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Screen Printing



A powerful printing process maintains its strong presence in the narrow web industry.



Published September 2, 2009
Related Searches: Rotary screen Label printer UV flexo Embossing
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Screen Printing



A powerful printing process maintains its strong presence in the narrow web industry.



By Jack Kenny



A RotaMesh screen from Stork Prints
Screen printing is an old process, and widely used in industrial, decorative, and artistic markets the world over. Textiles, particularly clothing, rely heavily on the screen printing process for decorations from the most simple to the complex. Packaging also has its share of the screen market, and today we are seeing the growth of this print process in an emerging industry: printed electronics.

The single greatest advantage that screen printing has over all other production printing processes is in the volume of ink that can be laid onto a substrate. UV letterpress can deposit ink as thick as 2 to 3.5 microns; UV offset is 1.5 to 2.5 microns; gravure is 2 to 5 microns, and UV flexo can lay down from 3 to 8 microns. UV screen, however, has an ink thickness that ranges from 4 to 30 microns and beyond.

Silkscreening came into being in China as an artistic medium around 1,000 years ago, and was soon adopted by Japan and other Asian cultures. It is thought to have been introduced to Western Europe in the late 1700s, but didn’t take off until the silk trade stabilized and a profitable market evolved.
Today, of course, silk has nothing to do with the process.

All screen printing was accomplished using flat screens until the mid-20th Century. Rotary screen printing was pioneered by George Reinke in the early 1960s. He grew up in the family’s flat screen printing business, and developed the first in-the-round screen and a two-color press in which to employ it.

Screens today are constructed of nickel and steel, available in a variety of meshes that will determine the volume of ink to be deposited. They are coated with photosensitive emulsions – proprietary formulas from each supplier – used to create the image to be printed.

In the conventional screen exposure process, a positive film image is placed over the emulsion-coated screen and exposed to light. The emulsion on the unexposed portion of the image washes away with water to become the image to be printed. The emulsion that remains on the exposed part of the screen has bonded together under the light, sealing the openings in the mesh to prevent ink from going through.

In flat screen technology, ink is applied to one end of the screen and spread by a moving squeegee across the entire length of the screen. The screen is prepared through a photo-imaging process so that the open parts of the screen mesh allow the ink through, and the closed areas prevent ink transfer.
In the rotary process, the ink is pumped into the center of the screen, which is formed into a cylinder (see photo on page 52). The squeegee is in a fixed position, and as the screen rotates, the ink is transfered to the passing substrate.

One of the most common uses of screen printing in the label business is the application of a coat of opaque white ink to a film as a dense base for the application of other colors via flexo or other process. This gives the colors a depth not achievable by other means. Rotary screen units used for this purpose have for years been manufactured as separate print stations, and can be positioned at any point on an inline press by use of a rail structure above the other print stations.

Brand sophistication and competition have taken screen printing well beyond the opaque white laydown. Many labels require more than one screen color, which gives it a more tactile feel. In addition, screen printing is used on flat substrates to create an overall texture to evoke richness and complexity.

Still another growing use of the screen process is in the area of product security. Microscopic taggants are blended into coatings and applied via screen printing onto labels and other packaging products. Such taggants, when scanned, deliver proprietary brand information and are aimed at foiling counterfeiters.

Screeny


Eight years ago, a product manager at Gallus, a manufacturer of narrow web presses, reported that nearly all of the presses the company was selling contained at least one rotary screen printing head. Today the picture has changed.

“Mostly everything we sell now goes out with several screen units,” says Jim Flynn, manager of print technology at Gallus Inc., Philadelphia, PA, the US arm of the Swiss company. “We’re selling four screen heads per press, and a lot of those sales have to do with security applications. Many of them are being used for doming varnishes instead of embossing, and are being used in the wine industry. The end users want a lot of special effects from the varnishes available today.

“The market for screen printing has not shrunk,” Flynn says. “We see it growing.”

Doming requires a heavy laydown of a coating, says Flynn, “45 microns or more. We have several different meshes of Screeny being used for that process.” Screeny is the brand of screen manufactured by Gallus in Switzerland. “We came out with a new line of screens a year ago – the S line – which are more stable, and we’ve added new meshes for coatings and for domings. Besides the 45 micron mesh for doming, we have a 250 micron mesh for Braille printing. A lot of techniques are being used to create these various effects using screens. Our Screeny portfolio now has 20-plus different meshes.” One such look that is gaining in popularity involves the use of reticulating varnishes, which results in a bubbly, wavy or foamy look when printed.

The increased stability in the Screeny S line, Flynn says, means that the printer is “able to get twice the amount of usage out of it than our standard screen. You get 200,000-plus revolutions out of one screen. It’s made of nickel-coated spring steel, and it doesn’t dent when you splice it.”

RotaMesh


Stork Prints is a global company, based in the Netherlands, whose most popular product in the narrow web industry is its RSI rotary screen unit. Stork’s RSI is the choice of many OEMs, and it appears in presses new and used throughout the industry.

José Maldonado, commercial manager for Stork Prints America, based in Charlotte, NC, USA, says that the screen printing market remains steady in the tough economy. “We see consistency selling not just rotary screen units but also in the consumables for the unit.”

Despite the proliferation of rotary screen printing units throughout the industry, Maldonado observes that many printers are not taking full advantage of the capabilities of the process. “Our main focus today is in providing information about the numerous applications our equipment can accomplish. I have found that in this market the printers limit themselves to certain uses of rotary screen because they’re not aware of the potential. They need more education, no question about it.”

Maldonado notes Clemson University in South Carolina is ahead of all other university-level programs in educating students about screen printing, and he is trying to make connections with other institutions of higher learning to encourage them to expand their course offerings. Charlotte-based Central Piedmont Community College, always active in print education, is one of those that will undertake more education about screen printing, he says.

Designers, Maldonado adds, also need more education about the benefits of rotary screen. “The process has a basic similarity to flat screen printing, but the applications, the level of ink deposits you can achieve with rotary screen in narrow web printing are enormous. Graphic designers will be able to produce much more effective packaging and label designs if they know how to include screen printing techniques in their designs.”

In terms of growing screen applications, Maldonado says that printers today “can almost duplicate a hot foil stamp on a label using metallic inks. Security inks and coatings are also on the rise. In Europe, with the use of special screens, printers are able to print bank notes, or put security inks into the bank notes.”

Stork Prints’ RSI units use RotaMesh screens, which have a base of electroformed nickel covered with a photosensitive emulsion for the developing process. Small printers order their screens directly from Stork; larger printers, however, can purchase a screen-making system from the manufacturer. “The advantage of using 100 percent nickel is that the screens can be re-used. Emulsion can be added as many times as you need,” Maldonado says. “And that can all be done in-house if you have the prepress system.”

CCL Label’s Home and Personal Care Division is investing in Stork Prints’ rotaLEN 5511 direct laser engraving system at four of its plants for the imaging of RotaMesh rotary screens. Until last year, CCL relied on the conventional UV exposure method for all its screens. But the process was viewed as cumbersome, and the company wanted more consistency.

One of the factories, CCL Moussy, near Paris, France, reports improved quality, consistency, and productivity as a result. “We could never imagine returning to analog now,” says Xavier Boutevillain, the front end director at CCL. “The switch-over has reduced scrap materials, improved quality levels, and given us a faster, more productive workflow.”

Direct laser engraving involves the thermal decomposition of emulsion covering the positive area, leaving behind the stencil’s open areas. The emulsion is burned away by a 250 watt laser. The screen spins at high rotation, moving steadily, under the beam’s path. Once the image is created, the screen is ready for the press.

Flat screen


Werner Kammann Maschinenfabrik has been manufacturing flatbed screen presses, as well as a wide range of other screen application machinery, for decades. The German company’s roll-to-roll presses are in use around the world and are a popular choice for label printers in need of flatbed screen equipment. The company also has been putting its screen presses to use in the printed electronics business since the 1990s.


A flat screen unit in a Kammann press
“Our presses have been used to manufacture printed electronics components for quite a long time,” says Steve Gilbertson, vice president of sales and marketing for Kammann USA, the company’s North American arm. “In the medical industry alone our machines are used for printing of EKG sensors, defibrillator patches, and biotabs for diabetic applications – measuring glucose levels.”

Gilbertson says that two critical areas in the printing of electronic materials are control and drying. “We have invested heavily in how to control materials,” he says. Using a thinner material typically allows the user to have a less expensive carrier. Then you can also dry better. We have a tension controlled web through the machine, and we can run at relatively decent speeds with screen.” Decent, he adds, is about 20 to 25 meters per minute (65-80 fpm).

Kammann presses typically have two or three print stations. Gilbertson says that the presses are used to print “a lot of silver, dielectrics, truly organic materials. In the medical world, our presses print live enzymes for biotabs.” The machines Kammann makes to produce the biotabs are quite sophisticated, he adds. “There are up to 500 biotabs in one repeat.” The presses also are used to print small OLEDs – organic light emitting diodes. “Our presses have done all of these using flat screen, not rotary screen. With flat screen we can control the deposition better; we can control the line so that there is no stairstepping or broken images.”


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