10. Mesh color
Dyed mesh inhibits light scatter, but prolongs exposure duration.
By the time we reach the exposure step, the stencil is a combination of mesh encapsulated by stencil material. Since exposure light must pass through the mesh, it plays an important role in image quality and durability. Ultra-violet (UV) exposure light reflects off of and travels through (linearally) white mesh threads. This is called undercutting, light scatter or halation. Shortening the exposure can reduce such undercutting, but has the disadvantage of rarely hardening the stencil enough for durability on the press. Colored mesh (dyed at the factory in yellow, amber, gold, orange, red and so on) has been carefully balanced to absorb UV exposure light so it doesn't get a chance to bounce around in the stencil (by reflecting off the threads or traveling along them). For this reason, dyed meshes are sometimes called "anti-halation" meshes. Their disadvantage is that they require a longer exposure time. Accordingly, if you switch from white to dyed mesh, don't rely on the exposure guide you've been using; your work will be underexposed. You must do a new exposure test using dyed mesh. Colored mesh is only recommended for counts of 200 and higher, because fine detail is rarely attempted with counts under 200, and the extra exposure time is rarely worth the effort. Orange and yellow are the most common dyed colors, offering a compromise between anti-halation characteristics and lengthened exposure times. Red is thought to be the most effective, but is rarely used because of its huge increase in exposure time-generally many times more than it is worth. |
11. Mesh tension
A measure of mesh deflection expressed in terms of Newtons per centimeter
(N/cm).
Tension should be looked at in two ways: the initial stretching and re-tensioning of mesh on your frames, and the ways in which tension interacts with other variables during printing. It's difficult to understand its dynamics without looking at tension relative to off-contact distance (Variable 42) and squeegee-blade pressure (Variable 33), so be sure to visit those variables after you've read about this one. The twofold job of mesh tension is to provide resistance for the blade-to be overcome only at the point where the blade forces mesh-to-substrate contact-and to pull the mesh out of the ink film after the blade passes.
Mesh tension is a measure of deflection expressed in Newtons per centimeter (N/cm). Until mesh stabilizes from continuously pulling the blade across it (orienting the molecular structure), it is unpredictable and unstable because of what's known as cold flow (the relaxing of the polyester molecules). This constant change can be very frustrating, especially to printers who don't use a tension meter. Under-stretched Tension of about 20 N/cm is an average minimum for mesh tension. Below 20 Newtons, the blade tends to over-manipulate the mesh, rolling it and dragging it in front of the stroke. And, as the mesh moves, it smears the ink film and blurs the image. Ink also picks up on the bottom of the screens when they're not adequately tensioned. What's more, because the mesh is moving, registration becomes impossible. Over-stretched When polyester is overstretched-from both excessive screen tensioning and overcoming excessive off-contact with the squeegee blade-the plastic deforms and it won't return to the neutral position required for good registration. The mesh will also fatigue faster and will lose tension faster than normal. You can also overstretch a mesh so much that it will rip. This is usually caused by tension concentrated in the corners where the fibers act as if they were shorter because of their heat set intersections. This corner concentration makes these fibers approach their breaking point faster than fibers in the center of the screen, and they rip. All screen makers should soften the corner fabric (utilizing longer fiber lengths) to prevent such damage. Mesh tension during stretching is limited by the thread diameter of the mesh, as well as how many of them there are per nch or per centimeter (see Variable 7: MESH COUNT). Low-elongation (LE) fibers have been treated during manufacture and are able to be stretched to higher initial tensions. This is a great benefit to stretch-and-glue screens because they can match the initial tensions of retensionable frames. Still, they will not be able to be retensioned during the life of the mesh, as the fibers elongate during printing. Retensionable frames can take advantage of the stabilization of the mesh through molecular orientation and retensioning, as well as recycle what might otherwise be unusable mesh. (In fact, such recycled, mature mesh is often more valuable than virgin mesh, fresh off the bolt.) On the press Once the screen is on the press, off-contact and blade pressure add still more stress to the screen. Engineers call this dynamic tension, static tension being tension without a squeegee-blade load. What's the relationship between off-contact and mesh tension? It is that printers typically compensate for inadequate tension with excessive off-contact. The result is that more blade pressure is required (essentially calling on the blade to perform as a tensioning device). On automatic equipment, a few turns of a knob adds more pressure, but it costs you registration, production time, and stencil life. For the poor manual printer, with tired wrists, less pressure is the most important requirement for sanity and health; the best way to avoid excessive pressure is to achieve adequate mesh tension. A low-tension screen doesn't provide resistance to the blade. Since you're already touching the shirt with the screen, you must use extra pressure just to get the ink to transfer. This works, but causes inferior prints. With textiles, too much pressure drives the ink right through the shirt and onto the platen. Whatever the force between mesh and blade, it isn't necessarily transmitted to the shirt. The force of the mesh must be enough to pull it back out of the ink; the blade pressure, though, needs to be only enough to cause the mesh to "kiss" the substrate. |
12. Mesh preparation
Abrasion and degreasing to make the mesh more receptive to stencil adhesion.
This variable precedes the discussion of other stencil variables because it is the source of various common stencil-breakdown problems-pinholes, fisheyes or poor stencil adhesion to the mesh-often blamed on the emulsion itself. The screen mesh must be prepared for stencil application by, in some cases, abrading and, in all cases, degreasing. Abrading is necessary when stencils which adhere to one side of the mesh-indirect emulsions and capillary films alike-are used, because the stencil may not adhere properly to the smooth surfaces of individual mesh filaments in today's monofilament polyester and nylon meshes. Abrading changes the surface structure of the mesh, literally scratching and scraping the filament surface. This results in minute cracks and crevasses that increase the surface area available for bonding. Oddly enough, despite its importance, there has been little definitive information available about how best this mesh-prep essential should be executed, so here are the basics. By far the most common means of abrading mesh is mechanical: an abrasive material, applied by hand either in the form of a dry powder or a paste/gel. The abrasive (either silicon carbide or aluminum oxide powder) is ground to a powder-like particle size small enough to pass easily through mesh counts as fine as 500 threads per inch, but with enough grit to roughen the filament surface-literally scratching and crevassing it-as they pass in, around and through the individual threads. This increases the surface area available for stencil bonding. The coarse abrasive powders and bleaches in the household cleaning products (such as Comet and Ajax) to which some screen makers resort are unsuitable for screen-print usage. (Save them for those tough pots and pans.) Most authorities agree that it is necessary to abrade only the print side of the screen fabric. The concern here is to minimize mesh damage, or wearing out the mesh. This process does, after all, "damage" the mesh which, if controlled, is a good thing. To much of that good thing will lead to premature mesh failure. Care should be taken to abrade the entire screen evenly, because ink deposit may differ between abraded and non-abraded areas. The screen should then be rinsed with a pressure washer or garden hose. Below are the generally recognized steps for a properly abraded screen: 1) Abrasive compound in a gel or paste form is preferable to dry powder. This is because finely powdered abraders easily become an airborne hazard. They can scratch anything with which they come into contact (including eye glasses and contact lenses) and can be accidentally inhaled. 2) The compound should be applied to a wet screen using a circular motion-similar to that used when waxing a car-with a high-density polyester or nylon-bristled brush rather than coarser brush materials. 3) Abrasion at moderate pressure will likely produce a balanced result in 5 to 10 seconds, and should certainly continue no longer than 15 seconds. In order for this procedure to produce repeatable results, consistent pressure is also very important. Since this is a mechanical process done by a human, a certain amount of variation in abrasion pressure is inevitable. Mesh may be abraded in a predetermined way for a predetermined time, yet the amount of force applied by the screen maker is likely to vary (depending on mood, fatigue level and so on) and will certainly vary between different people. Variations in pressure alone could result in fabric damage in some cases and insufficient adhesion in others. Therefore, in addition to setting parameters for technique and time of application, careful experimentation and training must also be done to maintain as much pressure consistency as possible. Finally, mesh must also be degreased or stripped of solvents and/or oily residues prior to further processing, because these contaminants will interfere with adhesion of the stencil. Even (perhaps especially) new mesh is not exempt, as it is still laden with lubricants with which it is contaminated during manufacture. Degreasing can be especially crucial with direct liquid emulsions, because contaminants can interfere with coating consistency by preventing an even flow of the liquid when it is applied. Although there are several products that may be used for degreasing, best results are achieved with commercial degreasing agents available from stencil manufacturers. (Household dishwashing liquids are sometimes used, but many contain fragrances and skin-care additives such as lanolin, which may remain on the mesh after rinsing. Lanolin is, after all, an oil.) With capillary films, the degreaser should also contain a wetting agent. Its presence on the mesh ensures good adhesion by promoting the smooth, unbroken "sheeting" of the water on the mesh that is so necessary to promote good capillary action (the water literally draws the stencil into the mesh) for which these films are named. Degreasing should not be confused with removal of residual ink or stencil material, which require screen-wash or stencil-reclaim chemistry. |
1997 National Business Media, Inc.
Among
mesh tension's various jobs are to provide resistance to the squeegee blade,
to pull the mesh out of the ink immediately following the print stroke and
to prevent mesh from rolling in front of the blade.