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 :: Technology 
 
FLOODCOATER FRIEND OR FOE ?
PREVIOUSLY IN PART 4, WE COVERED THE EFFECTS OF SQEEGEE ANGLES, OF -CONTACT DISTANCE AND PEEL OFF-CENTRING THE IMAGE, LENGTH OF SQEEGEE FLOODCOATER AND THE BENEFITS OF SQEEGEE "SNOWPLOUGH" -AS THEY PLAY A DIRECTLY ROL IN THE QUEST FOR A SUPERIOR QUALITY FINISH.

Image 'center' or not to 'center'

In PART 1, the image-to-frame size matrix was discussed in depth where several ratio samplings were given according to jobs’ degree of difficulties and tolerances. Each sampling was based on the image being positioned centred in both ‘X’ and ‘Y’ direction on the screen. This means that having a healthy image ratio could still 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 image distortion would no longer be acceptable for the job. In other words, it is not simply a case of image ratio to that of the frame but also how near to the inside of the frame one can safely print. Image-to-frame matrix is just but a guide, how close the image is to the frame is something entirely different.

No one can provide exact ratios or measurements for any company here because there are too many other factors that enter the equation. Actual in-house ratio percentages ought to be predetermined by each operation according to the level of jobs requiring special attention. As such, ratios developed internally may well be different from those given as a guide in this series, and certainly be more practical, since they would encompass one’s own operational skill levels and processing abilities.

For ease of developing your own suitable ratio matrix, consider printing fine graph lines on several sheets of clear film in both ‘’X’ and ‘Y’ directions. Space the lines say 5 mm apartover the whole image area using maximum frame size, standard pre-press/screen making, setup and printing procedures. Have three different squeegee lengths available, large (largest for the press), medium and small (shortest most commonly used) with good sharp edges. Use maximum length floodcoater for the test if checking registration capability but the correct lengths with each of the squeegees being used if testing for deposit uniformity.


Mike Young Imagetek Consulting International, USA.

Mr. Young has been a specialist in high-definition graphic and industrial screen printing for more than 30 years.He is a SGIA Fellow, a member of the Academy of Screen Printing Technology, recipient of the prestigious Swormstedt Award for technical writing. He is also a frequent contributing writer to trade publications, SGIA print judge, legal expert and a popular speaker at industry events. Mr Mike is the creator of the internationally known Troubleshooting Chart and published technical books on advance screen printing techniques, including The Register Guide about achieving print excellence. He reccently conducted a series of technical seminars at Screen Print India 2004 in Mumbai. He operates Imagetek Consulting International, a Connecticut USA-based consulting firm.

 
 


Starting with the small squeegee and lightest pressure, print some sheets using no more than half the stroke length, then change to the medium size squeegee (extent stroke length as required) and print more sheets. Continue until the test is completed with the longest squeegee and full stroke length - keeping in mind the squeegees must be kept centred in the press at all times and using same angle. It is not a problem if a little more pressure is required as the squeegee length is increased, which is one proof of what causes image distortion in the first place. Using a light table, one can then compare all three squeegee length prints against the master positive to show what the in-house limitations are according to the degree of image shift (elongation). In fact, comparing all three prints between themselves will show a long squeegee for a small print area also distorts the image, (which supports the illustration shown in Figure 29 later). This is a very simple and easy test to conduct but important to do if there are concerns about in-house capabilities.

Now repeat the same test again. This time, position the substrate to the outer edge of the print table (offset from the centre), with all three squeegees correspondingly centred to the substrate (not to the centre of the press). Net results will enable one to determine what the distortion-factor is, even with a so-called healthy image ratio to start of with, when the image is offset from the centre. This positioning test is rather important to conduct, especially for those operations who routinely places more than one image on a screen (to print subsequent images or colours.) As with the previous test, this exercise is worth every effort to conduct since it enables any operation to view precisely what their processing limitations are according to the degree of difficulties and image size limitations. Once this information is known, any operation will know if they can safely enlarge their overall printing area to print more multiple images for greater productivity - or need to make a reduction in size to reduce waste from misprints.


In the previous part of this series, the distortional effect was discussed from using too much off-contact distance. This was once again based on the image being centred in the screen, (as illustrated in Fig. 27a). What happens when the image is offset to one side or orientated to a corner (offset in both directions)? Such image positioning may not be a problem in many operations for the majority of jobs printed. Offsetting an image in this manner is a frequent requirement with many types of presses that utilize automatic take-off systems. For jobs that are critical, however, particularly with those that suffer from socalled “screen-stretch” (a very irritating problem when die-cutting multiple images), offsetting may not be the wisest choice for production.

Figure 27b clearly illustrates why additional distortion occurs - when a single or multiple images gradually shifting to one side, thus making die-cutting all but impossible to achieve without high scrap rates. Offsetting usually means the squeegee blade has to be offset from the screen’s centre too. Once offset, the squeegee requires more pressure on the other side to overcome the greater deflective pressure caused by the fabric’s powerful upward force, (as shown in Fig. 12, PART 2). Therefore, the closer a squeegee blade is to a frame’s inside edge the greater amount of print distortion results - regardless of printing techniques employed or screen tension level.

'... having a healthy image ratio could still pose aproblem if it were offset to one side because part of that image will now be inside the danger zone the area where image distortion would no longer be acceptable for the job.''

For those operations thinking it is not a problem using a full-length squeegee blade with an offset image - think again! Distortion will still be problematic, albeit less, because fabric clearance between the frame and image on one side (the right side shown in Fig. 27b) is still considerably greater than the other side. Consequently, fabric on the right side would “give” more overall (stretch further), thus making the image shift to the shorter (left) side. Despite what has been observed here, many operational style of presses are designed for offset image positioning in order to facilitate automate take-off systems. However, since quality usually becomes more important than productivity with crucial demanding type jobs, there is nothing wrong with centring the image.
"...the sqeegee blade actually collapses in the middle along its edges during the point strok".
When an image is offset from the centre, and the nearer it is positioned to the frames inside edge, the ratio of image-to-screen increases as it relates to distortion, (as shown in Fig. 4, PART 1). As mentioned, part of the image would now be printed in an area where expected distortion is not acceptable for the job - as determined previously by the operation’s own in-house processing abilities. In Figure 28, the same typical image distortion of a screen is reproduced from Figure 1, PART 1 earlier. The only difference this time is two coloured sheets are shown; the green printed in the centre of the screen while the red was positioned to the far right side. The difference in overall registration between the two same prints is clearly seen.
SQEEGEE LENGTH & DEPOSIT UNIFORMITY
Keeping the previously mentioned image- toscreen matrix in mind as a guide, actual ratio percentages established in-house depends entirely on the operation’s equipment used, processing techniques, skill levels, procedures employed, etc) against those required by demanding job specifications. With that understanding, it should be realised that reproduction by the screen printing method effectively produces a three-dimensional effect rather than that of an image seen twodimensionally - because it has “height” also. An image printed has length, width and, most importantly for many applications - depth (deposit thickness).
"...EVEN IF THE IMAGE IS HEALTHY IN SIZE AND CENTRED IN THE SCREEN, QUALITY COULD STILL BE A PROBLEM IF THE SQEEGEE USED TOO LONG FOR THE JOB".

Ink coating uniformity and controlled deposit thickness are often requirements to obtain good solid coverage or opacity, as well as for conductivity means, protection barrier or other purposes to yield a precise layer for many high-end/industrial requirements. It is becoming more common for high-end applications needing to print correctly in close tolerance, as a twodimensional requirement, are just but one objective. But it is entirely another matter if a controlled uniform deposit (a 3-D aspect of the same print) becomes as or even more important.

As deposit thickness and uniformity is the three-dimensional aspect of a printed image usually viewed two-dimensionally, it is not unreasonable to assume that if it were printed without distortion two-dimensionally-ink deposit should equally be consistent and uniform throughout. Although simplified here, the assumption is correct - although the degree of criticality needs to be defined. Where critical and precise deposit thickness and/or uniformity are crucial (even if registration is not), observing in-house pre-determined image-to-screen matrix becomes very important to reach the desired tolerance with ease.

Based on an acceptable matrix ratio to print the maximum size (shown as blue in Fig. 29), it is a reasonable expectation that a uniform coating can be obtained providing the correct squeegee length is used (also shown in blue). Using a much longer squeegee blade than necessary (shown in red), together with the same pressure, angle, speed, etc, the very same image will then distort two- and threedimensionally from the original that was printed with the correct length (blue) squeegee. The reason for distortion is that the longer squeegee influences the fabric in areas immediately outside the image (shown in red) and will therefore pull the image out from its original place. This is saying that even if the image is healthy in size and centred in the screen, quality could still be a problem if the squeegee used is too long for the job. Ideally, the squeegee blade should not be longer than 2 cm at either side of the image.

Consider for one moment that in many operations that it is the press operators who usually select the squeegees (and therefore its “length” unsuspectingly) to print a job. Making matters worse, image-to-image registration will then becomes another issue if previous colours or prints were printed with different length squeegees.
SQEEGEE LENGTH & OFF CONTACT JOINTLY COSCIDERD For many years, your writer has advocated to those seeking exceptional performance of anyquality level that off-contact and squeegee length are about the most problematic “on-press” variables during the final stages of press makeready. This is perhaps more to do with the not having the basic understanding by supervisors and press operators, believing these two factors are not the cause of the problems once printing starts. Figure 30 clearly demonstrates these two functions incorrectly used.
  
A squeegee blade that is too long for the job damages print integrity as highlighted in Figure 30a (see next subheading for reason). Even with the correct length blade, too much off-contact distance places more stress on the squeegee in overcoming the fabric’s upward force (Fig. 30b), which will continue to cause other unacceptable results. Figure 30c illustrates both in a desirable state to reduce any possible conflict against one another and final quality. There is no question that the lowest off-contact distance and shortest squeegee length are two important requirements during press make-ready for good printing performance. Final word on the matter: there are no other on-press devices between these two that can be substituted to reduce the mentioned negative effects once in production.              

             The realisation of a sqeegee that no body wants to know

Unknown to many, the squeegee blade actually collapses in the middle along its edge during the print stroke. Naturally, it is a greater problem with longer and softer blades, using more angle or pressure as well as not having good fabric clearance around the image in the screen (effectively too long for the screen).
When a squeegee blade is at zero degree from the vertical position, pressure is distributed equally along its entire length against the flat print surface (Fig. 31a.). Once angled for printing (Fig. 31b), contact pressure shifts towards the outer ends due to the compounding upward force created by the deflecting fabric (opposing force not shown for clarity). As such, the blade yields less integrity in the middle than its ends. Over pressure at both ends explains why the blade collapses in the centre (Fig. 31c) and the lost of image integrity. Reducing print angle helps, however, it is at the expense of depositing less ink with reduced filling-in/coverage abilities on non-smooth textured or grain-like surfaces.

Changing to dual or triple durometer squeegees (a soft print face supported with a harder backing) will improve the situation and overall print quality in most situations. It is easy to suggest harder durometer blades be used instead to reduce true “print angle” (from the mechanical set one), but the imageto- screen ratio is another factor to consider when requiring good quality finish. Once the overall image size has been kept within the accepted ratio (to that of the newlyestablished in-house matrix percentages), together with proper selected squeegee length, associated problems will notably disappear. If a very healthy image-toframe ratio was used to begin with, squeegee length becomes less of a factor in obtaining quality. There is an appropriate saying worth remembering that wisely states, “Use a squeegee that fits the job - not one that fits the screen.”

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