it’s entirely another. Job emphases may be completely different - but with creative techniques and processing skills, reaching a compromise between the two objectives is more common.
Because deposit thickness, or coating uniformity, is the 3-dimensional aspect of a printed image viewed 2-dimensionally, it is not unreasonable to assume that if it were printed without distortion, the ink deposit would be consistent and fairly uniform throughout. Although overly simplified here, the answer is pretty much yes - but the degree of criticality required has to be defined. Where critical and precise deposit thickness or uniformity is crucial for a job (even if registration isn’t), the relationship of image-to-screen ratios predetermined in-house becomes central to reaching the desired quality tolerance in performance.
Assuming the “image-to-screen” ratio to print the maximum size (Figure 2, shown in blue) is acceptable, it is reasonable to expect that using the correct squeegee length (Figure 2, shown in blue) will produce a uniform coating. Even with the same pressure, angle, speed, etc., using a much longer squeegee blade than necessary (shown in red), will result in an image distorted three-dimensionally and two-dimensionally. The reason for distortion is that the longer squeegee influences the fabric in areas immediately outside the image (Figure 2, shown in red), pulling the image out from its original place. This essentially is telling us that even if the image is healthy ratio-wise and centered in the screen, quality can be a challenge if the squeegee used is too long. Consider for one
moment that in many operations, press operators are the ones who have final say in selecting the squeegee (read “length”) used to complete a job. Image-toimage registration will become a big issue if previous colors/prints were printed with an array of different length squeegees, regardless of techniques otherwise used.
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Figure 5 : Affects b, e, g, h, j,
k, l, p, s, t and u. |
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Figure 6 : Affects b, f, g, h, j, k, p, s, and u. |
Off-Contact & Squeegee Length Jointly Considered
For many years, I have advocated to those seeking exceptional quality that off-contact and squeegee length are about the most problematic “on-press” variables during the final stages of press make-ready. This is perhaps to do with the lack of consideration or understanding by supervisors and press operators alike, as well as being quite incidental to the root cause of problems once printing commences. Figure 3 clearly demonstrates these two functions or entities erroneously employed. A squeegee blade that is too long for the job ruins print integrity - period (Figure 3a). Even with the correct length, too much off-contact places more stress on the blade to overcome the fabric’s upward force (Figure 3b), which may continue to cause unacceptable results. Figure 3c illustrates either entities, or functions, in a desirable state to reduce any possible conflict against quality. There is no question that the lowest off-contact setting and shortest squeegee length are two minimal, but essential, press make-ready requirements for high quality printing performance. A final word on the matter: These two are the only on-press devices that can be replaced to reduce the aforementioned negative effects once production has begun.
The Realisation of a Squeegee Nobody Wants to Know
The squeegee blade actually collapses somewhat - in the middle along its edge - during the print stroke. Naturally, this is a greater problem with longer blades and softer durometers, which require more angle or pressure and less than desirable fabric clearance, around the whole image. When a squeegee blade is at zero degrees (from the vertical), for all intents and purposes, pressure is distributed equally along its entire length against the hard print surface (Figure 4a.). Once angled for printing (Figure 4b), contact print pressure tends to shift toward the outer ends because of the compounding upward force created by the deflecting fabric. As such, the blade renders less integrity (unequal pressure) in the middle than at its ends - and will render more if the print angle increases without modifying pressure. As such, the blade renders less integrity (because of unequal pressure).
Over-pressure at both ends is the negative combined effect that explains why the blade literally collapses in the center (Figure 4c) and, subsequently, diminishes image integrity.
Reducing the print angle helps; however, it is inevitably at the expense of depositing less color with reduced encapsulation abilities on non-smooth/textured, grain-like surfaces.Switching over to dual or triple durometer blades (a soft print face supported by a harder backing) will certainly improve the situation and overall print quality in most instances.
That said, it is easy to suggest harder durometer blades to reduce true “print angle” (from the mechanically set one), but the imageto- screen ratio is another factor to contend with from the outset when seeking exceptional highend quality. If overall image size stays within the accepted ratio of the established in-house percentages, and the proper length squeegees are selected, associated problems will significantly diminish. If, on the other hand, an extremely healthy image-to-frame ratio was employed to begin with, squeegee length becomes less of a factor in obtaining quality. There is a appropriate adage worth remembering: “Use a squeegee that fits the job - not one that fits the screen.”
Figure 7 : Affects j, q, t, and u.
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Floodcoater Considerations
Up to now in this series, the concerns of a squeegee blade has been discussed as it relates to obtaining high quality print results - but what about the poor old floodcoater (also known as the floodbar or scraper lade) ? It may surprise you, but the flood-coater can be just s
important as the squeegee and, in many ighdemanding
situations, even more significant ! The common roblem here is that the floodcoater is seen principally as othing more than a tool that scrapes ink back to the start so that the squeegee can commence its next stroke. However, its real job is to “pre-fill” or “prime” the screen with the correct amount of ink. This will reduce queegee stress and screen wear, together with their ncontrollable negative effects, thereby making the ink-transferring phase of the process easier to complete (refer to Part III, SGIA Journal, 4th Quarter 2003, for more). |
Simply put, the purpose of the squeegee is nothing more than to transfer ink out of the
screen. The purpose of the floodcoater is to leave the correct amount of ink on the screen without the need for the squeegee to crush image integrity due to insufficient amounts left behind.
Now that it is apparent a floodcoater’s overall profile (angle, pressure, speed, etc.) can simply adjust how much ink is left on the screen to influence the final print finish, consider also the dominant role the edge profile plays. If, for example, a heavy opaque
coating were required, the scenario shown in Figure 5a would produce the most desirable results with the least amount of effort and stress on the squeegee blade. Conversely, the most advantageous setup to successfully reproduce fine lines and hold small characters is shown in Figure 5c, while 5b would yield a compromise between the extremes.
A clearer understanding of the effective results of creatively applying the flood-coater’s seven influential components (length, tip profile angle, tip outline [sharp or rounded, flooding angle, pressure, speed and print/floodflood print mode) is a big bonus in any type of high-end printing operation. This is because pre-filling the screen correctly with the floodcoater has a direct effect on image resolution and final finishing quality - but without negatively re-orientating the registration of the image, as the squeegee may do when it’s adjusted. Simply put, adjusting the floodcoater in any manner does not affect print registration at all, so always consider the floodcoater first to adjust deposit thickness and other characteristics of image/finishing quality.
Floodcoater Length
When an excessively long squeegee blade presses against the screen, the fabric’s highest pressure points naturally move towards the ends - the weakest being in the middle (Figure 6a), as previously observed. Although not seen with a naked eye, over-pressure with a hard surface (floodbar) against a softer one (screen mesh) causes unevenness or a sagging-like effect in the center of the screen, where the fabric is at its slackest (weakest) point. Nonetheless, this is not a concern; the fabric eventually balances itself out along the squeegee when it comes into contact with the substrate during the print stroke (Figure 6b).
The problem shown in Figures 6a and 6b is that the squeegee illustrated is too long for the job. Once the correct length is employed (Figure 6c), greater print integrity can be assured. However, life is not always that easy - it takes two to tango, as they say! If the
floodcoater’s length is not like-wise matched with that of the squeegee used, the so-called“sagging effect” becomes very real while printing - particularly if near the frame’s inside edges.
During the flooding part of the print cycle, the floodcoater traverses along the fabric (floods) while the screen is in midair, not in contact with any other entity and corrections
cannot be self-induced to compensate for the weak area of the screen. Consequently, a
greater amount of ink is flooded and left in the middle. (This is readily seen from light
reflecting off the wet flooded screen (Figure 6d).) Because there is now more ink ”prefilled” in the middle, the squeegee has little choice but to likewise transfer more in the center of the print. While this may not always be a problem, it can be a nightmare with fine line resolu- tion and halftone work as well as many 3-dimensional requirements. The best rule to ensure these problems do not occur is simply to have the’correct floodcoater length available for each squeegee - period, creating a “pair” or “set” if you will.
Squeegee Snow Plow
Originally devised to print and encapsulate soldermask over high/low tracks and spaces on printed circuit boards, the benefits of“snowplowing” have trickled down to many other screening applications that also require full coating or surface encapsulation. To snowplow is simply to skew a squeegee blade a few degrees in the direction of print travel, to print at an oblique angle - not to be confused with the squeegee blade “print angle” required for printing. Depending on the amount of skew and axis applied (rotated plus or minus from the parallel position), it helps to reduce or completely eliminate sawtooth and moire with process work, and can prevent banding with dot graduations/vignettes (Figure 7).
The reason why the snowplow feature helps to eliminate those faults is that it effectively
distorts the image slightly; in a S-shape fashion, thus breaking up the linear alignment of tone dots that appear as sawtooth and moire. Furthermore, it has also proven beneficial to achieving many other aspects of quality screening, such as deposit uniformity, edge resolution and superior coating encapsulation over un-even/textured surfaces (Figure 8), including fine line definition, regardless of image axis orientation. In respect to the latter, the argument of supporting the need to print down lines, rather than perpendicular, is greatly reduced. Having viewed the advantages of a snowplow squeegee, notwithstanding, caution is stressed since using too much of a skew can potentially distort the image beyond control.
