Combination Rotary Screen Printing

Converters looking to differentiate their capabilities with exciting and innovative imagery are making combination rotary screen printing their top choice.

Next time you're playing bartender, look behind (literally) the dashing graphics on that bottle of margarita mix. Chances are you'll see a brilliant white or colored background printed with rotary screen. From food and beverage to health and beauty, rotary screen is laying the foundation for some of today's most exciting and innovative imagery.

Since the 1980s, when it was first commercialized, rotary screen technology has found increasing favor in the label and package printing world. The main feature of screen printing—its ability to lay down a bold, opaque ink film on clear, metallized, or highly colored substrates—has become especially appealing with the explosion of film labels and packaging.

At the same time, the extreme simplicity and adaptability of the rotary screen process has made it a natural fit with a new generation of modular, combination web-fed presses. As a result of this marriage of need and convenience, combination rotary screen printing has become a key consideration for printers looking to differentiate their capabilities while exploiting sound and viable existing technology.

"More and more presses are being delivered with combination capabilities, and rotary screen is typically part of the combination," commented John Costenoble, sales manager for Stork's Graphical Print Systems (Charlotte, N.C.). Stork provides rotary screen presses and subsystems, and supplies engineering and retrofits for more than 35 different press manufacturers. Costenoble says combination rotary screen has quickly grown outside its initial pressure-sensitive label market. "Narrow-web printers start out with screen printing because they need the opacity it provides. They use it to print whites on films and foils to serve as a base for the four-color process. But many printers, once they're comfortable with the process, start selling the value-add features of screen."

Today, screen units can be found in-line on flexo, offset, and gravure presses, and their value-add is quite diverse. Because of its ability to apply an extraordinarily thick ink deposit (up to 260 microns), rotary screen can be used to create 3-dimensional tactile effects and Braille labeling. Used with UV varnishes, rotary screen can create high gloss or embossing effects. With metallic inks, it provides a cost-effective and environmentally friendly alternative to foil stamping. It's ideal for high-density requirements like RFID and magnetic printing, and for printing with fluorescent, thermochromatic, and other specialty inks with a variety of particle sizes in water-based, solvent, and UV formulations.

Inside the rotary screen subsystem

Screen printing is unlike any of the other printing processes in that the image is actually transferred to the substrate through the image carrier. A stenciled screen mesh cylinder is inked on the inside. As the web contacts the rotating mesh cylinder, a squeegee pushes the ink through openings in the screen onto the substrate. The size of the openings (or the mesh) in the screen, and the amount of pressure applied by the squeegee control the thickness of the ink deposit.

Stork has refined a technique for manufacturing the screen mesh that Costenoble says sets it apart from the competition. The company makes its RotaMesh of 100 percent nonwoven nickel using an electroform or galvanic process similar to the way car parts are chromed. A production mandrel with insulated and non-insulated areas is immersed in a liquid nickel bath, and an electrical charge is applied to the mandrel. Hexagonal nickel walls form where the mandrel is uninsulated and openings are left where the mandrel is insulated. According to Stork, the hexagonal shape provides superb stability for screens as much as 10 feet wide.

To create an imaged screen printing cylinder, end rings are glued in to the mesh. The newly formed cylinder is coated with a liquid, water-based photosensitive emulsion to block off all the holes in the mesh, then dried in a dryer. An undistorted film positive of the print image is wrapped around the coated cylinder and the emulsion is exposed in the round with a UV light source. The unexposed emulsion is then washed out in an automatic developing machine, leaving a stencil for the ink.

From there, the cylinder is ready to go to press. After the run, it can be cleaned with post-press equipment, and stored if needed for a repeat. Costenoble says Stork screens can also be stripped of all the emulsion—using the same developing machine with higher water pressure—and re-imaged as many as 10 to 20 times. That has some serious economies as Costenoble points out: "If you paid $400 for the cylinder, it can bring the cost per run for the screen down to $20."

Not counting the imaging cost, Stork provides that service (with screen and film positive in hand) in 3 to 5 days for around $175 per screen. Printers with enough volume, or who want control closer to the printing press, can bring the image processing equipment in-house. Costenoble says at least half of the printers that use Stork screens image their own.

Making it a winning combination with in-line flexo

Screen manufacturers, press manufacturers, and ink manufacturers have all recognized rotary screen's aesthetic, as well as logistical compatibility with in-line flexo printing—particularly with UV flexo. The instant curing nature of UV inks eliminates some ink-drying complexities, and the open press architecture makes it exceptionally easy to integrate rotary screen units into the production sequence.

In fact, Stork has made it even easier by codeveloping a new, less expensive drop-in subsystem with several narrow-web equipment providers. Danish Nilpeter (Davie, Fla.) was among the first to announce it was offering drop-in screen printing options for its FA- and FB-3000 series 10-, 13-, and 16-in. presses. The drop-in subsystem basically works by replacing the anilox and rubber roll flexo inking system with a rotary screen unit that acts as another impression roller.

Two of the first systems have been put into production at OPM Group in the United Kingdom. The company retrofitted the units to its 2-year-old, eight-color FA-3300 series press with cold foil stamping. OPM Chair Chris Ellison says the press operators found the units easy to use and he was delighted that they "… retained the same UV-lamp configuration, and because they operate with the same print register controls, there is no extra set-up time."

Ink manufacturers, too, are hastening to enhance screen's place in line with UV flexo. Even smaller companies, like Deco-Chem (Mishawaka, Ind.), are putting their research and development behind combination UV flexo and rotary screen. According to Brent Davis, VP of Deco-Chem, the company has just released their RSF silicone-free UV ink for rotary screen printing.

Rotary screen inks typically have silicone additives to aid in adhesion. When used to underprint flexo process inks, however, the silicone can cause beading and rivering—an issue already inherent in flexo printing on film substrates. Take the silicone out, though, and you get issues with ink adhesion. Davis says the field results for RSF inks have nonetheless been promising for a range of thin, clear, and metallized films and UV flexo inks—without the orange peel mottling or poor gloss performance that have plagued earlier attempts at eliminating silicone.

By: Terry McConnell