Pushing the Limit
Learn how to tension screens the right way and enhance the quality and efficiency of your work.
In this industry, you can find about a million different theories on just about every aspect of the printing process, whether it be taking the lid off an ink bucket or tensioning screens. Despite the countless hours represented by the combined experience of printers throughout our industry, we still can't agree on many fundamental aspects of the printing process. It doesn't help matters that very few of us can offer scientific evidence to explain why our ways are the best ways.
Many misconceptions make their way into screen-printing books, magazine articles, and other literature, and a lot of what I see in print I don't agree with, or at least take exception to. I think that sometimes, despite good intentions, authors don't follow through with the complete story. Or they present information that's accurate only for certain situations, environments, or applications, but treat it as a universal truth that can be applied across the board.
Based on what I've been reading recently, there is some confusion about mesh tensioning. In response, I'd like to present my theories on the subject.
To me, there is only one way to tension mesh, and that is tightly. I can think of only one type of application in the industry where loose screens can be of any benefit. And that is with jobs involving some round or odd-shaped objects where lower tension allows the mesh to conform to the shape of the part.
In every flat-stock application, however, high tension produces better quality and faster production speeds. The down side, according to one author I recently read, is that you ruin more screens by using high tension. In my opinion, you only ruin high-tension screens if something else is wrong with your printing procedures. If you keep a lower off-contact height, reduce the pressure of the flood and squeegee strokes, keep dirt off the mesh and substrate, and keep press beds level, you'll be amazed at what the higher tension will do.
The primary influences on achieving adequate screen tension include mesh count, screen size, stretching method/ equipment, and the experience of the screenmaker. The mesh count is an obvious factor--not because of the number of threads per inch, but because usually, as thread counts increase, thread diameter decreases. Will a 330-thread/in. screen fabric support a higher tension than a 380- or 420-thread/in. mesh? Sure it will. But will a 380-threads/in. mesh with a 34-micron thread diameter hold a higher tension than the same thread count using 31-micron threads? In fact, it will--by about 3-4 Newtons (N). So the mesh count alone is really an incomplete indicator of a mesh's ability to support higher tensions; thread diameter is an equally important concern.
Screen or frame size is the next factor. Your 380-thread/ in., 34-micron fabric cannot support the same tension level on a large 71 x 81-in. frame as it would support on a 24 x 24-in. frame, or even a 36 x 48-in. one. The smaller the frame size, the higher the tension you can get. But how high can you go on larger-format frames? One article I read contends that a graphics printer using a 380-thread/in. mesh on a frame 48 in. wide will not be able to achieve tensions above 20 N. We tension 380-thread/in. mesh on 41 x 51-in. frames up to 35 N on a daily basis, and on 41 x 88-in. frames, we use our pneumatic stretching system to get finished screens with tensions in the 25-N neighborhood. If you're ever in Joliet, IL, feel free to stop by and see for yourself.
In terms of stretching method, the big issue to me is how to handle relaxation of the mesh after stretching. If you tension a screen to 20 N and leave it overnight, you'll return to find that it is no longer at 20 N, but probably closer to 17-18 N. Now wash and degrease the mesh, then coat it. You drop another Newton or two. Next, put it on the press and print 100 passes. Two or three more Newtons are gone. Reclaim and re-image the screen, then print again. Tension drops by another 1-2 N. After two press runs and associated handling, your 20 N screen has dropped to 12-13 N.
John Coburn, who teaches a screenmaking workshop for the Screenprinting and Graphic Imaging Association, describes the nature of mesh by using a rubber-band analogy. Cut a rubber band, then attach one end to a wall with a thumb tack and put a washer to the other end. Measure the length of the rubber band and let it hang until the next day. When you return and measure the band, you'll find that it has stretched. Leave it for another day or two, and the band will have stretched even more, only stopping when it's reached its stabilization point. The behavior of screen mesh under tension is similar, which is why I don't recommend using newly stretched mesh for printing process color or jobs with tight registration until the mesh is stabilized.
I know of no way to completely stabilize the mesh unless you put it on the press and print. Retensionable frames were designed for this purpose, and they let you keep bringing up the tension after each use until it stabilizes. Note that I used the term "retensionable," not self-tensioning--I've never met a frame that can tension itself.
One of the keys to tensioning for stretch-and-glue screens is to ensure even tension over the entire screen. If you are producing a screen larger than 48 in. wide, I recommend you check nine points on the tensioned mesh--the center, the corners, and the midpoint of each side near the frame. Whatever the center reading is, the other points should measure within ±1.5 N of the center reading.
Why do you need consistent tension across the entire screen? Well, squeegees and floodbars aren't smart, so if you had 20 N in the center and across the right half of the mesh but only 17 N on the left, you'll have to adjust the squeegee/floodbar pressure to compensate for the difference. The result would be noticeable dot gain and color shifting from one side of the print to the other.
The tools used to stretch screens will limit or improve your ability to get high tensions. I remember in the mid-1970s using two 2 x 4s mounted on doll rods as my tensioning system. I would attach the mesh to the 2 x 4s, then rotate them with pliers to get the mesh as tight as possible before affixing it to the frame. Pulling the mesh tight by hand wasn't good enough, even then.
As for the people responsible for screenmaking and their level of experience, this is one variable that may always be a problem. Your ability to reduce the influence of this variable really depends on what kind of training screenmakers receive, who trains them, and what standards they're being trained to follow.
As a printer, you surely want to do the best you can, so make sure you really do the best you can. Don't accept less than the best. Like many of you, I remember coating screens with credit cards, exposing screens in the sun, using Ajax as a degreaser, working without a tension meter, and using lots of other shortcuts and make-do solutions. But I realized a long time ago that succeeding means moving your capabilities forward instead of going backwards or staying put. Push the limit, don't let it push you.