4. Mesh-fiber composition
The molecular and mechanical characteristics of screen-printing mesh.
At present, the only suitable, modern mesh fiber is high-quality monofilament polyester: round and smooth. It has superior elongation and strength characteristics, has no loss of strength when wet and, in fact, absorbs less than 0.8 percent moisture. Absorption of moisture is an important consideration, as this is such a wet process. Nylon, for example (or, by its generic name, polyamide, which can absorb up to 5 percent moisture), loses 10 to 20 percent of its strength when wet and elongates 26 to 40 percent before breaking-compared to polyester, which has elasticity of only 19 to 23 percent. Nylon mesh, therefore, should only be used when printing irregular-surfaced or three-dimensional objects (such as bottles) that might require such properties. Identifying nylon Why identify nylon? Because many such bolts of fabric are "out there," waiting for the unsuspecting printer to stretch them over a frame. Don't. Look on the edge of your roll of mesh for the brand name and model of the mesh you're using. Memorize the names different weavers give their monofilament polyester mesh. I often come across the odd "bargain" bolt of mesh that has "nylo" or "mide" (from the generic term for nylon) in the name. Get rid of it! If you have a bolt that isn't labeled, try a burn test. Both polyester and nylon fuse and shrink away from a flame, but polyester will put out a black smoke that nylon won't. Also notice (if you happen to get it into your nose) that the odor of burned polyester is "sweeter" than that of nylon. Molecular alignment When used as screen-printing mesh, polyester's most important trait is its ability to orient its molecules parallel to each other when the squeegee-blade drags across the mesh fibers during printing. This is called "work hardening," and is not a mystery, but a documented physical principle of long-chain molecules. They can change from low-orientation-molecules crisscrossing each other and not parallel-to high-orientation where the molecules are very parallel. High orientation is associated with good fiber strength and low elongation; low orientation offers nothing of value to screen printers. In polyester, molecules are joined to parallel molecules by associative forces known as "hydrogen bonding" (due to the effect of hydrogen atoms in parallel molecules bonding with other smaller electro-negative atoms). This bonding aids dimensional stability and lends mesh the ability to elongate-when acted on by off-contact and blade pressure-but then to return reliably to its original unstressed position. When molecules don't contain the polar groups necessary for hydrogen bonding, weaker associative forces can be established between molecules, provided they are very close to each other. The weaker forces depend on proximity and atomic charges-and, incidentally, are usually referred to as "van de Waal's forces." (I mention this mainly to illustrate that these principles are not just folk stories told by one generation to another, but actually have a scientific bearing on mesh performance.) The work-hardening phenomenon doesn't happen when you stretch mesh on a frame but, rather, as a result of the directional stroking of the blade on the mesh during printing. It is important, therefore, to document the direction in which the blade is used and never reverse the direction; this causes the molecules to re-orient themselves in the opposite direction, leading to chaos in the fibers and odd, unpredictable changes in tension and image shape-even unexpected stencil breakdown. Rapid tensioning During the development of high-tension printing in the 1980s, the best results were achieved by stretching mesh, then letting it relax (a mechanical stretcher works best for this, as pneumatic stretching doesn't allow the mesh to relax), then re-tensioning it until a higher than normal tension was achieved. These steps were used to allow the mesh to acclimate to the force on it gradually and prevent mesh failure-typically ripping. To make a critical screen dimensionally stable, some printers stretch and coat it, then cycle the squeegee blade across it in a single print head using a clear base as a lubricant-100 to 200 strokes, or until the screen stops losing tension-to workharden the mesh without an image. They then clean the base out, reclaim the emulsion, retension the mesh and re-coat it for a stencil. The introduction of low-elongation (LE) polyester fibers has again changed the approach to stretching. The molecules of this mesh are oriented during manufacture to help make it more dimensionally stable. This wasn't demanded of mesh manufacturers until the development of retensionable frames that could easily take meshes to their single-stretch tension limit and beyond. Until then, mesh had been stretched well below its potential. The techniques taught for retensionable frames of softening the corners to relieve stress facilitate high tensions. Tests by the SPTF (Screen Printing Technical Foundation) have demonstrated that, under controlled circumstances, a printer can proceed directly to the desired ultimate high tension in one stretch without significant loss of tension, compared to progressively elevating tension. Contact the SPTF and your mesh manufacturer for current instructions for using LE rather than traditional mesh. |
5. Thread structure
The differences between multi- and monofilament screen-printing mesh.
Here is an easy choice: when faced with multi- and monofilament polyester mesh, pick high-quality monofilament. Man-made fibers are made with either multiple or single filaments. Monofilament is a single smooth round fiber used in weaving mesh. These threads are extruded in very long lengths-even miles. Multifilaments differ from monofilaments in that the final thread for weaving is composed of several filaments combined into a single yarn. They are formed by having many holes in the polyester-extruding spinerette, instead of one. Before the widespread manufacture of monofilament mesh (in the early 80s), multifilament was cheaper. Today, there is little price advantage to buying multifilament, and the extra trouble to degrease, reclaim and get ink to transfer through it is not worth the effort. As a printing mesh, multifilament fibers are not as uniform as monofilament which makes the ink deposit irregular. Modern monofilament can be taken up to its ultimate mesh tension with little trouble, whereas multifilaments, because of all the internal friction, can't go as high. The development of modern capillary films that don't require the rough surface of multifilament to get the film to adhere to the mesh have also led to the diminishing popularity of this mesh. To identify monofilament, examine the mesh itself with a loupe or microscope and you'll have no trouble identifying the difference. If you discover you still have a roll of multifilament, decide if you can afford the increase in processing time it will force upon you ... then get rid of it. |
6. Thread diameter
Measured in microns, the differing diameters of individual mesh threads
affect ink transfer.
Compile your own list of meshes from manufacturers and use any available charts or tables that list comparative mesh characteristics. (I use Tension Chart No. 9 from Stretch Devices which lists the ultimate breaking strength and suggested working range for most meshes; there are others.)
Threads come in all sorts of diameters and the simple metric numbers are
easier to remember than the complex English ones. Don't use thousandths
of inches when referring to mesh diameters, because they'll distract you
with decimal points, zeros and those pesky little inch marks ("). Metric vs English*
... and so on. Both systems' increments are only used for comparison anyway-in our heads or in calculations-so I recommend using microns to compare mesh diameters, regardless of how you may feel about the metric system, overall. Even more interesting is the fact that thread of a given diameter can be woven into many different mesh counts. For example, 34-micron threads (wouldn't you rather say "thirty-four-micron" than "thirteen thousandths of an inch"?) is used to make counts of 420, 380, 355, 305 and 280. Because of the extra threads per inch, 420 mesh is actually stronger than 280 mesh. Although the threads may be thin, there are 140 more of them per inch. There is strength in numbers. For example, 305 mesh can be woven from 31-, 34- or 40-micron threads. As textile printers, we typically use the middle choice, because, although we can't afford to sacrifice too much strength, we need as much deposit as possible to overcome what is absorbed by the substrate. Measuring The thread measurements on our charts reflect the diameter after extrusion, rather than after the tortures of the manufacturing process. To measure true thread diameter, use a microscope or loupe with a measuring reticle. Measure the thread at the center of its span from intersection to intersection. You can also measure a single thread with a micrometer.
As is true in the area of mesh tension, the habits we have acquired over our years of experience can delay a transition as we use different thread diameters and don't notice how they change our results. Thicker threads can make a person who only pays attention to mesh count think that a mesh is behaving like a lower or higher count. The fact is that changing any of the variables of the mesh can make it behave like another mesh under other conditions. During the 1980s, for example, many printers commented about how a high-tension 160 mesh behaved like a low-tension 200. (This is why driving instructors hate to teach people how to drive in the rain.) Overall mesh thickness-although greater than thread diameter, because it's a function of two threads woven over and under each other-is usually something less than two times thread diameter and is the strongest influence on ink deposit. You have to measure thickness exactly because weaving and heat setting at the factory flattens the intersections rendering the threads wider than they are tall. Most ink-deposit changes are made by going to a higher or lower mesh count which usually changes the mesh thickness. (We say "usually" so you are forced to check your charts yourself and see what the difference will be.) Using a higher mesh count such as 305 means you're likely to be printing halftones and might get some benefit in terms of ink-deposit control by choosing different thread diameters from the same mesh count. For example, a thinner 305 thread-count (31-micron) mesh, will produce a thinner ink deposit than a thicker 305 thread-count (40-micron) mesh. These choices require actual testing because we all use different inks and substrates. Durability Because thread thickness plays a role in the overall stability and strength of the fabric, thicker meshes will offer more mechanical and chemical resistance, as well as longer mesh life, than their thinner counterparts. The thinnest threads (in this case, 31-micron) are not as durable, but offer better detail. The middle choice (34-micron) will be stronger without as much detail, but with a thicker ink deposit. |
1996 National Business Media, Inc.
