Blade Trade-offs

Plastic and composite doctor blades come up sharp in safety and wear, but steel's metering edge may be tough to beat.

By Susan Friedman

"Plastics make it possible," a recent American Plastics Council ad campaign trumpets. In the doctor blade arena, plastic and composite blades have offered an alternative to steel for over 20 years. Here's a review of what these materials make possible on-press, most notably, safety perks and wear resistance.

Meter credos

Supplier perspectives on blade metering capability leave plastics, in particular, a bit behind. Mark Hahn, director of sales and marketing for AAA Press International, Arlington Heights, IL, states in customer tests of doctor blades in narrow-web applications, plastics have shown a tendency to hydroplane, or lift up from the anilox roll, due to their lack of rigidity, while harder, more rigid steel blades actually shear the ink for better metering. He adds that plastic blades' hydroplaning tendencies can lengthen the life of older anilox rolls, providing more volume for achieving desired color hues.

Michael Clark, sales manager for graphic arts at Cincinnati-based International Knife and Saw, chalks steel blades' metering superiority up to their smaller contact zone on the roll, typically .006-inch to .008-inch. Thicker plastic blades (typically .014-inch and .020-inch) have a larger contact zone, and therefore don't wipe as cleanly.

Paul Sharkey, president of FLXON, Charlotte, NC, agrees that plastics' thickness can hinder performance. "The goal of the doctor blade is to get rid of ink's surface film on anilox rolls, which can overwhelm an image," he says. "This can't be done with plastic blades because the gauge required to achieve rigidity is far too thick to achieve uniform, precise ink films."

Steve Aldrich, product manager at Kidder, Agawam, MA, places time on plastics' side. "Plastic blades typically require a break-in period of several thousand feet to develop a suitable metering edge. During this period, screens and solids may exhibit inconsistent ink laydown, which can cause plugging or tonal variation," he says, adding that some plastic materials are prone to rippling or waviness in the blade holder at lengths beyond 45-inches.

Suppliers rallied around plastics' use in enclosed blade systems. FLXON, for instance, sells plastic blades as containment blades for the bottom side of chambered systems. International Knife and Saw's Clark explains, "In enclosed doctor blade systems, where blade angles are 30 degrees and the goal is simply to keep the ink in the system, either steel or plastic is fine" for containment.

Safety—first or last?

Plastic's safety niche is seen by industry experts as both a blessing and a burden. "Everybody I've talked to who is interested in such plastic blade materials as polyester has experienced a serious injury when removing a steel blade," remarks Karen Tichener, International Knife and Saw product specialist.

Avoiding steel doesn't automatically eradicate injuries, however. Hahn warns that cuts can occur with more rigid plastics, while Kidder's Aldrich confirms that "some composite blades can become sharp as a razor once they've worn in and become honed through contact with the anilox surface."

And should safety be the leading motivator in blade purchases? According to FLXON's Sharkey, "The only rhyme or reason for plastic blades is safety, and that to me is poor management. Steel blades cause injuries at some plants, and never cause injuries at others. Safety is not a good printing reason."

Customer requirements may determine when safety can speak loudest in blade selection. Clark says "converters will sacrifice quality for the safety factor [with plastic blades]," and Hahn states "plastics tend to be used successfully in lower-end, two-color applications, while for high quality printing, you must use steel."

Material values

The durability of a plastic or composite blade, like any other, has as much to do with blade material characteristics as the pressure applied.

"Our philosophy is to print a job with the least blade pressure possible to reduce wear," states Clark. "Not only do plastics wear faster because they're softer, but converters may tend to use more pressure on these blades because of their lack of rigidity."

On steel blades, too much pressure can send contaminating steel shavings into the ink and press components, necessitating clean-up with magnets or filters. "Converters who don't control pressure as closely can avoid this clean-up with plastics such as polyester," Clark adds.

Plastics' attractive no clean-up option may cloud selection methodology. "Magnets and filters as cleaning methods [for steel] take time, and rather than take that time, converters have thrown these methods out...and automatically go with plastic blades," comments Sharkey.

For carbon steel blades, corrosion is a major durability foe, especially in conjunction with water-based inks, says Clark. Leon Beaudoin, president of FlexoExport, Trumbull, CT, adds corroded steel can cause a damaging sawtooth effect on the anilox roll, while plastic's edge stays cleaner, actually slowing down its wear. He projects that steel blades last about a day in continuous use, while plastics such as nylon last longer, though the latter can be damaged just as easily as steel. Any nick in the blade will cause ink beading, he says.

Sharkey also cautions as plastic blades wear, strings on the edge of the blade can cause "hills and valleys" in the ink.

In addition to predicting wear, the material make-up of a plastic or composite blade can determine whether it is less expensive, comparable or more expensive than steel. Suppliers of various blade types gauged these blades as potentially all of the above.

Hahn states plastics can be twice as costly as steel, while composites made of fiberglass, graphite or other "exotic materials" used to increase rigidity and strength can be even more expensive, due to "higher quality controls" required in manufacture.

Beaudoin believes that nylon plastic blades are less expensive than steel, and "in the long run you use more steel than plastic."

Roll calls

Doctor blade and anilox roll usage trends suggest nearly parallel usage levels for plastic blades and chrome rolls, as well as steel blades and ceramic rolls. Hahn estimates anilox use to be 65 percent ceramic, 35 percent chrome, and places blade use at 75 percent steel, 25 percent plastics. These similar percentage patterns also reflect typical on-press pairings of these components.

"With chrome anilox rolls, we would recommend a plastic blade because steel can degrade chrome cell volume in seven to nine months," Hahn says. "Converters will choose plastic blades to extend chrome anilox roll life, and ultimately save money on the higher cost item—the roll. On ceramics, customer tests have shown that plastic blades do transfer to the cells, essentially melting at high speeds, and can potentially plug rolls."

Beaudoin agrees that plastic is less abrasive on chrome, while the wear ratio with steel on chrome is "very high." He also concurs that steel holds a sharper edge on a ceramic roll's harder surface, where it can better handle thicker, heavier viscosity inks typically used with ceramic's finer screen counts.

Dan Reilly, graphics specialist at Harper Corporation of America, Charlotte, NC, says steel blades can cause roll scoring when fine steel "wires" produced by the blade get trapped between it and the anilox. Metal particles from other sources can imbed in a plastic blade and cause scoring, he says.

FlexoExport's Beaudoin sums up that "ultimately blade selection is personal preference. Plastic is as utilitarian as steel and, ultimately, you get more life out of it."