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.
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.