Pushing the Limit










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 &#177;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.

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