By Geoff Baxter
The screen frame is one of the most critical links in screen printing. Unfortunately, it is also one of the most overlooked. Many problems attributed to flaws in the press are, in reality, often attributable to poor screens.
While the initial cost of wood frames may be attractive, the poor dimensional stability and its susceptibility to changes in atmospheric conditions make it a poor choice for all but the most rudimentary printing applications. The cost of press down-time resulting from the use of unstable wood frames far outweighs the cost of upgrading.
Although steel and aluminum frames are far superior to wood, they are still static frames. By static, we mean that once the mesh has been stretched and attached to the frame, the degree of tension is static (fixed). In other words, tension cannot be increased or decreased without removing the mesh from the frame and re-stretching it.
Synthetic materials: Polyester screen mesh, for example, goes through a phenomenon known as cold flow. Cold flow is the tendency of a material's molecules to elongate and realign under stress. This appears in a loss of screen tension known as mesh relaxation. Static frames, both wood and metal, cannot compensate for this loss of tension. While minor changes in screen tension may not seem terribly important, they can be the source of poor registration, premature screen breakdown, insufficient ink film deposit, and color inconsistency.
Self-tensioning frames are not a recent development. In fact, they have existed in one form or another for several decades. However, only in recent years have they gone mainstream. Self-tensioning frames are available in both round and square-bar formats. Either of these styles allows the mesh to be tensioned directly on the frame without the aid of an outside stretching deviceand to be re-tensioned for cold flow throughout its usable life.
There are several formats and manufacturers of self-tensioning frames. Consider them all, but base your purchase on frame quality, ease of use, and the manufacturer's reputation. Do not base your choice on price alone. Keep in mind that when properly cared for, quality re-tensionable frames will have a substantially longer lifespan than standard wooden frames, allowing you to amortize their cost over a much longer period of time.
Mesh Count: Proper mesh selection is critical to maintaining the extremely fine detail required in high-quality screen printing, and to eliminating objectionable moiré patterns.
Mesh count can affect moiré, color balance, resolution, and fine detail. Choosing the correct mesh becomes even more critical in technically challenging projects. The mesh count must provide the necessary image resolution while minimizing the potential for moiré patterns. A fine mesh count is required to reproduce the full tonal range present in today's halftones. While there are many mathematical formulas available to compute the appropriate dot-to-thread ratio, I believe that when using 55 or 65 line-dot screens, a mesh count of 355 for automatic printing (305 for hand printing) will usually yield the best results. The 355-count offers a higher degree of assurance that the print will be free of moiré. Unfortunately, due to its tighter weave, 355-count mesh may prove difficult to print manually.
Mesh Color: The use of a dyed fabric is critical in this application. Dyed fabrics help eliminate halation (the scattering of during screen exposure), by absorbing extraneous light rays. Dyed fabrics are available in yellow, red, and orange shades. Each of these colors will perform its function equally well. Keep in mind that the various colors will require increased exposure times based upon the density of the color. As a rule, exposure times for fabrics of the same mesh count will increase as follows, with white fabric as the reference:
Thread Diameter: Thread diameter can be just as critical as mesh count. Most fabrics are available in several grades. The standard notation for thread diameter has been "S" for small, "T" for thick, and "HD" for heavy duty. While these notations offer some insight into the deposition characteristics of the mesh, diameters with the same notation but coming from different weaving mills can vary by several microns. This variance can result in a noticeable difference the amount of ink deposited. This variable is just one more control point that can negatively affect consistency and repeatability. Variances between manufacturers can be minimized by specifying not only the mesh count, but also by specifying thread diameter in microns.
The most important aspect of mesh composition is its ability to hold registration. Resins are used to produce the threads that are woven into fabric. In modern screen printing, the only readily available options are Nylon and Polyester. Nylon is a poor choice for any type of close-tolerance printing since it exhibits hydrophilic characteristics (tendency to absorb moisture). That tendency can wreak havoc on the dimensional stability of the mesh. On the other hand, screen mesh woven from threads that are from polyester resin are far more stable, and are unaffected by changes in heat and humidity. Polyester is, in fact, referred to as hydrophobic, or antagonistic toward water. It's the best choice for printing closely-registered four-color process images.
LE (low elongation) mesh can be an excellent choice for full-color reproduction. LE fabrics are less susceptible to cold flow (fabric relaxation), rendering screens far more stable. However, although LE meshes will not, as a rule, tolerate extremely high screen tensions, they're a must for static frames.
High-tension fabrics are another recent development in the techno-mesh arena. They're able to tolerate tension levels that once would have been unimaginable. When properly stretched, these fabrics can greatly increase the quality and consistency of four-color process prints.
Weaving Technique: Weaving technique is probably the single most overlooked screen variable. We think of weaving as one thread over and one thread under (the term for this is plain weave). In reality, many screen fabrics, especially those with higher mesh counts, are woven with a twill technique. Twill fabrics have threads positioned either one over/two under or two over/two under. This variance in fabric weave can have some effect on the way ink is deposited, and may be a culprit in causing moiré patterns to form between the mesh and certain halftone separations in four-color process printing. For best results, plain weave fabrics should be specified for printing critical halftones.
© 2004 Geoffrey Baxter no unauthorized duplication without written consent of the author.
About the Author
- Email Address email@example.com
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Geoff Baxter previously served as President and co-founder of Atlantic Screen Supply, Inc. His 30-year industry career includes management positions with Logo-7, Harlequin Nature Graphics, Advance Process Supply Co., and Precision Screen Machines. He has also written numerous articles for trade publications and lectured at many industry events.
Geoff can be reached at firstname.lastname@example.org.