IT’S TRUE for just about every printing operation: Whatever they are printing today is much harder and more demanding than what they did five years ago. And what they printed back then was more challenging than five years earlier, and so on – all the while expecting superior results from printing equipment and processes that may have long passed their peak!
There are two predominate reasons why screen printing technology has handled this challenge relatively well. First, these higher degrees of difficulty and more stringent demands have slowly evolved over the course of time, not overnight. Parents, for example, do not notice their children growing each day, but they sure do. The changes only become apparent when looking back over the years.
The second factor in the apparently smooth transition toward more dynamic work with more controllable results is that screen making products have also advanced, in some cases significantly. This fact nicely dovetails into the theme of this article, because, like it or not, manufacturers have done their share – but have we, as printers, done ours by taking full advantage of these substantial developments? Are we taking these more technically advanced consumables and applying them in the manner prescribed? Or do we simply turn a blind eye to innovative materials, training, and astute instructions, thinking we can cruise along using techniques from the past because they still seem to work?
Often, the reality is closer to the latter, and it’s the barrier that prevents many printing operations from advancing their processing abilities. Ever wonder why screens that come from trade screen makers or other suppliers look absolutely magnificent? The truth is they really don’t — they were just painstakingly made following the instructions. Anyone can make “magnificent” looking screens, but only if they meticulously follow processing directives without cutting corners.
The problem I find myself wrestling with is that it doesn’t have to be this way, especially when an abundance of resources are available. Don’t get me wrong: Troubleshooting issues with high-performance printing applications keeps a nice roof over my head. My point is that many superb technical articles have been written on screen making, perhaps more than any subject, so what happens to them? I believe the truth of the matter is either apathy or WHADITW (we have always done it this way), or a bit of both. It’s wishful thinking for any printing operation to expect to profitably handle more challenging work without first preparing. Screen making goes hand in hand with the degree of print performance required – it’s impossible to deliver more than what was given!
From Ordinary to Extraordinary Results
Almost every challenging application requires some sort of critical result from the printing process. Usually, either the deposition uniformity is crucial or producing a sharp, crisp-looking image is imperative – or both. Whereas high-end graphic printing is primarily concerned with the two-dimensional aspect of a print from an aesthetic viewpoint, many functional applications necessitate the three-dimensional accuracy of the ink deposit in order for the product itself to work.Advertisement
This is why elevating the printing process has never been as important as it is today. New product innovation has accelerated at an alarming pace and so have performance requirements. This imposes more stringent controls than ever before on what is essentially an artistic process. If suitable improvements are not made to enhance the quality level, then gaining this lucrative work will be tough.
These five secrets for producing screens will elevate your results from ordinary to extraordinary, and then some. They are presented, except for the last item, in the sequence in which screens are made.
Secret #1: Frame Size Matters
A department supervisor concluded a hands-on training session in screen making with a smile and, in a raised voice, told her team: “It’s not the frame size – it’s the image-to-frame ratio, stupid!” Abruptly put, perhaps, but that’s what it comes down to: Is there sufficient mesh clearance between the image and frame to lessen troublesome image distortion and related issues? As squeegee pressure is applied, the mesh stretches outward due to off-contact distance and, if used, peel-off. More importantly, the closer the image and/or squeegee is to the frame’s inside edge, the worse the distortion will be due to the mesh’s severe angle of deflection. (See Figure 1.)
Other than guessing, the three most commonly used methods to establish the maximum image size for a given frame are by measurement, ratio, or proportion. The first approach uses a minimum dimension from the frame’s inside edge for placement of the image (such as 6 x 8 inches) regardless of the frame’s size; the second involves an image-to-frame ratio (IFR) that meets each job’s specific requirements; while the proportional method requires the frames to be physically twice the squeegee stroke length and triple its width, for example. The proportional method places a greater limitation on image size than going by the IFR, which I personally prefer for more stringent applications. Whatever system is used, the frame must be large enough to adequately support the whole image within the tolerance required by the application. Critical jobs are more likely to be doomed from the start – or at least made exceedingly more difficult – if frames are undersized.
The advantage of the IFR method is that it doesn’t assume all jobs are the same. Figure 2 shows different types of printing specialties loosely grouped together and at different general image sizes, providing a reasonable IFR that could be adopted as a benchmark or starting point. Each job may have different challenges to overcome. For example, the customer’s specs for one particular job required an IFR of 30 percent, while a similar job, size, and ink for another customer demanded the IFR be reduced to 22 percent – simply because the second job had tighter electrical specs.Advertisement
One could reasonably argue that the latter job was over-engineered based on the client’s specs, a subject we’ll revisit. But the example demonstrates why frame size does not always influence the acceptable maximum image size, as additional tolerances (print or otherwise) must be met according to the job specifications.
Secret #2: Mesh Makes a Difference
I learned a long time ago that many printers do not fully comprehend screen fabric geometry, at least beyond the mesh count and thread diameter of the fabrics they use. In fairness, an experienced, knowledgeable person selected those products, and they worked – it’s understandable why awareness of mesh stopped right then. The truth is those meshes may have met the necessities of that time, but as we’ve discussed, the challenges in today’s marketplace have changed radically and will continue to do so. What was adequate in the past may not necessarily be so today. The only way to know is to put newer fabric grades to the test.
Many of the finer meshes may not have been around when the current products were chosen. Not that long ago, these fabrics were woven for filtration purposes. They were designed to filter out particles, while our objective is to deliberately force particles through them. Since then, fabric mills have stepped up by adding higher mesh counts with finer thread diameters to their lineup, specifically to satisfy critical applications.
Screen printing was once widely hailed for its ability to lay down thick, solid, opaque coatings – and it’s a continued strength. Many of the recent innovative applications, however, require a more subtle ink deposition, often a tightly controlled, uniform depth; smoother topography; uniform tint/translucency; fine detail without clogging; blemish-free lines/curves (minus sawtoothed/serrated edges); and much more.
Without getting overly involved with mesh comparisons, consider two similar grades separated only by a 3-micron difference in thread diameter. One leading brand’s 355/34 plain weave polyester (meaning 355 threads per inch with a thread diameter of 34 microns) has a physical opening of 29 microns that provides an overall open area of 16 percent. (See Figure 3.) This means that only 16 percent of the ink can be transferred onto the substrate’s surface to create the image. The reason for the restriction is interference from the threads and crossover knuckles, which collectively consume 84 percent of the fabric. That’s good for sieving and filtration, but not for critical screen applications. If the ink contains large particulates – as many in functional applications do, such as conductive pastes and phosphor (for electroluminescence) – then the fabric’s petite openings may negatively affect coverage and/or image detail by impeding the ink transfer.
The second grade is a 355/31 mesh, with a 31-micron thread. Just this one difference vastly improves both the mesh opening size and percentage of open area to 38 microns and 28 percent, respectively, as shown in the lower half of Figure 3. Mesh interference is reduced by more than 14 percent, while particulate size becomes less of an issue. Many are loathe to venture down this avenue, however, believing it can affect the overall deposit thickness and/or opacity, and this could happen. After all, the second mesh is thinner (48 microns compared to 54).Advertisement
But the key word is could. In actuality, the thicker mesh with its higher peaks and deeper valleys creates long, narrow tunnels from which the ink can have difficulty “excavating” itself, promoting pinholes and streaks. (See Figure 4.) With the 355/31 mesh, the ink layer may be a little thinner, but the ensuing gains can be exceptional and may reduce the reject rate. Quality soars: The squeegee pressure is reduced, creating less distortion; opacity and uniformity of the ink deposit are superior; less dot gain/loss occurs; and colors are truer, to name but a few of the advantages.
Secret #3: Control the Tension
In virtually every functional and demanding application, using screens with a commensurable tension has a direct influence on the outcome. Suppliers advocate tension as a crucial prerequisite for success, but such guidance frequently falls through the cracks where it matters the most – in the screenroom. The problem is often twofold: The screens weren’t initially tensioned to the recommended level for the task, and inadequate steps were taken to minimize the substantial tension loss that occurs through continued use. In my experience, simply upping screen tension can instantly take a plant’s performance to a higher plateau with ease.
High screen tension dramatically reduces printing difficulties. No one suggests going beyond suppliers’ recommendations, but consider approaching the highest tension level they advise. Some screen makers complain that it is impossible to reach these levels, at least consistently – which indicates that their existing procedures aren’t working! With stretch and glue frames, follow the stretching system’s instructions carefully after ensuring the equipment and clamps are in good working order. The same applies to retensionable frames: Be sure they are properly maintained and are used consistently rather than everyone doing it their own way. Better yet, send personnel to the supplier’s training courses or have sessions conducted in-house.
Just as important as the tension level is the need to minimize tension loss from what’s known as work hardening. This is a term that explains why fabrics lose tension due to the natural forces of printing. Always leave newly stretched mesh to rest for at least 30 minutes before gluing or finalizing with retensionables. Afterward, it is a good practice to allow screens to rest at least a whole day before prepping them for coating, so that the fabric has a chance to reorient. Allowing this process to occur after the screen has been imaged or, worse, on press does no favors for anyone. The difference is night and day; well-made screens coated after this waiting period will experience only slight tension loss during production.
Some printing operations find it beneficial to allow two days before proceeding, particularly with larger screens (anything with images larger than 20 x 30 inches). If someone objects that there are not enough screens to implement this practice, then buy more. It’s very cost-effective when compared to constantly throwing money down the drain dumping nonshippable prints, to say nothing about the expensive materials and production time lost forever.
Secret #4: Don’t Assume You Have the Right Emulsion Over Mesh Ratio
As you would expect, suppliers of automatic coating machines talk highly about their products, but I am going to take a different route on the subject – discussing the coating trough rather than the machine itself and its role in determining the emulsion over mesh ratio (EOM). A coating trough is supposed to be a no-brainer – so why is it number four in our top five? It has to do with one specific aspect of the print, an issue commonly encountered with critical applications: controlling the ink thickness for lines, fine detail, halftones/gradations, and solid areas. When using capillary film, both the EOM and Rz value (the surface roughness) are uniformly maintained, but what about screens coated with direct emulsion? How can one be sure they’re uniform, too?
To achieve a consistent ink deposition for superb image resolution, the emulsion thickness over the whole screen should be similarly uniform. But is it? Use a thickness gauge to check screens, especially the center in comparison to the outer edges and corners within the image area to see if there are unacceptable differences. Alternatively, lay coating troughs carefully on a clean light table (with a glass surface, not plastic) facing downward at the correct coating angle, then slide sheets of paper between the edges and the surface of the glass to see how much space appears.
I have seen many trough edges that are badly worn (curved in a concave fashion), and even new ones in the 10-micron-plus range, and once a whopping 19 microns. It doesn’t take imagination to realize how this can potentially jinx any kind of challenging application from the start. Be mindful, too, that screens are generally coated several times – a great way to compound unevenness with any worn edges.
I have seen cheap, awful-looking troughs that were machined – no, strike that, whacked – into shape by sheet metal cowboys down the street. I was immensely puzzled once when troubleshooting three particular jobs at a high-profile research company that turned sour for no apparent reason – or so I thought. After going through each step of the way meticulously, I noticed the coating trough’s end caps protruded a little beyond the coating edge itself, just enough to allow it to coat far more emulsion than it should, thereby rendering the trough useless. Perhaps the issue might have been detected earlier if they’d had an operable thickness gauge!
Screen makers have also been known to deliberately coat their screens with far too much emulsion, claiming it lasts longer and prevents on-press breakdowns – often at the request of press operators themselves. This waste is good news for suppliers, but adds no value to the printing process. Just as the mesh tension and mesh itself should be of a uniform thickness, the emulsion must be, too; otherwise, it will be all but impossible to achieve the required results. If the EOM is too thick in certain areas, particularly with finer meshes, not only will resolution suffer, but edges around the image will be hideously thick and may spread. For a smoother edge profile with fine-line printing, it is better to reduce the EOM down to perhaps 10 percent of mesh thickness as a starting point. Taking the two mesh examples from above (the 355/34 and 355/31 screen fabrics), the EOM would be around 5.6 and 4.8 microns, respectively. At this EOM, printing will be less painful and will yield a more pleasing line plateau while improving conductivity, using less emulsion and ink.
To achieve a lower EOM, it is better to consider the “wet-on-dry” coating principle, rather than the more traditional “wet-on-wet” approach for superior results. With intermediate drying times between coats, the emulsion is built up in thinner, firmer layers that produce a uniform, controlled thickness. This also improves mesh bridging to promote sharper image detail and overall resolution. Additionally, it is better to use short-length coating troughs that cover the intended image size rather than coating the whole screen needlessly for every job. The results will be more consistent, and of course, blockout is considerably cheaper than emulsion.
Secret #5: Control the Client’s Expectations
Like any business, printers follow explicit job details, in part to deliver what their clients expect. But what about those “specifications” that outwardly appear excessive for the job, sometimes past the point of belief?
Without breaching nondisclosure agreements, here are some examples of over-engineered jobs: A multicolor cartoon character print on a child’s slipper that had to withstand 40,000 cycles of continuously flexing 180 degrees; a coffee mug transfer that had to endure 51 cycles through an automatic dishwasher on the longest and hottest settings; and a high-end graphic overlay/membrane switch printing operation that used the same specs and quality-control procedures to produce large signage and single-color labels. In that company, registration had to conform to a tolerance of ± 0.001 inches when the product itself required no better than 0.02 inches, while a 12-color job had to adhere to excruciating tolerances for each color on an unstable substrate – when only the first two colors actually mattered. In my estimation, these two examples occurred simply because that is what the customer, perhaps misguidedly, had specified.
Even in critical functional applications, similar product types can be poles apart specification-wise. Take transdermal patches, for example. The tolerances for nicotine or travel sickness patches would be generous, but that would understandably not be acceptable for diabetic insulin or birth control patches. Nicotine patches would become horrendously expensive if they were produced to the unnecessarily higher specs. It is a fact: Over-engineering a product will needlessly drive up its cost without making the print any better for its purpose.
The real-life rationale for such tight tolerances is that the finished product might have to meet certain criteria for its core use. Therefore, it may be subject to a battery of tests to determine whether it can withstand exposure to physical contact, environmental conditions (varying climates, lightfastness), chemicals, etc. Prints may further need to conform to recognized industry standards, such as ASTM (American Society for Testing and Materials).
The three jobs that failed at the research company mentioned earlier had more issues than just emulsion thickness. When the client was asked how the specs and some provisions of the jobs were determined, they admitted that most were culled from the internet. In many instances, researchers, developers, and project engineers adopt a set of pre-existing specs because it seems to meet their needs. Is it a fair burden for printers to conform to such rigid tolerances knowing that they are unwarranted and put their own bottom line at risk?
One way to bridge this gap is a price scale that reflects incremental cost increases when job specifications drift from the customary to the extreme. Take the first factor discussed in this article: the frame size. To stay within one customer’s specs, the IFR could be more than 30 percent of the screen. Yet for another customer, the exact same job could mean the ratio had to be reduced to 22 percent to meet the higher QC specifications. When presented with the cost increases, the client may see the specs as excessive, with too much fudge factor built in, or deem them absolutely essential if they are producing a top-shelf finished product.
A printed circuit board that costs 75 cents would be fitting for a calculator used as a promotional giveaway, but certainly unacceptable as part of an airline cockpit. Conversely, a $150 circuit board that would be more applicable for a plane would be overkill for the calculator. Merely applying common sense can identify excessive specs in many instances.
When a job’s specs appear to be an overindulgence that drives up the price, it is the printer’s prerogative to take exception to them in a business-like manner. We printers may not know much about product design and engineering – but chances are our clients know even less about printing. You might be surprised by the outcome. Many sticking points may be negotiable and alternatives readily acceptable. After all, customers always appreciate cost savings. When vendors show them how to eliminate unnecessary fat from their specifications, clients will be likely to reward them with more business.
The Groundwork for Success
Capitalizing on the lucrative high-performance and functional applications that are emerging requires screen making skills to be taken up to the next level. Applying just one of these principles can make a world of difference. When all five are under control, then taking on more elaborate and intricate work becomes a stress-free proposition that leaves the bottom line largely intact.
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