Seeing Applications Through

Screen material suppliers address vignette and metallic ink printing challenges, and look toward digital prepress compatibility solutions.

by Susan Friedman

With a wide-open attitude toward changing needs, screen material suppliers are addressing everything from larger pigment ink problems to process printing prospects, to compatibility with more presses.

With nickel sleeve constructions, the byword is stability. Stork Rotaform's line of Rotamesh rotary screens, which are produced in the round, feature a 100 percent nickel, non-woven material that is electroformed to create walls and holes, along with a smooth surface inside and outside. "Most other rotary screen materials are woven with the same materials typically used for flat screen printing, and then made round," explains John Costenoble, sales manager.

Costenoble says Stork's nickel construction is stable enough to allow production of screens as wide as 10´, complementing the current trend in this segment toward wider presses. Rotamesh screens can also be reused up to 15 times.

New product developments underway at Stork include a fixed-angle vignette screen, in which a quarter or a half of each hole can be blocked off, depending on how the film is prepared, to achieve vignettes and halftones.

Stainless steel mesh screenmakers tout dura-bility. In the case of Gallus' stainless steel Screeny rotary screens, which are nickel-plated and coated with an emulsion and a protective layer before being shipped in roll form, performance levels average 100,000 to 150,000 revolutions.

Coming advances in Screeny technology center on package printing's mad dash toward metallic inks, which harbor what Technical Service Manager Terry Trexler dubs "the problem of pigment size."

With metallics, he elaborates, the larger-size pigments needed to ensure better lustre typically clog screens over time. Gallus is currently working on a thicker mesh screen that will more easily pass metallic particles. However, Trexler relates the challenge with a thicker weave is to ensure pigments not only pass through, but rejoin on the substrate to achieve a smooth laydown.

Over in flatbed screen printing's neck of the woods, resistance to the elements keeps printers coming back to polyester mesh constructions with a PVA and PVC emulsion, reports Walt Waltmire, sales manager at Dynamesh, a company which manufactures both stainless steel and polyester mesh screens. Waltmire states polyester's popularity stems from its ability to hold shape and withstand almost all solvents.

Polyester mesh-makers continue to hone performance. Waltmire sees polyester being made stronger to allow for thinner threads, which in turn leave less mesh to interfere with the image. New treatments to reduce static electricity and improve emulsion adhesion are also coming on the scene. Dynamesh's new anti-static treated mesh, designed for its SR and EX polyester monofilament screens, maintains the same specifications as a regular mesh. Other supplier efforts have increased fabric thickness or weakened the mesh, he contends. Anti-static measures aim to improve ink flow and help print stay sharp, even on more difficult substrates.

Considering material and process

Ink laydown and tactile feel requirements are major considerations when choosing a screen material, but these specs can also show whether screen printing is indeed the appropriate process for a job.

Costenoble estimates 75 percent of all label screen printing applications can be satisified with a 10 micron laydown. "If a printer is trying to put down less than 8 microns, the application may be better suited for UV flexo," he advises, noting that printers can most easily justify screen as the process of choice if the desired ink deposit level is double typical UV flexo levels, which run 6 microns to 8 microns.

Trexler depicts a similar selection trend. Of the nine Gallus Screeny meshes available, package printers most often opt for the KS screen, a 325 mesh with a 10 micron ink laydown, or the KM screen, a 325 mesh with a 7 micron ink laydown.

"These two products provide the most opacity at a reasonable press speed," he explains. "Screens with lower ink laydown levels have their place for applications with small copy, but opacity suffers. On the other hand, a higher ink laydown may be a necessity for bolder graphics but will negatively impact press speeds."

An additional trend fueling the industry's allegiance to 7- and 10- micron deposit level screens is that the lion's share of screen printers are laying down screen for overprinting with process. According to Trexler, this usage pattern marks a resurgence of screen against UV flexo, particularly for laying down opaque white backgrounds. "Everybody was going to double-bump UV flexo and eliminate screen," he says, "but now printers are realizing that approach may not be as economical—mainly because it can tie up two print heads."