Elevating Production Efficiency, Part II

Workhardening: You don't need an electron microsope to observe polymer reorientation - a platter of pasta will do. Under stress, some bonds between noodlelike polymer chains break and mesh begins to elongate.  Additional new bonds form between now-more-parallel polymers, creating a stronger mesh strand.

      Let's take the last first. Mesh is unstable for two reasons. For one thing, it's elastic. We can illustrate this using something that arrives at your door everyday with your newspaper: a rubber band. If I stretch my rubber band out and let it go, it appears to snap back to its original shape. Mesh behaves in similar, but less exaggerated fashion. If stretched on a retensionable frame or stretching machine, it lengthens, but when pressure is released it appears to shrink back to original size. We call such dimensional recovery memory, because each object appears to remember it's original shape. Traditional thinking about mesh has assumed that mesh instability is due to its elastic properties alone. If we stretch our rubber band just enough to keep it from sagging, it is, of course, easily bent out of line or deflected. If we pull the band taut, you know what happens. It offers resistance to our downward pressure. This is the classic argument for high-tension screens: those more tightly strung exhibit less screen deflection when the squeegee displaces the mesh. As a result, there is far less image distortion.

      While there is nothing wrong with that argument (I intend to expand on it in a later installment), it's not the whole story behind mesh instability.

A slight case of amnesia
     On more careful examination, we see that our rubber band's memory is not as perfect as we thought. Lay the rubber band out and measure its length. Now stretch it over an object large enough to highly tension the band, and leave it for a while. Later, when it's removed, your second measurement will reveal the band is now longer. (Our's lengthened by a full quarter inch.)

      We may observe the relationship between screen tension and elongation by making two marks on a piece of mesh prior to stretching it. A tension meter placed on the mesh after tensioning will show the tension level falling as the mesh, like the rubber band, begins to relax. The marks will grow farther apart as the tension on the mesh drops. (Mesh, in fact. Elongates approximately 1/64 inch per foot, per 7-Newton tension drop.)

      When mesh "loses its memory" or elongates, increase in the stencil size is not just the temporary sort we observe when it is deflected by the squeegee, but permanent. And as the pencil marks grow farther apart, so do the elements of our stencil image.

       So to answer our first question, high tension alone isn't enough. Mesh is stable only when elongation has (for all practical purposes) been defeated, creating an image area as rigid and unchanging as possible.

      Unfortunately, mesh elongation is a much tougher nut to crack in practical terms. If, for example, we took an N300 mesh and cranked the tension up to 50 Newtons immediately, the elongation that would occur afterward as the mesh relaxed would create severe misregistration problems during printing. That doesn't mean you can't tension N300 mesh to 50 newtons, it just means you have to do it properly. But before we can properly put elongation in check, we've got to explore why it happens. As in last month's installment, we'll find our answers at the filament level.

      Under a powerful microscope, what appears to us a solid, cylindrical object—the mesh filament—is actually a collection of millions of molecules  arranged  in chain-like structures called polymers. In a piece of new mesh. these are randomly (or only partially) oriented to one another, resembling a plate of spaghetti. The polymer chains are bonded to one another where they intersect. Those bonds give the mesh what strength it has and, in simple terms, that strength is limited by the number of bond sites between chains.

      When we stretch the mesh, it's like taking a fork and pulling the spaghetti from opposite ends. What happens? First, the intersections between the spaghetti strands are disturbed. In like manner, by stretching mesh. we begin to break bonds between the polymer chains. The mesh is actually coming apart at the seams—some of tile bonds between polymer chains begin to disengage and the strand or filament begins to grow in length. The net result is a larger piece of mesh, overall, and explains why we experience that typical drop in tension after initial stretching.

Self-repair feature
     We would also have a considerably weaker piece of mesh were it not for the fact that the mesh has a remarkable ability to heal itself and, in fact, to get stronger. As we apply increasing force to the mesh during stretching, energy is created sufficient to break these molecular bonds between the polymer chains. When we stop increasing the force, that same energy is available to fuel the creation of new bonds. Experimentation has shown that in as little as ten minutes (though up to one hour is preferable), polymer chains within tensioned mesh can re-bond. This allows us to retension the mesh (this month's second concept) without weakening it. As a result, the retensionable frame has, in the last decade, gained wider acceptance as a means to raise tension to higher levels.

Spaghetti with Rubber Bands

      Last time I introduced three concepts that affect our ability to get and hold—to stabilize—mesh tension: elongation, retensioning and workhardening. You'll recall we established a mesh-tension goal, to obtain and stabilize high tension, but turned right around and stated flatly that mesh is, by nature, unstable.

      This month, we'll demonstrate how the three concepts just mentioned help resolve that contradictory state of affairs. But first, I've got to define some terms: What do we mean by stable? And why is mesh unstable?

Don Newman, president of Stretch Devices, is one of the industry's leading advocates of on-press production efficiency—primarily via the virtues of elevated screen tension. Here is the continuation of his comprehensive analysis of the subject, stripped of scientific jargon and mathematical formulae and revealed as a set of simple concepts.

Measurable memory loss: Beore and after (below) shots confirm permanet elongation.

Becoming "forgetful": Highly stressed over time, both rubber band and mesh filament relax or elongate and become incapable of full recovery to size.

Key word is "tends": Like the rubber band, the individual mesh filament tends to recover it's original shape and size when deflected.